LC 3 Assembly Lab Manual LC3 And Examples

LC3-AssemblyManualAndExamples

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111110001001110010010110100000101100100000011011000101000101100011111110110111101010001100101111000000010001001000011101000010
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101000011011001101001000000010011000001101011010100000010010001111110111110001010110111010011100101000111111101111010111111111
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100011101011001101000101101100100110100001110010101110110101011110110100010110001011000000000100100111000011100011101011111111
110100011010110111001000101000101111001010111100111100111100011011110101100100110100010101010110011111010001000011110100111110
111100111100111101000000010010100111001100101011000110101001010001010001111001110000010010001110010000000110110011000011000101
001111111111100011000101100011011100110110000111010000110101010100001111101101101100010110110110111001110000101001100111001001
011111011101011101110110010101000101111101011011000101110111010001100110001001011001001000000011110001110010110011110100100001

George M. Georgiou and Brian Strader
California State University, San Bernardino
August 2005

CONTENTS

Contents

ii

List of Code Listings

v

List of Figures

vi

Programming in LC-3

vii

LC-3 Quick Reference Guide
1

2

x

ALU Operations
1.1 Problem Statement . . . . . . . . . . . . . . . . .
1.1.1 Inputs . . . . . . . . . . . . . . . . . . . .
1.1.2 Outputs . . . . . . . . . . . . . . . . . . .
1.2 Instructions in LC-3 . . . . . . . . . . . . . . . . .
1.2.1 Addition . . . . . . . . . . . . . . . . . .
1.2.2 Bitwise AND . . . . . . . . . . . . . . . .
1.2.3 Bitwise NOT . . . . . . . . . . . . . . . .
1.2.4 Bitwise OR . . . . . . . . . . . . . . . . .
1.2.5 Loading and storing with LDR and STR . .
1.3 How to determine whether an integer is even or odd
1.4 Testing . . . . . . . . . . . . . . . . . . . . . . . .
1.5 What to turn in . . . . . . . . . . . . . . . . . . .

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1–1
1–1
1–1
1–1
1–2
1–2
1–2
1–2
1–3
1–3
1–3
1–3
1–4

Arithmetic functions
2.1 Problem Statement . . . . . . . . . . . . . .
2.1.1 Inputs . . . . . . . . . . . . . . . . .
2.1.2 Outputs . . . . . . . . . . . . . . . .
2.2 Operations in LC-3 . . . . . . . . . . . . . .
2.2.1 Loading and storing with LDI and STI
2.2.2 Subtraction . . . . . . . . . . . . . .
2.2.3 Branches . . . . . . . . . . . . . . .
2.2.4 Absolute value . . . . . . . . . . . .
2.3 Example . . . . . . . . . . . . . . . . . . . .
2.4 Testing . . . . . . . . . . . . . . . . . . . . .
2.5 What to turn in . . . . . . . . . . . . . . . .

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2–1
2–1
2–1
2–1
2–2
2–2
2–2
2–3
2–3
2–4
2–4
2–4

Revision: 1.17, January 20, 2007

ii

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CONTENTS
3

4

5

6

7

CONTENTS

Days of the week
3.1 Problem Statement . . . . . . . . . . . . . .
3.1.1 Inputs . . . . . . . . . . . . . . . . .
3.1.2 Outputs . . . . . . . . . . . . . . . .
3.2 The lab . . . . . . . . . . . . . . . . . . . .
3.2.1 Strings in LC-3 . . . . . . . . . . . .
3.2.2 How to output a string on the display
3.2.3 How to read an input value . . . . . .
3.2.4 Defining the days of the week . . . .
3.3 Testing . . . . . . . . . . . . . . . . . . . . .
3.4 What to turn in . . . . . . . . . . . . . . . .

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3–1
3–1
3–1
3–1
3–1
3–1
3–2
3–2
3–3
3–4
3–4

Fibonacci Numbers
4.1 Problem Statement
4.1.1 Inputs . . .
4.1.2 Outputs . .
4.2 Example . . . . . .
4.3 Fibonacci Numbers
4.4 Pseudo-code . . . .
4.5 Notes . . . . . . .
4.6 Testing . . . . . . .
4.7 What to turn in . .

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4–1
4–1
4–1
4–1
4–1
4–1
4–2
4–2
4–3
4–3

Subroutines: multiplication, division, modulus
5.1 Problem Statement . . . . . . . . . . . . .
5.1.1 Inputs . . . . . . . . . . . . . . . .
5.1.2 Outputs . . . . . . . . . . . . . . .
5.2 The program . . . . . . . . . . . . . . . . .
5.2.1 Subroutines . . . . . . . . . . . . .
5.2.2 Saving and restoring registers . . .
5.2.3 Structure of the assembly program .
5.2.4 Multiplication . . . . . . . . . . . .
5.2.5 Division and modulus . . . . . . .
5.3 Testing . . . . . . . . . . . . . . . . . . . .
5.4 What to turn in . . . . . . . . . . . . . . .

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5–1
5–1
5–1
5–1
5–1
5–1
5–2
5–2
5–3
5–3
5–5
5–5

Faster Multiplication
6.1 Problem Statement . . . . . . . . . .
6.1.1 Inputs . . . . . . . . . . . . .
6.1.2 Outputs . . . . . . . . . . . .
6.2 The program . . . . . . . . . . . . . .
6.2.1 The shift-and-add algorithm .
6.2.2 Examining a single bit in LC-3
6.2.3 The MULT1 subroutine . . .
6.3 Testing . . . . . . . . . . . . . . . . .
6.4 What to turn in . . . . . . . . . . . .

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6–1
6–1
6–1
6–1
6–1
6–1
6–2
6–2
6–2
6–2

Compute Day of the Week
7.1 Problem Statement . . . . .
7.1.1 Inputs . . . . . . . .
7.1.2 Outputs . . . . . . .
7.1.3 Example . . . . . .
7.2 Zeller’s formula . . . . . . .
7.3 Subroutines . . . . . . . . .
7.3.1 Structure of program

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7–1
7–1
7–1
7–1
7–1
7–2
7–2
7–2

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iii

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CONTENTS
7.4
7.5
8

9

CONTENTS

Testing: some example dates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7–3
What to turn in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7–3

Random Number Generator
8.1 Problem Statement . . . . . . . . . . . . . . . .
8.1.1 Inputs and Outputs . . . . . . . . . . . .
8.2 Linear Congruential Random Number Generators
8.3 How to output numbers in decimal . . . . . . . .
8.3.1 A rudimentary stack . . . . . . . . . . .
8.4 Testing . . . . . . . . . . . . . . . . . . . . . . .
8.5 What to turn in . . . . . . . . . . . . . . . . . .

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8–1
8–1
8–1
8–1
8–2
8–3
8–3
8–3

Recursive subroutines
9.1 Problem Statement . . . . . . . . . . . . . . . . .
9.1.1 Inputs . . . . . . . . . . . . . . . . . . . .
9.1.2 Output . . . . . . . . . . . . . . . . . . .
9.2 Recursive Subroutines . . . . . . . . . . . . . . .
9.2.1 The Fibonacci numbers . . . . . . . . . . .
9.2.2 Factorial . . . . . . . . . . . . . . . . . .
9.2.3 Catalan numbers . . . . . . . . . . . . . .
9.2.4 The recursive square function. . . . . . . .
9.3 Stack Frames . . . . . . . . . . . . . . . . . . . .
9.4 The McCarthy 91 function: an example in LC-3 . .
9.4.1 Definition . . . . . . . . . . . . . . . . . .
9.4.2 Some facts about the McCarthy 91 function
9.4.3 Implementation of McCarthy 91 in LC-3 .
9.5 Testing . . . . . . . . . . . . . . . . . . . . . . . .
9.6 What to turn in . . . . . . . . . . . . . . . . . . .

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9–1
9–1
9–1
9–1
9–1
9–1
9–1
9–2
9–2
9–3
9–5
9–5
9–5
9–5
9–7
9–7

iv

LIST OF CODE LISTINGS

1
1.1
1.2
1.3
1.4
1.5
1.6
2.1
2.2
2.3
2.4
2.5
2.6
3.1
3.2
4.1
4.2
5.1
5.2
5.3
5.4
5.5
6.1
7.1
8.1
8.2
8.3
8.4
9.1
9.2
9.3
9.4
9.5
9.6
9.7

“Hello World!” in LC-3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The ADD instruction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The AND instruction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The NOT instruction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Implementing the OR operation. . . . . . . . . . . . . . . . . . . . . . . . . .
Loading and storing examples. . . . . . . . . . . . . . . . . . . . . . . . . . .
Determining whether a number is even or odd. . . . . . . . . . . . . . . . . . .
Loading into a register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storing a register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subtraction: 5 − 3 = 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Condition bits are set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Branch if result was zero. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Absolute value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Days of the week data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Display the day. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pseudo-code for computing the Fibonacci number Fn iteratively . . . . . . . .
Pseudo-code for computing the largest n = N such that FN can be held in 16 bits
A subroutine for the function f (n) = 2n + 3. . . . . . . . . . . . . . . . . . . .
Saving and restoring registers R5 and R6. . . . . . . . . . . . . . . . . . . . .
General structure of assembly program. . . . . . . . . . . . . . . . . . . . . .
Pseudo-code for multiplication. . . . . . . . . . . . . . . . . . . . . . . . . . .
Pseudo-code for integer division and modulus. . . . . . . . . . . . . . . . . . .
The shift-and-add multiplication. . . . . . . . . . . . . . . . . . . . . . . . . .
Structure of the program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Generating 20 random numbers using Schrage’s method. . . . . . . . . . . . .
Displaying a digit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output a decimal number. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The code for the stack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The pseudo-code for the recursive version of the Fibonacci numbers function. .
The pseudo-code for the algorithm that implements recursive subroutines. . . .
The pseudo-code for the recursive McCarthy 91 function. . . . . . . . . . . . .
The pseudo-code for the McCarthy 91 recursive subroutine. . . . . . . . . . . .
The program that calls the McCarthy 91 subroutine. . . . . . . . . . . . . . . .
The stack subroutines PUSH and POP. . . . . . . . . . . . . . . . . . . . . . .
The McCarthy 91 subroutine . . . . . . . . . . . . . . . . . . . . . . . . . . .

Revision: 1.17, January 20, 2007

v

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vii
1–2
1–3
1–3
1–3
1–4
1–4
2–2
2–2
2–2
2–3
2–3
2–4
3–3
3–3
4–2
4–3
5–2
5–3
5–3
5–4
5–4
6–2
7–3
8–2
8–2
8–3
8–4
9–2
9–4
9–5
9–7
9–8
9–9
9–9

LIST OF FIGURES

1

LC-3 memory map: the various regions. . . . . . . . . . . . . . . . . . . . . . . .

1.1
1.2

Example run. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–4
The steps taken during the execution of the instruction LEA R2, xFF. . . . . . . . 1–5

2.1
2.2
2.3
2.4

The versions of the BR instruction. . . . . . . . . . . . . . . . . . . . . .
The steps taken during the execution of the instruction LDI R1, X. . . . .
The steps taken during the execution of the instruction STI R2, Y. . . . .
Decimal numbers with their corresponding 2’s complement representation

3.1

The string ”Sunday” in assembly and its corresponding binary representation . . . 3–2

4.1
4.2

Contents of memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–2
Fibonacci numbers table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–4

5.1
5.2

The steps taken during execution of JSR. . . . . . . . . . . . . . . . . . . . . . . 5–2
Input parameters and returned results for DIV. . . . . . . . . . . . . . . . . . . . . 5–4

6.1

Shift-and-add multiplication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–1

8.1

Sequences of random numbers generated for various seeds x0 . . . . . . . . . . . . 8–4

9.1
9.2
9.3
9.4
9.5
9.6
9.7

The first few values of f (n) = n!. . . . . . . . . . . . . . . . . . . . . . . . . . . .
The first few Catalan numbers Cn . . . . . . . . . . . . . . . . . . . . . . . . . . .
Some values of square(n). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The structure of the stack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A typical frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stack size in frames during execution. . . . . . . . . . . . . . . . . . . . . . . . .
Table that shows how many times the function M(n) is executed before it returns the
value for various n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maximum size of stack in terms of frames for n. . . . . . . . . . . . . . . . . . . .

9.8

Revision: 1.17, January 20, 2007

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ix

2–3
2–5
2–5
2–6

9–2
9–2
9–3
9–3
9–4
9–6
9–6
9–8

Programming in LC-3

Parts of an LC-3 Program
1 ; LC−3 Program t h a t d i s p l a y s
2 ; ” H e l l o World ! ” t o t h e c o n s o l e
3
.ORIG
x3000
4
LEA
R0 , HW ; l o a d a d d r e s s o f s t r i n g
5
PUTS
; output s t r i n g to console
6
HALT
; end p r o g r a m
7 HW
.STRINGZ ” H e l l o World ! ”
8
.END

Listing 1: “Hello World!” in LC-3.
The above listing is a typical hello world program written in LC-3 assembly language. The program
outputs “Hello World!” to the console and quits. We will now look at the composition of this
program.
Lines 1 and 2 of the program are comments. LC-3 uses the semi-colon to denote the beginning
of a comment, the same way C++ uses “//” to start a comment on a line. As you probably already
know, comments are very helpful in programming in high-level languages such as C++ or Java. You
will find that they are even more necessary when writing assembly programs. For example in C++,
the subtraction of two numbers would only take one statement, while in LC-3 subtraction usually
takes three instructions, creating a need for further clarity through commenting.
Line 3 contains the .ORIG pseudo-op. A pseudo-op is an instruction that you can use when
writing LC-3 assembly programs, but there is no corresponding instruction in LC-3’s instruction
set. All pseudo-ops start with a period. The best way to think of pseudo-ops are the same way you
would think of preprocessing directives in C++. In C++, the #include statement is really not a C++
statement, but it is a directive that helps a C++ complier do its job. The .ORIG pseudo-op, with its
numeric parameter, tells the assembler where to place the code in memory.
Memory in LC-3 can be thought of as one large 16-bit array. This array can hold LC-3 instructions or it can hold data values that those instructions will manipulate. The standard place for code
to begin at is memory location x3000. Note that the “x” in front of the number indicates it is in
hexadecimal. This means that the “.ORIG x3000” statement will put “LEA R0, HW” in memory
location x3000, “PUTS” will go into memory location x3001, “HALT” into memory location x3002,
and so on until the entire program has been placed into memory. All LC-3 programs begin with the
.ORIG pseudo-op.
Lines 4 and 5 are LC-3 instructions. The first instruction, loads the address of the “Hello World!”
Revision: 1.17, January 20, 2007

vii

Programming in LC-3
string and the next instruction prints the string to the console. It is not important to know how these
instructions actually work right now, as they will be covered in the labs.
Line 6 is the HALT instruction. This instruction tells the LC-3 simulator to stop running the
program. You should put this in the spot where you want to end your program.
Line 7 is another pseudo-op .STRINGZ. After the main program code section, that was ended
by HALT, you can use the pseudo-ops, .STRINGZ, .FILL, and .BLKW to save space for data that
you would like to manipulate in the program. This is a similar idea to declaring variables in C++.
The .STRINGZ pseudo-op in this program saves space in memory for the “Hello World!” string.
Line 8 contains the .END pseudo-op. This tells the assembler that there is no more code to assemble. This should be the very last instruction in your assembly code file. .END can be sometimes
confused with the HALT instruction. HALT tells the simulator to stop a program that is running.
.END indicates where the assembler should stop assembling your code into a program.

Syntax of an LC-3 Instruction
Each LC-3 instruction appears on line of its own and can have up to four parts. These parts in order
are the label, the opcode, the operands, and the comment.
Each instruction can start with a label, which can be used for a variety of reasons. One reason
is that it makes it easier to reference a data variable. In the hello world example, line 7 contains
the label “HW.” The program uses this label to reference the “Hello World!” string. Labels are also
used for branching, which are similar to labels and goto’s in C++. Labels are optional and if an
instruction does not have a label, usually empty space is left where one would be.
The second part of an instruction is the opcode. This indicates to the assembler what kind of
instruction it will be. For example in line 4, LEA indicates that the instruction is a load effective
address instruction. Another example would be ADD, to indicate that the instruction is an addition
instruction. The opcode is mandatory for any instruction.
Operands are required by most instructions. These operands indicate what data the instruction
will be manipulating. The operands are usually registers, labels, or immediate values. Some instructions like HALT do not require operands. If an instruction uses more than one operand like LEA in
the example program, then they are separated by commas.
Lastly an instruction can also have a comment attached to it, which is optional. The operand
section of an instruction is separated from the comment section by a semicolon.

LC-3 Memory
LC-3 memory consists of 216 locations, each being 16 bits wide. Each location is identified with an
address, a positive integer in the range 0 through 216 − 1. More often we use 4-digit hexadecimal
numbers for the addresses. Hence, addresses range from x0000 to xFFFF.
The LC-3 memory with its various regions is shown in figure 1 on page ix.

viii

Programming in LC-3

x0000

Key
x1000

x0000 − x00FF Trap Vector Table

x2000

x0100 − x01FF Interrupt Vector Table

x3000

x0200 − x2FFF OS and Supervisor Stack

x4000

x3000 − xFDFF User Program Area

x5000

xFE00 − xFFFF Device Register Addresses

x6000
x7000
x8000
x9000
xA000
xB000
xC000
xD000
xE000
xF000
xFFFF

Figure 1: LC-3 memory map: the various regions.

ix

LC-3 Quick Reference Guide

LC3 Quick Reference Guide
Instruction Set
Op
ADD

Format
ADD DR, SR1, SR2
ADD DR, SR1, imm5

Description
Adds the values in SR1 and
SR2/imm5 and sets DR to that
value.
Performs a bitwise and on the
values in SR1 and SR2/imm5
and sets DR to the result.

AND

AND DR, SR1, SR2
AND DR, SR1, imm5

BR

BR(n/z/p) LABEL
Note: (n/z/p) means
any combination of
those letters can appear
there, but must be in
that order.

Branch to the code section
indicated by LABEL, if the bit
indicated by (n/z/p) has been set
by a previous instruction. n:
negative bit, z: zero bit, p:
positive bit. Note that some
instructions do not set condition
codes bits.

JMP

JMP SR1

JSR

JSR LABEL

JSRR

JSSR SR1

Unconditionally jump to the
instruction based upon the
address in SR1.
Put the address of the next
instruction after the JSR
instruction into R7 and jump to
the subroutine indicated by
LABEL.
Similar to JSR except the
address stored in SR1 is used
instead of using a LABEL.

LD

LD DR, LABEL

LDI

LDI DR, LABEL

LDR

LDR DR, SR1, offset6

LEA

LEA DR, LABEL

NOT

NOT DR, SR1

Performs a bitwise not on SR1
and stores the result in DR.

RET

RET

Return from a subroutine using
the value in R7 as the base
address.

Load the value indicated by
LABEL into the DR register.
Load the value indicated by the
address at LABEL’s memory
location into the DR register.

Load the value from the memory
location found by adding the
value of SR1 to offset6 into DR.
Load the address of LABEL into
DR.

x

Example
ADD R1, R2, #5
The value 5 is added to the value in
R2 and stored in R1.
AND R0, R1, R2
A bitwise and is preformed on the
values in R1 and R2 and the result
stored in R0.
BRz LPBODY
Branch to LPBODY if the last
instruction that modified the
condition codes resulted in zero.
BRnp ALT1
Branch to ALT1 if last instruction
that modified the condition codes
resulted in a positive or negative
(non-zero) number.
JMP R1
Jump to the code indicated by the
address in R1.
JSR POP
Store the address of the next
instruction into R7 and jump to the
subroutine POP.
JSSR R3
Store the address of the next
instruction into R7 and jump to the
subroutine indicated by R3’s value.
LD R2, VAR1
Load the value at VAR1 into R2.
LDI R3, ADDR1
Suppose ADDR1 points to a
memory location with the value
x3100. Suppose also that memory
location x3100 has the value 8. 8
then would be loaded into R3.
LDR R3, R4, #-2
Load the value found at the address
(R4 –2) into R3.
LEA R1, DATA1
Load the address of DATA1 into
R1.
NOT R0, R1
A bitwise not is preformed on R1
and the result is stored in R0.
RET
Equivalent to JMP R7.

LC-3 Quick Reference Guide

RTI

RTI

ST

ST SR1, LABEL

STI

STI SR1, LABEL

STR

STR SR1, SR2, offset6

TRAP

TRAP trapvector8

Return from an interrupt to the
code that was interrupted. The
address to return to is obtained
by popping it off the supervisor
stack, which is automatically
done by RTI.
Store the value in SR1 into the
memory location indicated by
LABEL.
Store the value in SR1 into the
memory location indicated by
the value that LABEL’s memory
location contains.
The value in SR1 is stored in the
memory location found by
adding SR2 and offest6 together.
Performs the trap service
specified by trapvector8. Each
trapvector8 service has its own
assembly instruction that can
replace the trap instruction.

RTI
Note: RTI can only be used if the
processor is in supervisor mode.
ST R1, VAR3
Store R1’s value into the memory
location of VAR3.
STI R2, ADDR2
Suppose ADDR2’s memory
location contains the value x3101.
R2’s value would then be stored
into memory location x3101.
STR R2, R1, #4
The value of R2 is stored in
memory location (R1 + 4).
TRAP x25
Calls a trap service to end the
program. The assembly instruction
HALT can also be used to replace
TRAP x25.

Symbol Legend
Symbol
SR1, SR2
DR
imm5

Description
Source registers used by instruction.
Destination register that will hold
the instruction’s result.
Immediate value with the size of 5
bits.

Symbol
LABEL
trapvector8
offset6

Description
Label used by instruction.
8 bit value that specifies trap service
routine.
Offset value with the size of 6 bits.

TRAP Routines
Trap Vector
x20

Equivalent Assembly
Instruction
GETC

x21
x22

OUT
PUTS

x23

IN

x24

PUTSP

x25

HALT

Pseudo-op
.ORIG

Format
.ORIG #

.FILL
.BLKW
.STRINGZ
.END

.FILL #
.BLKW #
.STRINGZ “”
.END

Description
Read one input character from the keyboard and store it into R0
without echoing the character to the console.
Output character in R0 to the console.
Output null terminating string to the console starting at address
contained in R0.
Read one input character from the keyboard and store it into R0 and
echo the character to the console.
Same as PUTS except that it outputs null terminated strings with
two ASCII characters packed into a single memory location, with
the low 8 bits outputted first then the high 8 bits.
Ends a user’s program.

Pseudo-ops
Description
Tells the LC-3 simulator where it should place the segment of
code starting at address #.
Place value # at that code line.
Reserve # memory locations for data at that line of code.
Place a null terminating string  starting at that location.
Tells the LC-3 assembler to stop assembling your code.
xi

LC-3 Quick Reference Guide

xii

LAB 1
ALU Operations

1.1

Problem Statement

The numbers X and Y are found at locations x3100 and x3101, respectively. Write an LC-3 assembly
language program that does the following.
• Compute the sum X +Y and place it at location x3102.
• Compute X AND Y and place it at location x3103.
• Compute X OR Y and place it at location x3104.
• Compute NOT(X) and place it at location x3105.
• Compute NOT(Y ) and place it at location x3106.
• Compute X + 3 and place it at location x3107.
• Compute Y − 3 and place it at location x3108.
• If the X is even, place 0 at location x3109. If the number is odd, place 1 at the same location.
The operations AND, OR, and NOT are bitwise. The operation signified by + is the usual
arithmetic addition.

1.1.1

Inputs

The numbers X and Y are in locations x3100 and x3101, respectively:
x3100
x3101

1.1.2

X
Y

Outputs

The outputs at their corresponding locations are as follows:
Revision: 1.12, January 20, 2007

1–1

LAB 1

1.2. INSTRUCTIONS IN LC-3
x3102
x3103
x3104
x3105
x3106
x3107
x3108
x3109

X +Y
X AND Y
X OR Y
NOT(X)
NOT(Y )
X +3
Y −3
Z

(
0
Z=
1

if X is even
if X is odd.

where Z is defined as

1.2

(1.1)

Instructions in LC-3

LC-3 has available these ALU instructions: ADD (arithmetic addition), AND (bitwise and), NOT
(bitwise not).

1.2.1

Addition

Adding two integers is done using the ADD instruction. In listing 1.1, the contents of registers R1
and R2 and added and the result is placed in R3. Note the values of integers can be negative as well,
since they are in two’s complement format. ADD also comes in immediate version, where the second
operand can be a constant integer. For example, we can use it to add 4 to register R1 and place the
result in register R3. See listing 1.1. The constant is limited to 5 bits two’s complement format.
Note, as with all other ALU instructions, the same register can serve both as a source operand and
the destination register.
1
2
3
4
5
6
7
8

; Adding two r e g i s t e r s
ADD R3 , R1 , R2 ; R3 ← R1 + R2
; Adding a r e g i s t e r and a c o n s t a n t
ADD R3 , R1 , #4 ; R3 ← R1 + 4
; Adding a r e g i s t e r and a n e g a t i v e c o n s t a n t
ADD R3 , R1 , #−4 ; R3 ← R1 − 4
; Adding a r e g i s t e r t o i t s e l f
ADD R1 , R1 , R1 ; R1 ← R1 + R1
Listing 1.1: The ADD instruction.

1.2.2

Bitwise AND

Two registers can be bitwise ANDed using the AND instruction, as in listing 1.2 on page 1–3. AND
also comes in the immediate version. Note that an immediate operand can be given in hexadecimal
form using x followed by the number.

1.2.3

Bitwise NOT

The bits of a register can be inverted (flipped) using the bitwise NOT instruction, as in listing 1.3 on
page 1–3.
1–2

LAB 1

1.3. HOW TO DETERMINE WHETHER AN INTEGER IS EVEN OR ODD

1 ; Anding two r e g i s t e r s
2
AND R3 , R1 , R2 ; R3 ← R1 AND R2
3 ; Anding a r e g i s t e r and a c o n s t a n t
4
ADD R3 , R1 , xA ; R3 ← R1 AND 0000000000001010

Listing 1.2: The AND instruction.

1 ;
2

I n v e r t i n g t h e b i t s o f r e g i s t e r R1
NOT R2 , R1 ; R2 ← NOT( R1 )
Listing 1.3: The NOT instruction.

1.2.4

Bitwise OR

LC-3 does not provide the bitwise OR instruction. We can use, however, AND and NOT to built it.
For this purpose, we make use of De Morgan’s rule: X OR Y = NOT(NOT(X) AND NOT(Y )). See
listing 1.4.
1 ; ORing two r e g i s t e r s
2
NOT R1 , R1
3
NOT R2 , R2
4
AND R3 , R1 , R2
5
NOT R3 , R3

;
;
;
;

R1
R2
R3
R3

←
←
←
←

NOT( R1 )
NOT( R2 )
NOT( R1 ) AND NOT( R2 )
R1 OR R2

Listing 1.4: Implementing the OR operation.

1.2.5

Loading and storing with LDR and STR

The instruction LDR can be used to load the contents of a memory location into a register. Knowing
that X and Y are at locations x3100 and x3101, respectively, we can use the code in listing 1.5 on
page 1–4 to load them in registers R1 and R3, respectively. In the same figure one can see how
the instruction STR is used store the contents of a register to a memory location. The instruction
LEA R2, Offset loads register R2 with the address (PC + 1 + Offset), where PC is the address
of the instruction LEA and Offset is a numerical value, i.e. the immediate operand. Figure 1.2 on
page 1–5 shows the steps it takes to execute the LEA R2, xFF instruction.
If instead of a numerical value, a label is given, such as in instruction LEA R2, LABEL , then
the value of the immediate operand, i.e. the offset, is automatically computed so that R2 is loaded
with the address of the instruction with label LABEL.

1.3

How to determine whether an integer is even or odd

In binary, when a number is even it ends with a 0, and when it is odd, it ends with a 1. We can obtain
0 or 1, correspondingly, by using the AND instruction as in listing 1.6 on page 1–4. This method is
valid for numbers in two’s complement format, which includes negative numbers.

1.4

Testing

Test your program for several input pairs of X and Y . In figure 1.1 on page 1–4 an example is shown
of how memory should look after the program is run. The contents of memory are shown in decimal,
1–3

LAB 1
1
2
3
4
5
6
7
8
9
10
11
12
13

1.5. WHAT TO TURN IN

; V a l u e s X and Y a r e l o a d e d i n t o r e g i s t e r s R1 and R3.
.ORIG x3000 ; A d d r e s s where p r o g r a m c o d e b e g i n s
; R2 i s l o a d e d w i t h t h e b e g i n n i n g a d d r e s s o f t h e d a t a
LEA R2 , xFF ; R2 ← x3000 + x1 + xFF ( = x3100 )
; X, which i s l o c a t e d a t x3100 , i s l o a d e d i n t o R1
LDR R1 , R2 , x0 ; R1 ← MEM[ x3100 ]
; Y, which i s l o c a t e d a t x3101 , i s l o a d e d i n t o R3
LDR R3 , R2 , x1 ; R3 ← MEM[ x3100 + x1 ]
...
; S t o r i n g 5 i n memory l o c a t i o n x3101
AND R4 , R4 , x0 ; C l e a r R4
ADD R4 , R4 , x5 ; R4 ← 5
STR R4 , R2 , x1 ; MEM[ x3100 + x1 ] ← R4
Listing 1.5: Loading and storing examples.

AND R2 , R1 , x0001 ; R2 h a s t h e v a l u e o f t h e l e a s t
; s i g n i f i c a n t b i t o f R1.

1
2

Listing 1.6: Determining whether a number is even or odd.

hexadecimal, and binary format.
Address
x3100
x3101
x3102
x3103
x3104
x3105
x3106
x3107
x3108
x3108

Decimal
9
-13
-4
1
-5
65526
12
12
-16
1

Hex
0009
FFF3
FFFC
0001
FFFB
FFF6
000C
000C
FFF0
0001

Binary
0000 0000 0000 1001
1111 1111 1111 0011
1111 1111 1111 1100
0000 0000 0000 0001
1111 1111 1111 1011
1111 1111 1111 0110
0000 0000 0000 1100
0000 0000 0000 1100
1111 1111 1111 0000
0000 0000 0000 0001

Contents
X
Y
X +Y
X AND Y
X OR Y
NOT(X)
NOT(Y )
X +3
Y −3
z

Figure 1.1: Example run.

1.5

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of the (X,Y ) pairs (10, 20), (−11, 15), (11, −15), (9, 12), screenshots that show the
contents of location x3100 through x3108.

1–4

LAB 1

1.5. WHAT TO TURN IN

Step 2

Step 1
PC
3000

x3001

LEA R2, xFF

...

R2

0

0

Initial State of LC3 Simulator

x3002

...

Step 4

PC

LEA R2, xFF

x3001

Use PC to get instruction at x3000
and load it into IR.

Step 3

IR

x3000 LEA R2, xFF

IR

x3002

R2

3001

Memory

3000

x3000 LEA R2, xFF

IR
0

PC

Memory

PC

Memory

3001

x3000 LEA R2, xFF
x3001

IR

x3002

LEA R2, xFF

...

R2

R2

0

3100

Increment PC for the next instruction.

Memory
x3000 LEA R2, xFF
x3001
x3002

...

Execute LEA in IR by adding PC and the offset
and storing the result into R2.

Figure 1.2: The steps taken during the execution of the instruction LEA R2, xFF.

1–5

LAB 2
Arithmetic functions

2.1

Problem Statement

The numbers X and Y are found at locations x3120 and x3121, respectively. Write a program in
LC-3 assembly language that does the following:
• Compute the difference X −Y and place it at location x3122.
• Place the absolute values |X| and |Y | at locations x3123 and x3124, respectively.
• Determine which of |X| and |Y | is larger. Place 1 at location x3125 if |X| is, a 2 if |Y | is, or a
0 if they are equal.

2.1.1

Inputs

The integers X and Y are in locations x3120 and x3121, respectively:
x3120
x3121

2.1.2

X
Y

Outputs

The outputs at their corresponding locations are as follows:
x3122
x3123
x3124
x3125

X −Y
|X|
|Y |
Z

where Z is defined as


1
Z= 2


0
Revision: 1.11, January 26, 2007

if |X| − |Y | > 0
if |X| − |Y | < 0
if |X| − |Y | = 0
2–1

(2.1)

LAB 2

2.2
2.2.1

2.2. OPERATIONS IN LC-3

Operations in LC-3
Loading and storing with LDI and STI

In the previous lab, loading and storing was done using the LDR and STR instructions. In this lab,
the similar but distinct instructions LDI and STI will be used. Number X already stored at location
x3120 can be loaded into a register, say, R1 as in listing 2.1. The Load Indirect instruction, LDI, is
used. The steps taken to execute LDI R1, X are shown in figure 2.2 on page 2–5.
LDI R1 , X
...
...
HALT
...
. F I L L x3120

1
2
3
4
5
6 X

Listing 2.1: Loading into a register.
In listing 2.2, the contents of register R2 are stored at location x3121. The instruction Store
Indirect, STI, is used. The steps taken to execute STI R2, Y instruction are shown in figure 2.3 on
page 2–5.
STI R2 , Y
...
...
HALT
...
. F I L L x3121

1
2
3
4
5
6 Y

Listing 2.2: Storing a register.

2.2.2

Subtraction

LC-3 does not provide a subtraction instruction. However, we can build one using existing instructions. The idea here is to negate the subtrahend1 , which is done by taking its two complement, and
then adding it to the minuend.
As an example, in listing 2.3 the result of the subtraction 5 − 3 = 5 + (−3) = 2 is placed in
register R3. It is assumed that 5 and 3 are already in registers R1 and R2, respectively.
1
2
3
4
5
6
7

;
;
;
;

R e g i s t e r R1 h a s 5 and r e g i s t e r R2 h a s 3
R4 i s u s e d a s a t e m p o r a r y r e g i s t e r . R2 c o u l d h a v e b e e n u s e d
i n t h e p l a c e o f R4 , b u t t h e o r i g i n a l c o n t e n t s o f R2 would
h a v e b e e n l o s t . The r e s u l t o f 5−3=2 g o e s i n t o R3.
NOT R4 , R2
ADD R4 , R4 , #1 ; R4 ← −R2
ADD R3 , R1 , R4 ; R3 ← R1 − R2
Listing 2.3: Subtraction: 5 − 3 = 2.

1 Subtrahend

is a quantity which is subtracted from another, the minuend.

2–2

LAB 2

2.2.3

2.2. OPERATIONS IN LC-3

Branches

The usual linear flow of executing instructions can be altered by using branches. This enables us
to choose code fragments to execute and code fragments to ignore. Many branch instructions are
conditional which means that the branch is taken only if a certain condition is satisfied. For example
the instruction BRz TARGET means the following: if the result of a previous instruction was zero,
the next instruction to be executed is the one with label TARGET. If the result was not zero, the
instruction that follows BRz TARGET is executed and execution continues as normal.
The exact condition for a branch instructions depends on three Condition Bits: N (negative), Z
(zero), and P (positive). The value (0 or 1) of each condition bit is determined by the nature of the
result that was placed in a destination register of an earlier instruction. For example, in listing 2.4
we note that at the execution of the instruction BRz LABEL N is 0, and therefore the branch is not
taken.
...
AND R1 , R1 , x0
ADD R2 , R1 , x1
BRz LABEL
...
...

1
2
3
4
5
6 LABEL

; S i n c e R1 ← 0 , N = 0 , Z = 1 , P = 0
; S i n c e R2 ← 1 , N = 0 , Z = 0 , P = 1

Listing 2.4: Condition bits are set.

Table figure 2.1 shows a list of the available versions of the branch instruction. As an example
BR
BRz
BRn
BRp

branch unconditionally
branch if result was zero
branch if result was negative
branch is result was positive

BRnz
BRnp
BRzp
BRnzp

branch if result was negative or zero
branch if result was negative or positive
branch if result was zero or positive
branch unconditionally

Figure 2.1: The versions of the BR instruction.
consider the code fragment in listing 2.5. The next instruction after the branch instruction to be
executed will be the ADD instruction, since the result placed in R2 was 0, and thus bit Z was set.
The NOT instruction, and the ones that follow it up to the instruction before the ADD will never be
executed.
1
AND
2
BRz
3
NOT
4
...
5
...
6 TARGET ADD
7
...

R2 , R5 , x0 ; r e s u l t p l a c e d i n R2 i s z e r o
TARGET
; B r a n c h i f r e s u l t was z e r o ( i t was )
R1 , R3

R5 , R1 , R2
Listing 2.5: Branch if result was zero.

2.2.4

Absolute value

The absolute value of an integer X is defined as follows:
(
X if X ≥ 0
|X| =
−X if X < 0.
2–3

(2.2)

LAB 2

2.3. EXAMPLE

One way to implement absolute value is seen in listing 2.6.
1 ; Input :
R1 h a s v a l u e
2 ; O u t p u t : R2 h a s v a l u e
3
ADD R2 , R1 , #0
4
BRzp ZP
5
NOT R2 , R2
6
ADD R2 , R2 , #1
7 ZP
...
8
...

X.
|X | .
; R2 ← R1 , c a n now u s e c o n d i t i o n c o d e s
; I f z e r o o r p o s i t i v e , do n o t n e g a t e
; R2 = −R1
; At t h i s p o i n t R2 = | R1 |
Listing 2.6: Absolute value.

2.3

Example

At the end of a run, the memory locations of interest might look like this:
x3120
x3121
x3122
x3123
x3124
x3125

2.4

9
-13
22
9
13
2

Testing

Test your program for these X and Y pairs:
X
10
13
-10
10
-12

Y
12
10
12
-12
-12

Figure 2.4 on page 2–6 is table that shows the binary representations the integers -32 to 32, that can
helpful in testing.

2.5

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of the (X,Y ) pairs (10, 20), (−11, 15), (11, −15), (12, 12), screenshots that show the
contents of location x3120 through x3125.

2–4

LAB 2

2.5. WHAT TO TURN IN

Step 1
MAR
R1
0

Addr X

MAR

3120

X Addr

MDR
0

Step 2

Memory
R1
0

...
x311F

MDR

x3121

R1
0

MAR
3120
MDR
3120

Step 4

3120

R1
17

...
x311F
x3120

x311F
x3120

17

Value 3120 is loaded from memory and copied to MDR.

Memory
Addr X

...

x3121

Instruction loads MAR with X’s Address.
Use MAR to access memory.

Step 3

3120

X Addr

3120

17

x3120

Memory
Addr X

17

MAR
3120
MDR
17

x3121

Memory
Addr X

3120

...
x311F
x3120

17

x3121

Copy MDR to MAR.

Value 17 is loaded into MDR from memory.

Use MAR to access memory.

Copy MDR to R1.

Figure 2.2: The steps taken during the execution of the instruction LDI R1, X.

Step 1
MAR
R2
82

Addr Y

MAR

3121

Y Addr

MDR
0

Step 2

Memory
R2
82

...
x3120

MDR

x3122

R2
82

MAR
3121
MDR
3121

...

x3121

Step 4

3121

R2
82

x3120
x3121

MAR
3121
MDR
82

Memory
Addr Y

3121

...
x3120
x3121

82

x3122

x3122

Copy MDR to MAR.

x3120

Value 3121 is loaded from memory and copied to MDR.

Memory
Addr Y

...

x3122

Instruction loads MAR with Addr Y’s Address.
Use MAR to access memory.

Step 3

3121

Y Addr

3121

x3121

Memory
Addr Y

Copy value 82 from R2 to MDR.
Use MAR to access memory.
Store MDR’s value into memory.

Figure 2.3: The steps taken during the execution of the instruction STI R2, Y.
2–5

LAB 2

2.5. WHAT TO TURN IN

Decimal
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32

2’s Complement
0000000000000000
0000000000000001
0000000000000010
0000000000000011
0000000000000100
0000000000000101
0000000000000110
0000000000000111
0000000000001000
0000000000001001
0000000000001010
0000000000001011
0000000000001100
0000000000001101
0000000000001110
0000000000001111
0000000000010000
0000000000010001
0000000000010010
0000000000010011
0000000000010100
0000000000010101
0000000000010110
0000000000010111
0000000000011000
0000000000011001
0000000000011010
0000000000011011
0000000000011100
0000000000011101
0000000000011110
0000000000011111
0000000000100000

Decimal
-0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-12
-13
-14
-15
-16
-17
-18
-19
-20
-21
-22
-23
-24
-25
-26
-27
-28
-29
-30
-31
-32

2’s Complement
0000000000000000
1111111111111111
1111111111111110
1111111111111101
1111111111111100
1111111111111011
1111111111111010
1111111111111001
1111111111111000
1111111111110111
1111111111110110
1111111111110101
1111111111110100
1111111111110011
1111111111110010
1111111111110001
1111111111110000
1111111111101111
1111111111101110
1111111111101101
1111111111101100
1111111111101011
1111111111101010
1111111111101001
1111111111101000
1111111111100111
1111111111100110
1111111111100101
1111111111100100
1111111111100011
1111111111100010
1111111111100001
1111111111100000

Figure 2.4: Decimal numbers with their corresponding 2’s complement representation

2–6

LAB 3
Days of the week

3.1

Problem Statement

• Write a program in LC-3 assembly language that keeps prompting for an integer in the range
0-6, and each time it outputs the corresponding name of the day. If a key other than ’0’ through
’6’ is pressed, the program exits.

3.1.1

Inputs

At the prompt “Please enter number: ,” a key is pressed.

3.1.2

Outputs

If the key pressed is ’0’ through ’6’, the corresponding name of the day of the week appears on the
screen. Precisely, the correspondence is according to this table:
Code
0
1
2
3
4
5
6

Day
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday

When the day is displayed, the prompt “Please enter number: ” appears again and the program
expects another input. If any key other that ’0’ through ’6’ is pressed, the program exits.

3.2
3.2.1

The lab
Strings in LC-3

It will be necessary to define the prompt “Please enter number: ” and the days of the week as
strings in memory. All strings should terminate with the NUL character (ASCII 0). In LC-3 one
character per memory location is stored. Each location is 16 bits wide. The 8 most significant bits
are 0, while the 8 least significant bits hold the ASCII value of the character. Strings terminated with
the NUL character can be conveniently defined using the directive .STRINGZ ”ABC” , where
Revision: 1.6, August 4, 2005

3–1

LAB 3

3.2. THE LAB

“ABC” is any alphanumeric string. It automatically appends the NUL character to the string. As
an example, a string defined in assembly language and the corresponding contents of memory are
shown in figure 3.1.
x3100
x3101
x3102
x3103
x3104
x3105
x3106

.ORIG x3100
.STRINGZ ” Sunday ”

1
2

0053
0075
006 e
0064
0061
0079
0000

; S
; u
; n
; d
; a
; y
; NUL

Figure 3.1: The string ”Sunday” in assembly and its corresponding binary representation

3.2.2

How to output a string on the display

To output is a string on the screen, one needs to place the beginning address of the string in register R0, and then call the PUTS assembly command, which is another name for the instruction
TRAP x22 . For example, to output “ABC”, one can do the following:

1
2
3
4
5
6 ABCLBL
7

LEA R0 , ABCLBL ; Loads a d d r e s s o f ABC s t r i n g i n t o R0
PUTS
...
HALT
...
.STRINGZ ”ABC”
...

The PUTS command calls a system trap routine which outputs the NUL terminated string the
address of its first character is found in register R0.

3.2.3

How to read an input value

The assembly command GETC , which is another name for TRAP x20 , reads a single character
from the keyboard and places its ASCII value in register R0. The 8 most significant bits of R0 are
cleared. There is no echo of the read character. For example, one may use the following code to
read a single numerical character, 0 through 9, and place its value in register R3:

1
2
3
4
5

GETC
ADD R3 ,
ADD R3 ,
ADD R3 ,
ADD R3 ,

;
R0 ,
R3 ,
R3 ,
R3 ,

Place
x0 ;
#−16 ;
#−16
#−16 ;

ASCII v a l u e o f i n p u t c h a r a c t e r i n t o R0
Copy R0 i n t o R3
S u b t r a c t 4 8 , t h e ASCII v a l u e o f 0
R3 now c o n t a i n s t h e a c t u a l v a l u e

Notice that it was necessary to use three instructions to subtract 48, since the maximum possible
value of the immediate operand of ADD is 5 bits, in two’s complement format. Thus, -16 is the
most we can subtract with the immediate version of the ADD instruction. As an example, if the
pressed key was “5”, its ASCII value 53 will be placed in R0. Subtracting 48 from 53, the value 5
results, as expected, and is placed in register R3.
3–2

LAB 3

3.2.4

3.2. THE LAB

Defining the days of the week

For ease of programming one may define the days of the week so the they have the same length. We
note that “Wednesday” has the largest string length: 9. As a NUL terminated string, it occupies 10
locations in memory. In listing 3.1 define all days so that they have the same length.
1
2
3
4 DAYS
5
6
7
8
9
10

...
HALT
...
.STRINGZ
.STRINGZ
.STRINGZ
.STRINGZ
.STRINGZ
.STRINGZ
.STRINGZ

” Sunday
”
” Monday
”
” Tuesday ”
” Wednesday ”
” Thursday ”
” Friday
”
” Saturday ”
Listing 3.1: Days of the week data.

If the numerical code for a day is i (a value in the range 0 through 6, see section 7.1.2 on page 7–
1), the address of the corresponding day is found by this formula:
Address of(DAYS) + i ∗ 10

(3.1)

Address of(DAYS) is the address of label DAYS, which is the beginning address of the string “Sunday.” Since LC-3 does not provide multiplication, one has to implement it. One can display the
day that corresponds to i by means of the code in listing 3.2, which includes the code of listing 3.1.
Register R3 is assumed to contain i.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

...
; R3 a l r e a d y c o n t a i n s t h e n u m e r i c a l c o d e o f t h e day i
LEA R0 , DAYS
; A d d r e s s o f ” Sunday ” i n R0
ADD R3 , R3 , x0
; To be a b l e t o u s e c o n d i t i o n c o d e s
; The l o o p ( 4 i n s t r u c t i o n s ) i m p l e m e n t s R0 ← R0 + 10 ∗ i
LOOP
BRz DISPLAY
ADD R0 , R0 , #10
; Go t o n e x t day
ADD R3 , R3 , #−1
; Decrement loop v a r i a b l e
BR LOOP
DISPLAY PUTS
...
HALT
...
DAYS
.STRINGZ ” Sunday
”
.STRINGZ ” Monday
”
.STRINGZ ” T u e s d a y ”
.STRINGZ ” Wednesday ”
.STRINGZ ” T h u r s d a y ”
.STRINGZ ” F r i d a y
”
.STRINGZ ” S a t u r d a y ”
Listing 3.2: Display the day.

3–3

LAB 3

3.3

3.3. TESTING

Testing

Test the program with all input keys ’0’ through ’6’ to make sure the correct day is displayed, and
with several keys outside that range, to ascertain that the program terminates.

3.4

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of the input i = 0, 1, 4, 6, screenshots that show the output.

3–4

LAB 4
Fibonacci Numbers

4.1

Problem Statement

1. Write a program in LC-3 assembly language that computes Fn , the n−th Fibonacci number.
2. Find the largest Fn such that no overflow occurs, i.e. find n = N such that FN is the largest
Fibonacci number to be correctly represented with 16 bits in two’s complement format.

4.1.1

Inputs

The integer n is in memory location x3100:

4.1.2

4.2

x3100

n

x3101
x3102
x3103

Fn
N
FN

x3100
x3101
x3102
x3103

6
8
N
FN

Outputs

Example

Starting with 6 in location x3100 means that we intend to compute F6 and place that result in location
x3101. Indeed, F6 = 8. (See below.) The actual values of N and FN should be found by your
program, and be placed in their corresponding locations.

4.3

Fibonacci Numbers

The Fibonacci Fi numbers are the members of the Fibonacci sequence: 1, 1, 2, 3, 5, 8, . . .. The first
two are explicitly defined: F1 = F2 = 1. The rest are defined according to this recursive formula:
Fn = Fn−1 + Fn−2 . In words, each Fibonacci number is the sum of the two previous ones in the
Fibonacci sequence. From the sequence above we see that F6 = 8.
Revision: 1.8, August 14, 2005

4–1

LAB 4

4.4

4.4. PSEUDO-CODE

Pseudo-code

Quite often algorithms are described using pseudo-code. Pseudo-code is not real computer language
code in the sense that it is not intended to be compiled or run. Instead, it is intended to describe
the steps of algorithms at a high level so that they are easily understood. Following the steps in the
pseudo-code, an algorithm can be implemented to programs in a straight forward way. We will use
pseudo-code1 in some of the labs that is reminiscent of high level languages such as C/C++, Java,
and Pascal. As opposed to C/C++, where group of statements are enclosed the curly brackets “{”
and “}” to make up a compound statement, in the pseudo-code the same is indicated via the use of
indentation. Consecutive statements that begin at the same level of indentation are understood to
make up a compound statement.

4.5

Notes

• Figure 4.1 is a schematic of the contents of memory.

3000
LC3 Code

Inputs and Outputs
3100

Figure 4.1: Contents of memory
• The problem should be solved by iteration using loops as opposed to using recursion.
• The pseudo-code for the algorithm to compute Fn is in listing 4.1. It is assumed that n > 0.
1 i f n ≤ 2 then
2
F ← 1
3 else
4
a ← 1 / / Fn−2
5
b ← 1 / / Fn−1
6
f o r i ← 3 t o n do
7
F ← b + a / / Fn = Fn−1 + Fn−2
8
a ← b
9
b ← F

Listing 4.1: Pseudo-code for computing the Fibonacci number Fn iteratively

1 The pseudo-code is close to the one used in Fundamentals of Algorithmics by G. Brassard and P. Bratley, Prentice Hall,
1996.

4–2

LAB 4

4.6. TESTING

• The way to detect overflow is to use a similar for-loop to the one in listing 4.1 on page 4–2
which checks when F first becomes negative, i.e. bit 16 becomes 1. See listing 4.2.
Caution: upon exit from the loop, F does not have the value of FN . To obtain FN you have to
slightly modify the algorithm in listing 4.2.
1
2
3
4
5
6
7
8
9
10
11

a ← 1 / / Fn−2
b ← 1 / / Fn−1
i ← 2 / / loop index
repeat
F ← b + a / / Fn = Fn−1 + Fn−2
i f F < 0 then
N = i
exit
a ← b
b ← F
i ← i + 1
Listing 4.2: Pseudo-code for computing the largest n = N such that FN can be held in 16 bits

4.6

Testing

The table in figure 4.2 on page 4–4 will help you in testing your program.

4.7

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of n = 15 and n = 19, screen shots that show the contents of locations x3100, x3101,
x3102 and x3103, which show the values for F15 and F19 , respectively, and the values of N
and FN .

4–3

LAB 4

4.7. WHAT TO TURN IN

n
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

Fn
1
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
6765
10946
17711
28657
46368
75025

Fn in binary
0000000000000001
0000000000000001
0000000000000010
0000000000000011
0000000000000101
0000000000001000
0000000000001101
0000000000010101
0000000000100010
0000000000110111
0000000001011001
0000000010010000
0000000011101001
0000000101111001
0000001001100010
0000001111011011
0000011000111101
0000101000011000
0001000001010101
0001101001101101
0010101011000010
0100010100101111
0110111111110001
1011010100100000
0010010100010001

Figure 4.2: Fibonacci numbers table

4–4

LAB 5
Subroutines: multiplication, division,
modulus

5.1

Problem Statement

• Given two integers X and Y compute the product XY (multiplication), the quotient X/Y (integer division), and the modulus X (mod Y ) (remainder).

5.1.1

Inputs

The integers X and Y are stored at locations 3100 and 3101, respectively.

5.1.2

Outputs

The product XY , the quotient X/Y , and modulus X (mod Y ) are stored at locations 3102, 3103, and
3104, respectively. If X,Y inputs are invalid for X/Y and X (mod Y ) (see section 5.2.5 on page 5–3)
place 0 in both locations 3103 and 3104.

5.2
5.2.1

The program
Subroutines

Subroutines in assembly language correspond to functions in C/C++ and other computer languages:
they form a group of code that is intended to be used multiple times. They perform a logical task
by operating on parameters passed to them, and at the end they return one or more results. As an
example consider the simple subroutine in listing 5.1 on page 5–2 which implements the function
f n = 2n + 3. The integer n is located at 3120, and the result Fn is stored at location 3121. Register
R0 is used to pass parameter n to the subroutine, and R1 is used to pass the return value f n from the
subroutine to the calling program.
Execution is transfered to the subroutine using the JSR (“jump to subroutine”) instruction. This
instruction also saves the return address, that is the address of the instruction that follows JSR, in
register R7. See figure 5.1 on page 5–2 for the steps taken during execution of JSR. The subroutine
terminates execution via the RET “return from subroutine” instruction. It simply assigns the return
value in R7 to the PC.
The program will have two subroutines: MULT for the multiplication and DIV for division and
modulus.
Revision: 1.8, August 14, 2005

5–1

LAB 5

5.2. THE PROGRAM
LDI R0 , N
JSR F
STI R1 , FN
HALT
. F I L L 3120
. F I L L 3121

1
2
3
4
5 N
6 FN
7
8 F
9
10
11
12
13

AND
ADD
ADD
ADD
RET
END

R1 ,
R1 ,
R1 ,
R1 ,

R1 ,
R0 ,
R1 ,
R1 ,

; Argument N i s now i n R0
; Jump t o s u b r o u t i n e F .

; A d d r e s s where n i s l o c a t e d
; A d d r e s s where f n w i l l be s t o r e d .
; Subroutine F begins
x0 ; C l e a r R1
x0 ; R1 ← R0
R1 ; R1 ← R1 + R1
x3 ; R1 ← R1 + 3 .
R e s u l t i s i n R1
; R e t u r n from s u b r o u t i n e

Listing 5.1: A subroutine for the function f (n) = 2n + 3.

Step 1

Step 2

PC

Step 3

PC

PC

JSR Addr + 1

JSR Addr + 1

F Addr

IR

IR

IR

JSR F

JSR F

JSR F

R7

R7

R7

0

JSR Addr + 1

JSR Addr + 1

LC3 state right before

Copy PC to R7

Copy F’s address from

execution of JSR.

for the RET instruction.

IR to PC so execution
will proceed from there.

Figure 5.1: The steps taken during execution of JSR.

5.2.2

Saving and restoring registers

Make sure that at the beginning of your subroutines you save all registers that will be destroyed in
the course of the subroutine. Before returning to the calling program, restore saved registers. As an
example, listing 5.2 on page 5–3 shows how to save and restore registers R5 and R6 in a subroutine.

5.2.3

Structure of the assembly program

The general structure of the assembly program for this problem can be seen in listing 5.3 on page 5–
3.
5–2

LAB 5

5.2. THE PROGRAM

1 SUB
2
3
4
5
6
7
8
9
10 SaveReg5
11 SaveReg6

...
ST R5 , SaveReg5
ST R6 , SaveReg6
...
...

;
;
;
;

Subroutine is entered
Save R5
Save R6
u s e R5 and R6

LD R5 , SaveReg5
LD R6 , SaveReg6
RET
. F I L L x0
. F I L L x0

; R e s t o r e R5
; R e s t o r e R6
; Back t o t h e c a l l i n g p r o g r a m

Listing 5.2: Saving and restoring registers R5 and R6.

1
2
3
4
5
6
7
8
9 MULT
10
11
12
13
14
15 DIV
16
17
18

...
JSR MULT;
...
;
JSR DIV ;
HALT
...
...
...
...
...
...
RET

;
;
;
;
;
;
;

Jump t o t h e m u l t i p l i c a t i o n s u b r o u t i n e
Here p r o d u c t XY i s i n R2
Jump t o t h e d i v i s i o n and mod s u b r o u t i n e

Multiplication subroutine begins
Save r e g i s t e r s t h a t w i l l be o v e r w r i t t e n
M u l t i p l i c a t i o n Algorithm
Restore saved r e g i s t e r s
R2 h a s t h e p r o d u c t .
R e t u r n from s u b r o u t i n e
D i v i s i o n and mod s u b r o u t i n e b e g i n s

...
...
RET
END
Listing 5.3: General structure of assembly program.

5.2.4

Multiplication

Multiplication is achieved via addition:
XY = X + X + . . . + X
|
{z
}

(5.1)

Y times

Listing 5.4 on page 5–4 shows the pseudo-code for the multiplication algorithm. Parameters X and
Y are passed to the multiplication subroutine MULT via registers R0 and R1. The result is in R2.

5.2.5

Division and modulus

Integer division X/Y and modulus X (mod Y ) satisfy this formula:
X = X/Y ∗Y + X (mod Y )

(5.2)

Where X/Y is the quotient and X (mod Y ) is the remainder. For example, if X = 41 and Y = 7, the
equation becomes
41 = 5 ∗ 7 + 6
(5.3)
5–3

LAB 5
1
2
3
4
5
6
7
8
9
10
11
12
13
14

5.2. THE PROGRAM

/ / M u l t i p l y i n g XY. P r o d u c t i s i n v a r i a b l e p r o d .
s i g n ← 1 / / The s i g n o f t h e p r o d u c t
i f X < 0 then
X = −X
/ / Convert X to p o s i t i v e
s i g n = −s i g n
i f Y < 0 then
Y = −Y
/ / Convert Y to p o s i t i v e
s i g n = −s i g n
prod ← 0
/ / I n i t i a l i z e product
w h i l e Y 6= 0 do
prod ← prod + X
Y←Y − 1
i f sign < 0 then
p r o d ← −p r o d
/ / Adjust sign of product
Listing 5.4: Pseudo-code for multiplication.

Subroutine DIV will compute both the quotient and remainder. Parameter X is passed to DIV
through R0 and Y through R1. For simplicity division and modulus are defined only for X ≥ 0 and
Y > 0. Subroutine DIV should check if these conditions are satisfied. If, not it should return with
R2 = 0, indicating that the results are not valid. If they are satisfied, R2 = 1, to indicate that the
results are valid. Overflow conditions need not be checked at this time. Figure 5.2 summarizes the
input arguments and results that should be returned.
Register
R0
R1
R2

Input parameter
X
Y

Result
X/Y or 0 if invalid
X (mod Y ) or 0 if invalid
1 if results valid, 0 otherwise

Figure 5.2: Input parameters and returned results for DIV.
Listing 5.5 shows the pseudo-code for the algorithm that performs integer division and modulus
functions. The quotient is computed by successively subtracting Y from X. The leftover quantity is
the remainder.
1
2
3
4
5
6
7
8
9
10
11
12

/ / F i n d i n g t h e q u o t i e n t X/Y and r e m a i n d e r X mod Y .
quotient ← 0
// I n i t i a l i z e quotient
remainder ← 0 / / I n i t i a l i z e remainder ( in case input i n v a l i d )
valid ← 0
// I n i t i a l i z e valid
i f X < 0 or Y ≤ 0 then
exit
valid = 1
temp ← X
/ / Holds q u a n t i t y l e f t
w h i l e temp ≥ Y do
temp = temp − Y
quotient ← quotient + 1
r e m a i n d e r ← temp
Listing 5.5: Pseudo-code for integer division and modulus.

5–4

LAB 5

5.3

5.3. TESTING

Testing

You should first write the MULT subroutine, thoroughly test it, and then proceed to implement the
DIV subroutine. Thoroughly test DIV. Finally, test the program as a whole for various inputs.

5.4

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of the (X,Y ) pairs (100, 17), (211, 4), (11, −15), (12, 0), screenshots that show the
contents of locations 3100 through 3104.

5–5

LAB 6
Faster Multiplication

6.1

Problem Statement

Write a faster multiplication subroutine using the shift-and-add method.

6.1.1

Inputs

The integers X and Y are stored at locations 3100 and 3101, respectively.

6.1.2

Outputs

The product XY is stored at location x3102.

6.2

The program

The program should perform multiplication by subroutine MULT1, which is an implementation of
the so-called shift-and-add algorithm. Overflow is not checked.

6.2.1

The shift-and-add algorithm

Before giving the algorithm, we consider an example multiplication. We would like to multiply
X = 1101 and Y = 101011. This can be done with the shift-and-add method which resembles
multiplication by hand. Figure 6.1 shows the steps. The bold bits are the bits of the multiplier
scanned right-to-left. The result is initialized to zero, and then we consider the bits of the multiplier
from right to left: if the bit is 1 the multiplicand is added to the product and then shifted to the left
by one position. If the bit is 0, the multiplicand is shifted to the left, but no addition is performed.
101011
1101
101011
1010110
10101100
101011000
1000101111

← Multiplicand
← Multiplier
1: Add and shift
0: Shift (not added)
1: Add and shift
1: Add and shift
← Result

Figure 6.1: Shift-and-add multiplication
Revision: 1.8, August 14, 2005

6–1

LAB 6

6.3. TESTING

Let X = x15 x14 x13 . . . x1 x0 and Y = y15 y14 y13 . . . y1 y0 be the bit representations of multiplier X
and multiplicand Y . We would like to compute the product P = XY . For the time, we assume that
both X and Y are positive, i.e. x15 = y15 = 0. The multiplication algorithm is described in listing 6.1.
Recall that in binary, multiplication by 2 is equivalent to a left shift.
1
2
3
4
5
6
7
8

/ / Compute p r o d u c t P ← XY
/ / Y is the multiplicand
/ / X = x15 x14 x13 . . . x1 x0 i s t h e m u l t i p l i e r
P ← 0
/ / I n i t i a l i z e product
f o r i =0 t o 14 do
/ / Exclude the sign b i t
i f xi = 1 t h e n
P ← P + Y
/ / Add
Y←Y + Y
// Shift left
Listing 6.1: The shift-and-add multiplication.

6.2.2

Examining a single bit in LC-3

Suppose we would like to check whether the least significant bit (LSB) of R1 is 0 or 1. We can do
that with these instructions:
1
AND
2
ADD
3
AND
4
BRz
5
...
6 ISZERO . . .
7
...

R2 , R2 , x0
R2 , R2 , x1
R0 , R1 , R2
ISZERO

;
; I n i t i a l i z e R2 t o 1
;
; B r a n c h i f LSB o f R1 i s 0

To test the next bit of R1, we shift to the left the 1 in R2 with ADD R2, R2, R2 , and then again
we do:
AND R0 , R1 , R2
BRz ISZERO

1
2

;
; B r a n c h i f n e x t b i t o f R1 i s 0

We notice that by adding R2 to itself, the only bit in R2 that is 1 shifts to the left by one position.

6.2.3

The MULT1 subroutine

Subroutine MULT1 to be written should be used to perform the multiplication. Parameters X and
Y are passed to MULT1 via registers R0 and R1. The result is in R2. The multiplication should
work even if the parameters are negative numbers. To achieve this, use the same technique of the
algorithm in listing 5.4 on page 5–4 to handle the signs.
Registers that are used in the subroutine should be saved and then restored.

6.3

Testing

Test the MULT1 subroutine for various inputs, positive and negative.

6.4

What to turn in

• A hardcopy of the assembly source code.
6–2

LAB 6

6.4. WHAT TO TURN IN

• Electronic version of the assembly code.
• For each of the (X,Y ) pairs (100, 17), (−211, −4), (11, −15), (12, 0), screenshots that show
the contents of locations 3100 through 3102.

6–3

LAB 7
Compute Day of the Week

7.1

Problem Statement

Write an LC-3 program that given the day, month and year will return the day of the week.

7.1.1

Inputs

Before execution begins, it is assumed that locations x31F0, 31F1, and x31F2 contain the following
inputs:
x31F0
x31F1
x31F2

The usual number of the month
The day of the month
The year

For the example we have been using, June 1, 2005, we could use this code fragment in a different
module:
.ORIG
.FILL
.FILL
.FILL

7.1.2

x31F0
#6
#1
#2005

Outputs

The outputs are:
• A number between 0 and 6 that corresponds to the days of the week, starting with Sunday,
should be stored in location x31F3.
• The corresponding name of the day is displayed on the screen.

7.1.3

Example

The program to be written answers this question: what was the day of the week on January 1, 1900?
Answer: yadnoM
Revision: 1.6, August 26, 2005

7–1

LAB 7

7.2

7.2. ZELLER’S FORMULA

Zeller’s formula

The day of the week can be found by using Zeller’s formula1 :
f = k + (13m − 1)/5 + D + D/4 +C/4 − 2C,

(7.1)

where the symbol “/” represents integer division. For example 9/2 = 4. Using as example the date
June 1, 2005, the symbols in the formula have the following meaning:
• k is the day of the month. In the example, k = 1.
• m is the month number designated in a special way: March is 1, April is 2, . . . , December is
10; January is 11, and February is 12. If x is the usual month number, i.e. for January x is 1, for
February x is 2, and so on; then m can be computed with this formula: m = (x + 21)%12 + 1,
where % is the usual modulus (i.e. remainder) function. Alternatively, m can be computed in
this way:
(
x + 10, if x ≤ 2
(7.2)
m=
x − 2, otherwise.
In our example, m = 4.
• D is the last two digits of the year, but if it is January or February those of the previous year
are used. In our example, D = 05.
• C is for century, and it is the first two digits of year. In our example, C = 20.
• From the result f we can obtain the day of the week based on this code:
f %7
0
1
2
3
4
5
6

Day
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday

For example, if f = 123, then f %7 = 4, and thus the day was Thursday. Again, % is the
modulus function.

7.3

Subroutines

To compute the modulus (%), integer division (/), and multiplication, subroutines MULT and DIV,
which were written for a previous lab, should be used.
Make sure that MULT and DIV subroutines save and restore all registers they use, except those
that are used to return results. Use R0 and R1 to pass parameters, and R0, R1 and R2 to return the
results.

7.3.1

Structure of program

The general structure of the program appears in listing 7.1 on page 7–3. The problem of displaying
the name of the day on the screen was solved in Lab 3.
1

“Kalender-Formeln” von Rektor Chr. Zeller in Markgröningen, Mathematisch-naturwissenschaftliche Mitteilungen des
mathematisch-naturwissenschaftlichen Vereins in Wrttemberg, ser. 1, 1 (1885), pp.54-58 – in German.

7–2

LAB 7

7.4. TESTING: SOME EXAMPLE DATES

1
.ORIG x3000
2
...
3
. . . ; MULT and DIV a r e c a l l e d a number o f t i m e s
4
...
5
...
6
PUTS ; D i s p l a y day o f t h e week on s c r e e n
7
HALT
8 DAYS .STRINGZ ” Sunday
”
9
.STRINGZ ” Monday
”
10
.STRINGZ ” T u e s d a y ”
11
.STRINGZ ” Wednesday ”
12
.STRINGZ ” T h u r s d a y ”
13
.STRINGZ ” F r i d a y
”
14
.STRINGZ ” S a t u r d a y ”
15
...
16
17 MULT
...
; B e g i n n i n g o f MULT s u b r o u t i n e
18
19
...
20
RET
21 DIV
...
; B e g i n n i n g o f DIV s u b r o u t i n e
22
23
...
24
RET
25
.END

Listing 7.1: Structure of the program.

7.4

Testing: some example dates

Test your program using these dates:
September 11, 2001
June 6, 1944
September 1, 1939
November 22, 1963
August 8, 1974

7.5

Tuesday
Tuesday
Friday
Friday
Thursday

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of the random dates in the table below, screenshots that show the contents of memory
locations x31F0 through x31F3.
Date
January 3, 1905
June 6, 1938
June 23, 1941
May 7, 1961
Date this lab is due

7–3

Day of the week

LAB 8
Random Number Generator

8.1

Problem Statement

• Generate random numbers using a Linear Congruential Random Number Generator (LCRNG).

8.1.1

Inputs and Outputs

The seed, which is an integer in the range 1 to 32766, is found at location x3100. When the program
is executed, 20 random numbers in the interval 1 to 215 − 2 are generated and displayed.

8.2

Linear Congruential Random Number Generators

A LCRNG is defined by the this recurrence equation:
xn ← a xn−1 + c mod m

(8.1)

The multiplicative constant a, the constant c, and modulus m are integers that are chosen and fixed.
Given the seed x0 , a random number sequence is generated: x1 , x2 , x3 , . . . , with the xi ’s being in the
range 0 to m − 1. Eventually the sequence will repeat itself. In most cases, it is desirable that the
period of repetition is as long as possible.
Using the subroutines MULT and DIV, used in earlier labs, one can write a program in LC3 to generate random numbers based on equation (8.1). There is, however, the possibility that
intermediate operations, such as a xn−1 , cause an overflow. In the case where c = 0, to avoid overflow
we use Schrage’s method1 . In this method, the recurrence is
xn ← a xn−1

mod m,

and multiplication a x is performed in the following fashion:
(
a (x mod q) − r (x/q)
if ≥ 0
a x mod m =
a (x mod q) − r (x/q) + m otherwise,

(8.2)

(8.3)

where
q = m/a, r = m mod a.

(8.4)

As always, “/” denotes integer division. To ensure no overflow while performing the computations
in equation (8.3), multiplier a and modulus m must be chosen so that 0 ≤ r < q. Listing 8.1 on
page 8–2 has the algorithm to generate 20 random numbers.
1 Schrage,

L. 1979, ACM Transactions on Mathematical Software, vol. 5, pp. 132–138.

Revision: 1.6, August 4, 2005

8–1

LAB 8
1
2
3
4
5
6
7
8
9
10
11
12

8.3. HOW TO OUTPUT NUMBERS IN DECIMAL

/ / A l g o r i t h m f o r t h e i t e r a t i o n x ← a x mod m
/ / u s i n g S c h r a g e ’ s method
a ← 7
/ / a , the m u l t i p l i c a t i v e constant i s given
m ← 32767 / / m = 2 ˆ 1 5 −1, t h e modulus i s g i v e n
x ← 10
/ / x , the seed i s given
q = m/ a
r = m mod a
f o r 1 t o 20 do
x ← a ∗ ( x mod q ) − r ∗ ( x / q )
i f x < 0 then
x ← x + m
output x
Listing 8.1: Generating 20 random numbers using Schrage’s method.

For two’s complement 16-bit arithmetic, which is the LC-3 case, the largest possible m is 215 − 1.
Using this value for m, to produce a maximal non-repeating sequence2 of random numbers one can
choose a = 7. The seed x0 should never be 0; it should be any number from 1 to 215 − 2 = 32766.
Your program should implement equation (8.2) on page 8–1 with the algorithm found in
listing 8.1.

8.3

How to output numbers in decimal

The assembly command OUT, which is shorthand for TRAP x21, outputs the single ASCII character found in the 8 least significant bits of R0. (See listing 8.2 for an example.) We can use OUT,
1
2
3
4
5
6
7
8
9
10
11
12
13
14

; We would l i k e t o d i s p l a y i n d e c i m a l t h e d i g i t i n r e g i s t e r R3
; which h a p p e n s t o be n e g a t i v e
...
NOT R3 , R3
; N e g a t e R3 t o o b t a i n p o s i t i v e v e r s i o n
ADD R3 , R3 , #1
LD R0 , MINUS
; Output ’−’
OUT
LD R0 , OFFSET
; Output d i g i t
ADD R0 , R0 , R3
OUT
...
HALT
MINUS . F I L L x2D
; Minus s i g n i n ASCII
OFFSET . F I L L x30
; 0 i n ASCII
Listing 8.2: Displaying a digit.
therefore, to output the decimal digits of a number one by one. We can obtain the digits by successively applying the mod 10 on the number and truncating, until we obtain 0. This produces
the digits from right to left. For example if the number we would like to output is x219 = 537, by
applying the above procedure we obtain the digits in this order: 7, 3, 5. Thus, we have to output them
in reverse order of their generation. For this purpose we can use a stack, with operations PUSH and
POP.
2 I.e.,

all integers in the range 1 to 215 − 2, will be generated before the sequence will repeat itself.

8–2

LAB 8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

8.4. TESTING

/ / We would l i k e t o o u t p u t n a s a d e c i m a l
l e f t ← n / / remaining value
sign ← 1 / / sign of n
i f n < 0 then
s i g n = −s i g n
/ / n is negative
l e f t ← −n
i f l e f t = 0 then
digit ← 0
/ / in case n = 0
push d i g i t
w h i l e l e f t 6= 0 do
d i g i t ← l e f t mod 10 / / g e n e r a t e a d i g i t
push d i g i t
/ / p u s h d i g i t on s t a c k
l e f t ← l e f t /10
i f sign < 0 then
o u t p u t ’−’
/ / number i s n e g a t i v e
w h i l e n o t ( s t a c k e m p t y ) do
pop d i g i t
output d i g i t
Listing 8.3: Output a decimal number.

8.3.1

A rudimentary stack

The stack that is described here is a rudimentary one3 . It is intended for this problem only. There
are three operations, i.e. subroutines, that involve the stack: PUSH, POP, and ISEMPTY. PUSH
pushes the contents of register R0 on the stack, POP pops the top of the stack in register R0, and
ISEMPTY returns 1 in R0 if the stack is empty and 0 if the stack is non-empty. Register R6 points
to the top of the stack. The following have to be borne in mind when writing your program:
• R6 should be initialized to x4000, the base of the stack, and not be overwritten while manipulating the stack.
• R7 will be used (implicitly) to store the return address when calling a subroutine.
• Always ISEMPTY should be called before proceeding to call POP, to check whether the
stack is empty. If empty, POP should not be called.
Listing 8.4 on page 8–4 shows the implementation of the stack subroutines.

8.4

Testing

Using a = 7, m = 32767 in equation (8.2) on page 8–1, and starting with various seeds x0 , the first
10 random numbers generated in each case are listed in figure 8.1 on page 8–4.

8.5

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For a = 7, m = 32767 and seed x0 = 10010 , a screenshot showing the first 20 random numbers
generated.
3 For

a more sophisticated implementation of a stack see Chapter 10 of the textbook Introduction to Computing Systems
by Patt and Patel

8–3

LAB 8

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29

8.5. WHAT TO TURN IN

.ORIG x3000
; Your p r o g r a m g o e s h e r e
...
...
LD R6 , BASE
...
JSR PUSH
...
HALT
BASE
. F I L L x4000
...
...
...
PUSH
ADD R6 , R6 , #−1
STR R0 , R6 , #0
RET
POP
LDR R0 , R6 , #0
ADD R6 , R6 , #1
RET
ISEMPTY LD R0 , EMPTY
ADD R0 , R6 , R0
BRz I S
ADD R0 , R0 , #0
RET
IS
AND R0 , R0 , #0
ADD R0 , R0 , #1
RET
EMPTY
. F I L L xC000
END

; Top o f s t a c k p o i n t s t o b a s e
; Jump t o PUSH s u b r o u t i n e
; Your p r o g r a m e n d s h e r e
; More p r o g r a m d a t a h e r e
; Subroutines for stack begin
; Move t o p o f t h e s t a c k up
; S t o r e R0 t h e r e
; Load R0 w i t h t o p o f s t a c k
; Move t o p o f s t a c k down

; Branch i f a t base of s t a c k
; R0 ← 0 , s t a c k i s n o t empty

; R0 ← 1 , s t a c k i s empty
; −x4000

Listing 8.4: The code for the stack.

Decimal
Hex
Decimal
Hex
Decimal
Hex
Decimal
Hex
Decimal
Hex
Decimal
Hex

x0
1
0001
6
0006
9
0009
10
000A
178
00B2
1000
03E8

x1
7
0007
42
002A
63
003F
70
0046
1246
04DE
7000
1B58

x2
49
0031
294
0126
441
01B9
490
01EA
8722
2212
16233
3F69

x3
343
0157
2058
080A
3087
0C0F
3430
0D66
28287
6E7F
15330
3BE2

x4
2401
0961
14406
3846
21609
5469
24010
5DCA
1407
057F
9009
2331

x5
16807
41A7
2541
09ED
20195
4EE3
4235
108B
9849
2679
30296
7658

x6
19348
4B94
17787
457B
10297
2839
29645
73CD
3409
0D51
15470
3C6E

x7
4368
1110
26208
6660
6545
1991
10913
2AA1
23863
5D37
9989
2705

Figure 8.1: Sequences of random numbers generated for various seeds x0 .

8–4

x8
30576
7770
19621
4CA5
13048
32F8
10857
2A69
3206
0C86
4389
1125

x9
17430
4416
6279
1887
25802
64CA
10465
28E1
22442
57AA
30723
7803

x10
23709
5C9D
11186
2BB2
16779
418B
7721
1E29
26026
65AA
18459
481B

LAB 9
Recursive subroutines

9.1

Problem Statement

Implement the recursive square function in LC-3 as it is described in section 9.2.4 on page 9–2.

9.1.1

Inputs

The value n is found at location x3100.

9.1.2

Output

The value f (n) = n2 is saved at location x3101.

9.2

Recursive Subroutines

A subroutine, or function, is recursive when it calls itself. Mathematically, a recursive function is
one that is being used in its own definition. In what follows we will give the mathematical definitions
of some well-known recursive functions.

9.2.1

The Fibonacci numbers

The Fibonacci numbers Fn , which were encountered in an earlier lab, are defined as follows:
(
n,
if n ≤ 2
F(n) =
(9.1)
F(n − 1) + F(n − 2) otherwise.
Using pseudo-code, the algorithm for Fn is shown in listing 9.1 on page 9–2.

9.2.2

Factorial

The factorial function f (n) = n!, n ≥ 0, is defined as follows:
(
1,
if n = 0
f (n) =
n ∗ f (n − 1) if n > 0.
Revision: 1.3, August 14, 2005

9–1

(9.2)

LAB 9
1
2
3
4
5
6

9.2. RECURSIVE SUBROUTINES

/ / Compute t h e F i b o n a c c i number F ( n ) , n ≥ 1
function F(n)
if n ≤ 2
return 1
else
r e t u r n F ( n −1) + F ( n −2)
Listing 9.1: The pseudo-code for the recursive version of the Fibonacci numbers function.

Non-recursively, the factorial function is defined as follows:
(
1,
if n = 0
f (n) =
n ∗ (n − 1) ∗ . . . ∗ 1, if n > 0.

(9.3)

The first few values of f (n) = n! are shown in figure 9.1.
n
n!

0
1

1
1

2
2

3
6

4
24

5
120

6
720

7
5040

8
40320

9
362880

10
3628800

Figure 9.1: The first few values of f (n) = n!.

9.2.3

Catalan numbers

Catalan numbers Cn , n ≥ 0, are defined as follows:
 
n
1
(2n)!
Cn ≡
=
.
n + 1 2n
(n + 1)!n!

(9.4)

Recursively, the Catalan numbers can be defined as
Cn+1 =

2(2n + 1)
Cn ,
n+2

(9.5)

with C0 = 1. An alternative recursive definition is


if n = 0
1,
n−1
Cn =

 ∑ Ci Cn−1−i , if n > 0.

(9.6)

i=0

The first few values of Cn are shown in figure 9.2.
n
Cn

0
1

1
1

2
2

3
5

4
14

5
42

6
132

7
429

8
1430

9
4862

10
16796

Figure 9.2: The first few Catalan numbers Cn .

9.2.4

The recursive square function.

The familiar square function square(n) = n2 can be defined recursively as well:
(
0,
if n = 0
square(n) =
square(n − 1) + 2n − 1, if n > 0.
9–2

(9.7)

LAB 9

9.3. STACK FRAMES
n
square(n)

0
0

1
1

2
4

3
9

4
16

5
25

6
36

7
49

8
64

9
81

10
100

Figure 9.3: Some values of square(n).
The first few values of square(n) are shown in figure 9.3.
In this lab, you asked to implement the recursive square function as a subroutine, and call it from
the main program. Your program should work for negative numbers as well, however the square(n)
subroutine should never be called with a negative argument: there will be a stack overflow, which is
explained in the section that follows. In that and the other sections that follow you will find details
that will help you in the implementation of the square(n) subroutine.

9.3

Stack Frames

When a program (or subroutine) A calls a subroutine B with one of either instruction JSR and JSRR,
automatically the return address to A is saved in register R7. While executing, if subroutine B calls
another subroutine C, then the return address to B will again be saved in R7, which would overwrite
the previous value. When it is time to return to A, there will be no record of the proper return address.
This situation shows the need to have a bookkeeping method that will save return addresses. This
need is further demonstrated when having a subroutine that calls itself, i.e. a recursive subroutine.
In this case, beyond the return address other information, such as parameters and return value, needs
to be allocated for each invocation of the subroutine. The efficient solution to this problem is to have
that information saved on a stack.
The space on the stack associated with the invocation of a subroutine is called frame. The stack
consists of many frames, stacked in the order by which they are called from their corresponding
subroutines. If subroutine A calls subroutine, B calls subroutine C, and C calls itself two times, the
stack will have the structure of figure 9.4. When a subroutine returns, its corresponding frame is
removed from the stack.

Frame C
Frame C
Frame C
Frame B
Frame A
Figure 9.4: The structure of the stack.
A typical frame has the structure in figure 9.5 on page 9–4. The frame pointer, also known
known as dynamic link, points to the first parameter and is used to refer to items within the frame
via offsets. Register R5 is used hold the value of the current frame pointer. The frame pointer of
the calling subroutine is saved on the frame of the called subroutine. When the called subroutine
returns, the frame pointer is restored in R5, and is ready to be used in referring to items within the
current frame.
During the execution of a program, while subroutines are called and return, the stack grows and
shrinks accordingly. Every time a subroutine is entered, a frame is created; by the time a subroutine
returns, all the elements of the frame will have been popped from the stack, and the frame will
not exist anymore. If the size of the stack grows too large, i.e. there are too many outstanding
subroutines, there is the danger of not having sufficient space to accommodate it, and it will cause
an error, which is commonly referred to as stack overflow.
The pseudo-code algorithm to implement recursive subroutines is shown in listing 9.2 on page 9–
4. It demonstrates how subroutine frames are created on the run-time stack, and destroyed. It is a
9–3

LAB 9

9.3. STACK FRAMES

Local Variable 2
Local Variable 1
Frame Pointer
Return Address
Return Value
Parameter 2
Parameter 1












Frame











Figure 9.5: A typical frame
summary of the description in the textbook1 .
1 / / The c a l l i n g p r o g r a m
2 ...
3 PUSH P a r a m e t e r 1
4 CALL F ( )
5 ReturnValue
6 POP
7
8
9
10
11
12
13
14
15

16
17
18
19
20
21
22

← POP

/ / r e p e a t as needed f o r a d d i t i o n a l
// p a r a m e t e r s
/ / jump t o F ’ s c o d e
/ / pop t h e r e t u r n v a l u e o f f t h e s t a c k
/ / pop t h e p a r a m e t e r s o f f t h e s t a c k ,
// r e p e a t a s n e e d e d

...
/ / The f u n c t i o n ( s u b r o u t i n e ) F
F()
PUSH R e t u r n V a l u e

/ / beginning of f u n c t i o n F
/ / c r e a t e a p l a c e on t h e s t a c k f o r t h e
// r e t u r n v a l u e
PUSH R e t u r n A d d r e s s
/ / push t h e r e t u r n a d d r e s s onto t h e s t a c k
PUSH F r a m e P o i n t e r
/ / push t h e F r a m e P o i n t e r f o r p r e v i o u s
// f u n c t i o n
F r a m e P o i n t e r ← S t a c k P o i n t e r −1
/ / s e t t h e new f r a m e p o i n t e r t o
// t h e
/ / l o c a t i o n of the f i r s t l o c a l
// v a r i a b l e
//
PUSH L o c a l V a r 1
/ / push l o c a l f u n c t i o n v a r i a b l e s , r e p e a t
// a s n e e d e d
...
/ / f u n c t i o n body
L o c a l V a r 1 ← POP / / pop l o c a l v a r i a b l e s o f f t h e s t a c k , r e p e a t a s
// n e e d e d
F r a m e P o i n t e r ← POP
/ / r e s t o r e th e old frame p o i n t e r
R e t u r n A d d r e s s ← POP
/ / r e s t o r e ReturnAddress so t h e c a l l e r can
// be
//
returned to
return
/ / r e t u r n t o t h e c a l l e r , end o f F ( )
Listing 9.2: The pseudo-code for the algorithm that implements recursive subroutines.

Register R6 is used as the stack pointer, which points to the top of the stack. When referring to a
variable on the stack, one should access it through reference to the Frame Pointer, which is Register
R5. For example, suppose the function is nearly complete and the return value is in R0 and it is
1 Introduction

to Computing System, by Yale N. Patt and Sanjay J. Patel, pages 385–393

9–4

LAB 9

9.4. THE MCCARTHY 91 FUNCTION: AN EXAMPLE IN LC-3

desired to store it at the Return Value location on the stack. Assuming only one parameter and only
one register saved on the stack, the offset will be 3, as seen by the figure below:
Offset
0
1
2
3
4

Ptr Location
Current FramePointer −→

Stack
Register1
FramePointer (for last function)
ReturnAddress
ReturnValue
Parameter1

To store R0 at the ReturnValue location, following instruction is used:
1 STR

9.4
9.4.1

R0 , R5 , #3

; s t o r e t h e r e t u r n v a l u e on t h e s t a c k

The McCarthy 91 function: an example in LC-3
Definition

The McCarthy 91 function M(n) has been invented by John McCarthy, the inventor of the Lisp
programming language (late 1950’s). It is defined for n = 1, 2, 3, . . . , as follows:
(
M(M(n + 11)), if 1 ≤ n ≤ 100
M(n) =
(9.8)
n − 10,
if n > 100.
Remarkably, M(n) takes the value 91 for 1 ≤ n ≤ 101. For values n ≥ 102 it takes the value n − 10.
In listing 9.3 the algorithm of M(n) is specified in pseudo-code.
1
2
3
4
5
6
7

/ / Compute t h e McCarthy 91 f u n c t i o n M( n ) , n i s a p o s i t i v e i n t e g e r
f u n c t i o n M( n )
// n is ≥ 1
i f n ≤ 100
r e t u r n M(M( n + 11 ) )
else
r e t u r n n − 10
Listing 9.3: The pseudo-code for the recursive McCarthy 91 function.

9.4.2

Some facts about the McCarthy 91 function

The McCarthy 91 M(n) function for some numbers, 1 ≤ n ≤ 100, while executing calls itself a
number of times, while for n > 100 M(n) is called once. Figure 9.6 on page 9–6 shows the growth
and shrinkage of the stack during execution for n = 1, 20, 50, 80, and 99. A unit of time corresponds
to either creation or destruction of a frame on the stack.
For n = 1, since the curve becomes 0 at time = 402, M(n) is executed 201 times. Figure 9.7 on
page 9–6 shows the number of times M(n) is executed for various n.
The size of the stack measured as the number of frames on it for each n in the range 1..123 is
shown in figure 9.8 on page 9–8.

9.4.3

Implementation of McCarthy 91 in LC-3

As an example, in this section we give the implementation of the McCarthy 91 function in LC-3.
The general algorithm of listing 9.2 on page 9–4 is (slightly) modified in two ways:
9–5

LAB 9

9.4. THE MCCARTHY 91 FUNCTION: AN EXAMPLE IN LC-3

n=1
n = 20
n = 50
n = 80
n = 99

Stack Size (Frames)

20

15

10

5

0
0

50

100

150

200
Time

250

300

350

400

Figure 9.6: Stack size in frames during execution.
n
1
20
50
80
99
100
101
102

Number of times M(n) called
201
163
103
43
5
3
1
1

Figure 9.7: Table that shows how many times the function M(n) is executed before it returns the
value for various n.

• The Return Address register R7 is saved to a temporary location (R0) immediately after the
function F() is called because PUSH and POP will overwrite R7.
• The second change is that registers will be used for temporary storage, as opposed to using
local variables, and thus registers used will be saved and then restored.
The modified algorithm with these changes is shown in listing 9.4 on page 9–7.
The source code for the program that calls the McCarthy 91 subroutine appears in listing 9.5 on
page 9–8, the push and pop subroutines in listing 9.6 on page 9–9, and the McCarthy 91 subroutine
itself on listing 9.7 on page 9–9. The complete program, which is a concatenation of the code in
the three aforementioned figures, can be saved on your disk, if your pdf browser supports it, by
right-clicking here →
.
9–6

LAB 9

9.5. TESTING

1 / / The c a l l i n g p r o g r a m
2 ...
3 PUSH P a r a m e t e r 1
4 CALL F ( )
5 ReturnValue
6 POP
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

← POP

/ / r e p e a t as needed f o r a d d i t i o n a l
// p a r a m e t e r s
/ / jump t o F ’ s c o d e
/ / pop t h e r e t u r n v a l u e o f f t h e s t a c k
/ / pop t h e p a r a m e t e r s o f f t h e s t a c k ,
// r e p e a t a s n e e d e d

...
/ / The f u n c t i o n ( s u b r o u t i n e ) F
F()
TempVar ←R e t u r n A d d r e s s

/ / beginning of f u n c t i o n F
/ / s a v e R e t u r n A d d r e s s ( R7 ) t o a temp
// v a r i a b l e ( R0 )
PUSH R e t u r n V a l u e
/ / c r e a t e a p l a c e on t h e s t a c k f o r t h e
// r e t u r n v a l u e
PUSH TempVar
/ / push t h e ReturnAddress onto t h e s t a c k
PUSH F r a m e P o i n t e r
/ / push t h e F r a m e P o i n t e r f o r p r e v i o u s
// f u n c t i o n
F r a m e P o i n t e r ← S t a c k P o i n t e r −1
/ / s e t t h e new f r a m e p o i n t e r t o
// t h e l o c a t i o n o f t h e
//
f i r s t r e g i s t e r value
PUSH R e g i s t e r 1
/ / push r e g i s t e r s f o r saving , r e p e a t as
// n e e d e d
...
/ / f u n c t i o n body
R e g i s t e r 1 ← POP / / pop r e g i s t e r v a l u e s o f f t h e s t a c k , r e p e a t a s
// n e e d e d
F r a m e P o i n t e r ← POP
/ / r e s t o r e t he old frame p o i n t e r
R e t u r n A d d r e s s ← POP
/ / r e s t o r e ReturnAddress so t h e c a l l e r can
// be
//
returned to
return
/ / r e t u r n t o t h e c a l l e r , end o f F ( )
Listing 9.4: The pseudo-code for the McCarthy 91 recursive subroutine.

9.5

Testing

Test the square(n) subroutine for various inputs, positive and negative. Reminder: You should
never pass a negative parameter to square(n). First convert it to positive.

9.6

What to turn in

• A hardcopy of the assembly source code.
• Electronic version of the assembly code.
• For each of the inputs 0, 1, 7, −35, screenshots that show the contents of locations x3100
through x3101.
• Answer of this question: for each input above what is the maximum size of the stack in terms
of frames?
9–7

LAB 9

9.6. WHAT TO TURN IN

maximum

Stack Size (Frames)

20

15

10

5

0
0

20

40

60
n

80

100

120

Figure 9.8: Maximum size of stack in terms of frames for n.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

;
;
;
;

Program t h a t u s e s McCarthy 91 s u b r o u t i n e MC91
I t t a k e s t h e i n p u t from x3100
I t s t o r e s t h e o u t p u t a t x3101
and o u t p u t s t h e ASCII c h a r a c t e r o f t h e v a l u e t o t h e c o n s o l e
.ORIG
x3000
LD
R6 , STKBASE
; set the i n i t i a l stack pointer
; Push ( P a r a m e t e r 1 )
LDI
R0 , INPUT
JSR
PUSH
; c a l l McCarthy91
JSR
MC91
; R e t u r n V a l u e <− Pop ( )
JSR
POP
OUT
STI

; Pop ( )
JSR
HALT
STKBASE . F I L L
INPUT
.FILL
OUTPUT . F I L L

; l o a d f u n c t i o n i n p u t i n t o R0
; p u s h INPUT on s t a c k a s p a r a m e t e r 1

R0 , OUTPUT

;
; p r i n t ASCII v a l u e o f r e t u r n v a l u e
;
n o t e : ASCII ( 9 1 ) = [
; s t o r e t h e v a l u e a t x3101

POP

; pop o f f p a r a m e t e r

x4000
x3100
x3101

; stack base address
; McCarthy91 i n p u t
; McCarthy91 o u t p u t

Listing 9.5: The program that calls the McCarthy 91 subroutine.

9–8

LAB 9

9.6. WHAT TO TURN IN

1 ; p u s h and pop s u b s
2 PUSH
ADD
R6 , R6 ,
3
STR
R0 , R6 ,
4
RET
5 POP
LDR
R0 , R6 ,
6
ADD
R6 , R6 ,
7
RET

#−1
#0

; Move t o p o f t h e s t a c k up
; S t o r e R0 t h e r e

#0
#1

; Load R0 w i t h t o p o f s t a c k
; Move t o p o f s t a c k down

Listing 9.6: The stack subroutines PUSH and POP.

1 ; McCarthy91 f u n c t i o n
2
; TempVar <− R e t u r n A d d r e s s
3 MC91
ADD R0 , R7 , #0
; s a v e R7 t o R0 s o
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28 OVER100
29
30
31
32

i t c a n be p u s h e d
; on t h e s t a c k l a t e r
; Push ( R e t u r n V a l u e )
JSR
PUSH
; any v a l u e w i l l do f o r R e t u r n V a l u e
; space
; Push ( TempVar )
JSR
PUSH
; R e t u r n A d d r e s s i s a l r e a d y i n R0
; Push ( F r a m e P o i n t e r )
ADD
R0 , R5 , #0
; t r a n s f e r f r a m e p o i n t e r t o R0
JSR
PUSH
; save frame p o i n t e r
; F r a m e P o i n t e r <− S t a c k P o i n t e r −1
ADD
R5 , R6 , #−1
; c r e a t e a new f r a m e p o i n t e r b a s e d
;on R6
; Push ( R e g i s t e r 1 )
ADD
R0 , R1 , #0
; s a v e R1 by p u s h i n g i t on t h e
;stack
JSR
PUSH
; save frame p o i n t e r

; load Parameter1 into
LD
R1 , PARAM1
ADD
R1 , R5 , R1
LDR
R0 , R1 , #0

R0
; load o f f s e t
; get address of Parameter1
; l o a d P a r a m e t e r 1 i n t o R0

; t e s t t o s e e i f P a r a m e t e r 1 ≤ 100
LD
R1 , NEG100
; l o a d −100
ADD
R1 , R0 , R1
; R1 <− P a r a m e t e r 1 − 100
BRnz
LESS100
; i f i t i s ≤ 100 jump t o t h a t c o d e
; s i n c e P a r a m e t e r 1 > 1 0 0 , add −10 t o R0 and c l e a n u p
ADD
R0 , R0 , #−10
; R0 w i l l be s t o r e d i n t h e
;ReturnValue space
; at cleanup
BRnzp
CLEANUP

; s i n c e P a r a m e t e r 1 ≤ 1 0 0 , c a l l r e c u r s i v e l y MC91 ( MC91 (
; P a r a m e t e r 1 +11) )
33 LESS100 ADD
R0 , R0 , #11
; add 11 t o p a r a m e t e r 1 and p a s s i t
; t o MC91
34
35

; c a l l MC91 ( P a r a m e t e r 1 + 1 1 )
9–9

LAB 9
; Push ( P a r a m e t e r 1 )
JSR
PUSH
; c a l l McCarthy91
JSR
MC91
; R e t u r n V a l u e <− Pop ( )
JSR
POP
ADD
R1 , R0 , #0
; Pop ( )
JSR
POP

36
37
38
39
40
41
42
43
44
45
46
47
48

; p u s h R0 on s t a c k a s P a r a m e t e r 1

; t h e r e t u r n v a l u e i s now i n R0
; s a v e t h e r e t u r n v a l u e i n t o R1
; pop o f f P a r a m e t e r 1

; now c a l l MC( MC91 ( P a r a m e t e r 1 + 1 1 ) ) = MC( R1 )
; Push ( P a r a m e t e r 1 )
ADD
R0 , R1 , #0
; move t h e r e t u r n v a l u e o f MC91 (
; P a r a m e t e r 1 + 1 1 ) b a c k t o R0
JSR
PUSH
; p u s h R0 on s t a c k a s P a r a m e t e r 1
; c a l l McCarthy91
JSR
MC91
; R e t u r n V a l u e <− Pop ( )
JSR
POP
; t h e r e t u r n v a l u e i s now i n R0
ADD
R1 , R0 , #0
; s a v e t h e r e t u r n v a l u e i n t o R1
; Pop ( )
JSR
POP
; pop o f f P a r a m e t e r 1
ADD
R0 , R1 , #0
; move t h e r e t u r n v a l u e o f MC91 (
; P a r a m e t e r 1 + 1 1 ) b a c k t o R0
;
for cleanup

49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80

9.6. WHAT TO TURN IN

; s t o r e what i s i n R0 i n t o t h e R e t u r n A d d r e s s s p a c e on t h e s t a c k
CLEANUP LD
R1 , RETVAL
; load o f f s e t
ADD
R1 , R5 , R1
; get address of ReturnAddress
STR
R0 , R1 , #0
; s t o r e R0 a t R e t u r n A d d r e s s

; refer
RETVAL
PARAM1
NEG100

; R e g i s t e r 1 <−Pop ( )
JSR
POP
ADD
R1 , R0 , #0
; r e s t o r e R1 from s t a c k
; F r a m e P o i n t e r <− Pop ( )
JSR
POP
ADD
R5 , R0 , #0
; r e s t o r e R5 from s t a c k
; R e t u r n A d d r e s s <− Pop ( )
JSR
POP
ADD
R7 , R0 , #0
; r e s t o r e R e t u r n A d d r e s s from s t a c k
RET
t o v a r i a b l e s by o f f s e t s from t h e f r a m e p o i n t e r
.FILL
#3
.FILL
#4
.FILL
#−100
.END
Listing 9.7: The McCarthy 91 subroutine

9–10



---

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