A26 5988 2_2841_2302_2311_2321_2303_Component_Descriptions_Oct65 2 2841 2302 2311 2321 2303 Component Descriptions Oct65

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User Manual: A26-5988-2_2841_2302_2311_2321_2303_Component_Descriptions_Oct65

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File No. S360-07
Form A26-5988-2

Systems Reference Library

IBM System/360 Component Descriptions2841 Storage Control Unit
2302 Disk Storage, Models 3 and 4
2311 Disk Storage Drive
2321 Data Cell Drive, Model 1
2303 Drum Storage
This publication contains reference information for the
operation and programming of storage devices which attach
to the IBM 2841 Storage Control Unit. These storage
devices include the IBM 2311 Disk Storage Drive; the IBM
2302 Disk Storage, Models 3 and 4; the IBM 2321 Data Cell
Drive, Modell; and the IBM 2303 Drum Storage.

This is a reprint of an earlier publication (Form A26-5988 -1).
The following Technical Newsletter is incorporated in this
edition:
Form No.
N26-0131

Pages
iii and blank,
1 and 2, 13 and 14,
41 and 42

Date
10/22/65

Copies of this and other IBM publications can be obtained through IBM Branch Offices.
Comments concerning the contents of this publication may be addressed to:
IBM, Product Publications Department, San Jose, Calif. 95114

CONTENTS

Page
IBM 2841 STORAGE CONTROL UNIT. • • • • • • • • • • • • • ••
1
Introduction. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
IBM 2841 F1.Ulctions •••••••••••••••••••••••••••
Data Character Format ••• ••••• ••••••••••••••••••
Data Characters •••••••••••••••••••••••••••••
Data Checking ••••••••••••••••••••••••••••••
Data Character Transfer. • • • • • • • • • • • • • • • • • • • • • ••
2
Track Format. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ••
2
Index Marker. • • • • • • • • • • • • • • • • • • • • • • • • • • • • ••
2
Gaps. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ••
2
Home Address. • • • • • • • • • • • • • • • • • • • • • • • • • • • ••
3
Track Descriptor Record (RO) • • • • • • • • . • • • • • • • • • ••
3
Data Records (Rl - Rn) • • • • • • • • • • • • • • • • • • • • • ••
5
INPUT/OUTPUT OPERATIONS. • • • • • • • • • • • • • • • • • • ••
Instruc tions • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
Start I/O ••••••••••••.••••••••••••••••••••
Halt I/O ••••••••••••••••••••••••••••••••••
Test I/O ••••••••••••••••••••••••••••••••••
T est Channel ••••••••••• • • • • • • • • • • • • • • • • • • •
ChaIUlel Operation ••••••••••••••••••••••••••••
ChaIUlel Status Word ••••••••••••••••••••••••••

7
7
8
8
8
8
8
8

Channel Address Word •••••••••••••••••••••••••
ChaIUlel Command Word •••••••••••••••••••••••
Program Status Word ••••••••••••••••••••••••••
Channel Program Branching •••••••••••••••••••••
Control Commands. • • • • • • • • • • • • • • • • • • • • • • • • ••
Sense I/O Commands • • • • • • • • • • • • • • • • • • • • • • • ••
Search Commands • • • • • • • • • • • • • • • • • • • • • • • • • ••
Read Commands • • • • • • • • • • • • • • • • • • • • • • • • • • ••
Write Commands. • • • • • • • • • • • • • • • • • • • • • • • • • ••
End-of-File •••••••••••••• • . • • • • • • • • • • • • • ••
Multiple Track Operation • • • • • • • • • . • • • • • • • • • • ••
Two -ChaIUlel Switch •••• • • • . • . • . • . • • • • • • • • • .•

10
10

11
12

Record Overflow • • • • • • • • • • • • • • • • • • • • • • • • • • • ••

Page
28

IBM 2311 DISK STORAGE. • • • • • • • • • • • • • • • • • • • • • • ••
Introduction • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ••
Device Description. • • • • • • • • • • • • • • • • • • • • • • • • • ••
Data Storage. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ••
Operator Controls and Indicators • • • • • • • • • • • • • • • • • ••
Operating Procedures ••••••••••••••••••••••••••

29
29
29
31
31
32

IBM 2302 DISK STORAGE, MODELS 3 AND 4
Introduction • • • • • • • • • • • • • • • • • • • •
Device Description. • • • • • • • • • • • • • • •
Data Storage. • • • • • • • • • • • • • • • • • • •
Indicators ••••••••••••••••••••

••
••
••
••

34
34
34
36
36

IBM 2321 DATA CELL DRIVE ••••••••
Introduction ••••••••••••••••••••••••••••••••
Device Description ••••••••••••••••••••••••••••
Data Storage •••••••••••.••••••••••••••••••••
Operator Controls and Indicators •••••••••••••••••••
Operating Procedures •••••••••••••••••••••••••••

37
37
37
38
39
40

IBM 2303 DRUM STORAGE ••••••••••••••••••••••••
Introduction ••••••••••••••••••••••••••••••••
Device Description ••••••••••••••••••••••••••••
Data Storage •••••••••
••••••••••••••••••••••

41
41
41
41

APPENDIX A.

2841/2311 PROGRAMMING EXAMPLE •••••••

12
15
18
22
22
27
27
27

iii

•
•
•
•

APPENDIX B.

HEXADECIMAL-DECIMAL CONVERSION

APPENDIX C.

COMMAND SUMMARY

APPENDIX D.

TRACK ORIENTATION •••••••••••.•.••

IBM 2841 STORAGE CONTROL UNIT

INTRODUCTION

DATA CHARACTER FORMAT

The IBM 2841 Storage Control Unit provides for the
attachment of direct access storage devices to IBM
System/360. These storage devices are:

Data Characters

IBM 2311 Disk Storage Drive (standard feature)
IBM 2302 Disk Storage, Models 3 and 4
(special feature)
IBM 2321 Data Cell Drive (special feature)
IBM 2303 Drum Storage (special feature)
A single 2841 Storage Control Unit provides for
the attachment of any combination of the above storage devices up to a maximum of eight access mechanisms. With the 2841 Additional Storage special
feature, up to eight access mechanisms may be added,
bringing the total available access mechanisms to
sixteen.
A versatile set of instructions ensures optimum
data processing efficiency. Direct access to vast
quantities of operating information enables the user
to locate specific data records without sequential address searching. Voluminous master record files
can be stored on-line, ready for immediate reference
or updating.
Maintenance of master record files can be immediate and direct; the most current information
can be entered into the proper area of the master
record file as transactions occur. Complex accounting procedures can be Simplified, because intermediate manual operations, necessary to maintain offline record files, are eliminated.
IBM 2841 Functions
The 2841 performs the following functions:
• Interprets and executes commands from the
channel attached to the central processing
unit (CPU).
• Provides a path for data between the CPU and
attached storage devices.
• Translates data appropriately as it is transferred between the storage devices and the CPU.
• Furnishes operation status information to the CPU.
• Performs checks to ensure accurate transfer of
data.

The basic unit of data within all components of the
IBM System/360 is called a byte. A byte is eight
bits in length. A single byte can represent one
alphameric character, one 8 -bit binary number, or
two decimal digits. The eight bits of each byte can
be arranged in any of 256 combinations.

Decimal Values of Byte Positions

,128 I 64

I I I I I
32

A Byte Containing the Number 19

I I I I I I I
0

A group of related bytes is called a field. A series
of related fields is called a record. A series of
similar records is known as a logical file. The
length and organization of records and logical files
is versatile and is based on the needs of the data
processing application.

First Record
Field
Name:
Bytes
Required:

Second Re

Social Security
Number

Name

Address

Social Security
Number

N(
1

*Two Digits per Byte

Records and Fields within a Fi Ie

Data Checking
CPU (Central Processing Unit) - Parity
To ensure data accuracy, a parity bit is associated
with each byte within the CPU. When the byte is
formed, the parity bit is set to "zero" or "one" to
maintain an odd number of "one" bits within the
byte. This is called odd parity.
1

Whenever data is accessed by the CPU, its parity
is checked.
Storage Units - Cyclic Check
In 2841 controlled storage devices, data is stored
and retrieved in Areas, which contain one or more
fields. Storage capacity can be more efficiently used
by associating check bits with each area, rather than
with each byte.

Information is transferred between attached
storage devices and the 2841 one bit at a time (in
serial). This transfer method is called serial-bybit.
--The 2841 converts data from serial-by-bit to
parallel-by-bit or from parallel-by-bit to serial-bybit to provide data movement between the CPU and
the attached storage devices.

Parallel
By
Bit

Serial
By
Bit

ov----"
Count Area

Key Area

Dota Area

Count Areas Within Records

2
3
4
5
6
7
C

System
360
CPU

As data is transferred from the CPU to an
attached storage device, the 2841 removes the parity
bit from each byte. The 2841 then computes two
Cyclic Check (cc) bytes which are added to the end of
each Data Area. The two Cyclic Check bytes are
arithmetically coded to represent the data in the
associated area.
The Cyclic Check code detects the following types
of errors:
1. All errors occurring within a 16-bit span.
2. All errors involving an odd number of bits
over any span.
3. Errors involving an even number of bits
over a span greater than 16-bits, except in
certain cases.
During a transfer from a storage device, all
areas read are inspected by the 2841. Cyclic Check
bytes are recalculated for each area and compared
with those retrieved from storage. An unequal comparison will set Data Check Error indicators.
As the 2841 transmits data to the CPU, Cyclic
Check bytes are removed and parity bits are restored
as needed to maintain odd parity .
Data Character Transfer
Information is transmitted between the CPU and
2841 Storage Control Unit one byte at a time. A
ninth bit, the odd parity or check bit, is added as
needed and is associated with each byte. Thus, nine
bits are transferred simultaneously (in parallel) between these two units. This transfer method is called
parallel-by -bit.

76543210

2841
Storage
Control
Unit

Storage
Device

Data Transfer Format

TRACK FORMAT
All direct access st<.>rage units associated with the
2841 use the same track format:

Index Home
Mar ker Address

Record R.0

Dato Record R 1

Dota R

Troc k Format

Index Marker
The Index Marker indicates the physical beginning of
each track. There is one index marker per recording medium (disk pack, drum, strip). All tracks on
a device are synchronized by the same index marker.
No index indication appears on individual records.

Gaps (G)
Gaps (G) separate record areas on recording tracks.
Gap lengths will vary depending on storage device,
location within the record and the record length.

Home Address

Read/Write Head Number

The Home Address consists of seven bytes which
define track condition and physical location within
the storage device. There is one Home Address per
track. Home Addresses are transferred from the
CPU to the storage device only by a Write Home
Address operation, and from the storage device to
the CPU only by a Read Home Address operation.
Writing Home Addresses is usually accomplished by
utility programs.
Index Home
Merker Address

The read/write head number (2 bytes) identifies a
read/write head w\thin the selected cylinder.
The combination of cylinder and read/write
head numbers is used to locate a specific track.
A more detailed discussion of addressing
schemes will be found in the descriptions of the
various storage units.
Cyclic Check
A Cyclic Check is used for error detection as described in the section on Data Checking. Two bytes
are required for this check.
Gap
This is a fixed gap generated by the 2841 to separate
the Home Address from the next recorded area.
Track Descriptor Record (RO)

Home Address

Flag
A flag (1 byte) indicates track condition. It is normally all zero bits when Home Addresses are first
written. Bit significance is:
Bit

The first record following the home address on each
data track is the Track Descriptor Record (Figure 1),
or RO. Although it may be used to store data, RO
has been designed to enable entire tracks to be
moved to alternate tracks if a portion of the primary
track becomes defective. For description, a primary track is considered the original track on which
data was stored, and an alternate track contains
data which has been repositioned from a defective
primary track. This repositioning is independent
of the file organization scheme in use.
Count Area
This I1-byte area describes the Data Area and Key
Area which follow.

FWlction
Zero

Flag. Byte 0 of the Count Area is generated by the
2841 as RO is written. It is not sent from the CPU.

Zero
2

Zero

Track Condition

Bit
Flag
Byte

Function or Setting
Zero
Zero

o indicatE:s operative track
1 indicates defective track

Track Use

o indicates primaty track
1 indicates alternate track

Cylinder Number
The cylinder number (2 bytes) identifies the storage
unit cylinder within which the data is stored.

Flag
Byte

Zero

Track Condition

o indicates operative track
1 indicates defective track

Track Use

o indicates primary track
1 indicates alternate track

Home
Address

Index
Marker

Record R~

Data Record R1

Data R

__--------~A---------~r----------------JA~--------------~----------~

Home
Address

I CY~liC
Check

Count Area

Key Area

Data Area

Figure 1. Track Descriptor Record

Bits 6 and 7 are transmitted to the flag bytes of all
records on the track from the flag byte of the home
address of that track.
Cylinder Number. In a primary track, bytes 1
and 2 of RO contain the cylinder number of the
primary track on which this record was stored.
If this record has been moved to an alternate
track, the cylinder number of the alternate track
appears in the data area of RO of the defective
primary track.
Read/Write Head Number. In a primary track
bytes 3 and 4 of RO contain the read/write head
number of the primary track on which this record
was stored. If this area has been moved to an
alternate track, the head number of the alternate
track appears in the data area of RO of the
defective primary track.
Record Number. Byte 5 designates the sequential
number of the record on the track. For RO, the
record number is zero.
Key Length. Byte 6 specifies the number of bytes in
the Key Area of the record (excluding check bytes).
If the record has no key, this byte is zero. This
byte can indicate a Key Length from 0 to 255 bytes.
Because of its intended special use with alternate
track procedures, RO will normally have no Key
Area.
Data Length. Bytes 7 and 8 specify the number of
bytes in the Data Area of the record (excluding check

bytes). Two bytes (16 bits) can indicate Data Length
from 1 to 65,535 bytes.
Zero Data Length indicates the end of a logical
file. The 2841 sends special indicators to the CPU
when an End-of-File record is read or written.
Cyclic Check. Bytes 9 and 10 are used for error
detection as discussed in the section on Data Checking.
Key Area
Although a Key Area can be written and used in RO
by the commands used by the 2841, this use is purely
at the discretion of the programmer. Standard use
of RO by IBM Programming Systems does not include
a Key Area.
A more detailed discussion of Key Area may be
found in the section of this manual which describes
Key Area within Data Records (R1 - Rn).
Data Area
The design and use of this area is normally prescribed by IBM Programming Systems. Because of
this special use by the programming system, it is
recommended that this area not be used for application data.
If the Data Length is zero, indicating End -ofFile, the Data Area contains one byte of zeros in
addition to the check bytes. No data is transferred
to the channel when this record is read, but the Endof - File indicator is set.

Data Records (R - Rn)
1

Count Area

One or more data records may follow RO on a track.
Count areas make each record self-formatting for
maximum data organization flexibility and efficiency.

This II-byte area describes the Key and Data Areas
which follow it. Bytes 1 through 8 are created in the
CPU by the program used to write the record.

Index

Flag. Byte 0 of the Count Area is generated by the
2841 as each record is written. It is not sent from
the CPU.

Bit

Dcrlal
~

Address
Marker

* May

Not be Present

Address
Marker

Function

o for even-count records (RO,

~, R , R )
6
4
1 for odd-count records (R , R , Rs • • •• )
3
1
Used by the 2841 to ensure that all address
markers (and records) are present. The 2841
signals a missing Address Marker when two
consecutive, identical bits are encountered
(unless an Index Point intervenes).

Record R1 - Rn Format
Flag
Byte

Address Marker
This 2-byte area indicates the beginning of each
record after RO (Figure 2). Address markers are
supplied by the 2841 as records are written. They
are used by the 2841 to locate the beginning of a
record for searching, writing, and reading operations.

Used with Record Overflow feature.
o for all non-overflow records and for the last
record of an overflow chain.
for each record except the last record of an
overflow.

Record R1

Data Area

r'--------------------------~A~----------------------------~
~_D_a_ta_ _JI ~! A_r_ea_~
......_K_e_Y_~J J Area

T
Data Area

Address Mar ker

Count Area

Key Area

Address Marker and Count Area

Figure 2. Address Marker and Count Area

Flag
Byte

Function

Zero Data Length indicates the end of a logical
file. Special indicators are sent to the CPU when an
End-of-File record is read or written.

Zero

Track Condition

0 indicates operative track
1 indicates defective track

Key Area

Track Use

o indicates primary track

The Key Area concept has been provided in storage
units of the 2841 family to allow searching and data
accessing during a single disk, drum, or strip
revolution. The Key Area can contain identifying
information about a record, such as serial number,
social security number, or policy number. Special
commands are provided to search Key Areas for
this identifying information. When the desired
record is found, a read or write instruction can be
issued and the Data Area read or written during the
same revolution.
Comparison (during searching) is accomplished
within the 2841. Thus, use of Key Areas for searching allows searching and comparing of keys and
movement of the desired Data Area to or from the
CPU during a single disk, drum, or strip revolution.
Key Area length ranges from 1 to 255 bytes.
Two Cyclic Check bytes are added to the Key Area
by the 2841. If Key Length, in the Count Area, is
zero, no Key Area will be written.

Cyclic Check. Bytes 9 and 10 are used for error
detection as discussed in the section on Check
Characters.

1 indicates alternate track
Bits 6 and 7 are transmitted to the flag bytes of

all records on the track from the flag byte of
the Home Address of that track by the 2841.

Cy linder Number. Bytes 1 and 2 contain the cylinder
number of the track on which the data is stored.
Read/Write Head Number. Bytes 3 and 4 contain the
read/write head number of the track on which the
data is stored.
Record Number. Byte 5 designates the sequential
number of the record on the track.
Key Length. Byte 6 specifies the number of bytes in
the Key Area of the record (excluding check bytes).
If the record has no key, this byte is zero. This
byte can indicate a Key Length from zero to 255
bytes.
Data Length. Bytes 7 and 8 specify the number of
bytes in the Data Area of the record (excluding
check bytes). Two bytes (16 bits) can indicate Data
Length from 1 to 65,535 bytes. It should be noted
that maximum data length is a function
the track
capacity of the specific storage device. See the
description of the Overflow Feature for records that
exceed the track size.

Data Area
This area contains the information identified by the
Count and Key Areas. Data information is organized and arranged by the programmer.
Two Cyclic Check bytes are added to the Data
Area by the 2841. If Data Length was zero, indicating End-of-File, the Data Area will contain one
byte of zeros in addition to the check bytes, however
no data is transferred to the channel when this
record is read.

INPUT/OUTPUT OPERATIONS

Input/Output (I/O) operations involve the transfer of
information to or from CPU storage. Within this
concept, disk and drum storage drives and data cell
drives are considered I/O devices.
The CPU program initiates I/O operations with
the Start I/O instruction. Bit positions 24-31 of
this instruction identify the device. Start I/O causes
the channel to fetch the Channel Address Word (CAW)
from main storage location 72. The command address portion of the CAW designates the location in
main storage from which the channel subsequently
fetches the first Channel Command Word (CCW). The
CCW specifies the command to be executed and the
storage area to be used.
If the channel is not busy, the channel attempts
to select the device by sending the address of the
device to all attached control units. The control unit
specified in the address responds to its selection and
awaits further instructions. The command code is
sent to the selected control unit; the control unit
then responds with a device status byte to the CSW.
At this time, the start r/o is terminated. The
results of the attempt to initiate the execution of the
command are indicated by the condition code in the
Program Status Word, and, under certain conditions,
by status bytes in the Channel Status Word.
All data transfers from the channel to the 2841
are checked for parity. If a parity error is detected,
a unit check signal is sent to the CSW by the 2841
and the command will not be executed.
An I/O operation may involve transfer of data to
one storage area, designated by a single CCW. When
data chaining is specified, data is transferred to a
number of storage areas. In each case, a chain of
CCWs is used, in which each CCW designates an
area in main storage for a part of the operation.
The program can be notified of the progress of
chaining by specifying that the channel modify the
Channel Status byte upon fetching a new CCW. When
command chaining is specified, a series of commands
is executed.
Termination of an I/O device operation normally
is indicated by two CSW conditions: Channel End and
Device End. The channel end condition indicates that
the I/O device has received or provided all information associated with the operation and no longer needs
channel facilities. The device end signal indicates
that the I/O device has terminated execution of the
operation. The device end condition can occur concurrently with the channel end condition or later. If

command chaining has been specified, the next CCW
is fetched by the channel and the operation designated
is commenced. Unusual conditions and errors terminate the execution of a command chain.

INSTRUCTIONS
All I/O instructions use the following format:

1516

1920

I/O Instruction Format

Fields in the instruction are allocated as follows:
Bit Position

Field Designation

0-7

Operation (Op)
Code

8-15

Not Used

Function
Designates the operation to
be performed.

16-19

Base Address
Register
Location (B )
1

Designates the address of a
general register in main
storage. The register is 32
bits in length, but only the
low order 24 bits are used.

20-31

Displacement (D 1 )

The sum obtained by the
addition of the content of
the register at B1 and content
of the D1 field identifies the
channel and device addressed
by the instruction. The result has the format:

2021

2324

Bit Position
0-7

Operation (Op)
Code

8-20

Not Used

21-23

Function

Field DesilEation

Channel Address

Designates the operation to
be perfonned.

000 - deSignates multiplexer
channel.
001 - 110 - designates selector channel 1-6.
111 - invalid combination.

Shared Channel
Indicator

1 indicates multiplex channel
or sub-channel. On a selector channel, this bit is included in the control unit
address.

25-27

Control Unit

0-7 control units per channel.

28-31

Access Mechanism

0-7. Bit 28 will be 1 only if
additional access feature is
installed (indicates mechanism 8-15).

Bit positions 24 to 31 of the I/O instruction specify a
control unit and access mechanism.

Always 1 for 2841
on Multiplex Channel

Indicates Numbers 8-15
When Add'i Access Feature
Installed
Unit Address Format

A control unit number is permanently assigned
to each 2841 through internal wiring at the time the
unit is installed.
A maximum of 16 access mechanisms can be
addressed by each control unit. A standard 2841 can
control eight mechanisms; eight more can be
attached with the Additional Storage feature~
Start I/O
All I/O operations are initiated by a Start I/O instruction. If the channel facilities are free, Start I/O is
accepted and the CPU continues its program. The
channel independently selects the I/o device specified
by the instruction.
The CAW at main storage location 72 contains
the protection key for the sub-channel and the address
of the first CCW. The CCW so designated specifies
the operation to be performed, the main-storage

Halt I/O
Halt I/O terminates a channel operation, and the
2841 is disconnected from the channel.
Halt I/O does not cause a command byte to be
transferred to the 2841. If the operation in progress
was a write command, the 2841 completes the write
operation by inserting valid zeros to the end of the
field or track. An erase command also inserts
valid zeros to complete the operation.
Test I/O
Test I/O sets the condition code in the Program
Status Word to indicate the state of the addressed
channel, sub-channel, and I/O device. The Channel
Status Word is stored in location 64.
Test Channel

? I C~ntrol ~nit N~.
24

area to be used, and the action to be taken when the
operation is completed.
If any of the several conditions exist, Start I/O
will cause the status portion, bit positions 32-47, of
the CSW at main storage location 64 to be replaced
by a new set of status bits. The status bits pertain
to the device addressed by the instruction. The contents of the other fields of the CSW are not changed.

Test Channel sets the condition code in the Program
Status Word to indicate the state of the channel addressed by the instruction. The condition code then
indicates channel available, interruption condition
in channel, channel working, or channel not operational. The execution of this command does not
affect the 2841.
CHANNEL OPERATION
The IBM 2841 Storage Control is attached to the CPU
through a set of data paths called a channel (Figure
3). So that the CPU may control a wide variety of
input/output devices with a minimum of programming
differences, all control units are designed to respond to a standard set of commands from the channel. The control unit then translates these commands into specific operating orders for the particu1ar input/output unit. This enables the CPU to operate all input/output devices uniformly with the set of
basic instructions.
Channel Status Word (CSW)
The Channel Status Word informs the program of
the status of an I/O device or the conditions under
which an I/O operation has been terminated. The

System/360
CPU
Issues

I/o Instructions
Executes Commands (CCW's)

2841
Storage
Control

I NSTRUCTIO NS
and
COMMANDS

Data
Cell
Drive

Figure 3. Instructions, Commands and Orders

CSW is formed, or parts of it are replaced, during
I/O interruptions and during execution of I/O instructions. The CSW is placed in main storage at
location 64. It is available to the program at this
location until the next I/O interruption occurs or until another I/O instruction generates a new CSW,
whichever occurs first.
When the CSW is stored as a result of an I/O
interruption, the I/O device is identified by the I/O
address in the old PSW. The information placed in
the CSW by an I/O instruction pertains to the device
addressed by the instruction.

The CSW has the following format:

I I I
I
Key

Device

I
I

Command Address

0000

St1 tus

Channel

Count

4748

Channel Status Word

F.ields in the CSW are allocated for the following
purposes:

CSW Bit
Position

Field Designation

Function

0-3

Protection Key

Form the storage protection key
used in the chain of operation.

4-7

Not Used

Always zero.

8-31

Command Address

Form an address eight positions
higher than the address of the
last CCW used.

Attention

Not Used.

Status Modifier

Set whenever a Search High,
Search Equal, or a Search
High or Equal command has
been executed and the condition satisfied.
The Status Modifier is also set
whenever the 2841 is Busy.
This bit, in conjunction with
the Busy Bit, signifies Control
Unit Busy.

CSW Bit
Position

Field Designation

Function
When this condition is detected,
no data is transferred. If Key
Length is not zero, the Key Area
is transferred.

40-47

Channel Status

Indicate channel conditions as
follows:
Designation

Bit
40

Control Unit End

Busy

Channel End

Set if a Control Unit Busy status
has been generated previously
and the busy condition has been
terminated.
Indicates that the selected device
is busy. It is set when a new
command chain is initiated while
the selected access mechanism is
still in motion due to a previous
Seek command.
In conjunction with the Status
Modifier bit, indicates the control unit is busy. It is set when
a new command chain is initiated while the 2841 is causing
a track to be erased following a
Format Write command.
See section on Two Channel Switch.
Set at the end of each channel
command.

Device End

Indicates that an access mechanism
is free to be used.

Unit Check

Set whenever an unusual or error
condition is detected in the 2841
or the selected file device. A
Sense 1/0 Command may then
be used to identify the condition.

Unit Exception

41
42
43
44
45
46
47
48-63

Program-controlled
interruption
IncOlTect length
Program check
Protection check
Channel data check
Channel control check
Interface control check
Chaining check

Form the count of the last CCW used.

Gount

Channel Address Word (CAW)
Successful execution of start I/O causes the channel
to fetch a channel address word from main storage
location 72. The CAW specifies the location in main
s.torage where the channel program begins.
The CAW has the following format:

Command Address

Channel Address Word

CAW fields are allocated for the following purposes:
CAW Bit
Position·
0-3

Field Designation
Protection Key

4-7
8-31

Function
Forms the Storage Protection key
for all commands associated with
Start 1/0. This key is matched
with a storage key whenever
data is placed in storage.
Always Zero.

Command Address

Designates the lacation of tIie first
CCW in main storage.

Channel Command Word (CCW)
Indicates an End-of-File has been
detected during a Read RO CKD,
Read KD, Read D, Write KD, or
Write D operation. It results from
a Data Length of zero being detected in the Count Area of a record.

The byte location specified by the CAW is the first of
eight bytes of information (a double word) that the
channel fetches from main storage. These 64 bits
of information are called a Channel Command Word

(CCW). The address of the leftmost position of the
CCW must be divisible by four (in binary notation,
the two low-order positions of the address are zero).
One or more CCWs make up the channel program
that directs channel operations.
The CCW specifies the command to be executed.
For commands initiating I/O operations, it designates the storage area associated with the operation
and the action to be taken when transfer to or from
the area is completed. CCWs can be located anywhere in main storage and more than one can be
associated with a Start I/O. The channel refers to a
CCW in main storage only once. Once obtained, the
pertinent information is retained in the channel.
The first CCW is fetched during the execution of
Start I/O. Each additional CCW in the sequence is
obtained when the operation has progressed to a point
where the additional CCW is needed. Fetching of
CCWs by the channel does not affect the contents of
main storage.
Fields in the CCW are allocated for the following
purposes:
CCW Bit
Position
0-7

8-31

Field Designation
Command Code

CCW Bit
Position Field Designation

CD flag is zero in the last CCW
used, the incorrect length
indication is suppressed. When
both the CC and the SILl flags
are set to one, command
chaining takes place regardless
of the presence of an incorrect
length indication. Absence
of the SIll flag or the presence
of the CD flag causes the program to be notified of the incorrect length condition when
it occurs.
35

Skip (SKIP) Flag

When set to one, specifies suppression of a transfer of information to storage during a read,
read-backward, or sense operation. When bit 35 is zero,
normal transfer of data takes
place.

Program-ControlInterruption
(PC!) Flag

When set to one, causes the channel to generate an interruption
condition upon fetching the CCW.
When bit 36 is zero, normal
operation takes place.

37-39

Transfer-inChannel

Bit positions 37-39 of every CCW
other than one specifying transfer
in channel must contain zeros.
Violation of this restriction generates the program-check condition. For additional information,
see Control Commands Transfer-in-Channel.

Function
Specify the operation to be performed. The 4 high-order bits
specify the function to be performed by the addressed I/O
device; the 2 low-order bits
specify the channel function.

Data Address

Specify the location of an 8-bit
byte in main storage. This is
the first location of the area
designated by the CCW.

Chain Data (CD)
Flag

When set to one, specifies chaining of data. It causes the storage
area designated by the next CCW
to be used with the current operation. The command code of
the next CCW will be ignored.
When bit 32 is zero, the current
CCW is the last one for the
operation.

Not used.

40-47
32

Chain Command
(cq Flag

When set to one, and when the
CD flag is zero, specifies chaining of commands. It causes the
operation specified by the command code in the next CCW to
be initiated on normal completion of the current operation.

Suppress Incorrect
Length Indicator
(SIU)

Controls whether an incorrect
length condition is to be indicated to the program. When
this bit is set to one and the

Function

48-63

Count

Specify; the llumber of 8-bit byte
locations in the storage area
designated by the CCW.

Program Status Word (PSW)
A double word, the program status word (PSW) contains information required for proper program execution. In general, the PSW is used to control instruction sequencing and to hold and indicate the
status of the system in relation to the program being
executed. The active or controlling PSW is called
the "current PSW." By storing the current PSW
during an interruption, the status of the CPU can be
preserved for subsequent inspection. By loading a
new PSW or part of a PSW, the state of the CPU can
be initialized or changed.

The PSW has the following format:
Program Status Word

Interruption Code

ISystem Mask Key AMWP\

7 8

1112

15 16

Instruction Address
32.

3334

PSW Bit
Position
0-7

35 36

3940

Field Designation
System Mask

Function
Associated with I/O channels and
external signals. When a mask
bit is one, the source can interrupt the CPU. When a mask bit
is zero, the corresponding source
cannot interrupt the CPU and
interruptions remain pending.

all Search commands transmit.a status
modifier indication, this allows branching
from a command chain when the search
command condition has been satisfied.
The programmer can also modify the CCW
chain sequence by using the Transfer-inChannel (TIC) command. This command
directs the channel to fetch the next CCW
from an address specified within the TIC
CCW. See Control Commands - TransferIn -Channel for additional information.

These methods of modifying the sequence of a
chain of CCWs provide branching capability in the
channel program.
Control Commands

ASCII (A)*

Control operations on 110 devices do not involve a
transfer of data between a storage unit and the CPU.
However, in certain Control operations, a few bytes
or bits may be transferred between the CPU and
2841 to enable the operation to take place. These
bytes are parity checked during transfer.

Machine-Check
Mask (M)*

Erase

8-11

Interrupt Key*

Wait State (W)*

Problem State (P)*

16-31

Interruption Code*

32-33

Instruction Length
Code (ILC)*

34-35

Condition Code (CC)*

36-39

Program Mask*

40-63

Instruction Address

Identifies the cause of an I/O,
program, supervisor call, or
external interruption.

This command is used to erase the end of a track
after a track overflow has occurred. It has the
same chaining requirements as a Write Count-KeyData command. The execution of this command
causes one's to be written from the end of the Data
area of the record on which the preceding search
was satisfied, or the record just written by Write
CKD, to the end of the track. Channel End and
Device End signals are generated when Index Point
is reached. Both the channel and the control unit
are busy during execution of this command.

*Refer to IBM System/360 Principles of Operation, Form A22-6821.
Erase Command Code

Channel Program Branching
Normally the next CCW in a chain is fetched from a
core position eight bytes higher than the current
CCW. This sequence can be modified in two ways:
1. If command chaining is specified in the current CCW and execution of the CCW results
in a status modifier indication (without other
unusual conditions detected), the channel
will fetch the next CCW from a main storage
location sixteen positions higher than the
current CCW (one CCW is skipped). Since

Decimal

Hexadecimal

Binary

0001 0001

No Operation (No-Op)
This command causes the addressed device to respond with Channel End and Device End. No information other than the command itself is transferred to the 2841. The addressed device takes no
action.
Channel End and Device End are signalled
simultaneously to the CSW.

No-Operation Command Code
Decimal

Hexadecimal

Binary

00000011

Restore
This command is used with the 2321 only, It causes
the 2321 to restore the strip from the drum to the
cell. It causes Channel End to be generated upon
initiation of the operation by the Control Unit and
Device End when the strip is fully restored. The
Restore command operates exactly like a seek command except that no address is transferred to the
2841.
A Restore command is not restricted by the
file protect mask. Any device other than a 2321
performs a No-op when a Restore command is given.

Seek Address from main storage to the 2841. The
CCW count (pOSitions 48-63) should specify a 6-byte
count field. If the count is more than six, the 2841
operates on the first six bytes transferred and, if the
CCW SILl (Suppress Incorrect Length Indicator) bit
is zero, a Wrong Length Record is signalled to the
CSW. If the CCW count is less than six, the CSW
Unit Check bit is set, and a Sense I/O CCW may be
used to identify the Seek Check and Command Reject.
The six bytes specified must form a valid address.
At the completion of a successful address transfer
from main storage to the 2841, a Channel End indication is sent to the CSW. A Device End indicator is
set in the CSW when the selected access mechanism
has reached the addressed track.
A Seek command need not be preceded by any
other CCW.
Seek commands operate in conjunction with the
Set File Mask command.
The 6-byte seek address is arranged as follows:
Byte 1

Byte 2

Device

Byte 0

Decimal

Hexadecimal

Binary

2311

00010111

2302

2303

2321

0-9
cell

Byte 5

Byte 3

Byte 4

0-202
cylinder
0-249
cylinder
0-79
cylinder

0-9
head
0-45
head
0-9
head

0-19
sub-cell

0-9
strip

0-4
cylinder

0-19
head

Restore Command Code

X indicates not used, but all bits must be zero.

Recalibrate
This command is used with the 2311 only. It causes
the 2311 to seek to head zero and track zero. It
causes Channel End to be generated immediately
and Device End to be generated when the operation
is complete. Any device other than a 2311 performs
a No-op when a Recalibrate command is given. A
Recalibrate command works under the same File
Protect Mask as a Cylinder Seek command.

Seek. All six seek address bytes referenced by the
cCW are used to determine seek address.
Seek Cylinder. Only the four low-order bytes
(bytes 2-5) referenced by the CCW are used to determine seek address. With the 2321, only bytes 4
and 5 are used.
Seek Head. Only the two low-order bytes (bytes 4
and 5) referenced by the CCW are used to determine
seek address. With the 2321, only byte 5 is used.

Reca librate· Command Code
Decimal

Hexadec i ma I

Binary
0001 0011

Command

Seek Command Code
Decimal

Hexadecimal

Binary

Seek

00000111

Seek Cylinder

00001011

Seek Head

00011011

Seek
Three seek commands are associated with the 2841
Storage Control unit: Seek, Seek Cylinder, and Seek
Head. After a Start I/O instruction has selected the
proper channel, control unit, storage unit, and
access mechanism, the Seek CCW transfers a 6-byte

Set File Mask (Figure 4)
A Set File Mask command causes one byte of data to
be transferred from main storage to the 2841. At
the completion of the transfer, a Channel End and

Set File Mask

Data Address
7 8

Count
31 32

47 48

( 0000000000000001)

Specifies CPU Storage
Location of the Mask Byte,
At That Location is Found:

BO
0
0
0

Permit Write Commands

All Except Write Home Address
and Write Record RO

·0

All

None

0
0

Seek Head

Write Data or Write Key & Data
All

Permit Seek Commands

Seek Cylinder or Seek Head
None

For the 2841 Storage Control, B2, B5, B6, and B7 of
the mask must be zero. If these bits are not zero, the
mask is considered to be invalid and a CSW Unit Check
signal is generated. A subsequent Sense I/O command
will indicate Command Reject.

Figure 4. Set File Ma;k

Device End signal are sent to the CSW. The byte of
data that is sent to the 2841 describes the Write and
Seek functions that can be performed in the CCW
chain. Set File Mask can be issued anyplace within
a CCW chain. At the completion of the CCW chain,
the File Mask is reset to all zeros.
If a Seek or Write command is issued which
violates the File Mask, the command is not executed, and a Unit Check signal is generated in the CSVil
Status Byte. A subsequent Sense I/O command signals File Protect and Command Reject if the generation was a write. For a seek operation, File Protect is set; for a write operation, both File Protect
and Command Reject are set.
A Set File Mask command can be issued only
once within any given CCW chain. If an attempt
is made to issue more than one Set File Mask
command with a given CCW chain, a Unit Check
signal is generated in the CSW Status Byte. A

subsequent Sense I/O command indicates Command
Reject and Invalid Sequence.
Space Record
This command enables the 2841 to pass over the next
record on a track. It allows the 2841 to retrieve
subsequent records from a track when the Count
Area of a preceding record is not readable.
The execution of the Space Record command
causes the 2841 to search for the next Address
Marker on the track. Upon detection of the Address
Marker, Channel End and Device End signals are
genera~ed. Thus, the following CCW searches for
the Address Marker following the one detected by
the Space Record command.
When a Space Record command follows a Search
HA or Read HA CCW ~ the search for an Address
Marker is suspended and Channel End and Device

End signals are generated immediately. The effect
of this sequence is to cause Record RO to be passed
over.
The Space Record command does not detect a
Missing Address Marker. However, if an Address
Marker is missing, it is detected on all valid commands chained from the Space Record command
except Search ID commands.
The Space Record command must be chained
from either a Search or a Read command.

Space Record Command Code
Decimal

Hexadecimal

Binary

00001111

are transferred to the Channel by a Sense I/O command.
The Data Address portion of the CCW directs
the bytes to a specific CPU storage location.

Sense

I/o

Decimal

H exadec i ma I

Binary

0000 0100

The significance of a "1" condition for each bit is:
Bit

Designation

Command
Reject

Indicates that the 2841 has received
an invalid operation code, an invalid sequence of commands, an
invalid Seek Address*, or a file
mask is violated on a write command. (See Set File Mask. )

Intervention
Required

Indicates that the specified file is
not physically attached to the
system or, if physically attached
to the system, it is not available
for use because the file motor is
not on, a cover interlock is open,
etc.

Bus Out Parity
Check

Indicates that the 2841 has detected
a parity error during the transfer of
a command or data from the channel to the 2841. A parity error
detected during command transfer
signals a Parity Check, not a Com...
mand Rej ect.

Equipment
Check

Indicates that an unusual condition
is detected in the control or storage
unit. Conditions covered by this
bit are defined by Sense Byte 2.

Data Check

Indicates that a data error has been
detected in the information received by the 2841 from the
storage unit.

Overrun

Indicates that a chained CCW was
issued but that it was received too
late to be properly executed; or
that a byte was received during
Reading or Writing; or that a byte
was received too late (during a
read or write operation) to be
executed properly.

TIC Command Code
Decimal

Hexadecimal

Binary

XXXXIOOO

Positions Marked "X" Are Ignored

Sense I/O Commands
Four bytes of sense condition information are provided by the 2841 to identify the setting of the Unit
Check bit in the CSW Status Byte. These Sense Bytes

Significance of "1"

Byte

Transfer In Channel (TIC)
The Transfer in Channel command provides chaining
between CCWs not located in adjacent CPU storage
locations. The next CCW is fetched from the location specified by the Data Address field of the TIC
CCW.
TIC does not initiate any channel I/O operation
and the I/O device is not signalled that this command
is being executed.
TIC may not be the first CCW designated by a
CAW. One TIC command may not transfer directly
to a second TIC command.
When either of these errors is detected or when
an invalid address is specified in Transfer in Channel, the program-check condition is generated. Detection of these errors during data chaining causes
the operation of the I/O device to be terminated,
whereas during command chaining they cause an
interruption condition to be generated.
Bit positions 0-3 and 32-63 are ignored. Bits
29-31 must be zero to meet the boundary requirements for double words.

Command Code

*A track condition interrupt on an overflow record occurs during
a write command.

Byte

Bit

Designation

Significance of

11111

Byte

Bit

Designation

When Writing, the remaining portion of the record area will be
filled with valid zeros and the
Overrun check will be generated.
When Reading, the remaining
portion of the record will continue
to be read into the 2841 and the
Overrun Check will be generated.

Track
Condition
Check

Indicates defective track.

Seek Check

Indicates that the file has been unable to complete a Seek because:

write the Data Area, Read Home
Address, or Read RO. It is also set
in conjunction with Missing Address
Marker if there is no data on the
track. No Record Found is never
set if the Multi-Track bit in the
command (Bit 0) is on.
5

File Protected

Indicates that a Seek or Write CCW
was issued contrary to the file
mask. The Command Reject bit
is also set by this condition, if the
operation is a write operation.

Missing Address Marker

A missing Address Marker, which
may indicate a missing record is
detected during the execution of
command or chain of commands
which operates on successive
Count Areas on a track. The condition detected is two successive
records on a track with equal bit
conditions in bit 0 of the Flag
bytes, with no intervening Index
Point.
A missing Address Marker is also
detected if two Index Points are
passed with no intervening Address
Marker record on the track.

1. Transferred Seek address is outside the v.!lid address boundaries
of the storage device. Unused
seek address bytes must be a valid
address for the device selected.
Command Rej ect is also set.
2. Less than six seek address bytes
were sent. Equipment failed
which resulted in the access mechanism going to either the inner or
outer stop. In this case Command
Rej ect is not set.

Indicates that a data error has been
detected in a Count Area read from
the storage device. Data Check
(bit 4) in Byte 0 is also tumed on.
Error detection is the same as
described for Data Check.

Track Overrun

Indicates that writing has not been
completed by the time the Index
Point is detected. This type of error
is created during a Write RO or Write
Count, Key, and Data operation.

Cylinder End

Indicates that the CCW Command
Chain has not been completed, and
Cylinder End has been detected.

Invalid
Sequence

Indicates that an attempt has been
made to execute an invalid
sequence of CCWs or that two Set
File Mask commands appear in
the same command chain.
Valid command sequences are defined in the individual command
descriptions. Command Rej ect
(Byte 0 bit 0) is also set when an
invalid sequence is detected.

Count Area
Check

No Record
Found

Indicates that while executing a chain
of CCWs, the 2841 has detected two
Index Points without completing an
intervening command to read or

Significance of 11111

When a Missing Address Marker is
detected, this bitand bit 4 of Sense
Byte zero (Data Check) will be
tumed on for all commands or
chained commands except Search
ID CCWs. The Search ID CCW
may be used to pass over the Missing Address Marker so that the remaining data on the track can be
retrieved. Missing Address Marker
is set in conjunction with No Record
Found if there is no data on the track.

Overflow
Incomplete

This bit is used with the Record
Overflow special feature. It is
set with other indicators to signal
conditions as follows:

Condition

Sets Overflow
Incomple.te and
Other Indicator:

Overflow to a
Track Condition
defective track (Byte 0, bit 6)
Overflow from
an alternate
track

Track Condition
(Byte 0, bit 6)

Byte

Bit

Designation

Overflow
Incomplete

Significance of "1 II

Overflow to
File Protected
boundary

File Protected
(Byte 1, bit 5)
Command Rej ect
(Byte 0, bit 0)

Overflow to
wrong track
(Head number
unequal)

Set for write only.
Seek Check
(Byte 0, bit 7).

A Device Reserve command is rejected when a
Set File Mask command precedes it in the same
command chain. The Unit Check bit in the CSW is
set when the command is rejected, and the Command
Reject and Invalid Sequence bits are set to indicate
the conditions which caused the Unit Check.
The Device Reserve command performs all of
the functions of a Sense I/O command in addition to
the functions described in this section.
Device Reserve Command Code

A Track Condition check is generated under the
following conditions:
1. If an overflow record is being read, written,
or searched which overflows to a defective
track. The interrupt occurs after the last
byte on the previous track has been operated on and before the first byte for the defective track is requested from or sent to
the channel. In this case Overflow Complete
is also set. Command Reject is also set if
the operation was a write.
2. A Search HA, Read HA, or Read RO causes
a head switch to a defective track during a
multiple track operation, when a Search
operation other than Search HA is attempted.
The interruption occurs prior to transfer of
any data to or from the channel.
Write commands never set track condition checks.
Sense Bytes 2 and 3. These bytes are provided to
assist the Customer Engineer when using diagnostic
programs to locate equipment malfunctions.
Device Reserve (Two-Channel Switch Special Feature)
Without the Two-Channel Switch feature installed,
Device Reserve is rejected by the 2841 and the Unit
Check bit in the CSW Status Byte is set. The Command Reject bit in Sense Byte 0 is set to indicate
what caused the Unit Check condition.
With the Two-Channel Switch feature, a Device
Reserve command causes the addressed device to
be reserved to the channel issuing the command.
The device then remains reserved to the same
channel until that channel executes a Device Release
command addressed to the specific device, or until
the CPU is .reset.
A Device Reserve command is rejected with a
Busy indication in the CSW if any normal Busy condition exists. However, a Device Reserve command
is executed regardless of any abnormal file status
condition, such as off-line, unsafe, etc.

l>ecimal

Hexadecimal

Binary

180

1011 0100

Device Release (Two-Channel Switch Special Feature)
Without the Two-Channel Switch feature installed,
Device Release is rejected by the 2841 and the Unit
Check bit in the CSW Status Byte is set. The Command Reject bit in Sense Byte 0 is set to indicate what
caused the Unit Check condition.
With the Two-Channel Switch feature, a Device
Release command terminates the reservation of the
addressed device to the channel. This command is
rej ected with a Busy indication in the CSW if any
normal busy conditions exists. However, a Device
Release command is executed regardless of any abnormal file status condition such as off-line, unsafe,
etc.
A device is normally reserved to a particular
channel whenever that channel exectues a Device Reserve command. The device remains reserved to
the same channel until that channel causes the 2841 to
execute a Device Release command, or until the CPU
is reset.
A Device Release command is rejected when a
Set File Mask command precedes it in the same command chain. The Unit Check bit in the CSW is set
when the command is rejected, and the Command
Reject and Invalid Sequence bits in the Sense Bytes
are set to indicate the conditions which caused the
Unit Check.
A Device Release command performs all of the
functions of a Sense I/O command in addition to the
functions described in this section.
See Two-Channel Switch for additional information.
Device Release Command Code
Decimal

Hexadecimal

Binary

148

1001 0100

Search Commands
On all Search operations, the Channel operates in
the Write mode while the storage unit operates in
the Read mode. The 2841 compares the information
coming from CPU storage and the information coming from the storage unit.
If the search condition is satisfied, a status
modifier indication is sent to the CSW and the channel fetches the next CCW in the command chain from
a position sixteen positions higher than the current
(Search) CCW. This allows modification of a command chain as a function of the data recorded on
the direct access device.
On all Search commands, Command Code bit 0
determines whether this is to be a multiple track
operation; that is, whether switching to the next
read/write head in the cylinder is to occur when the
Index Point is detected. If bit O. is not set (0), head
switching does not take place; if bit 0 is set (1),
head switching does take place. If head switching
has occurred, the next track will be used if the
Search Command is repeated. This allows for sequential searching of an entire cylinder by repeating
the Search Command once for each record to be
searched.
The following command chain illustrates the
procedure for reading a record identified by a key
stored at location a in the CPU.
Command Chain
Search Key a
TIC *-8
Read Data

Search Identifier (Search ID)
Search ID commands (Figure 5) cause a comparison
to be made between five bytes of data from CPU
storage and the five byte record identifier portion of
a count area from the storage unit.

Function
Record Identifier

Compare Key with Search
Argument
Transfer Back to Search
Read Data Area if Status
Modifier was Returned
from Search

The channel is busy during a search operation.
Search Home Address Equal (Search HA)
This command causes the 2841 to search for the
Index Point, then compare four bytes of Home Address data (CCHH) coming from main storage with
four bytes of Home Address data coming from the
storage device. The Flag byte is not transferred or
compared during this command.
If a logical comparison is equal, a Channel End,
Device End, and Status Modifier signal is generated
in the CSW status byte. If the logical comparison is
lUlequal, then a Channel End and Device End are
generated.

Search Home Address does not generate a No
Record Found signal if .the specified Home Address
is not found.
If the CCW Count is greater than four bytes, the
Search operation is completed when the 2841 count
equals zero. The 2841 terminates the command with
a Channel End and Device End. The Status Modifier
is generated if the logical comparison was satisfied.
If the CCW Count is less than four bytes, the
logical comparison between the data coming from
CPU Storage and the data coming from the storage
unit continues until the CCW Count reaches zero. At
the time the 2841 count reaches zero, a Channel End
and Device End are generated. A Status Modifier is
generated if the search condition was satisfied on the
short field.
If a Parity Check, Overrun, or Data Check is
detected, Unit Check, Channel End, and Device End
signals are generated in the CSW at the completion of
the command.
A Search Home Address command does not have
to be preceded by any other CCW in order to be executed.

--------~~------

----....---~----------Count Area

The ID to be searched is the ID of the record
following the next Address Marker or Index point,
in which case ij.0 is searched.
If the CCW count is greater than five bytes, the
Search operation is completed when the 2841 count
equals zero. The 2841 terminates the command with
a Channel End and Device End. The Status Modifier
is generated if the logical comparison was satisfied.
If the CCW count is less than five bytes, the
logical comparison between the data coming from
core storage and the data coming from the file continues until the CCW COlUlt reaches zero. When the
2841 count reaches zero, a Channel End and Device
End are generated. A Status Modifier is generated
if the Search condition was satisfied on the short
field.

Search I D Command Code

Command

Decimal

Hexadecimal

Binary

Search ID Equal

00110001

Search I D High

01010001

Search I D Equal
or High

113

01110001

Search ID Multiple Track Command Code

Command

Decimal

Hexadecimal

Binary

Search I D Equa I

177

10110001

Search ID High

209

11010001

Search I D Equal
or High

241

11110001

Figure 5.

A Search ID command does not have to be preceded by any other CCW in order to be executed.
If Command Code bit (multiple track) is 0,
the search is confined to one track and can be repeated until either the Search Condition is satisfied
or until two Index Points are sensed; at which time
Unit Check (No Record Found), Channel End, and
Device End signals are generated.
If the multiple track bit is a 1, the search
can be repeated until the Search Condition is satisfi~d or until the End-of-Cylinder is detected. At
this time a Unit Check (End-of-Cylinder) signal is
generated.

Search Command Codes

If a Parity Check, Overrun, or Data Check is
detected during a Search-ID operation, Unit Check,
Channel End, and Device End signals are generated
at the completion of the command.

Search ID Equal. If a logical comparison on equal
is encountered, Channel End, Device End, and
Status Modifier signals are generated.
If the logical comparison is unequal, Channel
End and Device End signals are generated.

18. 1

Searcl;t. ID High. This command operates in a manner
similar to that of the Search ID Equal command, except that the comparison is made for a high condition.
The high condition indicates that the ID on the Storage
Unit is higher than the ID in main storage. The comparison is made byte by byte.

Search Key Equal. If a logical comparison on equal
is encountered, Channel End, Device End, and
Status Modifier signals are generated. If the logical
comparison is unequal or the Record has no Key
area, then Channel End and Device End signals are
generated.

Search ID Equal or High. This command operates in
a manner similar to that of the Search ID Equal command except that the comparison is made for either
an equal or high condition. The equal or high condition indicates that the ID on the storage device is
equal to or higher than the ID in main storage.

Search Key High. This command operates in a manner similar to that of the Search-Key Equal command except that the comparison is made for a high
condition. The high condition indicates that the key
in the storage unit is higher than the key in CPU
storage.

Search Key

Search Key Equal or High. This command operates
in a manner similar to that of the Search Key Equal
command except that the comparison is made for
either an equal or high condition. The equal or high
condition indicates that the key in the storage unit
is equal or higher than the key in CPU storage.

Execution of a Search Key command causes a comparison to be made between bytes of data from main
storage and a Key from the storage device. The Key
to be searched is the Key of the record following the
next Address Marker. Search Key will pass over
RO unless chained from a Search ID that has searched
the ID of RO.
If the CCW countis greater than the Key length,
the Search operation is completed when the 2841
count equals zero. The 2841 terminates the command with a Channel End and Device End. The
Status Modifier is generated if the logical comparison
was satisfied.
If the CCW count is less than the Key length,
the logical comparison between data from CPU storage and the data from the storage unit continues until the CCW count reaches zero. When the 2841 count
reaches zero, a Channel End and Device End are generated. A Status Modifier is generated if the Search
Condition was satisfied on the (short) field.
If a Parity Check, Overrun, or Data Check is
detected during the Search -Key operation, Unit
Check, Channel End, and Device End signals are
generated at the completion of the command.
A Search -Key command does not have to be preceded by any other CCW in order to be executed.
If the multiple track bit is 0, the search can
be confined to one track and can be repeated until
either the search condition is satisfied or until two
Index Points are sensed; at which time a Unit Check
(No Record Found), Channel End, and Device End
signals are generated. If the multiple track bit is
1, the search can be repeated until either the
search condition is satisfied or until End-of-Cylinder
is detected. If End-of-Cylinder is detected, an Endof-Cylinder indication is generated.
The Search Key command never returns a
Status Modifier if the Key Length of the search
record is zero.

Command

Search Key Command Codes
Decimal

Hexadecimal

Binary

Search Key Equal

00101001

Search Key High

01001001

Search Key Equal
or High

105

01101001

Command

Search Key Command Codes, Multiple Track
Decimal

Hexadecimal

Binary

Search Key Equal

169

10101001

Search Key High

201

11001001

Search Key Equal
or High

233

11101001

Search Key and Data (File Scan Special Feature)
The File Scan feature provides an automatic rapid
search for a specific set of conditions. The search
is carried out over both Key and Data areas of a
record. Prior to executing a File Scan operation,
a "control mask" is set up in main storage. The
mask consists of bytes of information on which a
comparison is or is not to be made. The bytes on
which a comparison is not to be made are filled
with l's prior to the search.
If an equal comparison is encountered, Channel
End, Device End, and Status Modifier signals are
generated. If the logical comparison is unequal,
only Channel End and Device End signals are generated.

If the multiple track bit is off (0), the search
can be confined to one track until the condition is
satisfied or until two Index Points are sensed, at
which time Unit Check (No Record Found), Channel
End, and Device End signals are generated. If the
multiple-track bit is 1, the search can be repeated
until the specified condition is met or until End-ofCylinder is encountered, at which time an End-ofCylinder signal is generated.
No more positions than the number specified by
the CCW Count are compared. If the CCW Count is
greater than Key Length plus Data Length, the Search
operation is limited by the record Key and Data
Lengths. A zero Key Length causes comparison of
data only. If the channel count is less than the Data
Length plus the Key Length, a truncated search is
performed.
If a Parity Check, Overrun, or Data Check is
detected during a Search Key and Data Operation,
Unit Check, Channel End, and Device End signals
are generated at the completion of the command.

Search Key and Data Equal. This command causes
the 2841 to make a logical comparison on equal between the Key and Data information from the storage
device with the mask information coming from main
storage (Figure 6).
Search Key and Data High. This command operates
in a manner similar to that of the Search Key and
Data Equal command except that the comparison is
made for a high condition. The high condition indicates that the storage device information is higher
than the mask information in main storage.

Command
1.
*2.

Seek
SRCH IDEQ (Previous)

3.
*4.

TIC *-8
Read Count a
Search Key and Data
EQ
TIC *-16
Seek Head a

*5.
*6.
7.
**8.
9.
**10.

Search ID EQ a
TIC *-8
Read Key and Data

Remark
Position Access
find record prior to beginning
of scan area
on Unequal Repeat Search
Read Count of Record into a
Scan Key and Data
On Unequal, Repeat steps 4 & 5
Reposition access (head
select only)
Search for ID read in step 4
On unequal repeat step 8
Read Key and data

*Multiple Track Bit On
**Multiple Track Bit Off

Step 4 of the sequence causes the Count information coming from the storage device to be read
into CPU storage. Key and data information from
the storage device are compared with the mask information from main storage on step 5. The comparis on is made based on the condition specified by
the Searcn Key and Data CCW. When the specified
condition is encountered, the program must reorient to the beginning of the record with steps 8 and
9. Then the desired key and data is read in step 10.
At each detection of Index Point in step 5, the
head address is incremented by one. As a result,
step 4 is done on the next track if the previous
record searched was the last one on a track.
Read Commands

Search Key and Data Equal or High. This command
operates in a manner similar to that of the Search
Key and Data Equal command except that the comparison is made for an equal or high condition. The
equal or high condition indicates that the storage
device information is equal to or higher than the
mask information in main storage.
A File Scan function over an entire cylinder can
be executed by the sequence of CCWs given below.
This sequence assumes that a Set File Mask CCW
was performed prior to the Scan sequence.

On all Read commands the 2841 checks the validity
of each area in a record as the area is transferred
from the storage device.
If a Data Check or Overrun is detected, a Unit
Check signal is generated upon completion of the
command except when the error is in the Count Area.
The command is terminated at the end of the Count
Area. At the completion of the last check, the 2841
sends Channel End and Device End signals to the
channel.

Search Key and Data Equa I
Search Key and Data High
Search Key and Data Equal or High
Command Code

Count
48

Specifies number of Bytes in
CPU storage {including "Mask
Bytes" to be used as Search
Argument. If CCW count equals
key length, command is effectiveIy a "Search Key."

Command Code
Command

Decimal

Hexadecimal

Binary

Search Key & Data Equa I

00101101

Search Key & Data High

01001101

Search Argument in CPU Storage

Byte 4 is not
Search Key & Data Equal

109

01101101

or High
is set to all l's
Command Code

and 8-11 are not compared
because of "Mask Bytes" in CPU

This example assumes

storage. CCW count of 20

CCW count

Decimal

Hexadecimal

Binary

Search Key & Data Equal

173

10101101

Search Key & Data High

205

11001101

Search Key & Data Equal
or High

237

11101101

Iimits search to 20 position
span.

Key Area

Command

Figure 6. Search Key and Data

N
....

Multiple-Track

Data Area
4

Key and Data Areas From Storage Unit

Read Home Address (Read HA)

executed immediately and does not cause a search
for Index Point.

This command causes the 2841 to search for the
Index Point. Detection of the Index Point causes the
five bytes of Home Address information to be transferred from the storage device to CPU storage.
Exactly five bytes are transferred including the flag
byte. If the channel count is less than five, only that
number of bytes is transferred.
Chaining requirement: None

Read RO Command Code
Decimal

Hexadecimal

Binary

00010110

Read RO Command Code, Multiple-Track
Decimal

H exadec i ma I

150
Read HA Command Code
Decimal

Hexadec i ma I

Binary

00011010

Read HA Command Code Multiple Track
Decimal

Hexadecimal

Binary

154

10011010

Binary
10010110

Read Data (Figure 7)
This command causes the Data Area of a record to
be transferred from the storage device to main storage.
Read Key and Data

Read Count
This command causes the eight bytes of the Count
Area following the next Address Marker (AM) to be
transferred from the storage device to main storage.
The number of bytes of information to be read is
always eight. If the channel COlIDt is less than eight,
only that number of bytes is transferred.
Chaining requirement: None. The Count Area
of RO cannot be read by a Read Count command.

Read Count Command Code
Decimal

Hexadecimal

Binary

00010010

Read Count Command Code, Multiple-Track

Read Count, Key, and Data (Figure 8)
This command causes the entire record (Count, Key,
and Data) following the next AM to be transferred
from the storage device to main storage. Record
RO is bypassed as it is not preceded by an address
mark.
Chaining requirements: None.
Write Commands

Decimal

Hexadecimal

Binary

146

10010010

Read Track Descriptor Record (Read RO)
This command causes the 2841 to search for Index
Point. Detection of Index Point causes the 2841 to
"count off" to Home Address and the following gap.
When these Areas have been traversed, record RO
(Count, Key and Data) is transferred from the storage device to main storage.
Chaining Requirement: None. A Read RO
command chained from a Search HA or Read HA is

This command causes the Key and Data areas of a
record to be transferred from the storage device to
main storage. If Key Length is zero, this command
operates like a Read Data command.
The record from which the Data or Key and
Data is read is dependent upon the previous operation.

Write commands cause data to be transferred from
main storage to the storage device. During the
transfer, the 2841 adds appropriate Cyclic Check
bits to each area written. At the completion of the
command, Channel End and Device End signals are
sent to the channel.
Format Write Commands
The following are Format Write commands:
Write Home Address
Write RO
Write Count, Key and Data

Read Data
Read Key and Data
Command Code

Data Address

Count

Specifies CPU storage
locations to which key
or key and data fields are
to be transferred. After the
command is executed, CPU
storage contains: .

Specifies number of
bytes to be transferred.
May be less than entire
data or key and data
length.

7 8

Key Area

Data Area

•

Specified
Location

Figure 7.

Read Data and Key and Data

Read Count, Key and Data
Command Code

Data Address

Count

7 8

Specifies CPU storage
location to which count,
key, and data areas are to
be transferred, after transfer, CPU storage contains:

I~__~:__~___~~o_u_n~~_A_re_~~~

Specifies number of bytes
to be transferred to CPU
storage, may be less than
entire record length.

~__~__K_e_y~~

Area

Data

~L._ _A_re_a_ _

..1

Read Data Command Codes
Decimal

Hexadecimal

Binary

Read Data

00000110

Read Key & Data

00001110

Read Count, Key
& Data

00011110

Read Data Command Codes, Multiple Track
Decimal

Figure 8.

Hexadecimal

Binary

Read Data

134

10000110

Read Key & Data

142

10001110

Read Count, Key
& Data

158

10011110

Read COWlt, Key, and Data

These commands are used to initiaJize tracks or
records. A command chain which includes one or
more Format Write commands must include an appropriate Set File Mask CCW preceding it.
After the last Format Write command in a chain
has been completed, and Channel End and Device
End sigrials generated, the 2841 causes the remaining portion of the track to be erased. If a new command chain is initiated before the end of the track is
reached, a Control Unit Busy signal is generated in
the CSW. At the end of the track, Control Unit End
is signalled to the channel.
If a command other than a Format Write is
chained from a Format Write command, the 2841
retains and executes it after the track has been
erased and Index Point is detected.
Detection of a Parity Check on an Overrun during a Format Write operation causes a Unit Check to
be signalled at the end of the operation. The 2841
writes valid zeros from the time the Overrun is detected to the end of the record. The Cyclic Check
bits written in this case may not be valid.

data from the CPU, and adds two bytes of Code
Check. At this point, Channel End and Device End
signals are generated.
If the dcw Count is less than five, the 2841
records valid zeros until five bytes have been written. If the CCW Count is greater than five, the 2841
transfers only the first five bytes from CPU storage.
A Write HA command is normally used to establish
track identity within a storage device. Each track
must be initialized with a Home Address before a
data operation which involves that track can take
place.
Chaining Requirements: Execution of this command is dependent upon a correct Set File Mask
. command preceding it in the same command chain.
Write Track Descriptor Record (Write RO). This
command causes specified data to be transferred
from main storage to the storage device (Figure 10).
The first eight bytes transferred from core
make up the Count Area. The Flag byte is generated by the 2841. The remaining data is written in
the Key and Data Areas as specified by Key Length
and Data Length in the Count Area. The 2841 writes
the correct Code Check at the end of each Area.
Channel End and Device End signals are generated
after the Code Check of the Data Area has been
written.

Write Home Address (Write HA). This command
causes the 2841 to search for the Index Point (Figure 9). When the Index Point is detected, the specified data is transferred from main storage to the
storage device. The 2841 transfers five bytes of

Write HA
Command Code

000

Data Address

Count

7 8

Specifies CPU storage
location from which five
,bytes of home address are
to be transferred.

Index
Marker

(0000000000000101)

Cyclic ~heck

Head Number

Cylinder 2Number

Home Address

Write Home Address Command
Decimal

Figure 9. Write HA

Usually 5

Hexadecimal

Binary

0001 1001

Write RO
Command Code

Data Address

Count

7 8

Specifies total RO field
lengths count (eight
bytes), plus key length,
plus data length.

Specifies CPU storage location
from wh ich count, key, and
data bytes are to be transferred.
Key and data field lengths are
specifi ed in the count area.

Write RO Command Code

Head No. Rec. Key
No. Len.

Hexadecimal

Binary

0001 0101

Dota
Length

Count Area

Gap

Key Area

Gap

Dota Area

Figure 10. Write RO

The CCW Count Field specifies the number of
bytes to be transferred from main storage by the
channel. This is eight (bytes) plus Key Length plus
Data Length. If CCW Count is less than this, the
2841 writes valid zeros so that the Key and Data
Areas conform to the lengths specified in the record
Count Area.
Chaining Requirements: This command causes
an Invalid Sequence signal to be generated, unless it
is chained from a Search HA that was equal on all
five bytes of the Home Address, or from a Write HA.
If the Write RO CCW is issued more than 60
microseconds after the preceding Search HA or
Write HA, an Overrun signal is generated.
Write Count, Key, and Data (Figure 11). This command is the same as the Write RO except that the
2841 causes an Address Marker to be written on the
track immediately before the Count.
This command causes an Invalid Sequence Signal
to be generated unless it is chained from a Write RO,
another Write Count, Key and Data, or from a successful Search Equal ill or Search Equal Key command. This search must not be a truncated search,
or one in which the CCW count is less than the length
of the area. A Read Data or Read Key and Data CCW
may be inserted between a Search CCW and Write
Count, Key and Data CCW.

Write Special Count, Key, and Data (Figure 12).
This command functions like the Write command
except that it causes a one-bit to be placed in bitposition one of the Flag byte when the 2841 generates
and writes the Flag byte. It is used with the Record
Overflow feature.
Data Write Commands
These commands are used for normal record updating. Detection of a Parity Check or Overrun
causes a Unit Check to be generated at the completion of the command. A data check which occurs in
record areas which must be passed over but not
written, terminates the command before data is
written.
Write Data. This command causes the specified
data to be transmitted from CPU storage to the
storage device. Writing continues as specified by
the Data Length portion of the Count Area. At this
point, the 2841 causes the Code Check to be written
and then sets Channel End and Device End. If the
CCW Count is less than the Data Length in the Count
Area, the 2841 causes valid zeros to be written in
the remaining portion of the Data Field.
Chaining Requirement: Write Data causes an
Invalid Sequence to be generated if it is not chained

Write Count, Key and Data
Command Code

Count

Address
48

7 8

Specifies CPU storage location
from wh iCh count, key, and
data bytes are to be transferred.
Key -cirid data field lengths are
specified in the count area.

Specifies total of all
field lengths; count
{eight bytes}, plus key
length, plus data length

Write Count Key, and Data Command

Flag

Cylinder
No.

Head
No.

Hexadeci rna I

Binary

00011101

Rec. Key
No. Len.

Count Area

Address
Marker

Decimal

Gap

Key Area

Data Area

Gap

Figure 11. Write Count, Key and Data

Write Special Count, Key, and Data
Command Code

Data Address

Count

7 8

Specifies CPU storage location
from wh ich count, key, and
data bytes are to be transferred.
Key and data field lengths are
specified in the count area.

Specifies total of all
field lengths; count
(eight bytes), plus key
length, plus data length
Write Special Count, Key
and Data Command Code

(

Cylinder
No.

'III
I

Fla g Cylinder
No.

Address
Marker

Head
No.
Count Area

Figure 12. Write Special Count, Key, and Data

Decimal

Hexadecimal

Binary

00000001

from a successful Search Equal ID or from a Search
Equal Key command which was not truncated.
Write Key and Data. This command is the same as
a Write Data command except that the Key Area is
also written. If the Key Length Field of the Count
Area is zero, the Write Key and Data command functions just like the Write Data command.
Write Data
Write Key and Data
Data Address

~--------~~~--------~
Specifies CPU storage location'

I The CCW count field specifie;
from which data or key and
the number of bytes to be
data fields are to be transferred. transferred.

Write Data Command Codes
Write Data
Write Key & Data

Decimal

Hexadecimal

Binary

05
13

05
00

00000101
00001101

An Invalid Sequence is generated if Write Key
and Data is not chained from a successful Search
Equal ID command, which was not truncated. Write
commands are never truncated.
End of File
The end of a logical file is indicated by a record
whose Count Area contains a Data Length of two
bytes of zeros. This may be any record on a track.
This record is written by a Write Count,Key
and Data CCW or a Write RO. The indicated zero
Data Length causes the 2841 to record a Data Area
of a single zero byte.
Formation of a Key Area in an end-of~file
record depends on the indicated Key Length. If Key
Length is not zero, the Key Area is transferred.
As a logical file is read or written, the Count
Area of each record is examined. Detection of a
zero Data Length causes Unit Exception to be signalled in the Channel Status Word at the completion
time of the read or write operation that is performed.
No part of the Data Area is transferred.
Record Rn
Key

End-of-File Record

Area

Area
oct.Q....1.

I. .

Multiple Track (M -T) Operation
The 2841 has the ability to automatically select the
next sequentially numbered head on an access mechanism under control of bit 0 (BO) of the Command
Byte. Head switching does not take place at Index
Point if BO is a zero. Head switching takes place at
Index Point if BO is a 1.
The M -T bit is recognized on all Read and
Search commands. Therefore, a certain amount of
discretion should be used when making BO a I bit.
For example, if during a Search operation the M-T
bit is a 1 and index is encountered before the
search condition is satisfied, the head automatically
switches to the next track. The operation continues
until the End-of-Cylinder is detected. This condition can occur if the search was initiated beyond the
point where the record was located on the track. On
the other hand, by correctly utilizing the M-T bit, it
is possible to search a complete cylinder of IDs or
Keys.

Two-Channel Switch (Special Feature)
The Two-Channel Switch feature provides the ability
for the 2841 Storage Control to be shared by two
channels and also allows individual devices (access
mechanisms) to be reserved for the exclusive use of
either of the channels. The two channels may be
attached to the same CPU or different CPUs. Channel switching and device reservation in the 2841 are
performed under control of the system program.
The Two-Channel Switch feature is limited to eight
access mechanisms.
Channel Selection Switch
This switch has three positions: Channel A,
Channel B, and neutral. With the switch in the
neutral position, the 2841 is selected by the first
channel to complete the selection sequence. Once
the 2841 is selected by a channel, it remains selected to that channel until an end status exists. The
channel selection switch then returns to neutral unless one of the following conditions exists.
1. The channel indicates command chaining.
2. The last status byte is associated with a
·channel-initiated signal sequence, it is
stacked by the channel.
3. The last status byte contains the unit check
bit.
4. No command other than an I/O or a No-op
has been initiated since the last unit check
condition occurred.

If Channel A (B) attempts to select the 2841

while the 2841 is selected to Channel B (A), the
2841 responds to Channel A (B) with control unit
busy. This, in turn, causes the 2841 to attempt to
present to Channel A (B) a status byte containing
control unit end after the channel selection switch
returns to the neutral position. The address byte
associated with this status condition is the base
address of the 2841 on that channel. This control
unit busy condition may occur on any attempt to
select the 2841, including initial program load, Test
I/O, etc.
Device Reservation (See Device Reserve and Device
Release Commands)
When a device is reserved to Channel A (B), any
command from Cha~mel B (A) addressed to that device
is rejected with a busy indication in the initial status
byte. This, in turn, causes the 2841 to attempt to
present to Channel B (A) a status byte containing
Device End after the reservation has been terminated.
The address byte associated with this status byte is
the same as that associated with the Busy status byte.
Device End status resulting from the completion
of a 'Seek command is presented to the channel that
issued the command.
A device that generates Device End status when
it changes from the not-ready to the ready state
causes the 2841 to present this type of Device End
status to both channels. However, no attempt is
made to present such status on one channel while the
device is reserved to the other channel.
A reset can be initiated by either channel at any
time. A reset causes all reservations and status
conditions stored in the 2841 and related to the resetting channel to be reset in the 2841. Reservations
and status conditions related to the other channel are
not affected.

Power Control
A power control interface is provided for each
channel. If either channel indicates power "ON" the
2841 turns on. The 2841 turns off only if both channels indicate power "OFF". If the Emergency
Power OFF switch of either channel is activated,
the 2841 power is turned off.
Record Overflow (Special Feature)
The Record Overflow feature is provided to allow a
logical record to overflow from one track to another.
It is useful in achieving a greater data packing
efficiency and in formatting records which exceed
the capacity of a track. The cylinder boundary is
the limiting factor to the size of a record.
Formatting OVerflow Records
A portion of an overflow record which is written on
one track is called a record segment. Each record
segment is processed as a normal record during
Format Write operations. The Write Special Count,
Key, and Data CCW is the command used for formatting all segments of an overflow record except
the last segment. The last segment is written by the
normal Write Count, Key, and Data CCW.
The Write Special CCW causes a bit to be written in Flag byte one in bit position one of the record
segment being written. Otherwise, the Write Special
CCW functions just like the normal Write command.
All overflow segments must be recorded as the
first record following RO on the overflow track.
Overflow segments are normally recorded without a Key Field, since only the Key Field of the first
segment has significance. All overflow record segments, except the last one, are full track records.
Processing OVerflow Records

Addressing
The base address (four high-order bits) of the 2841
on one channel is independent of the base address of
the 2841 on the other channel. However, the four
low-order address bits for any attached device must
be the same on both channels.
Usage Meter
A single usage meter records process time in the
2841; however, a separate Enable switch is provided
for each channel.

The following CCWs operate on an overflow record
as though it were a normal record if the Overflow
Record feature is installed:
Read Data
Read Key and Data
Read Count-Key and Data
Write Data
Write Key and Data
The 2841 detects that flag byte bit position
one is a 1-bit. After completing the read or write--

operation on the first segment based on the count
of the first segment, the 2841 searches for the
Index Point. At Index Point, the next sequential
Head is selected and the 2841 searches for the first
Address Marker on the track. Then, under control
of the Data Length in the Count Area, it processes
the Data Field of this record segment. This operation continues until the 2841 detects a record segment which contains a zero bit in flag byte bit
position one. At the end of this record segment, the
operation is terminated.
A CCW chain which starts operation on a record segment other than the first segment is processed as though it started on the first segment.
This type of operation may make it desirable to
repeat the Key Field in all record segments if the
chain of CCWs is dependent on a Search Key Equal.
Search ID, Search Key, and Read Count CCWs
operate on each record segment as though each
were a normal record.
Unusual Conditions. In addition to the checks provided in normal processing of any record, certain
conditions can occur which are unique to overflow
records. The commands stop immediately on detecting the following conditions:

Overflow to a Defective Track
Overflow Incomplete and Track Condition
Check sense bits will be set if an overflow
occurs to a track which has been flagged
as defective.

Overflow From an Alternate Track
Overflow Incomplete and Track Condition
Check sense bits are set if an attempt is
made to overflow from a track flagged as
an alternate.

Overflow Violating a File Mask
Attempting to overflow by issuing a command in violation of a file mask sets Overflow Incomplete, File Protected, and
Command Reject (write command only)
sense bits.

Overflow to a Track with Incorrect
Head Number
Overflow Incomplete and Seek Check sense
bits are set if the Head number compare is
unequal during an overflow. This condition
occurs if the last Seek Address issued to
the 2841 is not the address of the track
with the overflow record and an overflow
record is being read or written.

28.1

IBM 2311 DISK STORAGE

Compatibility between units:

Introduction
The IBM System/360 is used in many applications
which require that files of medium size be accessible
to the central processor. These files may contain
customer account balances, current inventory status,
payroll information, computer operating programs,
or other information, permanent or temporary, to
which the central processor' must refer to complete
the specific application.
If all files need not be directly accessible (on
line) to the central processor at all times, operating
and equipment economies can be realized if the file
can be separated from the file drive. As with magnetic tape, an unlimited volume of data can be stored
away from the central processor (off-line). Appropriate data files can be placed on-line as required
by the processing schedule.
It may also be desirable to be able to transfer
data files from one file drive to another (possibly
attached to a second central processor). This allows
one system to process and update a data file, and
another (possibly a smaller satellite system), to
print reports or answer inquiries. This capability
also enables a second system to complete a task if
the primary system is not available because of other
appl ications.
The IBM 2311 Disk Storage Drive, with IBM 1316
Disk Packs, offers processing features which answer
the needs of many data processing applications:

Storage capacity:

High speed accessibility:
Data file removability:

Fast data transfer to the
processor.
Multiple unit growth potential:

Over 7. 2 million
bytes per disk
pack
85 milliseconds,
average
Disk pack change
time: about one
minute
156,000 bytes per
second
Up to eight 2311's
58 million bytes,
on a single 2841
control unit

Large volume of data available
at a single access:

1316' s compatible
between any 2311 's
used on IBM
System/360
Over 36, 000 bytes
per cylinder

Device Description
The IBM 2311 Disk Storage consists of two main components: the 2311 Disk Storage Drive and the 1316
Disk Pack.
Storage Medium (1316 Disk Pack)
Each disk pack consists of six 14-inch disks, mounted
1/2 inch apart on a central hub. Data is recorded on
the inside ten disk surfaces. The two outer surfaces
are covered by protective plates. The entire assembly
of disks, hub, and protective plates is rotated at
2,400 revolutions per minute (25 ms per revolution).
Each disk pack weighs about ten pounds.
The two-piece plastic cover is designed to protect
disks against damage. A built-in handle on the top
cover makes carrying easy and efficient. A selflocking device in the handle permits removal of the
top cover only when the pack is mounted on the disk
storage drive.
Access Mechanism
When the 1316 Disk Pack is mounted in the 2311 Disk
Storage Drive, information is written on and read
from the ten disk surfaces by magnetic read/write
heads. These read/write heads are mounted in pairs
between each two disks on a movable comb-like access
mechanism. When in operation, the read/write heads
float over the disk surfaces on a thin film of air.
Access Time
Cylinder-to-cylinder (horizontal) access time varies
according to the number of cylinders traversed. Access time from a cylinder to an adjacent cylinder is
30 ms. Maximum access time (from cylinder 202 to
cylinder 000) is 145 ms, and the average time for
"random" accesses is approximately 85 ms (Figure 13)

140
~

120
~
~

tOO
~
~
~
Vl

~
~

........

:(
40

20
~
~
~

Figure 13.

, , ,

, , ,
20

tOO

120

140

160

180

200

NO. TRACKS TRAVELLED

Access Times

...----203 Cylinders

Once the access mechanism has reached a cylinder
position, additional time is required for disk rotation
to the desired record. At 2,400 revolutions per minute, rotation time is 25 ms, and one-half revolution
(12.5 ms) is the average rotational delay.
Because the access mechanism includes one
read/write head for each disk surface, no vertical
access motion is required.
Figure 13 shows the approximate times for access
mechanism movement in either direction, excluding
rotational delay. This may be used as an aid in programming for the most efficient utilization of the
storage unit.
Data Record Addressing

Cylinder Concept

As the access mechanism is moved horizontally, it
may be stopped at any of the 203 positions. This
provides 203 data tracks on each surface. Since all
ten read/write heads are moved by a single access
mechanism, a cylinder of ten data tracks is available
at each access mechanism position.

Data Storage

With the high density recording techniques used
in the 2311, minute contamination particles can affect
data reading and writing and may cause loss of bits.
Therefore, 203 tracks per disk surface are provided
to ensure that the stated capacity, based on 200
tracks, is maintained for the life of the disk pack.
Because each data record has non-data components, like Count Area and Gaps, track capacity for
data storage will vary with record design. As the
number of separate records on a track increases,
additional byte positions are used by gaps so that data
capacity is reduced. The track capacity formulas
(Figure 14) provide the means to determine total byte
requirements for records of various sizes on a track.

Format
Data is stored in the IBM 2311 Disk Storage Drive in
the format defined by the IBM 2841 Storage Control
Unit. This format is uniform for all storage devices
attached to the 2841.
Capacity
If IBM Programming Systems are not used, the first

record on each track (RO) may contain application
data.
Based on 200 tracks, with all records used for
application data, a single IBM 1316 Disk Pack can
contain over 7.3 million bytes, or over 14.7 million
packed decimal digits.
IBM Programming Systems reserve the use of
the first record on each track (Record RO) to store
various information about the track. This information is used by the Programming System, and no
application data is included. Using this format,
based on 200 tracks, each 1316 disk pack can contain
over 7.2 million bytes, or over 14.4 million packed
decimal digits (Figure 14). Record R1 is the first
application data record, and if R1 is the only data
record on the track, it may contain up to 3625 bytes
of information.

Index
Point

Home
Address

~~'-----Record RO

DB B

Storage

"I~
-Data......1

"""1

BB B
Basic Track Capacity

When RO is Used as Specified

When RO is Used for

2311

3625

~1:4

~I ~~~~;

Record Rn - - - - - ..

1GB

Da..........
ta

r--I

Data

Last Record

With Key

Without Key

81 + 1.049(K + D )
L
L

Without Key
61+1.049D

3694

Bytes Requ i red by Data Records
Data Records (except for last record)

Dato

By IBM Programming Systems.

Start/Stop Key. This key is lighted (on) when it is in
the Start position.
With the 2311 properly connected in a processing
system, press this key to the Start position to supply
power to the disk drive motor and other 2311 components. When the disk drive motor has come to
speed, and other components are ready for operation,
the read/write heads are moved into position and the
access mechanism performs an automatic seek cycle.
Pressing the Start/Stop key when it is in the Start
position changes it to the Stop position. This action
causes the access mechanism to retract from the disk

Record R1, R2, ---R(n_l)

Track Capacity Basis in Bytes,

Units

Operator Controls and Indicators

With Key
20+(K + D }
L
L

Number of Equa I Length Records Per 2311 Track

Record RO used as specified by
IBM Programming Systems. No
application data;KL = 0; DL
Without Key
With Key

3625 1739 1130 829 650

531

446

383

334

294 262

3605 1719 1110 809 630

511

426

363

314

274

242

235 212

193

176 161

148

137 127 117

215 192

173

156 141

128

117 107

Figure 14. 2311 Capacity

pack and removes power from the disk drive motor.
Automatic braking stops disk pack rotation in a few
seconds.
Select Lock Indicator. When on, this indicates a
machine condition which requires Customer Engineering attention. This condition causes the disk storage
drive to be disabled and stops the usage meter.
Enable/Disable Switch. When the CPU is in the
stopped state, this switch enables or disables the
communication of the storage drive with the CPU.
It also enables or disables the equipment usage meter.
If the CPU is running when the switch setting is
changed, the storage drive and usage meter operating
status are not changed until the CPU is placed in the
stopped state. (See also Select Lock Indicator. )
Operating Procedures
Disk Pack Handling
Each disk pack is protected in transit by special
carton inserts and special protective material.
When received, examine the carton closely. If
its condition is acceptable, remove the disk pack and
store it. Keep the carton and inserts; you may need
them later.
If the carton or its contents show any unusual
shipping damage, do not use the disk pack. Retain
the damaged carton and disk pack in its "as received"
condition and notify your IBM Customer Engineer
immediately.
Disk packs have been designed for ease of transport from location to location.
For best shipping results:
1. Be sure the pack is secure in its two-piece
cover.
2. Use only the specially designed IBM shipping
carton with its special protective padding
properly inserted. If the original carton is
worn or damaged, a new carton may be
ordered from your local IBM office.
Handle a disk pack only with its cover on.
If the disk pack is accidentally dropped, or

receives a sharp impact of any kind, call an IBM
Customer Engineer before using it.
Disk Pack Labeling. For positive identification,
small adhesive-backed labels can be placed on the
disk pack center hub. Labels in this location can be
read through the transparent disk pack cover.

The following operating procedures should be
followed when labeling disk packs:
1. Use adhesive-backed labels which can be
applied and removed easily.
2. Use a writing implement, like a pen or felttip marker, which does not produce loose
residue. Do not use a lead pencil.
3. Write on the label before it is applied to the
disk pack.
4. Place the label only on the center hub, not
on the disk pack cover or top disk surface.
5. Use a new label if changes are necessary.
Never use an eraser because microscopic
eraser particles can damage disk surfaces
and read/write heads.
Disk Pack Loading and Unloading. The following
procedures should be followed for rapid, effective
disk pack changing:
Loading.
1. Open the 2311 cover.
2. Remove the bottom disk pack cover by turning the bottom locking knob.
3. Place the 1316 disk pack (still contained in
top cover) on the 2311 spindle.
4. Turn the top cover in direction of ON arrow
until firm resistance is met.
5. Lift the top cover from the disk pack.
6. Close the 2311 cover.
7. Press the 2311 Start key.
8. Reassemble the top and bottom covers of the
disk pack.
9. Store the covers in a clean cabinet or on a
clean shelf.

CAUTION: Do not leave disk pack top cover inside
Disk Drive.

Unloading.
1. Press the 2311 Stop key.
2. Wait for the disk pack to stop rotating.
3. Separate the top and bottom disk pack covers.
4. Open the 2311 cover.
5. Place the disk pack top cover over disk pack.
6. Turn the top cover in direction of OFF arrow
at least two full turns.
7. Lift the top cover, now containing the disk
pack, from the spindle.
8. Fasten the bottom cover to disk pack (firmly).
9. Close the 2311 cover.
10. Store the disk pack in a clean cabinet or on a
clean shelf.

Disk Pack Storage. To assure maximum disk pack
life and reliability:
1. Store the disk packs flat, not on edge.
2. Each pack should rest on a shelf, not on
another disk pack.
3. Store in a clean, enclosed metal cabinet or
a similar fire-resistant container; never

4.
5.

in direct sunlight.
Store disk packs in a machine-room atmosphere (60 0 to 90 0 F, 10% to 80% humidity).
If disk packs must be stored in a different
environment, allow two hours for adjustment to machine room atmosphere before
use.

IBM 2302 DISK STORAGE, MODELS 3 AND 4

Introduction
The IBM 2302 provides the using system with:
• High data storage capacity
• Fast, direct access to stored data
• High data transfer rate
• Flexible data organization
The IBM 2302 Disk Storage provides fast access
to vast quantities of on-line, stored information.
Random access to specific areas of information
enables the user to locate any data record within a
maximum of 180 milliseconds, without sequential
address searching. Thus, voluminous master record
files may be stored on-line ready for immediate reference or updating.
Maintenance of master record files is immediate
and direct; the most current information available is
entered into the proper area of the master record
file as transactions occur. Complex accounting procedures can be simplified, because intermediate manual operations, necessary to maintain off-line record
files, are eliminated.
Device Description
storage Medium
Disk storage consists of thin metal disks uniformly
coated with a magnetic recording medium. Data and
control information are r~corded as magnetized spots
on the coated surfaces of the disks in concentric data
tracks.

Two access mechanisms are used to address the
500 data tracks on the disk surface. One access
mechanism services the inner 250 data tracks, and
the other access mechanism services the outer 250
data tracks (Figure 15).
The two access mechanisms of the 2302 are
addressed as Access 0 for the outer 250 cylinders
and Access 1 for the inner 250 cylinders.
The two access mechanisms on the 2302 operate
independently and may be in motion simultaneously.
Each mechanism is restricted to motion within its
own zone of operation; accordingly, one access mechanism cannot read a track written by the other access
mechanism.
Access Group. The access mechanism, together
with the attached read/write heads, and the 250 tracks
serviced by it, comprise an access group. Two access
groups are provided with the 2302 Model 3, and four
are provided with the 2302 Model 4.
Disk Storage Module. A stack of 25 magnetic disks
(50 disk surfaces) together with the associated read/
write heads and the horizontal positioning mechanisms
comprise a disk storage module.
Model 3 and Model 4 Disk Storage Designation.
Model (3 or 4) designation refers to the number of disk
storage modules provided. Model 3 disk storage contains one module of disk storage; Model 4 contains
two modules of disk storage, one above the other.
500 Data Tracks per Disk;
Two Access Mechanisms
per Module.

Access Mechanism
Horizontal positioning of the read/write heads is performed by a hydraulic access mechanism containing
46 data read/write heads. During a seek operation,
the access mechanism positions the read/write heads
at the program specified track location. No vertical
movement is necessary because there is a head for
each data surface. It is not necessary to perform an
additional seek operation if the desired record is in
the same vertical plane (cylinder) as the previous
record; only electronic head selection is necessary.

Figure 15. Access Mechanisms

TO Cylinder Address
0-

;:;

0N
0
I

0
N
0

0M
0
I

~I
0

8I
0

0-0
0
I

8 C! 8 8 0R

ex)

;! !!?

!!? ~
0

ex)

::I
~

-8
.=
>.

140-1

~
u..

150-1
160170-1

50 milliseconds

120 milliseconds

100ll'
.• .• .'• . •'. .• .• .'.•iI
~

180 milliseconds

NOTE: These times do not include rotational-delay time.
Figure 16. 2302 Disk Storage Access Time

Access Time
Access to one specific track on a given recording
surface is accomplished by the lateral movement of
the whole access mechanism from a current track
location. The time required for this movement is
called access motion time and is related to the lateral distance the arm moves. Figure 16 shows the
time requirements for access motion time for the
IBM 2302 Disk Storage.
In addition to access motion time, an additional
timing factor known as rotational delay time is encountered. Rotational delay time is the time required
for the disk to position the desired record at the
selected read/write head. Maximum rotation at delay
time is 34 milliseconds; average rotational delay
time is 17 milliseconds.

The selection of the proper read/write head is
performed simultaneously with access motion time.
The read/write head selection time consists solely of
electronic switching and is negligible.
Total data access time includes the summation of
access motion time and rotational delay time. Average rotational delay time (17 milliseconds) is generally
used in this calculation.
Data Record Addressing
The data tracks of the cylinders are numbered sequentially from bottom to top and from outermost cylinder
to the innermost cylinder of each access group. Data
track numbers begin with 0000 at the outermost cylinder of the lowest data disk surface, and continue up
through this outermost cylinder to track number 0045.

112 million bytes, or over 224 million packed decimal
digits. Record R 1 is the first application data record
and if it is the only data record on the track, it may
contain up to 4984 bytes of information.
With the high density recording techniques used
in the 2302, surface defects or contamination can
affect data reading and writing and require that alternate tracks be used. Therefore, rated capacity is
112.0 million bytes per module to ensure that the
stated capacity is maintained.
Because each data record has non;"data components,
such as Count Area and Gaps, the track capacity fqr
data storage will vary with record design. As the
number of separate records on a track increases,
additional byte positions are used by gaps so that data
capacity is reduced. The track capacity formulas
(Figure 17) provide the means to determine total
byte requirements for records of various sizes on a
track.

Numbering continues with the lowest data track
of the next inner cylinder, 0046 and proceeds upward
within the cylinder. Continuing through each of the
cylinders of the single access group in like manner,
the last track is the top track of the innermost
cylinder.
Data Storage
Format
The format of the data stored on the 2302 is determined by the 2841 Storage Control. It is identical
for all storage devices which attach to the 2841.
Capacity
If IBM Programming Systems are not used, the first
record on each track (RO) may contain application
data. Based on the use of all record areas for application data, a single IBM 2302 Disk Storage Drive
Model 3 can contain over 113 million bytes or
226 million packed decimal digits j the Model 4 can
contain over 226 million bytes or 452 million packed
decimal digits.
/'
IBM Programming Systems reserve the use of
the first record on each track (Record RO) to store
various information about the track. This information is used by the programming system, and no
application data is included. By using this format,
each 2302 Disk Storage module can contain over

Index
Point

Home toI_.----Record RO
Address

Storage

Count

•

File Ready indicates that the 2302 has power on, is
up to operating temperature, and is ready to accept
or retrieve data, on demand of the 2841 Storage Control. This light is used to determine when the storage device is ready after power is turned on. The
light remains on until power is dropped though
temperature may fall below normal operating range.

By IBM Programming Systems.

Record Rn

...

I I B I-D-ata-,
Count

ICount

When RO is Used for

Data Records (except for last record)

Data

Without Key

4984

5053

61 + 1.049 DL

Record RO used as specified by

-----I
..~

I GJ I

Index
Point

Data

Bytes Required By Data Records

Basic Track Capacity

When RO is Used as Specified
Unit

File Ready

Record RII R21 ---R(n-J)

I-~-ata-I B

Track Capacity Basis in Bytes,

2302

I•

Indicators

Last Record

With Key

Without Key

81 + 1.049(K + D )
L
L

With Key
20+(K + D )
L
L

Number of Equal length Records Per 2302 Track

IBM Programming Systems. No
application data;KL = 0; Dl = 8
Without Key
With Key

Figure 17. 2302 Capacity

4984 2402 1569 1157 912

749

632

4964 2383 1550 1138 892

729

613

546 478 424

380

343

526 459 405

361

324

312 285

263

243

224 208

195 182

293: 266

244

224

205 189

176 163

IBM 2321 DATA CELL DRIVE

Introduction

Cell Drive

Many data processing applications include the maintenance of very large files of operating information.
Direct access to a large file enables the central
processor to answer inquiries about any item in the
file. For processing economy, transactions may
still be batched and sequenced. However, processing
may be interrupted, an inquiry about another part of
the file answered, and processing resumed on an
"up-to-the-minute" basis. Thus, business decisions
can be based on the most current data available, and
applications not previously practical can be accomplished by data processing equipment.
The IBM 2321 Data Cell Drive includes processing capabilities which expand the sphere of data
processing applications:

Data Cell positioning is initiated by a Seek instruction.
The cell drive rotates the circular array of ten Data
Cells to one of 200 discrete subcell positions. The
array can rotate in either direction and always moves
in the direction that requires least travel.
When the array has placed the subcell containing
the addressed strip beneath the access station, a
position check is made by a subcell position detector,
which signals the 2841 Storage Control Unit that a
subcell is within the range of the access station.

Large storage capacity:
Medium speed accessibility:

Data file removability:

Fast data transfer to the
central processor:

Compatibility between units:

Large volume of data available at single access:

Over 418 million
bytes per 2321
600 milliseconds
maximum to any
record
Data Cell change
time: about one
minute
55; 000 bytes per
second
Up to eight 2321s
per 2841
Data .Cells compatible between
2321s used with
IBM System/360
198, 000 bytes per
strip

Device Description

From a circular array of 10 cells with 20 subcells each (Figure 18), a rotary positioning system
positions a selected subcell of ten strips beneath an
access station. At this station a selected strip is
first withdrawn from the subcell, then rotated past
a read/write head element for data transfer, and
finally returned to its original location in the subcell.

Access Station
The addressed strip is exposed by parting the adjacent
strips with separation fingers. The strip is selected
from a subcell of 10 and placed on a revolving drum.
It is then rotated past the read/write head block for
data transfer. When reading and/or writing is complete, the strip is returned to its original subcell
location by a restore function. The read/write head
block contains 20 magnetic elements. It can be
positioned to any of five positions (cylinders), thus
providing 100 recording tracks per strip. The head
block position is specified by the address in the Seek
instruction.
Access Time
Access time is defined as the length of time required
to place a selected strip in a data transfer position.
Average access time under varying conditions is
listed in Figure 19.
Data Record Addressing
The physical location of an individual recording track
is determined by considering the following areas:
Data Cell Drive
Data Cell
Subcell
Strip
Cylinder
Read/Write Head Element
When a record is addressed, the location of each
of the areas just listed is compared against the new

SUBCELL
10 Strips per Subcell

10 Ce 115 per Drive

CELL
20 Subcells per Cell

Figure 18. 2321 Drive, Cell, Subcell

Conditions
When only Read/
Write element
selection is required
When only Read/
Write head Block
motion is required
When No Strip
is on the Drum
When a previously
Addressed Strip is
on the Drum

175 ms

250 ms

350 ms

400 ms

375 ms

450 ms

550 ms

600 ms

Figure 19. 2321 Access Time

address. From this comparison, the necessary
electronic and electromechanical action required to
place the addressed record in the data transfer position is determined.
When the new address is the same as the old
address in a specific area, no access motion is
required. For example, if the only difference between
a new and old address is the selection of an adjacent
track within the same cylinder position, the only action
would be the electronic selection of the proper read/
write head element, providing the strip was not
restored in the interim.
Data Storage
Format
Data is stored in the IBM 2321 Data Cell Drive
in the format defined by the IBM 2841 Storage

Operator Controls and Indicators

Control. This format is uniform for all devices
attached to the 2841.

The operator's console on the Data Cell Drive contains indicator lights and manual controls. The
indicator lights provide the machine operator with
the following information.

Capacity
If IBM Programming Systems are not used, the first
record on each track (RO) may contain application
data. Based on the use of all records on a track for
application data, a single IBM 2321 Data Cell Drive
can contain 418 million bytes or 836 million packed
decimal digits.
IBM Programming Systems reserve the use of
the first record of each track (Record RO) to store
various information about the track. This information is used by the Programming System, and no
application data is included. By using this format,
each 2321 can contain over 400 million bytes, or
over 800 million packed decimal digits (Figure 20).
Record R1 is the first application data record, and
if R1 is the only data record on the track, it may
contain up to 2000 bytes of information.
Because each data record has non-data components, like Count Area and Gaps, track capacity
for data storage will vary with record design. As
the number of separate records on a track increases,
additional byte positions are used by gaps so that
data capacity is reduced. The track capacity formulas (Figure 20) provide the means to determine total
byte requirements for records of various sizes on a
track.

Index
Point

Home 1oI11i----Record
,..
Address
RO

1Count I

Storage

.. ..

"""1

D-ata--'I

Ac Power On. Indicates that primary ac power is
applied to the IBM 2321.
Drive Operative. Indicates that the 2321 has all power
on and no interlock conditions exist. Interlock conditions, such as an open entry door or an improperly
mounted data or ballast cell, render the 2321
inoperative.
Drive Ready. Indicates that the 2321 is ready to perform normal programmed operations under control
of the 2841.
Thermal. Indicates that a high temperature condition
exists within the 2321.
Drive Select. Indicates that the storage control unit
is communicating with the 2321.
The manual keys and switch enable the operator
to control the following functions.
Restart. This key allows the restart of the 2321
in the event of certain inoperative conditions, such
as a momentary interrupt in power.

ICount I@ I

Index
Record Rn - - - -__~-.I Point

....

Record Rl I R21 ---R(n-D

IB~ ~I

Data

Track Capacity Basis in Bytes,

Basic Track Capacity

When RO is Used as Specified

When RO is Used for

Dota Records (except for last record)

Data

Without Key

Unit

By IBM Programming Systems.

2321

2000

Bytes Required By Data Records

Record RO used as specified by

Lost Record

With Key

Without Key

100+ 1. 049 (K L + DL

84+ 1.049 DL

2092

1/1

Data

16+(K + D )
L
L

Number of Equal Length Records Per 2321 Track

IBM Programming Systems. No
application data;KL = 0; DL = 8
. Without Key
With Key

Figure 20.

2000 935

591

421 320

252

204

168 141

119

100

1984 919

575

406 304

237

189

153 125

103

52 A3

20
19

2321 Capacity

Reset. This key allows the 2321 indicators to be
reset in the event of an improper status condition.
This reset will not establish a Drive Operative
status.
DC On. Indicates that all dc power within the 2321
is on.

Operating Procedures
Data Cell Replacement. To facilitate Data Cell replacement, four operator aids are provided.
1. Entry door with interlock: The entry door
permits access to the Data Cell array. An
interlocking switch is provided for operator
safety. When the door is open, no machine
controlled motion can occur.
2. Data Cell location indicator: The Data Cell
location indicator identifies, by number,
the Data Cell located in the replacement
position. It also indicates the number of
the Data Cell positioned under the access
station.
3. Manual by-pass valve: The manual by-pass
valve allows an operator to manually rotate
the array to place any desired Data Cell in
the replacement position. The valve is
mechanically interlocked with the access
station and is closed automatically with the
closing of the entry door.
4. Data Cell mount interlock: An interlock
switch is provided to prevent 2321 operation
unless the switch is properly closed by either
a Data Cell or a ballast cell.

The following procedure should be followed when
replacing Data Cells.
1. Check the indicator lights for the following
pattern:
Ac Power should be on.
Drive Operative should be on.
Drive Ready should be on.
Drive Select should be off.
2. Open the entry door. The Drive Ready indicator should extinguish.
3. Check the Data Cell location indicator to
determine the physical position of the desired
Data Cell.
4. Open the manual by-pass valve and rotate the
array (in either direction) to place the desired Data Cell in the replacement position.
5. Place a Data Cell cover on the desired Data
Cell. This action engages all mechanical
and electrical interlocks and allows the Data
Cell to be removed from the machine.
NOTE: New Data Cells, replacement Data
Cells, or ballast cells must be inserted in
place of removed cells. When the Data Cell
cover is removed from a properly inserted
cell, the mechanical and electrical interlocks
are disconnected.
6.
7.

Close the entry door.
Check the indicator lights for the following
pattern:
Ac Power on
Drive Operative on
Drive Ready on
Drive Select off

IBM 2303 DRUM STORAGE

Introduction

Data Record Addressing

The IBM 2303 Drum Storage provides on-line random
storage of 4.006 million bytes on a magnetic
drum. Two 2303s may be attached to each 2841
Storage Control Unit for a total on-line random access storage of 8.012 million bytes or 16.024
million packed decimal digits.
The drum is divided into 800 data tracks; each
track has a read/write head and may contain up to
5,008 bytes of data. The maximum data transfer
rate is 312.5 thousand bytes per second.

Arrangement of read/write heads on vertical racks
retains the cylinder concept. Cylinder operations
with the 2303 allow up to 800 tracks to be
written or read with a single drum storage order.

acce~s

Device Description
The 2303 Drum Storage consists of a vertically
mounted drum and its associated electronic circuitry. The drum, coated with a magnetic recording material, rotates at about 3, 500 revolutions per
minute. The surface of the drum is divided into
tracks. These addressable tracks, extending
around the periphery of the drum, are used for
storing data as follows:
800
80

Standard Data Tracks
Alternate Data Tracks

The alternate tracks are provided to ensure that
each recorded bit can be stored in a magnetically
perfect medium. If a defect is encountered on a
track, the entire track is disabled and one of the
alternate tracks is substituted. This alternate
track is given the address of the disabled track.
Each data track has its own read/write head,
used for both recording and retrieving data. The
data read/write heads are fixed in position on 20
vertical racks that surround the drum. Each rack
contains 40 data read/write heads.
Access Time
Because of the assignment of an individual read/write
. head to each data track, data seek operations, with
their associated access motion time delay, are
eliminated. Therefore, the access time is composed
of only the rotational time of the drum.
Maximum rotational time 17 . 5 ms.
Average rotational time
8.6 ms.

Data Storage
Format
Data is stored in the IBM 2303 Drum storage in the
format defined by the 2841 Storage Control. This
format is uniform for all storage devices which
attach to the 2841.
Capacity
If IBM Programming Systems are not used, the first

record on each track may contain application data.
Based on the use of all records on a track for
application data, a single.IBM 2303 Drum Storage
Unit can contain over 4.006 million bytes or over
8.012 million packed decimal digits.
IBM Programming Systems reserve the use of
the first record of each track (Record RO), to store
various information about the track. This information is used by the Programming System, and no
application data is included. By using this format,
each 2303 can contain over 3. 913 million bytes, or
7.826 million packed decimal digits (Figure 21).
Record Rl is the first application data record, and
if Rl is the only data record on the track, it may
contain up to 4,892 bytes of information.
With the high density recording techniques used
in the 2303 minute contamination particles can affect
data reading and writing. Therefore, 80 alternate
tracks are provided to ensure that the stated
capacity, based on 800 tracks is maintained.
Because each data record has non-data components, like Count Area and Gaps, track capacity for
data storage will vary with record design. As the
number of separate records on a track increases,
additional byte pOSitions are used by gaps so that
data capacity is reduced. The track capacity formulas
(Figure 21) provide the means to determine total byte
requirements for records of various sizes on a track.

~~1~t A~d~~s 14
........---Record RO

0 I IB

Storage

Count

- I-Data-,

When RO is Used as Specified
By IBM Programming Systems

2303

Record R1, R2, ---R(n-l)

B I I@
Count

With Key

Figure 21.

2303 Capacity

Record Rn - - - -.........,1

ICount

IG I

Data

Without Key

Last Record

With Key

Without Key

146 + (K + D )
L
L

108 + DL

5008

Data

~~~~:

Bytes Required By Data Records

When RO is Used for

With Key
38 + (K + D )
L
L

Number of Equal Length Records Per 2303 Track

Record RO used as specified by
IBM Programming Systems. No

Without Key

,--Data-,

Data Records (except for lost record)

4892

application data;Kl = 0; Dl = 8

Basic Track Capacity

Track Capacity Basis in Bytes,

Unit

I-

4892 2392 1558 1142 892

725

606

4854 2354 1520 1104 854

687

568

517 447

392

346

479 409

354 308

308 276

249

225

270 238

211

187 166

204 186
148

169 155

142

131

117 104

APPENDIX A.

2841/2311 PROGRAMMING EXAMPLE

The following 2311 Programming example is included to illustrate the use of 2841 channel commands to perform operations on attached storage
devices. The program was assembled and simulated with the 7090/7094 Support Package for System/360. The example is solely for the purpose
of illustration, it does not necessarily reflect techniques in the use of Operating System/360 Programs.
Two separate operations are performed. The first is the writing of Home Addresses (HA) and Track Descriptor Records (RO) on all 2030
trocks of the 2311. Read Back Check of RO is performed in CPU storage. The second program writes three records on track number 37, reads
,them into a separate I/O area and searches for a fourth record which is not there.
The first routine is labeled HAROWT and uses the Channel command list WRDKHA. The three record read/write and fourth record search is
performed with the CCW list WR3REC. A subroutine (EXCP) is used to start I/O.

Not shown is an interrupt handling subroutine, a subroutine

(BZSIO) that acts on condition codes 1, 2 and 3 and DUMP which dumps CPU storage (as a debugging aid) in case of errors or conditions not
yet programmed. The system is in the supervisory mode and enabled for interrupts at all times unless EXCP has disabled or the interrupt subroutine
is being used. EXCP sets a bit equal to one in DEVTAB to indicate that the device is being used, this bit is cleared by the interrupt subroutine
when the device end bit appears in the channel status word.

000112
000114
000118
00011C
000120
000124
000128
00012A
00012E
000132
000136
00013A
00013E
000142
000146
00014A
000150
000154
000158
00015C

IB
58
40
40
40
58
IB
40
40
40
41
58
45
91
41
05
47
81
58
87

*
*HAROWT

40
70
10
10

C
C
C
C
50 C
66
60 C
60 C
60 C
00 C
10 C
EO C
01 C
10 C
17 C
10 C
64 C
50 C
14 C

2CA

4F2
4FA
4FE

CYLP

STH
STH
STH
L

HDLP

SR
STH
STH
STH
LA
L

BAL
TM
BC
CLC
BC
BXLE
L

BXLE

*
*
*
*
000160
000164
000168
00016C
000170
000114

41
S8
45
91
47
45

000178

45 EO C 166

00
10
EO
01
10
EO

C
C
C
C
C
C

lEE

•

2E6
0711
3EF
06A

166

R5,R5
R2,C1
RS,BBCCHH+4
R5,FCCHH+4
RS,ROCNT
R3,C9
R4,R4
R4,BBCCHH+6
R4,FCCHH+6
R4,ROCNT+2
SP,WRDKHA
PL,CI01
R,EXCP
CH1DV1,1
1,*-4
ROCMPR(Z4),ROCNT
7,HAROER
R4,RZ,HDLP
R3,C202
RS,R2,CYLP

SR
L

202

4F4
4FC
500
LAE
2E6
0711
3EF
040
51E C4FE
016
028
206
016

WRITE HOME ADDRESSES AND TRACK DESCRIPTOR RECORDS

•

*HARDER

SET CYLINDER REGISTER TO 0
SET INCREMENT TO 1
STORE CYLINDER NUMBER FOR SEEK
STORE CYLINDER NUMBER FOR WRITE
STORE CYLINDER IN TRACK OES REC
SET HEAD LOOP CO~PARE TO 9
S&T HEAD REGISTER TO 0
STORE HEAD NU~BER FOR SEEK
STORE HEAD NUMBER FOR WRITE
STORE HEAD IN TRACK DES. REC.
WRITE HOME ADORESS AND
TRACK DESCRIPTOR RECORD
ON THIS TRACK
WAIT FOR READ BACK COMPLETE
READ BACK CHECK
INCREMENT HEAD AND RECYCLE
SET CYLINDER LOOP COMPARE TO 202
INCRE~ENT CYLINDER AND RECYCLE

WRITE THREE RECORDS ON CYLINDER 3, TRACK 7
READ THEM BACK AND LOOK FOR A FOURTH (NOT THERE)
L
B'AL
TM
BC
BAL

SP,WR3REC
PL,CID1
R, EXCP
CHIDV1,l
1, *-4
R,DUMP

BAL

R,DUMP

READ BACK CHECK FAILED

11:

•
•
00017C
000180
000184
000188
00018C
000190
000194
000198
00019C
0001AO

80
50
9C
47
50
41
92
58
80
07

00
00
00
10
70
11

01
70
00
FE

C 2C7
0 048
1 000
C OAO
C 276
C 2EE
1 000
C 276
C 2C8

EXCP

CaNTIN

START 1/0 SUBROUTINE
SSM
ST
SIO
BC
ST
LA
MVI
L

SSM
BCR
EJECT

CISABL
SP,12(0,0)
O(PL)
7,BlS10
R5,REGS
RS,OEVTAB(Pl)
O(RS),l
RS,REGS
ENABLE
15,R

DISABLE SYSTEM FROM INTERRUPTS
SP CONTAINS CAW
PL CONTAINS DEVICE ADDRESS
TEST FOR UNUSUAL CONDITIONS
DEVICE STARTED.
SET BIT IN DEVICE TABLE
CALLER BAL R, EXCP AND
R CONTAINS RETURN TO CALLER

2A41/2311

PROGRA~MING

EXAMPLE

..•
•
•
..
0002BO
000288
0002eo
0002e8
000200
000208
O('l02EO
0002ES

IF
01
19
15
39
OS
16
03

0002FO
0002ER
000300
00030A
OQ 0 31 0
000318
000320
000328
OOOUO
000338
00034 0
000348
000350
000358
000360
000368

IF
07
39
0.8

OQOlIO

000378

oooHa

0003eB
0005F2
0005fB
000600
0005FC
0002E8
000620
0002E8

0003eB
00063A
00063C
000310
00063e
000370
000641
000A59
000A91
000641

08
10
10
10
31
08 oooua
OE OOOCAC
IE 0010Re
lE 0010f4
31 00130C
08 000360

ooouo

4COO
4COO
4COO
4COO
4000
OCOO
OCOO
COOO

0001
0006
0005
001S
0004
OOOQ
0018
0000

CHANNEl COfolMANC WORD 1I STS
WR ITE HOME ADDRESSES AND DESCRIPTOR RECORDS

WRCKHA

ecw
ccw
CCW
CCW
CCW
CCW
ecw
ccw

•
...
4COO
4COO
4COO
0000
4COO
CCOO
4COO
4COO
4COO
4COO
OCOO
4COO
4COO
4COO
4COO
CCOO

0001
0006
0004
0000
0005
0000
C418
0038
0218
0005
0000
0410
0038
0218
0005
0000
OCOO OQOQ

..NQRCH)
...•

CCW
CCW
Ccw
ecw
ccw
ccw
CCW
ecw
ccw
Ccw
CCW
CCW
ccw
CCW
CCW
CCW

3l.HAMASK.64.1
07,RECO-2,64,6
51.RECO.64.4
OA,N{)RCFD,Q,O
49.RECO.64.5
08,NORCFD,O,0
29.RECl.64.1048
29,REC2,64,56
29.REC3.64.536
49,RECl,64,5
08.*-8,0,0
14,RECIKO,64,1040
30.REC2CD.64.S6
30,REC3CO,64,536
49,REC4.64.5
08,*-8,0,0

COl

03 .. ,0,0

READ RO
ERROR EXIT

SE T FILE foIASK
SEEK eYl 3 TRK 7
SEARCH HA
SEARCH BO EOUAl
WRITE REe 1
WR I TE REC 2
WRITE REC 3
SEARCH Rl 10
TIC EQR SEARCH
READ Rl KD
REAP R2 CKO
READ R3 CKD
LOOK FOR R4
TIC FOR SEARCH
RECORD NCT THEBE

EXIT

SAVE AREA FOR GENERAL REG I STERS

os.

16F
00

EJECT

SET FILE MASK
SEEK TRACK
WR I TE HofolE ADORESS
IoiR I TE TRACK DESCRIPTCR RECORD
SEARCH HA EOUAl

WRITE AND REAP BACK THREE RECORPS

WfOREC

REGS

31,HAMASK,X'4C',1
01.BBCCHH+2.X'40'.6
25,FCCHH+3,X'40',5
21.ROCNT.X'40'.24
57;FCCHH+4,X'40',4
OS,.+16,O,C
22,ROCMPR,Q,24
03,·,0.0

§II]

2841/2311

PROGRA~MING

OOOJBA
OOOlBD
0003CO
0003C4
0003C8
0003Cq

0003CA
0003C8
0003CC
000100
000304
000308
0003DC
OOOlFO
00031:4
OOOlES
0003FO

EXA~PlE

•
•

0000000000
0001FC
FFOOOOCO

NICPSW

00
00
FF
CO
00000001
00000C04
00000009
OOCOOCCA
00000001
00000002
00000100
00000101

ECFFlG
DISA8l

APSW

E"'~BlE
H.6~.ASK

Cl
C4
C<.l
C202
COllI
COD2
CIDO
CllB
DEVTAA
0003FO

0003FO
0003Fl
000JF2

OC
00
00

0004FO
0004Fl
0004F2

00
00
00

CHOOVO
CHCOVI
CHODV2
0004FO
CHICVO
CHIDVI
CHICV2
0005FO

0005FO
0005F4
0005FA
0005FC
000600
000608
00C611
OC061A
0006lC
000620
0006J8
00C63C
000641
00064Q

000659
OOOA 'jQ
000A61
OOOA 71
0001\ 91
OOOA 99
00OAA2
000AA9
OOOC AC
0010t3C
0010F4
OODOC

00000000
00000000
00000000
OOOOOOCO
0000000000000010
000000000000000000
OOOOOOOOOCCCOO
OOOOOOOC
00000000

•

DC
DC
DC
OS
DC
DC
DC
DC
DC
DC
DC
DC
DC
DC
DC
DC
OS
ORG
DC
DC
DC
ORG
DC
DC
DC
ORG

Xl5'OOOOOOOOOO'
Al3{INTl
X'FFOOOOOO'
F
X'OO'
X'OO'
X'FF'
X'CC'
F '1'
F'4'
F'<.l'
F'202'
F I I'
F I 2'
F ' 256'
F ' 257'
64C
OEVTA8
X'OO'
X'OO'
X'OO'
CEVTAR+256
X'OO'
X'OO'
X'OO'
DEVTAB+512
RECORD AREAS
2F'0'

FILE MASK-ALL WRITES-All SEEKS

READER SYSINI
PRINTER SYSOU1
TAPE IN/Ol;T SYSUT4
2311
SYSCKI
STATUS OF DEVICES
ONE ECUAlS BUSY
lERO EQUALS FREE.

8BCCHH

FCCHH

2F'0'

HO~E

ROCNT
ROCATA

DC
DC

XLS ' I0'
Xl16'0'

RO COUNT F I HD
RO OATA FIElD

HAC~PR

2F'O'

HA COMPARE

ROC~PR

OS
CC
CC
DC
OS
OS
DC
OS
OS
OC
DC

6F
F'O'
X'00030007CO'
X'00030007011004CO'
Cll6
64Cl16
X'OO030007C21C0020'
Cll6
2CL16
X'OO030007031C0200'
CL16'RECORC 3 KEYKEY

TRACK RO

OS
OS
OS
OS

32CLl6
260F
14F
134F
X'0003000704'

OOOOOCOO
0003000700
00030007011C0400

RECO
RECI

00030007021C0020

REC2

00030007031 C0200
09C5C3060QC440F340
02C5E802C5E840

REC3

0003000704

CONSTANTS

RECIKO
REC2CC
REC3CC
REC4

•

SEEK ADDRESS

00 BB CC HH

ADDRESS

00 OF CC HH

00 OX XX XX

CC~PARE

AREA

RO 10
COUNT
KEY
DATA
COUNT
KEY
DA TA
COUNT
KEY
DATA
BUFFER FOR READ BACK
RECORD NOT THERE YET

APPENDIX B.

HEXADECIMAL-DECIMAL CONVERSION

The table in this appendix provides for direct conversion of decimal and hexadecimal numbers in these
ranges:

Hexadecimal
4000
5000
6000
7000
8000
9000
AOOO
BOOO
COOO
DOOO
EOOO
FOOO

Decimal
0000 to 4095

Hexadecimal
000 to FFF

For numbers outside the range of the table, add
the following values to the table figures:
Hexadecimal
1000
2000
3000

Decimal
4096
8192
12288

I
I
I
I
I
I

r---------------------J

I
I
I

I
I

Ir------------------------------------J
II
II
II

04 _
05 _
06_
07 _
08 _
09 _
OA_
OB_
OC_
OD_
OE _
OF _

10_
11_
12 _
13 _
14 _
15 _
16 _
17 _

r----- -- 9

r------ E

r------ 1

Decimal
16384
20480
24576
28672
32768
36864
40960
45056
49152
53248
57344
61440

r------------'
B

0000
0016
0032
0048
0064
0080
0096
0112
0128
0144
0160
0176
0192
0208
0224
0240

0001
0017
0033
0049
0065
0081
0097
0113
0129
0145
0161
0177
0193
0209
0225
0241

0002
0018
0034
0050
0066
0082
0098
0114
0130
0146
0162
0178
0194
0210
0226
0242

0003
0019
0035
0051
0067
0083
0099
0115
0131
0147
0163
0179
0195
0211
0227
0243

0004
0020
0036
0052
0068
0084
0100
0116
0132
0148
0164
0180
0196
0212
0228
0244

0005
0021
0037
0053
0069
0085
0101
0117
0133
0149
0165
0181
0197
0213
0229
0245

0006
0022
003S
0054
0070
0086
0102
0118
0134
0150
0166
0182
0198
0214
0230
0246

0007
0023
0039
0055
0071
0087
0103
0119
0135
0151
0167
0183
0199
0215
0231
0247

0010
0026
0042
0058
0074
0090
0106
0122
0138
0154
0170
0186
0202
0218
0234
0250

0011
0027
0043
0059
0075
0091
0107
0123
0139
0155
0171
0187
0203
0219
0235
0251

0012
0028
0044
0060
0076
0092
0108
0124
0140
0156
0172
0188
0204
0220
0236
0252

0013
0029
0045
0061
0077
0093
0109
0125
0141
0157
0173
0189
0205
0221
0237
0253

0014
0030
0046
0062
0078
0094
0110
0126
0142
0158
0174
0190
0206
0222
0238
0254

0015
0031
0047
0063
0079
0095
0111
0127
0143
0159
0175
0191
0207
0223
0239
0255

0256
0272
0288
0304
0320
0336
0352
0368
0384
0400
0416
0432

0257
0273
0289
0305
0321
0337
0353
0369
0385
0401
0417
0433

0258
0274
0290
0306
0322
0338
0354
0370

0259
0275
0291
0307
0323
0339
0355
0371
0387
0403
0419
0435

0260
0276
0292
0308
0324
0340
03.56
0372
0388
0404
0420
0436

0261
0277
0293
0309
0325
0341
0357
0373
0389
0405
0421
0437

0262
0278
0294
0310
0326
0342
0358
0374
0390
0406
0422
0438 -

0263
0279
0295
0311
0327
0343
0359
0375
0391
0407
0423
0439

0266
0282
0298
0314
0330
0346
0362
0378
0394
0410
0426
0442
0458
0474
0490
0506

0267
0283
0299
0315
0331
0347
0363
0379
0395
0411
0427
0443
0459
0475
0491
0507

0268
0284
0300
0316
0332
0348
0364
0380
0396
0412
0428
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0460
0476
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0269
0285
0301
0317
0333
0349
0365
0381
0397
0413
0429
0445
0461
0477
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0270
0286
0302
0318
0334
0350
0366
0382
0398
0414
0430
0446
0462
0478
0494
0510

0271

02~7

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0383
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0431
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0463
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20£+0:12
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0522
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065.5
0671
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0719
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0914
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1013

0774
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0806
0822
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0918
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1014

0775
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0919
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0824
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0888
0904
0920
0936
0952
0968
0984
1000
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0777
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0809
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0889
0905
0921
0937
0953
0969
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0778
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0810
0826
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0890
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0922
0938
0954
0970
0986
1002
1018

0779
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0811
0827
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0859
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0891
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0923
0939
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0971
0987
1003
1019

0780
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0812
0828
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0860
0876
0892
0908
0924
0940
0956
0972
0988
1004
1020

0781
0797
0813
0829
0845
0861
0877
0893
0909
0925
0941
0957
0973
0989
1005
1021

0782
0798
0814
0830
0846
0862
0878
0894
0910
0926
0942
0958
0974
0990
1006
1022

0783
0799
0815
0831
0847
0863
0879
0895
0911
0927
0943
0959
0975
0991
1007
1023

40_
41_
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46_
47 _
48_
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4C_
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1040
1056
1072
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1104
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1216
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1137
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1201
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1233
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1026
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1058
1074
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1122
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1027
1043
1059
1075
1091
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1028
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1060
1076
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1204
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1029
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1061
1077
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1173
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1205
1221
1237
1253
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1030
1046
1062
1078
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1032
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1160
1176
1192
1208
1224
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1256
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1033
1049
1065
1081
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1177
1193
1209
1225
1241
1257
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1034
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1066
1082
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1114
1130
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1194
1210
1226
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1035
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1179
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1227
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1036
1052
1068
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1100
1116
1132
1148
ll64
1180
1196
1212
1228
1244
1260
1276

1037
1053
1069
1085
1101
1117
1133
1149
ll65
1181
ll97
1213
1229
1245
1261
1277

1038
1054
1070
1086
1102
1118
ll34
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1166
1182
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1214
1230
1246
1262
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1039
1055
1071
1087
1103
ll19
ll35
1151
1167
ll83
ll99
1215
1231
1247
1263
1279

50 _
51 _
52 _
5354 _
55 56 _
57 _

1280
1296
1312
1328
1344
1360
1376
1392
1408
1424
1440
1456
1472
1488
1504
1520

1281
1297
1313
1329
1345
1361
1377
1393
1409
1425
1441
1457
1473
1489
1505
1521

1282
1298
1314
1330
1346
1362
1378
1394
1410
1426
1442
1458
1474
1490
1506
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1283
1299
1315
1331
1347
1363
1379
1395
14ll
1427
1443
1459
1475
1491
1507
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1284
1300
1316
1332
1348
1364
1380
1396
1412
1428
1444
1460
1476
1492
1508
1524

1285
1301
1317
1333
1349
1365
1381
1397
1413
1429
1445
1461
1477
1493
1509
1525

1286
1302
1318
1334
1350
1366
1382
1398
1414
1430
1446
1462
1478
1494
1510
1526

1287
1303
1319
1335
1351
1367
1383
1399
1415
1431
1447
1463
1479
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1527

1288
1304
1320
1336
1352
1368
1384
1400
1416
1432
1448
1464
1480
1496
1512
1528

1289
1305
1321
1337
1353
1369
1385
1401
1417
1433
1449
1465
1481
1497
1513
1529

1290
1306
1322
1338
1354
1370
1386
1402
1418
1434
1450
1466
1482
1498
1514
1530

1291
1307
1323
1339
1355
1371
1387
1403
1419
1435
1451
1467
1483
1499
1515
1531

1292
1308
1324
1340
1356
1372
1388
1404
1420
1436
1452
1468
1484
1500
1516
1532

1293
1309
1325
1341
1357
1373
1389
1405
1421
1437
1453
1469
1485
1501
1517
1533

1294
1310
1326
1342
1358
1374
1390
1406
1422
1438
1454
1470
1486
1502
1518
1534

1295
1311
1327
1343
1359
1375
1391
1407
1423
1439
1455
1471
1487
1503
1519
1535

5859 5A_
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61 _
1552
62 _
1568
63_
1584
64 _
1600
65 _
1616
66_
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67 _
1648
68_
1664
69 _
1680
6A_
1696
6B_
1712
6C_
1728
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1744
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1760
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1776
70_
1792
71_
1808
72 _
1824
73_
1840
74 _
1856
75 _
1872
76 _
1888
77 _
1904
78 _
1920
79_
1936
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1952
7B_
1968
7C_
1984
7D_
2000
7E_
2016
7F_
2032

80_
81_
82 _
83_
84 _
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86_
87 _
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89_
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90_
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94 _
95 _
96 _
97 _
98 99 _
9A_
9B_
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1547
1563
1579
1595
1611
1627
1643
1659
1675
1691
1707
1723
1739
1755
1771
1781

1548
1564
1580
1596
1612
1628
1644
1660
1676
1692
1708
1724
1740
1756
1772
1788

1549
1565
1581
1597
1613
1629
1645
1661
1677
1693
1709
1725
1741
1757
1773
1789

1550
1566
1582
1598
1614
1630
1646
1662
1678
1694
1710
1726
1742
1758
1774
1790

1551
1567
1583
1599
1615
1631
1647
1663
1679
1695
1711
1727
1743
1759
1775
1791

1804
1820
1836
1852
1868
1884
1900
1916
1932
1948
1964
1980
1996
2012
2028
2044

1805
1821
1837
1853
1869
1885
1901
1917
1933
1949
1965
1981
1997
2013
2029
2045

1806
1822
1838
1854
1870
1886
1902
1918
1934
1950
1966
1982
1998
2014
2030
2046

1807
1823
1839
1855
1871
1887
1903
1919
1935
1951
1967
1983
1999
2015
2031
2047

1537
1553
1569
1585
1601
1617
1633
1649
1665
1681
1697
1713
1729
1745
1761
1777

1538
1554
1570
1586
1602
1618
1634
1650
1666
1682
1698
1714
1730
1746
1762
1778

1539
1555
1571
1587
1603
1619
1635
1651
1667
1683
1699
1715
1731
1747
1763
1779

1540
1556
1572
1588
1604
1620
1636
1652
1668
1684
1700
1716
1732
1748
1764
1780

1541
1557
1573
1589
16005
1621
1637
1653
1669
1685
1701
1717
1733
1749
1765
1781

1542
1558
1574
1590
1606
1622
1638
1654
1670
1686
1702
1718
1734
1750
1766
1782

1543
1559
1575
1591
1607
1623
1639
1655
1671
1687
1703
1719
1735
1751
1767
1783

1544
1560
1576
1592
1608
1624
1640
1656
1672
1688
1704
1720
1736
1752
1768
1784

1545
1561
1577
1593
1609
1625
1641
1657
1673
1689
1705
1721
1737
1753
1769
1785

1546
1562
1578
1594
1610
1626
1642
1658
1674
1690
1706
1722
1738
1754
1770
1786

1793
1809
1825
1841
1857
1873
1889
1905
1921
1937
1953
1969
1985
2001
2017
2033

1794
1810
1826
1842
1858
1874
1890
1906
1922
1938
1954
1970
1986
2002
2018
2034

1795
1811
1827
1843
1859
1875
1891
1907
1923
1939
1955
1971
1987
2003
2019
2035

1796
1812
1828
1844
1860
1876
1892
1908
1924
1940
1956
1972
1988
2004
2020
2036

1797
1813
1829
1845
1861
1877
1893
1909
1925
1941
1957
1973
1989
200.5
2021
2037

1798
1814
1830
1846
1862
1878
1894
1910
1926
1942
1958
1974
1990
2006
2022
2038

1799
1815
1831
1847
1863
1879
1895
1911
1927
1943
1959
1975
1991
2007
2023
2039

1800
1816
1832
1848
1864
1880
1896
1912
1928
1944
1960
1976
1992
2008
2024
2040

1801
1817
1833
1849
1865
1881
1897
1913
1929
1945
1961
1977
1993
2009
2025
2041

1802 1803
1818 1819
1834 1835
1850 1851
1866 1867
1882 1883
1898 1899
1914 1915
1930 1931
1946 1947
1962 1963
1978 1979
1994 1995
2010 2011
2026 2027
2042 2043

2048
2064
2080
2096
2112
2128
2144
2160
2176
2192
2208
2224
2240
22.56
2272
2288

2049
2065
2081
2097
2113
2129
2145
2161
2177
2193
2209
222.5
2241
2257
2273
2289

2050
2066
2082
2098
2114
2130
2146
2162
2178
2194
2210
2226
2242
2258
2274
2290

2051
2067
2083
2099
2115
2131
2147
2163
2179
2195
2211
2227
2243
2259
2275
2291

2052
2068
2084
2100
2116
2132
2148
2164
2180
2196
2212
2228
2244
2260
2276
2292

2053
2069
2085
2101
2117
2133
2149
2165
2181
2197
2213
2229
2245
2261
2277
2293

2054
2070
2086
2102
2118
2134
2150
2166
2182
2198
2214
2230
2246
2262
2278
2294

2055
2071
2087
2103
2119
2135
2151
2167
2183
2199
2215
2231
2247
2263
2279
2295

20056
2072
2088
2104
2120
2136
2152
2168
2184
2200
2216
2232
2248
2264
2280
2296

2057
2073
2089
2105
2121
2137
2153
2169
2185
2201
2217
2233
2249
2265
2281
2297

2058
2074
2090
2106
2122
2138
2154
2170
2186
2202
2218
2234
2250
2266
2282
2298

2059
2075
2091
2107
2123
2139
2155
2171
2187
2203
2219
2235
2251
2267
2283
2299

2060
2076
2092
2108
2124
2140
2156
2172
2188
2204
2220
2236
2252
2268
2284
2300

2061
2077
2093
2109
2125
2141
2157
2173
2189
2205
2221
2237
2253
2269
2285
2301

2062
2078
2094
2110
2126
2142
2158
2174
2190
2206
2222
2238
2254
2270
2286
2302

2063
2079
2095
2111
2127
2143
2159
2175
2191
2207
2223
2239
2255
2271
2287
2303

2304
2320
2336
2352
2368
2384
2400
2416
2432
2448
2464
2480
2496
2512
2528
2544

2305
2321
2337
2353
2369
2385
2401
2417
2433
2449
2465
2481
2497
2513
2529
2545

2306
2322
2338
2354
2370
2386
2402
2418
2434
2450
2466
2482
2498
2514
2530
2546

2307
2323
2339
2355
2371
2387
2403
2419
2435
2451
2467
2483
2499
2515
2531
2547

2308
2324
2340
2356
2372
2388
2404
2420
2436
2452
2468
2484
2.500
2516
2532
2548

2309
2325
2341
2357
2373
2389
2405
2421
2437
2453
2469
2485
2501
2517
2533
2549

2310
2326
2342
2358
2374
2390
2406
2422
2438
2454
2470
2486
2502
2518
2534
2550

2311
2327
2343
2359
2375
2391
2407
2423
2439
2455
2471
2487
2503
2519
2535
2551

2312
2328
2344
2360
2376
2392
2408
2424
2440
2456
2472
2488
2504
2520
2536
2552

2313
2329
2345
2361
2377
2393
2409
2425
2441
.2457
2473
2489
2505
2521
2537
2553

2314
2330
2346
2362
2378
2394
2410
2426
2442
2458
2474
2490
2506
2522
2538
2554

2315
2331
2347
2363
2379
2395
2411
2427
2443
2459
2475
2491
2507
2523
2539
2555

2316
2332
2348
2364
2380
2396
2412
2428
2444
2460
2476
2492
2508
2524
2540
2556

2317
2333
2349
2365
2381
2397
2413
2429
2445
2461
2477
2493
2509
2525
2541
2557

2318
2334
2350
2366
2382
2398
2414
2430
2446
2462
2478
2494
2510
2526
2542
2558

2319
2335
2351
2367
2383
2399
2415
2431
2447
2463
2479
2495
2511
2527
2543
2559

2561
2577
2593
2609
262.5
2641
2657
2673
2689
2705
2721
2737
2753
2769
2785
2801

2562
2578
2594
2610
2626
2642
2658
2674
2690
2706
2722
2738
2754
2770
2786
2802

2563
2579
2595
2611
2627
2643
2659
2675
2691
2707
2723
2739
2755
2771
2787
2803

2564
2580
2596
2612
2628
2644
2660
2676
2692
2708
2724
2740
2756
2772
2788
2804

2565
2581
2597
2613
2629
2645
2661
2677
2693
2709
2725
2741
2757
2773
2789
2805

2566
2582
2598
2614
2630
2646
2662
2678
2694
2710
2726"
2742
2758
2774
2790
2806

2567
2583
2599
2615
2631
2647
2663
2679
2695
2711
2727
2743
2759
2775
2791
2807

2568
2584
2600
2616
2632
2648
2664
2680
2696
2712
2728
2744
2760
2776
2792
2808

2569
2585
2601
2617
2633
2649
2665
2681
2697
2713
2729
2745
2761
2777
2793
2809

2570
2586
2602
2618
2634
2650
2666
2682
2698
2714
2730
2746
2762
2778
2794
2810

2571
2587
2603
2619
2635
2651
2667
2683
2699
2715
2731
2747
2763
2779
2795
2811

2572
2588
2604
2620
2636
2652
2668
2684
2700
2716
2732
2748
2764
2780
2796
2812

2573
2589
2605
2621
2637
2653
2669
2685
2701
2717
2733
2749
2765
2781
2797
2813

2574
2590
2606
2622
2638
2654
2670
2686
2702
2718
2734
2750
2766
2782
2798
2814

2575
2591
2607
2623
2639
2655
2671
2687
2703
2719
2735
2751
2767
2783
2799
2815

2817
2833
2849
2865
2881
2897
2913
2929
2945
2961
2977
2993
3009
3025
3041
3057

2818
2834
2850
2866
2882
2898
2914
2930
2946
2962
2978
2994
3010
3026
3042
3058

2819
2835
2851
2867
2883
2899
2915
2931
2947
2963
2979
2995
3011
3027
3043
3059

2820
2836
2852
2868
2884
2900
2916
2932
2948
2964
2980
2996
3012
3028
3044
3060

2821
2837
2853
2869
2885
2901
2917
2933
2949
296.5
2981
2997
3013
3029
3045
3061

2822
2838
2854
2870
2886
2902
2918
2934
2950
2966
2982
2998
3014
3030
3046
3062

2823
2839
2855
2871
2887
2903
2919
2935
2951
2967
2983
2999
3015
3031
3047
3063

2824
2840
2856
2872
2888
2904
2920
2936
2952
2968
2984
3000
3016
3032
3048
3064

2825
2841
2857
2873
2889
2905
2921
2937
2953
2969
2985
3001
3017
3033
3049
3065

2826
2842
2858
2874
2890
2906
2922
2938
2954
2970
2986
3002
3018
3034
3050
3066

2827
2843
2859
2875
2891
2907
2923
2939
2955
2971
2987
3003
3019
3035
3051
3067

2828
2844
2860
2876
2892
2908
2924
2940
2956
2972
2988
3004
3020
3036
3052
3068

2829
2845
2861
2877
2893
2909
2925
2941
2957
2973
2989
3005
3021
3037
3053
3069

2830
2846
2862
2878
2894
2910
2926
2942
2958
2974
2990
3006
3022
3038
3054
3070

2831
2847
2863
2879
2895
2911
2927
2943
2959
2975
2991
3007
3023
3039
3055
3071

3072
3088
3104
3120
3136
3152
3168
3184
3200
3216
3232
3248
3264
3280
3296
3312

3073
3089
3105
3121
3137
3153
3169
3185
3201
3217
3233
3249
3265
3281
3297
3313

3074
3090
3106
3122
3138
3154
3170
3186
3202
3218
3234
3250
3266
3282
3298
3314

3075
3091
3107
3123
3139
3155
3171
3187
3203
3219
3235
3251
3267
3283
3299
3315

3076
3092
3108
3124
3140
3156
3172
3188
3204
3220
3236
3252
3268
3284
3300
3316

3077
3093
3109
3125
3141
3157
3173
3189
3205
3221
3237
3253
3269
3285
3301
3317

3078
3094
3110
3126
3142
3158
3174
3190
3206
3222
3238
3254
3270
3286
3302
3318

3079
3095
3111
3127
3143
3159
3175
3191
3207
3223
3239
3255
3271
3287
3303
3319

3080
3096
3112
3128
3144
3160
3176
3192
3208
3224
3240
3256
3272
3288
3304
3320

3081
3097
3113
3129
3145
3161
3177
3193
3209
3225
3241
3257
3273
3289
3305
3321

3082
3098
3114
3130
3146
3162
3178
3194
3210
3226
3242
3258
3274
3290
3306
3322

3083
3099
3115
3131
3147
3163
3179
3195
3211
3227
3243
3259
3275
3291
3307
3323

3084
3100
3116
3132
3148
3164
3180
3196
3212
3228
3244
3260
3276
3292
3308
3324

3085
3101
3117
3133
3149
3165
3181
3197
3213
3229
3245
3261
3277
3293
3309
3325

3086
3102
3118
3134
3150
3166
3182
3198
3214
3230
3246
3262
3278
3294
3310
3326

3087
3103
3119
3135
3151
3167
3183
3199
3215
3231
3247
3263
3279
3295
3311
3327

3328
3344
3360
3376
3392
3408
3424
3440
3456
3472
3488
3504
3520
3536
3552
3568

3329
3345
3361
3377
3393
3409
3425
3441
3457
3473
3489
3505
3521
3537
3553
3569

3330
3346
3362
3378
3394
3410
3426
3442
3458
3474
3490
3506
3522
3538
3554
3570

3331
3347
3363
3379
3395
3411
3427
3443
3459
3475
3491
3507
3523
3539
3555
3571

3332
3348
3364
3380
3396
3412
3428
3444
3460
3476
3492
3508
3524
3540
3556
3572

3333
3349
3365
3381
3397
3413
3429
3445
3461
3477
3493
3509
3525
3541
3557
3573

3334
3350
3366
3382
3398
3414
3430
3446
3462
3478
3494
3510
3526
3542
3558
3574

3335
3351
3367
3383
3399
3415
3431
3447
3463
3479
3495
3511
3527
3543
3559
3575

3336
3352
3368
3384
3400
3416
3432
3448
3464
3480
3496
3512
3528
3544
3560
3576

3337
3353
3369
3385
3401
3417
3433
3449
"3465
3481
3497
3513
3529
3545
3561
3577

3338
3354
3370
3386
3402
3418
3434
3450
3466
3482
3498
3514
3530
3546
3562
3578

3339
3355
3371
3387
3403
3419
3435
3451
3467
3483
3499
3515
3531
3547
3563
3579

3340
3356
3372
3388
3404
3420
3436
3452
3468
3484
3500
3516
3532
3548
3564
3580

3341
3357
3373
3389
3405
3421
3437
3453
3469
3485
3501
3517
3533
3549
3565
3581

3342
3358
3374
3390
3406
3422
3438
3454
3470
3486
3502
3518
3534
3550
3566
3582

3343
3359
3375
3391
3407
3423
3439
3455
3471
3487
3503
3519
3535
3551
3567
3583

~HO
AO
Al
A2
A3
A4
A5
A6
A7
A8
A9

_ 2.560
_ 2576
_ 2592
_ 2608
_ 2624
_ 2640
_ 2656
_ 2672
_ " 2688
_ 2704
AA_
2720
AB_ 2736
AC_ 2752
AD_ 2768
AE _ 2784
AF _
2800
BO _ 2816
Bl _ 2832
B2 _
2848
B3 _
2864
B4 _ 2880
2896
B5 B6 _
2912
B7 _
2928
B8 _
2944
B9 _
2960
BA_
2976
BB_
2992
BC_ 3008
BD_
3024
BE_
3040
BF _
3056

CO
Cl
C2
C3
C4
C5
C6
C7

_
_
_
_
_
_
_
_

C8 C9 _
CA_
-CB _
CC_
CD_
CE_
CF _
DO_
Dl_
D2 _
D3_
D4 _
D5 _
06 _

D7_
D8_
D9_
DA_
DB_
DC_
DD_
DE_
DF_

+-to

Eo_

3584
3600
3616
3632
3648
3664
3680
3696
3712
3728
3744
3760
3776
3792
3808
3824

3585
3601
3617
3633
3649
3665
3681
3697
3713
3729
3745
3761
3777
3793
3809
3825

3586
3602
3618
3634
3650
3666
3682
3698
3714
3730
3746
3762
3778
3794
3810
3826

3587
3603
3619
3635
3651
3667
3683
3699
3715
3731
3747
3763
3779
3795
3811
3827

3583
3604
3620
3636
3652
3668
3684
3700
3716
3732
3748
3764
3780
3796
3812
3828

3589
3605
3621
3637
3653
3669
3685
3701
3717
3733
3749
3765
3781
3797
3813
3829

3590
3606
3622
3638
3654
3670
3686
3702
3718
3734
3750
3766
3782
3798
3814
3830

3591
3607
3623
3639
3655
3671
3687
3703
3719
3735
3751
3767
3783
3799
3815
3831

3592
3608
3624
3640
3656
3672
3688
3704
3720
3736
3752
3768
3784
3800
3816
3832

3593
3609
3625
3641
3657
3673
3689
3705
3721
3737
3753
3769
3785
3801
3817
3833

3594 3595
3610 . 3611
3626 3627
3642 3643
3658 3659
3674 3675
3690 3691
3706 3707
3722 3723
3738 3739
3754 3755
3770 3771
3786 3787
3802 3803
3818 3819
3834 3835

3596
3612
3628
3644
3660
3676
3692
3708
3724
3740
3756
3772
3788
3804
3820
3836

3597
3613
3629
3645
3661
3677
3693
3709
3725
3741
3757
3773
3789
3805
3821
3837

3598
3614
3630
3646
3662
3678
3694
3710
3726
3742
3758
3774
3790
3806
3822
3838

3599
3615
3631
3647
3663
3679
3695
3711
3727
3743
3759
3775
3791
3807
3823
3839

FO_
Fl_
F2 _
F3 _
F4 _
F5 _
F6_

3840
3856
3872
3888
3904
3920
3936
3952
3968
3984
4000
4016
4032
4048
4064
4080

3841
3857
3873
3889
3905
3921
3937
3953
3969
3985
4001
4017
4033
4049
4065
4081

3842
3858
3874
3890
3906
3922
3938
3954
3970
3986
4002
4018
4034
4050
4066
4082

3843
3859
3875
3891
3907
3923
3939
3955
3971
3987
4003
4019
4035
4051
4067
4083

3844
3860
3876
3892
3908
3924
3940
3956
3972
3988
4004
4020
4036
4052
4068
4084

3845
3861
3877
3893
3909
3925
3941
3957
3973
3989
4005
4021
4037
4053
4069
4085

3846
3862
3878
3894
3910
3926
3942
3958
3974
3990
4006
4022
4038
4054
4070
4086

3847
3863
3879
3895
3911
3927
3943
3959
3975
3991
4007
4023
4039
4055
4071
4087

3848
3864
3880
3896
3912
3928
3944
3960
3976
3992
4008
4024
4040
4056
4072
4088

3849
3865
3881
3897
3913
3929
3945
3961
3977
3993
4009
4025
4041
4057
4073
4089

3850
3866
3882
3898
3914
3930
3946
3962
3978
3994
4010
4026
4042
4058
4074
4090

3852
3868
3884
3900
3916
3932
3948
3964
3980
3996
4012
4028
4044
4060
4076
4092

3853
3869
3885
3901
3917
3933
3949
3965
3981
3997
4013
4029
4045
4061
4077
4093

3854
3870
3886
3902
3918
3934
3950
3966
3982
3998
4014
4030
4046
4062
4078
4094

3855
3871
3887
3903
3919
3935
3951
3967
3983
3999
4015
4031
4047
4063
4079
4095

El_
E2E3E4_
E5_
E6_
E7 _
E8_
E9_
EA_
EB_
EC_
ED_
EE_
EF_

F7_

F8_
F9_

FA_
FB_
FCFD_
FE_
FF_

3851
3867
3883
3899
3915
3931
3947
3963
3979
3995
4011
4027
4043
4059
4075
4091

COMMAND CODE
Multiple Track On
(If Applicable)

Multiple Track Off
COMMAND
Control

Write

Binary

Decimal Hexadec irna I

DATA ADDRESS

Binary

03
17
13
07
OB
1B
04

0000
0001
0001
0000
0000
0001
0000

Set File Mask
Space Record
Transfer in Channel

3 1
15
X8

1F
OF
X8

0001 1111
0000 1111
XXXX 1000

57
49
8 1
1 13
4 1
73
105
45

39
3 1
5 1
71
29
49
69
20

0011
0011
0101
0111
0010
0100
0110
0010

1001
0001
0001
0001
1001
1001
1001
1101

185
177
209
241
169
:2 0 1
233
1 73

B9
8 1
o1
F 1
A9
C9
E9
AD

1011 1001
10110001
11010001
1111 0001
1010 1001
1100 1001
1110 1001
1010 1101

77
109

40
60

0100 1101
0110 1101

205
237

CD
ED

11001101
1110 1101

Home Address
Count
Record RO
Data
Key and Data
Count, Key and Data

26
18
22
06
14
30

lA
12
16
06
OE
1E

0001
0001
0001
0000
0000
0001

154
146
150
134
142
158

9A
92
96
86
8 E
9 E

1001 1010
1001 0010
10010110 \ CPU ,to'age 10coHon to whkh
read will be transferred
10000110
1000 1110
1001 1110

Home Address
Record RO'

25
2 1

19
15

0001 1001
0001 0101

Count, Key and Data
Special Count, Key and Data*
Data
Key and Data

29
01
05
13

10
a1
05
00

0001 1101
0000 0001
00000101
0000 1101

Home Address Equa I
Identifier Equal
Identifier High
Identifier Equal or High
Key Equal
Key High
Key Equal or High
Key and Data Equa 1*

* Special Feature
X Not Significant

0011
0111
0011
0111
1011
1011
0100

1010
0010
0110
0110
1110
1110

COUNT

X
X
X

03
23
19
07
11
2 7
04

Key and Data High*
Key and Data Equal or High*
Read

Decimal Hexadecimal

NoOp
Release*
Restore
Seek
Seek Cylinder
Seek Head
Sense I/O

} CPU storage location of seek address
CPU storage location to which four
sense bytes are sent
CPU storage location of mask byte
X
CPU storage location of next CCW (Must be divisible by 4)

)
! CPU storage location of search
argument

)

}

1
X
X
4 (usuaIlY~
5 (usually
5 (usually)
5 (usually)
From 1 to 255
From 1 to 255
From 1 to 255

(

X
X
X
6
6
6
4

0'00'

CPU "o'age 10coHon f'om whkh

0'00'

to be written will be transferred

Number of bytes lincluding
mask bytes} in search argument

5
8
Number
Number
Number
Number

of
of
of
of

bytes
bytes
bytes
bytes

to
to
to
to

be
be
be
be

transferred
transferred
transferred
transferred

5 {usually}
8+Key Length + Data Length of
Record RO
8+Key Length + Data Length
8+Key Length + Data Length
Data Length
Key Length + Data Length

A26-5988-2

APPENDIX D. TRACK ORIENTATION

As there is no unique physical address associated with each
record on a given track, the 2841 must have some means of
locating these records. There are seven states of orientation
with respect to a track in the 2841 that are used to accomplish this.
1. Index Point State - Gap between Index Point and
Home Address.
2. Home Address State - Gap between Home Address
and RO.
3. Count State - Gap between Count and Key Fields.
4. Key State - Gop between Key and Data Field.
5. Data State - Gap between Data Field and
succeeding Address Mark or Index Point if this is
the lost record on the track.
6. Address Marker State - Gop between Address
Mark and Count Field.
7. Reset Orientation State - None of the above.
The Reset Orientation State, while not associated with
any specific area of a track does not necessarily imply that
the 2841 has lost orientation. Any time a CCW chain is
broken or a Control Command is performed, the 2841 is set

to this state. The next data command (i. e., read, write or
search) further defines this state to one of the three substates below.
1. Reset Orientation to Index Point State (ROIP).
Orientation state is set to Index Point State upon
detection of the Index Point on the tra.ck.
2. Reset Orientation to Address Marker State
(ROAM). Orientation is set to Address Marker State
upon detection of any Address Marker.
3. Reset Orientation to Address Marker or Index Point
State (ROAM or IP). Orientation is set to either
Index Point State, upon detection of the Index
Poi fit, or to Address Marker State upon detection of
any Address Marker, whichever occurs first.
Read, Write, Search and some Control commands in the
2841 have two types of prerequisities that must be satisfied
to insure proper operation of the command. By considering
command sequence restrictions and orientation requirements,
all vol id command sequences can be constructed and the result of their execution predetermined. The following table
illustrates these two prerequisities and the resulting orientation
state for all data commands:

Command

Command
Prerequisite

Valid Orientation
State at Beginning
of Command

Read CKD

None

ROAM

Data

Read KD

None

Count
ROAM

Data

None

Count
Key
ROAM

Data

Write CKD
(also Write
Special CKD)

Search Equal
Count or Key
Write CKD
Write R~

Count
Key
Data

Data

Write KD

Search Equal
Count of Key

Count
ROAM

Data

Write 0

Search Equa I
Count or Key

Count
Key

Data

Orientation State
at Completion of
Command

Search 10

None

ROAM or IP

Count

Search Key

None

ROAM
Count

Key

Searc:h Home
Address

None

ROIP

Home Address

Read R~

None

Home Address
ROIP

Data

Write R~

Search Equal
Home Address
Write HA

Home Address

Data

Read HA

None

ROIP

Home Address

Write HA

None

ROIP

Home Address

Read IPl

None

ROAM

Data

Read Count

None

ROAM

Count

Control Space
Record

Search (any)
Read (any)

Count
Key

Reset Orientation

Control Erose

Write CKD
Write R~

Count
Key
Data

Reset Orientation

Control NOP

None

Reset Orientation

International Business Machines Corporation
. Data Processing Division
112 East Post Road, White Plains, N. Y. 10601

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