Systemverilog For Verification A Guide To Learning The Bench Language Features

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SystemVerilog for Verification

Chris Spear

●

Greg Tumbush

SystemVerilog
for Verification
A Guide to Learning the Testbench
Language Features
Third Edition

Chris Spear
Synopsys, Inc.
Marlborough, MA, USA

Greg Tumbush
University of Colorado, Colorado Springs
Colorado Springs, CO, USA

ISBN 978-1-4614-0714-0
e-ISBN 978-1-4614-0715-7
DOI 10.1007/978-1-4614-0715-7
Springer New York Dordrecht Heidelberg London
Library of Congress Control Number: 2011945681
© Springer Science+Business Media, LLC 2012
All rights reserved. This work may not be translated or copied in whole or in part without the written
permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York,
NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in
connection with any form of information storage and retrieval, electronic adaptation, computer software,
or by similar or dissimilar methodology now known or hereafter developed is forbidden.
The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are
not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject
to proprietary rights.
Printed on acid-free paper
Springer is part of Springer Science+Business Media (www.springer.com)

This book is dedicated to my wife Laura, who
takes care of everything, my daughter Allie,
long may you travel, my son Tyler, welcome
back, and all the mice.
– Chris Spear
This book is dedicated to my wife Carolye,
who shrugged off my “I need to work on the
book” requests with a patient smile, and to
my toddler son Lucca who was always
available for play time.
– Greg Tumbush

Preface

What is this Book About?
This book should be the first one you read to learn the SystemVerilog verification
language constructs. It describes how the language works and includes many examples on how to build a basic coverage-driven, constrained-random, layered testbench using Object-Oriented Programming (OOP). The book has many guidelines
on building testbenches, to help you understand how and why to use classes,
randomization, and functional coverage. Once you have learned the language, pick
up some of the methodology books listed in the References section for more information on building a testbench.

Who Should Read this Book?
If you create testbenches, you need this book. If you have only written tests using
Verilog or VHDL and want to learn SystemVerilog, this book shows you how to
move up to the new language features. Vera and Specman users can learn how one
language can be used for both design and verification. You may have tried to read
the SystemVerilog Language Reference Manual but found it loaded with syntax
but no guidelines on which construct to choose.
Chris originally wrote this book because, like many of his customers, he spent
much of his career using procedural languages such as C and Verilog to write tests,
and had to relearn everything when OOP verification languages came along. He made
all the typical mistakes, and wrote this book so you won’t have to repeat them.
Before reading this book, you should be comfortable with Verilog-1995. You do
not need to know about Verilog-2001 or SystemVerilog design constructs, or
SystemVerilog Assertions in order to understand the concepts in this book.

vii

viii

Preface

What is New in the Third Edition?
This new edition of SystemVerilog for Verification has many improvements over the
first two editions, written in 2006 and 2008, respectively.
• Our universities need to train future engineers in the art of verification. This
edition is suitable for the academic environment, with exercise questions at the
end of each chapter to test your understanding.
• Qualified instructors should visit http://extras.springer.com for additional materials such as slides, tests, homework problems, solutions, and a sample syllabus
suitable for a semester-long course.
• The 2009 version of the IEEE 1800 SystemVerilog Language Reference Manual
(LRM) has many changes, both large and small. This book tries to include the
latest relevant information.
• Accellera created UVM (Universal Verification Methodology) with ideas from
VMM (Verification Methodology Manual), OVM (Open Verification
Methodology), eRM (e Reuse Methodology), and other methodologies. Many of
the examples in this book are based on VMM because its explicit calling of phases
is easier to understand if you are new to verification. New examples are provided
that show UVM concepts such as the test registry and configuration database.
• When looking for a specific topic, engineers read books backwards, starting with
the index, so we boosted the number of entries.
• Lastly, a big thanks to all the readers who spotted mistakes in the previous
editions, from poor grammar to code that was obviously written on the morning
after an 18-hour flight from Asia to Boston, or, even worse, changing a diaper.
This edition has been checked and reviewed many times over, but once again,
all mistakes are ours.

Why was SystemVerilog Created?
In the late 1990s, the Verilog Hardware Description Language (HDL) became the
most widely used language for describing hardware for simulation and synthesis.
However, the first two versions standardized by the IEEE (1364-1995 and 13642001) had only simple constructs for creating tests. As design sizes outgrew the
verification capabilities of the language, commercial Hardware Verification
Languages (HVLs) such as OpenVera and e were created. Companies that did not
want to pay for these tools instead spent hundreds of man-years creating their own
custom tools.
This productivity crisis, along with a similar one on the design side, led to the
creation of Accellera, a consortium of EDA companies and users who wanted to
create the next generation of Verilog. The donation of the OpenVera language
formed the basis for the HVL features of SystemVerilog. Accellera’s goal was met

Preface

ix

in November 2005 with the adoption of the IEEE standard 1800-2005 for
SystemVerilog, IEEE (2005). In December 2009, the latest Verilog LRM, 13642005, was merged with the aforementioned 2005 SystemVerilog standard to create
the IEEE standard 1800-2009 for SystemVerilog. Merging these two standards into
a single one means there is now one language, SystemVerilog, for both design and
verification.

Importance of a Unified Language
Verification is generally viewed as a fundamentally different activity from design.
This split has led to the development of narrowly focused languages for verification
and to the bifurcation of engineers into two largely independent disciplines. This
specialization has created substantial bottlenecks in terms of communication
between the two groups. SystemVerilog addresses this issue with its capabilities for
both camps. Neither team has to give up any capabilities it needs to be successful,
but the unification of both syntax and semantics of design and verification tools
improves communication. For example, while a design engineer may not be able to
write an object-oriented testbench environment, it is fairly straightforward to read
such a test and understand what is happening, enabling both the design and verification engineers to work together to identify and fix problems. Likewise, a designer
understands the inner workings of his or her block, and is the best person to write
assertions about it, but a verification engineer may have a broader view needed to
create assertions between blocks.
Another advantage of including the design, testbench, and assertion constructs in
a single language is that the testbench has easy access to all parts of the environment
without requiring a specialized Application Programming Interface (API). The
value of an HVL is its ability to create high-level, flexible tests, not its loop constructs or declaration style. SystemVerilog is based on the Verilog, VHDL, and
C/C++ constructs that engineers have used for decades.

Importance of Methodology
There is a difference between learning the syntax of a language and learning how to
use a tool. This book focuses on techniques for verification using constrainedrandom tests that use functional coverage to measure progress and direct the verification. As the chapters unfold, language and methodology features are shown side
by side. For more on methodology, see Bergeron et al. (2006).
The most valuable benefit of SystemVerilog is that it allows the user to construct
reliable, repeatable verification environments, in a consistent syntax, that can be
used across multiple projects.

x

Preface

Overview of the Book
The SystemVerilog language includes features for design, verification, assertions,
and more. This book focuses on the constructs used to verify a design. There are
many ways to solve a problem using SystemVerilog. This book explains the tradeoffs between alternative solutions.
Chapter 1, Verification Guidelines, presents verification techniques to serve as
a foundation for learning and using the SystemVerilog language. These guidelines
emphasize coverage-driven random testing in a layered testbench environment.
Chapter 2, Data Types, covers the new SystemVerilog data types such as arrays,
structures, enumerated types, and packed arrays and structures.
Chapter 3, Procedural Statements and Routines, shows the new procedural
statements and improvements for tasks and functions.
Chapter 4, Connecting the Testbench and Design, shows the new SystemVerilog
verification constructs, such as program blocks, interfaces, and clocking blocks, and
how they are used to build your testbench and connect it to the design under test.
Chapter 5, Basic OOP, is an introduction to Object-Oriented Programming,
explaining how to build classes, construct objects, and use handles.
Chapter 6, Randomization, shows you how to use SystemVerilog’s constrainedrandom stimulus generation, including many techniques and examples.
Chapter 7, Threads and Interprocess Communication, shows how to create
multiple threads in your testbench, use interprocess communication to exchange
data between these threads and synchronize them.
Chapter 8, Advanced OOP and Testbench Guidelines, shows how to build a
layered testbench with OOP so that the components can be shared by all tests.
Chapter 9, Functional Coverage, explains the different types of coverage and
how you can use functional coverage to measure your progress as you follow a
verification plan.
Chapter 10, Advanced Interfaces, shows how to use virtual interfaces to simplify your testbench code, connect to multiple design configurations, and create
interfaces with procedural code so your testbench and design can work at a higher
level of abstraction.
Chapter 11, A Complete SystemVerilog Testbench, shows a constrained random testbench using the guidelines shown in Chapter 8. Several tests are shown to
demonstrate how you can easily extend the behavior of a testbench without editing
the original code, which always carries risk of introducing new bugs.
Chapter 12, Interfacing with C / C++, describes how to connect your C or
C++ Code to SystemVerilog using the Direct Programming Interface.

Preface

xi

Icons used in this book
Table i.1 Book icons
The compass shows verification methodology to guide
your usage of SystemVerilog testbench features.

The bug shows common coding mistakes such as
syntax errors, logic problems, or threading issues.

About the Authors
Chris Spear has been working in the ASIC design and verification field for 30
years. He started his career with Digital Equipment Corporation (DEC) as a CAD
Engineer on DECsim, connecting the first Zycad box ever sold, and then a hardware Verification engineer for the VAX 8600, and a hardware behavioral simulation accelerator. He then moved on to Cadence where he was an Application
Engineer for Verilog-XL, followed a a stint at Viewlogic. Chris is currently
employed at Synopsys Inc. as a Verification Consultant, a title he created a dozen
years ago. He has authored the first and second editions of SystemVerilog for
Verification. Chris earned a BSEE from Cornell University in 1981. In his spare
time, Chris enjoys road biking in the mountains and traveling with his wife.
Greg Tumbush has been designing and verifying ASICs and FPGAs for 13
years. After working as a researcher in the Air Force Research Labs (AFRL) he
moved to beautiful Colorado to work with Astek Corp as a Lead ASIC Design
Engineer. He then began a 6 year career with Starkey Labs, AMI Semiconductor,
and ON Semiconductor where he was an early adopter of SystemC and
SystemVerilog. In 2008, Greg left ON Semiconductor to form Tumbush
Enterprises, where he has been consulting clients in the areas of design, verification, and backend to ensure first pass success. He is also a 1/2 time Instructor at
the University of Colorado, Colorado Springs where he teaches senior and graduate level digital design and verification courses. He has numerous publications
which can be viewed at www.tumbush.com. Greg earned a PhD from the
University of Cincinnati in 1998.

xii

Preface

Final comments
If you would like more information on SystemVerilog and Verification, you can find
many resources at: http://chris.spear.net/systemverilog. This site
has the source code for many of the examples in this book. Academics who want to
use this book in their classes can access slides, tests, homework problems, solutions,
and a sample syllabus at http://extras.springer.com.
Most of the code samples in the book were verified with Synopsys’ Chronologic
VCS, Mentor’s QuestaSim, and Cadence Incisive. Any errors were caused by Chris’
evil twin, Skippy. If you think you have found a mistake in this book, please check
his web site for the Errata page. If you are the first to find a technical mistake in a
chapter, we will send you a free, autographed book. Please include “SystemVerilog”
in the subject line of your email.
Chris Spear
Greg Tumbush

Acknowledgments

We thank all the people who spent countless hours helping us learn SystemVerilog
and reviewing the book that you now hold in your hands. We especially would like
to thank all the people at Synopsys and Cadence for their help. Thanks to Mentor
Graphics for supplying Questa licenses through the Questa Vanguard program, and
to Tim Plyant at Cadence who checked hundreds of examples for us.
A big thanks to Mark Azadpour, Mark Barrett, Shalom Bresticker, James Chang,
Benjamin Chin, Cliff Cummings, Al Czamara, Chris Felton, Greg Mann, Ronald
Mehler, Holger Meiners, Don Mills, Mike Mintz, Brad Pierce, Tim Plyant, Stuart
Sutherland, Thomas Tessier, and Jay Tyer, plus Professor Brent Nelson and his
students who reviewed some very rough drafts and inspired many improvements.
However, the mistakes are all ours!
Janick Bergeron provided inspiration, innumerable verification techniques, and
top-quality reviews. Without his guidance, this book would not exist.
The following people pointed out mistakes in the second edition, and made
valuable suggestions on areas where the book could be improved: Alok Agrawal,
Ching-Chi Chang, Cliff Cummings, Ed D’Avignon, Xiaobin Chu, Jaikumar Devaraj,
Cory Dearing, Tony Hsu, Dave Hamilton, Ken Imboden, Brian Jensen,
Jim Kann, John Keen, Amirtha Kasturi, Devendra Kumar, John Mcandrew,
Chet Nibby, Eric Ohana, Simon Peter, Duc Pham, Hani Poly, Robert Qi, Ranbir
Rana, Dan Shupe, Alex Seibulescu, Neill Shepherd, Daniel Wei, Randy Wetzel,
Jeff Yang, Dan Yingling and Hualong Zhao.
Lastly, a big thanks to Jay Mcinerney for his brash pronoun usage.
All trademarks and copyrights are the property of their respective owners. If you
can’t take a joke, don’t sue us.

xiii

Contents

1

Verification Guidelines ...........................................................................
1.1
The Verification Process ................................................................
1.1.1
Testing at Different Levels ...............................................
1.1.2
The Verification Plan ........................................................
1.2
The Verification Methodology Manual ..........................................
1.3
Basic Testbench Functionality .......................................................
1.4
Directed Testing .............................................................................
1.5
Methodology Basics ......................................................................
1.6
Constrained-Random Stimulus ......................................................
1.7
What Should You Randomize? ......................................................
1.7.1
Device and Environment Configuration ...........................
1.7.2
Input Data .........................................................................
1.7.3
Protocol Exceptions, Errors, and Violations ....................
1.7.4
Delays and Synchronization.............................................
1.7.5
Parallel Random Testing ..................................................
1.8
Functional Coverage ......................................................................
1.8.1
Feedback from Functional Coverage to Stimulus ............
1.9
Testbench Components ..................................................................
1.10 Layered Testbench .........................................................................
1.10.1 A Flat Testbench ..............................................................
1.10.2 The Signal and Command Layers ....................................
1.10.3 The Functional Layer .......................................................
1.10.4 The Scenario Layer ..........................................................
1.10.5 The Test Layer and Functional Coverage ........................
1.11 Building a Layered Testbench ........................................................
1.11.1 Creating a Simple Driver..................................................
1.12 Simulation Environment Phases.....................................................
1.13 Maximum Code Reuse ...................................................................
1.14 Testbench Performance ..................................................................
1.15 Conclusion......................................................................................
1.16 Exercises ........................................................................................

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2

Contents

Data Types ...............................................................................................
2.1
Built-In Data Types ........................................................................
2.1.1
The Logic Type..............................................................
2.1.2
2-State Data Types ...........................................................
2.2
Fixed-Size Arrays ..........................................................................
2.2.1
Declaring and Initializing Fixed-Size Arrays ..................
2.2.2
The Array Literal..............................................................
2.2.3
Basic Array Operations — for and Foreach.................
2.2.4
Basic Array Operations – Copy and Compare .................
2.2.5
Bit and Array Subscripts, Together at Last ......................
2.2.6
Packed Arrays ..................................................................
2.2.7
Packed Array Examples ...................................................
2.2.8
Choosing Between Packed and Unpacked Arrays ...........
2.3
Dynamic Arrays .............................................................................
2.4
Queues ...........................................................................................
2.5
Associative Arrays .........................................................................
2.6
Array Methods ...............................................................................
2.6.1
Array Reduction Methods ................................................
2.6.2
Array Locator Methods ....................................................
2.6.3
Array Sorting and Ordering .............................................
2.6.4
Building a Scoreboard with Array Locator Methods .......
2.7
Choosing a Storage Type ...............................................................
2.7.1
Flexibility .........................................................................
2.7.2
Memory Usage .................................................................
2.7.3
Speed ................................................................................
2.7.4
Data Access ......................................................................
2.7.5
Choosing the Best Data Structure ....................................
2.8
Creating New Types with typedef ..........................................
2.9
Creating User-Defined Structures ..................................................
2.9.1
Creating a Struct and a New Type ...............................
2.9.2
Initializing a Structure ......................................................
2.9.3
Making a Union of Several Types ....................................
2.9.4
Packed Structures .............................................................
2.9.5
Choosing Between Packed and Unpacked Structures ......
2.10 Packages .........................................................................................
2.11 Type Conversion.............................................................................
2.11.1 The Static Cast .................................................................
2.11.2 The Dynamic Cast ............................................................
2.12 Streaming Operators.......................................................................
2.13 Enumerated Types ..........................................................................
2.13.1 Defining Enumerated Values ............................................
2.13.2 Routines for Enumerated Types .......................................
2.13.3 Converting to and from Enumerated Types......................

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2.14
2.15
2.16
2.17
2.18

xvii

Constants ........................................................................................
Strings ............................................................................................
Expression Width ...........................................................................
Conclusion......................................................................................
Exercises ........................................................................................

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3

Procedural Statements and Routines ....................................................
3.1 Procedural Statements ....................................................................
3.2 Tasks, Functions, and Void Functions ............................................
3.3 Task and Function Overview ..........................................................
3.3.1 Routine Begin…End Removed ........................................
3.4 Routine Arguments ........................................................................
3.4.1 C-style Routine Arguments ................................................
3.4.2 Argument Direction ...........................................................
3.4.3 Advanced Argument Types ................................................
3.4.4 Default Value for an Argument ..........................................
3.4.5 Passing Arguments by Name .............................................
3.4.6 Common Coding Errors .....................................................
3.5 Returning from a Routine...............................................................
3.5.1 The Return Statement.........................................................
3.5.2 Returning an Array from a Function ..................................
3.6 Local Data Storage .........................................................................
3.6.1 Automatic Storage..............................................................
3.6.2 Variable Initialization .........................................................
3.7 Time Values ....................................................................................
3.7.1 Time Units and Precision ...................................................
3.7.2 Time Literals ......................................................................
3.7.3 Time and Variables .............................................................
3.7.4 $time vs. $realtime .............................................................
3.8 Conclusion......................................................................................
3.9 Exercises ........................................................................................

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4

Connecting the Testbench and Design ..................................................
4.1 Separating the Testbench and Design ............................................
4.1.1 Communication Between the Testbench and DUT ............
4.1.2 Communication with Ports.................................................
4.2 The Interface Construct ..................................................................
4.2.1 Using an Interface to Simplify Connections ......................
4.2.2 Connecting Interfaces and Ports.........................................
4.2.3 Grouping Signals in an Interface Using Modports ............
4.2.4 Using Modports with a Bus Design ...................................
4.2.5 Creating an Interface Monitor ............................................
4.2.6 Interface Trade-Offs ...........................................................
4.2.7 More Information and Examples .......................................
4.2.8 Logic vs. Wire in an Interface ............................................

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Contents

4.3

Stimulus Timing .............................................................................
4.3.1
Controlling Timing of Synchronous Signals
with a Clocking Block......................................................
4.3.2
Timing Problems in Verilog .............................................
4.3.3
Testbench – Design Race Condition ................................
4.3.4
The Program Block and Timing Regions .........................
4.3.5
Specifying Delays Between the Design and Testbench ...
Interface Driving and Sampling .....................................................
4.4.1
Interface Synchronization ................................................
4.4.2
Interface Signal Sample ...................................................
4.4.3
Interface Signal Drive ......................................................
4.4.4
Driving Interface Signals Through a Clocking Block ......
4.4.5
Bidirectional Signals in the Interface ...............................
4.4.6
Specifying Delays in Clocking Blocks ............................
Program Block Considerations ......................................................
4.5.1
The End of Simulation .....................................................
4.5.2 Why are Always Blocks not Allowed
in a Program? ...................................................................
4.5.3 The Clock Generator ........................................................
Connecting It All Together.............................................................
4.6.1 An Interface in a Port List Must Be Connected ...............
Top-Level Scope.............................................................................
Program–Module Interactions........................................................
SystemVerilog Assertions ..............................................................
4.9.1 Immediate Assertions .......................................................
4.9.2 Customizing the Assertion Actions..................................
4.9.3 Concurrent Assertions ......................................................
4.9.4 Exploring Assertions ........................................................
The Four-Port ATM Router ............................................................
4.10.1 ATM Router with Ports ....................................................
4.10.2 ATM Top-Level Module with Ports .................................
4.10.3 Using Interfaces to Simplify Connections .......................
4.10.4 ATM Interfaces.................................................................
4.10.5 ATM Router Model Using an Interface ...........................
4.10.6 ATM Top Level Module with Interfaces ..........................
4.10.7 ATM Testbench with Interface .........................................
The Ref Port Direction ...................................................................
Conclusion......................................................................................
Exercises ........................................................................................

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Basic OOP ................................................................................................
5.1 Introduction ....................................................................................
5.2 Think of Nouns, not Verbs .............................................................
5.3 Your First Class ..............................................................................
5.4 Where to Define a Class .................................................................
5.5 OOP Terminology ..........................................................................

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4.4

4.5

4.6
4.7
4.8
4.9

4.10

4.11
4.12
4.13
5

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Contents

5.6

Creating New Objects ....................................................................
5.6.1
Handles and Constructing Objects ...................................
5.6.2
Custom Constructor .........................................................
5.6.3
Separating the Declaration and Construction...................
5.6.4
The Difference Between New() and New[]......................
5.6.5
Getting a Handle on Objects ............................................
Object Deallocation........................................................................
Using Objects .................................................................................
Class Methods ................................................................................
Defining Methods Outside of the Class .........................................
Static Variables vs. Global Variables .............................................
5.11.1 A Simple Static Variable ..................................................
5.11.2 Accessing Static Variables Through the Class Name ......
5.11.3 Initializing Static Variables ..............................................
5.11.4 Static Methods..................................................................
Scoping Rules.................................................................................
5.12.1 What is This? ................................................................
Using One Class Inside Another ....................................................
5.13.1 How Big or Small Should My Class Be? .........................
5.13.2 Compilation Order Issue ..................................................
Understanding Dynamic Objects ...................................................
5.14.1 Passing Objects and Handles to Methods ........................
5.14.2 Modifying a Handle in a Task ..........................................
5.14.3 Modifying Objects in Flight .............................................
5.14.4 Arrays of Handles ............................................................
Copying Objects .............................................................................
5.15.1 Copying an Object with the New Operator .......................
5.15.2 Writing Your Own Simple Copy Function.......................
5.15.3 Writing a Deep Copy Function ........................................
5.15.4 Packing Objects to and from Arrays Using
Streaming Operators.........................................................
Public vs. Local ..............................................................................
Straying Off Course .......................................................................
Building a Testbench ......................................................................
Conclusion......................................................................................
Exercises ........................................................................................

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Randomization ........................................................................................
6.1 Introduction ....................................................................................
6.2 What to Randomize ........................................................................
6.2.1
Device Configuration .......................................................
6.2.2
Environment Configuration ..............................................
6.2.3
Primary Input Data ...........................................................
6.2.4
Encapsulated Input Data ..................................................
6.2.5
Protocol Exceptions, Errors, and Violations ....................
6.2.6
Delays...............................................................................

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5.7
5.8
5.9
5.10
5.11

5.12
5.13

5.14

5.15

5.16
5.17
5.18
5.19
5.20
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Contents

6.3

6.4

6.5

6.6
6.7
6.8
6.9

6.10
6.11

6.12

6.13

Randomization in SystemVerilog...................................................
6.3.1
Simple Class with Random Variables ..............................
6.3.2
Checking the Result from Randomization .......................
6.3.3
The Constraint Solver ......................................................
6.3.4
What can be Randomized? ...............................................
Constraint Details ...........................................................................
6.4.1
Constraint Introduction ....................................................
6.4.2
Simple Expressions ..........................................................
6.4.3
Equivalence Expressions ..................................................
6.4.4
Weighted Distributions .....................................................
6.4.5
Set Membership and the Inside Operator .........................
6.4.6
Using an Array in a Set ....................................................
6.4.7
Bidirectional Constraints..................................................
6.4.8
Implication Constraints ....................................................
6.4.9
Equivalence Operator .......................................................
Solution Probabilities .....................................................................
6.5.1
Unconstrained ..................................................................
6.5.2
Implication .......................................................................
6.5.3
Implication and Bidirectional Constraints .......................
6.5.4
Guiding Distribution with Solve…Before .....................
Controlling Multiple Constraint Blocks .........................................
Valid Constraints ............................................................................
In-Line Constraints.........................................................................
The pre_randomize and post_randomize Functions ............
6.9.1
Building a Bathtub Distribution .......................................
6.9.2
Note on Void Functions ....................................................
Random Number Functions ...........................................................
Constraints Tips and Techniques....................................................
6.11.1 Constraints with Variables................................................
6.11.2 Using Nonrandom Values ................................................
6.11.3 Checking Values Using Constraints .................................
6.11.4 Randomizing Individual Variables ...................................
6.11.5 Turn Constraints Off and On ............................................
6.11.6 Specifying a Constraint in a Test Using In-Line
Constraints........................................................................
6.11.7 Specifying a Constraint in a Test with External
Constraints........................................................................
6.11.8 Extending a Class .............................................................
Common Randomization Problems ...............................................
6.12.1 Use Signed Variables with Care .......................................
6.12.2 Solver Performance Tips ..................................................
6.12.3 Choose the Right Arithmetic Operator to Boost
Efficiency .........................................................................
Iterative and Array Constraints ......................................................
6.13.1 Array Size.........................................................................
6.13.2 Sum of Elements ..............................................................

172
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Contents

6.13.3 Issues with Array Constraints ..........................................
6.13.4 Constraining Individual Array and Queue Elements .......
6.13.5 Generating an Array of Unique Values ............................
6.13.6 Randomizing an Array of Handles...................................
Atomic Stimulus Generation vs. Scenario Generation ..................
6.14.1 An Atomic Generator with History ..................................
6.14.2 Random Array of Objects ................................................
6.14.3 Combining Sequences ......................................................
6.14.4 Randsequence...................................................................
Random Control .............................................................................
6.15.1 Introduction to randcase................................................
6.15.2 Building a Decision Tree with randcase .......................
Random Number Generators..........................................................
6.16.1 Pseudorandom Number Generators .................................
6.16.2 Random Stability — Multiple Generators .......................
6.16.3 Random Stability and Hierarchical Seeding ....................
Random Device Configuration .......................................................
Conclusion......................................................................................
Exercises ........................................................................................

205
207
208
211
211
212
212
213
213
215
215
216
217
217
217
219
220
223
224

Threads and Interprocess Communication ..........................................
7.1 Working with Threads ....................................................................
7.1.1
Using fork…join and begin…end.................................
7.1.2
Spawning Threads with fork…join_none.....................
7.1.3
Synchronizing Threads with fork…join_any ...............
7.1.4
Creating Threads in a Class..............................................
7.1.5
Dynamic Threads .............................................................
7.1.6
Automatic Variables in Threads .......................................
7.1.7
Waiting for all Spawned Threads .....................................
7.1.8
Sharing Variables Across Threads ...................................
7.2 Disabling Threads ..........................................................................
7.2.1
Disabling a Single Thread ................................................
7.2.2 Disabling Multiple Threads..............................................
7.2.3
Disable a Task that was Called Multiple Times ...............
7.3 Interprocess Communication .........................................................
7.4 Events .............................................................................................
7.4.1
Blocking on the Edge of an Event....................................
7.4.2
Waiting for an Event Trigger............................................
7.4.3
Using Events in a Loop ....................................................
7.4.4
Passing Events ..................................................................
7.4.5
Waiting for Multiple Events .............................................
7.5 Semaphores ....................................................................................
7.5.1
Semaphore Operations .....................................................
7.5.2
Semaphores with Multiple Keys ......................................

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240
241
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6.14

6.15

6.16

6.17
6.18
6.19
7

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Contents

7.6

7.7

7.8
7.9
8

Mailboxes.........................................................................................
7.6.1 Mailbox in a Testbench ........................................................
7.6.2 Bounded Mailboxes .............................................................
7.6.3 Unsynchronized Threads Communicating
with a Mailbox .....................................................................
7.6.4 Synchronized Threads Using a Bounded Mailbox
and a Peek ............................................................................
7.6.5 Synchronized Threads Using a Mailbox and Event.............
7.6.6 Synchronized Threads Using Two Mailboxes .....................
7.6.7 Other Synchronization Techniques ......................................
Building a Testbench with Threads and IPC ...................................
7.7.1 Basic Transactor ..................................................................
7.7.2 Configuration Class .............................................................
7.7.3 Environment Class ...............................................................
7.7.4 Test Program ........................................................................
Conclusion .......................................................................................
Exercises ..........................................................................................

Advanced OOP and Testbench Guidelines ...........................................
8.1 Introduction to Inheritance ..............................................................
8.1.1 Basic Transaction .................................................................
8.1.2 Extending the Transaction Class.....................................
8.1.3 More OOP Terminology ......................................................
8.1.4 Constructors in Extended Classes ........................................
8.1.5 Driver Class .........................................................................
8.1.6 Simple Generator Class .......................................................
8.2 Blueprint Pattern ..............................................................................
8.2.1 The Environment Class .....................................................
8.2.2 A Simple Testbench .............................................................
8.2.3 Using the Extended Transaction Class ...........................
8.2.4 Changing Random Constraints with an Extended
Class .....................................................................................
8.3 Downcasting and Virtual Methods ..................................................
8.3.1 Downcasting with $cast ....................................................
8.3.2 Virtual Methods ...................................................................
8.3.3 Signatures and Polymorphism .............................................
8.3.4 Constructors are Never Virtual ............................................
8.4 Composition, Inheritance, and Alternatives.....................................
8.4.1 Deciding Between Composition and Inheritance ................
8.4.2 Problems with Composition ................................................
8.4.3 Problems with Inheritance ...................................................
8.4.4 A Real-World Alternative ....................................................
8.5 Copying an Object ...........................................................................
8.5.1 Specifying a Destination for Copy ......................................
8.6 Abstract Classes and Pure Virtual Methods.....................................

252
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256
257
259
261
262
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264
265
266
266
267
268
269
273
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275
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Contents

xxiii

8.7

Callbacks ........................................................................................
8.7.1
Creating a Callback ..........................................................
8.7.2
Using a Callback to Inject Disturbances ..........................
8.7.3
A Quick Introduction to Scoreboards ..............................
8.7.4
Connecting to the Scoreboard with a Callback ................
8.7.5
Using a Callback to Debug a Transactor ..........................
8.8 Parameterized Classes ....................................................................
8.8.1
A Simple Stack.................................................................
8.8.2
Sharing Parameterized Classes ........................................
8.8.3
Parameterized Class Suggestions .....................................
8.9 Static and Singleton Classes...........................................................
8.9.1
Dynamic Class to Print Messages ....................................
8.9.2
Singleton Class to Print Messages ...................................
8.9.3
Configuration Database with Static Parameterized
Class .................................................................................
8.10 Creating a Test Registry .................................................................
8.10.1 Test registry with Static Methods .....................................
8.10.2 Test Registry with a Proxy Class......................................
8.10.3 UVM Factory Build .........................................................
8.11 Conclusion......................................................................................
8.12 Exercises ........................................................................................

297
298
299
300
300
302
302
302
305
305
306
306
307

Functional Coverage ...............................................................................
9.1 Gathering Coverage Data ...............................................................
9.2 Coverage Types ..............................................................................
9.2.1 Code Coverage .................................................................
9.2.2 Functional Coverage ........................................................
9.2.3 Bug Rate ...........................................................................
9.2.4 Assertion Coverage ..........................................................
9.3 Functional Coverage Strategies ......................................................
9.3.1 Gather Information, not Data ...........................................
9.3.2 Only Measure What you are Going to Use ......................
9.3.3 Measuring Completeness .................................................
9.4 Simple Functional Coverage Example ...........................................
9.5 Anatomy of a Cover Group ............................................................
9.5.1 Defining a Cover Group in a Class...................................
9.6 Triggering a Cover Group ..............................................................
9.6.1
Sampling Using a Callback ..............................................
9.6.2
Cover Group with a User Defined Sample
Argument List ..................................................................
9.6.3
Cover Group with an Event Trigger .................................
9.6.4
Triggering on a System Verilog Assertion .......................
9.7 Data Sampling ................................................................................
9.7.1
Individual Bins and Total Coverage .................................
9.7.2
Creating Bins Automatically ............................................

323
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326
326
327
327
328
328
328
329
329
330
333
334
335
335

9

308
311
311
313
319
319
320

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Contents

9.7.3
Limiting the Number of Automatic Bins Created ............
9.7.4
Sampling Expressions ......................................................
9.7.5
User-Defined Bins Find a Bug .........................................
9.7.6 Naming the Cover Point Bins...........................................
9.7.7
Conditional Coverage.......................................................
9.7.8
Creating Bins for Enumerated Types ...............................
9.7.9
Transition Coverage .........................................................
9.7.10 Wildcard States and Transitions .......................................
9.7.11 Ignoring Values ................................................................
9.7.12 Illegal Bins .......................................................................
9.7.13 State Machine Coverage...................................................
Cross Coverage ..............................................................................
9.8.1
Basic Cross Coverage Example .......................................
9.8.2
Labeling Cross Coverage Bins .........................................
9.8.3
Excluding Cross Coverage Bins.......................................
9.8.4
Excluding Cover Points from the Total
Coverage Metric ...............................................................
9.8.5
Merging Data from Multiple Domains ............................
9.8.6
Cross Coverage Alternatives ............................................
Generic Cover Groups....................................................................
9.9.1
Pass Cover Group Arguments by Value ...........................
9.9.2
Pass Cover Group Arguments by Reference ....................
Coverage Options ...........................................................................
9.10.1 Per-Instance Coverage......................................................
9.10.2 Cover Group Comment ....................................................
9.10.3 Coverage Threshold .........................................................
9.10.4 Printing the Empty Bins ...................................................
9.10.5 Coverage Goal ..................................................................
Analyzing Coverage Data ..............................................................
Measuring Coverage Statistics During Simulation ........................
Conclusion......................................................................................
Exercises ........................................................................................

339
340
341
342
344
345
345
346
346
347
347
348
348
349
351

Advanced Interfaces ...............................................................................
10.1 Virtual Interfaces with the ATM Router.........................................
10.1.1 The Testbench with Just Physical Interfaces ....................
10.1.2 Testbench with Virtual Interfaces.....................................
10.1.3 Connecting the Testbench to an Interface
in Port List ........................................................................
10.1.4 Connecting the Test to an Interface with an XMR ...........
10.2 Connecting to Multiple Design Configurations .............................
10.2.1 A Mesh Design.................................................................
10.2.2 Using Typedefs with Virtual Interfaces .........................
10.2.3 Passing Virtual Interface Array Using a Port ...................
10.3 Parameterized Interfaces and Virtual Interfaces.............................

363
364
364
366

9.8

9.9

9.10

9.11
9.12
9.13
9.14
10

351
352
352
354
354
355
355
355
356
357
357
357
358
359
360
360

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377

Contents

xxv

10.4 Procedural Code in an Interface .....................................................
10.4.1 Interface with Parallel Protocol ........................................
10.4.2 Interface with Serial Protocol...........................................
10.4.3 Limitations of Interface Code ..........................................
10.5 Conclusion......................................................................................
10.6 Exercises ........................................................................................

379
380
380
381
382
382

11

A Complete SystemVerilog Testbench...................................................
11.1 Design Blocks ................................................................................
11.2 Testbench Blocks ...........................................................................
11.3 Alternate Tests ................................................................................
11.3.1 Your First Test - Just One Cell .........................................
11.3.2 Randomly Drop Cells.......................................................
11.4 Conclusion......................................................................................
11.5 Exercises ........................................................................................

385
385
390
411
411
412
413
414

12

Interfacing with C/C++ ..........................................................................
12.1 Passing Simple Values....................................................................
12.1.1 Passing Integer and Real Values.......................................
12.1.2 The Import Declaration ....................................................
12.1.3 Argument Directions ........................................................
12.1.4 Argument Types ...............................................................
12.1.5 Importing a Math Library Routine ...................................
12.2 Connecting to a Simple C Routine .................................................
12.2.1 A Counter with Static Storage..........................................
12.2.2 The Chandle Data Type ....................................................
12.2.3 Representation of Packed Values .....................................
12.2.4 4-State Values ...................................................................
12.2.5 Converting from 2-State to 4-State ..................................
12.3 Connecting to C++ .........................................................................
12.3.1 The Counter in C++ .........................................................
12.3.2 Static Methods..................................................................
12.3.3 Communicating with a Transaction Level
C++ Model .......................................................................
12.4 Simple Array Sharing.....................................................................
12.4.1 Single Dimension Arrays - 2-State ..................................
12.4.2 Single Dimension Arrays - 4-State ..................................
12.5 Open arrays ....................................................................................
12.5.1 Basic Open Array .............................................................
12.5.2 Open Array Methods ........................................................
12.5.3 Passing Unsized Open Arrays ..........................................
12.5.4 Packed Open Arrays in DPI .............................................
12.6 Sharing Composite Types...............................................................
12.6.1 Passing Structures Between SystemVerilog and C ..........
12.6.2 Passing Strings Between SystemVerilog and C ...............

415
416
416
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418
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432
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xxvi

Contents

12.7
12.8

Pure and Context Imported Methods ...........................................
Communicating from C to SystemVerilog ..................................
12.8.1 A simple Exported Function ..........................................
12.8.2 C function Calling SystemVerilog Function ..................
12.8.3 C Task Calling SystemVerilog Task ...............................
12.8.4 Calling Methods in Objects............................................
12.8.5 The Meaning of Context ................................................
12.8.6 Setting the Scope for an Imported Routine ....................
12.9
Connecting Other Languages ......................................................
12.10 Conclusion ...................................................................................
12.11 Exercises ......................................................................................

440
441
441
442
444
446
449
450
452
453
453

References ........................................................................................................ 455
Index ................................................................................................................. 457

List of Figures

Fig. 1.1
Fig. 1.2
Fig. 1.3
Fig. 1.4
Fig. 1.5
Fig. 1.6
Fig. 1.7
Fig. 1.8
Fig. 1.9
Fig. 1.10
Fig. 1.11
Fig. 1.12
Fig. 1.13

Directed test progress over time .....................................................
Directed test coverage ....................................................................
Constrained-random test progress over time vs.
directed testing ...............................................................................
Constrained-random test coverage .................................................
Coverage convergence....................................................................
Test progress with and without feedback .......................................
The testbench — design environment ............................................
Testbench components ...................................................................
Signal and command layers ...........................................................
Testbench with functional layer added ...........................................
Testbench with scenario layer added..............................................
Full testbench with all layers..........................................................
Connections for the driver ..............................................................

6
6
7
8
8
12
13
14
17
17
18
19
20

Fig. 2.1
Fig. 2.2
Fig. 2.3
Fig. 2.4

Unpacked array storage ..................................................................
Packed array layout ........................................................................
Packed array bit layout ...................................................................
Associative array ............................................................................

29
33
34
39

Fig. 4.1
Fig. 4.2
Fig. 4.3
Fig. 4.4
Fig. 4.5
Fig. 4.6
Fig. 4.7
Fig. 4.8
Fig. 4.9

The testbench – design environment ..............................................
Testbench – Arbiter without interfaces ..........................................
An interface straddles two modules ...............................................
Main regions inside a SystemVerilog time step .............................
A clocking block synchronizes the DUT and testbench ................
Sampling a synchronous interface .................................................
Driving a synchronous interface ....................................................
Testbench – ATM router diagram without interfaces .....................
Testbench - router diagram with interfaces ....................................

87
89
91
102
104
106
108
119
123

Fig. 5.1
Fig. 5.2

Handles and objects after allocating multiple objects .................... 138
Static variables in a class................................................................ 144
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Fig. 5.3
Fig. 5.4
Fig. 5.5
Fig. 5.6
Fig. 5.7

List of Figures

149
152
157
158

Fig. 5.8
Fig. 5.9

Contained objects Sample 5.22 ......................................................
Handles and objects across methods ..............................................
Objects and handles before copy with the new operator ................
Objects and handles after copy with the new operator...................
Both src and dst objects refer to a single statistics object
and see updated startT value ..........................................................
Objects and handles after deep copy ..............................................
Layered testbench...........................................................................

Fig. 6.1
Fig. 6.2
Fig. 6.3
Fig. 6.4
Fig. 6.5

Building a bathtub distribution.......................................................
Random strobe waveforms .............................................................
Sharing a single random generator.................................................
First generator uses additional values ............................................
Separate random generators per object ..........................................

194
204
218
218
219

Fig. 7.1
Fig. 7.2
Fig. 7.3
Fig. 7.4
Fig. 7.5
Fig. 7.6
Fig. 7.7
Fig. 7.8

Testbench environment blocks .......................................................
Fork…join blocks .........................................................................
Fork…join block ..........................................................................
Fork…join block diagram ............................................................
A mailbox connecting two transactors ...........................................
A mailbox with multiple handles to one object .............................
A mailbox with multiple handles to multiple objects ....................
Layered testbench with environment .............................................

230
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231
242
252
254
254
265

Fig. 8.1
Fig. 8.2
Fig. 8.3
Fig. 8.4
Fig. 8.5
Fig. 8.6
Fig. 8.7
Fig. 8.8

Simplified layered testbench ..........................................................
Base Transaction class diagram ..............................................
Extended Transaction class diagram ......................................
Blueprint pattern generator ............................................................
Blueprint generator with new pattern .............................................
Simplified extended transaction .....................................................
Multiple inheritance problem .........................................................
Callback flow..................................................................................

274
275
276
280
280
285
292
297

Fig. 9.1
Fig. 9.2
Fig. 9.3
Fig. 9.4
Fig. 9.5
Fig. 9.6

Coverage convergence....................................................................
Coverage flow.................................................................................
Bug rate during a project ................................................................
Coverage comparison .....................................................................
Uneven probability for packet length .............................................
Even probability for packet length with solve…before ..............

324
325
327
330
358
359

Fig. 10.1

Router and testbench with interfaces ............................................. 366

Fig. 11.1
Fig. 11.2

The testbench — design environment ............................................ 386
Block diagram for the squat design................................................ 386

Fig. 12.1
Fig. 12.2

Storage of a 40-bit 2-state variable ................................................ 424
Storage of a 40-bit 4-state variable ................................................ 425

158
160
163

List of Tables

Table 2.1

ALU Opcodes ..............................................................................

67

Table 4.1
Table 4.2

Primary SystemVerilog scheduling regions................................. 102
AHB Signal Description .............................................................. 128

Table 6.1
Table 6.2
Table 6.3
Table 6.4
Table 6.5
Table 6.6
Table 6.7
Table 6.8
Table 6.9

Solutions for bidirectional constraint ..........................................
Implication operator truth table ...................................................
Equivalence operator truth table ..................................................
Solutions for Unconstrained class ......................................
Solutions for Imp1 class ..............................................................
Solutions for Imp2 class ..............................................................
Solutions for solve x before y constraint ............................
Solutions for solve y before x constraint ............................
Solution probabilities...................................................................

Table 8.1

Comparing inheritance to composition ....................................... 289

Table 12.1
Table 12.2
Table 12.3
Table 12.4

Data types mapping between SystemVerilog and C ....................
4-state bit encoding......................................................................
Open array query functions .........................................................
Open array locator functions .......................................................

184
184
186
187
188
189
190
190
225

418
424
435
435

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List of Samples

Sample 1.1
Sample 1.2
Sample 1.3
Sample 1.4

Driving the APB pins ...............................................................
A task to drive the APB pins ....................................................
Low-level Verilog test ..............................................................
Basic transactor code ................................................................

15
16
16
20

Sample 2.1
Sample 2.2
Sample 2.3
Sample 2.4
Sample 2.5
Sample 2.6
Sample 2.7
Sample 2.8
Sample 2.9
Sample 2.10
Sample 2.11
Sample 2.12
Sample 2.13
Sample 2.14
Sample 2.15
Sample 2.16
Sample 2.17
Sample 2.18
Sample 2.19
Sample 2.20
Sample 2.21
Sample 2.22
Sample 2.23
Sample 2.24
Sample 2.25
Sample 2.26

Using the logic type..................................................................
Signed data types ......................................................................
Checking for 4-state values ......................................................
Declaring fixed-size arrays .......................................................
Calculating the address width for a memory ............................
Declaring and using multi-dimensional arrays.........................
Unpacked array declarations ....................................................
Initializing an array ..................................................................
Printing with %p print specifier ...............................................
Using arrays with for- and foreach loops ..........................
Initialize and step through a multi-dimensional array ..............
Output from printing multi-dimensional array values ..............
Printing a multi-dimensional array ...........................................
Output from printing multi-dimensional array values ..............
Array copy and compare operations .........................................
Using word and bit subscripts together ....................................
Packed array declaration and usage..........................................
Declaration for a mixed packed/unpacked array ......................
Using dynamic arrays ...............................................................
Using a dynamic array for an uncounted list............................
Multi-dimensional dynamic array ............................................
Queue methods .........................................................................
Queue operations ......................................................................
Declaring, initializing, and using associative arrays ................
Using an associative array with a string index .........................
Initializing and printing associative arrays ...............................

26
27
27
28
28
28
29
29
30
30
30
31
31
31
32
33
33
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35
36
36
37
38
39
40
41
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Sample 2.27
Sample 2.28
Sample 2.29
Sample 2.30
Sample 2.31
Sample 2.32
Sample 2.33
Sample 2.34
Sample 2.35
Sample 2.36
Sample 2.37
Sample 2.38
Sample 2.39
Sample 2.40
Sample 2.41
Sample 2.42
Sample 2.43
Sample 2.44
Sample 2.45
Sample 2.46
Sample 2.47
Sample 2.48
Sample 2.49
Sample 2.50
Sample 2.51
Sample 2.52
Sample 2.53

List of Samples

Sample 2.54
Sample 2.55
Sample 2.56
Sample 2.57
Sample 2.58
Sample 2.59
Sample 2.60
Sample 2.61
Sample 2.62
Sample 2.63

Array reduction operations .......................................................
Picking a random element from an associative array ...............
Array locator methods: min, max, unique...............................
Array locator methods: find ......................................................
Declaring the iterator argument ................................................
Array locator methods ..............................................................
Creating the sum of an array of single bits ...............................
Sorting an array ........................................................................
Sorting an array of structures ...................................................
A scoreboard with array methods .............................................
User-defined type-macro in Verilog .........................................
User-defined type in SystemVerilog .........................................
Definition of uint ......................................................................
User-defined array type ............................................................
User-defined associative array index ........................................
Creating a single pixel type ......................................................
The pixel struct .....................................................................
Initializing a struct ....................................................................
Using typedef to create a union ................................................
Packed structure ........................................................................
Package for ABC bus................................................................
Importing packages ..................................................................
Importing selected symbols from a package ............................
Converting between int and real with static cast ......................
Basic streaming operator ..........................................................
Converting between queues with streaming operator ...............
Converting between a structure and an array
with streaming operators ..........................................................
A simple enumerated type, not recommended .........................
Enumerated types, recommended style ....................................
Specifying enumerated values ..................................................
Incorrectly specifying enumerated values ................................
Correctly specifying enumerated values...................................
Stepping through all enumerated members ..............................
Assignments between integers and enumerated types ..............
Declaring a const variable ......................................................
String methods ..........................................................................
Expression width depends on context ......................................

41
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Sample 3.1
Sample 3.2
Sample 3.3
Sample 3.4
Sample 3.5
Sample 3.6

New procedural statements and operators ................................
Using break and continue while reading a file ....................
Case-inside statement with ranges............................................
Void function for debug ............................................................
Ignoring a function’s return value ............................................
Simple task without begin…end ..............................................

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List of Samples

Sample 3.7
Sample 3.8
Sample 3.9
Sample 3.10
Sample 3.11
Sample 3.12
Sample 3.13
Sample 3.14
Sample 3.15
Sample 3.16
Sample 3.17
Sample 3.18
Sample 3.19
Sample 3.20
Sample 3.21
Sample 3.22
Sample 3.23
Sample 3.24
Sample 3.25
Sample 3.26

xxxiii

Sample 3.27
Sample 3.28

Verilog-1995 routine arguments ...............................................
C-style routine arguments .........................................................
Verbose Verilog-style routine arguments ..................................
Routine arguments with sticky types ........................................
Passing arrays using ref and const .......................................
Using ref across threads..........................................................
Function with default argument values.....................................
Using default argument values .................................................
Binding arguments by name .....................................................
Original task header ..................................................................
Task header with additional array argument .............................
Task header with additional array argument .............................
Return in a task .........................................................................
Return in a function ..................................................................
Returning an array from a function with a typedef ..................
Passing an array to a function as a ref argument ......................
Specifying automatic storage in program blocks .....................
Static initialization bug .............................................................
Static initialization fix: use automatic ...................................
Static initialization fix: break apart declaration
and initialization .......................................................................
Time literals and $timeformat ..............................................
Time variables and rounding ....................................................

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Sample 4.1
Sample 4.2
Sample 4.3
Sample 4.4
Sample 4.5
Sample 4.6
Sample 4.7
Sample 4.8
Sample 4.9
Sample 4.10
Sample 4.11
Sample 4.12
Sample 4.13
Sample 4.14
Sample 4.15
Sample 4.16
Sample 4.17
Sample 4.18
Sample 4.19
Sample 4.20
Sample 4.21

Arbiter model using ports .........................................................
Testbench module using ports ..................................................
Top-level module with ports .....................................................
Simple interface for arbiter .......................................................
Arbiter using a simple interface................................................
Testbench using a simple arbiter interface ...............................
Top module with a simple arbiter interface ..............................
Bad test module includes interface ...........................................
Connecting an interface to a module that uses ports ................
Interface with modports ............................................................
Arbiter model with interface using modports ...........................
Testbench with interface using modports .................................
Top level module with modports ..............................................
Arbiter monitor with interface using modports ........................
Driving logic and wires in an interface.....................................
Interface with a clocking block ................................................
Race condition between testbench and design .........................
Testbench using interface with clocking block.........................
Signal synchronization .............................................................
Synchronous interface sample and drive from module ............
Testbench using interface with clocking block.........................

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xxxiv

Sample 4.22
Sample 4.23
Sample 4.24
Sample 4.25
Sample 4.26
Sample 4.27
Sample 4.28
Sample 4.29
Sample 4.30
Sample 4.31
Sample 4.32
Sample 4.33
Sample 4.34
Sample 4.35
Sample 4.36
Sample 4.37
Sample 4.38
Sample 4.39
Sample 4.40

List of Samples

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Sample 4.41
Sample 4.42
Sample 4.43
Sample 4.44
Sample 4.45
Sample 4.46
Sample 4.47
Sample 4.48
Sample 4.49
Sample 4.50
Sample 4.51
Sample 4.52

Interface signal drive ................................................................
Driving a synchronous interface ...............................................
Interface signal drive ................................................................
Bidirectional signals in a program and interface ......................
Clocking block with default statement .....................................
Clocking block with delays on individual signals ....................
A final block ............................................................................
Bad clock generator in program block .....................................
Good clock generator in module ..............................................
Top module with implicit port connections..............................
Module with just port connections ...........................................
Module with an interface ..........................................................
Top module connecting DUT and interface..............................
Top-level scope for arbiter design ............................................
Cross-module references with $root ......................................
Checking a signal with an if-statement...................................
Simple immediate assertion ......................................................
Error from failed immediate assertion ......................................
Creating a custom error message in an immediate
assertion ....................................................................................
Error from failed immediate assertion ......................................
Creating a custom error message ..............................................
Concurrent assertion to check for X/Z .....................................
ATM router model header with ports .......................................
Top-level module without an interface .....................................
Verilog-1995 testbench using ports ..........................................
Rx interface with modports and clocking block .......................
Tx interface with modports and clocking block .......................
ATM router model with interface using modports ...................
Top-level module with interface ...............................................
Testbench using an interface with a clocking block .................
Ref ports ...................................................................................

Sample 5.1
Sample 5.2
Sample 5.3
Sample 5.4
Sample 5.5
Sample 5.6
Sample 5.7
Sample 5.8
Sample 5.9
Sample 5.10
Sample 5.11
Sample 5.12

Simple transaction class ...........................................................
Class in a package ....................................................................
Importing a package in a program ............................................
Declaring and using a handle ...................................................
Simple user-defined new() function..........................................
A new() function with arguments .............................................
Calling the right new() function ...............................................
Allocating multiple objects.......................................................
Creating multiple objects ..........................................................
Using variables and routines in an object .................................
Routines in the class .................................................................
Out-of-block method declarations ............................................

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127

List of Samples

xxxv

Sample 5.13
Sample 5.14
Sample 5.15
Sample 5.16
Sample 5.17
Sample 5.18
Sample 5.19
Sample 5.20
Sample 5.21
Sample 5.22
Sample 5.23
Sample 5.24
Sample 5.25
Sample 5.26
Sample 5.27
Sample 5.28
Sample 5.29
Sample 5.30
Sample 5.31
Sample 5.32
Sample 5.33
Sample 5.34
Sample 5.35
Sample 5.36
Sample 5.37
Sample 5.38
Sample 5.39
Sample 5.40

Out-of-body method missing class name .................................
Class with a static variable .......................................................
The class scope resolution operator ..........................................
Static storage for a handle ........................................................
Static method displays static variable .......................................
Name scope ..............................................................................
Class uses wrong variable.........................................................
Move class into package to find bug.........................................
Using this to refer to class variable........................................
Statistics class declaration .................................................
Encapsulating the Statistics class ......................................
Using a typedef class statement ........................................
Passing objects..........................................................................
Bad transaction creator task, missing ref on handle ...............
Good transaction creator task with ref on handle .....................
Bad generator creates only one object ......................................
Good generator creates many objects .......................................
Using an array of handles .........................................................
Copying a simple class with new..............................................
Copying a complex class with new operator .......................
Simple class with copy function ..............................................
Using a copy function ..............................................................
Complex class with deep copy function ...................................
Statistics class declaration .................................................
Copying a complex class with new operator .......................
Transaction class with pack and unpack functions ...................
Using the pack and unpack functions .......................................
Basic Transactor .......................................................................

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Sample 6.1
Sample 6.2
Sample 6.3
Sample 6.4
Sample 6.5
Sample 6.6
Sample 6.7
Sample 6.8
Sample 6.9
Sample 6.10
Sample 6.11
Sample 6.12
Sample 6.13
Sample 6.14
Sample 6.15
Sample 6.16

Simple random class .................................................................
Randomization check macro and example ...............................
Constraint without random variables ........................................
Constrained-random class ........................................................
Bad ordering constraint ............................................................
Result from incorrect ordering constraint.................................
Constrain variables to be in a fixed order .................................
Weighted random distribution with dist.................................
Dynamically changing distribution weights .............................
Random sets of values ..............................................................
Inverted random set constraint..................................................
Random set constraint for an array ...........................................
Equivalent set of constraints .....................................................
Choose any value except those in an array ...............................
Printing a histogram .................................................................
Histogram for inside constraint ................................................

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xxxvi

Sample 6.17
Sample 6.18
Sample 6.19
Sample 6.20
Sample 6.21
Sample 6.22
Sample 6.23
Sample 6.24
Sample 6.25
Sample 6.26
Sample 6.27
Sample 6.28
Sample 6.29
Sample 6.30
Sample 6.31
Sample 6.32
Sample 6.33
Sample 6.34
Sample 6.35
Sample 6.36
Sample 6.37
Sample 6.38
Sample 6.39
Sample 6.40
Sample 6.41
Sample 6.42
Sample 6.43
Sample 6.44
Sample 6.45
Sample 6.46
Sample 6.47
Sample 6.48
Sample 6.49
Sample 6.50
Sample 6.51
Sample 6.52
Sample 6.53
Sample 6.54
Sample 6.55
Sample 6.56
Sample 6.57
Sample 6.58
Sample 6.59

List of Samples

Class to choose from an array of possible values .....................
Choosing from an array of values.............................................
Using randc to choose array values in random order ...............
Testbench for randc choosing array values
in random order ........................................................................
Bidirectional constraints ...........................................................
Constraint block with implication operator ..............................
Implication operator .................................................................
Constraint block with if implication operator ..........................
Constraint block with if-else operator ......................................
Constraint block with multiple if-else operator ........................
Equivalence constraint ..............................................................
Class Unconstrained ............................................................
Class with implication constraint .............................................
Class with implication constraint and additional
constraint ..................................................................................
Class with implication and solve…before ............................
Using constraint_mode .......................................................
Checking write length with a valid constraint ..........................
The randomize() with statement.............................
Building a bathtub distribution .................................................
$urandom range usage ..............................................................
Constraint with a variable bound ..............................................
dist constraint with variable weights .....................................
rand_mode disables randomization of variables .....................
Randomizing a subset of variables in a class ...........................
Class with an external constraint ..............................................
Program defining an external constraint ...................................
Signed variables cause randomization problems ......................
Randomizing unsigned 32-bit variables ...................................
Randomizing unsigned 8-bit variables .....................................
Expensive constraint with mod and unsized variable ...............
Efficient constraint with bit extract ..........................................
Constraining dynamic array size ..............................................
Random strobe pattern class .....................................................
First attempt at sum constraint: bad_sum1 ..............................
Program to try constraint with array sum .................................
Output from bad_sum1 ............................................................
Second attempt at sum constraint: bad_sum2..........................
Output from bad_sum2 ............................................................
Third attempt at sum constraint: bad_sum3.............................
Output from bad_sum3 ............................................................
Fourth attempt at sum constraint: bad_sum4..........................
Output from bad_sum4 ............................................................
Simple foreach constraint: good_sum5 ...................................

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List of Samples

xxxvii

Sample 6.60
Sample 6.61
Sample 6.62
Sample 6.63
Sample 6.64
Sample 6.65
Sample 6.66
Sample 6.67
Sample 6.68
Sample 6.69
Sample 6.70
Sample 6.71
Sample 6.72
Sample 6.73
Sample 6.74
Sample 6.75
Sample 6.76
Sample 6.77
Sample 6.78
Sample 6.79

Output from good_sum5 ..........................................................
Creating ascending array values with foreach .......................
Creating unique array values with foreach ............................
Creating unique array values with a randc helper class............
Unique value generator .............................................................
Class to generate a random array of unique values ..................
Using the UniqueArray class .................................................
Constructing elements in a random array class ........................
Simple random sequence with ascending values ......................
Command generator using randsequence .............................
Random control with randcase and $urandom_range ........
Equivalent constrained class .....................................................
Creating a decision tree with randcase ..................................
Simple pseudorandom number generator .................................
Test code before modification...................................................
Test code after modification .....................................................
Ethernet switch configuration class ..........................................
Building environment with random configuration ...................
Simple test using random configuration ...................................
Simple test that overrides random configuration ......................

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Sample 7.1
Sample 7.2
Sample 7.3
Sample 7.4
Sample 7.5
Sample 7.6
Sample 7.7
Sample 7.8
Sample 7.9
Sample 7.10
Sample 7.11
Sample 7.12
Sample 7.13
Sample 7.14
Sample 7.15
Sample 7.16
Sample 7.17
Sample 7.18
Sample 7.19
Sample 7.20
Sample 7.21
Sample 7.22
Sample 7.23
Sample 7.24

Interaction of begin…end and fork…join .............................
Output from begin…end and fork…join...............................
Fork…join_none code ...........................................................
Fork…join_none output .........................................................
Fork…join_any code..............................................................
Output from fork…join_any .................................................
Generator / Driver class with a run task ...................................
Dynamic thread creation...........................................................
Bad fork…join_none inside a loop .......................................
Execution of bad fork…join_none inside a loop ..................
Automatic variables in a fork…join_none ............................
Steps in executing automatic variable code ..............................
Automatic variables in a fork…join_none ............................
Using wait fork to wait for child threads .............................
Bug using shared program variable ..........................................
Disabling a thread .....................................................................
Limiting the scope of a disable fork .........................................
Using disable label to stop threads .......................................
Using disable label to stop a task ..........................................
Blocking on an event in Verilog ...............................................
Output from blocking on an event ............................................
Waiting for an event..................................................................
Output from waiting for an event .............................................
Waiting on event causes a zero delay loop ...............................

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xxxviii

List of Samples

Sample 7.25
Sample 7.26
Sample 7.27
Sample 7.28
Sample 7.29
Sample 7.30
Sample 7.31
Sample 7.32
Sample 7.33
Sample 7.34
Sample 7.35
Sample 7.36
Sample 7.37
Sample 7.38
Sample 7.39
Sample 7.40
Sample 7.41
Sample 7.42
Sample 7.43
Sample 7.44
Sample 7.45
Sample 7.46
Sample 7.47
Sample 7.48
Sample 7.49

Waiting for an edge on an event ...............................................
Passing an event into a constructor ...........................................
Waiting for multiple threads with wait fork .............................
Waiting for multiple threads by counting triggers ....................
Waiting for multiple threads using a thread count ....................
Semaphores controlling access to hardware resource ..............
Mailbox declarations ................................................................
Bad generator creates only one object ......................................
Good generator creates many objects .......................................
Good driver receives transactions from mailbox ......................
Exchanging objects using a mailbox: the Generator class .......
Bounded mailbox......................................................................
Output from bounded mailbox .................................................
Producer–consumer without synchronization ..........................
Producer–consumer without synchronization output ...............
Producer–consumer synchronized with bounded mailbox .......
Output from producer–consumer with bounded mailbox ........
Producer–consumer synchronized with an event .....................
Output from producer–consumer with event ............................
Producer–consumer synchronized with a mailbox ...................
Output from producer–consumer with mailbox .......................
Basic Transactor .......................................................................
Configuration class ...................................................................
Environment class.....................................................................
Basic test program ....................................................................

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Sample 8.1
Sample 8.2
Sample 8.3
Sample 8.4
Sample 8.5
Sample 8.6
Sample 8.7
Sample 8.8
Sample 8.9
Sample 8.10
Sample 8.11
Sample 8.12
Sample 8.13
Sample 8.14
Sample 8.15
Sample 8.16
Sample 8.17
Sample 8.18
Sample 8.19

Base Transaction class .....................................................
Extended Transaction class ...............................................
Constructor with arguments in an extended class.....................
Driver class ...............................................................................
Bad generator class ...................................................................
Generator class using blueprint pattern ....................................
Environment class ...............................................................
Simple test program using environment defaults .....................
Injecting an extended transaction into testbench ......................
Adding a constraint with inheritance ........................................
Base and extended class ...........................................................
Copying extended handle to base handle .................................
Copying a base handle to an extended handle ..........................
Using $cast to copy handles..................................................
Transaction and BadTr classes ........................................
Calling class methods ...............................................................
Building an Ethernet frame with composition..........................
Building an Ethernet frame with inheritance............................
Building a flat Ethernet frame ..................................................

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287
287
290
291
292

List of Samples

xxxix

Sample 8.20
Sample 8.21
Sample 8.22
Sample 8.23
Sample 8.24
Sample 8.25
Sample 8.26
Sample 8.27
Sample 8.28
Sample 8.29
Sample 8.30
Sample 8.31
Sample 8.32
Sample 8.33
Sample 8.34
Sample 8.35
Sample 8.36
Sample 8.37
Sample 8.38
Sample 8.39
Sample 8.40
Sample 8.41
Sample 8.42
Sample 8.43
Sample 8.44
Sample 8.45
Sample 8.46
Sample 8.47
Sample 8.48
Sample 8.49
Sample 8.50
Sample 8.51
Sample 8.52
Sample 8.53
Sample 8.54
Sample 8.55
Sample 8.56
Sample 8.57

Base transaction class with a virtual copy function ................
Extended transaction class with virtual copy method.............
Base transaction class with copy function ..............................
Extended transaction class with new copy function ...............
Abstract class with pure virtual methods..................................
Transaction class extends abstract class ...................................
Base callback class ...................................................................
Driver class with callbacks .......................................................
Test using a callback for error injection ...................................
Simple scoreboard for atomic transactions...............................
Test using callback for scoreboard ...........................................
Stack using the int type.............................................................
Parameterized class for a stack .................................................
Creating the parameterized stack class .....................................
Parameterized generator class using blueprint pattern .............
Simple testbench using parameterized generator class ............
Common base class for parameterized generator class ............
Dynamic print class with static variables .................................
Transactor class with dynamic print object ..............................
Static print class ........................................................................
Transactor class with static print class......................................
Configuration database with global methods ...........................
Configuration database with parameterized class.....................
Configuration database with static parameterized class ...........
Testbench for configuration database .......................................
Base test class ...........................................................................
Test registry class......................................................................
Simple test in a class.................................................................
Program block for test classes ..................................................
Test class that puts a bad transaction in the generator ..............
Common SVM base class .........................................................
Component class .......................................................................
Common base class for proxy class..........................................
Parameterized proxy class ........................................................
Factory class .............................................................................
Base test class and registration macro ......................................
Test program .............................................................................
UVM factory build example .....................................................

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311
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Sample 9.1
Sample 9.2
Sample 9.3
Sample 9.4
Sample 9.5
Sample 9.6

Incomplete D-flip flop model missing a path ...........................
Functional coverage of a simple object ....................................
Coverage report for a simple object .........................................
Coverage report for a simple object, 100% coverage ...............
Functional coverage inside a class............................................
Test using functional coverage callback ...................................

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xl

List of Samples

Sample 9.7
Sample 9.8
Sample 9.9
Sample 9.10
Sample 9.11
Sample 9.12
Sample 9.13
Sample 9.14
Sample 9.15
Sample 9.16
Sample 9.17
Sample 9.18
Sample 9.19
Sample 9.20
Sample 9.21
Sample 9.22
Sample 9.23
Sample 9.24
Sample 9.25
Sample 9.26
Sample 9.27
Sample 9.28
Sample 9.29
Sample 9.30
Sample 9.31
Sample 9.32
Sample 9.33
Sample 9.34
Sample 9.35
Sample 9.36
Sample 9.37
Sample 9.38
Sample 9.39
Sample 9.40
Sample 9.41
Sample 9.42
Sample 9.43
Sample 9.44
Sample 9.45
Sample 9.46
Sample 9.47

Callback for functional coverage ..............................................
Defining an argument list to the sample method ......................
Cover group with a trigger........................................................
Module with SystemVerilog Assertion .....................................
Triggering a cover group with an SVA .....................................
Using auto_bin_max set to 2 .................................................
Report with auto_bin_max set to 2 ........................................
Using auto_bin_max for all cover points ..............................
Using an expression in a cover point ........................................
Defining bins for transaction length .........................................
Coverage report for transaction length .....................................
Specifying bin names ...............................................................
Report showing bin names .......................................................
Specifying ranges with $ ..........................................................
Conditional coverage — disable during reset...........................
Using stop and start functions ............................................
Functional coverage for an enumerated type ............................
Coverage report with enumerated types ...................................
Specifying transitions for a cover point ....................................
Wildcard bins for a cover point ................................................
Cover point with ignore_bins ...................................................
Cover point with auto bin max and ignore bins............
Cover point with illegal_bins ............................................
Basic cross coverage .................................................................
Coverage summary report for basic cross coverage .................
Specifying cross coverage bin names .......................................
Cross coverage report with labeled bins ...................................
Excluding bins from cross coverage .........................................
Specifying cross coverage weight ............................................
Cross coverage with bin names ................................................
Cross coverage with binsof....................................................
Mimicking cross coverage with concatenation ........................
Covergroup with simple argument ...........................................
Pass-by-reference......................................................................
Specifying per-instance coverage .............................................
Specifying comments for a cover group ...................................
Specifying comments for a cover group instance .....................
Report all bins including empty ones .......................................
Specifying the coverage goal ....................................................
Original class for packet length ................................................
solve…before constraint for packet length ...........................

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Sample 10.1
Sample 10.2
Sample 10.3

Rx interface with clocking block .............................................. 364
Tx interface with clocking block .............................................. 364
Testbench using physical interfaces ......................................... 365

List of Samples

Sample 10.4
Sample 10.5
Sample 10.6
Sample 10.7
Sample 10.8
Sample 10.9
Sample 10.10
Sample 10.11
Sample 10.12
Sample 10.13
Sample 10.14
Sample 10.15
Sample 10.16
Sample 10.17
Sample 10.18
Sample 10.19
Sample 10.20
Sample 10.21
Sample 10.22
Sample 10.23
Sample 10.24
Sample 10.25
Sample 10.26
Sample 10.27
Sample 10.28

xli

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375
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376
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377
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378

Sample 10.29
Sample 10.30
Sample 10.31
Sample 10.32

Top level module with array of interfaces ..............................
Testbench using virtual interfaces ..........................................
Testbench using virtual interfaces ..........................................
Monitor class using virtual interfaces ....................................
Test harness using an interface in the port list........................
Test with an interface in the port list ......................................
Top module with a second interface in the test’s port list ......
Test with two interfaces in the port list ..................................
Test with virtual interface and XMR ......................................
Test harness without interfaces in the port list .......................
Test harness with a second interface ......................................
Test with two virtual interfaces and XMRs ............................
Interface for 8-bit counter.......................................................
Counter model using X if interface .....................................
Top-level module with an array of virtual interfaces .............
Counter testbench using virtual interfaces .............................
Driver class using virtual interfaces ...................................
Interface with a typedef ..........................................................
Testbench using a typedef for virtual interfaces .....................
Driver using a typedef for virtual interfaces ......................
Testbench using an array of virtual interfaces ........................
Testbench passing virtual interfaces with a port ....................
Parameterized counter model using X_if interface ..............
Parameterized interface for 8-bit counter ...............................
Parameterized top-level module with an array
of virtual interfaces.................................................................
Parameterized counter testbench using virtual interfaces ......
Driver class using virtual interfaces ...................................
Interface with tasks for parallel protocol ................................
Interface with tasks for serial protocol ...................................

Sample 11.1
Sample 11.2
Sample 11.3
Sample 11.4
Sample 11.5
Sample 11.6
Sample 11.7
Sample 11.8
Sample 11.9
Sample 11.10
Sample 11.11
Sample 11.12
Sample 11.13
Sample 11.14

Top level module ....................................................................
Testbench program .................................................................
CPU Management Interface ...................................................
Utopia interface ......................................................................
Environment class header .......................................................
Environment class methods ....................................................
Callback class connects driver and scoreboard ......................
Callback class connects monitor and scoreboard ...................
Callback class connects the monitor and coverage ................
Environment configuration class ............................................
Cell configuration type ...........................................................
Configuration class methods ..................................................
UNI cell format ......................................................................
NNI cell format ......................................................................

387
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xlii

List of Samples

Sample 11.15
Sample 11.16
Sample 11.17
Sample 11.18
Sample 11.19
Sample 11.20
Sample 11.21
Sample 11.22
Sample 11.23
Sample 11.24
Sample 11.25
Sample 11.26
Sample 11.27

ATMCellType .........................................................................
UNI_cell definition .................................................................
UNI_cell methods...................................................................
UNI_generator class ...............................................................
driver class ..............................................................................
Driver callback class...............................................................
Monitor callback class ............................................................
The Monitor class ...................................................................
The Scoreboard class ..............................................................
Functional coverage class .......................................................
The CPU_driver class .............................................................
Test with one cell ....................................................................
Test that drops cells using driver callback ..............................

398
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Sample 12.1
Sample 12.2
Sample 12.3
Sample 12.4
Sample 12.5
Sample 12.6
Sample 12.7
Sample 12.8
Sample 12.9
Sample 12.10
Sample 12.11
Sample 12.12
Sample 12.13
Sample 12.14
Sample 12.15
Sample 12.16
Sample 12.17
Sample 12.18
Sample 12.19
Sample 12.20
Sample 12.21
Sample 12.22
Sample 12.23
Sample 12.24

SystemVerilog code calling C factorial routine......................
C factorial function.................................................................
Changing the name of an imported function ..........................
Argument directions ...............................................................
C factorial routine with const argument .................................
Importing a C math function ..................................................
Counter routine using a static variable ...................................
Testbench for an 7-bit counter with static storage ..................
Counter routine using instance storage ..................................
Testbench for an 7-bit counter with per-instance storage.......
Testbench for counter that checks for Z or X values ..............
Counter routine that checks for Z and X values .....................
Counter class ..........................................................................
Static methods and linkage .....................................................
C++ counter communicating with methods ...........................
Static wrapper for C++ transaction level counter ...................
Testbench for C++ model using methods ...............................
C routine to compute Fibonacci series ...................................
Testbench for Fibonacci routine .............................................
C routine to compute Fibonacci series with 4-state array ......
Testbench for Fibonacci routine with 4-state array ................
Testbench code calling a C routine with an open array ..........
C code using a basic open array .............................................
Testbench calling C code with multi-dimensional
open array ...............................................................................
C code with multi-dimensional open array ............................
Testbench for packed open arrays ..........................................
C code using packed open arrays ...........................................
C code to share a structure .....................................................
Testbench for sharing structure ..............................................
Returning a string from C.......................................................

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Sample 12.25
Sample 12.26
Sample 12.27
Sample 12.28
Sample 12.29
Sample 12.30

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List of Samples

Sample 12.31
Sample 12.32
Sample 12.33
Sample 12.34
Sample 12.35
Sample 12.36
Sample 12.37
Sample 12.38
Sample 12.39
Sample 12.40
Sample 12.41
Sample 12.42
Sample 12.43
Sample 12.44
Sample 12.45
Sample 12.46
Sample 12.47
Sample 12.48
Sample 12.49
Sample 12.50
Sample 12.51
Sample 12.52
Sample 12.53
Sample 12.54

xliii

Returning a string from a heap in C .......................................
Importing a pure function .......................................................
Imported context tasks............................................................
Exporting a SystemVerilog function ......................................
Calling an exported SystemVerilog function from C .............
Output from simple export .....................................................
C code to read simple command file and call exported
function...................................................................................
SystemVerilog module for simple memory model .................
Command file for simple memory model ..............................
SystemVerilog module for memory model with exported
tasks ........................................................................................
C code to read command file and call exported function .......
Command file for simple memory model ..............................
Command file for exported methods with OOP
memories ................................................................................
SystemVerilog module with memory model class .................
C code to call exported tasks with OOP memory ..................
Second module for simple export example ............................
Output from simple example with two modules ....................
C code getting and setting context .........................................
Modules calling methods that get and set context ..................
Output from svSetScope code ................................................
SystemVerilog code calling C wrapper for Perl .....................
C wrapper for Perl script ........................................................
Perl script called from C and SystemVerilog .........................
VCS command line to run Perl script.....................................

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Chapter 1

Verification Guidelines

Some believed we lacked the programming language
to describe your perfect world…
(The Matrix, 1999)

Imagine that you are given the job of building a house for someone. Where should
you begin? Do you start by choosing doors and windows, picking out paint and
carpet colors, or selecting bathroom fixtures? Of course not! First you must consider
how the owners will use the space, and their budget, so you can decide what type of
house to build. Questions you should consider are: Do they enjoy cooking and want
a high-end kitchen, or will they prefer watching movies in their home theater room
and eating takeout pizza? Do they want a home office or an extra bedroom? Or does
their budget limit them to a more modest house?
Before you start to learn details of the SystemVerilog language, you need to
understand how you plan to verify your particular design and how this influences
the testbench structure. Just as all houses have kitchens, bedrooms, and bathrooms,
all testbenches share some common structure of stimulus generation and response
checking. This chapter introduces a set of guidelines and coding styles for designing
and constructing a testbench that meets your particular needs. These techniques use
some of the same concepts that are shown in the Verification Methodology Manual
for SystemVerilog (VMM), Bergeron et al. (2006), but without the base classes.
Other methodologies such as UVM and OVM share the same concepts.
The most important principle you can learn as a verification engineer is: “Bugs
are good.” Don’t shy away from finding the next bug, do not hesitate to ring a bell
each time you uncover one, and furthermore, always keep track of the details
of each bug found. The entire project team assumes there are bugs in the design, so
each bug found before tape-out is one fewer that ends up in the customer’s hands.
At each stage in the design cycle such as specification, coding, synthesis, manufacturing, the cost of fixing a bug goes up by a factor of 10, so find those bugs early and
often. You need to be as devious as possible, twisting and torturing the design to

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_1,
© Springer Science+Business Media, LLC 2012

1

1

2

Verification Guidelines

extract all possible bugs now, while they are still easy to fix. Don’t let the designers
steal all the glory — without your craft and cunning, the design might never work!
This book assumes you already know the Verilog language and want to learn the
System Verilog Hardware Verification Language (HVL). Some of the typical features of an HVL that distinguish it from a Hardware Description Language such as
Verilog or VHDL are:
• Constrained-random stimulus generation
• Functional coverage
• Higher-level structures, especially Object-Oriented Programming, and transactionlevel modeling
• Multi-threading and interprocess communication (IPC)
• Support for HDL types such as Verilog’s 4-state values
• Tight integration with event-simulator for control of the design
There are many other useful features, but these allow you to create testbenches
at a higher level of abstraction than you are able to achieve with an HDL or a
programming language such as C.

1.1

The Verification Process

What is the goal of verification? If you answered, “Finding bugs,” you are only
partly correct. The goal of hardware design is to create a device that performs a
particular task, such as a DVD player, network router, or radar signal processor,
based on a design specification. Your purpose as a verification engineer is to make
sure the device can accomplish that task successfully — that is, the design is an
accurate representation of the specification. Bugs are what you get when there is a
discrepancy. The behavior of the device when used outside of its original purpose is
not your responsibility, although you want to know where those boundaries lie.
The process of verification parallels the design creation process. A designer reads
the hardware specification for a block, interprets the human language description,
and creates the corresponding logic in a machine-readable form, usually RTL code.
To do this, he or she needs to understand the input format, the transformation function, and the format of the output. There is always ambiguity in this interpretation,
perhaps because of ambiguities in the original document, missing details, or conflicting descriptions. As a verification engineer, you must also read the hardware
specification, create the verification plan, and then follow it to build tests showing
the RTL code correctly implements the features. Therefore, as a verification engineer, not only do you have to understand the design and its intent, but also, you have
to consider all the corner test cases that the designer might not have thought about.
By having more than one person perform the same interpretation, you have
added redundancy to the design process. As the verification engineer, your job is
to read the same hardware specifications and make an independent assessment of
what they mean. Your tests then exercise the RTL to show that it matches your
interpretation.

1.1

The Verification Process

1.1.1

3

Testing at Different Levels

What types of bugs are lurking in the design? The easiest ones to detect are at the
block level, in modules created by a single person. Did the ALU correctly add two
numbers? Did every bus transaction successfully complete? Did all the packets make
it through a portion of a network switch? It is almost trivial to write directed tests to
find these bugs, as they are contained entirely within one block of the design.
After the block level, the next place to look for discrepancies is at boundaries
between blocks. This is known as the integration phase. Interesting problems arise
when two or more designers read the same description yet have different interpretations. For a given protocol, what signals change and when? The first designer builds
a bus driver with one view of the specification, while a second builds a receiver with
a slightly different view. Your job is to find the disputed areas of logic and maybe
even help reconcile these two different views.
To simulate a single design block, you need to create tests that generate stimuli
from all the surrounding blocks — a difficult chore. The benefit is that these lowlevel simulations run very fast. However, you may find bugs in both the design and
testbench, as the latter requires a great deal of code to provide stimuli from the
missing blocks. As you start to integrate design blocks, they can stimulate each
other, reducing your workload. These multiple block simulations may uncover more
bugs, but they also run slower. Analyzing the behavior to determine the root cause
of a bug is more time consuming at higher levels.
At the highest level of the Design Under Test (DUT), the entire system is tested,
but the simulation performance is greatly reduced. Your tests should strive to have
all blocks performing interesting activities concurrently. All I/O ports are active,
processors are crunching data, and caches are being refilled. With all this action,
data alignment and timing bugs are sure to occur.
At this level you are able to run sophisticated tests that have the DUT executing
multiple operations concurrently so that as many blocks as possible are active. What
happens if an MP3 player is playing music and the user tries to download new music
from the host computer? Then, during the download, the user presses several of the
buttons on the player? You know that when the real device is being used, someone
is going to do all this, so why not try it out before it is built? This testing makes the
difference between a product that is seen as easy to use and one that repeatedly
locks up.
Once you have verified that the DUT performs its designated functions correctly,
you need to see how it operates when there are errors. Can the design handle a partial transaction, or one with corrupted data or control fields? Just trying to enumerate all the possible problems is difficult, not to mention determining how the design
should recover from them. Error injection and handling can be the most challenging
part of verification.
As you move to system-level verification, the challenges also move to a higher
level. At the block level, you can show that individual cells flow through the blocks
of an ATM router correctly, but at the system level you might have to consider what

1

4

Verification Guidelines

happens if there are streams of different priority. Which cell should be chosen next
is not always obvious at the highest level. You may have to analyze the statistics
from thousands of cells to see if the aggregate behavior is correct.
One last point: you can never prove there are no bugs left, so you need to constantly come up with new verification tactics.

1.1.2

The Verification Plan

The verification plan is derived from the hardware specification and contains a
description of what features need to be exercised and the techniques to be used. These
steps may include directed or random testing, assertions, HW/SW co-verification,
emulation, formal proofs, and use of verification IP. For a more complete discussion
on verification see Bergeron (2006).

1.2

The Verification Methodology Manual

The book in your hands draws upon the VMM that has its roots in a methodology
developed by Janick Bergeron and others at Qualis Design. They started with
industry-standard practices and refined them based on their experience on many
projects. VMM’s techniques were originally developed for use with the OpenVera
language and were extended in 2005 for SystemVerilog. VMM and its predecessor,
the Reference Verification Methodology (RVM) for Vera, have been used successfully to verify a wide range of hardware designs, from networking devices to processors. Newer methodologies such as OVM and UVM use many similar ideas.
This book is based on many of the same concepts as all these methodologies,
though greatly simplified.
This book serves as a user guide for the SystemVerilog language. It describes
many language constructs and provides guidelines for choosing the ones best suited
to your needs. If you are new to verification, have little experience with ObjectOriented Programming (OOP), or are unfamiliar with constrained-random tests
(CRT), this book can show you the right path to choose. Once you are familiar with
them, you will find UVM and VMM to be an easy step up.
So why doesn’t this book teach you UVM or VMM? Like any advanced tool,
these methodologies were designed for use by an experienced user, and excel on
difficult problems. Are you in charge of verifying a 100 million-gate design with
many communication protocols, complex error handling, and a library of IP? If so,
UVM or VMM are the right tools for the job. However, if you are working on smaller
modules with a single protocol, you may not need such a robust methodology. Just
remember that your block is part of a larger system; UVM- or VMM-compliant code
is reusable both during a project and on later designs. The cost of verification goes
beyond your immediate project.

1.4

Directed Testing

5

The UVM and VMM have a set of base classes for data and environment, utilities
for managing log files and interprocess communication, and much more. This book
is an introduction to SystemVerilog and shows the techniques and tricks that go into
these classes and utilities, giving you insight into their construction.

1.3

Basic Testbench Functionality

The purpose of a testbench is to determine the correctness of the DUT. This is
accomplished by the following steps.
•
•
•
•
•

Generate stimulus
Apply stimulus to the DUT
Capture the response
Check for correctness
Measure progress against the overall verification goals

Some steps are accomplished automatically by the testbench, while others are
manually determined by you. The methodology you choose determines how the
preceding steps are carried out.

1.4

Directed Testing

Traditionally, when faced with the task of verifying the correctness of a design, you
probably used directed tests. Using this approach, you look at the hardware specification and write a verification plan with a list of tests, each of which concentrated
on a set of related features. Armed with this plan, you write stimulus vectors that
exercise these features in the DUT. You then simulate the DUT with these vectors
and manually review the resulting log files and waveforms to make sure the design
does what you expect. Once the test works correctly, you check it off in the verification plan and move to the next one.
This incremental approach makes steady progress, which is always popular with
managers who want to see a project making headway. It also produces almost immediate results, since little infrastructure is needed when you are guiding the creation
of every stimulus vector. Given ample time and staffing, directed testing is sufficient
to verify many designs.
Figure 1.1 shows how directed tests incrementally cover the features in the verification plan. Each test is targeted at a very specific set of design elements. If you
had enough time, you could write all the tests needed for 100% coverage of the
entire verification plan.
What if you do not have the necessary time or resources to carry out the directed
testing approach? As you can see, while you may always be making forward progress, the slope remains the same. When the design complexity doubles, it takes
twice as long to complete or requires twice as many people to implement it.

1

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Verification Guidelines

Coverage

100%

Time
Fig. 1.1 Directed test progress over time

Neither of these situations is desirable. You need a methodology that finds bugs
faster in order to reach the goal of 100% coverage. Brute force does not work; if
you tried to verify every combination of inputs for a 32-bit adder, your simulations
would still be running years after the project should have shipped.
Figure 1.2 shows the total design space and features that are covered by directed
test cases. In this space there are many features, some of which have bugs. You need
to write tests that cover all the features and find the bugs.

Feature
Feature

Test

Bug

Fig. 1.2 Directed test coverage

1.5

Methodology Basics

This book uses the following principles.
•
•
•
•
•

Constrained-random stimulus
Functional coverage
Layered testbench using transactors
Common testbench for all tests
Test case-specific code kept separate from testbench

1.5 Methodology Basics

7

All these principles are related. Random stimulus is crucial for exercising
complex designs. A directed test finds the bugs you expect to be in the design,
whereas a random test can find bugs you never anticipated. When using random
stimuli, you need functional coverage to measure verification progress. Furthermore,
once you start using automatically generated stimuli, you need an automated way
to predict the results — generally a scoreboard or reference model. Building the
testbench infrastructure, including self-prediction, takes a significant amount of
work. A layered testbench helps you control the complexity by breaking the problem into manageable pieces. Transactors provide a useful pattern for building these
pieces. With appropriate planning, you can build a testbench infrastructure that can
be shared by all tests and does not have to be continually modified. You just need
to leave “hooks” where the tests can perform certain actions such as shaping
the stimulus and injecting disturbances. Conversely, code specific to a single test
must be kept separate from the testbench to prevent it from complicating the
infrastructure.
Building this style of testbench takes longer than a traditional directed
testbench — especially the self-checking portions. As a result, there may be a
significant delay before the first test can be run. This gap can cause a manager to
panic, so make this effort part of your schedule. In Fig. 1.3, you can see the initial
delay before the first random test runs.

Coverage

100%
Random
Test

Directed
Test

Time
Fig. 1.3 Constrained-random test progress over time vs. directed testing

While this up-front work may seem daunting, the payback is high. Every random
test you create shares this common testbench, as opposed to directed tests where
each is written from scratch. Each random test contains a few dozen lines of code to
constrain the stimulus in a certain direction and cause any desired exceptions, such
as creating a protocol violation. The result is that your single constrained-random
testbench is now finding bugs faster than the many directed ones.
As the rate of discovery begins to drop off, you can create new random constraints to explore new areas. The last few bugs may only be found with directed
tests, but the vast majority of bugs will be found with random tests. If you create a
random testbench, you can always constrain it to created directed tests, but a directed
testbench can never be turned into a true random testbench.

1

8

1.6

Verification Guidelines

Constrained-Random Stimulus

Although you want the simulator to generate the stimulus, you don’t want totally
random values. You use the SystemVerilog language to describe the format of the
stimulus (“address is 32-bits; opcode is ADD, SUB or STORE; length < 32 bytes”),
and the simulator picks values that meet the constraints. Constraining the random
values to become relevant stimuli is covered in Chapter 6. These values are sent into
the design, and are also sent into a high-level model that predicts what the result
should be. The design’s actual output is compared with the predicted output.
Figure 1.4 shows the coverage for constrained-random tests over the total design
space. First, notice that a random test often covers a wider space than a directed one.
This extra coverage may overlap other tests, or may explore new areas that you did
not anticipate. If these new areas find a bug, you are in luck! If the new area is not
legal, you need to write more constraints to keep random generation from creating
illegal design functionality. Lastly, you may still have to write a few directed tests to
find cases not covered by any other constrained-random tests.
Figure 1.5 shows the paths to achieve complete coverage. Start at the upper
left with basic constrained-random tests. Run them with many different seeds.

New area

?
Test
overlap

Directed
testcase

?

?

Fig. 1.4 Constrained-random test coverage

Constrained
Many runs,
Many runs,
random tests Directed different
seeds
different seeds
Directed
testcase

Add
Add
constraints
constraints

Minimal code
modifications
Fig. 1.5 Coverage convergence

Functional
Coverage
Identify
Identify
holes
holes

1.7

What Should You Randomize?

9

When you look at the functional coverage reports, find the holes where there are
gaps in the coverage. Then you make minimal code changes, perhaps by using new
constraints, or by injecting errors or delays into the DUT. Spend most of your time
in this outer loop, writing directed tests for only the few features that are very
unlikely to be reached by random tests.

1.7

What Should You Randomize?

When you think of randomizing the stimulus to a design, you might first pick
the data fields. These values are is the easiest to create — just call $random (). The
problem is that this choice gives a very low payback in terms of bugs found. The
primary types of bugs found with random data are data path errors, perhaps with
bit-level mistakes. You need to find bugs in the control logic, source of the most
devious problems.
Think broadly about all design inputs, such as the following.
•
•
•
•
•
•

Device configuration
Environment configuration
Input data
Protocol exceptions
Errors and violations
Delays
These are discussed in sections 1.7.1 through 1.7.4.

1.7.1

Device and Environment Configuration

What is the most common reason why bugs are missed during testing of the RTL
design? Not enough different configurations are tried. Most tests just use the design
as it comes out of reset, or apply a fixed set of initialization vectors to put it into a
known state. This is like testing a PC’s operating system right after it has been
installed, but without any of the applications installed. Of course the performance is
fine and there aren’t any crashes.
In a real world environment, the DUT’s configuration becomes more random the
longer it is in use. For example, I helped a company verify a time-division multiplexor switch that had 2000 input channels and 12 output channels. The verification
engineer said, “These channels could be mapped to various configurations on the
other side. Each input could be used as a single channel, or further divided into
multiple channels. The tricky part is that although a few standard ways of breaking
it down are used most of the time, any combination of breakdowns is legal, leaving
a huge set of possible customer configurations.”
To test this device, the engineer had to write several dozen lines of directed testbench code to configure each channel. As a result, she was never able to try configurations with more than a handful of channels. Together, we wrote a testbench that

1

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Verification Guidelines

randomized the parameters for a single channel and then put this in a loop to configure
all the switch’s channels. Now she had confidence that her tests would uncover
configuration-related bugs that would have been missed before.
In the real world, your device operates in an environment containing other components. When you are verifying the DUT, it is connected to a testbench that mimics this
environment. You should randomize the entire environment configuration, including
the length of the simulation, number of devices, and how they are configured.
Of course you need to create constraints to make sure the configuration is legal.
In another Synopsys customer example, a company created an I/O switch chip
that connected multiple PCI buses to an internal memory bus. At the start of simulation they randomly chose the number of PCI buses (1–4), the number of devices on
each bus (1–8), and the parameters for each device (master or slave, CSR addresses,
etc.). They kept track of the tested combinations using functional coverage so that
they could be sure that they had covered almost every possible one.
Other environment parameters include test length, error injection rates, and delay
modes. See Bergeron (2006) for more examples.

1.7.2

Input Data

When you read about random stimulus, you probably thought of taking a transaction such as a bus write or ATM cell and filling the data fields with random values.
Actually, this approach is fairly straightforward as long as you carefully prepare
your transaction classes as shown in Chapters 5 and 8. You need to anticipate any
layered protocols and error injection, plus scoreboarding and functional coverage.

1.7.3

Protocol Exceptions, Errors, and Violations

There are few things more frustrating than when a device such as a PC or cell phone
locks up. Many times, the only cure is to shut it down and restart. Chances are that
deep inside the product there is a piece of logic that experienced some sort of error
condition from which it could not recover and thus prevented the device from working correctly.
How can you prevent this from happening to the hardware you are building? If
something can go wrong in the real hardware, you should try to simulate it. Look at
all the errors that can occur. What happens if a bus transaction does not complete?
If an invalid operation is encountered? Does the design specification state that two
signals are mutually exclusive? Drive them both and make sure the device continues
to operate properly.
Just as you are trying to provoke the hardware with ill-formed commands,
you should also try to catch these occurrences. For example, recall those mutually

1.7

What Should You Randomize?

11

exclusive signals. You should add checker code to look for these violations. Your
code should at least print a warning message when this occurs, and preferably generate an error and wind down the test. It is frustrating to spend hours tracking back
through code trying to find the root of a malfunction, especially when you could
have caught it close to the source with a simple assertion. See Vijayaraghavan
[2005] for more guidelines on writing assertions in your testbench and design code.
Just make sure that you can disable the code that stops simulation on error so that
you can easily test error handling.

1.7.4

Delays and Synchronization

How fast should your testbench send in stimulus? You should pick random delays
to help catch protocol bugs. A test with the shortest delays is easy to write, but won’t
create all possible stimulus combinations. Subtle bugs around boundary conditions
are often revealed when realistic delays are chosen.
A block may function correctly for all possible permutations of stimulus from a
single interface, but subtle errors may occur when transactions are flowing into
multiple inputs. Try to coordinate the various drivers so they can communicate at
different timing rates. What if the inputs arrive at the fastest possible rate, but the
output is being throttled back to a slower rate? What if stimulus arrives at multiple
inputs concurrently? What if it is staggered with different delays? Use functional
coverage, which will be discussed in Chapter 9, to measure what combinations have
been randomly generated.

1.7.5

Parallel Random Testing

How should you run the tests? A directed test has a testbench that produces a unique
set of stimulus and response vectors. To change the stimulus, you need to change the
test. A random test consists of the testbench code plus a random seed. If you run the
same test 50 times, each time with a unique seed, you will get 50 different sets of
stimuli. Running with multiple seeds broadens the coverage of your test and leverages your work.
You need to choose a unique seed for each simulation. Some people use the time
of day, but that can still cause duplicates. What if you are using a batch queuing
system across a CPU farm and tell it to start 10 jobs at midnight? Multiple jobs
could start at the same time but on different computers, and will thus get the same
random seed and run the same stimulus. You should blend in the processor name to
the seed. If your CPU farm includes multiprocessor machines, you could have two
jobs start running at midnight with the same seed, so you should also throw in the
process ID. Now all jobs get unique seeds.

1

12

Verification Guidelines

You need to plan how to organize your files to handle multiple
simulations. Each job creates a set of output files, such as log files
and functional coverage data. You can run each job in a different
directory, or you can try to give a unique name to each file. The easiest
approach is to append the random seed value to the directory name.

1.8

Functional Coverage

Sections 1.6 and 1.7 showed how to create stimuli that can randomly walk through
the entire space of possible inputs. With this approach, your testbench visits some
areas often, but takes too long to reach all possible states. Unreachable states will
never be visited, even given unlimited simulation time. You need to measure what
has been verified in order to check off items in your verification plan.
The process of measuring and using functional coverage consists of several
steps. First, you add code to the testbench to monitor the stimulus going into the
device, and its reaction and response, to determine what functionality has been
exercised. Run several simulations, each with a different seed. Next, merge the
results from these simulations into a report. Lastly, you need to analyze the results
and determine how to create new stimulus to reach untested conditions and logic.
Chapter 9 describes functional coverage in SystemVerilog.

1.8.1

Feedback from Functional Coverage to Stimulus

A random test evolves using feedback. The initial test can be run with many different seeds, thus creating many unique input sequences. Eventually the test, even with
a new seed, is less likely to generate stimulus that reaches areas of the design space.
As the functional coverage asymptotically approaches its limit, you need to change
the test to find new approaches to reach uncovered areas of the design. This is
known as “coverage-driven verification” and is shown in Fig. 1.6.

Coverage

100%

With
feedback
Without
feedback

Time
Fig. 1.6 Test progress with and without feedback

1.9

Testbench Components

13

What if your testbench were smart enough to do this for you? In a previous job,
I wrote a test that generated every bus transaction for a processor and additionally
fired every bus terminator (Success, Parity Error, Retry) in every cycle. This was
before HVLs, so I wrote a long set of directed tests and spent days lining up the
terminator code to fire at just the right cycles. After much hand analysis I declared
success — 100% coverage. Then the processor’s timing changed slightly! Now
I had to reanalyze the test and change the stimuli.
A more productive testing strategy uses random transactions and terminators.
The longer you run it, the higher the coverage. As a bonus, the test can be made
flexible enough to create valid stimuli even if the design’s timing changed. You can
accomplish this by adding a feedback loop that looks at the stimulus created so far
(generated all write cycles yet?) and then change the constraint weights (drop write
weight to zero). This improvement would greatly reduce the time needed to get to
full coverage, with little manual intervention.
This is not a typical situation however, because of the trivial feedback from functional coverage to the stimulus. In a real design, how should you change the stimulus to reach a desired design state? This requires deep knowledge of the design and
powerful formal techniques. There are no easy answers, so dynamic feedback is
rarely used for constrained-random stimulus. Instead, you need to manually analyze
the functional coverage reports and alter your random constraints.
Feedback is used in formal analysis tools such as Magellan (Synopsys 2003).
It analyzes a design to find all the unique, reachable states. It then runs a short simulation to see how many states were visited. Lastly, it searches from the state machine
to the design inputs to calculate the stimulus needed to reach any remaining states
and then Magellan applies this to the DUT.

1.9

Testbench Components

In simulation, the testbench wraps around the DUT, just as a hardware tester connects to a physical chip, as shown in Fig. 1.7. Both the testbench and tester provide
stimulus and capture responses. The difference between them is that your testbench
needs to work over a wide range of levels of abstraction, creating transactions and
sequences, which are eventually transformed into bit vectors. A tester just works at
the bit level.

Testbench

inputs

Design
Under
Test

Fig. 1.7 The testbench — design environment

outputs

1

14

Verification Guidelines

What goes into that testbench block? It is comprised of many Bus Functional
Models (BFM), which you can think of as testbench components — to the DUT
they look like real components, but they are part of the testbench, not the RTL
design. If the real device connects to AMBA, USB, PCI, and SPI buses, you have to
build equivalent components in your testbench that can generate stimulus and check
the response, as shown in Fig. 1.8. These are not detailed, synthesizable models, but
instead highlevel transactors that obey the protocol, and execute more quickly. On
the other hand, if you are prototyping using FPGAs or emulation, the BFMs do need
to be synthesizable.

Testbench

PCI
SPI

AMBA

USB

Design
Under
Test

Fig. 1.8 Testbench components

1.10

Layered Testbench

A key concept for any modern verification methodology is the layered testbench.
Although this process may seem to make the testbench more complex, it actually
helps to make your task easier by dividing the code into smaller pieces that can be
developed separately. Don’t try to write a single routine that can randomly generate
all types of stimuli, both legal and illegal, plus inject errors with a multi-layer
protocol. The routine quickly becomes complex and unmaintainable. In addition, a
layered approach allows reuse and encapsulation of Verification IP (VIP) which are
OOP concepts.

1.10.1

A Flat Testbench

When you first learned Verilog and started writing tests, they probably looked like
the low-level code in Sample 1.1, which does a simplified APB (AMBA Peripheral
Bus) Write. (VHDL users may have written similar code).

1.10

Layered Testbench

15

Sample 1.1 Driving the APB pins

After a few days of writing code like this, you probably realized that it is very
repetitive, so you created tasks for common operations such as a bus write, as shown
in Sample 1.2.

16

1

Verification Guidelines

Sample 1.2 A task to drive the APB pins

Now your testbench became simpler, as shown in Sample 1.3
Sample 1.3 Low-level Verilog test

By taking the common actions (such as reset, bus reads and writes) and putting
them in a routine, you became more efficient and made fewer mistakes. This creation of the physical and command layers is the first step to a layered testbench.

1.10

Layered Testbench

1.10.2

17

The Signal and Command Layers

Figure 1.9 shows the lower layers of a testbench.
Command

Driver

Assertions

Monitor

Signal
DUT

Fig. 1.9 Signal and command layers

At the bottom is the signal layer that contains the design under test and the signals that connect it to the testbench.
The next higher level is the command layer. The DUT’s inputs are driven by the
driver that runs single commands, such as bus read or write. The DUT’s output
drives the monitor that takes signal transitions and groups them together into commands. Assertions also cross the command/signal layer, as they look at individual
signals and also changes across an entire command.

1.10.3

The Functional Layer

Figure 1.10 shows the testbench with the functional layer added, which feeds down
into the command layer. The agent block (called the transactor in the VMM) receives
higher-level transactions such as DMA read or write and breaks them into individual commands or transactions. These commands are also sent to the scoreboard that
predicts the results of the transaction. The checker compares the commands from
the monitor with those in the scoreboard.
Functional

Agent

Scoreboard

Checker

Command

Driver

Assertions

Monitor

Signal
DUT

Fig. 1.10 Testbench with functional layer added

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18

1.10.4

Verification Guidelines

The Scenario Layer

The functional layer is driven by the generator in the scenario layer, as shown in
Fig. 1.11. What is a scenario? Remember that your job as a verification engineer is
to make sure that this device accomplishes its intended task. An example device is
an MP3 player that can concurrently play music from its storage, download new
music from a host, and respond to input from the user, such as adjusting the volume and track controls. Each of these operations is a scenario. Downloading a
music file takes several steps, such as control register reads and writes to set up the
operation, multiple DMA writes to transfer the song, and then another group of
reads and writes. The scenario layer of your testbench orchestrates all these steps
with constrained-random values for parameters such as track size and memory
location.

Scenario

Environment

Generator

Functional

Agent
Agent

Scoreboard

Checker

Command

Driver

Assertions

Monitor

Signal
DUT

Fig. 1.11 Testbench with scenario layer added

The blocks in the testbench environment (inside the dashed line of Fig. 1.11) are
written at the beginning of development. During the project they may evolve and
you may add functionality, but these blocks should not change for individual tests.
This is done by leaving “hooks” in the code so that a test can change the behavior
of these blocks without having to rewrite them. You create these hooks with factory
patterns (Section 8.2) and callbacks (Section 8.7).

1.10.5

The Test Layer and Functional Coverage

You are now at the top of the testbench, in the test layer, as shown in Fig. 1.12.
Design bugs that occur between DUT blocks are harder to find as they involve multiple people reading and interpreting multiple specifications.
This top-level test is the conductor: he does not play any musical instrument, but
instead guides the efforts of others. The test contains the constraints to create the
stimulus.
Functional coverage measures the progress of all tests in fulfilling the verification plan requirements. The functional coverage code changes through the project

1.11 Building a Layered Testbench

19

as the various criteria complete. This code is constantly being modified and thus it
is not part of the environment.
You can create a directed test in a constrained-random environment. Simply
insert a section of directed test code into the middle of a random sequence, or put
the two pieces of code in parallel. The directed code performs the work you want,
but the random “background noise” may cause a bug to become visible, perhaps in
a block that you never considered.

Test

Agent
Driver

Scoreboard

Checker

Assertions

Monitor

Functional Coverage

Environment

Generator

DUT

Fig. 1.12 Full testbench with all layers

Do you need all these layers in your testbench? The answer depends on what
your DUT looks like. A complicated design requires a sophisticated testbench. You
always need the test layer. For a simple design, the scenario layer may be so simple
that you can merge it with the agent. When estimating the effort to test a design,
don’t count the number of gates; count the number of designers. Every time you add
another person to the team, you increase the chance of different interpretations of
the specifications. Typical hardware teams need more than two verification engineers for every designer.
You may need more layers. If your DUT has several protocol layers, each should
get its own layer in the testbench environment. For example, if you have TCP traffic
that is wrapped in IP and sent in Ethernet packets, consider using three separate layers for generation and checking. Better yet, use existing verification components.
One last note about Fig. 1.12. It shows some of the possible connections between
blocks, but your testbench may have a different set. The test may need to reach
down to the driver layer to force physical errors. What has been described here is
just guidelines — let your needs guide what you create.

1.11

Building a Layered Testbench

Now it is time to take the preceding figures and learn how to map the components
into SystemVerilog constructs.

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20

1.11.1

Verification Guidelines

Creating a Simple Driver

First, take a closer look at one of the blocks, the driver.
Agent

Driver

DUT

Fig. 1.13 Connections for the driver

The driver shown in Fig. 1.13 receives commands from the agent. The driver
may inject errors or add delays. It then breaks the command down into individual
signal changes such as bus requests and handshakes. The general term for such a
testbench block is a “transactor,” which, at its core, is a loop. Sample code for a
transactor is shown in Sample 1.4.
Sample 1.4 Basic transactor code

Chapter 5 presents basic OOP and how to create an object that includes the routines and data for a transactor. Another example of a transactor is the agent. It might
break apart a complex transaction such as a DMA read into multiple bus commands.
Also in Chapter 5, you will see how to build an object that contains the data and
routines that make up a command. These objects are sent between transactors using
SystemVerilog mailboxes. In Chapter 7, you will learn about many ways to exchange
data between the different layers and to synchronize the transactors.

1.12

Simulation Environment Phases

Up until now you have been learning what parts make up the environment. When do
these parts execute? You want to clearly define the phases to coordinate the testbench so that all the code for a project works together. The three primary phases are
Build, Run, and Wrap-up. Each is divided into smaller steps. These three are a subset of the many phases of the UVM and VMM.

1.13

Maximum Code Reuse

21

The Build phase is divided into the following steps:
• Generate configuration: Randomize the configuration of the DUT and the surrounding environment.
• Build environment: Allocate and connect the testbench components based on the
configuration. A testbench component is one that only exists in the testbench, as
opposed to physical components in the design that are built with RTL code. For
example, if the configuration chose three bus drivers, the testbench would allocate and initialize them in this step.
• Reset the DUT.
• Configure the DUT: Based on generated configuration from the first step, load
the DUT command registers.
The Run phase is where the test actually runs. It has the following steps:
• Start environment: Run the testbench components such as BFMs and stimulus
generators.
• Run the test: Start the test and then wait for it to complete. It is easy to tell when
a directed test has completed, but doing so can be complex for a random test. You
can use the testbench layers as a guide. Starting from the top, wait for a layer to
drain all the inputs from the previous layer (if any), wait for the current layer
to become idle, and then wait for the next lower layer. You should also use timeout checkers to ensure that the DUT or testbench does not lock up.
The Wrap-up phase has two steps:
• Sweep: After the lowest layer completes, you need to wait for the final transactions to drain out of the DUT.
• Report: Once the DUT is idle, sweep the testbench for lost data. Sometimes the
scoreboard holds transactions that never came out, perhaps because they were
dropped by the DUT. Armed with this information, you can create the final report
on whether the test passed or failed. If it failed, be sure to delete any functional
coverage results, as they may not be correct.
As shown in Fig. 1.12, the test starts the environment, which, in turn, runs each of
the steps. More details can be found in Chapter 8.

1.13

Maximum Code Reuse

To verify a complex device with hundreds of features, you have to write hundreds
of directed tests. If you use constrained-random stimulus, you would write fewer
tests. Instead, the real work is put into constructing the testbench, which contains all
the lower testbench layers: scenario, functional, command, and signal. This testbench code is used by all the tests, so it remains generic.
These guidelines appear to recommend an overly complicated testbench, but
remember that every line that you put into a testbench can eliminate a line in every
single test. If you know you will be creating a few dozen tests, there is a high pay-

1

22

Verification Guidelines

back in making a more sophisticated testbench. Keep this in mind when you read
Chapter 8.

1.14

Testbench Performance

If this is the first time you have seen this methodology, you probably have some
qualms about how it works compared to directed testing. A common objection is
testbench performance. A directed test often simulates in a few seconds, whereas
constrained-random tests will wander around through the state space for minutes or
even hours. The problem with this argument is that it ignores a real verification
bottleneck: the time required by you to create a test. You may be able to hand-craft
a directed test in a day and debug it and manually verify the results by hand in
another day or two. The actual simulation run time is dwarfed by the amount of time
that you personally invested.
There are several steps to creating a constrained-random test. The first and most
significant step is building the layered testbench, including the self-checking portion. The benefit of this work is shared by all tests, so it is well worth the effort. The
second step is creating the stimulus specific to a goal in the verification plan. You
may be crafting random constraints, or devious ways of injecting errors or protocol
violations. Building one of these may take more time than making several directed
tests, but the payoff will be much higher. A constrained-random test that tries thousands of different protocol variations is worth more than the handful of directed
tests that could have been created in the same amount of time.
The third step in constrained-random testing is functional coverage. This task
starts with the creation of a strong verification plan with clear goals that can be easily
measured. Next you need to create the SystemVerilog code that adds instrumentation
to the environment and gathers the data. Finally, it is essential that you analyze the
results to determine if you have met the goals, and, if not, how you should modify
the tests.

1.15

Conclusion

The continuous growth in complexity of electronic designs requires a modern, systematic, and automated approach to creating testbenches. The cost of fixing a bug
grows by tenfold as a project moves from each step of specification to RTL coding,
gate synthesis, fabrication, and finally into the user’s hands. Directed tests only test
one feature at a time and cannot create the complex stimulus and configurations that
the device would be subjected to in the real world. To produce robust designs, you
must use constrained-random stimulus combined with functional coverage to create
the widest possible range of stimuli.

1.16

Exercises

1.16

23

Exercises

1. Write a verification plan for an Arithmetic Logic Unit (ALU) with:
•
•
•
•
•

Asynchronous active high input reset
Input clock
4-bit signed inputs, A and B
5-bit signed output C that is registered on the positive edge of input clock.
4 opcodes
–
–
–
–

2.
3.
4.
5.

Add: A + B
Sub: A − B
Bit-wise invert: A
Reduction Or: B

What are the advantages and disadvantages to testing at the block level? Why?
What are the advantages and disadvantages to testing at the system level? Why?
What are the advantages and disadvantages to directed testing? Why?
What are the advantages and disadvantages to constrained random testing? Why?

Chapter 2

Data Types

SystemVerilog offers many improved data structures compared with Verilog. Some
of these were created for designers but are also useful for testbenches. In this chapter
you will learn about the data structures most useful for verification.
System Verilog introduces new data types with the following benefits.
• Two-state: better performance, reduced memory usage
• Queues, dynamic and associative arrays: reduced memory usage, built-in support
for searching and sorting
• Classes and structures: support for abstract data structures
• Unions and packed structures: allow multiple views of the same data
• Strings: built-in string support
• Enumerated types: code is easier to write and understand

2.1

Built-In Data Types

Verilog-1995 has two basic data types: variables and nets, both which hold 4-state
values: 0, 1, Z, and X. RTL code uses variables to store combinational and sequential values. Variables can be unsigned single or multi-bit (reg [7:0] m), signed
32-bit variables (integer), unsigned 64-bit variables (time), and floating point
numbers (real). Variables can be grouped together into arrays that have a fixed
size. A net is used to connect parts of a design such as gate primitives and module
instances. Nets come in many flavors, but most designers use scalar and vector
wires to connect together the ports of design blocks. Lastly, all storage is static,
meaning that all variables are alive for the entire simulation and routines cannot use
a stack to hold arguments and local values. Verilog-2001 allows you to switch
between static and dynamic storage, such as stacks.
SystemVerilog adds many new data types to help both hardware designers and
verification engineers.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_2,
© Springer Science+Business Media, LLC 2012

25

2

26

2.1.1

Data Types

The Logic Type

The one thing in Verilog that always leaves new users scratching their heads is the
difference between a reg and a wire. When driving a port, which should you use?
How about when you are connecting blocks? SystemVerilog improves the classic
reg data type so that it can be driven by continuous assignments, gates, and modules, in addition to being a variable. It is given the synonym logic as some people
new to Verilog thought that reg declared a digital register, and not a signal. A logic
signal can be used anywhere a net is used, except that a logic variable cannot
be driven by multiple structural drivers, such as when you are modeling a bidirectional bus. In this case, the variable needs to be a net type such as wire so that
SystemVerilog can resolve the multiple values to determine the final value.
Sample 2.1 shows the SystemVerilog logic type.
Sample 2.1 Using the logic type

You can use the logic type to find netlist bugs as this type can
only have a single driver. Rather than trying to choose between
reg and wire, declare all your signals as logic, and you’ll get a
compilation error if it has multiple drivers. Of course, any signal
that you do want to have multiple drivers, such as a bidirectional
bus, should be declared with a net type such as wire or tri.

2.1.2

2-State Data Types

SystemVerilog introduces several 2-state data types to improve simulator performance and reduce memory usage, compared to variables declared as 4-state types.
The simplest type is the bit, which is always unsigned. There are four signed 2-state
types: byte, shortint, int, and longint. as shown in Sample 2.2.

2.2

Fixed-Size Arrays

27

Sample 2.2 Signed data types

You might be tempted to use types such as byte to replace more
verbose declarations such as logic [7:0]. Hardware designers should be careful as these new types are signed variables, so
a byte variable can only count up to 127, not the 255 you may
expect. (It has the range −128 to +127.) You could use byte unsigned, but that is
more verbose than just bit [7:0]. Signed variables can also cause unexpected
results with randomization, as discussed in Chapter 6.
Be careful connecting 2-state variables to the design under test,
especially its outputs. If the hardware tries to drive an X or Z, these
values are converted to a 2-state value, and your testbench code
may never know. Don’t try to remember if they are converted to
0 or 1; instead, always check for propagation of unknown values.
Use the $isunknown() operator that returns 1 if any bit of the expression is X or Z,
as shown in Sample 2.3.
Sample 2.3 Checking for 4-state values

The format %0t and the argument $time print the current simulation time, formatted as specified with the $timeformat() routine. Time values are explored in
more detail in Section 3.7.

2.2

Fixed-Size Arrays

SystemVerilog offers several flavors of arrays beyond the single-dimension, fixedsize Verilog-1995 arrays. Additionally, many new features have been added to support these data types.

2

28

2.2.1

Data Types

Declaring and Initializing Fixed-Size Arrays

Verilog requires that the low and high array limits must be given in the declaration.
Since almost all arrays use a low index of 0, SystemVerilog lets you use the shortcut
of just giving the array size, which is similar to C’s style, as shown in Sample 2.4.
Sample 2.4 Declaring fixed-size arrays

How can you compute the number of bits needed to address a given array size?
SystemVerilog has the $clog2() function that calculates the ceiling of log base 2,
as shown in Sample 2.5.
Sample 2.5 Calculating the address width for a memory

You can create multi-dimensional fixed-size arrays by specifying the dimensions
after the variable name. Sample 2.6 creates several two-dimensional arrays of integers, 8 entries by 4, and sets the last entry to 1. Multi-dimensional arrays were
introduced in Verilog-2001, but the compact declaration style is new.
Sample 2.6 Declaring and using multi-dimensional arrays

If your code accidently tries to read from an out-of-bounds address, SystemVerilog will return the default value for the array element type. That just means
that an array of 4-state types, such as logic, will return X’s, whereas an array of
2-state types, such as int or bit, will return 0. This applies for all array types –
fixed, dynamic, associative, or queue, and also if your address has an X or Z. An
undriven net is Z.
Many SystemVerilog simulators store each element on a 32-bit word boundary.
So a byte, shortint, and int are all stored in a single word, whereas a longint
is stored in two words.
An unpacked array, such as the one shown in Sample 2.7, stores the values in the
lower portion of the word, whereas the upper bits are unused. The array of bytes,
b_unpack, is stored in three words, as shown in Fig. 2.1.

2.2

Fixed-Size Arrays

29

Sample 2.7 Unpacked array declarations

76543210

b_unpack[0]
b_unpack[1]
b_unpack[2]

Unused space

76543210
76543210

Fig. 2.1 Unpacked array storage

Packed arrays are explained in Section 2.2.6.
Simulators generally store 4-state types such as logic and integer in two or
more consecutive words, using twice the storage as 2-state variables.

2.2.2

The Array Literal

Sample 2.8 shows how to initialize an array using an array literal, which is an apostrophe followed by the values in curly braces. (This is not the accent grave used for
compiler directives and macros.) You can set some or all elements at once. You can
replicate values by putting a count before the curly braces.
Sample 2.8 Initializing an array

Notice that in Sample 2.8, the declaration of the array ascend includes an initial
value. The 2009 LRM states that these variables must be declared either in a static
block, or have the static keyword. Since this book recommends always declaring
your test modules and programs as automatic, you need to add the static keyword to a declaration plus initialization when it is inside an initial block.
A great new feature in the 2009 LRM is printing with the %p format specifier.
This prints an assignment pattern that is equivalent to the data object’s value. You can
print any data type in SystemVerilog including arrays, structures, classes, and more.
Sample 2.9 shows how to print an array with the %p format specifier.

2

30

Data Types

Sample 2.9 Printing with %p print specifier

2.2.3

Basic Array Operations — for and Foreach

The most common way to manipulate an array is with a for or foreach loop. In
Sample 2.10, the variable i is declared local to the for loop. The SystemVerilog
function $size returns the size of the array. In the foreach loop, you specify the
array name and an index in square brackets, and SystemVerilog automatically steps
through all the elements of the array. The index variable is automatically declared
for you and is local to the loop.
Sample 2.10 Using arrays with for- and foreach loops

Note that in Sample 2.11, the syntax of the foreach loop for multi-dimensional
arrays may not be what you expected. Instead of listing each subscript in separate
square brackets, [i][j], they are combined with a comma: [i,j].
Sample 2.11 Initialize and step through a multi-dimensional array

The output from Sample 2.11 is shown in Sample 2.12.

2.2

Fixed-Size Arrays

31

Sample 2.12 Output from printing multi-dimensional array values

You can omit some dimensions in the foreach loop if you don’t need to step
through all of them. Sample 2.13 prints a two-dimensional array in a rectangle.
It steps through the first dimension in the outer loop, and then through the second
dimension in the inner loop.
Sample 2.13 Printing a multi-dimensional array

Sample 2.13 produces the output shown in Sample 2.14.
Sample 2.14 Output from printing multi-dimensional array values

Lastly, a foreach loop iterates using the ranges in the original declaration. The
array f[5] is equivalent to f[0:4], and a foreach (f[i]) is equivalent to for
(int i=0;i<=4; i++). With the array rev[6:2], the statement foreach(rev[i]) is
equivalent to for(int i=6; i>=2; i--).

2

32

2.2.4

Data Types

Basic Array Operations – Copy and Compare

You can perform aggregate compare and copy of arrays without loops. (An aggregate
operation works on the entire array as opposed to working on just an individual
element.) Comparisons are limited to just equality and inequality. Sample 2.15 shows
several examples of compares. The ? : conditional operator is a mini if-else statement. In Sample 2.15, it is used to choose between two strings. The final compare
uses an array slice, src[1:4], which creates a temporary array with 4 elements.
Sample 2.15 Array copy and compare operations

A copy between fixed arrays of different sizes causes a compile error. You can
not perform aggregate arithmetic such as addition or subtraction on arrays, for
example, a = b + c. Instead, use foreach loops. For logical operations such as xor,
you have to either use a loop or use packed arrays as described in Section 2.2.6.

2.2.5

Bit and Array Subscripts, Together at Last

A common annoyance in Verilog-1995 is that you cannot use array and bit subscripts together. Verilog-2001 removes this restriction for fixed-size arrays. Sample
2.16 prints the first array element (binary 101), its lowest bit (1), and the next two
higher bits (binary 10).

2.2

Fixed-Size Arrays

33

Sample 2.16 Using word and bit subscripts together

Although this change is not new to SystemVerilog, many users may not
know about this useful improvement in Verilog-2001. FYI - a double comma in a
$display statement inserts a space.

2.2.6

Packed Arrays

For some data types, you may want both to access the entire value and also to divide
it into smaller elements. For example, you may have a 32-bit register that sometimes
you want to treat as four 8-bit values and at other times as a single, unsigned value.
A SystemVerilog packed array is treated as both an array and a single value. It is
stored as a contiguous set of bits with no unused space, unlike an unpacked array.

2.2.7

Packed Array Examples

The packed bit and array dimensions are specified as part of the type, before the
variable name. These dimensions must be specified in the [msb:lsb] format, not
[size]. Sample 2.17 shows the variable bytes, a packed array of four bytes that
are stored in a single 32-bit word as shown in Fig. 2.2.
Sample 2.17 Packed array declaration and usage

bytes[3]
bytes

76543210 76543210 76543210 76543210

bytes[3][7]
Fig. 2.2 Packed array layout

2

34

Data Types

You can mix packed and unpacked dimensions. You may want to make an array
that represents a memory that can be accessed as bits, bytes, or longwords. Sample
2.18 shows barray, an unpacked array of five packed elements, each four bytes
wide, which are stored in memory as shown in Fig. 2.3.
Sample 2.18 Declaration for a mixed packed/unpacked array

barray[0][3]

barray[0][1][6]

barray[0] 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
barray[1] 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
barray[2] 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
barray[3] 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
barray[4] 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0

Fig. 2.3 Packed array bit layout

With a single subscript, you get a word of data, barray[0].With two subscripts,
you get a byte of data, barray[0][3]. With three subscripts, you can access a
single bit, barray[0][1][6]. Because one dimension is specified after the name,
barray[5], that dimension is unpacked, so you must always give at least one
subscript.
The last line of Sample 2.18 copies between two packed arrays. Since the underlying values are just bits, you can copy even if the arrays have different dimensions.

2.2.8

Choosing Between Packed and Unpacked Arrays

Which should you choose — a packed or an unpacked array? A packed array is
handy if you need to convert to and from scalars. For example, you might need to
reference a memory as a byte or as a word. The barray in Fig. 2.3 can handle this
requirement. Any array type can be packed, including dynamic arrays, queues and
associative arrays, which are explained in Sections 2.3, 2.4, and 2.5.
If you need to wait for a change in an array, you have to use a packed array.
Perhaps your testbench might need to wake up when a memory changes value, so
you want to use the @ operator. This is only legal with scalar values and packed
arrays. In Sample 2.18 you can block on the variables lw or barray[0], but not the
entire array barray unless you expand it: @(barray[0] or barray[1] or
barray[2] or barray[3] or barray[4]).

2.3

2.3

Dynamic Arrays

35

Dynamic Arrays

The basic Verilog array type shown so far is known as a fixed-size array, as its size
is set at compile time. What if you do not know the size of the array until run time?
For example, you may want generate a random number of transactions at the start
of simulation. If you stored the transactions in a fixed-size array, it would have to be
large enough to hold the maximum number of transactions, but would typically hold
far fewer, thus wasting memory. SystemVerilog provides a dynamic array that can
be allocated and resized during simulation so your simulation consumes a minimal
amount of memory.
A dynamic array is declared with empty word subscripts []. This means that you
do not specify the array size at compile time; instead, give it at run time. The array is
initially empty, so you must call the new[] constructor to allocate space, passing in
the number of entries in the square brackets. If you pass an array name to the new[]
constructor, the values are copied into the new elements, as shown in Sample 2.19.
Sample 2.19 Using dynamic arrays

In Sample 2.19, Line A calls new[5] to allocate 5 array elements. The dynamic
array dyn now holds 5 int’s. Line B sets the value of each element of the array to
its index value. Line C allocates another array and copies the contents of dyn into
it. Lines D and E show that the arrays dyn and d2 are separate. Line F allocates 20
new elements, and copies the existing 5 elements of dyn to the beginning of the
array. Then the old 5-element dyn array is deallocated. The result is that dyn points
to a 20-element array. The last call to new[] allocates 100 elements, but the existing
values are not copied. The old 20-element array is deallocated. Finally, line H
deletes the dyn array.
The $size function returns the size of a fixed or dynamic array. Dynamic arrays
have several built-in routines, such as delete and size.
If you want to declare a constant array of values but do not want to bother counting
the number of elements, use a dynamic array with an array literal. In Sample 2.20
there are 9 mask elements of 8-bits each. You should let SystemVerilog count them,
rather than making a fixed-size array and accidently choosing the wrong array size.

2

36

Data Types

Sample 2.20 Using a dynamic array for an uncounted list

You can make assignments between fixed-size and dynamic arrays as long as
they have the same base type such as int. You can assign a dynamic array to a fixed
array as long as they have the same number of elements.
When you copy a fixed-size array to a dynamic array, SystemVerilog calls the
new[] constructor to allocate space, and then copies the values.
You can have multi-dimensional dynamic arrays, so long as you are careful when
constructing the sub-arrays. Remember, a multi-dimensional array in SystemVerilog
can be thought of as an array of other arrays. First you need to construct the leftmost dimension. Then construct the sub-arrays. In Sample 2.21, each sub-array has
a different size.
Sample 2.21 Multi-dimensional dynamic array

2.4

Queues

SystemVerilog introduces a new data type, the queue, which combines the best of a
linked list and array. Like a linked list, you can add or remove elements anywhere
in a queue, without the performance hit of a dynamic array that has to allocate a new

2.4

Queues

37

array and copy the entire contents. Like an array, you can directly access any element with an index, without linked list’s overhead of stepping through the preceding elements.
A queue is declared with word subscripts containing a dollar sign: [$]. The elements of a queue are numbered from 0 to $. Sample 2.22 shows how you can add
and remove values from a queue using methods. Note that queue literals only have
curly braces, and are missing the initial apostrophe of array literals.
The SystemVerilog queue is similar to the Standard Template Library’s deque
data type. You create a queue by adding elements. SystemVerilog typically allocates
extra space so you can quickly insert additional elements. If you add enough elements that the queue runs out of that extra space, SystemVerilog automatically allocates more. As a result, you can grow and shrink a queue without the performance
penalty of a dynamic array, and SystemVerilog keeps track of the free space for you.
Note that you never call the new[] constructor for a queue.
Sample 2.22 Queue methods

The LRM does not allow inserting a queue in another queue using the above
methods, though some simulators permit this.
You can use word subscripts and concatenation instead of methods. As a shortcut, if you put a $ on the left side of a range, such as [$:2], the $ stands for the
minimum value, [0:2]. A $ on the right side, as in [1:$], stands for the maximum
value, [1:2], in first line of the initial block of Sample 2.23.

2

38

Data Types

Sample 2.23 Queue operations

The queue elements are stored in contiguous locations, so it is efficient to push
and pop elements from the front and back. This takes a fixed amount of time no
matter how large the queue. Adding and deleting elements in the middle of a queue
requires shifting the existing data to make room. The time to do this grows linearly
with the size of the queue.
You can copy the contents of a fixed or dynamic array into a queue.

2.5

Associative Arrays

Dynamic arrays are good if you want to occasionally create a big array, but what if
you want something really large? Perhaps you are modeling a processor that has a
multi-gigabyte address range. During a typical test, the processor may only touch a
few hundred or thousand memory locations containing executable code and data, so
allocating and initializing gigabytes of storage is wasteful.
SystemVerilog offers associative arrays that store entries in a sparse matrix. This
means that while you can address a very large address space, SystemVerilog only
allocates memory for an element when you write to it. In the following picture, the
associative array holds the values 0:3, 42, 1000, 4521, and 200,000. The memory
used to store these is far less than would be needed to store a fixed or dynamic array
with 200,000 entries, as shown in Figure 2.4.

2.5

Associative Arrays

39

data
index

0…..3

42

1000

4521

200,000

Fig. 2.4 Associative array

An associative array can be stored by the simulator as a tree or hash table. This
additional overhead is acceptable when you need to store arrays with widely separated index values, such as packets indexed with 32-bit addresses or 64-bit data
values. An associative array is declared with a data type in square brackets, such as
[int]. or [Packet]. Sample 2.24 shows declaring, initializing, printing, and stepping through an associative array.
Sample 2.24 Declaring, initializing, and using associative arrays

Sample 2.24 has the associative array, assoc, with very scattered elements: 1, 2,
4, 8, 16, etc. A simple for loop cannot step through them; you need to use a foreach
loop. If you want finer control, you can use the first and next functions in a
do…while loop. These functions modify the index argument, and return 0 or 1
depending on whether any elements are left in the array. You can find the number of
elements in an associative array with the num or size functions.
Associative arrays can also be addressed with a string index, similar to Perl’s
hash arrays. Sample 2.25 reads a file with strings and builds the associative array

40

2

Data Types

switch so you can quickly map from a string value to a number. Strings are
explained in more detail in Section 2.15.
If you try to read an element of an associative array that has not been written,
SystemVerilog returns the default value for the array base type, such as 0 for 2-state
types such as bit or int, or X for 4-state types such as logic. The simulator may
also give a warning message. You can use the function exists() to check if an
element has been allocated, as shown in Sample 2.25.
Sample 2.25 Using an associative array with a string index

You can initialize an associative array with the array literal with index:element
pairs as shown in Sample 2.26. When you print the array with %p, the elements are
displayed in the same format.

2.6

Array Methods

41

Sample 2.26 Initializing and printing associative arrays

You can also declare an associative array with wildcard subscripts, as in wild[*].
However, this style is not recommended as you are allowing an index of almost any
data type. One of the many resulting problems is with foreach–loops: what type is
the variable j in foreach(wild[j])? Integer, string, bit, or logic?

2.6

Array Methods

There are many array methods that you can use on any unpacked array types: fixed,
dynamic, queue, and associative. These routines can be as simple as giving the current array size or as complex as sorting the elements. The parentheses are optional
if there are no arguments.

2.6.1

Array Reduction Methods

A basic array reduction method takes an array and reduces it to a single value, as
shown in Sample 2.27. You can calculate the sum, product, or perform a logical
operation on all the elements.
Sample 2.27 Array reduction operations

Other array reduction methods are or, and xor.
SystemVerilog does not have a method specifically for choosing a random element from an array, so use the index $urandom_range(array.size()−1) for
queues and dynamic arrays, and $urandom_range($size(array)−1) for fixed
arrays, queues, dynamic and associative arrays. See Section 6.10 for more information on $urandom_range.
If you need to choose a random element from an associative array, you need to step
through the elements one by one as there is no one-line way to access the Nth element.
Sample 2.28 shows how to choose a random element from an associative array

2

42

Data Types

indexed by integers by first picking a random number, then stepping through the array.
If the array was indexed by a string, just change the type of idx to string.
Sample 2.28 Picking a random element from an associative array

2.6.2

Array Locator Methods

What is the largest value in an array? Does an array contain a certain value? The
array locator methods find data in an unpacked array. At first you may wonder why
these return a queue of values. After all, there is only one maximum value in an
array. However, SystemVerilog needs a queue for the case when you ask for a value
from an empty queue or dynamic array.
Sample 2.29 shows the array locator methods: min and max functions find the
smallest and largest elements in an array. These methods also work for associative
arrays. The unique method returns a queue of the unique values from the array —
duplicate values are not included.
Sample 2.29 Array locator methods: min, max, unique

You could search through an array using a foreach loop, but SystemVerilog can
do this in one operation with a locator method. The with expression tells
SystemVerilog how to perform the search, as shown in Sample 2.30. These methods
return an empty queue if the value you are searching for does not exist in the array.

2.6

Array Methods

43

Sample 2.30 Array locator methods: find

In a with clause, the name item is called the iterator argument and represents a
single element of the array. You can specify your own name by putting it in the argument list of the array method as shown in Sample 2.31.
Sample 2.31 Declaring the iterator argument

Sample 2.32 shows various ways to total up a subset of the values in the array.
The first line compares the item with 7. This relational returns a 1 (true) or 0 (false)
so the calculation is a sum of the array {1,0,1,0,0,0}. The second multiplies the boolean result with the array element being tested. So the total is the sum of {9,0,8,0,0,0},
which is 17. The third calculates the total of elements less than 8. The fourth total is
computed using the ? : conditional operator. The last counts the number of 4’s.
Sample 2.32 Array locator methods

When you combine an array reduction such as sum using the with clause, the
results may surprise you. In Sample 2.32, the sum operator totals the number of

2

44

Data Types

times that the expression is true. For the first statement in Sample 2.32, there are two
array elements that are greater than 7 (9 and 8) so count is set to 2.
The array locator methods that return an index, such as find_
index, return a queue of type int, not integer. Your code
may not compile if you use the wrong queue type with these
statements.
Be careful of SystemVerilog’s rules for the width of operations.
Normally, if you were to add a set of single bit values, SystemVerilog would make the calculations with enough precision not to
lose any bits. But the sum method uses the width of the array. So,
if you add the values of a single-bit array, the result is a single bit, which is probably
not what you expected. The solution is to use a with expression as shown in
Sample 2.33.
Sample 2.33 Creating the sum of an array of single bits

2.6.3

Array Sorting and Ordering

SystemVerilog has several methods for changing the order of elements in an array.
You can sort the elements, reverse their order, or shuffle the order as shown in
Sample 2.34. Notice that these change the original array, unlike the array locator
methods in Section 2.6.2, which create a queue to hold the results.
Sample 2.34 Sorting an array

2.6

Array Methods

45

The reverse and shuffle methods have no with-clause, so they work on the
entire array. Sample 2.35 shows how to sort a structure by sub-fields. Structures and
packed structures are explained in Section 2.9.
Sample 2.35 Sorting an array of structures

Only fixed and dynamic arrays, plus queues can be sorted, reversed, or shuffled.
Associative arrays can not be reordered.

2.6.4

Building a Scoreboard with Array Locator Methods

The array locator methods can be used to build a scoreboard. Sample 2.36 defines
the Packet structure, then creates a scoreboard made from a queue of these structures. Section 2.8 describes how to create structures with typedef.
Sample 2.36 A scoreboard with array methods

2

46

Data Types

The check_addr() function in Sample 2.36 looks up an address in the scoreboard. The find_index() method returns an int queue. If the queue is empty
(size==0), no match was found. If the queue has one member (size==1), a single
match was found, which the check_addr() function deletes. If the queue has multiple members (size > 1), there are multiple packets in the scoreboard whose address
matching the requested one.
A better choice for storing packet information is a class, which is described in
Chapter 5. You can read more about structures in Section 2.9.

2.7

Choosing a Storage Type

Here are some guidelines for choosing the right storage type based on flexibility,
memory usage, speed, and sorting. These are just rules of thumb, and results may
vary between simulators.

2.7.1

Flexibility

Use a fixed-size or dynamic array if it is accessed with consecutive positive integer
indices: 0, 1, 2, 3… Choose a fixed-size array if the array size is known at compile
time, or choose a dynamic array if the size is not known until run time. For example,
variable-size packets can easily be stored in a dynamic array. If you are writing
routines to manipulate arrays, consider using just dynamic arrays, as one routine
can work with any size dynamic array as long as the element types match: int,
string, etc. Likewise, you can pass a queue of any size into a routine as long as the
element type matches the queue argument. Associative arrays can also be passed
regardless of size. However, a routine with a fixed-size array argument only accepts
arrays of the specified length.
Choose associative arrays for nonstandard indices such as widely separated values because of random values or addresses. Associative arrays can also be used to
model content-addressable memories.
Queues are a good way to store values when the number of elements grows and
shrinks a lot during simulation, such as a scoreboard that holds expected values.

2.7.2

Memory Usage

If you want to reduce the simulation memory usage, use 2-state elements. You
should choose data sizes that are multiples of 32 bits to avoid wasted space.
Simulators usually store anything smaller in a 32-bit word. For example, an array of
1024 bytes wastes ¾ of the memory if the simulator puts each element in a 32-bit
word. Packed arrays can also help conserve memory.

2.7 Choosing a Storage Type

47

For arrays that hold up to a thousand elements, the type of array that you choose
does not make a big difference in memory usage (unless there are many instances of
these arrays). For arrays with a thousand to a million active elements, fixed-size and
dynamic arrays are the most memory efficient. You may want to reconsider your
algorithms if you need arrays with more than a million active elements.
Queues are slightly less efficient to access than fixed-size or dynamic arrays
because of additional pointers. However, if your data set grows and shrinks often,
and you store it in a dynamic memory, you will have to manually call new[] to
allocate memory and copy. This is an expensive operation and would wipe out any
gains from using a dynamic memory.
Modeling memories larger than a few megabytes should be done with an associative array. Note that each element in an associative array can take several times more
memory than a fixed-size or dynamic memory because of pointer overhead.

2.7.3

Speed

Choose your array type based on how many times it is accessed per clock cycle. For
only a few reads and writes, you could use any type, as the overhead is minor compared with the DUT. As you use an array more often, its size and type matters.
Fixed-size and dynamic arrays are stored in contiguous memory, so any element
can be found in the same amount of time, regardless of array size.
Queues have almost the same access time as a fixed-size or dynamic array for
reads and writes. The first and last elements can be pushed and popped with almost
no overhead. Inserting or removing elements in the middle requires many elements
to be shifted up or down to make room. If you need to insert new elements into a
large queue, your testbench may slow down, so consider changing how you store
new elements.
When reading and writing associative arrays, the simulator must search for the
element in memory. The LRM does not specify how this is done, but popular ways
are hash tables and trees. These require more computation than other arrays, and
therefore associative arrays are the slowest.

2.7.4

Data Access

Since SystemVerilog can sort any single-dimension array (fixed-size, dynamic, and
associative arrays plus queues), you should pick the array type based on how often
the values are added to it. If the values are received all at once, choose a fixed-size
or dynamic array so that you only have to allocate the array once. If the data slowly
dribbles in, choose a queue, as adding new elements to the head or tail is very
efficient.
If you have unique and noncontiguous values, such as ¢{1, 10, 11, 50}, you
can store them in an associative array by using them as an index. Using the routines

2

48

Data Types

first, next, and prev, you can search an associative array for a value and find
successive values. Lists are doubly linked, so you can find values both larger and
smaller than the current value. Both of these support removing a value. However,
the associative array is much faster in accessing any given element given an index.
For example, you can use an associative array of bits to hold expected 32-bit
values. When the value is created, write to that location. When you need to see if a
given value has been written, use the exists function. When done with an element,
use delete to remove it from the associative array.

2.7.5

Choosing the Best Data Structure

Here are some suggestions on choosing a data structure.
• Network packets. Properties: fixed size, accessed sequentially. Use a fixed-size or
dynamic array for fixed- or variable-size packets.
• Scoreboard of expected values. Properties: array size not known until run time,
accessed by value, and a constantly changing size. In general, use a queue, as you
are continually adding and deleting elements during simulation. If you can give
every transaction a fixed ID, such as 1, 2, 3, …, you could use this as an index
into the queue. If your transaction is filled with random values, you can just push
them into a queue and search for unique values. If the scoreboard may have hundreds of elements and you are often inserting and deleting them from the middle,
an associative array may be faster. If you model your transactions as objects, the
scoreboard can be a queue of handles. See Chapter 5 for more information of
classes.
• Sorted structures. Use a queue if the data comes out in a predictable order or an
associative array if the order is unspecified. If the scoreboard never needs to be
searched, just store the expected values in a mailbox as shown in Section 7.6.
• Modeling very large memories, greater than a million entries. If you do not need
every location, use an associative array as a sparse memory. If you do plan on
accessing every location, try a different approach where you do not need so much
live data. Be sure to use 2-state values packed into 32-bits to conserve simulation
memory.
• Command names or opcodes from a file. Property: translate a string to a fixed
value. Read string from a file, and then look up the commands or opcodes in an
associative array using the command as a string index.

2.8

Creating New Types with typedef

You can create new types using the typedef statement. For example, you may have
an ALU that can be configured at compile time to use 8, 16, 24, or 32-bit operands.
In Verilog you would define a macro for the operand width and another for the type
as shown in Sample 2.37.

2.8 Creating New Types with typedef

49

Sample 2.37 User-defined type-macro in Verilog

You are not really creating a new type; you are just performing text substitution.
In SystemVerilog you create a new type as shown in Sample 2.38. This book uses
the convention that user-defined types use the suffix “_t” except for the basic uint.
Sample 2.38 User-defined type in SystemVerilog

In general, SystemVerilog lets you copy between these basic types with no warning,
either extending or truncating values if there is a width mismatch.
Note that parameter and typedef statements can be put in a package so they
can be shared across the design and testbench, as shown in Section 2.10.
One of the most useful types you can create is an unsigned, 2-state,
32-bit integer as shown in Sample 2.39. Most values in a testbench are
positive integers such as field length or number of transactions
received, and so having a signed integer can cause problems. Put the
definition of uint in a package of common definitions so it can be
used anywhere.
Sample 2.39 Definition of uint

The syntax for defining a new array type is not obvious. You need to put the array
subscripts on the new name. Sample 2.40 creates a new type, fixed_array5_t,
a fixed array with 5 elements. It then declares an array of this type and initializes it.

2

50

Data Types

Sample 2.40 User-defined array type

A good use for a user defined type is an associative array, which must be declared
with an index that is a simple type. You could change Sample 2.24 to use 64 bit
values by changing the first line as shown in Sample 2.41.
Sample 2.41 User-defined associative array index

2.9

Creating User-Defined Structures

One of the biggest limitations of Verilog is the lack of data structures. In SystemVerilog you can create a structure using the struct statement, similar to what is available in C. However, a struct has just a subset of the functionality of a class, so use
a class instead for your testbenches, as shown in Chapter 5. Just as a Verilog module
combines both data (signals) and code (always/initial blocks plus routines), a class
combines data and routines to make an entity that can be easily debugged and
reused. A struct just groups data fields together. Without the code that manipulates the data, you are only creating half of the solution.
Since a struct is just a collection of data, it can be synthesized. If you want to
model a complex data type, such as a pixel, in your design code, put it in a struct.
This can also be passed through module ports. Eventually, when you want to generate constrained random data, look to classes.

2.9.1

Creating a Struct and a New Type

You can combine several variables into a structure. Sample 2.42 creates a structure
called pixel that has three unsigned bytes for red, green, and blue.
Sample 2.42 Creating a single pixel type

2.9

Creating User-Defined Structures

51

The problem with the preceding declaration is that it creates a single pixel of this
type. To be able to share pixels using ports and routines, you should create a new
type instead, as shown in Sample 2.43.
Sample 2.43 The pixel struct

Use the suffix “_s” when declaring a struct. This makes it easier to spot userdefined types, simplifying the process of sharing and reusing code.

2.9.2

Initializing a Structure

You can assign multiple values to a struct just like an array, either in the declaration
or in a procedural assignment. Just surround the values with an apostrophe and
braces, as shown in Sample 2.44.
Sample 2.44 Initializing a struct

2.9.3

Making a Union of Several Types

In hardware, the interpretation of a set of bits in a register may depend on the value
of other bits. For example, a processor instruction may have many layouts based on
the opcode. Immediate-mode operands might store a literal value in the operand
field. This value may be decoded differently for integer instructions than for floating
point instructions. Sample 2.45 stores both the unsigned bit vector b and the integer i
in the same location.

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52

Data Types

Sample 2.45 Using typedef to create a union

Use the suffix “_u” when declaring a union.
Unions are useful when you frequently need to read and write a
register in several different formats. However, don’t go overboard,
especially just to save memory. Unions may help squeeze a few
bytes out of a structure, but at the expense of having to create and
maintain a more complicated data structure. Instead, make a class
with a discriminant variable, as shown in Section 8.4.4. This “kind”
variable indicates which type of transaction you have, and thus which fields to read,
write, and randomize. If you just need an array of values, plus all the bits, use a
packed array as described Section in 2.2.6

2.9.4

Packed Structures

SystemVerilog allows you more control in how bits are laid out in memory by using
packed structures. A packed structure is stored as a contiguous set of bits with no
unused space. The struct for a pixel in Sample 2.43 has three values, so it is stored
in three longwords, even though it only needs three bytes. You can specify that it
should be packed into the smallest possible space with the packed keyword, as
shown in Sample 2.46.
Sample 2.46 Packed structure

Packed structures are used when the underlying bits represent a numerical value or
when you are trying to reduce memory usage. For example, you could pack together
several bit-fields to make a single register. Or you might pack together the opcode and
operand fields to make a value that contains an entire processor instruction.

2.9.5

Choosing Between Packed and Unpacked Structures

When you are trying to choose between packed and unpacked structures, consider
how the structure is most commonly used and the alignment of the elements. If you
plan on making aggregate operations on the structure, such as copying the entire
structure, a packed structure is more efficient. However, if your code accesses
the individual members more than the entire structure, use an unpacked structure.
The difference in performance is greater if the elements are not aligned on byte

2.10

Packages

53

boundaries, have sizes that don’t match the typical byte, or have word instructions
used by processors. Reading and writing elements with odd sizes in a packed structure requires expensive shift and mask operations.

2.10

Packages

At the start of a project, you need to create new types and parameters. For example,
if your processor communicates with your company’s ABC bus, your testbench
needs to define ABC data types, and parameters to specify the bus width and timing.
Another project may want to use these types, plus those for the XYZ bus.
You could create separate files for each bus and use the 'include statement to
bring in the files during compilation. But then every name associated with each bus
must be unique, even those that are internal variables, never intended to be visible.
How can you organize these types to avoid name conflicts?
The SystemVerilog package allows you to share declarations among modules,
packages, plus programs and interface, which are described in Chapter 4. Sample
2.47 shows the package for the ABC bus.
Sample 2.47 Package for ABC bus

You import symbols from a package with the import statement. The compiler only
looks in imported packages when a symbol is not defined in the usual search path. In
Sample 2.48, the first import statement makes the symbols abc_data_width,
abc_data_t, and timeout visible if there is no local variable with the same name.
The variable message in ABC is hidden by the one in the module.
Sample 2.48 Importing packages

2

54

Data Types

If you really want to see the message variable in ABC, use ABC::message.
You can import specific symbols from a package with the scope operator, ::.
Sample 2.49 imports all the symbols from ABC, plus just the timeout variable
from XYZ.
Sample 2.49 Importing selected symbols from a package

Packages can only see symbols defined inside themselves, or packages that they
import. You can not have hierarchical references to symbols such as signals, routines, or modules from outside the package. Think of a package as being completely
standalone, able to plug in where needed, with no outside dependencies.
A package can contain routines, plus classes, as shown in Section 5.4.

2.11

Type Conversion

SystemVerilog has several rules to ensure that expressions are evaluated with little
or no loss of accuracy. For example, if you add two 8-bit values, the addition is done
with 9-bit precision to avoid overflow. Multiply two 8-bit values, and SystemVerilog
calculates a 16-bit result.
The proliferation of data types in SystemVerilog means that you may need to
convert between them. If the layout of the bits between the source and destination
variables are the same, such as an integer and enumerated type, cast between the
two values. If the bit layouts differ, such as an array of bytes and words, use the
streaming operators to rearrange the bits as described in Section 2.12.

2.11.1

The Static Cast

The static cast operation converts between two types with no checking of values.
You specify the destination type, an apostrophe, and the expression to be converted
as shown in Sample 2.50. Note that Verilog has always implicitly converted between
types such as integer and real, and also between different width vectors.

2.12 Streaming Operators

55

Sample 2.50 Converting between int and real with static cast

2.11.2

The Dynamic Cast

The dynamic cast, $cast, allows you to check for out-of-bounds values. See Section
2.13.3 for an explanation and example with enumerated types.
Use a static cast when you want SystemVerilog to use a type with
more precision, like when using the sum method for a single bit
array. Use the dynamic cast when converting from a type with a
larger number of values than the destination, such as int to an enumerated variable.

2.12

Streaming Operators

When used on the right side of an assignment, the streaming operators << and >>
take an expression, structure, or array, and packs it into a stream of bits. The >>
operator streams data from left to right while << streams from right to left, as shown
in Sample 2.51. You can also give a slice size, used to break up the source before
being streamed. You can not assign the bit stream result directly to an unpacked
array. Instead, use the streaming operators on the left side of an assignment to
unpack the bit stream into an unpacked array.
Sample 2.51 Basic streaming operator

56

2

Data Types

You could do the same operations with many concatenation operators, {}, but
the streaming operators are more compact and easier to read.
If you need to pack or unpack arrays, use the streaming operator to convert
between arrays of different element sizes. For instance, you can convert an array of
bytes to an array of words. You can use fixed size arrays, dynamic arrays, and
queues. Sample 2.52 converts between queues, but would also work with dynamic
arrays. Array elements are automatically allocated as needed.
Sample 2.52 Converting between queues with streaming operator

A common mistake when streaming between arrays is mismatched array subscripts. The word subscript [256] in an array
declaration is equivalent to [0:255], not [255:0]. Since many
arrays are declared with the word subscripts [high:low],
streaming them to an array with the subscript [size] would result in the elements
ending up in reverse order. Likewise, streaming an unpacked array declared as bit
[7:0] src[255:0] to the packed array declared as bit [7:0] [255:0] dst will
scramble the order of values. The correct declaration for a packed array of bytes is
bit [255:0] [7:0] dst.
You can also use the streaming operator to pack and unpack structures, such as
an ATM cell, into an array of bytes. In Sample 2.53 a structure is streamed into a
dynamic array of bytes, then the byte array is streamed back into the structure.

2.13

Enumerated Types

57

Sample 2.53 Converting between a structure and an array with streaming operators

2.13

Enumerated Types

An enumerated type allows you to create a set of related but unique constants such
as states in a state machine or opcodes. In classic Verilog, you had to use text macros. Their global scope is too broad, and their value might not be visible in the
debugger. An enumeration creates a strongly typed variable that is limited to a set of
specified names. For example, the names ADD, MOVE, or ROTW make your code
easier to write and maintain than if you had used literals such as 8'h01 or macros.
A weaker alternative for defining constants is a parameter. These are fine for individual values, but an enumerated type automatically gives a unique value to every
name in the list.
The simplest enumerated type declaration contains a list of constant names and
one or more variables as shown in Sample 2.54. This creates an anonymous enumerated type, but it cannot be used for any other variables than the ones in this
declaration.
Sample 2.54 A simple enumerated type, not recommended

2

58

Data Types

It is recommended to create a named enumerated type so you can declare
multiple variables of the same type, especially if these are used as routine arguments or module ports. You first create the enumerated type, and then the variables of this type, as shown in Sample 2.55. You can get the string representation
of an enumerated variable with the built-in function name().

Sample 2.55 Enumerated types, recommended style

Use the suffix “_e” when declaring an enumerated type name.

2.13.1

Defining Enumerated Values

The actual values default to int starting at 0 and then increase. You can choose your
own enumerated values. The code in Sample 2.56 uses the default value of 0 for
INIT, then 2 for DECODE, and 3 for IDLE.
Sample 2.56 Specifying enumerated values

Enumerated constants, such as INIT in Sample 2.56, follow the same scoping
rules as variables. Consequently, if you use the same name in several enumerated
types (such as INIT in different state machines), they have to be declared in different scopes such as modules, program blocks, packages, routines, or classes.

2.13

Enumerated Types

59

An enumerated type is stored as int unless you specify otherwise. Be careful when assigning values to enumerated constants,
as the default value of an int is 0. In Sample 2.57, position is
initialized to 0, which is not a legal ordinal_e variable. This
behavior is not a tool bug – it is how the language is specified. So always specify an
enumerated constant with the value of 0, as shown in Sample 2.58, just to catch the
testbench error.
Sample 2.57 Incorrectly specifying enumerated values

Sample 2.58 Correctly specifying enumerated values

2.13.2

Routines for Enumerated Types

SystemVerilog provides several functions for stepping through enumerated types.
•
•
•
•
•
•

first () returns the first member of the enumeration.
last() returns the last member of the enumeration.
next() returns the next element of the enumeration.
next (N) returns the N th next element.
prev () returns the previous element of the enumeration.
prev(N) returns the N th previous element.

The functions next and prev wrap around when they reach the beginning or end
of the enumeration.
Note that there is no clean way to write a for loop that steps through all members
of an enumerated type if you use an enumerated loop variable. You get the starting
member with first function and the next member with next. A for loop ends when
the loop variable is outside the defined bounds, but the next function always returns
a value inside the enumeration. If you use the test current!= current.last(),
the loop ends before using the last value. If you use current<=current.last(), you
get an infinite loop, as next never gives you a value that is greater than the final
value. This is similar to trying to make a for loop that steps through the values 0..3
with an index declared as bit [1:0]. The loop never exits! You can get around this
limitation by either using an integer variable in the loop, or incrementing the
enumerated variable, but both of these solutions can give illegal values if your
enumerated values are not contigious, such as 1, 2, 3, 5, 8.
You can use a do…while loop to step through all the values, checking when the
value wraps around, as shown in Sample 2.59.

2

60

Data Types

Sample 2.59 Stepping through all enumerated members

2.13.3

Converting to and from Enumerated Types

The default type for an enumerated type is int (2-state). You can take the value of an
enumerated variable and assign it to a non-enumerated variable such as an int with
a simple assignment. SystemVerilog does not, however, let you store an integer value
in an enum without explicitly changing the type. Instead, it requires you to explicitly
cast the value to make you realize that you could be writing an out-of-bounds value.

Sample 2.60 Assignments between integers and enumerated types

When called as a function as shown in Sample 2.60, $cast() tried to assign
the right value to the left variable. If the assignment succeeds, $cast() returns 1.
If the assignment fails because of an out-of-bounds value, no assignment is made
and the function returns 0. If you use $cast() as a task and the operation fails,
SystemVerilog prints an error.
You can also cast the value using the type'(val) as shown in the example, but
this does not do any type checking, so the result may be out-of-bounds. For example,

2.15

Strings

61

after the static cast in Sample 2.60, c2 has an out-of-bounds value. You should avoid
this style of casting with enumerated types.

2.14

Constants

There are several types of constants in SystemVerilog. The classic Verilog way to
create a constant is with a text macro. On the plus side, macros have global scope
and can be used for bit field definitions and type definitions. On the negative side,
macros are global, so that they can cause conflicts if you just need a local constant.
Lastly, a macro requires the ` character so that it is recognized and expanded by the
compiler.
A Verilog parameter was loosely typed and was limited in scope to a single
module. Verilog-2001 added typed parameters, but their limited scope kept parameters from being widely used. In SystemVerilog, parameters can be declared in a
package so they can be used across multiple modules. This approach can replace
most Verilog macros that were just being used as constants.
SystemVerilog also supports the const modifier that allows you to make a variable that can be initialized in the declaration but not written by procedural code.
Sample 2.61 Declaring a const variable

In Sample 2.61, the value of colon is initialized at run time, when the initial
block is entered. In the next chapter, Sample 3.11 shows a const routine argument.

2.15

Strings

If you have ever tried to use a Verilog reg variable to hold a string of characters,
your suffering is over. The SystemVerilog string type holds variable-length
strings. An individual character is of type byte. The elements of a string of length
N are numbered 0 to N-1. Note that, unlike C, there is no null character at the end of
a string, and any attempt to use the character “\0” is ignored. Memory for strings is
dynamically allocated, so you do not have to worry about running out of space to
store the string.
Sample 2.62 shows various string operations. The function getc(N) returns the
byte at location N, while toupper returns an upper-case copy of the string and
tolower returns a lowercase copy. The curly braces {} are used for concatenation.
The task putc(M, C) writes a byte C into a string at location M, that must be between 0

2

62

Data Types

and the length as given by len. The substr(start,end) function extracts
characters from location start to end.
Sample 2.62 String methods

Note how useful dynamic strings can be. In other languages such as C, you have
to keep making temporary strings to hold the result from a function. In Sample 2.62,
the $sformatf function is used instead of $sformat, from Verilog-2001. This new
function returns a formatted temporary string that, as shown above, can be passed
directly to another routine. This saves you from having to declare a temporary string
and passing it between the formatting statement and the routine call. The undocumented function $psprintf has the same functionality as $sformatf, but is not in
the LRM, even though most vendors support this non-standard system function.
There are two ways to compare strings, but they behave differently. The equality operator, s1==s2, returns 1 if the strings are
identical, and 0 if they are not. The string comparison function,
s1.compare(s2), returns 1 if s1 is greater than s2, 0 if they are
equal, and −1 if s1 is less than s2. While this matches the ANSI C strcmp() behavior, it may not be what you expect.

2.16

Expression Width

A prime source for unexpected behavior in Verilog has been the width of expressions. Sample 2.63 adds 1+1 using four different styles. Addition A uses two 1-bit
variables, so with this precision 1+1=0. Addition B uses 8-bit precision because

2.17

Conclusion

63

there is an 8-bit variable on the left side of the assignment. In this case, 1+1=2.
Addition C uses a dummy constant to force SystemVerilog to use 2-bit precision.
Lastly, in addition D, the first value is cast to be a 2-bit value with the cast operator,
so 1+1=2.
Sample 2.63 Expression width depends on context

There are several tricks you can use to avoid this problem. First, avoid situations
where the overflow is lost, as in addition A. Use a temporary, such as b8, with the
desired width. Or, you can add another value to force the minimum precision, such
as 2'b0. Lastly, in SystemVerilog, you can cast one of the variables to the desired
precision.

2.17

Conclusion

SystemVerilog provides many new data types and structures so that you can create
high-level testbenches without having to worry about the bit-level representation.
Queues work well for creating scoreboards for which you constantly need to add
and remove data. Dynamic arrays allow you to choose the array size at run time for
maximum testbench flexibility. Associative arrays are used for sparse memories and
some scoreboards with a single index. Enumerated types make your code easier to
read and write by creating groups of named constants.
Don’t go off and create a procedural testbench with just these constructs. Explore
the OOP capabilities of SystemVerilog in Chapter 5 to learn how to design code at
an even higher level of abstraction, thus creating robust and reusable code.

2

64

2.18

Data Types

Exercises

1. Given the following code sample:

a.
b.
c.
d.
e.

What is the range of values my_byte can take?
What is the value of my_int in hex?
What is the value of my_bit in decimal?
What is the value of my_short_int1 in decimal?
What is the value of my_short_int2 in decimal?

2. Given the following code sample:

Evaluate the following statements in the given order and give the result for
each assignment
a.
b.
c.
d.
e.
f.
g.

my_mem[2] = my_logicmem[4];
my_logic = my_logicmem[4];
my_logicmem[3] = my_mem[3];
my_mem[3] = my_logic;
my_logic = my_logicmem[1];
my_logic = my_mem[1];
my_logic = my_logicmem[my_logicmem[41];

2.18

Exercises

65

3. Write the SystemVerilog code to:
a. Declare a 2-state array, my_array, that holds four 12-bit values
b. Initialize my_array so that:
*
*
*
*

my_array[0]
my_array[1]
my_array[2]
my_array[3]

=
=
=
=

12'h012
12'h345
12'h678
12'h9AB

c. Traverse my_array and print out bits [5:4] of each 12-bit element
* With a for loop
* With a foreach loop
4. Declare a 5 by 31 multi-dimensional unpacked array, my_array1. Each element
of the unpacked array holds a 4-state value.
a. Which of the following assignment statements are legal and not out of bounds?
* my_array1[4][30] = 1'b1;
* my_array1[29][4] = 1'b1;
* my_array1[4] = 32'b1;
b. Draw my_array1 after the legal assignments complete.
5. Declare a 5 by 31 multi-dimensional packed array, my_array2. Each element of
the packed array holds a 2-state value.
a. Which of the following assignment statements are legal and not out of bounds?
* my_array2[4][30] = 1'b1;
* my_array2[29][4] = 1'b1;
* my_array2[3] = 32'b1;
b. Draw my_array2 after the assignment statements complete.
6. Given the following code, determine what will be displayed.

2

66

Data Types

7. Write code for the following problems.
a. Create memory using an associative array for a processor with a word width of
24 bits and an address space of 220 words. Assume the PC starts at address 0 at
reset. Program space starts at 0´400. The ISR is at the maximum address.
b. Fill the memory with the following instructions:
*
*
*
*

24'hA50400; // Jump to location 0´400 for the main code
24'h123456; // Instruction 1 located at location 0´400
24'h789ABC; // Instruction 2 located at location 0´401
24'h0F1E2D; // ISR = Return from interrupt

c. Print out the elements and the number of elements in the array.
8. Create the SystemVerilog code for the following requirements
a.
b.
c.
d.
e.
f.
g.

Create a 3-byte queue and initialize it with 2, −1, and 127
Print out the sum of the queue in the decimal radix
Print out the min and max values in the queue
Sort all values in the queue and print out the resulting queue
Print out the index of any negative values in the queue
Print out the positive values in the queue
Reverse sort all values in the queue and print out the resulting queue

9. Define a user defined 7-bit type and encapsulate the fields of the following
packet in a structure using your new type. Lastly, assign the header to 7'h5A.
21 20

27

header

14 13

cmd

7

data

6

0

crc

10. Create the SystemVerilog code for the following requirements
a.
b.
c.
d.
e.
f.
g.
h.

Create a user-defined type, nibble, of 4 bits
Create a real variable, r, and initialize it to 4.33
Create a short int variable, i_pack
Create an unpacked array, k, containing 4 elements of your user defined type
nibble and initialize it to 4'h0, 4'hF, 4'hE, and 4'hD
Print out k
Stream k into i_pack right to left on a bit basis and print it out
Stream k into i_pack right to left on a nibble basis and print it out
Type convert real r into a nibble, assign it to k[0], and print out k

2.18

Exercises

67

11. An ALU has the opcodes shown in Table 2.1.
Table 2.1 ALU Opcodes
Opcode
Add: A + B
Sub: A − B
Bit-wise invert: A
Reduction Or: B

Encoding
2'b00
2'b01
2'b10
2'b11

Write a testbench that performs the following tasks.
a.
b.
c.
d.

Create an enumerated type of the opcodes: opcode_e
Create a variable, opcode, of type opcode_e
Loop through all the values of variable opcode every 10ns
Instantiate an ALU with one 2-bit input opcode

Chapter 3

Procedural Statements and Routines

As you verify your design, you need to write a great deal of code, most of which is in
tasks and functions. SystemVerilog introduces many incremental improvements to
make this easier by making the language look more like C, especially around argument passing. If you have a background in software engineering, these additions
should be very familiar.

3.1 Procedural Statements
SystemVerilog adopts many operators and statements from C and C++. You can
declare a loop variable inside a for loop that then restricts the scope of the loop
variable and can prevent some coding bugs. The new auto-increment ++ and autodecrement −− operators are available in both pre- and post-forms. The compound
assignments, +=, −=, ˆ=, and many more make your code tighter. If you have a label
on a begin or fork statement, you can put the same label on the matching end or
join statement. This makes it easier to match the start and finish of a block. You can
also put a label on other SystemVerilog end statements such as endmodule, endtask, endfunction, and others that you will learn in this book. Sample 3.1 demonstrates some of the new constructs.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_3,
© Springer Science+Business Media, LLC 2012

69

70

3 Procedural Statements and Routines

Sample 3.1 New procedural statements and operators

Two new statements help with loops. First, if you are in a loop, but want to skip
over the rest of the statements and do the next iteration, use continue. If you want
to leave the loop immediately, use break.
The compound assignment in Sample 3.1 is equivalent to sum = sum + array[j];
The loop in Sample 3.2 reads commands from a file using the file I/O system tasks
that are part of Verilog-2001. If the command is just a blank line, the code does a
continue, skipping any further processing of the command. If the command is
“done,” the code does a break to terminate the loop.
Sample 3.2 Using break and continue while reading a file

3.2

Tasks, Functions, and Void Functions

71

SystemVerilog expands the case statement so that you no longer have to give
every possible value, but can instead give a range values as shown in Sample 3.3. This
is a version of the inside operator shown more in more detail in Section 6.4.5.
Sample 3.3 Case-inside statement with ranges

3.2 Tasks, Functions, and Void Functions
Verilog makes a very clear differentiation between tasks and functions. The most
important difference is that a task can consume time whereas a function cannot.
A function cannot have a delay, #100, a blocking statement such as @(posedge
clock) or wait (ready), or call a task. Additionally, a Verilog function must
return a value and the value must be used, as in an assignment statement.
SystemVerilog relaxes this rule a little in that a function can call a task, but only in
a thread spawned with the fork… join_none statement, which is described in
Section 7.1.
If you have a SystemVerilog task that does not consume time, you
should make it a void function, which is a function that does not
return a value. Now it can be called from any task or function. For
maximum flexibility, any debug routine should be a void function
rather than a task so that it can be called from any task or function.
Sample 3.4 prints values from a state machine.
Sample 3.4 Void function for debug

In SystemVerilog, if you want to call a function and ignore its return value, cast the
result to void, as shown in Sample 3.5. Some simulators, such as VCS, allow you to
ignore the return value without using the void syntax. The LRM says this should be
a warning.
Sample 3.5 Ignoring a function’s return value

72

3 Procedural Statements and Routines

3.3 Task and Function Overview
SystemVerilog makes several small improvements to tasks and functions to make
them look more like C or C++ routines. In general, a routine definition or call with no
arguments does not need the empty parentheses (). This book includes them for added
clarity.

3.3.1 Routine Begin…End Removed
The first improvement you may notice in SystemVerilog routines is that begin…end
blocks are optional, while Verilog-1995 required them on all but single-line routines.
The task / endtask and function / endfunction keywords are enough to
define the routine boundaries, as shown in Sample 3.6.
Sample 3.6 Simple task without begin…end

3.4 Routine Arguments
Many of the SystemVerilog improvements for routines make it easier to declare arguments and expand the ways you can pass values to and from a routine.

3.4.1 C-style Routine Arguments
SystemVerilog and Verilog-2001 allow you to declare task and function arguments
more cleanly and with less repetition. The following Verilog task requires you to
declare some arguments twice: once for the direction, and once for the type, as shown
in Sample 3.7.
Sample 3.7 Verilog-1995 routine arguments

3.4 Routine Arguments

73

With SystemVerilog, you can use the less verbose C-style, shown in Sample 3.8.
Note that you should use the universal input type of logic.
Sample 3.8 C-style routine arguments

3.4.2 Argument Direction
You can take even more shortcuts with declaring routine arguments. The direction
and type default to “input logic” and are sticky, so you don’t have to repeat these for
similar arguments. Sample 3.9 shows a routine header written using the Verilog-1995
style and SystemVerilog data types.
Sample 3.9 Verbose Verilog-style routine arguments

You could rewrite this as shown in Sample 3.10.
Sample 3.10 Routine arguments with sticky types

The arguments a and b are input logic, 1-bit wide. The arguments u and v are
16-bit output bit types. Now that you know this, don’t depend on the defaults, as your
code will be infested with subtle and hard to find bugs, as explained in Section 3.4.6.
Always declare the type and direction for every routine argument.

3.4.3 Advanced Argument Types
Verilog had a simple way to handle arguments: an input or inout was copied to a
local variable at the start of the routine, whereas an output or inout was copied
when the routine exited. No memories could be passed into a Verilog routine, only
scalars.

3 Procedural Statements and Routines

74

In SystemVerilog, you can specify that an argument is passed by reference, rather
than copying its value. This argument type, ref, has several benefits over input,
output, and inout. First, you can now pass an array into a routine, here one that
prints the checksum.
Sample 3.11 Passing arrays using ref and const

The = compound assignment in Sample 3.11 is a shorthand way of writing the
statement: checksum = checksum ^ a[i];
SystemVerilog allows you to pass array arguments without the ref direction, but
the array is copied onto the stack, an expensive operation for all but the smallest
arrays.
The SystemVerilog LRM states that ref arguments can only be used in routines
with automatic storage. If you specify the automatic attribute for programs and
module, all the routines inside are automatic. See Section 3.6 for more details on
storage.
Sample 3.11 also shows the const modifier. As a result, the array a points to the
array in the routine call, but the contents of the array cannot be modified. If you try to
change the contents, the compiler prints an error.
Always use ref when passing arrays to a routine for best performance. If you don’t want the routine to change the array values, use
the const ref type, which causes the compiler to check that your
routine does not modify the array.
The second benefit of ref arguments is that a task can modify a variable and is
instantly seen by the calling function. This is useful when you have several threads
executing concurrently and want a simple way to pass information. See Chapter 7 for
more details on using fork-join.
In Sample 3.12, the thread2 block in the initial block can access the data from
memory as soon as enable is asserted, even though the bus_read task does
not return until the bus transaction completes, which could be several cycles later.

3.4 Routine Arguments

75

Sample 3.12 Using ref across threads

The data argument is passed as ref, and as a result, the @data statement triggers as
soon as data changes in the task. If you had declared data as output, the @data
statement would not trigger until the end of the bus transaction.

3.4.4 Default Value for an Argument
As your testbench grows in sophistication, you may want to add additional controls
to your code but not break existing code. For the function in Sample 3.11, you might
want to print a checksum of just the middle values of the array. However, you don’t
want to go back and rewrite every call to add extra arguments. In SystemVerilog you
can specify a default value that is used if you leave out an argument in the call.
Sample 3.13 adds low and high arguments to the print_csm function so you can
print a checksum of a range of values.

76

3 Procedural Statements and Routines

Sample 3.13 Function with default argument values

You can call this function in the following ways, as shown in Sample 3.14. Note
that the first call is compatible with both versions of the print_csm routine.
Sample 3.14 Using default argument values

Using a default value of −1 (or any out-of-range value) is a good way to see if the
call specified a value.
A Verilog for loop always executes the initialization (int i=low), and test
(i<=high) before starting the loop. Thus, if you accidently passed a low value that
was larger than high or the array size, the for loop would never execute the body.

3.4.5 Passing Arguments by Name
You may have noticed in the SystemVerilog LRM that the arguments to a task or function are sometimes called “ports,” just like the connections for a module. If you have
a task or function with many arguments, some with default values, and you only want
to set a few of those arguments, you can specify a subset by specifying the name of
the routine argument with a port-like syntax, as shown in Sample 3.15.

3.4 Routine Arguments

77

Sample 3.15 Binding arguments by name

3.4.6 Common Coding Errors
The most common coding mistake that you are likely to make
with a routine is forgetting that the argument type is sticky with
respect to the previous argument, and that the default type for the
first argument is a single-bit input. Start with the simple task
header in Sample 3.16.
Sample 3.16 Original task header

The two arguments are input integers. As you are writing the task, you realize that
you need access to an array, so you add a new array argument, and use the ref type
so it does not have to be copied. Your routine header now looks like Sample 3.17.
Sample 3.17 Task header with additional array argument

What argument types are a and b? They take the direction of the previous argument that is a ref. Using ref for a simple variable such as an int is not usually
needed, but you would not get even a warning from the compiler, and thus would not
realize that you were using the wrong direction.
If any argument to your routine is something other than the default input type,
specify the direction for all arguments as shown in Sample 3.18.
Sample 3.18 Task header with additional array argument

3 Procedural Statements and Routines

78

3.5 Returning from a Routine
Verilog had a primitive way to end a routine; after you executed the last statement in
a routine, it returned to the calling code. In addition, a function returned a value by
assigning that value to a variable with the same name as the function.

3.5.1 The Return Statement
SystemVerilog adds the return statement to make it easier for you to control the
flow in your routines. The task in Sample 3.19 needs to return early because of error
checking. Otherwise, it would have to put the rest of the task in an else clause,
which would cause more indentation and be more difficult to read.
Sample 3.19 Return in a task

The return statement in Sample 3.20 can simplify your functions.
Sample 3.20 Return in a function

3.5.2 Returning an Array from a Function
Verilog routines could only return a simple value such as a bit, integer, or vector. If
you wanted to compute and return an array, there was no simple way. In System
Verilog, a function can return an array, using several techniques.
The first way is to define a type for the array, and then use that in the function
declaration. Sample 3.21 uses the array type from Sample 2.40, and creates an function to initialize the array.

3.6

Local Data Storage

79

Sample 3.21 Returning an array from a function with a typedef

One problem with the preceding code is that the function init creates an array,
which is copied into the array f5. If the array was large, this could be a large performance problem.
The alternative is to pass the routine by reference. The easiest way is to pass the
array into the function as a ref argument, as shown in Sample 3.22.
Sample 3.22 Passing an array to a function as a ref argument

The last way for a function to return an array is to wrap the array inside of a class,
and return a handle to an object. Chapter 5 describes classes, objects, and handles.

3.6 Local Data Storage
When Verilog was created in the 1980s, it was tightly tied to describing hardware. As
a result, all objects in the language were statically allocated. In particular, routine
arguments and local variables were stored in a fixed location, rather than pushing
them on a stack like other programming languages. Why try to model dynamic code
such as a recursive routine when there is no way to build this in silicon? However,
software engineers verifying the designs, who were used to the behavior of stackbased languages such as C, were bitten by these subtle bugs, and were thus limited in
their ability to create complex testbenches with libraries of routines.

3 Procedural Statements and Routines

80

3.6.1 Automatic Storage
In Verilog-1995, if you tried to call a task from multiple places in your testbench, the
local variables shared common, static storage, and so the different threads stepped on
each other’s values. In Verilog-2001 you can specify that tasks, functions, and modules use automatic storage, which causes the simulator to use the stack for local
variables.
In SystemVerilog, routines still use static storage by default, for both
modules and program blocks. You should always make program
blocks (and their routines) use automatic storage by putting the
automatic keyword in the program statement. In Chapter 4 you
will learn about program blocks that hold the testbench code.
Section 7.1.6 shows how automatic storage helps when you are creating multiple threads.
Sample 3.23 shows a task to monitor when data are written into memory.
Sample 3.23 Specifying automatic storage in program blocks

You can call this task multiple times concurrently, as the addr and expect_data
arguments are stored separately for each call. Without the automatic modifier, if you
called wait_f or_bus a second time while the first was still waiting, the second call
would overwrite the two arguments.

3.6.2 Variable Initialization
A similar problem occurs when you try to initialize a local variable in a declaration, as it is actually initialized before the start of
simulation. The general solution is to avoid initializing a variable
in a declaration to anything other than a constant. Use a separate
assignment statement to give you better control over when initialization is done.

3.7 Time Values

81

The task in Sample 3.24 looks at the bus after five cycles and then creates a local
variable and attempts to initialize it to the current value of the address bus.
Sample 3.24 Static initialization bug

The bug is that the variable local_addr is statically allocated, so it is actually initialized at the start of simulation, not when the begin…end block is entered. Once
again, the solution is to declare the program as automatic as shown in Sample 3.25.

Sample 3.25 Static initialization fix: use automatic

Additionally, you can avoid this by never initializing a variable in the declaration,
but this is harder to remember, especially for C programmers. Sample 3.26 show the
recommended style of separating the declaration and initialization.
Sample 3.26 Static initialization fix: break apart declaration and initialization

3.7 Time Values
SystemVerilog has several new constructs to allow you to unambiguously specify
time values in your system.

3.7.1 Time Units and Precision
When you rely on the `timescale compiler directive, you must compile the files in
the proper order to be sure all the delays use the proper scale and precision. One way

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3 Procedural Statements and Routines

avoiding this compilation ordering problem is to require that every file that starts
with a `timescale compiler directive should end with one that resets it back to a
company-specific default such as 1ns/1ns.
The timeunit and timeprecision declarations eliminate this ambiguity by
precisely specifying the values for every module. Sample 3.27 shows these declarations. Note that if you use these instead of `timescale, you must put them in every
module that has a delay. See the LRM for more on these declarations.

3.7.2 Time Literals
SystemVerilog allows you to unambiguously specify a time value plus units. Your
code can use delays such as 0.1ns or 20ps. Just remember to use timeunit and
timeprecision or `timescale. You can make your code even more time aware by
using the classic Verilog $timeformat(), $time, and $realtime system tasks.
The four arguments to $timeformat are the scaling factor (−9 for nanoseconds, −12
for picoseconds), the number of digits to the right of the decimal point, a string to
print after the time value, and the minimum field width.
Sample 3.27 shows various delays and the result from printing the time when it is
formatted by $timeformat() and the %t specifier.
Sample 3.27 Time literals and $timeformat

3.7.3 Time and Variables
You can store time values in variables and use them in calculations and delays. The
values are scaled and rounded according to the current time scale and precision.
Variables of type time cannot hold fractional delays as they are just 64-bit integers,
so delays will be rounded. You should use realtime variables if this is a problem.
Sample 3.28 shows how realtime variables are rounded when used as a delay.

3.9

Exercises

83

Sample 3.28 Time variables and rounding

3.7.4 $time vs. $realtime
The system task $time returns an integer scaled to the time unit of the current module, but missing any fractional units, while $realtime returns a real number with
the complete time value, including fractions. This book uses $time in the examples
for brevity, but your testbenches may need to use $realtime.

3.8 Conclusion
The new SystemVerilog procedural constructs and task/function features make it
easier for you to create testbenches by making the language look more like other
programming languages such as C/C++. As a bonus, SystemVerilog has additional
HDL constructs such as timing controls, simple thread control, and 4-state logic.

3.9 Exercises
1. Create the SystemVerilog code with the following requirements:
a.
b.
c.
d.

Create a 512 element integer array
Create a 9-bit address variable to index into the array
Initialize the last location in the array to 5
Call a task, my_task(), and pass the array and the address

84

3 Procedural Statements and Routines

e. Create my_task() that takes two inputs: a constant 512-element integer array
passed by reference, and a 9-bit address. The task calls a function, print_
int(), and passes the array element indexed by the address, pre-decrementing
the address.
f. Create print_int() that prints out the simulation time and the value of the
input. The function has no return value.
2. For the following SystemVerilog code, what is displayed if the task my_task2()
is automatic?

3. For the same SystemVerilog code in Exercise 2, what is displayed if the task my_
task2() is not automatic?
4. Create the SystemVerilog code to specify that the time should be printed in ps
(picoseconds), display 2 digits to the right of the decimal point, and use as few
characters as possible

3.9

Exercises

85

5. Using the formatting system task from Exercise 4, what is displayed by the
following code?

Chapter 4

Connecting the Testbench and Design

There are several steps needed to verify a design: generate stimulus, capture
responses, determine correctness, and measure progress. However, first you need
the proper testbench, connected to the design, as shown in Fig. 4.1.
Your testbench wraps around the design, sending in stimulus and capturing the
design’s response. The testbench forms the “real world” around the design, mimicking the entire environment. For example, a processor model needs to connect to
various buses and devices, which are modeled in the testbench as bus functional
models. A networking device connects to multiple input and output data streams that
are modeled based on standard protocols. A video chip connects to buses that send
in commands, and then forms images that are written into memory models. The key
concept is that the testbench simulates everything not in the design under test.
Your testbench needs a higher-level way to communicate with the design than
Verilog’s ports and the error-prone pages of connections. You need a robust way to
describe the timing so that synchronous signals are always driven and sampled at
the correct time and all interactions are free of the race conditions so common to
Verilog models.

Testbench

inputs

Design
Under
Test

outputs

Fig. 4.1 The testbench – design environment

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_4,
© Springer Science+Business Media, LLC 2012

87

88

4.1

4 Connecting the Testbench and Design

Separating the Testbench and Design

In an ideal world, all projects have two separate groups: one to create the design and
one to verify it. In the real world, limited budgets may require you to wear both hats.
Each team has its own set of specialized skills, such as creating synthesizable RTL
code, or figuring out new ways to find bugs in the design. These two groups each
read the original design specification and make their own interpretations. The
designer has to create code that meets that specification, whereas your job as the
verification engineer is to create scenarios where the design does not match its
description.
Likewise, your testbench code is in a separate block from design code. In classic
Verilog, each goes in a separate module. However, using a module to hold the testbench often causes timing problems around driving and sampling, so SystemVerilog
introduces the program block to separate the testbench, both logically and temporally. For more details, see Section 4.3.
As designs grow in complexity, the connections between the blocks increase.
Two RTL blocks may share dozens of signals, which must be listed in the correct
order for them to communicate properly. One mismatched or misplaced connection
and the design will not work. You can reduce errors by using the connect-by-name
syntax, but this more than doubles your typing burden. If it is a subtle error, such as
swapping pins that only toggle occasionally, you may not notice the problem for
some time. Worse yet is when you add a new signal between two blocks. You have
to edit not only the blocks to add the new port but also the higher-level modules that
wire up the devices. Again, one wrong connection at any level and the design stops
working. Or worse, the system only fails intermittently!
The solution is the interface, the SystemVerilog construct that represents a bundle of wires. Additionally, you can specify timing, signal direction, and even add
functional code. An interface is instantiated like a module but is connected to ports
like a signal.

4.1.1

Communication Between the Testbench and DUT

The next few sections show a testbench connected to an arbiter, using individual
signals and again using interfaces. Figure 4.2 is a diagram of the top level design
including a testbench, arbiter, clock generator, and the signals that connect them.
This DUT (Design Under Test) is a trivial design, so you can concentrate on the
SystemVerilog concepts and not get bogged down in the design. At the end of the
chapter, an ATM router is shown.

4.1 Separating the Testbench and Design

89

Testbench
request[1:0]
rst

grant[1:0]

Arbiter

clk

Fig. 4.2 Testbench – Arbiter without interfaces

4.1.2

Communication with Ports

The following code shows the steps needed to connect an RTL block to a testbench.
First is the header for the arbiter model, shown in Sample 4.1. This uses the
Verilog-2001 style port declarations where the type and direction are in the header.
Some code has been left out for clarity.
As discussed in Section 2.1.1, SystemVerilog has expanded the classic reg type
so that you can use it like a wire to connect blocks. In recognition of its new capabilities, the reg type has the new name of logic. The only place where you cannot
use a logic variable is a net with multiple structural drivers, where you must use a
net such as wire.
Sample 4.1 Arbiter model using ports

The testbench in Sample 4.2 is kept in a module to separate it from the design.
Typically, it connects to the design with ports.

4 Connecting the Testbench and Design

90
Sample 4.2 Testbench module using ports

The top module connects the testbench and DUT, and includes a simple clock
generator.
Sample 4.3 Top-level module with ports

In Sample 4.3, the modules are simple, but real designs with hundreds of pins
require pages of signal and port declarations. All these connections can be error
prone. As a signal moves through several layers of hierarchy, it has to be declared
and connected over and over. Worst of all, if you just want to add a new signal, it
has to be declared and connected in multiple files. SystemVerilog interfaces can
help in each of these cases.

4.2

The Interface Construct

Designs have become so complex that even the communication between blocks may
need to be separated out into separate entities. To model this, SystemVerilog uses
the interface construct that you can think of as an intelligent bundle of wires. It con-

4.2

The Interface Construct

91

tains the connectivity, synchronization, and optionally, the functionality of the communication between two or more blocks and, optionally, error checking. They
connect design blocks and/or testbenches.
Design-level interfaces are covered in Sutherland (2006). This book concentrates
on interfaces that connect design blocks and testbenches.

4.2.1

Using an Interface to Simplify Connections

The first improvement to the arbiter example is to bundle the wires together into an
interface. Figure 4.3 shows the testbench and arbiter, communicating using an interface. Note how the interface extends into the two blocks, representing the drivers
and receivers that are functionally part of both the test and the DUT. The clock can
be part of the interface or a separate port.

Testbench

Interface

Arbiter

Fig. 4.3 An interface straddles two modules

The simplest interface is just a bundle of bidirectional signals as shown in Sample 4.4.
Use the logic data type so you can drive the signals from procedural statements.

Sample 4.4 Simple interface for arbiter

Sample 4.5 is the device under test, the arbiter, that uses an interface instead of
ports.

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4 Connecting the Testbench and Design

Sample 4.5 Arbiter using a simple interface

Sample 4.6 shows the testbench. You refer to a signal in an interface by making
a hierarchical reference using the instance name: arbif.request. Interface signals should always be driven using nonblocking assignments. This is explained in
more detail in Section 4.4.3 and 4.4.4.
Sample 4.6 Testbench using a simple arbiter interface

All these blocks are instantiated and connected in the top module as shown in
Sample 4.7.
Sample 4.7 Top module with a simple arbiter interface

4.2

The Interface Construct

93

You can see an immediate benefit, even on this small device: the connections
become cleaner and less prone to mistakes. If you wanted to put a new signal in an
interface, you would just have to add it to the interface definition and the modules
that actually used it. You would not have to change any module such as top that just
passes the interface through. This language feature greatly reduces the chance for
wiring errors.
This book only shows interfaces with a single clock that is connected to a generator at the top level. If your interface requires multiple clocks, treat them like the
other signals inside the interface, and connect the interface to a clock generator. You
are more productive if you work at a high level and treat the interface as a cycle
based construct. The next level up is transaction-based, which is beyond typical
RTL code.
Make sure you declare your interfaces outside of modules and
program blocks. If you forget, expect all sorts of trouble. Some
compilers may not support defining an interface inside a module. If allowed, the interface would be local to the module and
thus not visible to the rest of the design. Sample 4.8 shows the common mistake of
including the interface definition right after other include statements.
Sample 4.8 Bad test module includes interface

4.2.2

Connecting Interfaces and Ports

If you have a Verilog-2001 legacy design with ports that cannot be changed to use an
interface, you can just connect the interface’s signals to the individual ports. Sample 4.9
connects the original arbiter from Sample 4.1 to the interface in Sample 4.4.
Sample 4.9 Connecting an interface to a module that uses ports

4 Connecting the Testbench and Design

94

4.2.3

Grouping Signals in an Interface Using Modports

Sample 4.5 uses a point-to-point connection scheme with no signal directions in the
interface. The original modules using ports had this information that the compiler
uses to check for wiring mistakes. The modport construct in an interface lets you
group signals and specify directions. The MONITOR modport in Sample 4.10 allows
you to connect a monitor module to the interface.
Sample 4.10 Interface with modports

Sample 4.11 shows the arbiter model and testbench, with the modport in their
port connection list. Note that you put the modport name, DUT or TEST, after the
interface name, arb_if. Other than the modport name, these are identical to the
previous examples.
Sample 4.11 Arbiter model with interface using modports

Sample 4.12 Testbench with interface using modports

Even though the code didn’t change much (except that the interface grew larger),
this interface more accurately represents the real design, especially the signal
direction.
There are two ways to use these modport names in your design. You can specify
them in the modules that connect to the interface signals. In this case, the top
model does not change from Sample 4.7, except for the module names. This book

4.2

The Interface Construct

95

recommends this style, as the modport is an implementation detail that should not
clutter the top level module.
The alternative is to specify the modport when you instantiate the module as
shown in Sample 4.13.
Sample 4.13 Top level module with modports

With this style, you have the flexibility to instantiate a module more than once,
with each instance connected to a different modport, that is, a different subset of
interface signals. For example, a byte-wide RAM model could connect to one of
four slots on a 32-bit bus. In this case, you would need to specify the modport when
you instantiate the module, not in the module itself.
Note that modports are defined in the interface, and specified in the module port
list, but never in the signal name. The name arb_if.TEST.grant is illegal!

4.2.4

Using Modports with a Bus Design

Not every signal needs to go in every modport. Consider a CPU – memory bus modeled with an interface. The CPU is the bus master and drives a subset of the signals,
such as request, command, and address. The memory is a slave and receives
those signals and drives ready. Both master and slave drive data. The bus arbiter
only looks at request and grant, and ignores all other signals. So your interface
would have three modports for master, slave, and arbiter, plus an optional monitor
modport.

4.2.5

Creating an Interface Monitor

You can create a bus monitor using the MONITOR modport. Sample 4.14 shows a
trivial monitor for the arbiter. For a real bus, you could decode the commands and
print the status: completed, failed, etc.

4 Connecting the Testbench and Design

96

Sample 4.14 Arbiter monitor with interface using modports

4.2.6

Interface Trade-Offs

An interface cannot contain module instances, only instances of other interfaces.
There are trade-offs in using interfaces with modports as compared with traditional
ports connected with signals.
The advantages to using an interface are as follows.
• An interface is ideal for design reuse. When two blocks communicate with a
specified protocol using more than two signals, consider using an interface. If
groups of signals are repeated over and over, as in a networking switch, you
should additionally use virtual interfaces, as described in Chapter 10.
• The interface takes the jumble of signals that you declare over and over in every
module or program and puts it in a central location, reducing the possibility of
misconnecting signals.
• To add a new signal, you just have to declare it once in the interface, not in
higher-level modules, once again reducing errors.
• Modports allow a module to easily tap a subset of signals from an interface. You
can specify signal direction for additional checking.
The disadvantages of using an interface are as follows.
• For point-to-point connections, interfaces with modports are almost as verbose
as using ports with lists of signals. Interfaces have the advantage that all the
declarations are still in one central location, reducing the chance for making an
error.
• You must now use the interface name in addition to the signal name, possibly
making the modules more verbose, but more readable for debugging.
• If you are connecting two design blocks with a unique protocol that will not be
reused, interfaces may be more work than just wiring together the ports.

4.2

The Interface Construct

97

• It is difficult to connect two different interfaces. A new interface (bus_if) may
contain all the signals of an existing one (arb_if), plus new signals (address,
data, etc.). You may have to break out the individual signals and drive them
appropriately.

4.2.7

More Information and Examples

The SystemVerilog LRM specifies many other ways for you to use interfaces. See
Sutherland (2006) for more examples of using interfaces for design.

4.2.8

Logic vs. Wire in an Interface

This book recommends declaring the signals in your interface as logic while the
VMM has a rule that says to use a wire. The difference is ease-of-use vs.
reusability.
If your testbench drives an asynchronous signal in an interface with a procedural
assignment, the signal must be a logic type. A wire can only be driven with a
continuous assignment statement. Signals in a clocking block are always synchronous and can be declared as logic or wire. Sample 4.15 shows how the logic
signal can be driven directly, whereas the wire requires additional code.
Sample 4.15 Driving logic and wires in an interface

Another reason to use logic for interface signals is that the compiler will give
an error if you unintentionally use multiple structural drivers.
The VMM takes a more long-term approach. Take the case where you have created test code that works well on the current project and is later used in a new
design.

4 Connecting the Testbench and Design

98

What if your interface with all its logic signals is connected such that now a
signal has multiple structural drivers? The engineers will have to change that logic
to a wire, and, if the signal does not go through a clocking block, change the procedural assignment statements. Now there are two versions of the interface, and
existing tests must be modified before they can be reused. Rewriting good code
goes against the VMM principles.

4.3

Stimulus Timing

The timing between the testbench and the design must be carefully orchestrated.
At a cycle level, you need to drive and receive the synchronous signals at the proper
time in relation to the clock. Drive too late or sample too early, and your testbench
is off a cycle. Even within a single time slot (for example, everything that happens
at time 100ns), mixing design and testbench events can cause a race condition, such
as when a signal is both read and written at the same time. Do you read the old
value, or the one just written? In Verilog, nonblocking assignments help when a test
module drives the DUT, but the test could not always be sure it sampled the last
value driven by the design. SystemVerilog has several constructs to help you control
the timing of the communication.

4.3.1

Controlling Timing of Synchronous Signals
with a Clocking Block

An interface should contain a clocking block to specify the timing of synchronous
signals relative to the clocks. Clocking blocks are mainly used by testbenches but
also allow you to create abstract synchronous models. Signals in a clocking block
are driven or sampled synchronously, ensuring that your testbench interacts with the
signals at the right time. Synthesis tools do not support clocking blocks, so your
RTL code can not take advantage of them. The chief benefit of clocking blocks is
that you can put all the detailed timing information in here, and not clutter your
testbench.
An interface can contain multiple clocking blocks, one per clock domain, as there
is a single clock expression in each block. Typical clock expressions are @(posedge
clk) for a single edge clock and @(clk) for a DDR (double data rate) clock.
You can specify a clock skew in the clocking block using the default statement,
but the default behavior is that input signals are sampled just before the design executes, and the outputs are driven back into the design during the current time slot. The
next section provides more details on the timing between the design and testbench.
Once you have defined a clocking block, your testbench can wait for the clocking
expression with @arbif.cb rather than having to spell out the exact clock and
edge. Now if you change the clock or edge in the clocking block, you do not have
to change your testbench.

4.3 Stimulus Timing

99

Sample 4.16 is similar to Sample 4.10 except that the TEST modport now treats
request and grant as synchronous signals. The clocking block cb declares that
the signals are active on the positive edge of the clock. The signal directions are
relative to the modport where they are used. So request is a synchronous output
in the TEST modport, and grant is an synchronous input. The signal rst is asynchronous in the TEST modport.
Sample 4.16 Interface with a clocking block

4.3.2

Timing Problems in Verilog

Your testbench needs to be separate from the design, not just logically but also temporally. Consider how a hardware tester interacts with a chip for synchronous signals. In a real hardware design, the DUT’s storage elements latch their inputs from
the tester at the active clock edge. These values propagate through the storage element outputs, and then the logic clouds to the inputs of the next storage element. The
time from the input of the first storage to the next must be less than a clock cycle.

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4 Connecting the Testbench and Design

So a hardware tester needs to drive the chip’s inputs at the clock edge, and read the
outputs just before the following edge.
A testbench has to mimic this tester behavior. It should drive on or after the
active clock edge, and should sample as late as possible as allowed by the protocol
timing specification, just before the active clock edge.
If the DUT and testbench are made of Verilog modules only, this outcome is
nearly impossible to achieve. If the testbench drives the DUT at the clock edge,
there could be race conditions. What if the clock propagates to some DUT inputs
before the testbench stimulus, but is a little later to other inputs? From the outside,
the clock edges all arrive at the same simulation time, but in the design, some inputs
get the value driven during the last cycle, whereas other inputs get values from the
current cycle.
One way around this problem is to add small delays to the system, such as #0.
This forces the thread of Verilog code to stop and be rescheduled after all other
code. Invariably though, a large design has several sections that all want to execute
last. Whose #0 wins out? It could vary from run to run and be unpredictable between
simulators. Multiple threads using #0 delays cause indeterministic behavior. Avoid
using #0 as it will make your code unstable and not portable.
The next solution is to use a larger delay, #1. RTL code has no timing, other than
clock edges, so one time unit after the clock, the logic has settled. However, what if
one module uses a time precision of 1ns, whereas another used a resolution of just
10ps? Does that #1 mean 1ns, 10ps, or something else? You want to drive as soon
as possible after the clock cycle with the active clock edge, but not during that time,
and before anything else can happen. Worse yet, your DUT may contain a mix of
RTL code with no delays and gate code with delays. Just as you should avoid using
#0, stay away from #1 delays to fix timing problems. See Cummings (2000) and
other papers by him for additional guidelines.

4.3.3

Testbench – Design Race Condition

Sample 4.17 shows a potential race condition between the testbench and design.
The race condition occurs when the test drives the start signal and then the other
ports. The memory is waiting on the start signal and could wake up immediately,
whereas the write signal still has its old value, while addr and data have new
values. This behavior is perfectly legal according to the LRM. You could delay all
these signals slightly by using nonblocking assignments, as recommended by
Cummings (2000), but remember that the testbench and the design are both using
these assignments. It is still possible to get a race condition between the testbench
and design.
Sampling the design outputs has a similar problem. You want to grab the values
at the last possible moment, just before the active clock edge. Perhaps you know the
next clock edge is at 100ns. You can’t sample right at the clock edge at 100ns, as
some design values may have already changed. You should sample at Tsetup just
before the clock edge.

4.3 Stimulus Timing

101

Sample 4.17 Race condition between testbench and design

4.3.4

The Program Block and Timing Regions

The root of the problem is the mixing of design and testbench events during the
same time slot, though even in pure RTL the same problem can happen. Good coding guidelines such as proper use of nonblocking assignments can reduce these race
conditions, but improperly coded assignments have the habit of creeping in. What if
there were a way you could separate these events temporally, just as you separated
the code? At 100ns, your testbench could sample the design outputs before the clock
has had a chance to change and any design activity has occurred. By definition,
these values would be the last possible ones from the previous time slot. Then, after
all the design events are done, your testbench would start.
How does SystemVerilog know to schedule the testbench events separately from
the design events? In SystemVerilog, your testbench code is in a program block,
which is similar to a module in that it can contain code and variables and be instantiated in other modules. However, a program cannot have any hierarchy such as
instances of modules, interfaces, or other programs.
A new region of the time slot was introduced in SystemVerilog as shown in Fig.
4.4. In Verilog, most events are executed in the Active region. There are dozens of
other regions for nonblocking assignments, PLI execution, etc., but they can be

4 Connecting the Testbench and Design

102
Fig. 4.4 Main regions inside
a SystemVerilog time step

From previous
time slot
Active
(design)

Loop back
if more events

Observed
(assertions)

Reactive
(testbench)

Postponed
(sample)
To next
time slot

ignored for the purposes of this book. See Table 4.1, the LRM, and Cummings
(2006) for more details on the SystemVerilog event regions.
First to execute during a time slot is the Active region, where design events run.
These include your traditional RTL and gate code plus the clock generator. The
second region is the Observed region, where SystemVerilog Assertions are evaluated. Following that is the Reactive region where the testbench code in a program
executes. Note that time does not strictly flow forwards — events in the Observed
and Reactive regions can trigger further design events in the Active region in the
current cycle. Last is the Postponed region, which samples signals at the end of the
time slot, in the readonly period, after design activity has completed.

Table 4.1 Primary SystemVerilog scheduling regions
Name
Activity
Active
Simulation of design code in modules
Observed
Evaluation of SystemVerilog Assertions
Reactive
Execution of testbench code in programs
Postponed
Sampling design signals for testbench input

Sample 4.18 shows part of the testbench code for the arbiter. Note that the statement @arbif.cb waits for the active edge of the clocking block, @(posedge
clk), as shown in Sample 4.16. This sample shows that your testbench code is

4.3 Stimulus Timing

103

written at a slightly higher level of abstraction, using cycle-by-cycle timing instead
of worrying about individual clock edges.
Section 4.4 explains more about the driving and sampling of interface signals.
Sample 4.18 Testbench using interface with clocking block

Your test should be contained in a single program. You should use
OOP to build a dynamic, hierarchical testbench from objects instead
of modules. A simulation may have multiple program blocks if you
are using code from other people or combining several tests.
As discussed in Section 3.6.1, you should always declare your program block as automatic so that it behaves more like the routines
in stack-based languages you may have worked with, such as C.

Note that not all vendors regard program blocks equally. See Rich (2009) for an
alternate opinion.

4.3.5

Specifying Delays Between the Design and Testbench

The default timing of the clocking block is to sample inputs with a skew of #1step
and to drive the outputs with a delay of #0. The 1step delay specifies that signals
are sampled in the Postponed region of the previous time slot, before any design
activity. So you get the output values just before the clock changes. The testbench
outputs are synchronous by virtue of the clocking block, so they flow directly into the
design. The program block, running in the Reactive region, generates the stimulus

4 Connecting the Testbench and Design

104

that is applied to the DUT, which is then evaluated in the Active region during the
same time slot. The DUT evaluates its logic and drives its outputs, which are the
inputs to the testbench through the clocking blocks. These are then sampled in
the Postponed region and the cycle repeats. If you have a design background, you can
remember this by imagining that the clocking block inserts a synchronizer between
the design and testbench, as shown in Fig. 4.5. With proper use of program and
clocking blocks, race conditions between the testbench and DUT can be all but
eliminated.

Testbench
test
out

in

Design out
Under Test

d

q

test
in

clk

Fig. 4.5 A clocking block synchronizes the DUT and testbench

4.4

Interface Driving and Sampling

Your testbench needs to drive and sample signals from the design, primarily through
interfaces with clocking blocks. The next section uses the arbiter interface from
Sample 4.16 and the top-level module from Sample 4.9.
Asynchronous signals such as rst pass through the interface with no delays. The
signals in the clocking block get synchronized as shown in the sections below.

4.4.1

Interface Synchronization

You can use the Verilog @ and wait constructs to synchronize the signals in a testbench. Sample 4.19 shows the various constructs.

4.4

Interface Driving and Sampling

105

Sample 4.19 Signal synchronization

4.4.2

Interface Signal Sample

When you read a signal from a clocking block, you get the value sampled from just
before the last clock edge, i.e., from the Postponed region. Sample 4.20 shows a
program block that reads the synchronous grant signal from the DUT. The arb
module drives grant to 1 & 2 in the middle of the 100ns cycle, and then to 3 exactly
at the clock edge. This code is for illustration only and is not real, synthesizable
RTL.
Sample 4.20 Synchronous interface sample and drive from module

The waveforms in Fig. 4.6 show that in the program, arbif.cb.grant gets the
value from just before the clock edge. When the interface input changes right at a
clock edge, such as 250ns, the value does not propagate to the testbench until the
next cycle, which starts at 350ns.

4 Connecting the Testbench and Design

106

clk
DUT arb.grant

X

1

2

X

TEST arbif.cb.grant
50ns

1
150ns

250ns

3
2
350ns

3

Fig. 4.6 Sampling a synchronous interface

4.4.3

Interface Signal Drive

Sample 4.21 has an abbreviated version of the arbiter test program, which uses the
arbiter interface defined in Sample 4.16.
Sample 4.21 Testbench using interface with clocking block

When using modports with clocking blocks, a synchronous
interface signal such as request must be prefixed with both the
interface name, arbif, and the clocking block name, cb. So in
Sample 4.21, arbif.cb.request is legal, but arbif.
request is not. This is the most common coding mistake with interfaces and clocking blocks.

4.4.4

Driving Interface Signals Through a Clocking Block

You should always drive interface signals in a clocking block with a synchronous
drive using a nonblocking assignment. This is because the design signal does not
change immediately after your assignment – remember that your testbench executes

4.4

Interface Driving and Sampling

107

in the Reactive region while design code is in the Active region. If your testbench
drives arbif.cb.request at 100ns, the same time as arbif.cb (which is
@(posedge clk) according to the clocking block), request changes in the design
at 100ns. However, if your testbench tries to drive arbif.cb.request at time
101ns, between clock edges, the change does not propagate until the next clock
edge. In this way, your drives are always synchronous. In Sample 4.20, arbif.
grant is driven by a module and can use a blocking assignment.
If the testbench drives the synchronous interface signal at the active edge of the
clock, as shown in Sample 4.22, the value propagates immediately to the design.
This is because the default output delay is #0 for a clocking block. If the testbench
drives the output just after the active edge, the value is not seen in the design until
the next active edge of the clock.
Sample 4.22 Interface signal drive

Sample 4.23 shows what happens if you drive a synchronous interface signal at
various points during a clock cycle. This uses the interface from Sample 4.16 and
the top module and clock generator from Sample 4.9.
Sample 4.23 Driving a synchronous interface

Note that in Fig. 4.7, the value 3, driven in the middle of the first cycle, is seen
by the DUT at the start of the second cycle. The value 2 is driven in the middle of
the second cycle. It is never seen by the DUT as the testbench drives a 1 at the end
of the second cycle.

4 Connecting the Testbench and Design

108

clk
TEST arb.cb.request
DUT

X

3

2

X

arbif.request
50ns

3
150ns

1
1

250ns

1
350ns

Fig. 4.7 Driving a synchronous interface

Driving clocking block signals asynchronously can lead to dropped values.
Instead, drive at the clock edge by using a cycle delay prefix on your drives as
shown in Sample 4.24.
Sample 4.24 Interface signal drive

If you want to wait for two clock cycles before driving a signal, you can either
use “repeat (2) @arbif.cb;” or use the cycle delay ##2. This latter delay only
works as a prefix to a drive of a signal in a clocking block, as it needs to know which
clock to use for the delay.
The cycle delay of ##0 in an assignment that drives the value immediately if the
clock was asserted in this time slot, according to the clocking block. If the clock was
not just asserted, the signal is driven at the next active edge of the clock. The cycle
delay of ##1 always waits for the next active edge of the clock, even if the clock was
asserted in the current time slot.
The naked cycle delay statement ##3; works if you have a default clocking block
for your program or module. This book only recommends putting a clocking block in
an interface and not creating a default clocking block. You should always be specific
about which clock is being referenced.

4.4.5

Bidirectional Signals in the Interface

In Verilog-1995, if you want to drive a bidirectional signal such as a port from
procedural code, you need a continuous assignment to connect the reg to the wire.
In SystemVerilog, synchronous bidirectional signals in interfaces are easier to use
as the continuous assignment is added for you, as shown in Sample 4.25. When you
write to the net from a program, SystemVerilog actually writes to a temporary variable that drives the net. Your program reads directly from the wire, seeing the value
that is resolved from all the drivers. Design code in a module still uses the classic
register plus continuous assignment statement.

4.4

Interface Driving and Sampling

109

Sample 4.25 Bidirectional signals in a program and interface

The SystemVerilog LRM is not clear on driving an asynchronous bidirectional
signal using an interface. Two possible solutions are to use a cross-module reference
and continuous assignment or to use a virtual interface as shown in Chapter 10.

4.4.6

Specifying Delays in Clocking Blocks

A clocking block ensures that your signals are driven and sampled at the specified
clock edge. You can skew these times with either a default statement, or by specifying the delays for individual signals. This can be useful when simulating netlists
with real delays. Sample 4.26 shows a clocking block with a default statement that
has the skews for all signals. In this example, the inputs are sampled 15ns before the
posedge of the clock and the outputs are driven 10ns after the posedge of the clock.
Sample 4.26 Clocking block with default statement

Sample 4.27 shows the equivalent clocking block, but with the delays specified
on the individual signals.

4 Connecting the Testbench and Design

110

Sample 4.27 Clocking block with delays on individual signals

4.5
4.5.1

Program Block Considerations
The End of Simulation

In Verilog, simulation continues while there are scheduled events, or until a $finish
is executed. SystemVerilog adds an additional way to end simulation. A program
block is treated as if it contains a test. If there is only a single program, simulation
ends when you complete the last statement in every initial block in the program,
as this is considered the end of the test. Simulation ends even if there are threads
still running in the program or modules. As a result, you don’t have to shut down
every monitor and driver when a test is done.
If there are several program blocks, simulation ends when the last program completes. This way simulation ends when the last test completes. You can terminate
any program block early by executing $exit. Of course you can still explicitly call
$finish to end simulation, but this might cause issues if you have multiple
programs.
However, simulation is not yet over. A module or program can have a final
block that contains code to be run just before the simulator terminates, as shown in
Sample 4.28. This is a great place to perform clean up work such as closing files,
and printing a report of the number of errors and warnings encountered. You cannot
schedule any events, or have any delays in a final block that could cause time to
elapse. You do not have to worry about freeing any memory that was allocated as
this will be done automatically.
Sample 4.28 A final block

4.5

Program Block Considerations

4.5.2

111

Why are Always Blocks not Allowed in a Program?

In SystemVerilog you can put initial blocks in a program, but not always blocks.
This may seem odd if you are used to Verilog modules, but there are several reasons
SystemVerilog programs are closer to a program in C, with one (or more) entry
points, than Verilog’s many small blocks of concurrently executing hardware. In a
design, an always block might trigger on every positive edge of a clock from the
start of simulation. In contrast, a testbench has the steps of initialization, stimulate
and respond to the design, and then wrap up simulation. An always block that runs
continuously would not work.
When the last initial block completes in the program, simulation implicitly
ends just as if you had executed $finish. If you had an always block, it would run
for ever, so you would have to explicitly call $exit to signal that the program completed. But don’t despair. If you really need an always block, you can use initial
forever to accomplish the same thing.

4.5.3

The Clock Generator

Now that you have seen the program block, you may wonder if the clock generator
should be in a module. The clock is more closely tied to the design than the testbench, and so the clock generator should remain in a module. The generator should
be instantiated at the same level as the DUT so it can drive both the DUT and testbench As you refine the design, you create clock trees, and you have to carefully
control the skews as the clocks enter the system and propagate through the blocks.
The testbench is much less picky. It just wants a clock edge to know when to
drive and sample signals. Functional verification is concerned with providing the
right values at the right cycle, not with fractional nanosecond delays and relative
clock skews.
The program block is not the place to put a clock generator. Sample 4.29 tries to
put the generator in a program block but just causes a race condition. The clk and
data signals both propagate from the Reactive region to the design in the Active
region and could cause a race condition depending on which one arrived first.
Sample 4.29 Bad clock generator in program block

4 Connecting the Testbench and Design

112

Avoid race conditions by always putting the clock generator in a
module. If you want to randomize the generator’s properties, create
a class with random variables for skew, frequency, and other characteristics, as shown in Chapter 6. You can use this class in the
generator module, or in the testbench.
Sample 4.30 shows a good clock generator in a module. It deliberately avoids an
edge at time 0 to prevent race conditions. All clock edges are generated with a
blocking assignment to trigger events during the Active region. If you must generate
a clock edge at time 0, use a nonblocking assignment to set the initial value so all
clock sensitive logic such as always blocks will have started before the clock
changes value.
Sample 4.30 Good clock generator in module

Lastly, don’t try to verify the low-level timing with functional verification. The testbenches described in this book check the behavior
of the DUT but not the timing, which is better done with a static
timing analysis tool. Your testbenches should be flexible enough to
be compatible with gate-level simulations run with back-annotated
timing.

4.6

Connecting It All Together

Now you have a design described in a module, a testbench in a program block, and
interfaces that connect them together. Sample 4.31 has the top-level module that
instantiates and connects all the pieces.
Sample 4.31 Top module with implicit port connections

4.6 Connecting It All Together

113

This is almost identical to Sample 4.7. It uses a shortcut notation.* (implicit port
connection) that automatically connects module instance ports to signals at the current level if they have the same name and data type.

4.6.1

An Interface in a Port List Must Be Connected

The SystemVerilog compiler won’t let you compile a single module or program that
uses an interface in the port list. Why not? After all, a module or program with ports
made of individual signals can be compiled without being instantiated, as shown in
Sample 4.32.
Sample 4.32 Module with just port connections

The compiler creates wires and connects them to the dangling signals. However,
a module or program with an interface in its port list must be connected to an
instance of the interface.
Sample 4.33 Module with an interface

For Sample 4.33, the compiler is not able to build even a simple interface. If you
have modports or a program block using clocking blocks in an interface, the compiler has an even more difficult time. Even if you are just looking to wring out
syntax bugs, you must complete the connections. This can be done as shown in
Sample 4.34.
Sample 4.34 Top module connecting DUT and interface

4 Connecting the Testbench and Design

114

4.7

Top-Level Scope

Sometimes you need to create things in your simulation that are outside of a
program or module so that they are seen by all blocks. In Verilog, only macros
extend across module boundaries, and so are used for creating global constants.
SystemVerilog introduces the compilation unit, that is a group of source files
that are compiled together. The scope outside the boundaries of any module,
macromodule, interface, program, package, or primitive is known as the
compilation-unit scope, also referred to as $unit. Anything such as a parameter
defined in this scope is similar to a global because it can be seen by all lower-level
blocks. However, it is not truly global as the parameter cannot be seen during
compilation of other files.
This leads to some confusion. Some simulators compile all the SystemVerilog
code together, so $unit is global. Other simulators and synthesis tools compile a
single module or group of modules at a time, so $unit may be just the contents of
one or a few files. As a result, $unit is not portable. Packages allow you to have
code outside of a program or module while eliminating the requirement of compiling all the modules at the same time.
This book calls the scope outside blocks the “top-level scope.” You can define
variables, parameters, data types and even routines in this space. Sample 4.35
declares a top-level parameter, TIMEOUT, that can be used anywhere in the hierarchy. This example also has a const string that holds an error message. You can
declare top-level constants either way.
Sample 4.35 Top-level scope for arbiter design

The instance name $root allows you to unambiguously refer to names in the
system, starting with the top-level scope. In this respect, $root is similar to “/” in
the Unix file system. For tools such as VCS that compile all files at once, $root
and $unit are equivalent. The name $root also solves an old Verilog problem.

4.8 Program–Module Interactions

115

When your code refers to a name in another module, such as i1.var, the compiler
first looks in the local scope, then looks up to the next higher scope, and so on until
it reaches the top. You may have wanted to use i1.var in the top module, but an
instance named i1 in an intermediate scope may have sidetracked the search, giving
you the wrong variable. You use $root to make unambiguous cross module references by specifying the absolute path.
Sample 4.36 shows a program that is instantiated in the module top that is
implicitly instantiated in the top-level scope. The program can use a relative or
absolute reference to the clk signal in the module. You can use a macro to hold the
hierarchical path so that when the path changes, you only have to change one piece
of code. The LRM does not allow modules to be explicitly instantiated in the toplevel scope.
Sample 4.36 Cross-module references with $root

4.8

Program–Module Interactions

The program block can read and write all signals in modules, and can call routines
in modules, but a module has no visibility into a program. This is because your testbench needs to see and control the design, but the design should not depend on
anything in the testbench.
A program can call a routine in a module to perform various
actions. The routine can set values on internal signals, also known
as “backdoor load.” Next, because the current SystemVerilog standard does not define how to force signals from a program block,
you need to write a task in the design to do the force, and then call
it from the program.

4 Connecting the Testbench and Design

116

Lastly, it is a good practice for your testbench to use a function to get information
from the DUT. Reading signal values can work most of the time, but if the design
code changes, your testbench may interpret the values incorrectly. A function in the
module can encapsulate the communication between the two and make it easier for
your testbench to stay synchronized with the design. Chapter 10 shows how to
embed functions and SystemVerilog Assertions in an interface.

4.9

SystemVerilog Assertions

You can create temporal assertions about signals in your design to check their
behavior and temporal relationship with SystemVerilog Assertions (SVA). The simulator keeps track of what assertions have triggered, so you can gather functional
coverage data on them.

4.9.1

Immediate Assertions

An immediate assertion checks if an expression is true when the statement is executed. Your testbench procedural code can check the values of design signals and
testbench variables and take action if there is a problem. For example, if you have
asserted the bus request, you expect that grant will be asserted two cycles later. You
could use an if-statement as shown in Sample 4.37.
Sample 4.37 Checking a signal with an if-statement

An assertion is more compact than an if-statement. However, note that the logic
is reversed compared to the if-statement above. You want the expression inside the
parentheses to be true; otherwise, print an error as shown in Sample 4.38.
Sample 4.38 Simple immediate assertion

If the grant signal is asserted correctly, the test continues. If the signal does not
have the expected value, the simulator produces a message similar Sample 4.39.
Sample 4.39 Error from failed immediate assertion

4.9 SystemVerilog Assertions

117

This says that on line 7 of the file test.sv, the assertion top.t1.a1 started at
55ns to check the signal arbif.cb.grant, but failed immediately. The label a1
should be unique so that you can quickly locate the failing assertion.
You may be tempted to use the full SystemVerilog Assertion syntax
to check an elaborate sequence over a range of time, but use care as
they can be hard to debug. Assertions are declarative code, and execute very differently than the surrounding procedural code. In just a
few lines of assertions, you can verify temporal relations; the equivalent procedural code would be more complicated and verbose, but easier for the next
person to understand when they have to read your code.
If you are a VHDL programmer, you may be tempted at this
point to start sprinkling immediate assertions across your code.
Resist the temptation! Your code will work correctly for weeks
or months until someone decides to improve simulation performance by disabling assertions. The simulator will no longer execute the expression
in the assertion. If the expression has a side effect such as incrementing a value or
calling a function, it will no longer occur.

4.9.2

Customizing the Assertion Actions

An immediate assertion has optional then- and else-clauses. If you want to augment
the default message, you can add your own as shown in Sample 4.40.
Sample 4.40 Creating a custom error message in an immediate assertion

If grant does not have the expected value, you’ll see an error message similar to
Sample 4.41.
Sample 4.41 Error from failed immediate assertion

SystemVerilog has four functions to print messages: $info, $warning, $error,
and $fatal. These are allowed only inside an assertion, not in procedural code,
though future versions of SystemVerilog may allow this.
You can use the then-clause to record when an assertion completed successfully as shown in Sample 4.42.

4 Connecting the Testbench and Design

118
Sample 4.42 Creating a custom error message

4.9.3

Concurrent Assertions

The other type of assertion is the concurrent assertion that you can think of as a small
model that runs continuously, checking the values of signals for the entire simulation.
These are instantiated similarly to other design blocks and are active for the entire
simulation. You need to specify a sampling clock in the assertion. Sample 4.43 has
a small assertion to check that the arbiter request signal does not have X or Z values
except during reset. This code is placed outside of procedural blocks such as initial
and always. Sample 4.43 is for illustration only. See one of the books listed below
for a more information.
Sample 4.43 Concurrent assertion to check for X/Z

4.9.4

Exploring Assertions

There are many other uses for assertions. For example, you can put assertions in an
interface. Now your interface not only transmits signal values but also checks the
protocol.
This Section has provided a brief introduction to SystemVerilog Assertions. For
more information, see Vijayaraghhavan and Ramanathan (2005) and Haque et al.
(2007).

4.10

The Four-Port ATM Router

4.10

119

The Four-Port ATM Router

The arbiter example is a good introduction to interfaces, but real designs have more
than a single input and output. This section discusses a four-port ATM (Asynchronous
Transfer Mode) router, shown in Fig. 4.8.

Testbench
Tx0

Rx0
Rx1
Rx2

4x4
ATM
router

Rx3

Tx1
Tx2
Tx3

Fig. 4.8 Testbench – ATM router diagram without interfaces

4.10.1

ATM Router with Ports

The following code fragments show the tangle of wires you would have to endure to
connect an RTL block to a testbench. First is the header for the ATM router model.
This uses the Verilog-1995 style port declarations, where the type and direction are
separate from the header.
The actual code for the router in Sample 4.44 is crowded out by nearly a page of
port declarations.

4 Connecting the Testbench and Design

120
Sample 4.44 ATM router model header with ports

So what sort of synthesizable code goes in the “…” at the end of Sample 4.44 ?
See Sutherland (2006) for more information and examples of using interfaces in
modules and other SystemVerilog design constructs.

4.10.2

ATM Top-Level Module with Ports

Sample 4.45 contains the top-level module.

4.10

The Four-Port ATM Router

121

Sample 4.45 Top-level module without an interface

Sample 4.46 shows the top of the testbench module. Once again, note that the
ports and wires take up the majority of the module.

122
Sample 4.46 Verilog-1995 testbench using ports

4 Connecting the Testbench and Design

4.10

The Four-Port ATM Router

123

You just saw three pages of code, and it was all just connectivity — no testbench,
no design! Interfaces provide a better way to organize all this information and eliminate the repetitive parts that are so error prone.

4.10.3

Using Interfaces to Simplify Connections

Figure 4.9 shows the ATM router connected to the testbench, with the signals
grouped into interfaces.

Testbench
Rx

Tx

4x4 ATM
router

Fig. 4.9 Testbench - router diagram with interfaces

4 Connecting the Testbench and Design

124

4.10.4

ATM Interfaces

Sample 4.47 and 4.48 show the Rx and Tx interfaces with modports and clocking
blocks.
Sample 4.47 Rx interface with modports and clocking block

Sample 4.48 Tx interface with modports and clocking block

4.10.5

ATM Router Model Using an Interface

Sample 4.49 contains the ATM router model and testbench, which need to specify
the modport in their port connection list. Note that you put the modport name after
the interface name, Rx_if.

4.10

The Four-Port ATM Router

125

Sample 4.49 ATM router model with interface using modports

4.10.6

ATM Top Level Module with Interfaces

The top module, shown in Sample 4.50, has shrunk considerably, along with the
chances of making a mistake.

Sample 4.50 Top-level module with interface

4.10.7

ATM Testbench with Interface

Sample 4.51 shows the part of the testbench that captures cells coming in from the
TX port of the router. Note that the interface names are hard-coded, so you have to
duplicate the same code four times for the 4´4 ATM router. For example, only the
task receive_cell0 is shown, and the final code would also have receive_
cell1, receive_cell2, and receive_cell30. Chapter 10 shows how to simplify the code by using virtual interfaces.

4 Connecting the Testbench and Design

126

Sample 4.51 Testbench using an interface with a clocking block

4.11

The Ref Port Direction

SystemVerilog introduces a new port direction for connecting modules: ref. You
should be familiar with the input, output, and inout directions. The last is for
modeling bidirectional connections. If you drive a signal with multiple inout ports,
SystemVerilog will calculate the value of the signal by combining the values of all
drivers, taking in to account driver strengths and Z values.
A ref port is a different beast. It is essentially a way to make two names that
both reference the same variable. There is only one storage location, but multiple
aliases. Ref ports can only connect to variables, not signals. See Section 3.4.3 for
information on the ref direction for routine arguments.

4.12

Conclusion

127

In Sample 4.52, the incr module has two ref ports, c and d. These two variables
share storage with the c and d variables in the top module. When top changes the
value of c, it is seen immediately by incr. Then incr increments c and the result
is seen back in the top module. If the port c was declared as inout, you would
have had to build tristate drivers such as continuous assignment statements, and
make sure you properly drove an enable signal and Z values. Don’t consider ref
ports as a convenient replacement for inout ports as only the latter are supported
for synthesis.
Sample 4.52 Ref ports

4.12

Conclusion

In this chapter you have learned how to use SystemVerilog’s interfaces to organize
the communication between design blocks and your testbench. With this design
construct, you can replace dozens of signal connections with a single interface,
making your code easier to maintain and improve, and reducing the number of wiring mistakes.
SystemVerilog also introduces the program block to hold your testbench and to
reduce race conditions between the device under test and the testbench. With a
clocking block in an interface, your testbenches will drive and sample design signals correctly relative to the clock.

4 Connecting the Testbench and Design

128

4.13

Exercises

1. Design an interface and testbench for the ARM Advanced High-performance
Bus (AHB). You are provided a bus master as verification IP that can initiate
AHB transactions. You are testing a slave design. The testbench instantiates the
interface, slave, and master. Your interface will display an error if the transaction
type is not IDLE or NONSEQ on the negative edge of HCLK. The AHB signals
are described in Table 4.2.
Table 4.2 AHB Signal Description
Signal
Width
Direction
HCLK
1
Output
HADDR
21
Output
HWRITE
1
Output
HTRANS
2
Output
HWDATA
HRDATA

8
8

Output
Input

Description
Clock
Address
Write flag: 1=write, 0=read
Transaction type:
2¢b00=IDLE, 2¢b10=NONSEQ
Write data
Read data

2. For the following interface, add the following code.
a. A clocking block that is sensitive to the negative edge of the clock, and all I/O
that are synchronous to the clock.
b. A modport for the testbench called master, and a modport for the DUT called
slave

c. Use the clocking block in the I/O list for the master modport.

4.13

Exercises

129

3. For the clocking block in Exercise 2, fill in the data_in and data_out signals
in the following timing diagram.

test/reg_bus/clk
test/reg_bus/data_in
16 h0000

test/reg_bus/data_out
reg_bus/cb/data_in

16 h0000

16 h0001

16 h0001

16 h0002

16 h0003

reg_bus/cb/data_out

4. Modify the clocking block in Exercise 2 to have:
a. output skew of 25ns for output write and address
b. input skew of 15ns
c. restrict data_in to only change on the positive edge of the clock
5. For the clocking block in Exercise 4, fill in the following timing diagram, assuming
a clock period of 100ns.

reg_bus/clk
reg_bus/write
reg_bus/data_in
16 h0000

reg_bus/data_out
reg_bus/cb/data_in
reg_bus/cb/data_out
reg_bus/cb/write

16 h0000

16 h0001

16 h0001
16 h0002

16 h0003

2
4

Chapter 5

Basic OOP

5.1

Introduction

With procedural programming languages such as Verilog and C, there is a strong
division between data structures and the code that uses them. The declarations and
types of data are often in a different file than the algorithms that manipulate them.
As a result, it can be difficult to understand the functionality of a program, as the
two halves are separate.
Verilog users have it even worse than C users, as there are no structures in Verilog,
only bit vectors and arrays. If you wanted to store information about a bus transaction, you would need multiple arrays: one for the address, one for the data, one for
the command, and more. Information about transaction N is spread across all the
arrays. Your code to create, transmit, and receive transactions is in a module that
may or may not be actually connected to the bus. Worst of all, the arrays are all
static, so if your testbench only allocated 100 array entries, and the current test
needed 101, you would have to edit the source code to change the size and recompile. As a result, the arrays are sized to hold the greatest conceivable number of
transactions, but during a normal test, most of that memory is wasted.
Object-Oriented Programming (OOP) lets you create complex data types and tie
them together with the routines that work with them. You can create testbenches and
system-level models at a more abstract level by calling routines to perform an action
rather than toggling bits. When you work with transactions instead of signal transitions, you are working at a higher level, and your code is more easily written and
understood. As a bonus, your testbench is decoupled from the design details,
making it more robust and easier to maintain and reuse on future projects.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_5,
© Springer Science+Business Media, LLC 2012

131

5 Basic OOP

132

If you already are familiar with OOP, skim this chapter, as SystemVerilog
follows OOP guidelines fairly closely. Be sure to read Section 5.18 to learn how
to build a testbench. Chapter 8 presents advanced OOP concepts such as inheritance and more testbench techniques; it should be read by everyone.

5.2

Think of Nouns, not Verbs

Grouping data and code together helps you in creating and maintaining large testbenches. How should data and code be brought together? You can start by thinking
of how you would perform the testbench’s job.
The goal of a testbench is to apply stimulus to a design and then check the result
to see if it is correct. The data that flows into and out of the design is grouped
together into transactions such as bus cycles, opcodes, packets, or data samples. The
best way to organize the testbench is around the transactions, and the operations that
you perform on them. In OOP, the transaction is the focus of your testbench.
You can think of an analogy between cars and testbenches. Early cars required
detailed knowledge about their internals (nouns) to operate. You had to advance or
retard the spark, open and close the choke, keep an eye on the engine speed and be
aware of the traction of the tires if you drove on a slippery surface such as a wet
road. Today your interactions with the car are at a high level. When you get into a
car, you perform discrete actions (verbs), such as starting, moving forward, turning,
stopping, and listening to music while you drive. If you want to start a car, just turn
the key in the ignition, and you are done. Get the car moving by pressing the gas
pedal; stop it with the brakes. Are you driving on snow? Don’t worry: the anti-lock
brakes help you stop safely and in a straight line. You don’t have to think about the
low level details.
Your testbench should be structured the same way. Traditional testbenches were
oriented around the operations that had to happen: create a transaction, transmit it,
receive it, check it, and make a report. Instead, you should think about the structure
of the testbench, and what each part does. The generator creates transactions and
passes them to the next level. The driver talks with the design that responds with
transactions that are received by a monitor. The scoreboard checks these against the
expected data. You should divide your testbench into blocks, and then define how
they communicate. This chapter shows many examples of these components.
How do you represent these blocks in SystemVerilog? A class may describe a
data-centric block such as a bus transaction, network packet, or CPU instruction. Or
a class might represent a control block such as a driver or scoreboard. Either way, a
class encapsulates the data together with the routines that manipulate it. The details
of how the class implements actions such as data generation or checking is hidden
from the outside, making the class more reusable.

5.4

5.3

Where to Define a Class

133

Your First Class

Sample 5.1 shows a class for a generic transaction. It contains an address, a checksum, and an array of data values. There are two routines in the Transaction class:
one that displays the address, and another that computes a checksum of the data.
To make it easier to match the beginning and end of a named block,
you can put a label on the end of it. In Sample 5.1 these end labels
may look redundant, but in complex code, with many nested blocks,
the labels help you find the mate for a simple end, endtask,
endfunction, or endclass.
Sample 5.1 Simple transaction class

Every company has its own naming style. This book uses the following convention: Class names start with a capital letter and avoid
using underscores, as in Transaction or Packet. Constants are all
upper case, as in CELL_SIZE, and variables are lower case, as in
count or opcode. You are free to use whatever style you want.

5.4

Where to Define a Class

You can define and use classes in SystemVerilog in a program, module, package,
or outside of any of these.
When you start a project, you might store a single class per file. When the number of files gets too large, you can group a set of related classes and type definitions
into a SystemVerilog package as shown in Sample 5.2. For instance, you might
group together all USB3 transactions and BFMs into a single package. Now you
can compile the package separately from the rest of the system. Unrelated classes,
such as those for other transactions, scoreboards, or different protocols, should
remain in separate files.

5 Basic OOP

134

Code samples in this book leave out the packages to keep the text more compact.
Sample 5.2 Class in a package

Sample 5.3 shows how to import a package into a program.
Sample 5.3 Importing a package in a program

5.5

OOP Terminology

What separates you, an OOP novice, from an expert? The first thing is the words
you use. You already know some OOP concepts from working with Verilog. Here
are some OOP terms, definitions, and rough equivalents in Verilog 2001.
• Class – a basic building block containing routines and variables. The analogue in
Verilog is a module.
• Object – an instance of a class. In Verilog, you need to instantiate a module to use it.
• Handle – a pointer to an object. In Verilog, you use the name of an instance when
you refer to signals, and routines from outside the module. A handle is like the
address of the object, but stored in a pointer that can only refer to one type.
A handle is similar to a reference in other OOP languages.
• Property – a variable that holds data. In Verilog, this is a signal such as a register
or wire.
• Method – the procedural code that manipulates variables, contained in tasks and
functions. Verilog modules have tasks and functions plus initial and always
blocks.
• Prototype – the header of a routine that shows the name, type, and argument list
plus return type. The body of the routine contains the executable code. See
Section 5.10 for more on prototypes and out-of-body experiences.

5.6

Creating New Objects

135

This book uses the more traditional terms from Verilog of “variable” and “routine”
when discussing non-OOP code, and “property” and “method” for classes.
In Verilog you build complex designs by creating modules and instantiating them
hierarchically. In OOP you create classes and construct them (creating objects) to
create a similar hierarchy. Modules are instantiated during compilation while classes
are constructed at run time.
Here is an analogy to explain these OOP terms. Think of a class as the blueprint for
a house. This plan describes the structure of the house, but you cannot live in a blueprint; you need to build the physical house. An object is the actual house. Just as one
set of blueprints can be used to build a whole subdivision of houses, a single class can
be used to build many objects. The house address is like a handle in that it uniquely
identifies your house. Inside your house you have things such as lights (on or off),
with switches to control them. A class has variables that hold values, and routines that
control the values. A class for the house might have many lights. A single call to
turn_on_porch_light() sets the light variable ON in a single house.

5.6

Creating New Objects

Both Verilog and OOP have the concept of instantiation, but there are some differences in the details. A Verilog module, such as a counter, is instantiated when you
compile your design. A SystemVerilog class, such as a network packet, is instantiated during simulation, when needed by the testbench. Verilog instances are static,
as the hardware does not change during simulation; only signal values change.
SystemVerilog stimulus objects are constantly being created and used to drive the
DUT and check the results. Later, the objects may be freed so their memory can be
used by new ones. Back to the house analogy: the address is normally static, unless
your house burns down, causing you to construct a new one. Garbage collection at
home is not automatic, especially if you have teenagers.
The analogy between OOP and Verilog has a few other exceptions. The top-level
Verilog module is implicitly instantiated. However, a SystemVerilog class must be
instantiated before it can be used. Next, a Verilog instance name only refers to a
single instance, whereas a SystemVerilog handle can refer to many objects, though
only one at a time.

5.6.1

Handles and Constructing Objects

In Sample 5.4, tr is a handle that points to an object of type Transaction. For
brevity, you can just say tr is a Transaction handle.

5 Basic OOP

136
Sample 5.4 Declaring and using a handle

When you declare the handle tr, it is initialized to the special value null.
On the next line, call the new() function to construct the Transaction object.
This special new function allocates space for the Transaction, initializes the
variables to their default value (0 for 2-state variables and X for 4-state ones), and
returns the address where the object is stored. For example, the Transaction class
has two 32-bit registers (addr and csm) and an array with eight values (data), for a
total of 10 longwords, or 40 bytes. So when you call new, SystemVerilog allocates
at least 40 bytes of storage. If you have used C, this step is similar to calling the
malloc function. Note that SystemVerilog requires additional memory for 4-state
variables and housekeeping information such as the object’s type.
This process is called instantiation as you are making an instance of the class.
The new function is sometimes called the constructor, as it builds the object, just as
your carpenter constructs a house from wood and nails. For every class, SystemVerilog
creates a default new function to allocate and initialize an object.

5.6.2

Custom Constructor

You can define your own new() function to set your own values. Note that you must
not give a return value type as the constructor is a special function and automatically
returns a handle to an object of the same type as the class.
Sample 5.5 Simple user-defined new() function

In Sample 5.5, first SystemVerilog allocates the space for the object automatically. Next it sets addr and data to fixed values but leaves csm at its default value
of X. You can use arguments with default values to make a more flexible constructor,
as shown in Sample 5.6. Now you can specify the value for addr and data when
you call the constructor, or use the default values.

5.6

Creating New Objects

137

Sample 5.6 A new() function with arguments

How does SystemVerilog know which new()function to call? It looks at the type
of the handle on the left side of the assignment. In Sample 5.7, the call to new inside
the Driver constructor calls the new() function for Transaction, even though
the one for Driver is closer. Since tr is a Transaction handle, SystemVerilog
does the right thing and creates an object of type Transaction.
Sample 5.7 Calling the right new() function

5.6.3

Separating the Declaration and Construction

You should avoid declaring a handle and calling the constructor,
new, all in one statement. While this is legal syntax and less verbose,
it can create ordering problems, as the constructor is called before
the first procedural statement. You might need to initialize objects in
a certain order, but if you call new() in the declaration, you won’t
have the same control. Additionally, if you forget to use automatic storage, the
constructor is called at the start of simulation, not when the block is entered.

5 Basic OOP

138

5.6.4

The Difference Between New() and New[]

You may have noticed that this new() function looks a lot like the new[] operator
described in Section 2.3, used to set the size of dynamic arrays. They both allocate
memory and initialize values. The big difference is that the new() function is called
to construct a single object, whereas the new[] operator is building an array with
multiple elements. new() can take arguments for setting object values, whereas
new[] only takes a single value for the number of elements in the array. Just remember that the new with square brackets [] is for arrays, while the one with parentheses () is for classes, which usually contain methods.

5.6.5

Getting a Handle on Objects
New OOP users often confuse an object with its handle. The two
are very distinct. You declare a handle and construct an object.
Over the course of a simulation, a handle can point to many
objects. This is the dynamic nature of OOP and SystemVerilog.
Don’t get the handle confused with the object.

In Sample 5.8, t1 first points to one object, then another. Fig. 5.1 shows the
resulting handles and objects.
Sample 5.8 Allocating multiple objects

t1

Second
Transaction
object

t2

First
Transaction
object

Fig. 5.1 Handles and objects after allocating multiple objects

Why would you want to create objects dynamically? During a simulation you
may need to create hundreds or thousands of transactions. SystemVerilog lets you
create objects automatically, when you need them. In Verilog, you would have to
use a fixed-size array large enough to hold the maximum number of transactions.
Note that this dynamic creation of objects is different from anything else offered
before in the Verilog language. An instance of a Verilog module and its name are
bound together statically during compilation. Even with automatic variables, which
come and go during simulation, the name and storage are always tied together.

5.7

Object Deallocation

139

An analogy for handles is people who are attending a conference. Each person is
similar to an object. When you arrive, a badge is “constructed” by writing your
name on it. This badge is a handle that can be used by the organizers to keep track
of each person. When you take a seat for the lecture, space is allocated. You may
have multiple badges for attendee, presenter, or organizer. When you leave the conference, your badge may be reused by writing a new name on it, just as a handle can
point to different objects through assignment. Lastly, if you lose your badge and
there is nothing to identify you, you will be asked to leave. The space you take, your
seat, is reclaimed for use by someone else.

5.7

Object Deallocation

Now you know how to create an object — but how do you get rid of it? For example,
your testbench creates and sends thousands of transactions, such as packets, instructions, frames, interrupts, etc. into your DUT. Once you know the transaction has
completed successfully, you don’t need to keep it around. You should reclaim the
memory; otherwise, a long simulation might run out of memory.
Garbage collection is the process of automatically freeing objects that are no
longer referenced. One way SystemVerilog can tell if an object is no longer being
used is by keeping track of the number of handles that point to it. When the last
handle no longer references an object, SystemVerilog releases the memory for it.
(The actual algorithm to find unused objects varies between simulators. This section
describes reference counting, which is the easiest to understand).
Sample 5.9 Creating multiple objects

The second line in Sample 5.9 calls new() to construct an object and store the
address in the handle t. The next call to new() constructs a second object and stores
its address in t, overwriting the previous value. Since there are no handles pointing
to the first object, SystemVerilog can deallocate it. The object may be deleted immediately, or after a short wait. The last line explicitly clears the handle so that now the
second object can be deallocated.
If you are familiar with C++, these concepts of objects and handles are familiar,
but there are some important differences. A SystemVerilog handle can only point to
objects of one type, so they are called “type-safe.” In C, a typical void pointer is only
an address in memory, and you can set it to any value or modify it with operators
such as pre-increment. You cannot be sure that a pointer is valid. A C++ typed pointer
is much safer, but you may be tempted by C’s flexibility. SystemVerilog does not

5 Basic OOP

140

allow any modification of a handle or using a handle of one type to refer to an object
of another type. (SystemVerilog’s OOP specification is closer to Java than C++).
Since SystemVerilog garbage collects an object when no more handles refer to it,
you can be sure your code always uses valid handles. In C / C++, a pointer can refer
to an object that no longer exists. Garbage collection in those languages is manual, so
your code can suffer from “memory leaks” when you forget to deallocate objects.
SystemVerilog cannot garbage collect an object that is still referenced somewhere by a handle. For example, if you keep objects in
a linked list, SystemVerilog cannot deallocate the objects until you
manually clear all handles by setting them to null. If an object
contains a routine that forks off a thread, the object is not deallocated while the thread is running. Likewise, any objects that are
used by a spawned thread may not be deallocated until the thread terminates. See
Chapter 7 for more information on threads.

5.8

Using Objects

Now that you have allocated an object, how do you use it? Going back to the Verilog
module analogy, you can refer to variables and routines in an object with the “.”
notation, as shown in Sample 5.10.
Sample 5.10 Using variables and routines in an object

In strict OOP, the only access to variables in an object should be through accessor
functions such as get() and put(). This is because accessing variables directly
limits your ability to change the underlying implementation in the future. If a better
(or simply different) algorithm comes along in the future, you may not be able to
adopt it because you would also need to modify all of the references to the variables.
The problem with this methodology is that it was written for large
software applications with lifetimes of a decade or more. With dozens
of programmers making modifications, stability is paramount.
However, you are creating a testbench, where the goal is maximum
control of all variables to generate the widest range of stimulus values.
One of the ways to accomplish this is with constrained-random stimulus generation, which cannot be done if a variable is hidden behind a screen of methods. While the get() and put() methods are fine for compilers, GUIs, and APIs,
you should stick with public variables that can be directly accessed anywhere in
your testbench.

5.9

Class Methods

141

The exception to this rule is for verification IP that is created and maintained by
a group such as a company that has no direct relationship to the end user. For example, if you purchase a PCI transactor from another company, they will restrict access
to the internals, forcing you to treat it as a black box. The developer must give you
enough methods to generate both good transactions and inject all flavors of errors.

5.9

Class Methods

A method in a class is just a task or function defined inside the scope of the class.
Sample 5.11 defines display() methods for the Transaction and PCI_Tran.
SystemVerilog calls the correct one, based on the handle type.
Sample 5.11 Routines in the class

A method in a class uses automatic storage by default, so you don’t have to worry
about remembering the automatic modifier.

5 Basic OOP

142

5.10

Defining Methods Outside of the Class
A good rule of thumb is you should limit a piece of code to one
“page” or screen in your favorite editor to keep it understandable.
You may be familiar with this rule for routines, but it also applies
to classes. If you can see everything in a class on the screen at one
time, it is easier to understand.

However, if each method takes a page, how can the whole class fit on a page? In
SystemVerilog you can break a method into the prototype (method name and arguments) inside the class, and the body (the procedural code) that goes after the class.
Here is how you create out-of-block declarations. Copy the first line of the
method, with the name and arguments, and add the extern keyword at the beginning. Now take the entire method and move it after the class body, and add the class
name and two colons (:: the scope operator) before the method name. The above
classes could be defined as shown in Sample 5.12.
Sample 5.12 Out-of-block method declarations

A common coding mistake is when the prototype does not match
the out-of-body. SystemVerilog requires that the prototype be
identical to the out-of-block method declaration, except for the
class name and scope operator, ::. The prototype can have qualifiers such as local, protected, or virtual, but not the out-of-body. If any
arguments have default values, they must be given in the prototype, but they are
optional in the out-of-body.

5.11 Static Variables vs. Global Variables

143

Another common mistake is to leave out the class name when you
declare the method outside of the class. As a result, it is defined at
the next higher scope (probably the program or package scope),
and the compiler gives an error when the task tries to access classlevel variables and methods. This is shown in Sample 5.13.
Sample 5.13 Out-of-body method missing class name

5.11

Static Variables vs. Global Variables

Every object has its own local variables that are not shared with any other object. If
you have two Transaction objects, each has its own addr, csm, and data variables.
Sometimes though, you need a variable that is shared by all objects of a certain type.
For example, you might want to keep a running count of the number of transactions
that have been created. Without OOP, you would probably create a global variable.
Then you would have a global variable that is used by one small piece of code, but
is visible to the entire testbench. This “pollutes” the global name space and makes
variables visible to everyone, even if you want to keep them local.

5.11.1

A Simple Static Variable

In SystemVerilog you can create a static variable inside a class. This variable is
shared amongst all instances of the class, but its scope is limited to the class. In
Sample 5.14, the static variable count holds the number of objects created so far. It
is initialized to 0 in the declaration because there are no transactions at the beginning of the simulation. Each time a new object is constructed, it is tagged with a
unique value, and count is incremented.

5 Basic OOP

144
Sample 5.14 Class with a static variable

In Sample 5.14, there is only one copy of the static variable count, regardless of
how many Transaction objects are created. You can think that count is stored
with the class and not the object. The variable id is not static, so every Transaction
has its own copy, as shown in Fig. 5.2. Now you don’t need to make a global variable for the count.

class Transaction;
static int count = 0;
int id;
endclass

id: 0
count
id: 1
count

Fig. 5.2 Static variables in a class

Using the ID field is a good way to track objects as they flow
through a design. When debugging a testbench, you often need a
unique value. SystemVerilog does not let you print the address of
an object, but you can make an ID field. Whenever you are tempted
to make a global variable, consider making a class-level static variable. A class should be self-contained, with as few outside references as possible.

5.11.2

Accessing Static Variables Through the Class Name

Sample 5.14 showed how you can reference a static variable using a handle. You
don’t need a handle; you could use the class name followed by ::, the class scope
resolution operator, shown in Sample 5.15.

5.11 Static Variables vs. Global Variables

145

Sample 5.15 The class scope resolution operator

5.11.3

Initializing Static Variables

A static variable is usually initialized in the declaration. You can’t easily initialize it
in the class constructor, as this is called for every single new object. You would need
another static variable to act as a flag, indicating whether the original variable had
been initialized. If you have a more elaborate initialization, you could use an initial
block. Make sure static variables are initialized before the first object is constructed.
Another use for a static variable is when every instance of a class needs information from a single object. For example, a transaction class may refer to a configuration object that has the number of transactions. If you have a non-static handle in the
Transaction class, every object will have its own copy, wasting space. Sample
5.16 shows how to use a static variable instead.
Sample 5.16 Static storage for a handle

5.11.4

Static Methods

As you employ more static variables, the code to manipulate them may grow into a full
fledged routine. In SystemVerilog you can create a static method inside a class that can
read and write static variables, even before the first instance has been created.
Sample 5.17 has a simple static function to display the values of the static variables.
SystemVerilog does not allow a static method to read or write non-static variables,
such as id. You can understand this restriction based on the code below. When the
function display_statics is called at the end of the example, no Transaction
objects have been constructed, so no storage has been created for id variables.

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146
Sample 5.17 Static method displays static variable

5.12

Scoping Rules

When writing your testbench, you need to create and refer to many variables.
SystemVerilog follows the same basic rules as Verilog, with a few helpful
improvements.
A scope is a block of code such as a module, program, task, function, class, or
begin/end block. The for and foreach loops automatically create a block so
that an index variable can be declared or created local to the scope of the loop.
You can only define new variables in a block. New in SystemVerilog is the ability to declare a variable in an unnamed begin-end block.
A name can be relative to the current scope or absolute starting with $root. For
a relative name, SystemVerilog looks up the list of scopes until it finds a match. If
you want to be unambiguous, use $root at the start of a name. Variables can not be
declared in $root, that is, outside of any module, program or package.
Sample 5.18 uses the same name in several scopes. Note that in actual code, you
would use more meaningful names! The name limit is used for a global variable,
a program variable, a class variable, a function variable, and a local variable in an
initial block. The latter is in an unnamed block, so the label created is tool dependent, along with the signal’s hierarchical name.

5.12

Scoping Rules

147

Sample 5.18 Name scope

For testbenches, you can declare variables in the program or in the initial
block. If a variable is only used inside a single initial block, such as a counter,
you should declare it there to avoid possible name conflicts with other blocks. Note
that if you declare a variable in an unnamed block, such as the initial in Sample
5.18, there is no hierarchical name that works consistently across all tools.
Declare your classes outside of any program or module in a
package. This approach can be shared by all the testbenches, and
you can declare temporary variables at the innermost possible
level. This style also eliminates a common bug that happens when
you forget to declare a variable inside a class. SystemVerilog looks
for that variable in higher scopes.
If a block uses an undeclared variable, and another variable with
that name happens to be declared in the program block, the class
uses it instead, with no warning. In Sample 5.19, the function
Bad::display did not declare the loop variable i, so SystemVerilog uses the program level i instead. Calling the function changes the value of
test.i, probably not what you want!

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Sample 5.19 Class uses wrong variable

If you move the class into a package, the class cannot see the program-level
variables, and thus won’t use them unintentionally as shown in Sample 5.20.
Sample 5.20 Move class into package to find bug

5.12.1

What is This?

When you use a variable name, SystemVerilog looks in the current scope for it, and
then in the parent scopes until the variable is found. This is the same algorithm
used by Verilog. What if you are deep inside a class and want to unambiguously
refer to a class-level object? This style code is most commonly used in constructors,

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where the programmer uses the same name for a class variable and an argument.
In Sample 5.21, the keyword “this” removes the ambiguity to let SystemVerilog
know that you are assigning the local variable, name, to the class variable, name.
Sample 5.21 Using this to refer to class variable

Some people think this argument naming style makes the code easier to read;
others think it is a shortcut by a lazy programmer.

5.13

Using One Class Inside Another

A class can contain an instance of another class, using a handle to an object. This is
just like Verilog’s concept of instantiating a module inside another module to build
up the design hierarchy. A common reason for using containment are code reuse and
controlling complexity. For example, every one of your transactions may have a
statistics block, including timestamps indicating when the transaction started and
ended transmission, and information about all transactions, as shown in Fig. 5.3 and
Sample 5.22.

class Transaction;
bit [31:0] addr, crc, data[8];
Statistics stats;
endclass
class Statistics;
time startT, stopT;
static int ntrans= 0;
static time total_elapsed_time;
endclass
Fig. 5.3 Contained objects Sample 5.22

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Sample 5.22 Statistics class declaration

Now you can use the Statistics class inside another class such as a transaction as
can been seen in Sample 5.23.

Sample 5.23 Encapsulating the Statistics class

The outermost class, Transaction, can refer to things in the Statistics class
using the usual hierarchical syntax, such as stats.startT.
Remember to instantiate the object; otherwise, the handle stats is null and the
call to start fails. This is best done in the constructor of the outer class,
Transaction.

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As your classes become larger, they may become hard to manage. When your
variable declarations and method prototypes grow larger than a page, you should
see if there is a logical grouping of items in the class so that it can be split into
several smaller ones.
This is also a potential sign that it’s time to refactor your code, i.e., split it into
several smaller, related classes. See Chapter 8 for more details on class inheritance.
Look at what you’re trying to do in the class. Is there something you could move
into one or more base classes, i.e., decompose a single class into a class hierarchy?
A classic indication is similar code appearing at various places in the class. You
need to factor that code out into a function in the current class, one of the current
class’s parent classes, or both.

5.13.1

How Big or Small Should My Class Be?

Just as you may want to split up classes that are too big, you should
also have a lower limit on how small a class should be. A class with
just one or two members makes the code harder to understand as it
adds an extra layer of hierarchy and forces you to constantly jump
back and forth between the parent class and all the children to
understand what it does. In addition, look at how often it is used. If a small class is
only instantiated once, you might want to merge it into the parent class.
One Synopsys customer put each transaction variable into its own class for fine
control of randomization. The transaction had a separate object for the address,
checksum, data, etc. In the end, this approach only made the class hierarchy more
complex. On the next project they flattened the hierarchy.
See Section 8.4 for more ideas on partitioning classes.

5.13.2

Compilation Order Issue

Sometimes you need to compile a class that includes another class that is not yet
defined. The declaration of the handle causes an error, as the compiler does not
recognize the new type. Declare the class name with a typedef statement, as shown
in Sample 5.24.

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Sample 5.24 Using a typedef class statement

5.14

Understanding Dynamic Objects

In a statically allocated language such as Verilog, every signal has a unique variable
associated with it. For example, there may be a wire called grant, the integer
count, and a module instance i1. In OOP, there is not the same one-to-one correspondence. There can be many objects, but only a few named handles. A testbench
may allocate a thousand transaction objects during a simulation, but may only have
a few handles to manipulate them. This situation takes some getting used to if you
have only written Verilog code.
In reality, there is a handle pointing to every active object. Some handles may be
stored in arrays or queues, or in another object, like a linked list. For objects stored
in a mailbox, the handle is in an internal SystemVerilog structure. See Section 7.6
for more information on mailboxes. Remember that as soon as you assign a new
value to the last handle pointing to an object, that object can be garbage collected.

5.14.1

Passing Objects and Handles to Methods

What happens when you pass an object into a method? Perhaps the method only
needs to read the values in the object, such as transmit above. Or, your method
may modify the object, like a method to create a packet. Either way, when you call
the method, you pass a handle to the object, not the object itself.
task generator;
Transaction t;
t = new;
transmit(t);
endtask
task transmit(input Transaction t);
…
endtask
Fig. 5.4 Handles and objects across methods

Transaction

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In Fig. 5.4, the generator task has just called transmit. There are two
handles, generator.t and transmit.t, that both refer to the same object.
When you call a method, if you pass a scalar variable such as a handle into a ref
argument, SystemVerilog passes the address of the variable so the method can modify it. If you don’t use ref, SystemVerilog copies the scalar’s value into the argument variable, so any change to the argument in the method does not affect the
original value.
Sample 5.25 Passing objects

In Sample 5.25, the initial block allocates a Transaction object and calls the
transmit task with the handle that points to the object. Using this handle, transmit can read and write values in the object. However, if transmit tries to modify
the handle, the result won’t be seen in the initial block, as the t argument was not
declared as ref.
A method can modify an object, even if the handle argument
does not have a ref modifier. This frequently causes confusion
for new users, as they mix up the handle with the object. As
shown above, transmit can modify data[0] in the object
without changing the value of t. If you don’t want an object modified in a method,
pass a copy of it so that the original object is untouched. See Section 5.15 for more
on copying objects.

5.14.2

Modifying a Handle in a Task
A common coding mistake is to forget to use ref on method
arguments that you want to modify, especially handles. In
Sample 5.26, the argument tr is not declared as ref, so any
change to it is not be seen by the calling code. The argument tr
has the default direction of input.

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Sample 5.26 Bad transaction creator task, missing ref on handle

Even though create modified the argument tr, the handle t in the calling
block remains null. You need to declare the argument tr as ref as can be seen in
Sample 5.27.
Sample 5.27 Good transaction creator task with ref on handle

If a method is only going to modify the properties of the object, the
method should declare the handle as an input argument. If a method
is going to modify the handle, for example to make it point to a new
object, the method must declare the handle as a ref argument.

5.14.3

Modifying Objects in Flight
A very common mistake is forgetting to create a new object for
each transaction in the testbench. In Sample 5.28, the generate_
bad task creates a Transaction object with random values, and
transmits it into the design over several cycles.

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155

Sample 5.28 Bad generator creates only one object

What are the symptoms of this mistake? The code above creates only one
Transaction, so every time through the loop, generator_bad changes the object
at the same time it is being transmitted. When you run this, the $display shows
many addr values, but all transmitted Transaction objects have the same value
of addr. The bug becomes visible when transmit spawns off a thread that takes

several cycles to send the transaction, and so the values in the object are re-randomized
in the middle of transmission. If your transmit task makes a copy of the object,
you can recycle the same object over and over. This bug can also happen with
mailboxes as shown in Sample 7.32
To avoid this bug, you need to create a new Transaction during each pass
through the loop as seen in Sample 5.29.
Sample 5.29 Good generator creates many objects

5.14.4

Arrays of Handles

As you write testbenches, you need to be able to store and reference many objects.
You can make arrays of handles, each of which refers to an object. Sample 5.30
shows storing ten bus transaction handles in an array.

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Sample 5.30 Using an array of handles

The array tarray is made of handles, not objects. So you need to construct each
object in the array before using it, just as you would for a normal handle. There is
no way to call new on an entire array of handles.
There is no such thing as an “array of objects”, though you may use this term as
a shorthand for an array of handles that points to objects. Keep in mind that some
handles may be set to null, or that multiple handles could point to a single object.

5.15

Copying Objects

You may want to make a copy of an object to keep a method from modifying the original, or in a generator to preserve the constraints. You can either use the simple, built-in
copy available with new operator or you can write your own for more complex classes.
See Section 8.2 for more reasons why you should make a copy method.

5.15.1

Copying an Object with the New Operator

Copying an object with the new operator is easy and reliable as shown in Sample
5.31. Memory for the new object is allocated and all variables from the existing
object are copied. However any new() function that you may have defined is not
called.
Sample 5.31 Copying a simple class with new

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157

This is a shallow copy, similar to a photocopy of the original, blindly transcribing
values from source to destination. If the class contains a handle to another class,
only the handle’s value is copied by the new operator, not a full copy of the lower
level object. In Sample 5.32, the Transaction class contains a handle to the
Statistics class, originally shown in Sample 5.22.
Sample 5.32 Copying a complex class with new operator

The initial block creates the first Transaction object and modifies a variable in
the contained object stats as shown in Fig. 5.5.

src

id=0
stats

startT=42

dst
Fig. 5.5 Objects and handles before copy with the new operator

When you use the new operator to make a copy, the Transaction object is
copied, but not the Statistics one. This is because the new operator does not call
your own new() function. Instead, the values of variables and handles are copied.
So now both Transaction objects have the same id as shown in Fig. 5.6.

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158

src

dst

id=0
stats

startT=42

id=0
stats

Fig. 5.6 Objects and handles after copy with the new operator

Worse yet, both Transaction objects point to the same Statistics object so
modifying startT with the src handle affects what is seen with the dst handle as
you can see in Figure 5.7.

src

dst

id=0
stats

startT=96

id=0
stats

Fig. 5.7 Both src and dst objects refer to a single statistics object and see updated startT value

5.15.2

Writing Your Own Simple Copy Function

If you have a simple class that does not contain any references to other classes, writing
a copy function is easy as you can see in Samples 5.33 and 5.34. Instead of calling
the new() function and copying each individual variable, the copy function could
have instead used the new operator, but then it would need to replicate any processing
done in new(), such as setting the id.

Sample 5.33 Simple class with copy function

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159

Sample 5.34 Using a copy function

5.15.3

Writing a Deep Copy Function

For nontrivial classes, you should always create your own copy function as seen in
Sample 5.35. You can make it a deep copy by calling the copy functions of all the
contained objects. Your own copy function makes sure all your user fields (such as
id) remain consistent. The downside of making your own copy function is that you
need to keep it up to date as you add new variables – forget one and you could spend
hours debugging to find the missing value.

Sample 5.35 Complex class with deep copy function

The new() constructor is called by copy so every object gets a unique id. Add
a copy() method for the Statistics class as shown in Sample 5.36, and every
other class in the hierarchy.

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160
Sample 5.36 Statistics class declaration

Now when you make a copy of the Transaction object, it will have its own
Statistics object as shown in Sample 5.37.

Sample 5.37 Copying a complex class with new operator

src

dst

id=0
stats

startT=42

id=1
stats

startT=96

Fig. 5.8 Objects and handles after deep copy

The good news is that the UVM data macros create the copy function automatically, so you are spared from having to write them by hand. Manually creating these
is very error prone, especially when you add new variables.

5.15

Copying Objects

5.15.4

161

Packing Objects to and from Arrays Using
Streaming Operators

Some protocols, such as ATM, transmit control and data values one byte at a time.
Before you send out a transaction, you need to pack together the variables in the
object to a byte array. Likewise, after receiving a string of bytes, you need to unpack
them back into a transaction object. For both of these functions, use the streaming
operators, as shown in Section 2.12.
You can’t just stream the entire object as this would gather all properties,
including both data and also meta-data such as timestamps and self-checking
information that you may not want packed. You need to write your own pack
function like the one in Samples 5.38 and 5.39 that only uses the properties that
you choose.
More good news - the UVM data macros create the pack and unpack methods.

Sample 5.38 Transaction class with pack and unpack functions

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162
Sample 5.39 Using the pack and unpack functions

5.16

Public vs. Local

The core concept of OOP is encapsulating data and related methods into a class.
Variables are kept local to the class by default to keep one class from poking around
inside another. A class provides a set of accessor methods to access and modify the
data. This would also allow you to change the implementation without needing to
let the users of the class know. For instance, a graphics package could change its
internal representation from Cartesian coordinates to polar as long as the user interface (accessor methods) have the same functionality.
Consider the Transaction class that has a payload and a checksum so that the
hardware can detect errors. In conventional OOP, you would make a method to set
the payload also set the checksum so they would stay synchronized. Thus your
objects would always be filled with correct values.
However, testbenches are not like other programs, such as a web browser or word
processor. A testbench needs to create errors. You want to have a bad checksum so
you can test how the hardware reacts to errors.
OOP languages such as C++ and Java allow you to specify the visibility of variables and methods. By default, everything in a class is local unless labeled otherwise.
In SystemVerilog, everything is public unless labeled local or
protected. You should stick with this default so you have the
greatest control over the operation of the DUT, which is more
important than long-term software stability. For example, making
the checksum visible allows you to easily inject errors into the

5.18

Building a Testbench

163

DUT. If the checksum was local, you would have to write extra code to bypass the
data-hiding mechanisms, resulting in a larger and more complex testbench.

5.17

Straying Off Course

As a new OOP student, you may be tempted to skip the extra thought needed to
group items into a class, and just store data in a few variables. Avoid the temptation!
A basic DUT monitor samples several values from an interface. Don’t just store them
in some integers and pass them to the next stage. This saves you a few minutes at
first, but eventually you need to group these values together to form a complete transaction. Several of these transactions may need to be grouped to create a higher-level
transaction such as a DMA transfer. Instead, immediately put those interface values
into a transaction class. Now you can store related information (port number, receive
time) along with the data, and easily pass this object to the rest of your testbench.

5.18

Building a Testbench

Now that you have seen the basics of OOP, you can see how to create a layered testbench from a set of classes. Figure 5.9 is the diagram from Chapter 1. Obviously,
the transactions flowing between the blocks are objects, but each block is also
modeled with a class.

Environment

Generator

Agent

Scoreboard

Checker

Driver

Assertions

Monitor

DUT
Fig. 5.9 Layered testbench

Functional Coverage

Test

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164

The Generator, Agent, Driver, Monitor, Checker, and Scoreboard are
all classes, modeled as transactors (described below). They are instantiated inside
the Environment class. For simplicity, the test is at the top of the hierarchy, as is the
program that instantiates the Environment class. The Functional coverage definitions can be put inside or outside the Environment class. See Section 1.10 for a
description of the layered verification environment and its components.
A transactor is made of a simple loop that receives a transaction object from a
previous block, makes some transformations, and sends it to the following one as
you can see in Sample 5.40. Some, such as the Generator, have no upstream
block, so this transactor constructs and randomizes every transaction, while others,
such as the Driver, receive a transaction and send it into the DUT as signal
transitions.

Sample 5.40 Basic Transactor

How do you exchange transactions between blocks? With procedural code you
could have one object call the next, or you could use a data structure such as a FIFO
to hold transactions in flight between blocks. In Chapter 7, you will learn how to use
mailboxes, which are FIFOs with the ability to stall a thread until a new value is
added.

5.19

Conclusion

Using Object-Oriented Programming is a big step, especially if your first computer
language was Verilog. The payoff is that your testbenches are more modular and
thus easier to develop, debug, and reuse.

5.20

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165

Have patience — your first OOP testbench may look more like Verilog with a
few classes added. As you get the hang of this new way of thinking, you begin to
create and manipulate classes for both transactions and the transactors in the testbench that manipulate them.
In Chapter 8 you will learn more OOP techniques so your test can change
the behavior of the underlying testbench without having to change any of the existing code.

5.20

Exercises

1. Create a class called MemTrans that contains the following members, then construct a MemTrans object in an initial block.
a. An 8-bit data_in of logic type
b. A 4-bit address of logic type
c. A void function called print that prints out the value of data_in and address
2. Using the MemTrans class from Exercise 1, create a custom constructor, the new
function, so that data_in and address are both initialized to 0.
3. Using the MemTrans class from Exercise 1, create a custom constructor so that
data_in and address are both initialized to 0 but can also be initialized through
arguments passed into the constructor. In addition, write a program to perform
the following tasks.
a. Create two new MemTrans objects.
b. Initialize address to 2 in the first object, passing arguments by name.
c. Initialize data_in to 3 and address to 4 in the second object, passing arguments by name.
4. Modify the solution from Exercise 3 to perform the following tasks.
a. After construction, set the address of the first object to 4’hF.
b. Use the print function to print out the values of data_in and address for
the two objects.
c. Explicitly deallocate the 2nd object.
5. Using the solution from Exercise 4, create a static variable last_address that
holds the initial value of the address variable from the most recently created
object, as set in the constructor. After allocating objects of class MemTrans (done
in Exercise 4) print out the current value of last_address.
6. Using the solution from Exercise 5, create a static method called print_last_
address that prints out the value of the static variable last_address. After
allocating objects of class MemTrans, call the method print_last_address
to print out the value of last_address.

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5 Basic OOP

7. Given the following code, complete the function print_all in class MemTrans
to print out data_in and address using the class PrintUtilities.
Demonstrate using the function print_all.

5.20

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167

8. Complete the following code where indicated by the comments starting with //.

9. For the following class, create a copy function and demonstrate its use. Assume
the Statistics class has its own copy function.

Chapter 6

Randomization

6.1

Introduction

As designs grow larger, it becomes more difficult to create a complete set of stimuli
needed to check their functionality. You can write a directed testcase to check a
certain set of features, but you cannot write enough directed testcases when the
number of features keeps doubling on each project. Worse yet, the interactions
between all these features are the source for the most devious bugs and are the least
likely to be caught by going through a laundry list of features.
The solution is to create test cases automatically using constrained-random tests
(CRT). A directed test finds the bugs you think are there, but a CRT finds bugs you
never thought about, by using random stimulus. You restrict the test scenarios to
those that are both valid and of interest by using constraints.
Creating the environment for a CRT takes more work than creating one for
directed tests. A simple directed test just applies stimulus, and then you manually
check the result. These results are captured as a golden log file and compared with
future simulations to see whether the test passes or fails. A CRT requires an environment to predict the result, using a reference model, transfer function, or other techniques, plus functional coverage to measure the effectiveness of the stimulus.
However, once this environment is in place, you can run hundreds of tests without
having to hand-check the results, thereby improving your productivity. This tradeoff of test-authoring time (your work) for CPU time (machine work) is what makes
CRT so valuable.
A CRT is made of two parts: the test code that uses a stream of random values to
create input to the DUT, and a seed to the pseudo-random number generator (PRNG),
shown in Section 6.16.1 at the end of this chapter. You can make a CRT behave differently just by using a new seed. This feature allows you to leverage each test so
each is the functional equivalent of many directed tests, just by changing seeds. You
are able to create more equivalent tests using these techniques than with directed
testing.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_6,
© Springer Science+Business Media, LLC 2012

169

6

170

Randomization

You may feel that these random tests are like throwing darts. How do you know
when you have covered all aspects of the design? The stimulus space is too large to
generate every possible input, so you need to generate a useful subset. In Chapter 9
you will learn how to measure verification progress by using functional coverage.
There are many ways to use randomization, and this chapter gives many examples. It highlights the most useful techniques, but choose what works best for you.

6.2

What to Randomize

When you think of randomizing the stimulus to a design, the first thing you may
think of are the data fields. These are the easiest to create – just call $random. The
problem is that this approach has a very low payback in terms of bugs found: you
only find data-path bugs, perhaps with bit-level mistakes. The test is still inherently
directed. The challenging bugs are in the control logic. As a result, you need
to randomize all decision points in your DUT. Everywhere control paths diverge,
randomization increases the probability that you’ll take a different path in each
test case.
You need to think broadly about all design input such as the following items.
•
•
•
•
•
•
•
•

Device configuration
Environment configuration
Primary input data
Encapsulated input data
Protocol exceptions
Delays
Transaction status
Errors and violations

6.2.1

Device Configuration

What is the most common reason why bugs are missed during testing of the RTL
design? Not enough different configurations have been tried! Most tests just use the
design as it comes out of reset, or apply a fixed set of initialization vectors to put it
into a known state. This is like testing a PC’s operating system right after it has been
installed, and without any applications; of course the performance is fine, and there
are no crashes.
Over time, in a real world environment, the DUT’s configuration becomes more
and more random. In a real world example, a verification engineer had to verify a
timedivision multiplexor switch that had 600 input channels and 12 output channels. When the device was installed in the end-customer’s system, channels would
be allocated and deallocated over and over. At any point in time, there was little

6.2

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171

correlation between adjacent channels. In other words, the configuration would
seem random.
To test this device, the verification engineer had to write several dozen lines of
Tcl code to configure each channel. As a result, she was never able to try configurations with more than a handful of channels enabled. Using a CRT methodology, she
wrote a testbench that randomized the parameters for a single channel, and then put
this in a loop to configure the whole device. Now she had confidence that her tests
would uncover bugs that previously would have been missed.

6.2.2

Environment Configuration

The device that you are designing operates in an environment containing other
devices. When you are verifying the DUT, it is connected to a testbench that mimics
this environment. You should randomize the entire environment, including the number of objects and how they are configured.
Another company was creating a PCI switch that connected multiple buses to an
internal memory bus. At the start of simulation the customer used randomization to
choose the number of PCI buses (1–4), the number of devices on each bus (1–8),
and the parameters for each device (master or slave, CSR addresses, etc.). Even
though there were many possible combinations, this company knew all had been
covered.

6.2.3

Primary Input Data

This is what you probably thought of first when you read about random stimulus:
take a transaction such as a bus write or ATM cell and fill it with some random
values. How hard can that be? Actually it is fairly straightforward as long as you
carefully prepare your transaction classes. You should anticipate any layered protocols and error injection.

6.2.4

Encapsulated Input Data

Many devices process multiple layers of stimulus. For example, a device may create
TCP traffic that is then encoded in the IP protocol, and finally sent out inside
Ethernet packets. Each level has its own control fields that can be randomized to try
new combinations. So you are randomizing the data and the layers that surround it.
You need to write constraints that create valid control fields but that also allow
injecting errors.

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6.2.5

Randomization

Protocol Exceptions, Errors, and Violations

Anything that can go wrong, will, eventually. The most challenging part of design
and verification is how to handle errors in the system. You need to anticipate all the
cases where things can go wrong, inject them into the system, and make sure the
design handles them gracefully, without locking up or going into an illegal state.
A good verification engineer tests the behavior of the design to the edge of the functional specification and sometimes even beyond.
When two devices communicate, what happens if the transfer stops partway
through? Can your testbench simulate these breaks? If there are error detection and
correction fields, you must make sure all combinations are tried.
The random component of these errors is that your testbench should be able to
send functionally correct stimuli and then, with the flip of a configuration bit, start
injecting random types of errors at random intervals.

6.2.6

Delays

Many communication protocols specify ranges of delays. The bus grant comes one
to three cycles after request. Data from the memory is valid in the fourth to tenth bus
cycle. However, many directed tests, optimized for the fastest simulation, use the
shortest latency, except for that one test that only tries various delays. Your testbench should always use random, legal delays during every test to try to find that
(hopefully) one combination that exposes a design bug.
Below the cycle level, some designs are sensitive to clock jitter. By sliding the
clock edges back and forth by small amounts, you can make sure your design is not
overly sensitive to small changes in the clock cycle.
The clock generator should be in a module outside the testbench so that it creates
events in the Active region along with other design events. However, the generator
should have parameters such as frequency and offset that can be set by the testbench
during the configuration phase.
Note that the methodology described in this book is for finding functional
errors, not timing errors. Your constrained random testbench should not purposefully violate setup and hold and hold requirements. These are better validated using
timing analysis tools.

6.3

Randomization in SystemVerilog

The random stimulus generation in SystemVerilog is most useful when used with
OOP. You first create a class to hold a group of related random variables, and then
have the random-solver fill them with random values. You can create constraints to
limit the random values to legal values, or to test specific features.

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173

You can randomize individual variables, but this case is the least interesting. True
constrained-random stimuli is created at the transaction level, not one value at a time.

6.3.1

Simple Class with Random Variables

Sample 6.1 shows a packet class with random variables and constraints, plus testbench code that constructs and randomizes a packet.
Sample 6.1 Simple random class

This class has four random variables. The first three use the rand modifier, so
that every time you randomize the class, the variables are assigned a value. Think of
rolling dice where each roll could be a new value or repeat the current one. The
kind variable is randc, which means random cyclic, so that the random solver
does not repeat a random value until every possible value has been assigned. Think
of dealing cards from a deck where you deal out every card in the deck in random
order, then shuffle the deck, and deal out the cards in a different order. Note that the
cyclic pattern is for a single variable. A randc array with five elements has five different patterns, like five decks of cards, dealt in parallel. Simulators are only required
to implement randc variables up to 8 bits wide with 256 different values, but most
support much larger ranges.
A constraint is just a set of relational expressions that must be true for the chosen
value of the variables. In this example, the src variable must be greater than 10 and
less than 15. Note that the constraint expression is grouped using curly braces: {}.
This is because this code is declarative, not procedural, which uses begin…end.
The randomize function returns 0 if a problem is found with the constraints.
The code checks the result and stops simulation with $finish if there is a problem.
Alternatively, you might want to call a special routine to end simulation, after doing
some housekeeping chores like printing a summary report. The rest of the book uses
a macro instead of this extra code.

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Randomization

You should not randomize an object in the class constructor. Your
test may need to turn constraints on or off, change weights, or even
add new constraints before randomization. The constructor is for
initializing the object’s variables, and if you called randomize at
this early stage, you might end up throwing away the results.
Variables in your classes should be random and public. This gives
your test the most control over the DUT’s stimulus and control.
You can disable randomization of a variable, as shown in Section
6.11.2. If you forget to make a variable random, you must edit
the environment, which you want to avoid. The exception is that
configuration variables such as weights and limits should not be
random in transaction classes as their values are chosen at the start of simulation and
do not change.

6.3.2

Checking the Result from Randomization
The randomize function assigns random values to any variable in
the class that has been labeled as rand or randc, and also makes
sure that all active constraints are obeyed. Randomization can fail
if your code has conflicting constraints (see next section), so you
should always check the status. If you don’t check, the variables
may get unexpected values, causing your simulation to fail.

The remaining code samples in this book employ the macro in Sample 6.2 to check
for the result of randomization. If you adopt this style, you can easily add code to give
meaningful error messages and gracefully wind down simulation. The macro shows
off several coding tricks, including wrapping the generated code in a do…while statement so it can be used like a normal statement terminated with a semicolon, including
in an if-else statement, something that VMM log macros did right, but not OVM.
Sample 6.2 Randomization check macro and example

6.4

Constraint Details

6.3.3

175

The Constraint Solver

The process of solving constraint expressions is handled by the SystemVerilog
constraint solver. The solver chooses values that satisfy the constraints. The values
come from SystemVerilog’s PRNG, that is started with an initial seed. If you give a
SystemVerilog simulator the same seed and the same testbench, it should always
produce the same results. Note that changing the tool version or switches such as
debug level can change results. See the exercises at the end of this chapter to see
how to specify the initial seed.
The solver is specific to the simulation vendor, and a constrained-random test
may not give the same results when run on different simulators, or even on different
versions of the same tool. The SystemVerilog standard specifies the meaning of the
expressions, and the legal values that are created, but does not detail the precise
order in which the solver should operate. See Section 6.16 for more details on random number generators.

6.3.4

What can be Randomized?

SystemVerilog allows you to randomize integral variables, that is, variables that
contain a simple set of bits. This includes 2-state and 4-state types, though randomization only generates 2-state values. You can have integers, bit vectors, etc. You
cannot have a random string, or refer to a handle in a constraint. Randomizing real
variables is not yet defined in the LRM.

6.4

Constraint Details

Useful stimulus is more than just random values — there are relationships between
the variables. Otherwise, it may take too long to generate interesting stimulus values, or the stimulus might contain illegal values. You define these interactions in
SystemVerilog using constraint blocks that contain one or more constraint expressions. SystemVerilog chooses random values so that the expressions are true.
At least one variable in each expression should be random, either
rand or randc. The following class fails when randomized,
unless age happens to be in the right range. The solution is to
add the modifier rand or randc before age.
Sample 6.3 Constraint without random variables

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Randomization

The randomize function tries to assign new values to random variables and to
make sure all constraints are satisfied. In Sample 6.3, since there are no random variables, randomize just checks the value of age to see if it is in the bounds specified
by the constraint c_teenager. Unless the variable falls in the range of 13:19, randomize fails. While you can use a constraint to check that a non-random variable has
a valid value, use an assert or if-statement instead. It is much easier to debug your
procedural checker code than read through an error message from the random solver.

6.4.1

Constraint Introduction

Sample 6.4 shows a simple class with random variables and constraints. The specific constructs are explained in the following sections. Notice that in constraint
blocks, you use curly braces, { }, to group together multiple expressions. The
begin…end keywords are for procedural code.
Sample 6.4 Constrained-random class

6.4.2

Simple Expressions

Sample 6.4 showed a constraint block with several expressions. The first two control
the values for the len variable. As you can see, a variable can be used in multiple
expressions.
There should be a maximum of only one operator in an expression, such as <, <=, ==, >=, or >. Sample 6.5 shows a
SystemVerilog gotcha in that it incorrectly tries to generate three
variables in a fixed order.

6.4

Constraint Details

177

Sample 6.5 Bad ordering constraint

Sample 6.6 Result from incorrect ordering constraint

Sample 6.6 shows the results, which are not what was intended. The constraint
bad in Sample 6.5 is broken down into multiple binary relational expressions, going
from left to right: ((lo < med) < hi). First, the expression (lo < med) is evaluated,
which gives 0 or 1. Then hi is constrained to be greater than the result. The variables
lo and med are randomized but not constrained. The correct constraint is shown in

Sample 6.7. For more examples, see Sutherland (2007).
Sample 6.7 Constrain variables to be in a fixed order

6.4.3

Equivalence Expressions

The most common mistake with constraints is trying to make
an assignment in a constraint block, which can only contain
expressions. Instead, use the equivalence operator to set a random variable to a value, e.g., len==42. You can build complex
relationships between one or more random variables: len == (header.addr_
mode * 4 + payload.size()).

6.4.4

Weighted Distributions

A bug in the DUT may be found with constrained random stimulus if you apply
enough patterns. However, it may take a long time for a particular corner case to be
generated. When reviewing functional coverage result, see if corner cases are being
generated. If not, you can use a weighted distribution to skew the stimulus in a

178

6

Randomization

particular direction, and thus accelerate finding bugs. The dist operator allows you
to create weighted distributions so that some values are chosen more often than
others.
The dist operator takes a list of values and weights, separated by the : = or
the : / operator. The values and weights can be constants or variables. The values
can be a single value or a range such as [lo:hi]. The weights are not percentages
and do not have to add up to 100. The : = operator specifies that the weight is the
same for every specified value in the range, whereas the : / operator specifies that
the weight is to be equally divided between all the values.
Sample 6.8 Weighted random distribution with dist

In Sample 6.8, src gets the value 0, 1, 2, or 3. The weight of 0 is 40, whereas, 1,
2, and 3 each have the weight of 60, for a total of 220. The probability of choosing
0 is 40/220, and the probability of choosing 1, 2, or 3 is 60/220 each.
Next, dst gets the value 0, 1, 2, or 3. The weight of 0 is 40, whereas 1, 2, and 3
share a total weight of 60, for a total of 100. The probability of choosing 0 is 40/100,
and the probability of choosing 1, 2, or 3 is only 20/100 each.
Once again, the values and weights can be constants or variables. You can use
variable weights to change distributions on the fly or even to eliminate choices by
setting the weight to zero, as shown in Sample 6.9.

6.4

Constraint Details

179

Sample 6.9 Dynamically changing distribution weights

In Sample 6.9, the len enumerated variable has three values. With the default
weighting values, longword lengths are chosen more often, as w_lwrd has the largest value. Don’t worry, you can change the weights on the fly during simulation to
get a different distribution.

6.4.5

Set Membership and the Inside Operator

You can create sets of values with the inside operator. The SystemVerilog solver
chooses between the values in the set with equal probability, unless you have other
constraints on the variable. As always, you can use variables in the sets.
Sample 6.10 Random sets of values

In Sample 6.10, SystemVerilog uses the values for lo and hi to determine the
range of possible values. You can use the variables as parameters for your constraints so that the testbench can alter the behavior of the stimulus generator without
rewriting the constraints. Note that if lo > hi, an empty set is formed, and the constraint fails.
If you want any value, as long as it is not inside a set, invert the constraint with
the NOT operator: ! as shown in Sample 6.11.

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Randomization

Sample 6.11 Inverted random set constraint

6.4.6

Using an Array in a Set

Sample 6.12 shows how you can choose from a set of values by storing them in an
array.
Sample 6.12 Random set constraint for an array

This is expanded into the constraints in Sample 6.13.
Sample 6.13 Equivalent set of constraints

Likewise, you can use the NOT operator to tell SystemVerilog to choose any
value except those in an array as shown in Sample 6.14.
Sample 6.14 Choose any value except those in an array

Always make sure your constraints work as you expect. You could create functional coverage groups and generate reports, or print a histogram of values with the
code in Sample 6.15, with the output in Sample 6.16.

6.4

Constraint Details

181

Sample 6.15 Printing a histogram

Sample 6.16 Histogram for inside constraint

Samples 6.17 and 6.18 choose a day of the week from a list of enumerated values. You can change the list of choices on the fly. If you make choice a randc
variable, the simulator tries every possible value before repeating.
Sample 6.17 Class to choose from an array of possible values

182

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Randomization

Sample 6.18 Choosing from an array of values

The name function returns a string with the name of an enumerated value.
If you want to dynamically add or remove values from a set, think
twice before using the inside operator because of its performance. Perhaps you have a set of values that you want to choose
just once. You could use inside to choose values from a queue,
and delete them to slowly shrink the queue. This requires the solver
to solve N constraints, where N is the number of elements left in
the queue. Instead, use a randc variable that is an index into an array of choices as
shown in Samples 6.19 and 6.20. Choosing a randc value takes a short, constant
time, whereas solving a large number of constraints is more expensive, especially if
your array has more than a few dozen elements.
Sample 6.19 Using randc to choose array values in random order

6.4

Constraint Details

183

Sample 6.20 Testbench for randc choosing array values in random order

Note that constraints and routines can be mixed in any order.

6.4.7

Bidirectional Constraints

By now you may have realized that constraint blocks are not procedural code, executing from top to bottom. They are declarative code, all active at the same time. If
you constrain a variable with the inside operator with the set [10:50] and have
another expression that constrains the variable to be greater than 20, SystemVerilog
solves both constraints simultaneously and only chooses values between 21 and 50.
SystemVerilog constraints are solved bidirectionally, which means that the constraints on all random variables are solved concurrently. Adding or removing a constraint on any one variable affects the value chosen for all variables that are related
directly or indirectly. Consider the constraint in Sample 6.21.
Sample 6.21 Bidirectional constraints

The SystemVerilog solver looks at all four constraints simultaneously. The variable
r has to be less than t, which has to be less than 10. However, r is also constrained
to be equal to s, which is greater than 5. Even though there is no direct constraint on

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Randomization

the lower value of t, the constraint on s restricts the choices. Table 6.1 shows the
possible values for these three variables.
Table 6.1 Solutions for bidirectional constraint
Solution
r
s
A
6
6
B
6
6
C
6
6
D
7
7
E
7
7
F
8
8

6.4.8

t
7
8
9
8
9
9

Implication Constraints

Normally, all constraint expressions are active in a block. What if you want to have
an expression active only some of the time? Set the highest address, but only for IO
space mode. SystemVerilog supports two implication operators, -> and if.
Sample 6.22 Constraint block with implication operator

The expression A->B is equivalent to the expression (!A || B). When the implication operator appears in a constraint, the solver picks values for A and B so the
expression is true. Truth Table 6.2 shows the value of the expression for the logical
values of A and B.
Table 6.2 Implication operator truth table
A->B
B=false
B=true
A=false
true
true
A=true
false
true

6.4

Constraint Details

185

When A is true, B must be true, but when A is false, B can be true or false. Note
that this is a partly bidirectional constraint, but that A->B does not imply that B->A.
The two expressions produce different results.
In Sample 6.23, when d==1, the variable e must be 1, but when e==1, d can be
0 or 1.
Sample 6.23 Implication operator

If you add the constraint {e==0;}, the variable d must be 0; But if you add a
constraint {e==1;} the values of d are not constrained, it can still be 0 or 1.
Sample 6.24 shows how Sample 6.22 could be written with an if implication
constraint.
Sample 6.24 Constraint block with if implication operator

The if-else operator is a great way to choose between multiple expressions.
For example, the bus defined in Sample 6.9 might support byte, word, and longword
reads, but only longword writes if written like Sample 6.25.
Sample 6.25 Constraint block with if-else operator

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Randomization

The constraint if (A) B else C; is equivalent to the two constraints (A && B); and
(!A && C);. Sample 6.26 shows how you can chain together multiple choices.
Sample 6.26 Constraint block with multiple if-else operator

6.4.9

Equivalence Operator

The equivalence operator <-> is bidirectional. A<−>B is defined as ((A->B) &&
(B->A)). Table 6.3 is the truth table for the logical values of A and B as constrained
in Sample 6.27.
Table 6.3 Equivalence operator truth table
A<->B
B=false
B=true
A=false
true
false
A=true
false
true
Sample 6.27 Equivalence constraint

When d is true, e must also be true, and when d is false, e must also be false. So this
operator is the same as a logical XNOR. If you start with the constraint d<−>e, and add
a constraint such as d==1, e is set to 1 by the solver. The constraint d<−>e and e==0
cause d to be set to 0 by the solver. If your class has all three of the constraints,
d<−>e, d==1, and e==0, the solver will not be able to choose values for d and e.

6.5

Solution Probabilities

Whenever you deal with random values, you need to understand the probability
of the outcome. SystemVerilog does not guarantee the exact solution found by
the random constraint solver, but you can influence the distribution. Any time you

6.5

Solution Probabilities

187

work with random numbers, you have to look at thousands or millions of values to
average out the noise. Some simulators, such as Synopsys VCS, have multiple solvers to allow you to trade memory usage vs. performance. The distributions will vary
between different simulators. The tables were generated with Synopsys VCS
2011.03.

6.5.1

Unconstrained

Start with two random variables in a class with no constraints as shown in Sample
6.28.
Sample 6.28 Class Unconstrained

Table 6.4 shows the eight possible solutions. Since there are no constraints, each
has the same probability. You have to run thousands of randomizations to see the
actual results approach the listed probabilities.
Table 6.4 Solutions for Unconstrained
Solution
x
y
A
0
0
B
0
1
C
0
2
D
0
3
E
1
0
F
1
1
G
1
2
H
1
3

6.5.2

class
Probability
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8

Implication

In Sample 6.29, the value of y depends on the value of x. This is indicated with the
implication operator in the following constraint. This example and the rest in this
section also behave in the way same with the if implication operator.

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Randomization

Sample 6.29 Class with implication constraint

Table 6.5 shows the possible solutions and probability. You can see that the random solver recognizes that there are eight combinations of x and y, but all the solutions where x==0 (solutions A–D) have been merged together.

Table 6.5 Solutions for Imp1 class
Solution
x
y
Probability
A
0
0
1/2
B
0
1
0
C
0
2
0
D
0
3
0
E
1
0
1/8
F
1
1
1/8
G
1
2
1/8
H
1
3
1/8

6.5.3

Implication and Bidirectional Constraints

Note that the implication operator says that when x==0, y is forced to 0, but when
y==0, there is no constraint on x. However, implication is bidirectional in that if y
were forced to a nonzero value, x would have to be 1. Sample 6.30 has the constraint
y>0, so x can never be 0 and Table 6.6 shows the solutions.

6.5

Solution Probabilities

189

Sample 6.30 Class with implication constraint and additional constraint

Table 6.6 Solutions for Imp2 class
Solution
x
y
Probability
A
0
0
0
B
0
1
0
C
0
2
0
D
0
3
0
E
1
0
0
F
1
1
1/3
G
1
2
1/3
H
1
3
1/3

6.5.4

Guiding Distribution with Solve…Before

You can guide the SystemVerilog solver using the “solve…before” constraint as
seen in Sample 6.31.
Sample 6.31 Class with implication and solve…before

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Randomization

The solve…before constraint does not change the solution space, just the probability of the results. The solver chooses values of x (0, 1) with equal probability.
In 1000 calls to randomize, x is 0 about 500 times, and 1 about 500 times. When
x is 0, y must be 0. When x is 1, y can be 0, 1, 2, or 3 with equal probability as
shown in Table 6.7.
Table 6.7 Solutions for solve x before y
constraint
Solution
x
y
Probability
A
0
0
1/2
B
0
1
0
C
0
2
0
D
0
3
0
E
1
0
1/8
F
1
1
1/8
G
1
2
1/8
H
1
3
1/8

If you use the constraint solve y before x, you get a very different distribution as shown in Table 6.8.
Table 6.8 Solutions for solve y before x
constraint
Solution
x
y
Probability
A
0
0
1/8
B
0
1
0
C
0
2
0
D
0
3
0
E
1
0
1/8
F
1
1
1/4
G
1
2
1/4
H
1
3
1/4

Only use solve…before if you are dissatisfied with how often
some values occur. Excessive use can slow the constraint solver
and make your constraints difficult for others to understand.

For the simple class in Sample 6.31, the equivalence operator, <->, gives the same
solution as the implication operator ->. Try adding additional constraints and plot
the results for your favorite simulator.

6.6 Controlling Multiple Constraint Blocks

6.6

191

Controlling Multiple Constraint Blocks

A class can contain multiple constraint blocks. One block might ensure you have a
valid transaction, as described in Section 6.7, but you might need to disable this
when testing the DUT’s error handling. Or you might want to have a separate constraint for each test. Perhaps one constraint would restrict the data length to create
small transactions (great for testing congestion), whereas another would make long
transactions.
You can turn constraints on and off with the constraint_mode function. You can
control a single constraint with handle. constraint. constraint_mode (arg).
To control all constraints in an object, use handle. constraint_mode (arg), as
shown in Sample 6.32. When the argument for constraint_mode is 0, the constraint is turned off, and when it is 1, the constraint is turned on.
Sample 6.32 Using constraint_mode

While many small constraints may give you more flexibility, the process of turning them on and off is more complex. For example, when you turn off all constraints
that create data, you are also disabling all the ones that check the data’s validity.
If you just want to make a random variable non-random, use rand_mode as
described in Section 6.11.2.

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192

6.7

Randomization

Valid Constraints

A good randomization technique is to create several constraints to ensure the
correctness of your random stimulus, known as “valid constraints.” In Sample
6.33, a bus read-modify-write command is only allowed for a longword data
length.
Sample 6.33 Checking write length with a valid constraint

Now you know the bus transaction obeys the rule. Later, if you want to violate
the rule, use constraint_mode to turn off this one constraint. You can turn these
off with constraint_mode when you want to generate errors. For example, what
if a packet has a zero-length payload? You should have a naming convention to
make these constraints stand out, such as using the prefix valid as shown above.

6.8

In-Line Constraints

As you write more tests, you can end up with many constraints. They can interact
with each other in unexpected ways, and the extra code to enable and disable them
adds to the test complexity. Additionally, constantly adding and editing constraints
to a class could cause problems in a team environment.
Many tests only randomize objects at one place in the code. SystemVerilog
allows you to add an extra constraint using randomize with. This is equivalent to
adding an extra constraint to any existing ones in effect. Sample 6.34 shows a base
class with constraints, then two randomize with statements.

6.9

The pre _ randomize and Post _ Randomize Functions

193

Sample 6.34 The randomize() with statement

The extra constraints are added to the existing ones in effect. Use constraint_
mode if you need to disable a conflicting constraint. Note that inside the with{}
statement, SystemVerilog uses the scope of the class. That is why Sample 6.34 used
just addr, not t.addr.
A common mistake is to surround your in-line constraints with
parenthesis instead of curly braces {}. Just remember that constraint blocks use curly braces, so your in-line constraint must
use them too. Braces are for declarative code.

6.9

The pre_randomize and post_randomize Functions

Sometimes you need to perform an action immediately before every randomize
call or immediately afterwards. For example, you may want to set some nonrandom
class variables (such as limits or weights) before randomization starts, or you may
need to calculate the error correction bits for random data. SystemVerilog lets you
do this with two functions, pre_randomize and post_randomize that are created automatically in any class with random variables.

6.9.1

Building a Bathtub Distribution

For some applications, you want a nonlinear random distribution. For instance,
small and large packets are more likely to find a design bug such as buffer overflow

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Randomization

Probability

than medium-sized packets. So you want a bathtub shaped distribution; high on
both ends, and low in the middle. You could build an elaborate dist constraint, but
it might require lots of tweaking to get the shape you want. Verilog has several functions for nonlinear distribution, such as $dist_exponential, but none for a bathtub. The graph in Fig. 6.1 shows how you can combine two exponential curves to
make a bathtub curve. The pre_randomize method in Sample 6.35 calculates a
point on an exponential curve, then randomly chooses to put this on the left curve,
or right. As you pick points on either the left and right curves, you gradually build a
distribution of the combined values.

Left
Exponential

0

Sum is a
bathtub

Values

Right
Exponential

WIDTH

Fig. 6.1 Building a bathtub distribution
Sample 6.35 Building a bathtub distribution

Every time this object is randomized, the variable value gets updated. Across
many randomizations, you will see the desired nonlinear distribution. Since the
variable is calculated procedurally, not through the random constraint solver, it does
not need the rand modifier.
See Sample 6.64 for another example of post_randomize.

6.10

Random Number Functions

6.9.2

195

Note on Void Functions
The functions pre_randomize and post_randomize can only
call other functions, not tasks that could possibly consume time.
You cannot have a delay in the middle of a call to randomize.
When you are debugging a randomization problem, you can call
your display routines if you planned ahead and made them void
functions.

Chapter 8 describes advanced OOP concepts including extended classes and virtual
methods. The pre_randomize and post_randomize functions are not virtual and
so they are called based on the type of the handle, not the object. Additionally, if your
extended class’s pre_randomize or post_randomize need functionality in the
base class’s pre_randomize and post_randomize functions, they should call
these methods using the super prefix, as in super.pre_randomize.

6.10

Random Number Functions

You can use all the Verilog-1995 distribution functions, plus several that are new for
SystemVerilog. Consult a statistics book for more details on the “dist” functions.
Some of the useful functions include the following.
•
•
•
•
•
•
•

$random — Flat distribution, returning signed 32-bit random
$urandom — Flat distribution, returning unsigned 32-bit random
$urandom_range — Flat distribution over a range
$dist_exponential — Exponential decay, as shown in Fig. 6.1
$dist_normal — Bell-shaped distribution
$dist_poisson — Bell-shaped distribution
$dist_uniform — Flat distribution

The $urandom_range function takes two arguments, an optional low value, and
a high value as shown in Sample 6.36.
Sample 6.36 $urandom range usage

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196

6.11

Randomization

Constraints Tips and Techniques

How can you create constrained-random tests that can be easily modified? There are
several tricks you can use. The most general technique is to use OOP to extend the
original class as described in sections 6.11.8 and 8.2.4 but this requires more planning. So first learn some simple techniques, but keep your mind open to other ways.

6.11.1

Constraints with Variables

Most constraint examples in this book use constants to make them more readable.
In Sample 6.37, length is randomized over a range that uses a variable for the
upper bound.
Sample 6.37 Constraint with a variable bound

By default, this class creates random lengths between 1 and 100, but by changing
the variable max_length, you can vary the upper limit.
You can use variables in the dist constraint to turn on and off values and ranges.
In Sample 6.38, each bus command has a different weight variable.
Sample 6.38 dist constraint with variable weights

By default, this constraint produces each command with equal probability. If you
want to have a greater number of READ8 commands, increase the read8_wt weight
variable. Most importantly, you can turn off generation of a command by dropping
its weight to 0.

6.11

Constraints Tips and Techniques

6.11.2

197

Using Nonrandom Values

If you have a set of constraints that produces stimulus that is almost what you want,
but not quite, you could call randomize, and then set a variable to the value you
want — you don’t have to use the random value. However, your stimulus values
may not be correct according to the constraints you created to check validity.
If there are just a few random variables that you want to override, use the rand_
mode function to make them nonrandom. When you call this method with the argument 0 for a random variable, the rand or randc qualifier is disabled and the
variable’s value is no longer changed by the random solver, but the value is still
checked in if it appears in a constraint. Setting the random mode to 1 turns the qualifier back on so the variable can changed by the solver.
Sample 6.39 rand_mode disables randomization of variables

In Sample 6.39, the packet size is stored in the random variable length. The
first half of the test randomizes both the length variable and the contents of the
payload dynamic array. The second half calls rand_mode to make length a nonrandom variable, sets it to 42, then calls randomize. The constraint sets the payload size at the constant 42, but the array is still filled with random values.

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198

6.11.3

Randomization

Checking Values Using Constraints

If you randomize an object and then modify some variables, you can check that the
object is still valid by checking if all constraints are still obeyed. Call handle.
randomize(null) and SystemVerilog treats all variables as nonrandom (“state
variables”) and just ensures that all constraints are satisfied, i.e all expressions are
true. If any constraints are not satisfied, the randomize function returns 0.

6.11.4

Randomizing Individual Variables

Suppose you want to randomize a few variables inside a class. You can call randomize with the subset of variables. Only those variables passed in the argument
list will be randomized; the rest will be treated as state variables and not randomized. All constraints remain in effect. In Sample 6.40, the first call to randomize
only changes the values of two rand variables med and hi. The second call only
changes the value of med, whereas hi retains its previous value. Surprisingly, you
can pass a non-random variable, as shown in the last call, and low is given a random
value, as long as it obeys the constraint.
Sample 6.40 Randomizing a subset of variables in a class

This trick of only randomizing a subset of the variables is not commonly used in
real testbenches as you are restricting the randomness of your stimulus. You want
your testbench to explore the full range of legal values, not just a few corners.

6.11.5

Turn Constraints Off and On

Sections 6.6 and 6.7 discuss valid constraints and constraint_mode. Turning off
individual constraints is fine for error generation, but should be used in moderation.

6.11

Constraints Tips and Techniques

6.11.6

199

Specifying a Constraint in a Test Using In-Line
Constraints

If you keep adding constraints to a class, it becomes hard to manage and control.
Soon, everyone is checking out the same file from your source control system. Many
times a constraint is only used by a single test, so why have it visible to every test?
One way to localize the effects of a constraint is to use in-line constraints, randomize with, shown in Section 6.8. This works well if your new constraint is additive
to the default constraints. If you follow the recommendations in Section 6.7 to create “valid constraints”, you can quickly constrain valid sequences. For error injection, you can disable any constraint that conflicts with what you are trying to do.
A test that injects a particular flavor of corrupted data would first turn off the particular validity constraint that checks for that error.
There are several tradeoffs with using in-line constraints. The first is that now
your constraints are in multiple locations which can make it more difficult to understand all the active constraints. If you add a new constraint to the original class, it
may conflict with the in-line constraint. The second is that it can be very hard for
you to reuse an in-line constraint across multiple tests. By definition, an in-line
constraint only exists in one piece of code. You could put it in a routine in a separate
file and then call it as needed. At that point it has become nearly the same as an
external constraint.

6.11.7

Specifying a Constraint in a Test with External
Constraints

The body of a constraint does not have to be defined within the class, just as a routine body can be defined externally, as shown in Section 5.10 . Your data class could
be defined in one file, with one empty constraint. Then each test could define its own
version of this constraint to generate its own flavors of stimulus as shown in Samples
6.41 and 6.42.

Sample 6.41 Class with an external constraint

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Sample 6.42 Program defining an external constraint

External constraints have several advantages over in-line constraints. They can
be put in a file and thus reused between tests. An external constraint applies to all
instances of the class, whereas an in-line constraint only affects the single call to
randomize. Consequently, an external constraint provides a primitive way to
change a class without having to learn advanced OOP techniques. Keep in mind that
with this technique, you can only add constraints, not alter existing ones, and you
need to define the external constraint prototype in the original class.
Like in-line constraints, external constraints can cause problems, as the constraints are spread across multiple files. The LRM requires external constraints to be
defined in the same scope as the original class. A class defined in a package must
have its external constraint also defined in the same package, limiting its usefulness.
That is why Sample 6.42 includes the class definition rather than using a package.
A final consideration is what happens when the body for an external constraint is
never defined. The SystemVerilog LRM does not currently specify what should
happen in this case. Before you build a testbench with many external constraints,
find out how your simulator handles missing definitions. Is this an error that prevents simulation, just a warning, or no message at all?

6.11.8

Extending a Class

In Chapter 8, you will learn how to extend a class. With this technique, you can take
a testbench that uses a given class, and swap in an extended class that has additional
or redefined constraints, routines, and variables. See Sample 8.10 for a typical testbench. Note that if you define a constraint in an extended class with the same name
as one in the base class, the extended constraint replaces the base one.
Learning OOP techniques requires a little more study, but the flexibility of this
new approach repays with great rewards.

6.12

Common Randomization Problems

You may be comfortable with procedural code, but writing constraints and understanding random distributions requires a new way of thinking. Here are some issues
you may encounter when trying to create random stimulus.

6.12

Common Randomization Problems

6.12.1

201

Use Signed Variables with Care

When creating a testbench, you may be tempted to use the int, byte, or other
signed types for counters and other simple variables. Don’t use them in random
constraints unless you really want signed values. What values are produced when
the class in Sample 6.43 is randomized? It has two random variables and wants to
make the sum of them 64.
Sample 6.43 Signed variables cause randomization problems

Obviously, you could get pairs of values such as (32, 32) and (2, 62). Additionally,
you could see (−63, 127), as this is a legitimate solution of the equation, even though
it may not be what you wanted. To avoid meaningless values such as negative
lengths, use only unsigned random variables, as shown in Sample 6.44.
Sample 6.44 Randomizing unsigned 32-bit variables

Even this version causes problems, as large values of pkt1_len and pkt2_len,
such as 32'h80000040 and 32'h80000000, wrap around when added together
and give 32'd64 or 32'h40. You might think of adding another pair of constraints
to restrict the values of these two variables, but the best approach is to make them
only as wide as needed, and to avoid using 32-bit variables in constraints. In Sample
6.45, the sum of two 8-bit variables is compared to a 9-bit value.
Sample 6.45 Randomizing unsigned 8-bit variables

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6.12.2

Randomization

Solver Performance Tips

Each constraint solver has its strengths and weaknesses but there are some guidelines that you can follow to improve the speed of your simulations with constrained
random variables. Tools are always being improved, so check with your vendor for
more specific information.
If you just need to fill an array with raw data, don’t use the solver as it has some
overhead choosing values, even for a variable that has no constraints. Don’t declare
these arrays as rand, instead calculate the values in pre_randomize with $urandom
or $urandom_range. These functions calculate a value up to 100 times faster than the
solver, which is important when you need a 1000 values quickly. Generally, the larger
the array, the less important are the individual values, and the less likely that there
is a need to use a solver. Even if you need a non-uniform range of values, or there is
a simple relationship between values, you might be able to employ an if statement.

6.12.3

Choose the Right Arithmetic Operator to Boost Efficiency
Simple arithmetic operators such as addition and subtraction, bit
extracts, and shifts are handled very efficiently by the solver in a
constraint. However, multiplication, division, and modulo are
very expensive with 32-bit values. Remember that any constant
without an explicit size, such as 42, is treated as a 32-bit value,
32'd42.

If you want to generate random addresses that are near a page boundary, where a
page is 4096 bytes, you could write the following code, but the solver may take a
long time to find suitable values for addr if you use the constraint in Sample 6.46.
Sample 6.46 Expensive constraint with mod and unsized variable

Many constants in hardware are powers of 2, so take advantage of this with a bit
extraction rather than division and modulo. Only constrain the bits that matter, not
the upper bits. Likewise, multiplication by a power of two can be replaced by a shift.
Note that some constraint solvers make these optimizations automatically Sample
6.47 replaces the MOD operator with a bit extract.
Sample 6.47 Efficient constraint with bit extract

6.13 Iterative and Array Constraints

6.13

203

Iterative and Array Constraints

The constraints presented so far allow you to specify limits on single variables.
What if you want to randomize an array? The foreach constraint and several array
functions let you shape the distribution of the values.
Using the foreach constraint creates many constraints that can
slow down simulation. A good solver can quickly solve hundreds
of constraints but may slow down with thousands. Especially slow
are nested foreach constraints, as they produce N2 constraints for
an array of size N. See Section 6.13.5 for an algorithm that used
randc variables instead of nested foreach.

6.13.1

Array Size

The easiest array constraint to understand is the size function. In Sample 6.48, you
are specifying the number of elements in a dynamic array or queue.
Sample 6.48 Constraining dynamic array size

Using the inside constraint lets you set a lower and upper boundary on the
array size. In many cases you may not want an empty array, that is, size==0.
Remember to specify an upper limit; otherwise, you can end up with thousands or
millions of elements, which can cause the random solver to take an excessive
amount of time.

6.13.2

Sum of Elements

You can send a random array of data into a design, but you can also use it to control
the flow. Perhaps you have an interface that has to transfer four data words.
The words can be sent consecutively or over many cycles. A strobe signal tells when
the data signal is valid. Figure 6.2 shows some legal strobe patterns, sending four
values over ten cycles.

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Randomization

Fig. 6.2 Random strobe waveforms

You can create these patterns using a random array as shown in Sample 6.49.
Constrain it to have four bits enabled out of the entire range using the sum function.
Sample 6.49 Random strobe pattern class

As you remember from Chapter 2, the sum of an array of single-bit elements
would normally be a single bit, e.g., 0 or 1. Sample 6.49 compares strobe.sum to
a 4-bit value (4'h4), so the sum is calculated with 4-bit precision. The example uses
4-bit precision to store the maximum number of elements, which is 10.

6.13 Iterative and Array Constraints

6.13.3

205

Issues with Array Constraints

The sum function looks simple but can cause several problems because of Verilog’s
arithmetic rules. The following is a simple problem that one of the authors experienced creating constrained random stimulus. You want to generate from one to eight
transactions, such that the total length of all of them is less than 1024 bytes. Sample
6.50 shows a first attempt, 6.51 has the test program, and 6.52 shows the output. The
len field is a byte in the original transaction.
Sample 6.50 First attempt at sum constraint: bad_sum1

Sample 6.51 Program to try constraint with array sum

Sample 6.52 Output from bad_sum1

This generates some smaller lengths, but the sum is sometimes negative and is
always less than 127 — definitely not what you wanted! Sample 6.53 shows another
attempt, but this time replace the byte data type with an unsigned field. The display
function is unchanged. Sample 6.54 shows the output.

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Randomization

Sample 6.53 Second attempt at sum constraint: bad_sum2

Sample 6.54 Output from bad_sum2

Sample 6.53 has a subtle problem. The sum of all transaction lengths is always
less than 256, even though you constrained the array sum to be less than 1024. The
problem here is that in Verilog, the sum of many 8-bit values is computed using an
8-bit result. Sample 6.55 bumps the len field up to 32 bits using the uint type from
Section 2.8 .
Sample 6.55 Third attempt at sum constraint: bad_sum3

Sample 6.56 Output from bad_sum3

Wow – what happened here in Sample 6.56? This is similar to the signed problem
in Section 6.12.1, in that the sum of two very large numbers can wrap around to a
small number. You need to limit the size based on the comparison in the constraint.
Samples 6.57 and 6.58 show the next attempt and result.

6.13

Iterative and Array Constraints

207

Sample 6.57 Fourth attempt at sum constraint: bad_sum4

Sample 6.58 Output from bad_sum4

This does not work either as the individual len fields are more than 8 bits, so the
len values are often greater than 255. You need to specify that each len field is
between 1 and 255, but use a 10-bit field so they sum correctly. This requires constraining every element of the array, as shown in the following section.

6.13.4

Constraining Individual Array and Queue Elements

SystemVerilog lets you constrain individual elements of an array using foreach.
While you might be able to write constraints for a fixed-size array by listing every
element, the foreach style is more compact. The only practical way to constrain a
dynamic array or queue is with foreach as shown in Samples 6.59 and 6.60.
Sample 6.59 Simple foreach constraint: good_sum5

Sample 6.60 Output from good_sum5

6

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Randomization

The addition of the constraint for individual elements fixed the example. Note
that the len array can be 10 or more bits wide, but must be unsigned.
You can specify constraints between array elements as long as you are careful
about the endpoints. The class in Sample 6.61 creates an ascending list of values by
comparing each element to the previous, except for the first.
Sample 6.61 Creating ascending array values with foreach

How complex can these constraints become? Constraints have been written to
solve Einstein’s problem (a logic puzzle with five people, each with five separate
attributes), the Eight Queens problem (place eight queens on a chess board so that
none can capture each other), and even Sudoku.

6.13.5

Generating an Array of Unique Values

How can you create an array of random unique values? If your array has N elements, and the element values range from 0..N-1, you can simply use the array
shuffle function as described in Section 2.6.3 .
What if the range of values is greater than the number of array elements? If you
try to make a randc array, each array element will be randomized independently, so
you are almost certain to get repeated values.
You may be tempted to use a constraint solver to compare every element with
every other with nested foreach loops as shown in Sample 6.62. This creates over
4000 individual constraints, which could slow down simulation.
Sample 6.62 Creating unique array values with foreach

6.13 Iterative and Array Constraints

209

Instead, you should use procedural code as shown in Sample 6.63 with a helper
class containing a randc variable so that you can randomize the same variable over
and over.
Sample 6.63 Creating unique array values with a randc helper class

Samples 6.64 and 6.65 give a more general solution. For example, you may need
to assign ID numbers to N bus drivers, which are in the range of 0 to MAX-1 where
MAX >=N.
Sample 6.64 Unique value generator

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Randomization

Sample 6.65 Class to generate a random array of unique values

Sample 6.66 has a program. Here is a program that uses the UniqueArray
class.
Sample 6.66 Using the UniqueArray class

6.14

Atomic Stimulus Generation vs. Scenario Generation

6.13.6

211

Randomizing an Array of Handles

If you need to create multiple random objects, you might create a random array of
handles. Unlike an array of integers, you need to allocate all the elements before
randomization as the random solver never constructs objects. If you have a dynamic
array, allocate the maximum number of elements you may need, and then use a
constraint to resize the array as shown in Sample 6.67. A dynamic array of handles can
remain the same size or shrink during randomization, but it can never increase in size.
Sample 6.67 Constructing elements in a random array class

The above code works well for a single array randomization. If you need to repeatedly
randomize the same array over and over, allocate the array and construct the elements
in pre_randomize. See Section 5.14.4 for more on arrays of handles.

6.14

Atomic Stimulus Generation vs. Scenario Generation

Up until now, you have seen atomic random transactions. You have learned how to
make a single random bus transaction, a single network packet, or a single processor
instruction. This is a good start, however your job is to verify that the design works

6

212

Randomization

with real-world stimuli. A bus may have long sequences of transactions such as
DMA transfers or cache fills. Network traffic consists of extended sequences of
packets as you simultaneously read e-mail, browse a web page, and download music
from the net, all in parallel. Processors have deep pipelines that are filled with the
code for routine calls, for loops, and interrupt handlers. Generating transactions
one at a time is unlikely to mimic any of these scenarios.

6.14.1

An Atomic Generator with History

The easiest way to create a stream of related transactions is to have an atomic generator base some of its random values on ones from previous transactions. The class
might constrain a bus transaction to repeat the previous command, such as a write,
80% of the time, and also use the previous destination address plus an increment.
You can use the post_randomize function to make a copy of the generated transaction for use by the next call to randomize.
This scheme works well for smaller cases but gets into trouble when you need
information about the entire sequence ahead of time. A DUT may need to know the
length of a sequence of network transactions before it starts.

6.14.2

Random Array of Objects

If you want to generate stimulus for a complex, multi-level protocol, you could
build up a combination of code and arrays of random objects. The UVM and VMM
both allow you to generate random sequences through a sophisticated set of classes
and macros. This section shows a simplified random sequence.
One way to generate random sequences is to randomize an entire array of objects.
You can create constraints that refer to the previous and next objects in the array,
and the SystemVerilog solver solves all constraints simultaneously. Since the entire
sequence is generated at once, you can then extract information such as the total
number of transactions or a checksum of all data values before the first transaction
is sent. Alternatively, you can build a sequence for a DMA transfer that is constrained to be exactly 1024 bytes, and let the solver pick the right number of transactions to reach that goal.
Sample 6.68 shows a simple sequence of transactions, each one with a destination address that is greater than the one before. It builds on the array constraint
shown in Sample 6.61.

6.14

Atomic Stimulus Generation vs. Scenario Generation

213

Sample 6.68 Simple random sequence with ascending values

6.14.3

Combining Sequences

You can combine multiple sequences together to make a more realistic flow of
transactions. For example, for a network device, you could make one sequence that
resembles downloading e-mail, a second that is viewing a web page, and a third that
is entering single characters into web-based form.The techniques to combine these
flows is beyond the scope of this book, but you can learn more from the VMM, as
described in Bergeron, et al. (2005).

6.14.4

Randsequence

You may find it challenging to write random constraints as they don’t execute
sequentially like procedural statements. An alternative way to create random
sequences is to describe the grammar of a protocol with a declarative style using a
syntax similar to BNF (Backus-Naur Form) and random weighted case statements.

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Randomization

SystemVerilog’s randsequence construct resembles the algorithmic code that you
have traditionally used but can still be challenging.
Sample 6.69 generates a sequence called stream. A stream can be either cfg_
read, io_read, or mem_read. The random sequence engine randomly picks one.
The cfg_read label has a weight of 1, io_read has twice the weight and so is
twice as likely to be chosen as cfg_read. The label mem_read is most likely to be
chosen, with a weight of 5.
Sample 6.69 Command generator using randsequence

A cfg_read can be either a single call to cfg_read_task, or a call to the task
followed by another cfg_read. As a result, the task is always called at least once,
and possibly many times.
One big advantage of randsequence is that it is procedural code and you can
debug it by stepping though the execution, or adding $display statements. When
you call randomize for an object, it either all works or all fails, but you can’t see
the steps taken to get to a result.
There are several problems with using randsequence. The code to generate the
sequence is separate and a very different style from the classes with data and constraints used by the sequence. So if you use both randomize and randsequence,
you have to master two different forms of randomization. More seriously, if you
want to modify a sequence, perhaps to add a new branch or action, you have to
modify the original sequence code. You can’t just make an extension. As you will
see in Chapter 8, you can extend a class to add new code, data, and constraints without having to edit the original class.

6.15

Random Control

6.15

215

Random Control

At this point you may be thinking that this process is a great way to create long
streams of random input into your design. Or you may think that this is a lot of work
if all you want to do is occasionally to make a random decision in your code. You may
prefer a set of procedural statements that you can step through using a debugger.

6.15.1

Introduction to randcase

You can use randcase to make a weighted choice between several actions, without
having to create a class and instance. Sample 6.70 chooses one of the three branches
based on the weight. SystemVerilog adds up the weights (1+8+1 = 10), chooses a
value in this range, and then picks the appropriate branch. The branches are not
order dependent, the weights can be variables, and they do not have to add up to
100%. The function $urandom_range is described in Section 6.10.
Sample 6.70 Random control with randcase and $urandom_range

You can write Sample 6.70 using a class and the randomize function. For this
small case, the OOP version in Sample 6.71 is a little larger. However, if this were
part of a larger class, the constraint would be more compact than the equivalent
randcase statement.
Sample 6.71 Equivalent constrained class

6

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Randomization

Code using randcase is more difficult to override and modify than random
constraints. The only way to modify the random results is to rewrite the code or use
variable weights.
Be careful using randcase, as it does not leave any tracks behind. For example,
you could use it to decide whether or not to inject an error in a transaction. The
problem is that the downstream transactors and scoreboard need to know of this
choice. The best way to inform them would be to use a variable in the transaction or
environment. However, if you are going to create a variable that is part of these
classes, you could have made it a random variable and used constraints to change its
behavior in different tests.

6.15.2

Building a Decision Tree with randcase

You can use the randcase statement to create a decision tree. Sample 6.72 has just
two levels of procedural code, but you can see how it can be extended to use more.
Sample 6.72 Creating a decision tree with randcase

6.16

Random Number Generators

6.16

217

Random Number Generators

How random is SystemVerilog? On the one hand, your testbench depends on an
uncorrelated stream of random values to create stimulus patterns that go beyond any
directed test. On the other hand, you need to repeat the patterns over and over during
debug of a particular test, even if the design and testbench make minor changes.

6.16.1

Pseudorandom Number Generators

Verilog uses a simple PRNG that you could access with the $random function. The
generator has an internal state that you can set by providing a seed to $random. All
IEEE-1364-compliant Verilog simulators use the same algorithm to calculate
values.
Sample 6.73 shows a simple PRNG, not the one used by SystemVerilog. The
PRNG has a 32-bit state. To calculate the next random value, square the state to
produce a 64-bit value, take the middle 32 bits, then add the original value.
Sample 6.73 Simple pseudorandom number generator

You can see how this simple code produces a stream of values that seem random,
but can be repeated by using the same seed value. SystemVerilog calls its own
PRNG to generate a new value for randomize and randcase.

6.16.2

Random Stability — Multiple Generators

Verilog has a single PRNG that is used for the entire simulation. What would happen if SystemVerilog kept this approach? Testbenches often have several stimulus
generators running in parallel, creating data for the design under test. If two streams
share the same PRNG, they each get a subset of the random values.

6

218

Randomization

PRNG
PRNG
a
d
g

b,c
e,f
h,i

class Gen1;
Transaction tr;

class Gen2;
Transaction tr1, tr2;

forever @(int1.cb)
tr.randomize()
endclass

forever @(int2.cb)
tr1.randomize();
tr2.randomize();
endclass

Fig. 6.3 Sharing a single random generator

In Fig. 6.3, there are two stimulus generators and a single PRNG producing
values a, b, c, etc. Gen2 has two random objects, so during every cycle, it uses
twice as many random values as Gen1.
A problem can occur when one of the classes changes as shown in Fig. 6.4. Gen1
gets an additional random variable, and so consumes two random values every time
it is called. This approach changes the values used not only by Gen1, but also by
Gen2.

PRNG
PRNG
a,b
e,f
i,j

c,d
g,h
k,l

class Gen1;
Transaction tra, trb;

class Gen2;
Transaction tr1, tr2;

forever @(int1.cb)
tra.randomize();
trb.randomize();
endclass

forever @(int2.cb)
tr1.randomize();
tr2.randomize();
endclass

Fig. 6.4 First generator uses additional values

In SystemVerilog, there is a separate PRNG for every object and thread. Figure 6.5
shows how changes to one object don’t affect the random values seen by others.

6.16

Random Number Generators

PRNG 1
a,b
c,d
e,f

219

PRNG 2
m,n
o,p
q,r

class Gen1;
Transaction tra, trb;

class Gen2;
Transaction tr1, tr2;

forever @(int1.cb)
tra.randomize();
trb.randomize();
endclass

forever @(int2.cb)
tr1.randomize();
tr2.randomize();
endclass

Fig. 6.5 Separate random generators per object

6.16.3

Random Stability and Hierarchical Seeding

In SystemVerilog, every object and thread has its own PRNG and unique seed.
When a new object or thread is started, its PRNG is seeded from its parent’s PRNG.
Thus a single seed specified at the start of simulation can create many streams of
random stimulus, each distinct.
When you are debugging a testbench, you add, delete, and move code. Even with
random stability, your changes may cause the testbench to generate different random values. This can be very frustrating if you are in the middle of debugging a
DUT failure, and the testbench no longer creates the same stimulus. You can minimize the effect of code modifications by adding any new objects or threads after
existing ones. Sample 6.74 shows a routine from testbench that constructs objects,
and runs them in parallel threads.
Sample 6.74 Test code before modification

Sample 6.75 adds a new generator, and runs it in a new thread. The new object is
constructed after the existing ones, and the new thread is spawned after the old ones.

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Randomization

Sample 6.75 Test code after modification

As new code is added, you may not be able to keep the random streams the same as
the old ones, but you might be able to postpone any side effects from these changes.

6.17

Random Device Configuration
An important part of your DUT to test is the configuration of both
the internal DUT settings and the system that surrounds it. As
described in Section 6.2.1, your tests should randomize the environment so that you can be confident it has been tested in as many
modes as possible.

Sample 6.76 shows a random testbench configuration that can be modified as needed
at the test level. The EthCfg class describes the configuration for a 4-port Ethernet
switch. It is instantiated in an environment class, which in turn is used in the test.
The test overrides one of the configuration values, enabling all 4 ports.

6.17

Random Device Configuration

221

Sample 6.76 Ethernet switch configuration class

The configuration class is used in the Environment class during several phases.
The configuration is constructed in the Environment constructor, but not randomized until the gen_cfg phase as shown in Sample 6.77. This allows you to turn
constraints on and off before randomize is called. Afterwards, you can override
the generated values before the build phase creates the virtual components around
the DUT. (The classes such as EthGen and EthMii are not shown).
Sample 6.77 Building environment with random configuration

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Randomization

Now you have all the components to build a test, which is described in a program
block. The test in Sample 6.78 instantiates the environment class and then runs each
step.
Sample 6.78 Simple test using random configuration

You may want to override the random configuration, perhaps to reach a corner
case. The test in Sample 6.79 randomizes the configuration class and then enables
all the ports.

6.18

Conclusion

223

Sample 6.79 Simple test that overrides random configuration

Notice how in Sample 6.77 all generators were constructed, but
only a few were run, depending on the random configuration.
If you only constructed the generators that are in-use, you would
have to surround any reference to gen [i] with a test of in_
use [i], otherwise your testbench would crash when it tried to
refer to the non-existent generator. The extra memory taken up by these generators
that are not used is a small price to pay for a more stable testbench.

6.18

Conclusion

Constrained-random tests are the only practical way to generate the stimulus needed
to verify a complex design. SystemVerilog offers many ways to create a random
stimulus and this chapter presents many of the alternatives.
A test needs to be flexible, allowing you either to use the values generated by
default or to constrain or override the values so that you can reach your goals.
Always plan ahead when creating your testbench by leaving sufficient “hooks” so
that you can steer the testbench from the test without modifying existing code.

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6.19

Randomization

Exercises

1. Write the SystemVerilog code for the following items.
a. Create a class Exercise1 containing two random variables, 8-bit data and
4-bit address. Create a constraint block that keeps address to 3 or 4.
b. In an initial block, construct an Exercise1 object and randomize it.
Check the status from randomization.
2. Modify the solution for Exercise 1 to create a new class Exercise2 so that:
a.
b.
c.
d.

data is always equal to 5
The probability of address==0 is 10%
The probability of address being between [1:14] is 80%
The probability of address==15 is 10%

3. Using the solution to either Exercise 1 or 2, demonstrate its usage by generating
20 new data and address values and check for success from the constraint
solver.
4. Create a testbench that randomizes the Exercise2 class 1000 times.
a. Count the number of times each address value occurs and print the results in
a histogram. Do you see an exact 10% / 80% / 10% distribution? Why or why
not?
b. Run the simulation with 3 different random seeds, creating histograms, and
then comment on the results. Here is how to run a simulation with the seed 42.
VCS: > simv +ntb_random_seed=42
IUS: > irun exercise4.sv −svseed 42
Questa: > vsim −sv_seed 42
5. For the code in Sample 6.4, describe the constraints on the len, dst, and src
variables.
6. Complete Table 6.9 below for the following constraints.

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225
Table 6.9 Solution probabilities
Solution
x
y
Probability
A
0
0
B
0
1
C
0
2
D
0
3
E
1
0
F
1
1
G
1
2
H
1
3

7. For the following class, create:
a. A constraint that limits read transaction addresses to the range 0 to 7,
inclusive.
b. Write behavioral code to turn off the above constraint. Construct and randomize a MemTrans object with an in-line constraint that limits read transaction addresses to the range 0 to 8, inclusive. Test that the in-line constraint
is working.

8. Create a class for a graphics image that is 10x10 pixels. The value for each
pixel can be randomized to black or white. Randomly generate an image that is,
on average, 20% white. Print the image and report the number of pixels of each
type.
9. Create a class, StimData, containing an array of integer samples. Randomize
the size and contents of the array, constraining the size to be between 1 and
1000. Test the constraint by generating 20 transactions and reporting the size.

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Randomization

10. Expand the Transaction class below so back-to-back transactions of the
same type do not have the same address. Test the constraint by generating 20
transactions.

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227

11. Expand the RandTransaction class below so back-to-back transactions of
the same type do not have the same address. Test the constraint by generating
20 transactions.

Chapter 7

Threads and Interprocess Communication

In real hardware, the sequential logic is activated on clock edges, whereas combinational logic is constantly changing when any inputs change. All this parallel activity is
simulated in Verilog RTL using initial and always blocks, plus the occasional gate
and continuous assignment statement. To stimulate and check these blocks, your testbench uses many threads of execution, all running in parallel. Most blocks in your
testbench environment are modeled with a transactor and run in their own thread.
The SystemVerilog scheduler is the traffic cop that chooses which thread runs
next. You can use the techniques in this chapter to control the threads and thus your
testbench.
Each of these threads communicates with its neighbors. In Fig. 7.1, the generator
passes the stimulus to the agent. The environment class needs to know when the
generator completes and then tell the rest of the testbench threads to terminate. This
is done with interprocess communication (IPC) constructs such as the standard
Verilog events, event control and wait statements, and the SystemVerilog mailboxes and semaphores.1

1
The SystemVerilog LRM uses “thread” and “process” interchangeably. The term “process” is
most commonly associated with Unix processes, in which each contains a program running in its
own memory space. Threads are lightweight processes that may share common code and memory,
and consume far fewer resources than a typical process. This book uses the term “thread.” However,
“interprocess communication” is such a common term that it is used in this book.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_7,
© Springer Science+Business Media, LLC 2012

229

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Environment

Generator

Agent
Agent

Scoreboard

Checker

Assertions

Monitor

Driver

DUT
Fig. 7.1 Testbench environment blocks

7.1

Working with Threads

While all the thread constructs can be used in both modules and program blocks,
your testbenches belong in program blocks. As a result, your code always starts with
initial blocks that start executing at time 0. You cannot put an always block in
a program. However, you can easily get around this by using a forever loop in an
initial block.
Classic Verilog has two ways of grouping statements — with a begin…end or
fork…join. Statements in a begin…end run sequentially, whereas those in a
fork…join execute in parallel. The latter is very limited in that all statements
inside the fork…join have to finish before the rest of the block can continue. As a
result, it is rare for Verilog testbenches to use this feature.
SystemVerilog introduces two new ways to create threads — with the fork…
join_none and fork…join_any statements, shown in Fig. 7.2.
fork

join

Fig. 7.2 Fork…join blocks

fork

fork

join_any

join_none

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231

Your testbench communicates, synchronizes, and controls these threads with
existing constructs such as events, @ event control, the wait and disable statements, plus new language elements such as semaphores and mailboxes.

7.1.1

Using fork…join and begin…end

Sample 7.1 has a fork…join parallel block with an enclosed begin…end sequential block, and shows the difference between the two.
Sample 7.1 Interaction of begin…end and fork…join

Parent

initial
$display
#10 $display
fork
.
join
$display
#80 $display

Fig. 7.3 Fork…join block

$display
#50 $display
#10 $display
#30 $display
#10 $display

Child
threads

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In the output below, the code in the fork…join executes in parallel, so statements
with shorter delays execute before those with longer delays. As shown in Sample 7.2,
the fork…join completes after the last statement, which starts with #50.
Sample 7.2 Output from begin…end and fork…join

7.1.2

Spawning Threads with fork…join_none

A fork…join_none block schedules each statement in the block, but execution
continues in the parent thread. Sample 7.3 is identical to Sample 7.1 except that the
join has been converted to join_none.
Sample 7.3 Fork…join_none code

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233

The diagram for this block is similar to Fig. 7.3. Note that the statement after the
join_none block in Sample 7.4 executes before any statement inside the fork…
join_none.
Sample 7.4 Fork…join_none output

7.1.3

Synchronizing Threads with fork…join_any

A fork…join_any block schedules each statement in the block. Then, when the
first statement completes, execution continues in the parent thread. All other remaining threads continue. Sample 7.5 is identical to the previous examples, except that
the join has been converted to join_any.
Sample 7.5 Fork…join_any code

Note in Sample 7.6, the statement $display(“after join_any”) completes
after the first statement in the parallel block.

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Sample 7.6 Output from fork…join_any

7.1.4

Creating Threads in a Class

You can use a fork…join_none to start a thread, such as the code for a random
transactor generator. Sample 7.7 shows a generator / driver class with a run task
that creates N packets. The full testbench has classes for the driver, monitor, checker,
and more, all with transactors that need to run in parallel.
Sample 7.7 Generator / Driver class with a run task

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235

There are several points to note with Sample 7.7. First, the transactor is not started in the new() function. The constructor should
just initialize values, not start any threads. Separating the constructor from the code that does the real work allows you to change
any variables before you start executing the code in the object.
This allows you to inject errors, modify the defaults, and alter the
behavior of the object. Next, the run task starts a thread in a fork…join_none
block. The thread is a part of the transactor and should be spawned there, not in the
parent class.

7.1.5

Dynamic Threads

Verilog’s threads are very predictable. You can read the source code and count the
initial, always, and fork…join blocks to know how many threads were in a
module. On the other hand, SystemVerilog lets you create threads dynamically, and
does not require you to wait for them to finish.
In Sample 7.8, the testbench generates random transactions and sends them to a
DUT that stores them for some predetermined time, and then returns them. The
testbench has to wait for the transaction to complete, but does not want to stop the
generator.
Sample 7.8 Dynamic thread creation

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When the check_trans task is called, it spawns off a thread to watch the bus
for the matching transaction data. During a normal simulation, many of these threads
run concurrently. In this simple example, the thread just prints a message, but you
could add more elaborate controls.

7.1.6

Automatic Variables in Threads

A common but subtle bug occurs when you have a loop that
spawns threads and you don’t save variable values before the
next iteration. Sample 7.8 only works in a program or module
with automatic storage. If check_trans used static storage,
each thread would share the same variable tr, so later calls
would overwrite the value set by earlier ones. Likewise, if the example had the
fork…join_none inside the repeat loop, it would try to match incoming transactions using tr, but its value would change the next time through the loop. Always
use automatic variables to hold values in concurrent threads.
Sample 7.9 has a fork…join_none inside a for loop. SystemVerilog schedules
the threads inside a fork…join_none but they are not executed until after the
original code blocks, here because of the #0 delay. So Sample 7.9 prints “3 3 3”
which are the values of the index variable j when the loop terminates.
Sample 7.9 Bad fork…join_none inside a loop

7.1

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237

Sample 7.10 Execution of bad fork…join_none inside a loop

The #0 delay blocks the current thread and reschedules it to start later during the
current time slot. In Sample 7.10, the delay makes the current thread run after the
threads spawned in the fork…join_none statement. This delay is useful for blocking a thread, but you should be careful, as excessive use causes race conditions and
unexpected results.
You should use automatic variables inside a fork…join statement to save a
copy of a variable as shown in Sample 7.11.
Sample 7.11 Automatic variables in a fork…join_none

The fork…join_none block is split into two parts, declarations and procedural
code. The automatic variable declaration with initialization runs in the thread inside
the for loop. During each loop, a copy of k is created and set to the current value
of j. Then the body of the fork…join_none ($write) is scheduled, including a
copy of k. After the loop finishes, #0 blocks the current thread, so the three threads
run, printing the value of their copy of k. When the threads complete, and there is
nothing else left during the current time-slot region, SystemVerilog advances to the
next statement and the $display executes.

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Threads and Interprocess Communication

Sample 7.12 traces the code and variables from Sample 7.11. The three copies of
the automatic variable k are called k0, k1, and k2 for this sample.
Sample 7.12 Steps in executing automatic variable code

Another way to write Sample 7.11 is to declare the automatic variable outside of
the fork…join_none. Sample 7.13 works inside a program with automatic storage.
Sample 7.13 Automatic variables in a fork…join_none

7.1.7

Waiting for all Spawned Threads

In SystemVerilog, when all the initial blocks in the program are done, the simulator exits. Sample 7.14 shows how you can spawn many threads, which might still
be running. Use the wait fork statement to wait for all child threads.

7.1

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239

Sample 7.14 Using wait fork to wait for child threads

7.1.8

Sharing Variables Across Threads

Inside a class’s routines, you can use local variables, class variables,
or variables defined in the program. If you forget to declare a
variable, SystemVerilog looks up the higher scopes until it finds
a match. This can cause subtle bugs if two parts of the code are
unintentionally sharing the same variable, perhaps because you forgot to declare it
in the innermost scope.
For example, if you like to use the index variable, i, be careful that two different
threads of your testbench don’t concurrently modify this variable by each using it in
a for loop. Or you may forget to declare a local variable in a class, such as Buggy,
shown below. If your program block declares a global i, the class just uses the
global instead of the local that you intended. You might not even notice this unless
two parts of the program try to modify the shared variable at the same time.

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Sample 7.15 Bug using shared program variable

The solution is to declare all your variables in the smallest scope that encloses all
uses of the variable. In Sample 7.15, declare index variables inside the for loops,
not at the program or class level. Better yet, use the foreach statement whenever
possible.

7.2

Disabling Threads

Just as you need to create threads in the testbench, you also need to stop them. The
Verilog disable statement works on SystemVerilog threads. The following sections show how you can asynchronously disable threads. This can cause unexpected
behavior, so you should watch out for side effects when a thread is stopped midstream. You may, instead, want to design your algorithm to check for interrupts at
stable points, then gracefully give up its resources.

7.2

Disabling Threads

7.2.1

241

Disabling a Single Thread

Here is the check_trans task, this time using a fork…join_any plus a disable
to create a watch with a time-out. In this case, you are disabling a labelled block, to
precisely specify what to stop.
The outermost fork…join_none is identical to Sample 7.8. This version implements a time-out with two threads inside a fork…join_any so that the simple wait
statement is executed in parallel with a delayed $display. If the correct bus data
comes back quickly enough, the wait construct completes, the join_any executes,
and then the disable kills off the remaining thread. However, if the bus data does
not get the right value before the TIME_OUT delay completes, the error message is
printed, the join_any executes, and the disable kills the thread with the wait.
Sample 7.16 Disabling a thread

Watch out, as you might unintentionally stop too many threads
with disable label. This statement stops every process executing the labeled block, as might occur if you have multiple driver
or monitor objects running in parallel. If your code only has one
instance, disable label is a safe way to stop a thread.

7.2.2

Disabling Multiple Threads

Sample 7.16 used the classic Verilog disable statement to stop the threads in a
named block. SystemVerilog introduces the disable fork statement so you can
stop all child threads that have been spawned from the current thread.

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Watch out, as you might unintentionally stop too many threads
with disable fork, such as those created from surrounding
task calls. You should always surround the target code with
a fork…join to limit the scope of a disable fork
statement.
The next few samples use the check_trans task from Sample 7.16. You can just
think of this task as doing a #TIME_OUT. Sample 7.17 has an additional begin…end
block inside the fork…join to make the statements sequential.
Sample 7.17 Limiting the scope of a disable fork

Fig. 7.4 shows a diagram of the spawned threads.
initial begin
check_trans(tr0)
fork
...
join
end

thread 0
thread 1
check_trans(tr1)
fork
...
join
#TIME_OUT/2
disable fork

thread 2
thread 3
check_trans(tr2)
thread 4

Fig. 7.4 Fork…join block diagram

The code calls check_trans that starts thread 0. Next a fork…join creates
thread 1. Inside this thread, one is spawned by the check_trans task and one by
the innermost fork…join, which spawns thread 4 by calling the task. After a delay,
a disable fork stops and all the child threads, 2-4. Thread 0 is outside the fork…
join block that has the disable, so it is unaffected.
Sample 7.18 is the more robust version of Sample 7.17, with disable with a
label that explicitly names the threads that you want to stop.

7.2

Disabling Threads

243

Sample 7.18 Using disable label to stop threads

7.2.3

Disable a Task that was Called Multiple Times

Be careful when you disable a block from inside that block - you might end up stopping
more than you expected. As expected, if you disable a task from inside the task, it is like
a return statement, but it also kills all threads started by the task. Additionally, a single
disable label terminates all threads using that code, not just the current one.
In Sample 7.19, the wait_for_time_out task is called three times, spawning
three threads. Then, thread 0 also disables the task after #2ns. When you run this code,
you will see the three threads starting, but none finishes, because of the disable in
thread 0 stops all three threads, not just one. If this task was inside a driver class that
was instantiated multiple times, a disable label in one could stop all the blocks.
Sample 7.19 Using disable label to stop a task

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7

7.3

Interprocess Communication

Threads and Interprocess Communication

All these threads in your testbench need to synchronize and exchange data. At the
most basic level, one thread waits for another, such as the environment object waiting for the generator to complete. Multiple threads might try to access a single
resource such as bus in the DUT, so the testbench needs to ensure that one and only
one thread is granted access. At the highest level, threads need to exchange data
such as transaction objects that are passed from the generator to the agent. All of
this data exchange and control synchronization is called interprocess communication (IPC), which is implemented in SystemVerilog with events, semaphores, and
mailboxes. These are described in the remainder of this chapter.
There are generally three parts to IPC: a producer that creates the information, a
consumer that accepts the information, and the channel that carries the information.
The producer and consumer are in separate threads.

7.4

Events

A Verilog event synchronizes threads. It is similar to a phone, where one person
waits for a call from another person. In Verilog a thread waits for an event with the
@ operator. This operator is edge sensitive, so it always blocks, waiting for the event
to change. Another thread triggers the event with the -> operator, unblocking the
first thread.
System Verilog enhances the Verilog event in several ways. An event is now a
handle to a synchronization object that can be passed around to routines. This feature allows you to share events across objects without having to make the events
global. The most common way is to pass the event into the constructor for an
object.
There is always the possibility of a race condition in Verilog where one thread
blocks on an event at the same time another triggers it. If the triggering thread executes before the blocking thread, the trigger is missed. SystemVerilog introduces the
triggered status that lets you check whether an event has been triggered, including during the current time-slot. A thread can wait on this function instead of blocking with the @ operator.

7.4

Events

7.4.1

245

Blocking on the Edge of an Event

When you run Sample 7.20, one initial block starts, triggers its event, and then
blocks on the other event, as shown in the output in Sample 7.21. The second block
starts, triggers its event (waking up the first), and then blocks on the first event.
However, the second thread locks up because it missed the first event, as it is a zerowidth pulse.
Sample 7.20 Blocking on an event in Verilog

Sample 7.21 Output from blocking on an event

7.4.2

Waiting for an Event Trigger

Instead of the edge-sensitive block @e1, use the level-sensitive wait(e1.triggered). This does not block if the event has been triggered during this time step.
Otherwise, it waits until the event is triggered.

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Sample 7.22 Waiting for an event

When you run Sample 7.22, one initial block starts, triggers its event, and then
blocks on the other event. The second block starts, triggers its event (waking up the
first) and then blocks on the first event, producing the output in Sample 7.23.
Sample 7.23 Output from waiting for an event

Several of these samples have race conditions and may not execute exactly the
same on every simulator. For example, the output in Sample 7.23 assumes that when
the second block triggers e2, execution jumps back to the first block. It would also
be legal for the second block to trigger e2, wait on e1, and display a message before
control is returned back to the first block.

7.4.3

Using Events in a Loop

You can synchronize two threads with an event, but use caution.
If you use wait (handshake.triggered) in a loop, be sure to
advance the time before waiting again. Otherwise your code will
go into a zero delay loop as the wait continues over and over
again on a single event trigger. Sample 7.24 incorrectly uses a
level-sensitive blocking statement for notification that a transaction is ready.

7.4

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247

Sample 7.24 Waiting on event causes a zero delay loop

Just as you learned to always put a delay inside an always block you need to put
a delay in a transaction process loop. The edge-sensitive delay statement in Sample
7.25 continues once and only once per event trigger.
Sample 7.25 Waiting for an edge on an event

You should avoid events if you need to send multiple notifications in a single
time slot, and look at other IPC methods with built-in queuing such as semaphores
and mailboxes, discussed later in this chapter.

7.4.4

Passing Events

As described above, an event in SystemVerilog can be passed as an argument to a
routine. In Sample 7.26, an event is used by a transactor to signal when it has
completed.

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Sample 7.26 Passing an event into a constructor

7.4.5

Waiting for Multiple Events

In Sample 7.26, you had a single generator that fired a single event. What if your
testbench environment class must wait for multiple child processes to finish, such as
N generators? The easiest way is to use wait fork, that waits for all child processes to end. The problem is that this also waits for all the transactors, drivers, and
any other threads that were spawned by the environment. You need to be more
selective. You still want to use events to synchronize between the parent and child
threads.
You could use a for loop in the parent to wait for each event, but that would only
work if thread 0 finished before thread 1, which finished before thread 2, etc. If the
threads finish out of order, you could be waiting for an event that triggered many
cycles ago.
The solution is to make a new thread and then spawn children from there that
each block on an event for each generator, as shown in Sample 7.27. Now you can
do a wait fork because you are being more selective.

7.4

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249

Sample 7.27 Waiting for multiple threads with wait fork

Another way to solve this problem is to keep track of the number of events that
have triggered, as shown in Sample 7.28.
Sample 7.28 Waiting for multiple threads by counting triggers

That was slightly less complicated. Why not get rid of all the events and just wait
on a count of the number of running generators? This count can be a static variable

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Threads and Interprocess Communication

in the Generator class. Note that most of the thread manipulation code has been
replaced with a single wait construct. The last block in Sample 7.29 waits for the
count using the class scope resolution operator, ::. You could have used any handle,
such as gen[0], but that would be less direct.
Sample 7.29 Waiting for multiple threads using a thread count

7.5

Semaphores

A semaphore allows you to control access to a resource. Imagine that you and your
spouse share a car. Obviously, only one person can drive it at a time. You can manage this situation by agreeing that whoever has the key can drive it. When you are
done with the car, you give up the car so that the other person can use it. The key is
the semaphore that makes sure only one person has access to the car. In operating

7.5

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251

system terminology, this is known as “mutually exclusive access,” so a semaphore
is known as a “mutex” and is used to control access to a resource.
Semaphores can be used in a testbench when you have a resource, such as a bus,
that may have multiple requestors from inside the testbench but, as part of the physical design, can only have one driver. In SystemVerilog, a thread that requests a key
when one is not available always blocks. Multiple blocking threads are queued in
FIFO order.

7.5.1

Semaphore Operations

There are three basic operations for a semaphore. You create a semaphore with one
or more keys using the new method, get one or more keys with the blocking task
get(), and return one or more keys with put(). If you want to try to get a semaphore, but not block, use the try_get() function. If keys are available, try_get()
obtains them and returns 1. If there are not sufficient keys, it just returns a 0. Sample
7.30 shows how to control access to a resource with a semaphore.
Sample 7.30 Semaphores controlling access to hardware resource

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7.5.2

Threads and Interprocess Communication

Semaphores with Multiple Keys

There are two things you should watch out for with semaphores. First, you can put
more keys back than you took out. Suddenly you may have two keys but only one car!
Secondly, be careful if your testbench needs to get and put multiple keys. Perhaps you
have one key left, and a thread requests two, causing it to block. Now a second thread
requests a single semaphore – what should happen? In SystemVerilog the second
request, get(1), sneaks ahead of the earlier get(2), bypassing the FIFO ordering.
If you are mixing different sized requests, you can always write your own class.
That way you can be very clear on who gets priority.

7.6

Mailboxes

How do you pass information between two threads? Perhaps your generator needs to
create many transactions and pass them to a driver. You might be tempted to just have
the generator thread call a task in the driver. If you do that, the generator needs to know
the hierarchical path to the driver task, making your code less reusable. Additionally,
this style forces the generator to run at the same speed as the driver, that can cause
synchronization problems if one generator needs to control multiple drivers.
Think of your generator and driver as transactors that are autonomous
objects that communicate through a channel. Each object gets a transaction from an upstream object (or creates it, as in the case of a generator), does some processing, and then passes it to a downstream
object. The channel must allow its driver and receiver to operate asynchronously. You may be tempted to just use a shared array or queue, but it can be
difficult to create threads that read, write, and blocks safely.
The solution is a SystemVerilog mailbox. From a hardware point of view, the easiest
way to think about a mailbox is that it is just a FIFO, with a source and sink. The
source puts data into the mailbox, and the sink gets values from the mailbox.
Mailboxes can have a maximum size or can be unlimited. When the source thread
tries to put a value into a sized mailbox that is full, that thread blocks until the value
is removed. Likewise, if a sink threads tries to remove a value from a mailbox that
is empty, that thread blocks until a value is put into the mailbox.
Figure 7.5 shows a mailbox connecting a generator and driver.

generator

driver
mailbox

Fig. 7.5 A mailbox connecting two transactors

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Mailboxes

253

A mailbox is an object and thus has to be instantiated by calling the new function. This takes an optional size argument to limit the number of entries in the
mailbox. If the size is 0 or not specified, the mailbox is unbounded and can hold an
unlimited number of entries.
You put data into a mailbox with the put() task, and remove it with the blocking
get() task. A put() blocks if the mailbox is full, and get() blocks if the mailbox
is empty. Use try_put() if you want to see if the mailbox is full. and try_get()
to see if it is empty. The peek() task gets a copy of the data in the mailbox but does
not remove it.
The data is a single value, such as an integer, or logic of any size or
a handle. A mailbox never contains objects, only references to
them. By default, a mailbox does not have a type, so you could put
any mix of data into it. Don’t do it! Enforce one data type per mailbox by sticking with parameterized mailboxes as shown in Sample
7.31 to catch type mismatches at compile time.
Sample 7.31 Mailbox declarations

A classic mailbox bug, shown in Sample 7.32, is a loop that randomizes objects and puts them in a mailbox, but the object is
only constructed once, outside the loop. Since there is only one
object, it is randomized over and over.
Sample 7.32 Bad generator creates only one object

Figure 7.6 shows all the handles pointing to a single object. A mailbox only
holds handles, not objects, so you end up with a mailbox containing multiple handles that all point to the single object. The code that gets the handles from the mailbox just sees the last set of random values.

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All handles refer
to the same object
Fig. 7.6 A mailbox with multiple handles to one object

The solution, shown in Sample 7.33, is to make sure your loop has all three steps
of constructing the object, randomizing it, and putting it in the mailbox. This bug is
so common that it is also mentioned in Section 5.14.3.
Sample 7.33 Good generator creates many objects

The result, shown in Fig. 7.7, is that every handle points to a unique object. This
type of generator is known as the Blueprint Pattern and described in Section 8.2.

Fig. 7.7 A mailbox with multiple handles to multiple objects

Sample 7.34 shows the driver that waits for transactions from the generator.
Sample 7.34 Good driver receives transactions from mailbox

7.6

Mailboxes

255

If you don’t want your code to block when accessing the mailbox, use the try_
get() and try_peek() functions. If they are successful, they return a nonzero
value; otherwise, they return 0. These are more reliable than the num() function, as
the number of entries can change between when you measure it and when you next
access the mailbox.

7.6.1

Mailbox in a Testbench

Sample 7.35 shows a program with a Generator and Driver exchanging transactions
using a mailbox.
Sample 7.35 Exchanging objects using a mailbox: the Generator class

7

256

7.6.2

Threads and Interprocess Communication

Bounded Mailboxes

By default, mailboxes are similar to an unlimited FIFO — a producer can put any
number of objects into a mailbox before the consumer gets the objects out. However,
you may want the two threads to operate in lockstep so that the producer blocks
until the consumer is done with the object.
You can specify a maximum size for the mailbox when you construct it. The
default mailbox size is 0 which creates an unbounded mailbox. Any size greater
than 0 creates a bounded mailbox. If you attempt to put more objects than this limit,
put() blocks until you get an object from the mailbox, creating a vacancy.
Sample 7.36 Bounded mailbox

7.6

Mailboxes

257

Sample 7.36 creates the smallest possible mailbox, which can hold a single message.
The Producer thread tries to put three messages (integers) in the mailbox, and the
Consumer thread slowly gets messages every 1ns. As Sample 7.37 shows, the first
put() succeeds, then the Producer tries put(2) which blocks. The Consumer
wakes up, gets a message 1 from the mailbox, so now the Producer can finish putting the message 2.
Sample 7.37 Output from bounded mailbox

The bounded mailbox acts as a buffer between the two processes. You can see how
the Producer generates the next value before the Consumer reads the current value.

7.6.3

Unsynchronized Threads Communicating with a Mailbox

In many cases, two threads that are connected by a mailbox should
run in lockstep, so that the producer does not get ahead of the consumer. The benefit of this approach is that your entire chain of stimulus generation now runs in lock step. The highest level generator
only completes when the last low level transaction completes transmission. Now your testbench can tell precisely when all stimulus has
been sent. In another example, if your generator gets ahead of the driver, and you are
gathering functional coverage on the generator, you might record that some transactions were tested, even if the test stopped prematurely. So even though a mailbox
allows you to decouple the two sides, you may still want to keep them synchronized.
If you want two threads to run in lockstep, you need a handshake in addition to the
mailbox. In Sample 7.38 the Producer and Consumer are now classes that exchange

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Threads and Interprocess Communication

integers using a mailbox, with no explicit synchronization between the two objects.
As a result, as shown in Sample 7.39, the producer runs to completion before the
consumer even starts.
Sample 7.38 Producer–consumer without synchronization

The above sample holds the mailbox in a global variable to make the code more
compact. In real code, you should pass the mailbox into the class through the constructor and save a reference to it in a class-level variable.

7.6

Mailboxes

259

Sample 7.38 has no synchronization so the Producer puts all three integers into
the mailbox before the Consumer can get the first one. This is because a thread
continues running until there is a blocking statement, and the Producer has none.
The Consumer thread blocks on the first call to mbx.get.
Sample 7.39 Producer–consumer without synchronization output

This example has a race condition, so on some simulators the consumer could
activate earlier. The result is still the same as the values are determined by the producer, not by how quickly the consumer sees them.

7.6.4

Synchronized Threads Using a Bounded Mailbox
and a Peek

In a synchronized testbench, the Producer and Consumer operate in lock step. This
way, you can tell when the input stimuli is complete by waiting for any of the
threads. If the threads operate unsynchronized, you need to add extra code to detect
when the last transaction is applied to the DUT.
To synchronize two threads, the Producer creates and puts a transaction into a
mailbox, then blocks until the Consumer finishes with it. This is done by having the
Consumer remove the transaction from the mailbox only when it is finally done
with it, not when the transaction is first detected.
Sample 7.40 show the first attempt to synchronize two threads, this time with a
bounded mailbox. The Consumer uses the built-in mailbox method peek() to look
at the data in the mailbox without removing. When the Consumer is done processing the data, it removes the data with get(). This frees up the Producer to generate
a new value. If the Consumer loop started with a get() instead of the peek(), the
transaction would be immediately removed from the mailbox, so the Producer could
wake up before the Consumer finished with the transaction. Sample 7.41 has the
output from this code.

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Threads and Interprocess Communication

Sample 7.40 Producer–consumer synchronized with bounded mailbox

Sample 7.41 Output from producer–consumer with bounded mailbox

You can see that the Producer and Consumer are in lockstep, but the Producer is
still one transaction ahead of the Consumer. This is because a bounded mailbox
with size=1 only blocks when you try to do a put of the second transaction.2
2
This behavior is different from the VMM channel. If you set a channel’s full level to 1, the very first
call to put() places the transaction in the channel, but does not return until the transaction is removed.

7.6

Mailboxes

7.6.5

261

Synchronized Threads Using a Mailbox and Event

You may want the two threads to use a handshake so that the Producer never gets ahead
of the Consumer. The Consumer already blocks, waiting for the Producer using a mailbox. The Producer needs to block, waiting for the Consumer to finish the transaction.
Do this by adding a blocking statement to the Producer such as an event, a semaphore,
or a second mailbox. Sample 7.42 uses an event to block the Producer after it puts data
in the mailbox. The Consumer triggers the event after it consumes the data.
If you use wait (handshake.triggered) in a loop, be sure to
advance the time before waiting again, as previously shown in
Section 7.4.3. This wait blocks only once in a given time slot, so
you need move into another. Sample 7.42 uses the edge-sensitive
blocking statement @handshake instead to ensure that the
Producer stops after sending the transaction. The edge-sensitive statement works
multiple times in a time slot but may have ordering problems if the trigger and block
happen in the same time slot.
Sample 7.42 Producer–consumer synchronized with an event

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Threads and Interprocess Communication

Now the Producer does not advance until the Consumer triggers the event, as
shown in Sample 7.43.
Sample 7.43 Output from producer–consumer with event

You can see that the Producer and Consumer are successfully running in lockstep by
the fact that the Producer never produces a new value until after the old one is read.

7.6.6

Synchronized Threads Using Two Mailboxes

Another way to synchronize the two threads is to use a second mailbox that sends a
completion message back to the Producer, as shown in Sample 7.44.

7.6

Mailboxes

263

Sample 7.44 Producer–consumer synchronized with a mailbox

The return message in the rtn mailbox is just a negative version of the original
integer. You could use any value, but this one can be checked against the original for
debugging purposes.

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Sample 7.45 Output from producer–consumer with mailbox

You can see from Sample 7.45 that the Producer and Consumer are successfully
running in lockstep.

7.6.7

Other Synchronization Techniques

You can also complete the handshake by blocking on a variable or a semaphore. An
event is the simplest construct, followed by blocking on a variable. A semaphore is
comparable to using a second mailbox, but no information is exchanged. SystemVerilog’s bounded mailbox just does not work as well as these other techniques as there
is no way to block the producer when it puts the first transaction in. Sample 7.41
shows that the Producer is always one transaction ahead of the Consumer.

7.7

Building a Testbench with Threads and IPC

Way back in Section 1.10 you learned about layered testbenches. Figure 7.8 shows
the relationship between the different parts. Now that you know how to use threads
and IPC, you can construct a basic testbench with transactors.

7.7 Building a Testbench with Threads and IPC

Scenario

265

Environment

Generator

Functional

Agent
Agent

Scoreboard

Checker

Command

Driver

Assertions

Monitor

Signal
DUT
Fig. 7.8 Layered testbench with environment

7.7.1

Basic Transactor

Sample 7.46 is the Agent class that sits between the Generator and the Driver.
Sample 7.46 Basic Transactor

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7.7.2

Threads and Interprocess Communication

Configuration Class

The configuration class allows you to randomize the configuration of your system
for every simulation. Sample 7.47 has just one variable and a basic constraint.
Sample 7.47 Configuration class

7.7.3

Environment Class

The Environment class, shown as a dashed line in Fig. 7.8, holds the Generator,
Agent, Driver, Monitor, Checker, Scoreboard, and Config objects, and the mailboxes between them. Sample 7.48 shows a basic Environment class.
Sample 7.48 Environment class

7.7 Building a Testbench with Threads and IPC

267

Chapter 8 shows more details on how to build these classes.

7.7.4

Test Program

Sample 7.49 shows the main test, which is in a program block. As discussed in
Section 4.3.4, you can also put a test in a module, but at a slight increase in the
chances of race conditions.

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Threads and Interprocess Communication

Sample 7.49 Basic test program

7.8

Conclusion

Your design is modeled as many independent blocks running in parallel, so your
testbench must also generate multiple stimulus streams and check the responses
using parallel threads. These are organized into a layered testbench, orchestrated by
the toplevel environment. SystemVerilog introduces powerful constructs such as
fork…join_none and fork…join_any for dynamically creating new threads, in
addition to the standard fork…join. These threads communicate and synchronize
using events, semaphores, mailboxes, and the classic @ event control and wait
statements. Lastly, the disable command is used to terminate threads.
These threads and the related control constructs complement the dynamic nature
of OOP. As objects are created and destroyed, they can run in independent threads,
allowing you to build a powerful and flexible testbench environment.

7.9

7.9

Exercises

269

Exercises

1. For the following code determine the order and time of execution for each statement if a join or join_none or join_any is used. Hint: the order and time of
execution between the fork and join/join_none/join_any is the same,
only the order and execution time of the statements after the join are different.

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Threads and Interprocess Communication

2. For the following code what would the output be with and without a wait fork
inserted in the indicated location?

7.9

Exercises

271

3. What would be displayed with the following code? Assume that the events and
task trigger is declared inside a program declared as automatic.

4. Create a task called wait10 that for 10 tries will wait for 10ns and then check
for 1 semaphore key to be available. When the key is available, quit the loop and
print out the time.

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Threads and Interprocess Communication

5. What would be displayed with the following code that calls the task from Exercise 4?

6. What would be displayed with the following code?

7. Look at Fig. 7.8 “Layered testbench with environment” on page 265 and create
the Monitor class. You can make the following assumptions.
a. The Monitor class has knowledge of class OutputTrans with member variables out1 and out2.
b. The DUT and Monitor are connected with an interface called my_bus, with
signals out1 and out2.
c. The interface my_bus has a clocking block, cb.
d. On every active clock edge, the Monitor class will sample the DUT outputs,
out1 and out2, assign them to an object of type OutputTrans, and place
the object in a mailbox.

Chapter 8

Advanced OOP and Testbench Guidelines

How would you create a complex class for a bus transaction that also performs error
injection and has random delays? The first approach is to put everything in a large,
flat class. This approach is simple to build, easy to understand (all the code is right
there in one class) but can be slow to develop and debug. Additionally, such a large
class is a maintenance burden, as anyone who wants to make a new transaction
behavior has to edit the same file. Just as you would never create a complex RTL
design using just one Verilog module, you should break classes down into smaller,
reusable blocks.
Another approach is composition. As you learned in Chapter 5, you can instantiate one class inside another, just as you instantiate modules inside another, building up a hierarchical testbench. You write and debug your classes from the top
down or bottom up, always looking for natural partitions when deciding what variables and method go into the various classes. A pixel could be partitioned into its
color and coordinate. A packet might be divided into header and payload. You
might break an instruction into opcode and operands. See Section 8.4 for guidelines
on partitioning.
Sometimes it is difficult to divide the functionality into separate parts. Consider
injecting errors during a bus transaction. When you write the original class for the
transaction, you may not think of all the possible error cases. Ideally, you would like
to make a class for a good transaction, and later add different error injectors.
The transaction has data fields and an error-checking checksum field generated from
the data. One form of error injection is corruption of the checksum field. If you use
composition, you need separate classes for good transactions and error transactions.
Testbench code that used good objects would have to be rewritten to process the
new error objects. What you need is a class that resembles the original class but adds
a few new variables and methods. This result is accomplished through inheritance.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_8,
© Springer Science+Business Media, LLC 2012

273

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Advanced OOP and Testbench Guidelines

Inheritance allows a new class to be extended from an existing one by adding
new variables and methods. The original class is known as the base class. Since the
new class extends the capability of the base class, it is called the extended class.
Inheritance provides reusability by overlaying features, such as error injection, on
an existing class, without modifying that class.
A real power of OOP is that it gives you the ability to take an existing class,
such as a transaction, and selectively update parts of its behavior by replacing
methods, but without having to change the surrounding infrastructure. All your
original tests that depend on the base class keep working, and you can now create
new tests with the extended class. With some planning, you can create a testbench
solid enough to send basic transactions, but able to accommodate any extensions
needed by the test.
Note that this chapter goes into a wide range of advanced OOP topics, many of
which you won’t need when learning SystemVerilog. Feel free to skip the later sections for now, and save them for when you are digging into the internals of UVM
and VMM.

8.1

Introduction to Inheritance

Figure 8.1 shows a simple testbench. The test controls the generator. The generator
creates transactions, randomizes them, and sends them to the driver along the dotted
line. The driver breaks down the transaction into pin wiggles and sends it into the
DUT along the dashed line. The rest of the testbench is left out.

Test

Environment

Generator
transactions

Driver

pin wiggles

Fig. 8.1 Simplified layered testbench

DUT

8.1

Introduction to Inheritance

8.1.1

275

Basic Transaction

The basic transaction class in Sample 8.1 has variables for the source and destination addresses, eight data words, and a checksum for error checking, plus methods
for displaying the contents and calculating the checksum. The calc_csm function
is tagged as virtual so that it can be redefined if needed, as shown in the next section. Virtual methods are explained in more detail later in this chapter in Section
8.3.2. The class is simple enough that it uses the default SystemVerilog constructor
that allocates memory and initializes variables to their default value.
Sample 8.1 Base Transaction class

Normally calculating the checksum would be done in post_randomize(), but in
this example it has been separated from the randomization to show how to inject errors.
Figure 8.2 shows a diagram for the class with both the variables and methods.

Transaction

src

data[0]

calc_csm()

dst

data[1]

display()

csm

data[7]

Fig. 8.2 Base Transaction class diagram

8.1.2

Extending the Transaction Class

Suppose you have a testbench that sends good transactions through the DUT and
now you want to inject errors. If you follow the guidelines from Chapter 1, you
would want to make as few code changes as possible to your existing testbench.
So how can you reuse the existing Transaction class? Take the existing class and

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extend it to create a new class. This is done by declaring a new class, BadTr, as an
extension of the current class. Transaction is the base class, and BadTr is the
extended class. The code is shown in Sample 8.2 and in a diagram in Fig. 8.3.
Sample 8.2 Extended Transaction class

Note that in Sample 8.2, the variable csm is does not need a hierarchical identifier. The BadTr class can see all the variables from the original Transaction plus
its own variables such as bad_csm, as shown in Fig. 8.3. The calc_csm function
in the extended class calls calc_csm in the base class using the super prefix. You
can call a single level up, but going across multiple levels such as super.super.
new is not allowed in SystemVerilog. This style, that reaches across multiple levels,
would violate the rules of encapsulation by reaching across multiple boundaries.
The original display method printed a single line, starting with the prefix. So
the extended display method prints the prefix, class name, and bad_csm with
$write so the result is still on a single line.

Transaction

BadTr

src

data[0]

calc_csm()

dst

data[1]

display()

csm

data[7]

bad_csm

calc_csm()
display()

Fig. 8.3 Extended Transaction class diagram

8.1

Introduction to Inheritance

277

Always declare methods inside a class as virtual so that they can be
redefined in an extended class. This applies to all tasks and functions except the new function, which is called when the object is
constructed, so there is no way to extend it. SystemVerilog always
calls the new function based on the handle’s type. Virtual methods
are described fully in Section 8.3.2.

8.1.3

More OOP Terminology

Here is a quick glossary of terms. As explained in Chapter 5, the OOP term for a
variable in a class is “property,” and a task or function is called a “method.” A base
class is one that is not derived from any other class. When you extend a class, the
original class (such as Transaction) is called the parent class or superclass. The
extended class (BadTr) is also known as the derived or subclass. The “prototype”
for a method is just the first line that shows the argument list and return type, if
any. The prototype is used when you move the body of the method outside the
class, but is needed to describe how the method communicates, as shown in
Section 5.10.

8.1.4

Constructors in Extended Classes

When you start extending classes, there is one rule about constructors (new functions) to keep in mind. If your base class constructor has any arguments, the
extended class must have a constructor and must call the base’s constructor on its
first line. In Sample 8.3, since Base::new has an argument, Extended::new
must call it.

Sample 8.3 Constructor with arguments in an extended class

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8.1.5

Advanced OOP and Testbench Guidelines

Driver Class

The driver class in Sample 8.4 receives transactions from the generator and drives
them into the DUT.
Sample 8.4 Driver class

This class receives Transaction objects from the generator though the mailbox
gen2drv, breaks them down into signal changes in the interface to stimulate the
DUT. What happens if your generator instead sends a BadTr object into the class?
OOP rules say that if you have a handle of the base type (Transaction), it can also
point to an object of an extended type (BadTr). The handle tr can only reference
things in the base class such as the variables src, dst, csm, and data, and the
method calc_csm. So you can send BadTr objects into the driver without changing
the Driver class.
See Chapter 10 and 11 for examples of fully functional drivers with advanced
features such as virtual interfaces and callbacks.
When the driver calls tr.calc_csm, which one will be called, the one in
Transaction or BadTr? Since calc_csm was declared as a virtual method in the
base class in Sample 8.1, SystemVerilog chooses the proper method based on the
type of object stored in tr. If the object is of type Transaction, SystemVerilog
calls the task Transaction::calc_csm. If it is of type BadTr, SystemVerilog calls
the function BadTr::calc_csm.

8.1

Introduction to Inheritance

8.1.6

279

Simple Generator Class

The generator in Sample 8.5 for this testbench creates a random transaction and puts
it in the mailbox to the driver. The following (bad) example shows how you might
create the class from what you have learned so far. Note that this avoids a very common testbench bug by constructing a new transaction object every pass through the
loop instead of just once outside. This bug is discussed in more detail in Section 7.6
on mailboxes.
Sample 8.5 Bad generator class

There is a big limitation with this generator. The run task constructs a transaction and immediately randomizes it. This means that the transaction uses whatever
constraints are turned on by default. The only way you can change this would be to
edit the Transaction class, which goes against the verification guidelines presented in this book. Worse yet, the generator only uses Transaction objects —
there is no way to use an extended object such as BadTr. The fix is to separate the
construction of tr from its randomization as shown below in Section 8.2.
As you build data-oriented classes such as network and bus transactions, you
will see that they have common properties (id) and methods (display). Controloriented classes such as the Generator and Driver classes also have a common
structure. You can enforce this by making both of these classes extensions of a
base Transactor class, with virtual methods for run, and wrap_up. Both the
UVM and VMM has an extensive set of base classes for transactors, data, and
much more.

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8.2

Advanced OOP and Testbench Guidelines

Blueprint Pattern

A useful OOP technique is the “blueprint pattern.” If you have a
machine to make signs, you don’t need to know the shape of every
possible sign in advance. You just need a stamping machine and
then change the die to cut different shapes. Likewise, when you
want to build a transactor generator, you don’t have to know how to
build every type of transaction; you just need to be able to stamp
new ones that are similar to a given transaction.
Instead of constructing and then immediately using an object, as in Sample 8.5,
construct a blueprint object (the cutting die), and then modify its constraints with
constraint_mode, or even replace it with an extended object, as shown in Fig. 8.4.
Now when you randomize this blueprint, it will have the random values that you want.
Make a copy of this object and send the copy to the downstream transactor.
Fig. 8.4 Blueprint pattern
generator

Generator
Generator
Copy

Blueprint
(from test)

Generated
stream

The beauty of this technique is that if you change the blueprint object, your generator creates an object of a different type. Using the sign analogy, you change the
cutting die from a square to a triangle to make Yield signs, as shown in Fig. 8.5.
Fig. 8.5 Blueprint generator
with new pattern

Generator
Generator
Blueprint
(from test)

Copy

Generated
stream

The blueprint is the “hook” that allows you to change the behavior of the generator class without having to change its code. You need to make a copy method that
can make a copy of the blueprint to transmit, so that the original blueprint object is
kept around for the next pass through the loop.
Sample 8.6 shows the generator class using the blueprint pattern. The important
thing to notice is that the blueprint object is constructed in one place (the new function)

8.2

Blueprint Pattern

281

Sample 8.6 Generator class using blueprint pattern

and used in another (the run task). Previous coding guidelines in this book said to
separate the declaration and construction; similarly, you need to separate the construction and randomization of the blueprint object.
The copy method, which makes a duplicate of an object by copying its variables
into a new object, is discussed in Sections 5.15 and 8.5. For now, remember that you
must add it to the Transaction and BadTr classes. Sample 8.34 on page 304
shows an advanced generator using templates.
This generator constructs a new transaction every time the blueprint is randomized. This coding style prevents the classic OOP mailbox bug, as the mailbox will
store handles to multiple unique objects, not that same single object.
Another advantage of randomizing the blueprint object over and over is that
randc variables work correctly. The bad generator in Sample 8.5 constructed new
objects every pass through the loop. Every object with a randc variable maintains
a history of previous values generated for the variable. Every time you construct a
new object, that history is lost, and the bad generator creates objects with separate
randc variables. In Sample 8.6, only the blueprint object is randomized, so the
randc history is maintained.
Section 8.2.3 shows how to change the blueprint.

8.2.1

The Environment Class

Chapter 1 discussed the three phases of execution: Build, Run, and Wrap-up. Sample
8.7 shows the environment class that instantiates all the testbench components, and
runs these three phases. Also notice how the mailbox gen2drv carries transactions
from the generator to the driver, and so is passed into the constructor for each.

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Sample 8.7 Environment class

8.2.2

A Simple Testbench

The test is contained in the top-level program shown in Sample 8.8. The basic test
just lets the environment run with all the defaults.
Sample 8.8 Simple test program using environment defaults

8.2

Blueprint Pattern

8.2.3

283

Using the Extended Transaction Class

To inject an error, you need to change the blueprint object from a
Transaction object to a BadTr. You do this between the build
and run phases in the environment. The top-level testbench in
Sample 8.9 runs each phase of the environment and changes the
blueprint. Note how all the references to BadTr are in this one file,
so you don’t have to change the Environment or Generator
classes. You want to restrict the scope of where BadTr can be used, so a standalone
begin…end block is used in the middle of the initial block. This makes a visually distinctive block of code. You can take a shortcut and construct the extended
class in the declaration.
Sample 8.9 Injecting an extended transaction into testbench

8.2.4

Changing Random Constraints with an Extended Class
In Chapter 6 you learned how to generate constrained random data.
Most of your tests are going to need to further constrain the data,
which is best done with inheritance. In Sample 8.10, the original
Transaction class is extended to include a new constraint that keeps
the destination address in the range of +/−100 of the source address.

Sample 8.10 replaces the generator’s blueprint with an extended object that has an
additional constraint. As you will learn later in this chapter, the Nearby class should
have a copy method, but hold on for a few sections.

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Sample 8.10 Adding a constraint with inheritance

Note that if you define a constraint in an extended class with the same name as
one in the base class, the extended constraint replaces the base one. This allows you
to change the behavior of existing constraints.

8.3

Downcasting and Virtual Methods

As you start to use inheritance to extend the functionality of classes, you need a few
OOP techniques to control the objects and their functionality. In particular, a handle
can refer to an object for a certain class, or any extended class. So what happens
when a base handle points to an extended object? What happens when you call a
method that exists in both the base and extended classes? This section explains what
happens using several examples.

8.3.1

Downcasting with $cast

Downcasting or conversion is the act of casting a base class handle to point to an
object that is a class extended from that base type. Consider the base and extended
classes in Sample 8.11 and Fig. 8.6.

8.3 Downcasting and Virtual Methods

285

Sample 8.11 Base and extended class

Transaction

src

display()

BadTr

bad_csm

display()

Fig. 8.6 Simplified extended transaction

You can assign an extended handle to a base handle, and no special code is
needed, as shown in Sample 8.12. When a class is extended, all the base class variables and methods are included, so src is in the extended object. The assignment to
tr is permitted, as any reference using the base handle tr is valid, such as tr.src
and tr.display.
Sample 8.12 Copying extended handle to base handle

What if you try going in the opposite direction, copying a handle to a base object
into an extended handle, as shown in Sample 8.13? This fails because the base
object is missing properties that only exist in the extended class, such as bad_csm.
The SystemVerilog compiler does a static check of the handle types and will not
compile the second line.

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Sample 8.13 Copying a base handle to an extended handle

It is not always illegal to assign a base handle to an extended handle, but you
must always use $cast. The assignment is allowed when the base handle points to
an extended object. The $cast method checks the type of object referenced by the
handles, not just the handle. If the source object is the same type as the destination,
or a class extended from the destination’s class, you can copy the address of the
extended object from the base handle, tr, into the extended handle, bad2.
Sample 8.14 Using $cast to copy handles

When you use $cast as a task, SystemVerilog checks the type of the source
object at run time and gives an error if it is not compatible with the destination.
When you use $cast as a function, SystemVerilog still checks the type, but no
longer prints an error if there is a mismatch. The $cast function returns zero when
the types are incompatible, and one for compatible types.
As an alternative to the if statement in Sample 8.14, you could use something
like the SV_RAND_CHECK macro from Section 6.3.2. You should not use an immediate assert statement as the assertion expression is not evaluated if you disable
assertions, which means the $cast and bad2 assignment will never execute.

8.3.2

Virtual Methods

By now you should be comfortable using handles with extended classes. What happens if you try to call a method using one of these handles? Sample 8.15 and 8.16
show base and extended classes and code that calls methods inside these classes.

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Sample 8.15 Transaction and BadTr classes

Sample 8.16 contains a block of code that uses handles of different types.
Sample 8.16 Calling class methods

To decide which virtual method to call, SystemVerilog uses the object’s type, not
the handle’s type. In the last statement of Sample 8.16, tr points to an extended
object (BadTr) and so BadTr::calc_csm is called.
If you leave out the virtual modifier on Transaction::calc_csm,
SystemVerilog checks the type of the handle tr (Transaction), not the object.
That last statement in Sample 8.16 calls Transaction::calc_csm – probably not
what you wanted.
The OOP term for multiple methods sharing a common name is “polymorphism.”
It solves a problem similar to what computer architects faced when trying to make
a processor that could address a large address space but had only a small amount of
physical memory. They created the concept of virtual memory, where the code and

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data for a program could reside in memory or on a disk. At compile time, the program
didn’t know where its parts resided — that was all taken care of by the hardware
plus operating system at run time. A virtual address could be mapped to some RAM
chips, or the swap file on the disk. Programmers no longer needed to worry about
this virtual memory mapping when they wrote code — they just knew that the
processor would find the code and data at run time. See also Denning (2005).

8.3.3

Signatures and Polymorphism

There is a downside to using virtual methods: once you define one, all extended
classes that define the same method must use the same “signature,” i.e., the same
number and type of arguments, plus return value, if any. You cannot add or remove
an argument in an extended virtual method. This means you need to plan ahead.
There is a good reason that SystemVerilog and other OOP languages require that
a virtual method must have the same signature as the one in the parent (or grandparent). If you were able to add an additional argument, or turn a task into a function,
polymorphism would no longer work. Your code needs to be able to call a virtual
method with the assurance that a method in a extended class will have the same
interface.

8.3.4

Constructors are Never Virtual

When you call a virtual method, SystemVerilog checks the type of the object to
decide if it should call the method in the base class or the extended. Now you can
see why a constructor can not be virtual. When you call it, there is no object whose
type can be checked. The object only exists after the constructor call starts.

8.4

Composition, Inheritance, and Alternatives

As you build up your testbench, you have to decide how to group related variables
and methods together into classes. In Chapter 5 you learned how to build basic
classes and include one class inside another. Previously in this chapter, you saw the
basics of inheritance. This section shows you how to decide between the two styles,
and also shows an alternative.

8.4.1

Deciding Between Composition and Inheritance

How should you tie together two related classes? Composition uses a “has-a” relationship. A packet has a header and a body. Inheritance uses an “is-a” relationship.

8.4

Composition, Inheritance, and Alternatives

289

A BadTr is a Transaction, just with more information. Table 8.1 is a quick guide,
with more detail below.
Table 8.1 Comparing inheritance to composition
Question
1. Do you need to group multiple extended classes
together? (SystemVerilog does not support
multiple inheritance)
2. Does the higher-level class represent objects at a
similar level of abstraction?
3. Is the lower-level information always present or
required?
4. Does the additional data need to remain attached
to the original class while it is being processed
by pre-existing code?

Inheritance
(is-a relationship)
No

Composition
(has-a relationship)
Yes

Yes

No

Yes

No

Yes

No

1. Are there several small classes that you want to combine into a larger class? For
example, you may have a data class and header class and now want to make a
packet class. SystemVerilog does not support multiple inheritance, where one
class extends from several classes at once. Instead you have to use composition.
Alternatively, you could extend one of the classes to be the new class, and manually add the information from the others.
2. In Sample 8.15, the Transaction and BadTr classes are both bus transactions
created in a generator and driven into the DUT, so inheritance makes sense.
3. The lower-level information such as src, dst, and data must always be present
for the Driver to send a transaction.
4. In Sample 8.15, the new BadTr class has a new field bad_csm and the extended
calc_csm function. The Generator class just transmits a transaction and does
not care about the additional information. If you use composition to create the
error bus transaction, the Generator class would have to be rewritten to handle
the new type.
If two objects seem to be related by both “is-a” and “has-a,” you may need to
break them down into smaller components.

8.4.2

Problems with Composition

The classical OOP approach to building a class hierarchy partitions functionality
into small blocks that are easy to understand. However, testbenches are not standard

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software development projects, as was discussed in Section 5.16 on public vs. local
attributes. Concepts such as information hiding (using local variables) conflict with
building a testbench that needs maximum visibility and controllability. Similarly,
dividing a transaction into smaller pieces may cause more problems than it solves.
When you are creating a class to represent a transaction, you may want to partition it to keep the code more manageable. For example, you may have an Ethernet
MAC frame and your testbench uses two flavors, normal (II) and Virtual LAN
(VLAN). Using composition, you could create a basic cell EthMacFrame with all the
common fields such as da and sa and a discriminant variable, kind, to indicate the
type as shown in Sample 8.17. There is a second class to hold the VLAN information, which is included in EthMacFrame.
Sample 8.17 Building an Ethernet frame with composition

There are several problems with composition. First, it adds an extra layer of
hierarchy, so you are constantly having to add an extra name to every reference. The
VLAN information is called eth_h.vlan_h.vlan. If you start adding more layers, the hierarchical names become a burden.
A more subtle issue occurs when you want to instantiate and randomize
the hierarchy of classes. What does the EthMacFrame constructor create? Since
kind is random, you don’t know whether to construct a Vlan object when new
is called. When you randomize the class, the constraints set variables in both the
EthMacFrame and Vlan objects based on the random kind field. You have a circular dependency in that randomization only works on objects that have been
instantiated, but you can’t instantiate these objects until kind has been chosen.
The only solution to the construction and randomization problems is to always
instantiate all objects in EthMacFrame::new. However, if you are always using all
alternatives, why divide the Ethernet cell into two different classes?

8.4

Composition, Inheritance, and Alternatives

8.4.3

291

Problems with Inheritance

Inheritance can solve some of these issues. Variables in the extended classes can be
referenced without the extra hierarchy as in eth_h.vlan. You don’t need a discriminant, but you may find it easier to have one variable to test rather than doing
type-checking as shown in Sample 8.18.
Sample 8.18 Building an Ethernet frame with inheritance

On the downside, a set of classes that use inheritance always requires more effort
to design, build, and debug than a set of classes without inheritance. Your code must
use $cast whenever you have an assignment from a base handle to an extended
handle. Building a set of virtual methods can be challenging, as they all have to have
the same signature. If you need an extra argument, you need to go back and edit the
entire set, and possibly the method calls too.
There are also problems with randomization. How do you make a constraint that
randomly chooses between the two kinds of frame and sets the proper variables?
You can’t put a constraint in EthMacFrame that references the vlan field.
The final issue is with multiple inheritance. In Fig. 8.7, you can see how the
VLAN frame is extended from a normal MAC frame. The problem is that these different standards reconverged. SystemVerilog does not support multiple inheritance,
so you could not create the VLAN / Snap / Control frame through inheritance.

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Ethernet MAC Frame

Ethernet VLAN Frame

Ethernet II Frame

Ethernet Control Frame

Ethernet Snap Frame

Ethernet VLAN Snap Control Frame

Fig. 8.7 Multiple inheritance problem

8.4.4

A Real-World Alternative

If composition leads to large hierarchies, but inheritance requires extra code and
planning to deal with all the different classes, and both have difficult construction
and randomization, what can you do? You can instead make a single, flat class that
has all the variables and methods. This approach leads to a very large class, but it
handles all the variants cleanly. You have to use the discriminant variable often to
tell which variables are valid, as shown in Sample 8.19. It contains several conditional constraints, which apply in different cases, depending on the value of kind.
Sample 8.19 Building a flat Ethernet frame

Regardless of how you build your classes, define the typical behavior and constraints in the class, and then use inheritance to inject new behavior at the test level.

8.5

8.5

Copying an Object

293

Copying an Object

In Sample 8.6, the generator first randomized, and then copied the blueprint to make
a new transaction. Take a closer look at the copy function in Sample 8.20. Also see
Section 5.15 for more examples of copy functions.
Sample 8.20 Base transaction class with a virtual copy function

When you extend the Transaction class to make the class BadTr, the copy
function still has to return a Transaction object. This is because the extended
virtual function must match the base Transaction::copy, including all arguments and return type, as shown in Sample 8.21
Sample 8.21 Extended transaction class with virtual copy method

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Specifying a Destination for Copy

The previous copy methods always constructed a new object. An improvement for
copy is to specify the location where the copy should be put. This technique is useful
when you want to reuse an existing object, and not allocate a new one.
Sample 8.22 Base transaction class with copy function

The only difference is the additional argument to specify the destination, and the
code to test that a destination object was passed to this method. If nothing was
passed (the default), construct a new object, or else use the existing one.
Since you have added a new argument to a virtual method in the base class, you
will have to add it to the same method in the extended classes, such as BadTr.
Sample 8.23 Extended transaction class with new copy function

Notice how BadTr::copy only needs to copy the fields in the extended class
and can use the base class method, Transaction::copy to copy its own fields.

8.6 Abstract Classes and Pure Virtual Methods

8.6

295

Abstract Classes and Pure Virtual Methods

By now you have seen classes with methods to perform common operations such as
copying and displaying. One goal of verification is to create code that can be shared
across multiple projects. If your company standardizes on a common set of classes
and methods, it is easier to reuse code between projects.
OOP languages such as SystemVerilog have two constructs to allow you to build
a shareable base class. The first is an abstract class, which is a class that can be
extended, but not instantiated directly. It is defined with the virtual keyword. The
second is a pure virtual method, which is a prototype without a body. A class
extended from an abstract class can only be instantiated if all pure virtual methods
have bodies. The pure keyword specifies that a method declaration is a prototype,
and not just an empty virtual method. A pure method has no endfunction or endtask. Lastly, pure virtual methods can only be declared in an abstract class. An
abstract class can contain pure virtual methods, virtual methods with and without a
body, and non-virtual methods. Note that if you define a virtual method without a
body, i.e. no code inside, you can call it but it just immediately returns.
Sample 8.24 shows an abstract class, BaseTr, which is a base class for transactions. It starts with a some useful properties such as id and count. The constructor
makes sure every instance has a unique ID. Next are pure virtual methods to compare, copy, and display the object.
Sample 8.24 Abstract class with pure virtual methods

You can declare handles of type BaseTr, but you cannot construct objects of this
type. You need to extend the class and provide implementations for all the pure
virtual methods.

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Sample 8.25 shows the definition of the Transaction class, which has been
extended from BaseTr. Since Transaction has bodies for all the pure virtual
methods extended from BaseTr, you can construct objects of this type in your
testbench.
Sample 8.25 Transaction class extends abstract class

Abstract classes and pure virtual methods let you build testbenches that have a
common look and feel. This allows any engineer to read your code and quickly
understand the structure.

8.7

Callbacks

8.7

297

Callbacks

One of the main guidelines of this book is to create a single verification environment that you can use for all tests with no changes. The key requirement is that this
testbench must provide a “hook” where the test program can inject new code without modifying the original classes. Your driver may want to do the following.
•
•
•
•
•
•

Inject errors
Drop the transaction
Delay the transaction
Synchronize this transaction with others
Put the transaction in the scoreboard
Gather functional coverage data

Rather than try to anticipate every possible error, delay, or disturbance in the flow
of transactions, the driver just needs to “call back” a method that is defined in the
top-level test. The beauty of this technique is that the callback method can be defined
differently in every test. As a result, the test can add new functionality to the driver
using callbacks, without editing the Driver class. For some drastic behaviors such
as dropping a transaction, you need to code this in the class ahead of time, but this is
a known pattern. The reason why the transaction is dropped is left to the callback.

task Driver::run;
forever begin
...

transmit(tr);

...
end
endtask

task pre_callback;
...
endtask
task post_callback;
...
endtask

Fig. 8.8 Callback flow

In Fig. 8.8, the Driver::run task loops forever with a call to a transmit task.
Before sending the transaction, run calls the pre-transmit callback, if any. After
sending the transaction, it calls the post-callback task, if any. By default, there are
no callbacks, so run just calls transmit.
You could make Driver::run a virtual method and then override its behavior
in an extended class, perhaps MyDriver::run. The drawback to this is that you
might have to duplicate all the original method’s code in the new method if you are

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injecting new behavior. Now if you made a change in the base class, you would
have to remember to propagate it to all the extended classes. Additionally, you can
inject a callback without modifying the code that constructed the original object.

8.7.1

Creating a Callback

A callback task is created in the top-level test and called from the driver, the lowest
level of the environment. However, the driver does not have to have any knowledge
of the test – it just has to use a generic class that the test can extend. The driver in
Sample 8.27 uses a queue to hold the callback objects, which allows you to add
multiple objects. The base callback class in Sample 8.26 is an abstract class that
must be extended before being used. Your callback is a task so it can have delays.
Sample 8.26 Base callback class

Sample 8.27 Driver class with callbacks

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Callbacks

299

Note that while Driver_cbs is an abstract class, pre_tx and post_tx are not
pure virtual methods. This is because a typical callback uses only one of them. If a
class has even one pure virtual method without an implementation, OOP rules won’t
allow you to instantiate it.
Callbacks are part of both VMM and UVM. This callback technique is not related
to Verilog PLI callbacks or SVA callbacks.

8.7.2

Using a Callback to Inject Disturbances

A common use for a callback is to inject some disturbance such as causing an error
or delay. The testbench in Sample 8.28 randomly drops packets using a callback
object. Callbacks can also be used to send data to the scoreboard or to gather functional coverage values. Note that you can put callback objects in the queue with the
push_back() or push_front() depending on the order in which you want these
to be called. For example, you probably want the scoreboard called after any tasks
that may delay, corrupt, or drop a transaction. You should only gather coverage after
a transaction has been successfully transmitted.
Sample 8.28 Test using a callback for error injection

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A Quick Introduction to Scoreboards

The design of your scoreboard depends on the design under test. A DUT that processes
atomic transactions such as packets may have a scoreboard that contains a transform function to turn the input transactions into expected values, a memory to hold
these values, and a compare method. A processor design needs a reference model to
predict the expected output, and the comparison between expected and actual values
may happen at the end of simulation.
Sample 8.29 shows a simple scoreboard that stores transactions in a queue of
expected values. The first method saves an expected transaction, and the second
tries to find an expected transaction that matches an actual one that was received by
the testbench. Note that when you search through a queue, you can get 0 matches
(transaction not found), 1 match (ideal case) or multiple matches (you need to do a
more sophisticated match).
Sample 8.29 Simple scoreboard for atomic transactions

8.7.4

Connecting to the Scoreboard with a Callback

The testbench in Sample 8.30 creates its own extension of the driver’s callback class
and adds a reference to the driver’s callback queue. Note that the scoreboard callback needs a handle to the scoreboard so it can call the method to save the expected
transaction. This example does not show the monitor side, which will need its own
callback to send the actual transaction to the scoreboard for comparison.

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Sample 8.30 Test using callback for scoreboard

The VMM recommends that you use callbacks for scoreboards and functional
coverage. The monitor transactor can use a callback to compare received transactions with expected ones. The monitor callback is also the perfect place to gather
functional coverage on transactions that are actually sent by the DUT.
You may have thought of putting the scoreboard or functional coverage group in
a transactor, and connect it to the testbench using a mailbox. This is a poor solution
for several reasons. These testbench components are almost always passive and
asynchronous, so they only wake up when the testbench has data for them, plus they
never pass information to a downstream transactor. Thus a transactor that has to
monitor multiple mailboxes concurrently is an overly complex solution. Additionally,
you may sample data from several points in your testbench, but a transactor is
designed for a single source. Instead, put methods in your scoreboard and coverage
classes to gather data, and connect them to the testbench with callbacks.

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The UVM recommends a TLM analysis port for connecting monitors / drivers to
scoreboards and functional coverage. A description of this construct is beyond the
scope of this book, but you can think of it as a mailbox with an optional consumer.

8.7.5

Using a Callback to Debug a Transactor

If a transactor with callbacks is not working as expected, you can add a debug callback. You can start by adding a callback to display the transaction. If there are multiple instances of the transactor, create a unique identifier for each. Put debug code
before and after the other callbacks to locate the one that is causing the problem.
Even for debug, you want to avoid making changes to the testbench environment.

8.8

Parameterized Classes

As you become more comfortable with classes, you may notice that a class, such as
a stack or generator, only works on a single data type. This section shows how you
can define a single parameterized class that works with multiple data types.

8.8.1

A Simple Stack

A common data structure is a stack, which has push and pop methods to store and
retrieve data. Sample 8.31 shows a simple stack that works with the int data type.

Sample 8.31 Stack using the int type

8.8 Parameterized Classes

303

The problem with this class is that it only works with integers. If you want to
make a stack for real numbers, you would have to copy the class, and change the
data type from int to real. This quickly leads to a proliferation of classes, which
can become a maintenance problem if you ever want to add new operations such as
traversing or printing the stack contents.
In SystemVerilog you can add a data type parameter to a class and then specify a
type when you declare handles to that class. This is similar to, but more powerful
than, a parameterized module, where you can specify a value such as bus width
when it is instantiated. SystemVerilog’s parameterized classes are similar to templates in C++.
Sample 8.32 is a parameterized class for a stack. Notice how the type T is defined
on the first line with a default type of int.
Sample 8.32 Parameterized class for a stack

The step of specifying values to a parameterized class is called specialization.
Sample 8.33 declares a handle to the stack class with a real data type.
Sample 8.33 Creating the parameterized stack class

Generators are a great example of a class that can be parameterized. Once you
have defined the class for one, the same structure works for any data type. Sample
8.34 takes the atomic generator from Sample 8.6 and adds a parameter so you can

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generate any random object. The generator should be part of a package of verification
classes. It needs to specify a the default type, so it uses BaseTr from Sample 8.24
as this abstract class should also be part of the verification package.
Sample 8.34 Parameterized generator class using blueprint pattern

Using the Transaction class from Sample 8.25 and the generator in Sample 8.34,
you can build a simple testbench like in Sample 8.35. It starts the generator and prints
the first five transactions, using the mailbox synchronization shown in Sample 7.40.
Sample 8.35 Simple testbench using parameterized generator class

8.8 Parameterized Classes

8.8.2

305

Sharing Parameterized Classes

When you specialize a parameterized class, as in the real stack in Sample 8.33, you
are creating a new data type, with no OOP relationship to any other specialization. For
example, you can not use $cast() to convert between a stack of real variables and
one of integers. For that, you need a common base class as shown in Sample 8.36.
Sample 8.36 Common base class for parameterized generator class

Upcoming sections show more examples of parameterized classes.

8.8.3

Parameterized Class Suggestions

When creating parameterized classes, you should start with a non-parameterized
class, debug it thoroughly, and then add parameters. This separation reduces your
debug effort.
A common set of virtual methods in your transaction class help you when creating parameterized classes. The Generator class uses the copy method, knowing
that it always has the same signature. Likewise, the display method allows you to
easily debug transactions as they flow through your testbench components.
The system functions $typename() and $bits() are helpful when your class
needs to know the name and width of the parameter. The $typename(T) function
returns the name of the parameter type such as int, real, or the class name for a
handle. The $bits() function returns the width of the parameter. For complex
types such as structures and arrays, it returns the number of bits required to hold an
expression as a bit stream. The UVM transaction print methods use this function to
get the fields to line up correctly.
Macros are an alternative to parameterized classes. For example, you could define
a macro for the generator and pass it the transaction data type. Macros are harder to
debug than parameterized classes, unless your compiler outputs the expanded code.

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If you need to define several related classes that all share the same transaction
type, you could use parameterized classes or a single large macro. In the end, how
you define your classes is not as important as what goes into them.

8.9

Static and Singleton Classes

This section and the next show advanced OOP concepts that are used extensively in
the UVM and VMM. You could try to understand UVM’s factory mechanism by
reading the source code with its many methods, but this section should save you
several days of experimentation with a greatly simplified example. This chapter
shows several alternatives so you can understand why the UVM did not pick a more
simple alternative.
One of the goals of OOP is to eliminate global variables and methods as the
resulting code is hard to maintain and reuse. Their names exist in the global name
space, potentially causing name space collisions. Does packet_count refer to
TCP/IP packets or some other protocol? Instead, put a variable called count in the
Packet class to avoid any ambiguity.

8.9.1

Dynamic Class to Print Messages

Sometimes, however, you really need globals. For example, all verification methodologies provide a print service so you can filter messages and count errors. If you try
to build such a class with what you have learned so far, it might look something like
Sample 8.37.
Sample 8.37 Dynamic print class with static variables

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307

This is a greatly simplified version of the VMM log class. The VMM code allows
you to filter messages by the class and instance names, and many other features.
Sample 8.38 has a class that prints an error message with the Print class from
Sample 8.37.
Sample 8.38 Transactor class with dynamic print object

The biggest limitation for the Print class is that every component in your testbench needs to instantiate it. The simple Print class above has a small footprint,
but a realistic one, like VMM’s, could have many strings and arrays, consuming a
significant amount of memory. This overhead, when added to a transactor class
might not be significant, but could overwhelm a small transaction class, such as an
ATM cell, which only has 53 bytes.

8.9.2

Singleton Class to Print Messages

An alternative to constructing all these print objects is to not construct any. As you
saw in section 5.11.4, you could declare the methods in the Print class to be static.
These methods can only reference static variables, as shown in Sample 8.39.
Sample 8.39 Static print class

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Now that the class is static, you can no longer have per-instance information such as
the parent class’s name and instance. Any filtering has to be based on other criteria.
Sample 8.40 Transactor class with static print class

Sample 8.40 shows the call to the error() method using the Print class name.
This style of class is known as a singleton class, as there is only one copy, the one
allocated at elaboration time with the static variables.
As your static classes, such as the one in Sample 8.39, grow larger, you have to
label everything with the static keyword, a small annoyance. Next, the class is
allocated before simulation time, even if you never use it. Additionally, there is no
handle to this class, so you can not pass it around your testbench. The alternative to
a static class is a singleton class (or singleton pattern) with a single instance, which
is a non-static class that is only constructed once. They are more difficult to create
initially, but they can simplify your program’s architecture. Many of the UVM’s
classes are singletons.
The singleton pattern is implemented by creating a class with a method that creates a new instance of the class if one does not exist. If an instance already exists, it
simply returns a handle to that object. To make sure that the object cannot be instantiated any other way, you must make the constructor protected. Don’t make it
local, because an extended class might need to access the constructor.

8.9.3

Configuration Database with Static Parameterized Class

Another good use for static classes in verification is a database of configuration
parameters. At the start of simulation you randomize the configuration of your system. In a small system, you can simply store these in a single class or hierarchy of
classes and pass them around the testbench as needed. At some point though, this
becomes too complicated as handles are passed up and down the hierarchy. Instead,
create a global database of parameters, indexed by a name, that you can access anywhere in the testbench. UVM 1.0 introduced this concept, which is the basis for the
following set of examples. This code has a single string index into the database,
while a real database such as UVM’s could have a property name, instance name, and
other values. You could concatenate these to create a more complex index string.
One issue with a database is that you need to store values of different types, such
as bit vectors, integers, real numbers, enumerated values, string, class handles, virtual
interfaces, and more in a single database. While you could find a few common types
such as bit vectors and a common base class, there are some type such as virtual

8.9 Static and Singleton Classes

309

interfaces that are unique, so there is no easy way to store them in a common database.
Earlier versions of OVM and UVM recommended creating a class wrapper around
virtual interfaces, but this required extra coding and was a common source of bugs.
What if you made a different database for each data type? You could use an associative array indexed by the parameter name. A real database might also have an
instance name, but for this simple example, you can just concatenate all the names
together to make a single index. Sample 8.41 shows the code for an integer database
made from global methods.
Sample 8.41 Configuration database with global methods

You can generalize this into a parameterized class with the concepts from Section
8.8, as shown in Sample 8.42.
Sample 8.42 Configuration database with parameterized class

You can now construct objects for an integer database, a real database, etc. The
final problem is that each instance of the database is local to the scope where this

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class is instantiated. The solution shown in Sample 8.43 is to go global and make
this a static class, that is a class with static properties and methods.
Sample 8.43 Configuration database with static parameterized class

You can test the above code with Sample 8.44 and see how the parameterized
class creates a new database for each type.
Sample 8.44 Testbench for configuration database

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311

With singletons implemented as single instances instead of static class members,
you can initialize the singleton lazily, creating it only when it is needed.
The UVM database allows wildcards and other regular expressions, which
requires a more complex lookup scheme than associative arrays.

8.10

Creating a Test Registry

In a real design, compiling your test and DUT takes a significant amount of time.
If you want to run 100 tests, each in a separate program block, you need to recompile before each test, 100 times in all. This is a waste of CPU time as most of the
code has not changed. If you make 100 program blocks, each with a single test,
and connect all these programs in the model, you then need a way to disable all
but one program block. The best solution is to include all tests and testbenches
inside one program block, compile this once with the DUT. This section shows
how you can select one test per run with a Verilog command line switch.

8.10.1

Test registry with Static Methods

Earlier examples in this book have a program that contains one test. For this new
approach, each test is a separate class, all which are in a single program block, either
imported from a package or included at compile time. The test classes are constructed, registered in a test registry, and then, at run time, you can choose the
desired test at runtime. This follows an early VMM style.
First you need a base test class that your tests can extend from. Sample 8.45
shows an abstract class that contains a handle for the Environment class and a pure
virtual task that is a placeholder for the method that contains your test code.
Sample 8.45 Base test class

The core of the test registry class is an associative array of handles to all the tests,
indexed by the test name. The TestRegistry class, shown in Sample 8.46, is a
static class with only static variables and methods, and is never constructed. The
get_test() method reads the Verilog command line argument to determine which
test to execute.

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Sample 8.46 Test registry class

Sample 8.47 show how you can extend TestBase to create a simple test that
runs all the environment phases. The last line of the example is a declaration that
calls the constructor, which also registers the test. All the test objects are constructed,
but only one is run.
Sample 8.47 Simple test in a class

The program in Sample 8.48 now just asks the test registry for a test object and
runs it. The test classes can be declared in a package and imported, or declared
inside or outside the program block.

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Sample 8.48 Program block for test classes

Sample 8.49 shows how you can create a test class that injects new behavior by
changing the generator’s blueprint to create bad transactions.
Sample 8.49 Test class that puts a bad transaction in the generator

This short example allows you to compile many tests into a single simulation
executable and choose your test at runtime, saving many recompiles. This pattern is
fine when you are starting out with a handful of tests, but the next section shows
more powerful approach.

8.10.2

Test Registry with a Proxy Class

The previous section’s test registry works well for smaller test environments, but
has several limitations for real projects. First, you need to remember to construct every test class, otherwise the registry can not locate it. Second, every test
gets constructed at the start of simulation, even though only one is actually run.

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When verifying a large design, there could be hundreds of tests, so constructing all
of them wastes valuable simulation time and memory.
Consider this analogy. When you are looking to buy a car, you can go to a dealer
to see the choices. If there are only a few variants, white or black, with or without
sunroof, the dealer can stock one of each model with little overhead. This is what you
saw in the previous section, where the test registry had an object of each test type.
What if there are many different models, each in one of a dozen colors, with variants such as radios, sunroofs, air conditioning, sports packages, and engines? The
dealer could never have one of each type on his lot as there are hundreds of combinations. Instead he would show you a catalog with all the choices. You pick the
options that you want, and the factory builds one to your specification. Likewise, the
test registry can have a lot of small classes, each which knows how to build a complete test. The small class has low overhead, so even a thousand objects would not
consume much memory. Now when you want to run test N, imagine flipping through
the catalog (test registry) until you find a picture of your test, and you then tell the
factory to build an object of that type.
The test registry needs a table (analogous to the above catalog) that goes from
test names to objects. In section 8.10.1, this table is an associative array of TestBase
handles, indexed by a string, shown in Sample 8.46. What if instead, you had a
parameterized class whose only job is to construct a test? The UVM uses a design
pattern called a proxy class whose only role is to build the actual desired class. The
proxy class is lightweight in that it only contains a few properties and methods, and
thus consumes little memory or CPU time. It acts like the picture in the car dealer’s
catalog, holding a representation of what you can build.
The next few code samples show how the UVM class factory works. Because the
code in this book is a simplified version of the real UVM classes, the name has been
changed to SVM, SystemVerilog Methodology, so that you won’t confuse it with
the real thing. Hopefully you will find this explanation of a simple factory easier to
understand than trying to read the UVM source code.
First is Sample 8.50 which has the common base class from which everything
else is built. It is a abstract class because you should never construct an object of this
type, only classes extended from this one.
Sample 8.50 Common SVM base class

8.10 Creating a Test Registry

315

Next is the component class in Sample 8.51. In the UVM, a component is a timeconsuming object that forms the testbench hierarchy, similar to a VMM transactor.
In this simplified example, the hierarchical parent handle has been removed.
Sample 8.51 Component class

Now define svm_object_wrapper, the abstract common base class for the
proxy class as shown in Sample 8.52. It has pure virtual methods to return the name
of the class type, and create an object of this type.
Sample 8.52 Common base class for proxy class

Now for the crucial class, svm_component_registry shown in Sample 8.53.
This is a lightweight class that can be constructed with little overhead. It is parameterized with the test class type and name. Once you have an instance of this class,
your testbench can construct the actual test class at any time, using the create_
object method. This is a singleton class as you only need one copy to create an
instance of the test class. At the start of simulation, the static handle me is initialized
by calling the get() method that constructs the first instance if needed.

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Sample 8.53 Parameterized proxy class

The last major class is svm_factory, which, at its core, is just a singleton class
that holds the array, m_type_names, to go from test case name to the proxy class
that creates an instance of the test class. Also in this class in Sample 8.54 is the
get_test method that reads the test name from the simulation run command line
and constructs an instance of the test class. Unlike Sample 8.46, you even get a little
self checking.

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317

Sample 8.54 Factory class

Lastly is a base test class, extended from svm_component shown in Sample 8.55.
It uses the macro svm_component_utils to define a new data type, type_id, that
points to the proxy class. The macro stringifies the token T that holds the class name,
and turns it into a string containing the value of T with the syntax: `"T`".

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Sample 8.55 Base test class and registration macro

Sample 8.56 Test program

Here are the steps that happen when you start a simulation with the command line
switch +SVM_TESTNAME=TestBase.
• With the macro svm_component_utils, the class TestBase defines the type
type_id based on the class svm_component_registry, with the parameters
TestBase and "TestBase". Because this is a new type, the simulator initializes the static variable svm_component_registry::me by calling the get
method that instantiates the class. This instance is registered in the factory. What
does all this mean? There is now an object that can construct the TestBase
class, and you can get to it through the factory.
• Simulation now starts and the factory’s get_test method reads the test name
from the command line. This string is used an index into the registry to get a
handle to the proxy object. This object’s create_object method constructs an
instance of the TestBase object.

8.11

Conclusion

319

• The program calls the test object’s run_test method, which calls the steps for
the specific class. Now the TestBase class in Sample 8.55 does not do anything
interesting, but add a call to svm_component_utils macro to the test classes
in Sample 8.47 and Sample 8.49 and you can run tests.
Now you can see the basic UVM flow to start tests. The registry contains a list of
proxy classes that can construct test objects.

8.10.3

UVM Factory Build

The UVM factory can also construct objects for any class in the testbench with the
create method in Sample 8.53. Sample 8.57 show how to build a driver.

Sample 8.57 UVM factory build example

The above code calls the static method create to construct an object of type
driver. In UVM, the second argument points to the parent of the component being
created.
The UVM factory allows you to override the component so that when you build
a component, you get an extended one instead.
You may have noticed a change in terminology. In classic OOP, you “construct”
a class by calling the new method, based on the handle type and assigning the
address to the handle on the left side of the assignment statement. With the UVM
factory pattern, you “build” an object by calling the static create method. This
could make an object of the same type as the handle, or an extended type.

8.11

Conclusion

The software concept of inheritance, where new functionality is added to an existing class, parallels the hardware practice of extending the design’s features for each
generation, while still maintaining backwards compatibility.
For example, you can upgrade your PC by adding a larger capacity disk. As long
as it uses the same interface as the old one, you do not have to replace any other part
of the system, yet the overall functionality is improved.
Likewise, you can create a new test by “upgrading” the existing driver class to
inject errors. If you use an existing callback in the driver, you do not have to change
any of the testbench infrastructure.
You need to plan ahead if you want use these OOP techniques. By using virtual
methods and providing sufficient callback points, your test can modify the behavior

8

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Advanced OOP and Testbench Guidelines

of the testbench without changing its code. The result is a robust testbench that does
not need to anticipate every type of disturbance (error-injection, delays, synchronization) that you may want as long as you leave a hook where the test can inject its
own behavior.
The testbench is more complex than what you have previously constructed, but
there is a payback in that the tests become smaller and easier to write. The testbench
does the hard work of sending stimulus and checking responses, so the test only has
to make small tweaks to cause specialized behavior. An extra few lines of testbench
code might replace code that would have to be repeated in every single test.
Lastly, OOP techniques improve your productivity by allowing you to reuse
classes. For example, a parameterized class for a stack that operates on any other
class, rather than a single type, saves you from having to create duplicate code.

8.12

Exercises

1. Given the following class, create a method in an extended class ExtBinary that
multiplies val1 and val2 and returns an integer.

2. Starting with the solution to Exercise 1, use the ExtBinary class to initialize
val1=15, val2=8, and print out the multiplied value.
3. Starting with the solution to Exercise 1, create an extended class Exercise3
that constrains val1 and val2 to be less than 10.
4. Starting with the solution to Exercise 3, use the Exercise3 class to randomize
val1 and val2, and print out the multiplied value.

8.12

Exercises

321

5. Given the class in Exercise 1, and the following declarations, and an extended
class ExtBinary, what will handles mc, mc2, and b point to after executing
each code snippet a-d, or will a compile error occur?

a. mc = new(15,8);
b = mc;

b. b = new(15, 8);
mc = b;

c. mc = new(15, 8);
b = mc;
mc2 = b;
d. mc = new(15, 8);
b = mc;
if($cast(mc2, b))
$display(“Success”);
else
$display(“Error: cannot assign”);

6. Given the classes Binary and Ext Binary in Exercise 1 and the following
copy function for class Binary, create the function Ext Binary::copy.

7. From the solution to Exercise 6, use the copy function to copy the object pointed
to by the extended class handle mc to the extended class handle mc2.
8. Using code Sample 8.26 to Sample 8.28 in Section 8.7.1 and 8.7.2 of the text,
add the ability to randomly delay a transaction between 0 and 100ns.
9. Create a class that can compare any data type using the case equality operators,
=== and !==. It contains a compare function that returns a 1 if the two values
match, 0 otherwise. By default it compares two 4-bit data types.
10. Using the solution from Exercise 9, use the comparator class to compare two
4-bit values, expected_4bit and actual_4bit. Next, compare two values
of type color_t, expected_color and actual_color. Increment an error
counter if an error occurs.

Chapter 9

Functional Coverage

As designs become more complex, the only effective way to verify them effectively
is with constrained-random testing (CRT). This approach elevates you above the
tedium of writing individual directed tests, one for each feature in the design.
However, if your testbench is taking a random walk through the space of all design
states, how do you know if you have reached your destination? Even directed tests
should be double checked with functional coverage. Over the life of a project, small
changes in the DUT’s timing or functionality can subtly alter the results from a
directed test, so it no longer verifies the same features. Whether you are using random or directed stimulus, you can gauge progress using coverage.
Functional coverage is a measure of which design features have been exercised by the
tests. Start with the design specification and create a verification plan with a detailed list
of what to test and how. For example, if your design connects to a bus, your tests need to
exercise all the possible interactions between the design and bus, including relevant
design states, delays, and error modes. The verification plan is a map to show you where
to go. For more information on creating a verification plan, see Bergeron (2006).
In many complex systems, you may never achieve 100% coverage as schedules
don’t allow you to reach every possible corner case. After all, you didn’t have time
to write directed tests to get sufficient coverage, and even CRT is limited by the time
it takes you to create and debug test cases, and analyze the results.
Figure 9.1 shows the feedback loop to analyze the coverage results and decide on
which actions to take in order to converge on 100% coverage. Your first choice is to
run existing tests with more seeds; the second is to build new constraints. Only
resort to creating directed tests if absolutely necessary.
Back when you exclusively wrote directed tests, the verification planning was
limited. If the design specification listed 100 features, all you had to do was write
100 tests. Coverage was implicit in the tests — the “register move” test moved all
combinations of registers back and forth. Measuring progress was easy: if you had
completed 50 tests, you were halfway done. This chapter uses “explicit” and
“implicit” to describe how coverage is specified. Explicit coverage is described

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_9,
© Springer Science+Business Media, LLC 2012

323

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Constrained
random tests

Functional Coverage

Many runs,
runs,
Many
Many seeds
runs,
different
different
seeds
different seeds

Directed
testcase

Add
Add
constraints
constraints

Minimal code
modifications

Functional
Coverage
Identify
Identify
holes
holes

Fig. 9.1 Coverage convergence

directly in the test environment using SystemVerilog features. Implicit coverage is
implied by a test — when the “register move” directed test passes, you have hopefully
covered all register transactions.
With CRT, you are freed from hand crafting every line of input stimulus, but now
you need to write code that tracks the effectiveness of the test with respect to the verification plan. You are still more productive, as you are working at a higher level of
abstraction. You have moved from tweaking individual bits to describing the interesting design states. Reaching for 100% functional coverage forces you to think more
about what you want to observe and how you can direct the design into those states.

9.1

Gathering Coverage Data

You can run the same random testbench over and over, simply by changing the
random seed to generate new stimulus. Each individual simulation generates a database of functional coverage information, the trail of footprints from the random
walk. You can then merge all this information together to measure your overall
progress using functional coverage as shown in Figure 9.2.
You then analyze the coverage data to decide how to modify your tests. If the
coverage levels are steadily growing, you may just need to run existing tests with
new random seeds, or even just run longer tests. If the coverage growth has started
to slow, you can add additional constraints to generate more “interesting” stimuli.
When you reach a plateau, some parts of the design are not being exercised, so you
need to create more tests. Lastly, when your functional coverage values near 100%,
check the bug rate. If bugs are still being found, you may not be measuring true
coverage for some areas of your design. Don’t be in too big of a rush to reach 100%
coverage, which just shows that you looked for bugs in all the usual places. While
you are trying to verify your design, take many random walks through the stimulus
space; this can create many unanticipated combinations, as shown in van der Schoot
(2007).

9.1

Gathering Coverage Data

325

Design
Specification

Design

Verification
Verification
Plan
Plan

Coverage
Database

Tests
Tests

Coverage
Analysis

Debug
Pass?
No

Yes

Fig. 9.2 Coverage flow

Each simulation vendor has its own format for storing coverage data and as well
as its own analysis tools. You need to perform the following actions with those tools.
• Run a test with multiple seeds. For a given set of constraints and coverage
groups, compile the testbench and design into a single executable. Now you need
to run this constraint set over and over with different random seeds. You can use
the Unix system clock as a seed, but be careful, as your batch system may start
multiple jobs simultaneously. These jobs may run on different servers or may
start on a single server with multiple processors. So combine all these values to
make a truly unique seeds. The seed must be saved with the simulation and coverage results for repeatability.
• Check for pass/fail. Functional coverage information is only valid for a successful simulation. When a simulation fails because there is a design bug, the coverage information must be discarded. The coverage data measures how many items
in the verification plan are complete, and this plan is based on the design specification. If the design does not match the specification, the coverage values are
useless. Some verification teams periodically measure all functional coverage
from scratch so that it reflects the current state of the design.
• Analyze coverage across multiple runs. You need to measure how successful
each constraint set is, over time. If you are not yet getting 100% coverage for the
areas that are targeted by the constraints, but the amount is still growing, run more
seeds. If the coverage level has plateaued, with no recent progress, it is time to
modify the constraints. Only if you think that reaching the last few test cases for
one particular section may take too long for constrained-random simulation should
you consider writing a directed test. Even then, continue to use random stimulus
for the other sections of the design, in case this “background noise” finds a bug.

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9.2

9

Functional Coverage

Coverage Types

Coverage is a generic term for measuring progress to complete design verification.
Your simulations slowly paint the canvas of the design, as you try to cover all of the
legal combinations. The coverage tools gather information during a simulation and
then post-process it to produce a coverage report. You can use this report to look for
coverage holes and then modify existing tests or create new ones to fill the holes.
This iterative process continues until you are satisfied with the coverage level.

9.2.1

Code Coverage

The easiest way to measure verification progress is with code coverage. Here you
are measuring how many lines of code have been executed (line coverage), which
paths through the code and expressions have been executed (path coverage),
which single-bit variables have had the values 0 or 1 (toggle coverage), and which
states and transitions in a state machine have been visited (FSM coverage). You
don’t have to write any extra HDL code. The tool instruments your design automatically by analyzing the source code and adding hidden code to gather statistics. You
then run all your tests, and the code coverage tool creates a database.
Most simulators include a code coverage tool. A post-processing tool converts
the database into a readable form. The end result is a measure of how much your
tests exercise the design code. Note that you are primarily concerned with analyzing
the design code, not the testbench. Untested design code could conceal a hardware
bug, or may be just redundant code.
Code coverage measures how thoroughly your tests exercised the “implementation”
of the design specification, but not the verification plan. Just because your tests have
reached 100% code coverage, your job is not done. What if you made a mistake that
your test didn’t catch? Worse yet, what if your implementation is missing a feature?
The module in Sample 9.1 is for a D-flip flop. Can you see the mistake?
Sample 9.1 Incomplete D-flip flop model missing a path

The reset logic was accidently left out. A code coverage tool would report that
every line had been exercised, yet the model was not implemented correctly. Go
back to the functional specification that describes reset behavior and make sure your
verification plan includes a requirement to verify this. Then gather functional coverage information on the design during reset.

9.2

Coverage Types

9.2.2

327

Functional Coverage

The goal of verification is to ensure that a design behaves correctly in its real environment, be that an MP3 player, network router, or cell phone. The design specification details how the device should operate, whereas the verification plan lists how
that functionality is to be stimulated, verified, and measured. When you gather measurements on what functions were covered, you are performing “design” coverage.
For example, the verification plan for a D-flip flop would mention not only its data
storage but also how it resets to a known state. Until your test checks both these
design features, you will not have 100% functional coverage.
Functional coverage is tied to the design intent and is sometimes called “specification coverage,” while code coverage measures how well you have tested the RTL
code and is known as, “implementation coverage.” These are two very different
metrics. Consider what happens if a block of code is missing from the design. Code
coverage cannot catch this mistake and could report that you have executed 100%
of the lines, but functional coverage will show that the functionality does not exist.

9.2.3

Bug Rate

An indirect way to measure coverage is to look at the rate at which fresh bugs are found,
show in the graph in Fig. 9.3. You should keep track of how many bugs you found each
week, over the life of a project. At the start, you may find many bugs through inspection
as you create the testbench. As you read the design spec, you may find inconsistencies,
which hopefully are fixed before the RTL is written. Once the testbench is up and running, a torrent of bugs is found as you check each module in the system. The bug rate
drops, hopefully to zero, as the design nears tape-out. However, you are not yet done.
Every time the rate sags, it is time to find different ways to create corner cases.
Integration

Bugs/week

New DUT code

Design
review

New verif.
engineer

Tape-out

Time
Fig. 9.3 Bug rate during a project

The bug rate can vary per week based on many factors such as project phases,
recent design changes, blocks being integrated, personnel changes, and even vacation schedules. Unexpected changes in the rate could signal a potential problem.

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As shown in Fig. 9.3, it is not uncommon to keep finding bugs even after tape-out,
and even after the design ships to customers.

9.2.4

Assertion Coverage

Assertions are pieces of declarative code that check the relationships between design
signals, either once or over a period of time. These can be simulated along with the
design and testbench, or proven by formal tools. Sometimes you can write the
equivalent check using SystemVerilog procedural code, but many assertions are
more easily expressed using SystemVerilog Assertions (SVA).
Assertions can have local variables and perform simple data checking. If you need
to check a more complex protocol, such as determining whether a packet successfully went through a router, procedural code is often better suited for the job. There
is a large overlap between sequences that are coded procedurally or using SVA. See
Vijayaraghavan and Ramanadhan (2005), Cohen et al. (2005), and Chapters 3 and 7
in the VMM book, Bergeron et al. (2006) for more information on SVA.
The most familiar assertions look for errors such as two signals that should be
mutually exclusive or a request that was never followed by a grant. These error
checks should stop the simulation as soon as they detect a problem. Assertions can
also check arbitration algorithms, FIFOs, and other hardware. These are coded with
the assert property statement.
Some assertions might look for interesting signal values or design states, such as a
successful bus transaction. These are coded with the cover property statement.
You can measure how often these assertions are triggered during a test by using assertion coverage. A cover property observes sequences of signals, whereas a cover group
(described below) samples data values and transactions during the simulation. These
two constructs overlap in that a cover group can trigger when a sequence completes.
Additionally, a sequence can collect information that can be used by a cover group.

9.3

Functional Coverage Strategies

Before you write the first line of test code, you need to anticipate what are the key
design features, corner cases, and possible failure modes. This is how you write
your verification plan. Don’t think in terms of data values only; instead, think about
what information is encoded in the design. The plan should spell out the significant
design states.

9.3.1

Gather Information, not Data

A classic example is a FIFO. How can you be sure you have thoroughly tested a
1K FIFO memory? You could measure the values in the read and write indices,

9.3

Functional Coverage Strategies

329

but there are over a million possible combinations. Even if you were able to simulate
that many cycles, you would not want to read the coverage report.
At a more abstract level, a FIFO can hold from 0 to N–1 possible values. So what
if you just compare the read and write indices to measure how full or empty the
FIFO is? You would still have 1K coverage values. If your testbench pushed 100
entries into the FIFO, then pushed in 100 more, do you really need to know if the
FIFO ever had 150 values? Not as long as you can successfully read out all values.
The corner cases for a FIFO are Full and Empty. If you can make the FIFO go from
Empty (the state after reset) through Full and back down to Empty, you have covered
all the levels in between. Other interesting states involve the indices as they pass
between all 1’s and all 0’s. A coverage report for these cases is easy to understand.
You may have noticed that the interesting states are independent of the FIFO
size. Once again, look at the information, not the data values.
Design signals with a large range (more than a few dozen possible values) should
be broken down into smaller ranges, plus corner cases. For example, your DUT may
have a 32-bit address bus, but you certainly don’t need to collect 4 billion samples.
Check for natural divisions such as memory and IO space. For a counter, pick a few
interesting values, and always try to rollover counter values from all 1’s back to 0.

9.3.2

Only Measure What you are Going to Use

Gathering functional coverage data can be expensive, so only measure what you will
analyze and use to improve your tests. Your simulations may run slower as the simulator monitors signals for functional coverage, but this approach has lower overhead
than gathering waveform traces and measuring code coverage. Once a simulation
completes, the database is saved to disk. With multiple testcases and multiple seeds,
you can fill disk drives with functional coverage data and reports. But if you never
look at the final coverage reports, don’t perform the initial measurements.
There are several ways to control cover data: at compilation, instantiation, or
triggering. You could use switches provided by the simulation vendor, conditional
compilation, or suppression of the gathering of coverage data. The last of these is
less desirable because the post-processing report is filled with sections with 0%
coverage, making it harder to find the few enabled ones.

9.3.3

Measuring Completeness

Like your kids in the backseat on a family vacation, your manager constantly asks
you, “Are we there yet?” How can you tell if you have fully tested a design? You
need to look at all coverage measurements and consider the bug rate to see if you
have reached your destination.
At the start of a project, both code and functional coverage are low. As you
develop tests, run them over and over with different random seeds until you no longer see increasing values of functional coverage. Create additional constraints and

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Functional Coverage

tests to explore new areas. Save test/seed combinations that give high coverage, so
that you can use them in regression testing.

High
Low

Functional Coverage

Fig. 9.4 Coverage comparison
Need more FC
points, including
corner cases

Good coverage:
check bug rate

Start of
project

Is design complete?
Perhaps try
formal tools
High

Low

Code Coverage

What if the functional coverage is high but the code coverage is low as shown in
the upper left of Figure 9.4? Your tests are not exercising the full design, perhaps
from an inadequate verification plan. It may be time to go back to the hardware
specifications and update your verification plan. Then you need to add more functional coverage points to locate untested functionality.
A more difficult situation is high code coverage but low functional coverage.
Even though your testbench is giving the design a good workout, you are unable to
put it in all the interesting states. First, see if the design implements all the specified
functionality. If it is there, but your tests can’t reach it, you might need a formal verification tool that can extract the design’s states and create appropriate stimulus.
The goal is both high code and functional coverage. However, don’t plan your
vacation yet. What is the trend of the bug rate? Are significant bugs still popping up?
Worse yet, are they being found deliberately, or did your testbench happen to
stumble across a particular combination of states that no one had anticipated? On
the other hand, a low bug rate may mean that your existing strategies have run out
of steam, and you should look into different approaches. Try different approaches
such as new combinations of design blocks and error generators.

9.4

Simple Functional Coverage Example

To measure functional coverage, you begin with the verification plan and write an
executable version of it for simulation. In your System Verilog testbench, sample
the values of variables and expressions. These sample locations are known as cover
points. Multiple cover points that are sampled at the same time (such as when a
transaction completes) are placed together in a cover group.
Sample 9.2 has a transaction that comes in eight flavors. The testbench generates
the dst variable randomly, and the verification plan requires that every value be tried.

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Sample 9.2 Functional coverage of a simple object

Sample 9.2 creates a random transaction and drives it out to an interface. The
testbench samples the value of the dst field using the CovDst2 cover group. Eight
possible values, 32 random transactions — did your testbench generate them all?
Samples 9.3 and 9.4 have part of a coverage report from VCS. Because of randomization, every simulator will give different results.
As you can see, the testbench generated dst values of 1, 2, 3, 4, 5, 6, and 7, but
never generated a 0. The at least column specifies how many hits are needed
before a bin is considered covered. See Section 9.10.3 for the at_least option.
To improve your functional coverage, the easiest strategies are to run
more simulation cycles, or to try new random seeds. For Sample
9.2, the very next random transaction (#33) has a dst value of 0, giving 100% coverage. Or, if you started simulation with a different
seed, you may reach 100% in fewer transactions, for this trivial
case. On a real design, you may see a plateau in coverage, with most
coverage points getting hit more and more, but a few stubborn points that are never
hit, no matter how long you run, regardless of seed values. In this case, you probably
have to try a new strategy, as the testbench is not creating the proper stimulus. The
most important part of any coverage report are the points with 0 hits.

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Sample 9.3 Coverage report for a simple object

Sample 9.4 Coverage report for a simple object, 100% coverage

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Anatomy of a Cover Group

333

This book gives a rough explanation of how coverage is calculated. The LRM
has a very detailed explanation of coverage computation across four pages, with
more details across an entire chapter. Consult it for the most accurate details.

9.5

Anatomy of a Cover Group

A cover group is similar to a class — you define it once and then instantiate it one
or more times. It contains cover points, options, formal arguments, and an optional
trigger. A cover group encompasses one or more data points, all of which are sampled at the same time.
You should create very clear cover group names that explicitly indicate what you
are measuring and, if possible, reference to the verification plan. The name Parity_
Errors_In_Hexaword_Cache_Fills may seem verbose, but when you are trying to read a coverage report that has dozens of cover groups, you will appreciate
the extra detail. You can also use the comment option for additional descriptive
information, as shown in Section 9.9.2.
A cover group can be defined in a class or at the program or module level. It can
sample any visible variable such as program/module variables, signals from an
interface, or any signal in the design (using a hierarchical reference). A cover group
inside a class can sample variables in that class, as well as data values from embedded objects.
Don’t define the cover group in a data class, such as a transaction,
as doing so can cause additional overhead when gathering coverage data. Imagine you are trying to track how many beers were
consumed by patrons in a pub. Would you try to follow every bottle
as it flowed from the loading dock, over the bar, and into each person? No, instead you could just have each patron check off the type and number of
beers consumed, as shown in van der Schoot (2006).
In SystemVerilog, you should define cover groups at the appropriate level of abstraction. This level can be at the boundary between your testbench and the design, in the
transactors that read and write data, in the environment configuration class, or wherever is needed. The sampling of any transaction must wait until it is actually received
by the DUT. If you inject an error in the middle of a transaction, causing it to be
aborted in transmission, you need to change how you treat it for functional coverage. You need to use a different cover point that has been created just for error
handling.
A class can contain multiple cover groups. This approach allows you to have
separate groups that can be enabled and disabled as needed. Additionally, each
group may have a separate trigger, allowing you to gather data from many
sources.

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A cover group must be instantiated for it to collect data. If you
forget, no error message about null handles is printed at run
time, but the coverage report will not contain any mention of the
cover group. This rule applies for cover groups defined either
inside or outside of classes.

9.5.1

Defining a Cover Group in a Class

A cover group can be defined in a program, module, or class. In all cases, you must
explicitly instantiate it to start sampling. If the cover group is defined in a class, it is
known as an embedded covergroup. In this case, you do not make a separate name
when you construct it; just use the original cover group name. You must construct
an embedded covergroup in the class’s constructor, as opposed to a non-embedded
cover group that can be constructed at any time.
Sample 9.5 is very similar to the first example of this chapter except that it
embeds a cover group in a transactor class, and thus does not need a separate instance
name.
Sample 9.5 Functional coverage inside a class

9.6 Triggering a Cover Group

9.6

335

Triggering a Cover Group

The two major parts of functional coverage are the sampled data values and the time
when they are sampled. When new values are ready (such as when a transaction has
completed), your testbench triggers the cover group. This can be done directly
with the sample function, as shown in Sample 9.5, or by using a coverage event
in the covergroup definition. The coverage event can use a @ to block on signals
or events.
Use sample if you want to explicitly trigger coverage from procedural code, if
there is no existing signal or event that tells when to sample, or if there are multiple
instances of a cover group that trigger separately.
Use the coverage event in the covergroup declaration if you want to tap into
existing events or signals to trigger coverage.

9.6.1

Sampling Using a Callback

One of the better ways to integrate functional coverage into your testbench is to use
callbacks, as originally shown in Section 8.7. This technique allows you to build a
flexible testbench without restricting when coverage is collected. You can decide
for every point in the verification plan where and when values are sampled. And if
you need an extra “hook” in the environment for a callback, you can always add
one in an unobtrusive manner, as a callback only “fires” during simulations when
the test registers a callback object. You can create many separate callbacks for each
cover group, with little overhead. As explained in Section 8.7.4, callbacks are
superior to using a mailbox to connect the testbench to the coverage objects. You
might need multiple mailboxes to collect transactions from different points in your
testbench. A mailbox requires a transactor to receive transactions, and multiple
mailboxes cause you to juggle multiple threads. Instead of an active transactor, use
a passive callback.
Sample 8.26 – 8.28 shows a driver class that has two callback points, before and
after the transaction is transmitted. Sample 8.26 shows the base callback class, and
Sample 8.28 has a test with an extended callback class that sends data to a scoreboard. Make your own extension, Driver_cbs_coverage, of the base callback
class, Driver_cbs, to call the sample task for your cover group in post_tx. Push
an instance of the coverage callback class into the driver’s callback queue, and your
coverage code triggers the cover group at the right time. Samples 9.6 and 9.7 define
and use the callback Driver_cbs_coverage.

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Sample 9.6 Test using functional coverage callback

The UVM recommends gathering coverage by monitoring the DUT and sending
transactions to a coverage component through an analysis port, similar to a mailbox.
Sample 9.7 Callback for functional coverage

9.6.2

Cover Group with a User Defined Sample Argument List

In Sample 9.5, the cover group samples a variable in transaction object that is
defined inside the class. If your cover group is defined outside of a class, you can
pass variables through the sample method by defining your own argument list.
Now you can sample variables from anywhere in the testbench.
In Sample 9.8, the cover group is expanded to also cover the low data bit. The last
statement of the run method passes the destination address and also the configuration variable for high speed mode.

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337

Sample 9.8 Defining an argument list to the sample method

9.6.3

Cover Group with an Event Trigger

In Sample 9.9, the cover group CovDst9 is sampled when the testbench triggers the
trans_ready event.
Sample 9.9 Cover group with a trigger

The advantage of using an event over calling the sample method directly is that
you may be able to use an existing event such as one triggered by an assertion, as
shown in Sample 9.11.

9.6.4

Triggering on a System Verilog Assertion

If you already have an SVA that looks for useful events like a complete transaction,
you can add an event trigger to wake up the cover group as shown in 9.10.

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Sample 9.10 Module with SystemVerilog Assertion

Sample 9.11 Triggering a cover group with an SVA

9.7

Data Sampling

How is coverage information gathered? When you specify a variable or expression
in a cover point, SystemVerilog creates a number of “bins” to record how many
times each value has been seen. These bins are the basic units of measurement for
functional coverage. If you sample a one-bit variable, a maximum of two bins are
created. You can imagine that System Verilog drops a token in one or the other bin
every time the cover group is triggered. At the end of each simulation, a database is
created with all bins that have a token in them. You then run an analysis tool that
reads all databases and generates a report with the coverage for each part of the
design and for the total coverage.

9.7.1

Individual Bins and Total Coverage

To calculate the coverage for a point, you first have to determine the total number of
possible values, also known as the domain. There may be one value per bin or multiple

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values. Coverage is the number of sampled values divided by the number of bins in
the domain.
A cover point that is a 3-bit variable has the domain 0:7 and is normally divided
into eight bins. If, during simulation, values belonging to seven bins are sampled,
the report will show 7/8 or 87.5% coverage for this point. All these points are combined to show the coverage for the entire group, and then all the groups are combined to give a coverage percentage for all the simulation databases.
This is the status for a single simulation. You need to track coverage over time.
Look for trends so you can see where to run more simulations or add new constraints or tests. Now you can better predict when verification of the design will be
completed.

9.7.2

Creating Bins Automatically

As you saw in the report in Sample 9.3, System Verilog automatically creates bins for
cover points. It looks at the domain of the sampled expression to determine the range
of possible values. For an expression that is N bits wide, there are 2N possible values.
For the 3-bit variable dst, there are 8 possible values. The range of an enumerated
type is shown in Section 9.6.8. The domain for enumerated data types is the number
of named values. You can also explicitly define bins as shown in Section 9.6.5.

9.7.3

Limiting the Number of Automatic Bins Created

The cover group option auto_bin_max specifies the maximum number of bins to
automatically create, with a default of 64 bins. If the domain of values in the cover
point variable or expression is greater than this option, System Verilog divides the
range into auto_bin_max bins. For example, a 16-bit variable has 65,536 possible
values, so each of the 64 bins covers 1024 values.
In reality, you may find this approach impractical, as it is very difficult to find the
needle of missing coverage in a haystack of auto-generated bins. Lower this limit to
8 or 16, or better yet, explicitly define the bins as shown in Section 9.6.5.
Sample 9.12 takes the chapter’s first example and adds a cover point option that
sets auto_bin_max to two bins. The sampled variable is still dst, which is three
bits wide, for a domain of eight possible values. The first bin holds the lower half of
the range, 0–3, and the other hold the upper values, 4–7.
Sample 9.12 Using auto_bin_max set to 2

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The coverage report from VCS shows the two bins. This simulation achieved
100% coverage because the eight dst values were mapped to two bins. Since both
bins have sampled values, your coverage is 100% as shown in Sample 9.13.
Sample 9.13 Report with auto_bin_max set to 2

Sample 9.12 used auto_bin_max as an option for the cover point only. You can
also use it as an option for the entire group as shown in Sample 9.14.
Sample 9.14 Using auto_bin_max for all cover points

9.7.4

Sampling Expressions

You can sample expressions, but always check the coverage report to be sure you
are getting the values you expect. You may have to adjust the width of the computed
expression, as shown in Section 2.16. For example, sampling a 3-bit header length
(0:7) plus a 4-bit payload length (0:15) creates only 24 or 16 bins, which may not be
enough if your transactions can actually be from 0 to 22 bytes long.
Sample 9.15 Using an expression in a cover point

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Sample 9.15 has a cover group that samples the total packet length. The cover
point has a label to make it easier to read the coverage report. Also, the expression
has an additional dummy constant so that the transaction length is computed with
5-bit precision, for a maximum of 32 auto-generated bins.
A long run with random packets showed that the len16 had 100% coverage, but
this is across only 16 bins. (The cover point only has 16 bins as the sum of a 3-bit
and 4-bit value is only 4-bits in Verilog.) The cover point len32 had 72% coverage
across 32 bins. (The addition of a 5-bit value to the expression for bin32 results in
a 5-bit result.) Neither of these cover points are correct, as the maximum length has
a domain of 0:22 (0+0:7+15). The auto-generated bins just don’t work, as the maximum length is not a power of 2. You need a way to precisely define bins.

9.7.5

User-Defined Bins Find a Bug

Automatically generated bins are okay for anonymous data values, such as counter
values, addresses, or values that are a power of 2. For other values, you should
explicitly name the bins to improve accuracy and ease coverage report analysis.
System Verilog automatically creates bin names for enumerated types, but for other
variables you need to give names to the interesting states. The easiest way to specify
bins is with the [] syntax, as shown in Sample 9.16.
Sample 9.16 Defining bins for transaction length

After sampling many random transactions, the group has 95.83% coverage.
A quick look at the report in Sample 9.17 shows the problem — the length of 23 (17
hex) was never seen. The longest header is 7, and the longest payload is 15, for a
total of 22, not 23! If you change to the bins declaration to use 0:22, the coverage
jumps to 100%. The user-defined bins found a bug in the test.

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Sample 9.17 Coverage report for transaction length

9.7.6

Naming the Cover Point Bins

Sample 9.18 samples a 4-bit variable, kind, that has 16 possible values. The first
bin is called zero and counts the number of times that kind is 0 when sampled. The
next four values, 1–3 and 5, are all grouped into a single bin, lo. The upper eight
values, 8–15, are kept in separate bins, hi_8, hi_9, hi_a, hi_b, hi_c, hi_d,
hi_e, and hi_f. Note how $ in the hi bin expression is used as a shorthand notation for the largest value for the sampled variable. Lastly, misc holds all values that
were not previously chosen: 4, 6, and 7.

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Sample 9.18 Specifying bin names

Note that the additional information about the coverpoint is grouped using
curly braces: {}. This is because the bin specification is declarative code, not procedural code that would be grouped with begin…end. Lastly, the final curly brace
is NOT followed by a semicolon, just as an end never is.
Now you can easily see in Sample 9.19 which bins have no hits — hi_8 in this case.
Sample 9.19 Report showing bin names

When you define the bins, you are restricting the values used for coverage to
those that are interesting to you. SystemVerilog no longer automatically creates
bins, and it ignores values that do not fall into a predefined bin. More importantly,
only the bins you create are used to calculate functional coverage. You get 100%
coverage only as long as you get a hit in every specified bin.
Values that do not fall into any specified bin are ignored. This rule
is useful if the sampled value, such as transaction length, is not a
power of 2. If you are specifying bins, you can use the default bin
specifier to catch values that you may have forgotten. However, the
LRM says that default bins are not used in coverage calculation.
In Sample 9.18, the range for hi uses a dollar sign ($) on the right side to specify
the upper value. This is a very useful shortcut - now you can let the compiler calculate

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the limits for a range. You can use the dollar sign on the left side of a range to
specify the lower limit. In Sample 9.20, the $ in the range for bin neg represents the
negative number furthest from zero: 32¢h8000_0000, or -2,147,483,648, whereas
the $ in bin pos represents the largest signed positive value, 32’h7FFF_FFFF, or
2,147,483,647.
Sample 9.20 Specifying ranges with $

9.7.7

Conditional Coverage

You can use the iff keyword to add a condition to a cover point. The most common
reason for doing so is to turn off coverage during reset so that stray triggers are
ignored. Sample 9.21 gathers only values of dst when rst is 0, where rst is
active-high.
Sample 9.21 Conditional coverage — disable during reset

Alternately, you can use the start and stop functions to control individual
instances of cover groups as shown in Sample 9.22.
Sample 9.22 Using stop and start functions

9.7

Data Sampling

9.7.8

345

Creating Bins for Enumerated Types

For enumerated types, SystemVerilog creates a bin for each value as you can see in
Sample 9.23.
Sample 9.23 Functional coverage for an enumerated type

Here is part of the coverage report from VCS, Sample 9.24 showing the bins for
the enumerated types.
Sample 9.24 Coverage report with enumerated types

If you want to group multiple values into a single bin, you have to define your
own bins. Any bins outside the enumerated values are ignored unless you define a
bin with the default specifier. When you gather coverage on enumerated types,
auto_bin_max does not apply.

9.7.9

Transition Coverage

You can specify state transitions for a cover point. In this way, you can tell not only
what interesting values were seen but also the sequences. For example, you can
check if dst ever went from 0 to 1, 2, or 3 as shown in Sample 9.25.
Sample 9.25 Specifying transitions for a cover point

You can quickly specify multiple transitions using ranges. The expression (1, 2
=> 3, 4) creates the four transitions (1=>3), (1=>4), (2=>3), and (2=>4).
You can specify transitions of any length. Note that you have to sample once for
each state in the transition. So (0 => 1 => 2) is different from (0 => 1 => 1 => 2)

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or (0 => 1 => 1 => 1 => 2). If you need to repeat values, as in the last
sequence, you can use the shorthand form:(0 => 1[*3] => 2). To repeat the
value 1 for 3, 4, or 5 times, use 1[*3:5].

9.7.10

Wildcard States and Transitions

You use the wildcard keyword to create multiple states and transitions. Any X, Z,
or ? in the expression is treated as a wildcard for 0 or 1. Sample 9.26 creates a cover
point with a bin for even values and one for odd.
Sample 9.26 Wildcard bins for a cover point

9.7.11

Ignoring Values

With some cover points, you never get all possible values. For instance, a 3-bit variable may be used to store just six values, 0–5. If you use automatic bin creation, you
never get beyond 75% coverage. There are two ways to solve this problem. You can
explicitly define the bins that you want to cover as shown in Section 9.6.5.
Alternatively, you can let SystemVerilog automatically create bins, and then use
ignore_bins to tell which values to exclude from functional coverage calculation like
in Sample 9.27.
Sample 9.27 Cover point with ignore_bins

The original range of low_ports_0_5, a three-bit variable is 0:7. The ignore_
bins excludes the last two bins, which reduces the range to 0:5. So total coverage
for this group is the number of bins with samples, divided by the total number of
bins, which is 5 in this case.

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Sample 9.28 Cover point with auto bin max and ignore bins

If you define bins either explicitly or by using the auto_bin_max option, and
then ignore them, the ignored bins do not contribute to the calculation of coverage.
In Sample 9.28, four bins are initially created using the auto_bin_max option: 0:1,
2:3, 4:5, and 6:7. However, then the uppermost bin is eliminated by ignore_bins,
so in the end only three bins are created. This cover point can have coverage of 0%,
33%, 66%, or 100%.

9.7.12

Illegal Bins

Some sampled values not only should be ignored, but also should cause an error if
they are seen. This is best done in the testbench’s monitor code, but can also be done
by labeling a bin with illegal_bins as shown in Sample 9.29. Use this to catch
states that were missed by the test’s error checking. This also double-checks the
accuracy of your bin creation: if an illegal value is found by the cover group, it is a
problem either with the testbench or with your bin definitions.
Sample 9.29 Cover point with illegal_bins

9.7.13

State Machine Coverage

You should have noticed that if a cover group is used on a state machine, you can
use bins to list the specific states, and transitions for the arcs. However, this does not
mean you should use SystemVerilog’s functional coverage to measure state machine
coverage. You would have to extract the states and arcs manually. Even if you did
this correctly the first time, you might miss future changes to the design code.
Instead, use a code coverage tool that extracts the state register, states, and arcs
automatically, saving you from possible mistakes.
However, an automatic tool extracts the information exactly as coded, mistakes
and all. You may want to monitor small, critical state machines manually using
functional coverage.

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Cross Coverage

A cover point records the observed values of a single variable or expression. You
may want to know not only what bus transactions occurred but also what errors happened during those transactions, and their source and destination. For this you need
cross coverage that measures what values were seen for two or more cover points at
the same time. Note that when you measure cross coverage of a variable with N
values, and of another with M values, SystemVerilog needs N ´ M cross bins to store
all the combinations.

9.8.1

Basic Cross Coverage Example

Previous examples have measured coverage of the transaction kind, and destination
port number, but what about the two combined? Did you try every kind of transaction into every port? The cross construct in SystemVerilog records the combined
values of two or more cover points in a group. The cross statement takes only
cover points or a simple variable name. If you want to use expressions, hierarchical
names or variables in an object such as handle.variable, you must first specify
the expression in a coverpoint with a label and then use the label in the cross
statement.
Sample 9.30 creates cover points for tr.kind and tr.dst. Then the two points
are crossed to show all combinations. SystemVerilog creates a total of 128 (8 ´ 16)
bins. Be careful: even a simple cross can result in a very large number of bins.
Sample 9.30 Basic cross coverage

A random testbench created 56 transactions and produced the coverage report in
Sample 9.31. Note that even though all possible kind and dst values were generated, only 1/3 of the cross combinations were seen. This is a very typical result.
Also note that the total coverage for the group is the cross coverage plus the coverage for kind and dst.

9.8

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Sample 9.31 Coverage summary report for basic cross coverage

9.8.2

Labeling Cross Coverage Bins

If you want more readable cross coverage bin names, you can label the individual cover
point bins as demonstrated in Sample 9.32, and SystemVerilog will use these names
when creating the cross bins.

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Sample 9.32 Specifying cross coverage bin names

If you define bins that contain multiple values, the coverage statistics change. In the
report below, the number of bins has dropped from 128 to 80. This is because kind
has 10 bins: zero, lo, hi_8, hi_9, 9hi_a, hi_b, hi_c, hi_d, hi_e, and hi_f.
Remember that the misc bin, which defined its values with default, does not add
to the coverage total. The percentage of coverage jumped from 87.5% to 90.91%
as shown in Sample 9.33 because any single value in the lo bin, such as 2, allows
that bin to be marked as covered, even if the other values, 1 or 3, are not seen.
Sample 9.33 Cross coverage report with labeled bins

9.8

Cross Coverage

9.8.3

351

Excluding Cross Coverage Bins

To reduce the number of bins, use ignore_bins. With cross coverage, you specify
the cover point with binsof and the set of values with intersect so that a single
ignore_bins construct can sweep out many individual bins.
Sample 9.34 Excluding bins from cross coverage

The first ignore_bins in Sample 9.34 just excludes bins where dst is 7 and any
value of kind. Since kind is a 4-bit value, this statement excludes 12 bins, as misc’s
values of 4–7 don’t count because of the default. The second ignore_bins is more
selective, ignoring bins where dst is 0 and kind is 9, 10, or 11, for a total of 3 bins.
The ignore_bins can use the bins defined in the individual cover points. The
ignore_bins lo uses bin names to exclude kind.lo that is 1, 2, or 3. The bins
must be names defined at compile time, such as zero and lo. The bins hi_8, hi_9,
hi_a,… hi_f, and any automatically generated bins do not have names that can be
used at compile time in other statements such as ignore_bins; these names are
created at run time or during the report generation.
Note that binsof uses parentheses () while intersect specifies a range and
therefore uses curly braces {}.

9.8.4

Excluding Cover Points from the Total Coverage Metric

The total coverage for a group is based on all simple cover points and cross coverage.
If you are only sampling a variable or expression in a coverpoint to be used in a
cross statement, you should set its weight to 0 so that it does not contribute to the
total coverage.

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Sample 9.35 Specifying cross coverage weight

There are two types of options: those that are specific to an instance of a covergroup and those that specify an option for the covergroup type as a whole. The
instance specific options are like local variables and are specified with the option
keyword, as in option.auto_bin_max=2 from Sample 9.12. The alternatives are
specified with the type_option keyword and are tied to the cover group, like static
variables in a class. In Sample 9.35, type_option.weight applies to all instances
of this group. The LRM has a detailed explanation of the difference, and this book
shows the most common options and their usage.

9.8.5

Merging Data from Multiple Domains

One problem with cross coverage is that you may need to sample values from different timing domains. You might want to know if your processor ever received an
interrupt in the middle of a cache fill. The interrupt hardware is separate from and
may use different clocks than the cache hardware, making it difficult to know when
to trigger the cover group. On the other hand, you want to make sure you have tested
this case, as a previous design had a bug of this very sort.
The solution is to create a timing domain separate from the cache or interrupt
hardware. Make copies of the signals into temporary variables and then sample
them in a new coverage group that measures the cross coverage.

9.8.6

Cross Coverage Alternatives

As your cross coverage definition becomes more elaborate, you may spend considerable time specifying which bins should be used and which should be ignored. You
may have two random bits, a and b with three interesting states, {a==0, b==0},
{a==1, b==0}, and {b==1}.

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353

Sample 9.36 shows how you can name bins in the cover points and then gather
cross coverage using those bins.
Sample 9.36 Cross coverage with bin names

Sample 9.37 gathers the same cross coverage, but now uses binsof to specify
the cross coverage values.
Sample 9.37 Cross coverage with binsof

Alternatively, you can make a cover point that samples a concatenation of values.
Then you only have to define bins using the less complex cover point syntax.

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Sample 9.38 Mimicking cross coverage with concatenation

Use the style in Sample 9.36 if you already have bins defined for the individual
cover points and want to use them to build the cross coverage bins. Use Sample 9.37
if you need to build cross coverage bins but have no pre-defined cover point bins.
Use Sample 9.38 if you want the tersest format.

9.9

Generic Cover Groups

As you start writing cover groups, you will find that some are very similar to one
another. SystemVerilog allows you to create a generic cover group so that you can
specify a few unique details when you instantiate it.

9.9.1

Pass Cover Group Arguments by Value

Sample 9.39 shows a cover group that uses an argument to split the range into two
halves. Just pass the midpoint value to the cover groups’ new function.
Sample 9.39 Covergroup with simple argument

9.10

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9.9.2

355

Pass Cover Group Arguments by Reference

You can specify a variable to be sampled with pass-by-reference. Here you want the
cover group to sample the value during the entire simulation, not just to use the
value when the constructor is called.
Sample 9.40 Pass-by-reference

Like a task or function, the arguments to a cover group have a
sticky direction. In Sample 9.40, if you forgot the input direction, the mid argument will have the direction ref. The example
would not compile because you cannot pass a constant (4 or 2)
into a ref argument.

9.10

Coverage Options

You can specify additional information in the cover group using options. There are
two flavors of options: instance options that apply to a specific cover group instance
and type options that apply to all instances of the cover group, and are analogous to
static data members of classes. Options can be placed in the cover group so that they
apply to all cover points in the group, or they can be put inside a single cover point
for finer control. You have already seen the auto_bin_max and weight options.
Here are several more.

9.10.1

Per-Instance Coverage

If your testbench instantiates a coverage group multiple times, by default SystemVerilog groups together all the coverage data from all the instances. However, if you

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have several generators, each creating very different streams of transactions, you
will need to see separate reports. For example, one generator may be creating long
transactions while another makes short ones. The cover group in Sample 9.41 can
be instantiated in each separate generator. It keeps track of coverage for each
instance, and has a unique comment string with the hierarchical path to the cover
group instance.
Sample 9.41 Specifying per-instance coverage

The per-instance option can only be given in the cover group, not in the cover
point or cross point.

9.10.2

Cover Group Comment

You can add a comment into coverage reports to make them easier to analyze.
A comment could be as simple as the section number from the verification plan to
tags used by a report parser to automatically extract relevant information from the
sea of data. If you have a cover group that is only instantiated once, use the type_
option as shown in Sample 9.42.
Sample 9.42 Specifying comments for a cover group

However, if you have multiple instances, you can give each a separate comment,
as long as you also use the per-instance option as shown in Sample 9.43.
Sample 9.43 Specifying comments for a cover group instance

9.10

Coverage Options

9.10.3

357

Coverage Threshold

You may not have sufficient visibility into the design to gather robust coverage
information. Suppose you are verifying that a DMA state machine can handle bus
errors. You don’t have access to its current state, but you know the range of cycles
that are needed for a transfer. So if you repeatedly cause errors during that range,
you have probably covered all the states. So you could set option.at_least to 8
or more to specify that after 8 hits on a bin, you are confident that you have exercised that combination.
If you define option.at_least at the cover group level, it applies to all cover
points. If you define it inside a point, it only applies to that single point.
However, as Sample 9.2 showed, even after 32 attempts, the random kind variable still did not hit all possible values. So only use at_least if there is no direct
way to measure coverage, like when the testbench can not probe the DUT details.

9.10.4

Printing the Empty Bins

By default, the coverage report shows only the bins with samples. Your job is to verify
all that is listed in the verification plan, so you are actually more interested in the bins
without samples. Use the option cross_num_print_missing to tell the simulation
and report tools to show you all bins, especially the ones with no hits. Set it to a
large value, as shown in Sample 9.44, but no larger than you are willing to read.
Sample 9.44 Report all bins including empty ones

9.10.5

Coverage Goal

The goal for a cover group or point is the level at which the group or point is
considered fully covered. The default is 100% coverage. If you set this level below
100% like in Sample 9.45, you are requesting less than complete coverage, which is
probably not desirable. This option affects only the coverage report.

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Sample 9.45 Specifying the coverage goal

9.11

Analyzing Coverage Data

In general, assume you need more seeds and fewer constraints. After all, it is easier
to run more tests than to construct new constraints. If you are not careful, new constraints can easily restrict the search space.
If your cover point has only zero or one sample, your constraints are probably
not targeting these areas at all. You need to add constraints that “pull” the solver
into new areas. In Sample 9.16, the transaction length had an uneven distribution.
Sample 9.46 shows the full class. This situation is similar to the distribution seen
when you roll two dice and look at the total value.
Sample 9.46 Original class for packet length

The problem with this class is that len is not evenly weighted. Look in the coverage report and note how the low and high values are rarely hit. Figure 9.5 is a
graph of the values from the report.

160
140

Count

120
100
80
60
40
20
0
0

5

10

15

Packet length

Fig. 9.5 Uneven probability for packet length

20

25

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359

If you want to make the total length be evenly distributed, use a solve…before
constraint as shown in Sample 9.47 and plotted in Fig. 9.6.
Sample 9.47 solve…before constraint for packet length

120
100

Count

80
60
40
20
0
0

5

10

15

20

25

Packet length

Fig. 9.6 Even probability for packet length with solve…before

The normal alternative to solve…before is the dist constraint. However, this
does not work, as len is also being constrained by the sum of the two lengths.

9.12

Measuring Coverage Statistics During Simulation

You can query the level of functional coverage on the fly during simulation. This
allows you to check whether you have reached your coverage goals, and possibly to
control a random test.
At the global level, you can get the total coverage of all cover groups with
$get_coverage, which returns a real number between 0. and 100. This system
task looks across all cover groups.
You can narrow down your measurements with the get_coverage() and get_
inst_coverage() methods. The first function works with both cover group names
and instances to give coverage across all instances of a cover group, for example
CoverGroup::get_coverage() or cgInst.get_coverage(). The second
function returns coverage for a specific cover group instance, for example cgInst.
get_inst_coverage(). You need to specify option.per_instance=1 if you
want to gather per-instance coverage.

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The most practical use for these functions is to monitor coverage over a long test.
If the coverage level does not advance after a given number of transactions or cycles,
the test should stop. Hopefully, another seed or test will increase the coverage.
While it would be nice to have a test that can perform some sophisticated actions
based on functional coverage results, it is very hard to write this sort of test. Each
test + random seed pair may uncover new functionality, but it may take many runs
to reach a goal. If a test finds that it has not reached 100% coverage, what should it
do? Run for more cycles? How many more? Should it change the stimulus being
generated? How can you correlate a change in the input with the level of functional
coverage? The one reliable thing to change is the random seed, which you should
only do once per simulation. Otherwise, how can you reproduce a design bug if the
stimulus depends on multiple random seeds?
You can query the functional coverage statistics if you want to create your own
coverage database. Verification teams have built their own SQL databases that are
fed functional coverage data from simulation. This setup allows them greater control over the data, but requires a lot of work outside of creating tests.
Some formal verification tools can extract the state of a design and then create
input stimulus to reach all possible states. Don’t try to duplicate this in your
testbench!

9.13

Conclusion

When you switch from writing directed tests, hand-crafting every bit of stimulus, to
constrained-random testing, you might worry that the tests are no longer under your
command. By measuring coverage, especially functional coverage, you regain control by knowing what features have been tested.
Using functional coverage requires a detailed verification plan and much time
creating the cover groups, analyzing the results, and modifying tests to create the
proper stimulus. This may seem like a lot of work, but is less effort than would be
required to write the equivalent directed tests. Additionally, the time spent in gathering coverage helps you better track your progress in verifying your design.

9.14

Exercises

1. For the class below, write a covergroup to collect coverage on the test plan
requirement, “All ALU opcodes must be tested.” Assume the opcodes are valid
on the positive edge of signal clk.

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361

2. Expand the solution to Exercise 1 to cover the test plan requirement, “Operand1
shall take on the values maximum negative (−128), zero, and maximum positive
(127).” Define a coverage bin for each of these values as well as a default bin.
Label the coverpoint operand1_cp.
3. Expand the solution to Exercise 2 to cover the following test plan requirements:
a. “The opcode shall take on the values ADD or SUB” (hint: this is 1 coverage
bin).
b. “The opcode shall take on the values ADD followed by SUB” (hint: this is a
second coverage bin).
Label the coverpoint opcode_cp.
4. Expand the solution to Exercise 3 to cover the test plan requirement, “Opcode
must not equal DIV” (hint: report an error using illegal_bins).
5. Expand the solution to Exercise 4 to collect coverage on the test plan requirement,
“The opcode shall take on the values ADD or SUB when operand1 is maximum
negative or maximum positive value.” Weight the cross coverage by 5.
6. Assuming that your covergroup is called Covcode and the instantiation name of
the covergroup is ck, expand Exercise 4 to:
a. Display the coverage of coverpoint operand1_cp referenced by the instantiation name.
b. Display the coverage of coverpoint opcode_cp referenced by the covergroup
name.

Chapter 10

Advanced Interfaces

In Chapter 4 you learned how to connect the design and testbench with interfaces.
These physical interfaces represent real signals, similar to the wires that connected
ports in Verilog-1995. A testbench uses these interfaces by statically connecting to
them through ports. However, for many designs, the testbench needs to connect
dynamically to the design.
For example, in a network switch, a single driver class may connect to many
interfaces, one for each input channel of the DUT. You wouldn’t want to write a
unique driver for each channel — instead you want to write a generic driver, instantiate it N times, and have it connect to each of the N physical interfaces. You can do
this in SystemVerilog by using a virtual interface, which is merely a handle or
pointer to a physical interface. A better name for a virtual interface would be a “ref
interface.”
You may need to write a testbench that attaches to several different configurations of your design. In another example, a chip may have multiple configurations.
In one, the pins might drive a USB bus, whereas in another the same pins may drive
an I2C serial bus. Once again, you can use a virtual interface so you can decide at
run time which drivers to run in your testbench.
A SystemVerilog interface is more than just signals — you can put executable
code inside. This might include routines to read and write to the interface, initial and
always blocks that run code inside the interface, and assertions to constantly check
the status of the signals. However, do not put testbench code in an interface. Program
blocks have been created expressly for building a testbench, including scheduling
their execution in the Reactive region, as described in the SystemVerilog LRM.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_10,
© Springer Science+Business Media, LLC 2012

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Virtual Interfaces with the ATM Router

The most common use for a virtual interface is to allow objects in a testbench to
refer to items in a replicated interface using a generic handle rather than the actual
name. Virtual interfaces are the only mechanism that can bridge the dynamic world
of objects with the static world of modules and interfaces.

10.1.1 The Testbench with Just Physical Interfaces
Chapter 4 showed how to build an interface to connect a 4x4 ATM router to a testbench. Sample 10.1 and 10.2 show the ATM interfaces for the receive and transmit
directions.
Sample 10.1 Rx interface with clocking block

Sample 10.2 Tx interface with clocking block

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Virtual Interfaces with the ATM Router

365

These interfaces can be used in a program block shown in Sample 10.3. This
procedural code is hard coded with interface names such as Rx0 and Tx0. Note that
in these examples, the top module does not pass a clock to the testbench; instead the
tests synchronize with clocking blocks in the interfaces, thus allowing you to work
at a higher level of abstraction.
Sample 10.3 Testbench using physical interfaces

Figure 10.1 shows the testbench communicating with the design through virtual
interfaces.

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Testbench
d0
d1
d2

Rx

Tx

4x4 ATM

router

d3

m0
m1
m2
m3

top
Fig. 10.1 Router and testbench with interfaces

The top level module must connect an array of interfaces to work with the
testbench in Sample 10.6. The module in Sample 10.4 instantiates an array of interfaces, and passes this array to the testbench. Since the DUT was written with four
RX and four TX interfaces, you need to pass the individual interface array elements
into the DUT instance.
Sample 10.4 Top level module with array of interfaces

10.1.2 Testbench with Virtual Interfaces
A good OOP technique is to create a class that uses a handle to reference an object,
rather than a hard-coded object name. In this case, you can make a single Driver
class and a single Monitor class, have them operate on a handle to the data, and then
pass in the handle at run time.

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Virtual Interfaces with the ATM Router

367

The program block in Sample 10.5 is still passed the 4 Rx and 4 Tx interfaces
as ports, as in Sample 10.3, but it creates an array of virtual interfaces, vRx and vTx.
These can now be passed into the constructors for the drivers and monitors.
Sample 10.5 Testbench using virtual interfaces

You can also skip the virtual interface array variables, and make an array in the
port list. These interfaces are passed to the constructors as shown in Sample 10.6.
Sample 10.6 Testbench using virtual interfaces

The task monitor::receive_cell in Sample 10.7 is similar to the task
receive_cell0 in Sample 10.3, except it uses the virtual interface name Tx
instead of the physical interface Tx0.

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Sample 10.7 Monitor class using virtual interfaces

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Virtual Interfaces with the ATM Router

369

A common mistake when creating a testbench is to leave off the
modport name from a virtual interface declaration. The program in
Sample 10.5 declares Tx_if.TB Tx0 in the port list, so it can only
assign Tx0 to a virtual interface declared with the TB modport. See
the declaration of the virtual interface Tx in Sample 10.7.

10.1.3 Connecting the Testbench to an Interface in Port List
This book shows tests that connect to the DUT with interfaces in the port list. This
style is comfortable to Verilog users who have always connected modules using
signals in ports. Sample 10.8 is the top level module, also known as a test harness,
which connects the DUT and test using an interface in the port list.
Sample 10.8 Test harness using an interface in the port list

Sample 10.9 shows the program block with an interface in the port list.

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Sample 10.9 Test with an interface in the port list

What happens if you add a new interface to your design? The test harness in
Sample 10.10 declares the new bus and puts it in the port lists.
Sample 10.10 Top module with a second interface in the test’s port list

Now you have to change the test in Sample 10.9 to include another interface in
the port list, giving the test in Sample 10.11.
Sample 10.11 Test with two interfaces in the port list

Adding a new interface to your design means you need to edit all existing tests
so they can plug into the test harness. How can you avoid this extra work? Avoid
port connections!

10.1.4 Connecting the Test to an Interface with an XMR
Your test needs to connect to the physical interface in the harness, so use a cross
module reference (XMR) and a virtual interface in the program block as shown in
Sample 10.12. You must use a virtual interface so you can assign it the physical
interface in the top level module.
Sample 10.12 Test with virtual interface and XMR

The program connects to the test harness shown in Sample 10.13.

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Virtual Interfaces with the ATM Router

371

Sample 10.13 Test harness without interfaces in the port list

This approach is recommended by methodologies such as the VMM to make
your test code more reusable. If you add a new interface to your design, as shown in
Sample 10.14, the test harness changes, but existing tests don’t have to change.
Sample 10.14 Test harness with a second interface

The harness in Sample 10.14 works with the test in Sample 10.12 that does not
know about the new interface, as well as the test in Sample 10.15 that does.
Sample 10.15 Test with two virtual interfaces and XMRs

Some methodologies have a rule that makes the connection between
tests and harnesses slightly more complicated than with traditional
ports, but means you won’t have to modify existing tests, even if
the design changes. The examples in this book use the simple style
of interfaces in the port lists, but you should decide if test reuse is
important enough to change your coding style.

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Advanced Interfaces

Connecting to Multiple Design Configurations

A common challenge to verifying a design is that it may have several configurations. You could make a separate testbench for each configuration, but this could
lead to a combinatorial explosion as you explore every alternative. Instead, you can
use virtual interfaces to dynamically connect to the optional interfaces.

10.2.1 A Mesh Design
Sample 10.16 is built of a simple replicated component, an 8-bit counter. This
resembles a DUT that has a device such as a network chip or processor that is
instantiated repeatedly in a mesh configuration. The key idea is that the top-level
module creates an array of interfaces and counters. Now the testbench can connect
its array of virtual interfaces to the physical ones.
Sample 10.16 shows the code for the counter’s interface, X_if. If the code
printed the signal values with a $monitor, they would display when any signal
changed. Instead, the always block waits until the clocking block changes, then
prints the values of the signals at the end of the time slot with $strobe. The result
is you are now working at a higher level of abstraction, seeing the values cycle by
cycle instead of the individual events.
Sample 10.16 Interface for 8-bit counter

The simple counter is shown in Sample 10.17.

10.2 Connecting to Multiple Design Configurations

373

Sample 10.17 Counter model using X_if interface

The top-level module in Sample 10.18 uses a generate statement to instantiate
NUM_XI interfaces and counters, but only one testbench.
Sample 10.18 Top-level module with an array of virtual interfaces

In Sample 10.19, the key line in the testbench is where the local virtual interface array, vxi, is assigned to point to the array of physical interfaces in the top
module, top.xi. (Note that this example takes some shortcuts compared to the

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recommendations in Chapter 8. To simplify Sample 10.18, the environment class
has been merged with the test, whereas the generator, agent, and driver layers have
been compressed into the driver.)
The testbench assumes there is at least one counter and thus at least one X interface. If your design could have zero counters, you would have to use a dynamic array
to hold the virtual interfaces, as a fixed-size array cannot have a size of zero. The
actual number of interfaces is passed as a parameter from the top-level module.
Sample 10.19 Counter testbench using virtual interfaces

Of course in this simple example, you could just pass the interface directly into
the Driver’s constructor, rather than make a separate variable.
In Sample 10.20, the Driver class uses a single virtual interface to drive and
sample signals from the counter.

10.2 Connecting to Multiple Design Configurations

375

Sample 10.20 Driver class using virtual interfaces

10.2.2 Using Typedefs with Virtual Interfaces
You can reduce the amount of typing, and ensure you always use the correct modport by replacing “virtual X_if.TB” with a typedef, as shown in Sample 10.21
through 10.23, of the interface, testbench, and driver.
Sample 10.21 Interface with a typedef

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Sample 10.22 Testbench using a typedef for virtual interfaces

Sample 10.23 Driver using a typedef for virtual interfaces

10.2.3 Passing Virtual Interface Array Using a Port
The previous examples passed the array of virtual interfaces using a cross module
reference (XMR). An alternative is to pass the array in a port. Since the array in the
top module is static and so only needs to be referenced once, the XMR style makes
more sense than using a port that normally is used to pass changing values.
Sample 10.24 uses a global parameter to define the number of X interfaces. Here
is a snippet of the top module.
Sample 10.24 Testbench using an array of virtual interfaces

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377

The testbench that uses the virtual interfaces is shown in Sample 10.25. It creates
an array of virtual interfaces so that it can pass them into the constructor for the
driver class, or just pass the interface directly from the port.
Sample 10.25 Testbench passing virtual interfaces with a port

10.3

Parameterized Interfaces and Virtual Interfaces

The example in Section 10.2 shows an 8-bit counter and matching busses. What if
you want to vary the counter’s width? Verilog-1995 allows you to parameterize
modules, and System Verilog extends this concept with parameterized interfaces
and virtual interfaces.
First, update the counter, originally shown in Sample 10.17 with parameters.
This only requires changing the first few lines. Sample 10.26 now passes the number of interfaces in as a parameter too.

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Sample 10.26 Parameterized counter model using X_if interface

Next, Sample 10.27 adds the bit width parameter to the interface in Sample 10.16.

Sample 10.27 Parameterized interface for 8-bit counter

Sample 10.28 shows the parameter being passed into the testbench.
Sample 10.28 Parameterized top-level module with an array of virtual interfaces

Lastly are the testbench module and Driver class are shown in Samples 10.29
and 10.30. These have virtual interfaces that must be parameterized. The syntax for

10.4

Procedural Code in an Interface

379

this is a little tricky, especially when you have a modport. First, the testbench,
updated from Sample 10.19. Notice how the parameter goes between the type name
and the modport.
Sample 10.29 Parameterized counter testbench using virtual interfaces

Sample 10.30 Driver class using virtual interfaces

10.4

Procedural Code in an Interface

Just as a class contains both variables and routines, an interface can contain code
such as routines, assertions, and initial and always blocks. Recall that an interface includes the signals and functionality of the communication between two
blocks. So the interface block for a bus can contain the signals and also routines to
perform commands such as a read or write. The inner workings of these routines are
hidden from the external blocks, allowing you to defer the actual implementation.
Access to these routines is controlled using the modport statement, just as with
signals. A task or function is imported into a modport so that it is then visible to
any block that uses the modport.
These routines can be used by both the design and the testbench. This approach
ensures that both are using the same protocol, eliminating a common source of testbench bugs. However, not all synthesis tools can handle routines in an interface.
A problem with sharing code between the design and testbench is that the independence between the design and verification teams is lost. If only one person
implements the interface protocol for both parts, who checks it?
You can verify a protocol with assertions in an interface. An assertion can check
for illegal combinations, such as protocol violations and unknown values. These can
display state information and stop simulation immediately so that you can easily
debug the problem. An assertion can also fire when good transactions occur.
Functional coverage code uses this type of assertion to trigger the gathering of coverage information.

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10.4.1 Interface with Parallel Protocol
When creating your system, you may not know whether to choose a parallel or
serial protocol. The interface in Sample 10.31 has two tasks, initiatorSend
and targetRcv, that send a transaction between two blocks using the interface
signals. It sends the address and data in parallel across two 8-bit buses.
Sample 10.31 Interface with tasks for parallel protocol

10.4.2 Interface with Serial Protocol
The interface in Sample 10.32 implements a serial interface for sending and receiving the address and data values. It has the same interface and routine names as
Sample 10.31, so you can swap between the two without having to change any
design or testbench code.

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Procedural Code in an Interface

381

Sample 10.32 Interface with tasks for serial protocol

10.4.3

Limitations of Interface Code

Tasks in interfaces are fine for RTL, where the functionality is strictly defined.
However, these tasks are a poor choice for any type of verification IP. Interfaces and
their code cannot be extended, overloaded, or dynamically instantiated based on
configuration. An interface cannot have private data. Any code for verification needs

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maximum flexibility and configurability, and so should go in classes that run in a
program block.

10.5

Conclusion

The interface construct in SystemVerilog provides a powerful technique to group
together the connectivity, timing, and functionality for the communication between
blocks. In this chapter you saw how you can create a single testbench that connects
to many different design configurations containing multiple interfaces. Your signal
layer code can connect to a variable number of physical interfaces at run time with
virtual interfaces. Additionally, an interface can have routines that drive the signals
and assertions to check the protocol, but put the test in a program block, not an
interface.
In many ways, an interface can resemble a class with pointers, encapsulation,
and abstraction. This lets you create an interface to model your system at a higher
level than Verilog’s traditional ports and wires. Just remember to keep the testbench
in the program block.

10.6

Exercises

1. Complete the following code, as indicated by the comments.

10.6

Exercises

383

2. Using the solution to Exercise 1, complete the following code as indicated by the
comments.

3. Modify the following program declaration to use cross module references (XMR).
Assume the top module that contains the interface is named top.

Modify the following instantiation of program test to use cross module
references (XMR).

4. Expand the solution to Exercise 3 to create NUM_RISC_BUS environments and
create NUM_RISC_BUS interfaces.
5. Expand the solution to Exercise 3 to use a typedef for the virtual interface.

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6. Modify the following interface to use a parameter, ADDRESS_WIDTH. By default
the addressing space supports 256 words.

Chapter 11

A Complete SystemVerilog Testbench

This chapter applies the many concepts you have learned about SystemVerilog
features to verify a design. The testbench creates constrained random stimulus, and
gathers functional coverage. It is structured according to the guidelines from Chapter 8
so you can inject new behavior without modifying the lower-level blocks.
The design is an ATM switch that was shown in Sutherland [2006], who based
his SystemVerilog description on an example from Janick Bergeron’s Verification
Guild. Sutherland took the original Verilog design and used SystemVerilog design
features to create a switch that can be configured from 4×4 to 16×16. The testbench
in the original example creates ATM cells using $urandom, overwrites certain fields
with ID values, sends them through the device, then checks that the same values
were received.
The entire example, with the testbench and ATM switch, is available for
download at http://chris.spear.net/systemverilog. This chapter shows
just the testbench code.

11.1

Design Blocks

The overall connection between the design and testbench, shown in Fig. 11.1,
follows the pattern shown in Chapter 4.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_11,
© Springer Science+Business Media, LLC 2012

385

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A Complete SystemVerilog Testbench

Testbench

inputs

Design
Under
Test

outputs

Fig. 11.1 The testbench — design environment

The top level of the design is called squat, as shown in Fig. 11.2. The module
has 1..N Utopia Rx interfaces that are sending UNI formatted cells. Inside the
DUT, cells are stored, converted to NNI format, and forwarded to the Tx interfaces.
The forwarding is done according to a lookup table that is addressed with the VPI field
of the incoming cell. The table is programmed through the management interface.

Tx
Utopia

Rx
Utopia
Rewrite
Cell

Tx
Utopia

Rx
Utopia
Mgmt
I/F

Register
File

Fig. 11.2 Block diagram for the squat design

The top level module in Sample 11.1 defines arrays of interfaces for the Rx and
Tx ports.

11.1

Design Blocks

387

Sample 11.1 Top level module

The testbench program in Sample 11.2 passes the interfaces and signals through
the port list. See Section 10.1.4 for a discussion on ports vs. cross module references. The actual testbench code is in the Environment class. The program steps
through the phases of the environment. In order to work at a higher level of abstraction, the testbench only uses clocking blocks in the interfaces to synchronize with
the DUT, not low level clocks.

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Sample 11.2 Testbench program

The testbench loads control information into the ATM switch through the
Management interface, also known as the CPU interface, shown in Sample 11.3.
In this chapter’s examples, the interface is only used to load the lookup table that
maps VPI to forwarding masks.
Sample 11.3 CPU Management Interface

Sample 11.4 shows the Utopia interface, which is used by the testbench to
communicate with the squat design by transmitting and receiving ATM cells.
The interface has clocking blocks for the transmit and receive paths, and modports
for the design and testbench connections to the interface.

11.1

Design Blocks

Sample 11.4 Utopia interface

389

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11.2

A Complete SystemVerilog Testbench

Testbench Blocks

The environment class, as shown in Section 8.2.1, is the scaffolding that supports
the testbench structure. Inside this class lies the blocks of your layered testbench,
such as generators, drivers, monitors, and scoreboard. The environment also controls the sequencing of the four testbench steps: generate a random configuration,
build the testbench environment, run the test and wait for it to complete, and a wrapup phase to shut down the system and generate reports. Sample 11.5 shows the ATM
environment class. It uses the virtual interface vCPU_T defined in Sample 11.3.
Sample 11.5 Environment class header

With the $test$plusargs() system task, the Environment class constructor
in Sample 11.6 looks for the VCS switch +ntb_random_seed, which sets the
random seed for the simulation. The system task $value$plusargs() extracts
the value from the switch. Your simulator may have a different way to set the seed.
It is important to print the seed in the log file so that if the test fails, you can run it
again with the same value.

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Sample 11.6 Environment class methods

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The method Environment::build in Sample 11.6 connects the scoreboard to
the driver and monitor with the callback class, which is shown in Sample 11.7,
Scb_Driver_cbs. This class sends the expected values to the scoreboard. The
base driver callback class, Driver_cbs, is shown in Sample 11.20.

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Sample 11.7 Callback class connects driver and scoreboard

The callback class in Sample 11.8, Scb_Monitor_cbs, connects the monitor
with the scoreboard. The base monitor callback class, Monitor_cbs, is shown in
Sample 11.21.
Sample 11.8 Callback class connects monitor and scoreboard

The environment connects the monitor to the coverage class with the final
callback class, Cov_Monitor_cbs, shown in Sample 11.9.

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Sample 11.9 Callback class connects the monitor and coverage

The random configuration class header is shown in Sample 11.10. It starts with
nCells, a random value for the total number of cells that flow through the system.
The constraint c_nCells_valid ensures that the number of cells is valid by being
greater than zero, whereas c_nCells_reasonable limits the test to a reasonable

size, 1000 cells. You can disable or override this if you want longer tests.
Next is a dynamic bit array, in_use_Rx, to specify which Rx channels into the
switch are active. This is used in Sample 11.6 in the run method so that only active
channels run.
The array cells_per_chan is used to randomly divide the total number of cells
across the active channels. The constraint zero_unused_channels sets the number of cells to zero for inactive channels. To help the solver, the active channel mask
is solved before dividing up the cells between channels. Otherwise, a channel would
be inactive only if the number of cells assigned to it was zero, which is very
unlikely.

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Sample 11.10 Environment configuration class

The cell rewriting and forwarding configuration type is shown in Sample 11.11.

Sample 11.11 Cell configuration type

The methods for the configuration class are shown in Sample 11.12

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Sample 11.12 Configuration class methods

The ATM switch accepts UNI formatted cells and sends out NNI formatted cells.
These cells are sent through both an OOP testbench and a structural design, so they
are defined using typedef. The major difference between the two formats is that
the UNI’s GFC and VPI field are merged into the NNI’s VPI. The definitions in
Sample 11.13 through 11.15 are from Sutherland [2006].
Sample 11.13 UNI cell format

Sample 11.14 NNI cell format

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The UNI and NNI cells are merged with a byte memory to form a universal type,
shown in Sample 11.15.
Sample 11.15 ATMCellType

The testbench generates constrained random ATM cells, shown in Sample 11.16,
that are extended from the BaseTr class, defined in Sample 8.24.
Sample 11.16 UNI_cell definition

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Sample 11.17 shows the methods for the UNI cell.
Sample 11.17 UNI_cell methods

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The NNI_cell class is almost identical to UNI_cell, except that it does not
have a GFC field, or a method to convert to a UNI_cell.
Sample 11.18 shows the UNI cells random atomic generator, as originally shown
in Section 8.2. The generator randomizes the blueprint instance of the UNI cell, and
then sends out a copy of the cell to the driver.

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Sample 11.18 UNI_generator class

Sample 11.19 shows the Driver class that sends UNI cells into the ATM switch.
This class uses the driver callbacks in Sample 11.20. Note that there is a circular
relationship here. The Driver class has a queue of Driver_cbs objects, and the
pre_tx() and post_tx() methods in Driver_cbs are passed Driver objects.
When you compile the two classes, you may need either typedef class Driver;
before the Driver_cbs class definition, or typedef class Driver_cbs; before
the Driver class definition.
Sample 11.19 driver class

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Sample 11.20 shows the driver callback class which has simple callbacks that are
called before & after a cell is transmitted. This class has empty tasks, which are used
by default. A test case can extend this class to inject new behavior in the driver
without having to change any code in the driver

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Sample 11.20 Driver callback class

The Monitor class in Sample 11.21 has a very simple callback, with just one
task that is called after a cell is received.
Sample 11.21 Monitor callback class

Sample 11.22 shows the Monitor class. Like the Driver class, this uses a
typedef to break the circular compile dependency with Monitor_cbs.
Sample 11.22 The Monitor class

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The scoreboard in Sample 11.23 gets expected cells from the driver through the
function save_expected, and the cells actually received by the monitor with the
function check_actual. The function save_expected() is called from the callback Scb_Driver_cbs::post_tx(), shown in Sample 11.7. The function
check_actual () is called from Scb_Monitor_cbs::post_rx() in Sample 11.8.

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Sample 11.23 The Scoreboard class

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Sample 11.24 shows the class used to gather functional coverage. Since the coverage only looks at data in a single class, the cover group is defined and instantiated
inside the Coverage class. The data values are read by the class’s sample()method,
then the cover group’s sample() method is called to record the values.
Sample 11.24 Functional coverage class

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Sample 11.25 shows the CPU_driver class that contains the methods to drive
the CPU interface.
Sample 11.25 The CPU_driver class

11.3 Alternate Tests

11.3

411

Alternate Tests

The simplest test program is shown in Sample 11.2 and runs with very few constraints. During verification, you will be creating many tests, depending on the
major functionality to be tested. Each test can then be run with different seeds.

11.3.1

Your First Test - Just One Cell

The first test you run should probably have just one cell, such as the test in Sample
11.26. You can add a new constraint to the Config class by extending it, and then

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injecting a new object into the environment before randomization. Once this test
works, you can try two cells, then rewrite the constraint on the number of cells to
run longer sequences.
Sample 11.26 Test with one cell

11.3.2

Randomly Drop Cells

The next test you may run creates errors by occasionally dropping cells, as shown
in Sample 11.27. You need to make a callback for the driver that sets the drop bit.
Then, in the test, inject this new functionality after the driver class has been constructed during the build phase.

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Sample 11.27 Test that drops cells using driver callback

11.4

Conclusion

This chapter shows how you can build a layered testbench, following the guidelines
in this book. You can then create new tests by just modifying a single file and injecting new behavior, utilizing the hooks such as callbacks and multiple environment
phases.
The testbench was able to get to 100% functional coverage of the ATM switch,
at least for the basic cover group. You can use this example to explore more about
SystemVerilog testbenches.

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A Complete SystemVerilog Testbench

Exercises

1. In Sample 11.2, why is clk not passed into the port list of program test?
2. In Sample 11.6, could numRx be substituted for Rx.size() ? Why or why not?
3. For the following code snippet from Sample 11.6, explain what is being created
for each statement.

4. In Sample 11.9, what coverage object does the handle cov point to?
5. In Sample 11.17, the function UNI_cell::copy assumes that the handle to the
object UNI_cell points to an object of class UNI_cell as depicted in the following
drawing. Draw what object the handle dst points to for the following function calls.
handle

UNI_cell

a. copy() ;
b. copy(handle);
6. In Sample 11.18, why are the $cast () required?
7. In Sample 11.19 and 11.20, why are the typedef declaration needed?
8. In Sample 11.19, why is peek() used first and then later a get()?
9. In Sample 11.23, is the error message “…cell not found…” in the function
check_actual printed every time it are called? Why or why not?
10. Why do classes Environment, Scoreboard, and CPU_driver all define a
handle to class Config? Are 3 objects of class Config created?

Chapter 12

Interfacing with C/C++

In Verilog, you can communicate with C routines using the Programming Language
Interface. With the three generations of the PLI: TF (Task / Function), ACC (Access),
and VPI (Verification Procedural Interface), you can create delay calculators, connect and synchronize multiple simulators, and add debug tools such as waveform
displays. However the PLI’s greatest strength is also its greatest weakness. If you
just want to connect a simple C routine using the PLI, you need to write dozens
of lines of code, and understand many different concepts such as synchronizing
with multiple simulation phases, call frames, and instance pointers. Additionally,
the PLI adds overhead to your simulation as it copies data between the Verilog and
C domains, in order to protect Verilog data structures from corruption.
SystemVerilog introduces the Direct Programming Interface (DPI), an easier
way to interface with C, C++, or any other foreign language. Once you declare or
“import” the C routine with the import statement, you can call it as if it were
any SystemVerilog routine. Additionally, your C code can call SystemVerilog
routines. With the DPI you can connect C code that reads stimulus, contains a
reference model, or just extends SystemVerilog with new functionality. Currently
SystemVerilog only supports an interface to the C language. C++ code has to be
wrapped to look like C.
If you have a SystemC model that does not consume time, and that you want
to connect to SystemVerilog, you can use the DPI. SystemC models with timeconsuming methods are best connected with the utilities built into your favorite
simulator.
The first half of this chapter is data-centric and shows how you can pass different
data types between SystemVerilog and C. The second half is control centric, showing how you can pass control back and forth between SystemVerilog and C. While
the actual C code is trivial, with the factorial function, the Fibonacci series, and
counters, they are easy to understand so you can quickly substitute your own code.

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7_12,
© Springer Science+Business Media, LLC 2012

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Passing Simple Values

The first few examples in this chapter show you how to pass integral values between
SystemVerilog and C, and the mechanics of how to declare routines and their arguments on both sides. Later sections show how to pass arrays and structures.

12.1.1

Passing Integer and Real Values

The most basic data type that you can pass between SystemVerilog and C is an int,
the 2-state, 32-bit type. Sample 12.1 shows the SystemVerilog code that calls a C
factorial routine, shown in Sample 12.2.
Sample 12.1 SystemVerilog code calling C factorial routine

The import statement declares that a SystemVerilog routine factorial is
implemented in a foreign language such as C. The modifier "DPI-C" specifies that
this is a Direct Programing Interface routine, and the rest of the statement describes
the routine arguments.
Sample 12.1 passes 32-bit signed values using the SystemVerilog int data type
that maps directly to the C int type. The SystemVerilog int is always 32 bits,
whereas the width of an int in C is operating system dependent. The C function in
Sample 12.2 takes an integer as an input and so the DPI passes the argument by
value.
Sample 12.2 C factorial function

12.1.2

The Import Declaration

The import declaration defines the prototype of the C task or function, but using
SystemVerilog types. A C function with a return value is mapped to a SystemVerilog

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417

function. A void C function can be mapped to a SystemVerilog task or void function.
If the name of the imported C function conflicts with a SystemVerilog name, you can
import the function with a new name. In Sample 12.3, the C function expect is
mapped to the SystemVerilog name fexpect, as the name expect is a reserved
keyword in SystemVerilog. The name expect becomes a global symbol, used to link
with the C code, whereas fexpect is a local SystemVerilog symbol. In the second
half of the example, the C function stat is given a new name in SystemVerilog,
file_exists. SystemVerilog does not support overloading a routine, for example by
importing expect once with a real argument and once with an int.
Sample 12.3 Changing the name of an imported function

You can import routines anywhere in your SystemVerilog code where you can
declare a routine including inside programs, modules, interfaces, packages, and $unit,
the compilation-unit space. The imported routine will be local to the declaration
space in which it is declared. If you need to call an imported routine in several locations in your code, put the import statement in a package which you import where
it is needed. Any changes to the import statements are localized to the package.

12.1.3

Argument Directions

Imported C routines can have zero or more arguments. By default the argument
direction is input (data goes from SystemVerilog to C), but can also be output
and inout. The direction ref is not supported. A function can return a simple value
such as an integer or real number, or have no return value if you make it void.
Sample 12.4 shows how to specify argument directions.

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Sample 12.4 Argument directions

You can reduce the chances of bugs in your C code by declaring any input arguments as const as shown in Sample 12.5 so the C compiler will give an error for
any write to an input.
Sample 12.5 C factorial routine with const argument

12.1.4

Argument Types

Each variable that is passed through the DPI has two matching definitions: one for
the SystemVerilog side, and one for the C side. It is your responsibility to use compatible types. The SystemVerilog simulator cannot compare the types as it is unable
to read the C code. (The VCS compiler produces vc_hdrs.h and Questa creates
incl.h with the C header for any routine that you have imported. You can use this
file as a guide to matching the types.)
Table 12.1 shows the data type mapping between SystemVerilog and the inputs
and outputs of C routines. The C structures are defined in the include file svdpi.h.
Arrays mapping is discussed in Section 12.4 and 12.5, and structures are discussed
in Section 12.6.
Table 12.1 Data types mapping between SystemVerilog and C
SystemVerilog
C (input)
byte
char
shortint
short int
int
int
longint
long long int
shortreal
float
real
double
string
const char*
string [N]
const char**
bit
svBit or unsigned char
logic, reg
svLogic or unsigned char
bit[N:0]
const svBitVecVal*
reg[N:0] logic[N:0]
const svLogicVecVal*
unsized array[]
const svOpenArrayHandle
chandle
const void*

C (output)
char*
short int*
int*
long int*
float*
double*
char**
char**
svBit* or unsigned char*
svLogic* or unsigned char*
svBitVecVal*
svLogicVecVal*
svOpenArrayHandle
void*

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Note that some mappings are not exact. For example, a bit in
SystemVerilog maps to svBit in C, which ultimately maps to
unsigned char in the svdpi.h include file. As a result, you
could write illegal values into the upper bits.
The LRM limits imported function results “small values”, which include: void, byte,
shortint, int, longint, real, shortreal, chandle, and string, plus single
bit values of type bit and logic. A function cannot return a vector such as bit
[6:0] as this would require returning a pointer to a svBitVecVal structure.

12.1.5

Importing a Math Library Routine

Sample 12.6 shows how you can call many functions in the C math library directly,
without a C wrapper, thereby reducing the amount of code that you need to write.
The Verilog real type maps to a C double.
Sample 12.6 Importing a C math function

12.2

Connecting to a Simple C Routine

Your C code might contain a simulation model, such as a processor, that is instantiated side by side with Verilog models. Or your code could be a reference model
that is compared to a Verilog model at the transaction or cycle level. Many examples in this chapter show an 7-bit counter written in C or C++. Though very simple,
the counter has the same parts as a complex model, with inputs, outputs, storage
of internal values between calls, and the need to support multiple instances. The
counter is 7 bits to show what happens when a hardware type does not match a
C type.

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12

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A Counter with Static Storage

Sample 12.7 is the C code for an 7-bit counter. The count is stored in a static variable,
as you might do if you wrote the model before thinking about simulation.
Sample 12.7 Counter routine using a static variable

The reset and load signals are 2-state single bit signals, and so they are passed
as svBit which reduces to unsigned char. Your code could declare the value
either way, but play it safe by using the SystemVerilog DPI types. The input i is
2-state, and 7 bits wide, and is passed as svBitVecVal. Notice that it is passed as
a const pointer, which means the underlying value can change, but you cannot
change the value of the pointer, such as making it point to another value. Likewise,
the reset and load inputs are also marked as const. In this example, the 7-bit
counter value is stored in a char, so you have to mask off the upper bit.
The file svdpi.h contains the definitions for SystemVerilog DPI structures and
methods. The C code examples in the rest of this chapter leave off the #include
statements, unless they are important to the discussion.

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Sample 12.8 shows a SystemVerilog program that imports and calls the C function
for the 7-bit counter.
Sample 12.8 Testbench for an 7-bit counter with static storage

12.2.2

The Chandle Data Type

The chandle data type allows you to store a C or C++ pointer in your SystemVerilog
code. A chandle variable is wide enough to hold a pointer on the machine where
the code was compiled, i.e. 32- or 64-bits. The counter in Sample 12.7 works well
as long as it is the only one in the design. You could wrap the counter7 calls from
Sample 12.8 in a module, and instantiate multiple copies in a design. However,
since the counter value is stored in a C static, every instance shares a single value.
If you need more than one instance of a module that calls C code, the C code needs
to store its variables somewhere other than in static variables. A better way is to
allocate storage, and pass a handle to it, along with the input and output signal
values. Sample 12.9 shows a counter that stores the 7-bit count in the structure c7.
This is overkill for a simple counter, but if you are creating a model for a larger
device, you can build from this example.

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Sample 12.9 Counter routine using instance storage

The routine counter7_new constructs the counter instance. This returns a
chandle that must be passed into future calls to counter7. The counter value is
stored in a struct of type c7. The function counter7_new calls malloc to allocate
the struct, and casts the result into a local pointer c.
The C code uses the PLI task io_printf to display debug messages. The
routine is helpful when you are debugging C and SystemVerilog code side-by-side
as it writes to the same outputs, including log files, as $display, including
the simulator’s log file. The routine is defined in veriuser.h.
The testbench for this counter in Sample 12.10 differs from the static one in
several ways. First, the counter must be constructed before it can be used. Next, the
counter is called on a clock edge, rather than calling it in-line with the stimulus. For
simplicity, the counter is invoked when the clock goes high, and stimulus is applied
when the clock goes low, to avoid any race conditions.

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Sample 12.10 Testbench for an 7-bit counter with per-instance storage

12.2.3

Representation of Packed Values

The string "DPI-C"1 specifies that you are using the canonical representation of
packed values. This representation stores a SystemVerilog variable as a C array of
one or more elements. A 2-state variable is stored using the type svBitVecVal.
A 2-state array is stored with multiple elements of this type.
1

Early versions of the LRM used “DPI” but this is now obsolete and should not be used.

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31:0
Unused

39:32

Fig. 12.1 Storage of a 40-bit 2-state variable

For performance reasons. the SystemVerilog simulator may not mask the
upper bits after calling a DPI routine, and so the SystemVerilog variable could be
corrupted. Make sure your C code treats these values properly.
If you need to convert between bits and words, use the macro SV_PACKED_
DATA_NELEMS. For example, to convert 40 bits to two 32-bit words (as seen in
Fig. 12.1), use SV_PACKED_DATA_NELEMS(40).

12.2.4

4-State Values

Each 4-state bit in SystemVerilog is stored in the simulator using two bits known as
aval and bval, as shown in Table 12.2
Table 12.2 4-state bit encoding
4-state value
bval
aval
0
0
0
1
0
1
Z
1
0
X
1
1

A single bit 4-state variable, such as logic f, is stored in an unsigned byte, with
the aval bit stored in the least significant bit, and the bval in the next higher bit.
So the value 1'b0 is seen as 0x0 in C, 1'b1 is 0x1, 1'bz is 0x2, and 1'bx is 0x3.
A 4-state vector such as logic [31:0] lword is stored using pairs of 32 bits,
svLogicVecVal, which contains the aval and bval bits as shown in Figure 12.2.
The 32-bit variable lword is stored in a single svLogicVecVal. Variables wider
than 32-bits are stored in multiple svLogicVecVal elements, with the first element contains the 32 least significant bits, the next element contains the next 32
bits, up to the most significant bits. A 40-bit logic variable is stored as one
svLogicVecVal for the least significant 32 bits, and a second for the upper 8 bits
(Fig. 12.2). The unused 24-bits in this upper value are undetermined, and you are
responsible for masking or extending the sign bit, as needed. The svLogicVecVal
type is equivalent to s_vpi_vecval, which is used to represent 4-state types such
as logic in the VPI.

12.2 Connecting to a Simple C Routine
aval 31:0
bval 31:0
Unused
Unused

425

aval 39:32
bval 39:32

Fig. 12.2 Storage of a 40-bit 4-state variable

Beware of arguments declared without bit subscripts or those
declared with a single bit. An argument declared as input
logic a is stored in an unsigned char. The argument input
logic [0:0] b is svLogicVecVal, even though it contains
only a single bit.
Sample 12.11 shows the import statements for a 4-state counter. The only difference
from Sample 12.10 is that the bit types are now logic.
Sample 12.11 Testbench for counter that checks for Z or X values

The counter previously shown in Sample 12.9 assumes all the inputs are 2-state.
Sample 12.12 extends this code to check for Z and X values on reset, load, and i.
The actual count is still kept as a 2-state value.

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Sample 12.12 Counter routine that checks for Z and X values

If you want to force the simulation to terminate cleanly because of a condition found in an imported routine, you can call the VPI routine vpi_
control(vpiFinish, 0). This routine and constant are defined in the include file
vpi_user.h. The value vpiFinish tells the simulator to execute a $finish after
your imported routine returns.

12.2.5

Converting from 2-State to 4-State

If you have a DPI application that works with 2-state types and you want to convert
it to work with 4-state types, follow the following guidelines.
On the SystemVerilog side, change the import declaration from using 2-state
types such as bit and int to 4-state types such as logic and integer. Make sure
you are using 4-state variables in the function call.
On the C side, switch the argument declarations from svBitVecVal to svLogicVecVal. Any reference to the arguments will have to use the .aval suffix to

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427

correctly access the data. When you read from a 4-state variable, check the bval
bits to see if there are any Z or X values. When you write to a 4-state variable, clear
the bval bits unless you need to write Z or X values.

12.3

Connecting to C++

You can use the DPI to connect routines written in C or C++ to SystemVerilog.
There are several ways your C++ code can communicate using the DPI, depending
on your model’s level of abstraction.

12.3.1

The Counter in C++

Sample 12.13 shows a C++ class for the 7-bit counter, with 2-state inputs. It connects to the SystemVerilog testbench in Sample 12.10 and the C++ wrapper code in
Sample 12.14.
Sample 12.13 Counter class

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Static Methods

The DPI can only call a C or C++ function that is known at link time. As a result,
your SystemVerilog code cannot call a C++ routine in an object as the object does
not exist when the linker runs.
So what if you need to call a method in a C++ class? The solution, as shown in
Sample 12.14, is to create a function with a fixed address, that then can communicate with the C++ dynamic objects and methods. The first routine, counter7_new,
constructs an object for the counter and returns a handle to the object. The second
static routine, counter7, calls the C++ method that performs the counter logic,
using the object handle.
Sample 12.14 Static methods and linkage

The extern “C” code tells the C++ compiler that the external information
sent to the linker should use C calling conventions and not perform name mangling.
You can put this before each routine that is called by SystemVerilog, or put extern
"C" { ... } around a set of methods.
From the testbench point of view, the C++ counter looks the same as the counter
that stored the value in per-instance storage, shown in Sample 12.9, so you can use
the same testbench, Sample 12.10, for both.

12.3.3

Communicating with a Transaction Level C++ Model

The previous C / C++ code examples were low-level models that communicated with
the SystemVerilog at the signal level. This is not efficient; for example the counter is
called every clock cycle, even if the data or control inputs have not changed. When
you create models for complex devices such as processors and networking devices,
communicate with them at the transaction level for faster simulations.

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429

The C++ counter model in Sample 12.15 has a transaction-level interface,
communicating with methods instead of signals and a clock.
Sample 12.15 C++ counter communicating with methods

The dynamic C++ methods such as reset, load, and count are wrapped in
static methods that use the object handle, passed from SystemVerilog, as shown in
Sample 12.16.

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Sample 12.16 Static wrapper for C++ transaction level counter

The OOP interface for the transaction level counter is carried up to the testbench.
Sample 12.17 has the SystemVerilog import statements and a class to wrap the C++
object. This allows you to hide the C++ handle inside the class.
Note that the counter7_get() function returns an int (32-bit, signed) rather
than bit [6:0], as the latter would require returning a pointer to a svBitVecVal, as
shown in Table 12.1. An imported function can not return a pointer. It can only return
a value of type void, byte, shortint, int, longint, real, shortreal,
chandle, and string, plus single bit values of type bit and logic.

12.3 Connecting to C++
Sample 12.17 Testbench for C++ model using methods

431

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Simple Array Sharing

So far you have seen examples of passing scalar and vectors between SystemVerilog
and C. A typical C model might read an array of values, perform some computation,
and return another array with the results.

12.4.1

Single Dimension Arrays - 2-State

Sample 12.18 shows a routine that computes the first 20 values in the Fibonacci
series. It is called by the SystemVerilog code in Sample 12.19.
Sample 12.18 C routine to compute Fibonacci series

Note that in C, you could have alternatively declared the argument as a pointer,
*data or an array, data[20]. In this example, they are interchangeable.

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433

Sample 12.19 Testbench for Fibonacci routine

Notice that the array of Fibonacci values is allocated and stored in SystemVerilog,
even though it is calculated in C. There is no way to allocate an array in C and reference it in SystemVerilog.

12.4.2

Single Dimension Arrays - 4-State

Sample 12.20 shows the Fibonacci C routine for a 4-state array with the testbench
in Sample 12.21.
Sample 12.20 C routine to compute Fibonacci series with 4-state array

Sample 12.21 Testbench for Fibonacci routine with 4-state array

Section 12.2.5 describes how to convert a 2-state application to 4-state.

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12

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Open arrays

When sharing arrays between SystemVerilog and C, you have two options. For the
fastest simulations, you can reverse-engineer the layout of the elements in SystemVerilog, and write your C code to use this mapping. This approach is fragile,
meaning that you will have to rewrite and debug your C code if any of the array
sizes change. A more robust approach is to use “open arrays”, and their associated
SystemVerilog routines to manipulate them. These allow you to write generic C
routines that can operate on any size array.

12.5.1

Basic Open Array

Sample 12.22 and 12.23 show how to pass a simple array between SystemVerilog
and C with open arrays. Use the empty square brackets [] in the SystemVerilog
import statement to specify that you are passing an open array.
Sample 12.22 Testbench code calling a C routine with an open array

Your C code references the open array with a handle of type svOpenArrayHandle. This points to a structure with information about the array such as the
declared word range. You can locate the actual array elements with calls such as
svGetArrayPtr. Note that svSize() is an open array query method, as described
in the next section.
Sample 12.23 C code using a basic open array

12.5

Open arrays

12.5.2

435

Open Array Methods

There are many DPI methods to access their contents and ranges, as defined in
svdpi.h. These only work with open array handles declared as svOpenArrayHandle, not with pointers such as svBitVecVal or svLogicVecVal. The
methods in Table 12.3 give you information about the size of an open array.

Table 12.3 Open array query functions
function
Description
int svLeft(h, d)
Left bound for dimension d
int svRight(h, d)
Right bound for dimension d
int svLow(h, d)
Low bound for dimension d
int svHigh(h, d)
High bound for dimension d
int svIncrement(h, d)
If left >= right, 1, else −1
int svSize(h, d)
Number of elements in dimension d: svHigh−svLow+1
int svDimensions(h)
Number of dimensions in open array
int svSizeOfArray(h)
Total size of array in bytes

In Table 12.3, the variable h is a svOpenArrayHandle and d is an int. The
dimensions are numbered starting with d=1.
The functions in Table 12.4 return the locations of the C storage for the entire
array or a single element.

Table 12.4 Open array locator functions
Function

void
void
void
void
void

*svGetArrayPtr(h)
*svGetArrElemPtr(h, i1, ...)
*svGetArrElemPtr1(h, i1)
*svGetArrElemPtr2(h, i1, i2)
*svGetArrElemPtr3(h, i1, i2, i3)

12.5.3

Returns pointer to:
storage for the entire array
an element in the array
an element in a 1-D array
an element in a 2-D array
an element in a 3-D array

Passing Unsized Open Arrays

Sample 12.24 calls C code with a 2-dimensional array. The C code uses the svLow
and svHigh methods to find the array ranges, which, in this example, don’t follow
the usual 0..size-1.

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Sample 12.24 Testbench calling C code with multi-dimensional open array

This calls the C code in Sample 12.25 that reads the array using the open array
methods. The routine svLow(handle, dimension) returns the lowest index
number for the specified dimension. So svLow(h,1) returns 1 for the array declared
with the range [6:1]. Likewise, svHigh(h, 1) returns 6. You should use svLow
and svHigh with C for loops.
The methods svLeft and svRight return the left and right index from the array
declaration, 6 and 1 respectively for the range [6:1]. At the center of Sample 12.25, the
call svGetArrElemPtr2 returns a pointer to an element in a two dimensional array.
Sample 12.25 C code with multi-dimensional open array

12.5.4

Packed Open Arrays in DPI

An open array in the DPI is treated as having a single packed dimension and one or
more unpacked dimensions. You can pass an array with multiple packed dimensions, as long as they pack into an element that is the same size as a single element

12.6 Sharing Composite Types

437

of the formal argument. For example, if you have the formal argument bit[63:0]
b64[] in the import statement, you could pass in the actual argument bit [1:0]
[0:3][6:−1] bpack [9:1]. Sample 12.26 shows the SystemVerilog code with
packed open arrays.
Sample 12.26 Testbench for packed open arrays

Sample 12.27 C code using packed open arrays

Notice that the C code in Sample 12.27 prints a 64-bit value using %llx, and
casts the result from svGetArrayElemPtr1 to long long int.

12.6

Sharing Composite Types

By this point you may wonder how to pass objects between SystemVerilog and C.
The layout of class properties does not match between the two languages, so you
cannot share objects directly. Instead, you must create similar structures on each

438

12

Interfacing with C/C++

side, plus pack and unpack methods to convert between the two formats. Once you
have all this in place, you can share composite types.

12.6.1

Passing Structures Between SystemVerilog and C

The following example shares a simple structure for a pixel made of three bytes
packed into a word. Sample 12.28 shows the C structure. Notice that C treats a char
as signed variable, which can give you unexpected results, so the structure marks
the char as unsigned. The bytes are in reverse order from the SystemVerilog because
this code was written for a Intel x86 processor that is little-endian, which means that
the least significant byte is stored at a lower address than the most significant. A Sun
SPARC is big endian, so the bytes are stored in the same order as in SystemVerilog:
r, g, b.
Sample 12.28 C code to share a structure

The SystemVerilog testbench in Sample 12.29 has a packed struct that holds a
single pixel, and class to encapsulate the pixel operations. The RGB_T struct is
packed so SystemVerilog will store the bytes in consecutive locations. Without the
packed modifier, each 8-bit value would be stored in a separate word.

12.6 Sharing Composite Types

439

Sample 12.29 Testbench for sharing structure

12.6.2

Passing Strings Between SystemVerilog and C

Using the DPI, you can pass strings from C back to SystemVerilog. You might need
to pass a string for the symbolic value of a structure, or get a string representing the
internal state of your C code for debug.
The easiest way to pass a string from C to SystemVerilog is for your C function
to return a pointer to a static string as shown in Sample 12.30. The string must be

440

12

Interfacing with C/C++

declared as static in C, and not as a local string. Non-static variables are stored
on the stack and are reclaimed when the function returns.
Sample 12.30 Returning a string from C

A danger with static storage is that multiple concurrent calls could end up
sharing storage. For example, a SystemVerilog $display statement that is printing
several pixels might call the above print routine multiple times. Depending on
how the SystemVerilog compiler orders these calls, later calls to print() could
overwrite results from earlier calls, unless the SystemVerilog compiler makes a
copy of the string. Note that a call to an imported routine can never be interrupted
by the SystemVerilog scheduler. Sample 12.31 stores the strings in a heap to support
concurrent calls.
Sample 12.31 Returning a string from a heap in C

12.7

Pure and Context Imported Methods

Imported methods are classified as pure, context, or generic. A pure function
calculates its output strictly based on its inputs, with no outside interactions.
Specifically, a pure function does not access any global or static variables, perform
any file operations, or interact with anything outside the function such as the operating

12.8 Communicating from C to SystemVerilog

441

system, processes, shared memory, sockets, etc. The SystemVerilog compiler may
optimize away calls to a pure function if the result is not needed, or replace the call
with the results from a previous call with the same arguments. The factorial
function in Sample 12.5, and the sin function in 12.6 are both pure functions as
their result is only based on their inputs. Sample 12.32 shows how to import a pure
function.
Sample 12.32 Importing a pure function

An imported routine may need to know the context of where it is called so it can
call a PLI TF, ACC, or VPI methods, or a SystemVerilog task that has been exported.
Use the context attribute for these methods as shown in Sample 12.33.
Sample 12.33 Imported context tasks

An imported routine may use global storage, so it is not pure, but might not have
any PLI references, so it does not need the overhead of a context routine.
Sutherland (2004) uses the term “generic” for these methods as the SystemVerilog
LRM does not have a specific name. By default, an imported routine is generic, as
are many of the examples in this chapter.
There is overhead invoking a context imported routine as the simulator needs
to record the calling context, so only declare a routine as context if needed. On the
other hand, if a generic imported routine calls an exported task or a PLI routine that
accesses SystemVerilog data objects, the simulator could crash.
A context-aware PLI routine is one that needs to know where it was called from
so that it can access information relative to that location.

12.8

Communicating from C to SystemVerilog

The examples so far have shown you how to call C code from your SystemVerilog
models. The DPI also allows you to call SystemVerilog routines from C code. The
SystemVerilog routine can be a simple task to record the result from an operation
in C, or a time-consuming task representing part of a hardware model.

12.8.1

A simple Exported Function

Sample 12.34 shows a module that imports a context function, and exports a SystemVerilog function.

442

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Sample 12.34 Exporting a SystemVerilog function

The export declaration in Sample 12.34 looks naked because
the LRM forbids putting a return value declaration or any arguments. You can’t even give the usual empty parentheses. This
information in the export declaration would duplicate the
information in the function declaration at the end of the module and could thus
become out of sync if you ever changed the function.
Sample 12.35 shows the C code that calls the exported function.
Sample 12.35 Calling an exported SystemVerilog function from C

This example prints the line from the C code, followed by the $display output
from the SystemVerilog, as shown in Sample 12.36.
Sample 12.36 Output from simple export

12.8.2

C function Calling SystemVerilog Function

While the majority of your testbench should be in SystemVerilog, you may have
legacy testbenches in C or other languages, or applications that you want to reuse.
This section creates a SystemVerilog memory model that is stimulated by C code
that reads transactions from an external file.
The first version of the memory model, shown in Sample 12.38 and 12.37,
is coded with just functions, so everything runs in zero time. The C code in

12.8 Communicating from C to SystemVerilog

443

Sample 12.37 opens the file, reads a command, and calls the exported function.
Error checking has been removed for compactness.
Sample 12.37 C code to read simple command file and call exported function

The SystemVerilog code calls the C task read_file which opens a file. The only
command in the file sets the memory size, so the C code calls an exported
function.
Sample 12.38 SystemVerilog module for simple memory model

444

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Interfacing with C/C++

Notice that in Sample 12.38, the export statement does not have any arguments
as this information is already in the function declaration.
The command file is trivial, with one command to construct a memory with 100
elements as shown in Sample 12.39.
Sample 12.39 Command file for simple memory model

12.8.3

C Task Calling SystemVerilog Task

A real memory model has operations such as read and write that consume time, and
thus must be modeled with tasks.
Sample 12.40 shows the SystemVerilog code for the second version of the memory
model. It has several improvements compared to Sample 12.38. There are two new
tasks, mem_read and mem_write, which respectively take 20ns and 10ns to complete. The imported routine read_file is now a SystemVerilog task as it is calling
other tasks that consume time. The import statement now specifies that read_file is
a context task, as the simulator needs to create a separate stack when it is called.
Sample 12.40 SystemVerilog module for memory model with exported tasks

12.8 Communicating from C to SystemVerilog

445

The C code in Sample 12.41 primarily expands the case statement that
decodes commands and calls the exported tasks, which are declared as extern
int according to the LRM.2
Sample 12.41 C code to read command file and call exported function

The command file in Sample 12.42 has new commands that write two locations,
and then reads back one of them, and includes the expected value.
2

VCS declared exported tasks as void functions in C.

446

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Sample 12.42 Command file for simple memory model

12.8.4

Calling Methods in Objects

You can export SystemVerilog methods, except for those defined inside a class. This
restriction is similar to the restriction of importing static C methods, as shown in
Section 12.3.2 as objects do not exist when SystemVerilog elaborates your code.
The solution is to pass a reference to the object between the SystemVerilog and C
code. However, unlike a C pointer, a SystemVerilog handle cannot be passed through
the DPI. You can instead have an array of handles, and pass the array index between
the two languages.
The following examples build on the previous versions of the memory. The
SystemVerilog code in Sample 12.44 has a class that encapsulates the memory. Now
you can have multiple memories, each in a separate object. The command file in
Sample 12.43 creates two memories, M0, and M1. Then it performs several writes
to initialized locations in both memories, and lastly tries to read back the values.
Notice that location 12 is used for both memories.
Sample 12.43 Command file for exported methods with OOP memories

The SystemVerilog code in Sample 12.44 constructs a new object for every M
command in the file. The exported function mem_build calls the Memory constructor. It then stores the handle to the Memory object in a SystemVerilog queue, and
returns the queue index, idx, to the C code as shown in Sample 12.45. The handles
are stored in a queue so you can dynamically add new memories. The exported tasks
mem_read and mem_write now have an additional argument, the index of the
memory handle in the queue.

12.8 Communicating from C to SystemVerilog
Sample 12.44 SystemVerilog module with memory model class

447

448
Sample 12.45 C code to call exported tasks with OOP memory

12

Interfacing with C/C++

12.8 Communicating from C to SystemVerilog

12.8.5

449

The Meaning of Context

The context of an imported routine is the location where it was defined, such as
$unit, module, program, or package scope, just like a normal SystemVerilog
routine. If you import a routine in two different scopes, the corresponding C code
executes in the context of where the import statement occurred. This is similar to
defining a SystemVerilog run() task in each of two separate modules. Each task
accesses variables in its own module, with no ambiguity.
Sample 12.46 shows that if you add a second module to Sample 12.34 that
imports the same C code and exports its own function, the C routine will call different
SystemVerilog methods, depending on the context of the import and export
statements.
Sample 12.46 Second module for simple export example

The output in Sample 12.47 shows that one C routine calls two separate
SystemVerilog methods, depending on where the C routine was called.
Sample 12.47 Output from simple example with two modules

450

12.8.6

12

Interfacing with C/C++

Setting the Scope for an Imported Routine

Just as your SystemVerilog code can call a routine in the local scope, an imported C
routine can call a routine outside its default context. Use the routine svGetScope to
get a handle to the current scope, and then use that handle in a call to svGetScope
to make the C code think it is inside another context. Sample 12.48 shows the C code
for two methods. The first, save_my_scope( ), saves the scope of where it was
called from the SystemVerilog side. The second routine, c_display(), sets its
scope to the saved one, prints a message, then calls your function, sv_display().
Sample 12.48 C code getting and setting context

The C code calls svGetNameFromScope() that returns a string of the current
scope. The scope is printed twice, once with the scope where the C code was first
called from, and again with the scope that was previously saved. The routine
svGetScopeFromName() takes a string with a SystemVerilog scope and returns a
pointer to a svScope handle that can be used with svSetScope().
In the SystemVerilog code in Sample 12.49, the first module, block, calls a C
routine that saves the context. When the module top calls c_display(), the
routine sets scope back to block, and so it calls the sv_display() routine in
the block module, not the top module.

12.8 Communicating from C to SystemVerilog

451

Sample 12.49 Modules calling methods that get and set context

This produces the output shown in Sample 12.50.
Sample 12.50 Output from svSetScope code

You could use this concept of scope to allow a C model to know where it was
instantiated from, and differentiate each instance. For example, a memory model
may be instantiated several times, and needs to allocate unique storage for every
instance.

452

12.9

12

Interfacing with C/C++

Connecting Other Languages

This chapter has shown the DPI working with C and C++. With a little work, you
can connect other languages. The easiest way is to call the Verilog $system() task.
If you need the return value from the command, use the Unix system() function
and the WEXITSTATUS macro. The SystemVerilog code in Sample 12.51 calls a
C wrapper for system().
Sample 12.51 SystemVerilog code calling C wrapper for Perl

Sample 12.52 is the C wrapper that calls system() and translates the return value.
Sample 12.52 C wrapper for Perl script

Sample 12.53 is a Perl script that prints a message and returns a value.
Sample 12.53 Perl script called from C and SystemVerilog

Now you can run the Unix command in Sample 12.54 to run the simulation and
call the hello.pl script.

12.11

Exercises

453

Sample 12.54 VCS command line to run Perl script

12.10

Conclusion

The Direct Programing Interface allows you to call C routines as if they are just
another SystemVerilog routine, passing SystemVerilog types directly into C. This
has less overhead than the PLI, which builds argument lists, and always has to keep
track of the calling context, not to mention the complexity of having up to four
C routines for every system task.
Additionally, with the DPI, your C code can call SystemVerilog routines, allowing
external applications to control simulation. With the PLI you would need trigger
variables and more argument lists, and you have to worry about subtle bugs from
multiple calls to time-consuming tasks.
The most difficult part of the DPI is mapping SystemVerilog types to C,
especially if you have structures and classes that are shared between the two
languages. If you can master this problem, you can connect almost any application
to SystemVerilog.

12.11

Exercises

1. Create a C function, shift_c, that has two input arguments: a 32-bit unsigned
input value i and an integer for the shift amount n. The input i is shifted n
places. When n is positive, values are shifted left, when n is negative, shifted
right, and when n is 0, no shift is performed. The function returns the shifted
value. Create a SystemVerilog module that calls the C function and tests each
feature. Provide the output.
2. Expand Exercise 1 to add a third argument to shift_c, a load flag ld. When ld
is true, i is shifted by n places and then loaded into an internal 32-bit register.
When ld is false, the register is shifted n places. The function returns the value
of the register after these operations. Create a SystemVerilog module that calls
the C function and tests each feature. Provide the output.
3. Expand Exercise 2 to create multiple instances of the shift_c function. Each
instance in C needs a unique identifier, so use the address where the internal
register is stored. Print this address along with the arguments when the function
shift_c is called. Instantiate the function twice, and call each instance twice.
Provide the output.

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4. Expand the C code from Exercise 3 to display the total number of times the
shift_c function has been called, even if the function is instantiated more
than once.
5. Expand Exercise 4 to provide the ability to initialize the stored value at
instantiation.
6. Expand Exercise 5 to encapsulate the shift_c function in a class.
7. For the code in Sample 12.24 and 12.25, what is returned by the following open
array methods?

8. Modify Exercise 1 so that instead of shifting the value in C, the function calls
an exported SystemVerilog void function named shift_sv that does the
shifting.
9. Expand Exercise 8 to call the SystemVerilog function shift_sv for two different SystemVerilog objects as demonstrated in Section 12.8.4 of the text. Assume
the SystemVerilog function shift_build has been exported to the C code.
10. Expand Exercise 8 to:
a. Create a SystemVerilog class Shift containing the function shift_sv that
stores the result in a class-level variable, and a shift_print function that
displays the stored result.
b. Define and export SystemVerilog function shift_build.
c. Support the creation of multiple Shift objects with the handles to these
objects stored in a queue.
d. Create a testbench that constructs multiple Shift objects. Demonstrate that
each object holds a separate result after performing calculations.

References

Bergeron, Janick. Writing Testbenches Using SystemVerilog. Norwell, MA: Springer, 2006
Bergeron, Janick, Cerny, Eduard, Hunter, Alan, and Nightingale, Andrew. Verification Methodology
Manual for SystemVerilog. Norwell, MA: Springer, 2006
Cohen, Ben, Venkataramanan, Srinivasan, and Kumari, Ajeetha. SystemVerilog Assertions
Handbook for Formal and Dynamic Verification: VhdlCohen Publishing 2005
Cummings, Cliff. Nonblocking Assignments in Verilog Synthesis, Coding Styles That Kill!
Synopsys User Group, San Jose, CA, 2000
Cummings, Cliff, Salz, Arturo. SystemVerilog Event Regions, Race Avoidance & Guidelines,
Synopsys User Group, Boston, CA, 2006
Denning, Peter. The Locality Principle, Communications of the ACM, 48(7), July 2005,
pp. 19–24
Haque, Faisal, Michelson, Jonathan. The Art of Verification with SystemVerilog Assertions.
Verification Central 2006
IEEE IEEE Standard for SystemVerilog — Unified Hardware Design, Specification, and Verification
Language. New York: IEEE 2009 (a.k.a. SystemVerilog Language Reference Manual,
or LRM.)
IEEE IEEE Standard Verilog Hardware Design, Description Language. New York: IEEE 2001
Rich, Dave Are SystemVerilog Program Blocks Needed? http://blogs.men-tor.com/verificationhorizons/
blog/2009/05/07/programblocks/ 2009
Sutherland, Stuart. Integrating SystemC Models with Verilog and SystemVerilog Using the
SystemVerilog Direct Programing Interface. Synopsys User Group Europe, 2004
Sutherland, Stuart, Davidmann, Simon, Flake, Peter, and Moorby, Phil. System-Verilog for Design: A
Guide to Using SystemVerilog for Hardware Design and Modeling. Norwell, MA: Springer, 2006
Sutherland, Stuart, Mills, Don. Verilog and SystemVerilog Gotchas. Norwell, MA: Springer, 2007
Synopsys, Inc., Hybrid RTL Formal Verification Ensures Early Detection of Corner-Case Bugs,
http://synopsys.com/products/magellan/magellan_wp.html, 2003
van der Schoot, Hans, and Bergeron, Janick Transaction-Level Functional Coverage in
SystemVerilog. San Jose, CA: DVCon, February 2006
Vijayaraghavan, Srikanth, and Ramanathan, Meyyappan. A Practical Guide for SystemVerilog
Assertions. Norwell, MA: Springer, 2005
Wachowski Andy, and Wachowski Larry. The Matrix. Hollywood, CA: Warner Brothers
Studios, 1999

C. Spear and G. Tumbush, SystemVerilog for Verification: A Guide to Learning
the Testbench Language Features, DOI 10.1007/978-1-4614-0715-7,
© Springer Science+Business Media, LLC 2012

455

Index

!, 179
--, 69
## cycle delay, 108, 204, 375
$, 37, 343–344
$bits, 305
$cast, 55, 60, 284, 286, 291, 305
$clog2, 28
$dist_exponential, 195
$dist_normal, 195
$dist_poisson, 195
$dist_uniform, 195
$error, 117
$exit, 110–111
$fatal, 117
$fclose, 70
$feof, 40, 70
$finish, 110–111, 426
$fopen, 40, 70
$fscanf, 40, 70–71
$get_coverage, 359
$info, 117
$isunknown, 27, 118
$monitor, 372
$psprintf, 62
$random, 195, 217
$realtime, 82–83
$root, 115
$sformat, 62
$sformatf, 62
$size, 30, 35, 41
$strobe, 372
$system, 452
$test$plusargs, 390–391
$time, 27, 82–83
$timeformat, 27, 82–83
$typename, 305, 310
$unit, 114, 417, 449
$urandom, 195, 202, 251
$urandom_range, 41, 194, 195, 202, 215

$value$plusargs, 311, 390–391, 452
$warning, 117
%0t, 27
%p, 29–30, 41, 309
%t, 82
++, 69
+=, 69–70
+ntb_random_seed, 224, 390
::, 54, 142, 144, 250
<<, 55
<->, 186, 190
-=, 69
->, 184, 188–189, 192
->, 190
>>, 55–57, 161
? : operator, 32, 43
[$], 37
[], 35
^=, 69, 74, 76
__FILE__, 174
__LINE__, 174
`define, 49
‘include, 53, 93
‘SV_RAND_CHECK, 174
Numerics
2-state types, 27, 416, 420, 423, 426
4-state types, 27, 424–426
A
Abstract class, 295–296, 298–299, 311,
314, 315
Accellera, viii
Accessor methods, 162
Active region, 101–103, 106, 112
Agent, 17, 163
AHB, 128
Always block, 230
457

458
Always block in programs, 111
and method, 41
Anonymous enumerated type, 57
API see Application Programming
Interface (API)
Application Programming Interface (API), ix
Arguments
default value, 75–76
sticky, 73
task and function, 72
type, 73
Array
assignment, 37
associative, 38–42, 47
compare, 32
constraint, 203
copy, 32
dynamic, 35–36, 38, 46, 47
fixed-size, 28, 36, 38, 46, 47
handle, 155
literal, 29, 35, 37, 40
locator methods, 42
methods, 41
multi-dimensional, 28, 30–31, 36, 436
open, 435–436
packed, 33–34
queue, 36, 38, 46, 47
reduction methods, 41
slice, 32
unpacked, 29, 34
Assertion
concurrent, 118
coverage, 328
immediate, 116–117, 286
in an interface, 379
Assignment pattern, 29
Associative array, 38–42, 47, 50
at_least option, 331, 357
ATM router, 119–121, 123–125
Atomic stimulus generation, 211
auto_bin_max, 339–340, 347, 355
Automatic, 74, 80–81, 103, 137, 138, 141, 237
in threads, 236
Aval, 426, 428

B
Backdoor load, 115
Base class, 276–277
BaseTr class, 295, 304–305
Begin…end, 72, 230
optional in tasks and functions, 72
Bergeron, Janick, xiii, 4, 387, 455
BFM see Bus Functional Model (BFM)

Index
Bus Functional Model (BFM), 13
Bidirectional constraint, see Constraint
bidirectional
Bidirectional signal, 108
Binsof, 351, 353
Bit data type, 27, 421, 432
Bit streaming see Streaming operator
Blueprint pattern, 279
BNF see Backus-Naur Form (BNF)
Backus-Naur Form (BNF), 213
Bounded mailbox, 256–257
Break, 70
Bval, 424, 426
Byte data type, 27, 418, 430

C
calc_csm method, 275–276, 287
Callback
coverage, 335
creation, 298
inject distrubance, 297
scoreboard, 300
usage, 299
Case, 71
Cast, 54, 60, 63, 71
function return value, 71
Cerny, Eduard, 455
Chandle data type, 419, 421–423, 430
Char, 419
Checker, 17, 163
Class, 132, 134
Class constructor, 136
Class scope resolution operator, 144, 250
Clock generator, 111–112
Clocking block, 98–99, 103, 108, 109, 123,
124, 365
Cohen, Ben, 455
Command layer, 17
Comment
covergroup, 356
Comment option, 333, 356
Compilation unit, 113
Composition, 273, 288, 290, 292
Compound assignment, 69–70, 74
Concatenation
bits, 56
string, 61
Concurrent assertion, 118
Conditional operator, 32, 43
Config class, 266
Configuration database, 308
Const, 76
Const type, 61, 74, 114, 176

Index
Constrained, 7
Constraint
array, 203
bidirectional, 184, 188
block, 173
dist, 178–179, 196, 359
implication, 188
in extended class, 284
inside, 176, 179, 183, 202, 203
solve…before, 190–191, 359, 396
constraint_mode, 191–193, 280
Constructor, 136–137, 277, 295
Consumer, 244
Containment, 149
Context, 444
Continue, 70
Copy
deep, 159
method, 159–160, 280, 293, 294
object, 156, 293
shallow, 158
Coverage event, 335
Covergroup
comment, 356
embedded, 334
generic, 354–355
option, 355
sample, 331, 335–337
trigger, 335
Coverpoint, 330, 334, 337, 340, 343, 351,
353, 358
Cross coverage, 348–354
Cross module reference, 114, 370, 376
cross_num_print_missing option, 357
CRT see constrained-random test (CRT)
Constrained-Random Test (CRT), 4, 7–8, 169,
196, 323
Cummings, Cliff, xiii, 100, 102, 455
Cyclic random, 173

D
Data type
bit, 26–27
byte, 26–27
chandle, 419, 421–423, 430
int, 27
integer, 25
logic, 26
longint, 27
real, 25
reg, 25–26
shortint, 27
time, 25

459
wire, 26
Deallocation, 139
DDR see Double data rate (DDR)
Decrement, 69
Default clocking block, 108
Default coverage value, 343, 345, 350–351
Default statement, 98
Default value, 75–76
Delete method, 35, 37, 39, 48
Derived class, 277
Design pattern, 314
Design Under Test (DUT), 3, 88
Direct Programming Interface (DPI), 415–454
Disable, 231, 240–243
Disable fork, 241–242
Disable label, 242–243
Display method, 141–142, 275, 276, 285
Dist constraint, 178, 179, 196, 359
do…while, 39, 59, 70, 174
Domain, 338–339
Double data rate (DDR) clock, 98
Double data type, 418–419
Double precision floating point, 28
Downcasting, 284
DPI see Direct Programming Interface (DPI)
DPI-C, 416, 423
Driver, 17, 163
Driver class, 375, 379
DUT see Design Under Test (DUT)
Dynamic array, 35–36, 38, 46, 47
Dynamic cast, see $cast
Dynamic threads, 235

E
e Reuse Methodology (eRM), viii
Embedded covergroup, 334
Enum, 57–60
Enumerated types, see enum
Enumerated values, 58
Enumeration, 57
Environment, 18, 163
Environment class, 163
Equivalence operator, 186, 190
eRM see e Reuse Methodology (eRM)
Event, 244–249, 264
Event triggered, 244–249, 261
Exists, 40, 48
Expression width, 62
Extended class, 276–277
Extern, see External routine declaration
External constraint, 199–200
External routine declaration, 142–143,
266, 296

460
F
Fabs routine, 417
File I/O, 70
File_exists method, 417
Final block, 110
Find_first method, 43
Find_index method, 43–45
Find_last method, 43
Find_last_index method, 43
First, 39
First method, 39, 47, 59–60
Fixed-size array, 28, 35, 38, 46, 47
Float data type, 418
for loop, 30, 39, 59, 69, 197
Force design signals, 115
foreach constraint, 203, 207–209, 396
foreach loop, 30–31, 35, 37, 40
Fork…join, 230–231
Fork…join_any, 230, 233
Fork…join_none, 230, 232, 234, 235, 237,
238
Four-state types, see 4-state types
Function, 71
arguments, 72
Functional coverage, 18, 455
using callbacks, 301
Functional layer, 17

G
Garbage collection, 139–140
Generator, 18, 163
Generator class, 234, 248, 250, 255, 279, 281,
304, 305
Get method, 251–254, 261, 263, 265
get_coverage, 359
get_inst_coverage, 359
Getc method, 61–62
Goal option, 358

H
Handle, 134–135, 139
array, 155
Haque, Faisal, 455
Hardware Description Language (HDL), 2, 83
Hardware Verification Language (HVL), viii,
2, 13
HDL see Hardware Description Language
(HDL)
Histogram, 180
Hook, 280, 297
HVL see Hardware Verification Language
(HVL)

Index
I
IEEE 1800, see LRM
If constraint, 185–186
Iff, 118, 344
Ignore_bins, 346–347, 351
Illegal_bins, 347
Immediate assertion, 116–117, 286
Implication, 184
Implication operator, 184, 190
Implicit port connection, 112
Import, 53, 418–419
Incl.h, 418
Increment, 69
Inheritance, 274, 288
Initial, 29
Initialization in declaration, 80
In-line constraint, 199
Inout
argument type, 73
port type, 126
Input
argument type, 73
port type, 126
Insert queue, 37
Inside, 71, 176, 180
Inside constraint, 176, 179, 183, 202, 203
Instance, 136
Instantiation, 136
Int data type, 27, 418, 419, 430
Integer data type, 25, 27, 51
Interface, 91, 97, 113, 369
connecting to port, 93
procedural code, 379
virtual, 363–364, 366, 368, 370, 379
Interprocess, 2
Interprocess communication (IPC), 229,
244, 264
Intersect, 351, 353
io_printf, 422, 436
IPC see Interprocess Communication (IPC)
Iterator argument, 43

L
Language Reference Manual (LRM), vii–viii,
47, 76, 100, 102, 109, 175, 200,
229, 333, 352, 363
Last method, 59
Local, 162, 302, 303, 308
Logic, 49, 91
Logic data type, 26, 27, 97, 419, 430
Long long int, 437
Longint data type, 26, 418, 419, 430
LRM see Language Reference Manual (LRM)

Index
M
Macro, 48
Macromodule, 114
Magellan, 455
Mailbox, 252, 257, 262–264
bounded, 256, 257
unbounded, 256
Makes Jack a dull boy
Malloc, 136, 422
Max method, 42
Method, 134, 277
virtual, 286–288, 293, 294
Min method, 42
Modport, 94, 123
Module, 133
Monitor, 17, 163
Multi-dimensional array, 28, 30–31, 36, 436

N
Name function, 182
Name method, 57
New
constructor, 136–137
copying objects, 156
New function, 137
New[] operator, 35, 138
Next method, 39, 48, 59
Nonblocking assignment, 92, 98, 100,
101, 106
Null, 136, 139, 140, 198
Num method, 39, 255

O
Object, 134, 135
copy, 156, 293
deallocation, 139
Object-Oriented Programming
(OOP), vii, 131, 132, 273
analogy badge, 139
analogy car, 132
analogy house, 135
terminology, 134, 277
Observed region, 102
OOP, see Object-Oriented
Programming (OOP)
Open array, 434
Open array, see array open
Open Verification Methodology
(OVM), viii, 4, 309
OpenVera, viii
Option, 352
at_least, 331, 357

461
auto_bin_max, 339, 340, 347, 355
comment, 333, 356
cross_num_print_missing, 357
goal, 358
per_instance, 356, 359
weight, 351–353, 355
Output
argument type, 73
port type, 126
OVM, see Open Verification Methodology
(OVM)

P
Pack
method, 160, 161, 438–439
operation, 55
Package, 49, 53, 61, 114, 133, 146, 200
Packed array, 33–34
Packed structure, 52
Parameter, 49, 61, 114
Parameterized
class, 302
interface, 377
mailbox, 253
module, 377
virtual interface, 377–379
Parent class, 277
Peek method, 253
Per_instance option, 356, 359
Perl hash array, 39
Physical interfaces, 363
PLI, 415, 422
Plyant, Tim
a good guy
Polymorphism, 287–288
Pop_back method, 37
Pop_front method, 37
Port
connecting to interface, 93
Post_randomize, 193–195, 210, 212
Post-decrement, 69
Post-increment, 69
Postponed region, 102–105
Pre_randomize, 193–195, 202
Pre-decrement, 69
Pre-increment, 69
Prev method, 47, 59
Primitive, 114
Print class, 306
Pseudo-random number generator
(PRNG), 175, 217, 219
Process, 229
Producer, 244

462
Product method, 41
Program, 80, 103, 114, 133, 146, 238,
267–268
Program blocks
single vs. multiple, 103
Property, 136, 277
Protected, 308
Prototype, 134, 277, 295
Proxy class, 314–315
PRNG see Pseudo-random number
generator(PRNG)
Public, 162
Pure
imported method, 440–441
virtual method, 295–296, 299, 311
virtual methods, 315
Push_back method, 43, 299
Push_front, 37
Push_front method, 37, 299
Put method, 251, 253, 254, 257–258, 261,
263, 265
Putc method, 62

Q
Queue, 36, 38, 46–47
literal, 37–38

R
Ramanathan, Meyyappan, 455
Rand, 173
Rand_mode, 191, 197
Randc, 173, 182, 203, 208, 281
Randcase, 215–216
Random seed, 11–12, 170, 175, 217,
219, 224
Random stability, 217, 219
Randomize function, 173, 176
Randomize() with, 192–193, 199
Randomize(null), 198
Reactive region, 102–103, 106
Real data type, 25, 27, 175,
418, 430
Realtime data type, 82–83
Ref
argument type, 76, 418–419
port type, 126
Reg data type, 25–26
Regions
scheduling, 101, 102
Return, 78
Reverse method, 45
Rich, Dave, 455

Index
Routine arguments, 72
Run method, 279, 281–282
RVM, 4

S
S_vpi_vecval, 424
Sample method, 331, 335–337
Scenario generation, 211–212
Scenario layer, 18
Scheduling region, 101
Scheduling regions, 102
Scope operator, 54, 142
Scoreboard, 18, 45–46, 163, 297,
300–301
using callbacks, 297
seed, see Random seed
Semaphore, 250–252, 264
Shortint data type, 29, 418, 430
Shortreal data type, 418, 430
Shuffle method, 45
Signal layer, 17
Signature, 288
Signed data types, 27
Sine function, 441
Singleton class, 306, 308, 315
Size function, 203
Size method, 35, 39
Solve…before, see Constraint solve…before
Sort method, 45
Sparse matrix, 38
Specialization, 303, 305
Start method, 344
Stat method, 417
State variables, 198
Static, 29
cast, 54
class, 306, 311
method, 145, 311
storage, 80
variable, 143–146, 311
Stop method, 344
Streaming operator, 55–56, 160–161
String concatenation, 61
String data type, 61–62, 419, 430
Stringify, 174, 317
Struct, 50–53, 57
Subclass, 277
Substr method, 62
Sum, 55
Sum method, 41, 43–44, 204–207, 396
Superclass, 277
Sutherland, Stuart, 385
SV_PACKED_DATA_NELEMS, 424

Index
SV_RAND_CHECK, 174, 286
SVA, 118, 328, 338
SvBit, 419–420, 422, 427–428
SvBitVecVal, 419–420, 423, 426, 435
SvDimension, 435
Svdpi.h, 418, 420, 422, 435
SvGetArrayElemPtr1, 437
SvGetArrayPtr, 434, 435
SvGetArrElemPtr, 435
SvGetArrElemPtr1, 435
SvGetArrElemPtr2, 435–436
SvGetArrElemPtr3, 435
SvGetNameFromScope, 450
SvGetScope, 450
SvGetScopeFromName, 450
SvHigh, 435–436
SvIncrement, 435
SvLeft, 435–436
SvLogic, 418, 426
SvLogicVecVal, 419, 424–426, 435
SvLow, 435–436
SVM, 314
SvOpenArrayHandle, 434, 435
SvRight, 435–436
SvScope, 450
-Svseed, 224
SvSetScope, 450
SvSize, 435
SvSizeOfArray, 435
Synchronous drive, 106
System(), 452
SystemC, 415
SystemVerilog, see LRM
SystemVerilog Assertion, see SVA
SystemVerilog Methodology, 314

T
Task, 71
Arguments, 72
Template, see Parameterized class
Test class, 311
Test layer, 18
The Matrix, 1
This, 148–149
Thread, 229, 232–236, 238–240
Time data type, 25, 27, 82
Time literals, 82
Time step, 102
Timeprecision, 82
Timescale, 81–83, 114
Timeunit, 82
TLM analysis port, 302
Tolower method, 61

463
Toupper method, 61–62
Transactor, 163–164
Transition coverage, 345–346
Tri data type, 26
Triggered, 246
Triggered method, 244–249, 261
Try_get method, 251, 255
Try_put method, 253
Two-state types, see 2-State types
Type conversion, 54
Type_option, 352
Typedef, 48–49
array, 50, 79
associative array index, 50
class, 152
enum, 58
struct, 51–52, 57
union, 52
virtual interface, 375

U
Uint user-defined type, 49, 206
Unbounded mailbox, 256
Union, 51–52
Unique method, 42
Universal Verification Methodology (UVM),
viii, 4–5, 20, 160, 212, 279, 299,
305, 309, 314, 336
Unpack
method, 160–161, 438–439
operation, 55
Unpacked array, 29, 33–34
Unsigned, 27, 49, 195, 201, 208
Unsized array, see array open
User defined type, see typedef
UVM see Universal Verification Methodology
(UVM)

V
Van der Schoot, Hans, 455
Vc_hdrs.h, 418
Verification IP, see VIP
Verification Methodology Manual for
SystemVerilog, see VMM
Verification Procedural Interface,
see VPI
Verilog-1995, vii–viii, 25, 28, 32, 72–73, 80,
119, 195, 363, 377
Verilog-2001, vii–viii, 26, 28, 32, 33, 61, 62,
70, 72, 80, 89, 93
Veriuser.h, 422
Vijayaraghavan, Srikanth, 455

464
VIP, 14
Virtual
class, see Abstract classinterface, 309,
363–379
memory, 287
method, 279, 286–288, 293–295
VMM, viii, 4–5, 17, 20, 97, 212–213, 279,
299, 307, 314, 328, 371
Void
data type, 71, 419, 430
function, 71, 195
VPI, 415, 424, 426
Vpi_control, 426
VpiFinish, 426

Index
W
Wait, 231, 235, 241, 245–250, 268
Wait fork, 238–239, 248
Weight option, 351–353,
355
Wildcard, 346, 354
Wire data type, 26, 97
With, 42–44
Wrap_up method, 282

X
XMR, 370–371, 376
Xor method, 41
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