C28 6628 3_OS_Rel_17_System_Control_Blocks_Nov68 3 OS Rel 17 System Control Blocks Nov68

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User Manual: C28-6628-3_OS_Rel_17_System_Control_Blocks_Nov68

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File No. 5360-36
Form C28-6628-3

Systems Reference Library

IBM System/3S0 Operating System
System Control Blocks
This publication shows the formats of the major
control blocks and tables used by more than one
component of the System/360 operating System control program. Descriptions of each field within
the control blocks or tables follow each format
illustration.
The system control blocks described in this
publication will be changed by IBM from time to
time to extend the capabi1ities of the operating
system. Programs should refer to these control
blocks only through the system macro instruction
facilities provided in the operating system.
(For
example, a field of the Data Control Block shou1d
only be referred to by use of the DCBD macro
instruction.) Programs that refer to the contr01
blocks by other means do so at the risk of not
executing correctly in the future.

OS

Preface

This publication contains reference
information about the contents and format
of system control blocks. Most of the control blocks covered in this publication are
used by more than one component of the
System/360 operating System control program. A diagram of each block is followed
by descriptions of its fields. The block
descriptions are ordered alphabetically by
acronym. When a block has different access
method versions, they are arranged under
the block name in this order: SAM, I SAM,
DAM, TAM, GAM.
The pointer diagram previously shown in
this publication has been superseded by a
similar one now shown in the IBM publication IBM System/360 Operating System: Programmer's Guide to Debugging, Form
C28-6670.

The reader of this manual must be familiar with the following publications:
IBM System/360: Principles of Operation, Form A22-6821
IBM System/360 Operating System:
duction, Form C28-6534

Intro-

IBM System/360 Operating system: Concepts and Facilities, Form C28-6535
IBM System/360 Operating System: Supervisor and Data Management Services, Form
C28-6646

Fourth Edition (November, 1968)
This is a major revision of, and obsoletes, C28-6628-2 and
Technical Newsletter N28-2360. The changes in content made
in this edition for this release are enumerated in a summary
of revisions which follows the table of contents. Changes
to the text, and small changes to illustrations, are indicated by a vertical line to the left of the change; changed
or added illustrations are denoted by the symbol • to the
left of the caption.
This edition applies to release 17 of IBM System/360
Operating System and to all subsequent releases until otherwise indicated in new editions or Technical Newsletters.
Changes are continually made to the specifications herein;
before using this publication in connection with the operation of IBM systems, consult the latest IBM System/360 SRL
Newsletter, Form N20-0360, for the editions that are
applicable and current.
Requests for copies of IBM publications should be made to
your IBM representative or to the IBM branch office serving
your locality.
A form for readers' comments is provided at the back of
this publication.
If the form has been removed, comments may
be addressed to IBM Corporation, Programming Systems Publications, Department D58, PO Box 390, Poughkeepsie, N. Y.
12602

@ Copyright International Business Machines Corporation 1966,1967,1968

Contents

SUMMARY OF REVISIONS •

6

SYSTEM CONTROL BLOCKS
MBBCCHHR - Actual Address Format •
Revised Page Format
communication Vector Table •
Data Control Blocks
Data Control Block
QSAM. BSAM. BPAM. EXCP Access Method
Data Control Block
ISAM
Data Control Block
BDAM
Data Control Block
QTAM
Data Control Block
BTAM
Data Control Block
GAM
Data Extent Blocks •
Data Extent Block -- Ordinary
Data Extent Block -- QTAM Message Processing Program •
Data Event Control Blocks
Data Event Control Block
BSAM
Data Event Control Block
ISAM
Data Event Control Block
BDAM
Data Event Control Block
QTAM
Data Event Control Block
BTAM
Data Set Control Blocks
Data Set Control Block
Format 1
Data set Control Block
Format 2
Data Set control Block
Format 3
Data Set control Block
Format 4
Data Set Control Block
Format 5
Data Set Control Block
Format 6
Data Set Labels -- Magnetic Tape •
Data Set Label 1
FLl
Data Set Label 2 -- FL2
DEVICE NAME TABLE
Event Control Block
Interruption Control Block '.
Input/Output Block •
Job File Control Block •
Line Control Block '.
Partitioned Data Set Directory Entries •
Partitioned Data Set Directory Entry
General Format
Partitioned Data Set Directory Entry
Format 1
Partitioned Data Set Directory Entry
Format 2
REQUEST BLOCKS
Request Block -- PCP. MFT configurations •
Request Blocks -- MVT configuration
Interruption Request Block -- MVT
Program Request Block -- MVT •
System Interruption Request Block
Supervisor Request Blocks -- MVT •
Supervisor Request Block
MVT
Resident SVC Routines
Supervisor Request Block -- MVT -- Transient SVC Routines
Task Control Block •
Task Input/output Table
Uni t Control Block '.
Volume Label •
Volwne Table of Contents '.
VOlwne Table of Contents '.

7

7
8

11
21
23
39
53
61
69

77
81
83

93
99

.101
.103
.107
.111
.113
.121
.123
.129
.135
.137
.141
.143
.145
.147
.151
.155
.159
.161
.165
.177
.189

.195
.197
.199
.203
.207
.209
.215
.217
,.221
.225
.229
.231
.235
,.239
.249
.253
.275
.277
.279

Illustrations

Figures
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure

1.
2A.
2B.
2C.
3.
4.
5.
6.
7.
8.

9A.
9B.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30A.
30B.
30C.
31A.
31B.
31C.
31D.
31E.
31F.
32.
33.
34.
35.
36.
37.

· ···········

Communication Vector Table (Part 1 of 4)
·
Data Control Block - QSAM, BSAM, BP.Mi , EXCP - Device Type Segments
Data Control Block - QSAM , BSAM. BPAM. EXCP - Foundation Segments
Data Control Block - QSAM , BSAM , BPAM , EXCP - Access Method Segments
Data Control Block - ISAM (Part 1 of 4)
Data Control Block - BDAM (Part 1 of 2)
Data Control Block - QTAM (Part 1 of 2)
Data Control Block - BTAM (Part 1 of 2)
Data control Block - GAM
Data Extent Block
Ordinary (Part 1 of 4)
Data Extent Block -- QTAM -- Message Process Queue
Data Extent Block -- QTAM -- Destination Queue
Data Event Control Block
BSAM .. II
Data Event Control Block
ISAlo1
10
Data Event control Block
BDAM
Data Event Control Block
QTAM
Data Event Control Block
BTAM
Data Set control Block
Format 1 (Part 1 of 2)
Data set Control Block
Format 2 (Part 1 of 2)
Data set Control Block
Format 3
Data Set Control Block
Format 4
Data Set Control Block
Format 5
Data Set Control Block
Format 6
Data Set Label 1
Data Set Label 2
Device Name Table
Event Control Block
Interruption Control Block
Input/Output Block (Part 1 of 3)
Job File Control Block (Part 1 of 2)
..
JFCB Extension Block
Line Control Block (Part 1 of 2)
·
·
·
PDS Directory Entry
General Format
PDS Directory Entry -- Format 1
PDS Directory Entry -- Format 2
Request Blocks -- PCP, MFT ..
Interruption Request Block -- MVT
Program Request Block -- MVT
System Interruption Request Block -- MVT
Supervisor Request Block -- Resident SVC Routines
Supervisor Request Block
MVT -- Transient SVC Routines
Task Control Block (Part 1 of 2)
Task Input/Output Table
Unit Control Block (Part 1 of 3)
The UCBTYP Field
Volume Label
Volume Table of Contents

····

·····

·······
· ···
·· ··· ··
·· ·
· '. · · ·

·

·····
·· ·
···
·· · · · ·

····

·· ·· · ·
· · ·· ·· ··

· ·· · ·

· · · · · · ··

····
····

··

--

····

..··
·

····

·
·
··

·

·
··

·

····

· ·····
····
··
· ·· ·· ··
·
· · · ··
···
··· · ····
· · · · · · · · ·· ·
·
··
·
·
··
·
·····
·
······
·
·· ·· ·· · ·
··
·· ·
·
··
·
······
· ··
·
·
··
·
·····

·········
···· ·
·
····
··
···

··
··
· ·

··
· ··
·

····

12
24
25
26
40
54
62
70
78
84
94
95
.102
.104
.108
.112
.114
.124
.130
.136
.138
.142
.144
.148
.152
.156
.159
.162
.166
.178
.187
.190
.198
.200
.204
.210
.218
.222
.226
.232
.236
.240
.250
.254
.265
.276
.278

·· ··

·· ··
· · · · · · · ·· · ·

·

...···

··

;~

Tables
Table

1.

Relation of BBCCHH Number to Devices

7

(

Summary of Revisions

Subject

Blocks or Fields Affected

2361 Core Storage

DCBHIARC , JFCBHIAR, XSTAB,
RBSTAB, XLIST, TCBMSS, TCBPQE

World Trade Telegraph

DCB-WTT Interface, DCBCPOLL,
DECB-QTAM, BTAM, LCBINCAM,
UCBTYP-Communications

Advanced
Checkpoint-Restart

CVTCRTTR, CVTSTUSA, CVTSCTRK,
JFCBTSDM, JFCBOTTR, JFCBMASK,
JFCBIND2, JFCB + 96, XSTAB,
TCBTRN

Model 65 Multiprocessing

CVTDCB, CVTMPCVT, TCBFLGS, UCBFL3

Variable Length Record
Format Extension

DCBRECFM, DCBLRECL, DCBPRECL,
DS1RECFM, DS1LRECL,
FL2RECL, FL2BLKA

2420 Model 7
Magnetic Tape unit
SGJP, 1130-360 Data
Transmission
User Label in os
Flush Data after DD DATA

UCBTYP-Magnetic Tape

Punch Check Recovery
FORTRAN Use of IN-OUT
Reduced Tape Error Recovery
Read Opposite
Work Disposition
STAE
Quiesce Abnormal
Termination
Time Slice Element Address
- MFT, MVT
SVC number format
DECB - BSAM, BDAM
Correction
ECB - BSAM, BDAM, BISAM
TIOT - Device is a Data
Cell Drive
TIOT - Device Entry
Description
TCB Format Change
BTAM
Field Names
M= 1
GJP
Model 91
ISAM Status
Miscellaneous

6

OS System Control Blocks

DCB-BTAM
DS1EXT1, JFCBLTYP, VTOC
CVTSTUSA
DCBIND1
JFCBMASK
DCBOPTCD, DEBOFLGS, JFCOPTCD
UCBROR
TIOELINK
TCBNSTAE
CVTXWTO
CVTTSCE
XRBNM
DECTYPE
CVTRELNO
DECSDECB, DECBECB
TIOEFSRT
TIOT - Device Entry Description TCB
DECB - BTAM. ECB - BTAM
FL2TRTCW, FL2CNTR
Device Address
TCBFLGS, TCBTRN
TCBTRN
DCBST, DS2STIND
DS1DSIND, DS1EXPDT , UCBTYP - Graphics.,
Deletion of TCBJSE, CVTIXAVL, DCBIFLG, TCBFLAGS.,
XSTAB, RBSTAB, DS4HPCHR, DCBBUFCT

(

System Control Blocks

System control blocks are the primary means for communicating information among the major
parts of the System/360 Operating System control program. The information is stored in
the control blocks and tables in a highly compact, readily accessible form. These blocks
and tables have a standardized format, so that the information is usable by all parts of
the control program. The addresses maintained in the control blocks and tables permit
the control program to locate other control blocks and tables.
If a field or block is used in different manners by the different configurations of
the System/360 Operating System control program, the control program configurations are
shown as:
PCP - The primary control program configuration of the System/360 Operating System.
MFT - The multiprogramming with a fixed number of tasks configuration of the System/
360 Operating system.
MVT - The multiprogramming with a variable number of tasks configuration of the
System/360 Operating System.
This publication consists of descriptions of the major system control blocks and
tables. It illustrates their formats and describes their fields.
Both the format illustrations and the field descriptions show the decimal (Dec.) and hexadecimal (Hex.) displacements of the fields. Each block description begins on a right-hand page for ease in
turning to a particular block and to segment material about different blocks into sets of
separate pages; thus, users may readily remove selected parts of the publication.
MBBCCHHR - Actual Address Format
In the operating system, the actual address for a location on direct-access storage is
expressed in the 8 byte format MBBCCHHR. These 8 bytes contain:
M

- The extent number. A one-byte binary number specifying the relative location
of an entry in a data extent block (DEB). Each extent entry describes a set of
contiguous tracks allocated for the data set. For the first extent M=O except
when ISAM is used. In that case, M=l for the first extent of user data.
BB
- The bin number. The number of the bin of a 2321 data cell drive, in which a
data cell is mounted.
(For devices other than 2321, this number is zero.)
CCHH - The CCHH number. The number that identifies:
• A track of a 2301 drum •
• A subcell, strip, cylinder, and track of a 2321 data cell drive •
• A cylinder and track of other direct-access storage devices.
- The record number. The number of a record on its track.

R

Table 1 shows how the BBCCHH number relates to the various types of devices.
Table

1.

Relation of BBCCHH Number to Devices

Device

B

B

C

C

H

2301

Drum

Track

2303

Disk
Data Ce II Dri ve

H

Bin

Subcell

Cylinder

Track

Cylinder

Track

Strip

Cylinder

Track

Note: Unused fields are zero.

System Control Blocks

7

Revised Page Format
The page format used to contain the control block field descriptions has been revised to
that shown in the following:

r---------------------------------------------------------------------------------------,

I
Bytes and Field
Hex.
I
Offset
Alignment Name
Dig. Field Description, Contents, Meaning
IL _______________________________________________________________________________________
JI
The new format puts field identification data to one side of the page, and thereby gives
major emphasis to describing the use of the field. The longer text lines make it easier
and quicker to read and contrast successive field descriptions.
The new format also
includes a new item: alignment of the field with respect to the word boundary. When
reading a dump, this helps to locate, for example, a flag field that is not the highorder byte of a word. For coding, it shows the relative position of the subject data or
mask in a register.
The columns of the new page format and their use are:
Offset - The numeric address of the field relative to the beginning of the block.
The first number is the offset in decimal, followed (in parentheses) by the hexadecimal equivalent.
Example:
16

(10)

Bytes and Alignment - The size (number of bytes) of the field and its alignment relative to the full-word boundary.
Examples:
4
2
1
3

A 4-byte field beginning on a word boundary.
- A 2-byte field beginning on a halfword boundary.
- A 1-byte field in the low-order byte of a word.
- A 3-byte field beginning at the low-order byte of a word (and running
into the next word).

Field Name - A name that identifies the field.
This column is also used to show the bit settings of flag fields, that is, the
state of bits in a byte. When the column is used to show the state of bits (0, 1)
in a flag byte, it is shown as follows:

x...
1...
0...
•• xx

The 8 bit positions (0 - 7) in a byte. For ease of scanning, the highorder (left-hand) 4 bits are separated from the low-order 4 bits.
A reference to bit o.
Bit 0 is on.
Bit 0 is off •
A reference to bits 6 and 7.

Bit settings that are significant are shown and described. Bit settings that are
not presently significant are described as reserved bits. Users should not use
these bits because future features of the Operating system may make use of them.
Hex.

Dig.

(Hexadecimal Digits) - The contents of the field expressed as hexadecimal
digits .•

Examples:
F F
8
-

0

J K

-

8

- A 1-byte field with all bits on.
A 1-byte field in which the high-order bit has a meaning independent of the
setting of the 4 low-order bits.
- A 1-byte field in which the off-state of the 4 low-order bits has a significance independent of the state of the 4 high-order bits.
- A general reference to the high-order 4 bits .•
- A general reference to the low-order 4 bits .•

os System Control Blocks

Field Description, Contents~ Meaning - The use of the field.
where a field's contents relate directly to a value coded by the user (generally in
job control statements) the value coded is shown under the heading:
Code - The value coded by the user that resulted in the described contents.

POINTER DIAGRAM

r---------------------------------------------------,

I
I
I The pointer diagram previously shown here has been I
I
I superseded by a similar one now shown in the
I IBM publication IBM System/360 Operating System: I
I
I Programmer's Guide to Debugging, Form C28-6628.
IL___________________________________________________ I
~

System Control Blocks

9

(

Communication Vector Table

The communication vector table (CVT) provides the means whereby nonresident routines may refer to information in the nucleus of the control program. The CVT is
part of the resident nucleus. During the
nucleus initialization process (NIP) the
address of the CVT is placed in main
storage in the full word at decimal address
16 (hexadecimal address 10).
(Location
16(dec.)/10(hex.) points to the CVT but is
not a part of the CVT. Figure 1 illustrates the format of the CVT. Descriptions
of the fields follow the illustrations.
The symbolic displacements shown for the
various fields are generated in nonresident
routines by use of the CVT macro
instruction .•

»
CVT

11

COMMUNICATION VECTOR TABLE
-4 (-4)
CVTRELNO
Reserved
0(0)
CVTTCBP
Pointer to Address for Next and Current TCB
4 (4)

CVTOEFOO
Address of Routine to Schedule Asynchronous Exits

8 (8)
CVTLlNK
Address of DCB for SYSl. LlNKLlB
12 (C)
CVTJOB
Address of Work Queue Control Blocks
16 (10)
CVTBUF
Address of Buffer for Resident Console Interruption Routine
20 (14)
CVTXAPG
Address of lOS Appendage Table
24 (18)
CVTOVLOO
Address of Entry-Point of Address Validity Checking Routine
28 (1C)
CVTPCNVT
Address of Entry-Point of Routine for Converting Relative Track Address to Absolute
32 (20)
CVTPRLTV
Address of Entry-Point of Routine for Converting Absolute Track Address to Relative
36 (24)
CVTlLK1
Address of Channel and Control Unit Section in UCB Lookup Table
40 (28)
CVTlLK2
Address of UCB Address List Portion in UCB Lookup Table
44 (2C)
CVTXTLER
Address of Entry-Point to XCTL Routine for Systems Error Routines
48 (30)
CVTSYSAD
Address of System Residence Volume Entry in UCB Table
52 (34)
CVTBTERM
Address of Entry-Point of ABTERM Routine
56 (38)
CVTDATE
Current Date in Packed Decimal
60 (3C)

CVTMSLT
PCP: Address of Master Scheduler Linkage Table
MFT, MVT: Address of Master Scheduler Resident Data Area
Continued

• Figure

1.

communication Vector Table (Part 1 of 4)

(
12

OS System Control Blocks

COMMUNICATION VECTOR TABLE

Continued
64 (40)

Address of

I/o

CVTZDTAB
Device Characteristic Table

68 (44)
CVTXITP
Address of Error Interpreter Routine
72 (48)

CVTXWTO
PCP: Address of Console Output Routine
MFT, MVT: Address of System Quiesce Routine

76 (4C)
CVTOFNOO
Entry-Point Address to FINCH
80 (50)

82 (52)
CVTBRET
A BCR 15, 14 Instruction

CVTEXIT
An SVC 3 Instruction
84 (54)
CVTSVDCB
Address of DCB for SYS 1. SVCLlB
88 (58)
CVTTPC
Address of Pseudo Clock for Timer Routine
92 (5C)
CVTPBLDL
Address of BAL Entry-Point to BLDL Routine
96 (60)

CVTSJQ
Address of Selected Job Queue

100 (64)
CVTCUCB
Address of Tobie with Console UCB Address
104 (68)
CVTQTEOO
Address of Timer Enqueue Routine
108 (6C)
CVTQTDOO
Address of Timer Dequeue Routine
112 (70)
Address of

I/o

CVTSTB
Device Statistics Table

116 (74)
CVTDCB
System Configuration, Address of DCB for SYS1. LOGREC
120 (78)
CVTlOQET
Address of Request Element Table
124 (7C)
CVTlXAVL
Address of lOS Freelist Pointer
Continued

-Figure

1.

Communication Vector Table (Part 2 of 4)

CVT

13

COMMUNICATION VECTOR TABLE
Continued
128 (80)

CYTNUCB
Lowest Storage Address Not in Nucleus

132 (84)

CYTBOSY
Address of Program Fetch Routine

136 (88)

CYTODS
Address of Entry-Point of Dispatcher

140 (8C)
CYTILCH
Address of Logical Channel Word Table
144 (90)
CYTIERLC
Address of Asynchronous Exit Queue
148 (94)

CYTMSER
PCP: Address of Major QCB
MFT, MYT: Address of Master Scheduler Resident Data Area

152 (98)

CYTOPTOl
Address of Branch Entry-Point for Post Routine

156 (9C)
CYTTRMTB
Address of Terminal Table for QTAM
160 (AO)
CYTHEAD
Address of Highest Priority TCB in Ready Queue
164 (A4)

CYTMZOO
Highest Storage Address in Machine

168 (A8)
CYTlEFOO
Address of IRB Creation Routine
172(AC)

CYTQOCR
PCP: Reserved
MFT, MYT: Address of a GFX Parameter List Word, or Zeros

176 (BO)

CYTQMWR
PCP: Reserved
MFT, MYT: Address of Queue Manager's Communication Data Area

180 (84)

184 (B8)

CYTSNCTR
PCP, MFT, MYT: Serial Number Counter

182 (B6)
PCP, MFT, MYT: Reserved

PCP: CYTCRTTR - TTR of JCT for Restart
~ (BA) PCP: CYTSTUSA
-------------------------------------------------MFT: CYTQCDSR - Reserved
MYT: CYTQCDSR - Address of Search Routine for Contents Directory

188 (BC) _ _ _ _ -"CP:~YTSCTRK_ _ _ _ _ _ _ _ _.~O~E~ _ _ _ _ _ _PCP: Reserved _ _ _ _ _ _ _ _
PCP, MFT: CYTQLPAQ - Reserved
MYT: CYTQLPAQ - Address of Top of Contents Directory in LPA Queue
Continued

-Figure

1.

Communication Vector Table (Part 3 of 4)

c
14

OS System Control Blocks

COMMUNICATION VECTOR TABLE
Continued
192 (CO)

CYTMPCYT
PCP, MFT: Reserved
MYT: Address of M65MP Secondary CYT

196 (C4)
PCP, MFT, MYT: Reserved
200 (C8)

CYTABEND
PCP, MFT: Reserved
MYT: Address of Secondary CYT

204 (CC)
CYTUSER
PCP, MFT, MYT: Field Available to the User

207 (CF)

MFT, MVT Extension
208 (DO)
MFT, MYT: Reserved
212 (D4)

CYTQABST
MFT: Reserved
MYT: An SYC 13 Instruction

216 (DC)

-Figure

214 (D6)
MFT, MYT: Reserved
CYTTSCE
MFT: Address of TSCE
MYT: Address of First TSCE

1.

219 (DF)

Communication Vector Table (Part 4 of 4)

CVT

15

Offset

16

Bytes and
Alignment

Field
Name

COMMUNICATION VECTOR TABLE
Hex.
Dig. Field Description. contents. Meaning

-4

(-4)

4

CVTRELNO

Reserved.

o

(0)

4

CVTTCBP

Address of a double word. the first containing the
next-to-be dispatched TCB address. the second containing the last (current) TCB address. Both words
are identical unless the task goes into a WAIT state.
When in a WAIT state. the first word is set to zero
until the waiting is over; then both words are once
again identical.

4

(4)

4

CVTOEFOO

Address of routine to schedule asynchronous exits.

8

(8)

4

CVTLINK

Address of the DCB for the SYS1 .• LINKLIB data set .•

12

(C)

4

CVTJOB

Address of work queue control blocks used by the job
scheduler.

16

(10)

4

CVTBUF

Address of the buffer of the resident console interrupt routine.

20

(14)

4

CVTXAPG

Address of the I/O supervisor appendage table.

24

(18)

4

CVTOVLOO

Address of entry point of the task supervisor's
address validity checking routine.

28

(1C)

4

CVTPCNVT

Address of entry point of the routine which converts
a relative track address (TTR) to an absolute track
address (MBBCCHHR).

32

(20)

4

CVTPRLTV

Address of entry point of the routine which converts
an absolute track address (MBBCCHHR) to a relative
track address (TTR).

36

(24)

4

CVTILK1

Address of the channel and control unit portion of
the UCB lookup table.

40

(28)

4

CVTILK2

Address of the UCB address list portion of the UCB
lookup table.

44

(2C)

4

CVTXTLER

Address of entry point of an XCTL routine that brings
system error routines into the error transient area.

48

(30)

4

CVTSYSAD

Address of the system residence volume entry in the
UCB table.

52

(34)

4

CVTBTERM

Address of entry point of the ABTERM routine.

56

(38)

4

CVTDATE

Current date in packed decimal.

60

(3C)

4

CVTMSLT

PCP:
MFT.MVT:

64

(40)

4

CVTZDTAB

Address of the I/O device characteristic table.

68

(44)

4

CVTXITP

Address of the error interpreter routine.

(48)

4

CVTXWTO

PCP:
MFT, MVT:

(4C)

4

CVTOFNOO

Address of entry point of the FINCH routine.

OS System Control Blocks

Address of master scheduler linkage table.
Address of master scheduler resident data
area.
(compare the use of this field with the use of the
CVTMSER field at offset 148 dec., 94 hex .• )

Address of the console output routine.
Address of the system quiesce routine.

(

Offset

Bytes and
Alignment

Field
Name

COMMUNICATION VECTOR TABLE
Hex.
Dig. Field Description, Contents, Meaning

80

(50)

2

CVTEXIT

An SVC 3 instruction (EXIT).

82

(52)

•• 2

CVTBRET

A BCR 15,14 instruction (used by data management
routines).

84

(54)

4

CVTSVDCB

Address of the DCB for the SYS1.SVCLIB data set.

88

(58)

4

CVTTPC

Address of the 6-hour pseudo clock 
Read Punch
Reader
Punch
Reader
Read Punch

Reserved
PRINTER INTERFACE

16

(10)

1

DCBPRTSP

Number indicating normal printer spacing ..
Code

xxx.. • xx.
.. .. . 1
1 ... 1
..... 1 •.. 1
..... 1 1 ... 1
17

(11)

• 1

DCBDEVT

x.xx .. x.x
.1 .... 1 .....
.. 1 ... 1 .. 1 ..
18

(12)

... 1

o
1
2
3

Device type.
(Reserved bits>
1403 Printer and 1404 Printer
(continuous form support only>
1443 Printer
Test-for-printer-overflow mask (PRTOV mask).
If printer overflow is to be tested for, the PRTOV
macro instruction sets the mask as follows:

xx •• xxxx
... 1.
... .. 1

19

(13)

(Reserved bits)
No spacing ..
Space one line ..
Space two lines •
Space three lines •

••• 1

9
12

(Reserved bits)
Test for channel 9 overflow •
Test for Channel 12 overflow.

Reserved
END OF DEVICE INTERFACE SEGMENTS

(
30

OS System Control Blocks

DATA CONTROL BLOCK -- SAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
COMMON INTERFACE

20

(14)

1

DCBBUFNO

Number of buffers required for this data set.
May range from 0 to a maximum of 255.

21

(15)

• 3

DCBBUFCB

Address of buffer pool control block.

24

(18)

2

DCBBUFL

Length at buffer.
May range from 0 to a maximum of 32,767.

26

(lA)

••

DCBDSORG

Data set organization to be used.
Code

26

(lA)

2

Byte 1
IS
PS
DA

1 •••
• 1 ••
• . 1.

••• x xx ••
. . 1.
••• 1

PO
U

Indexed sequential organization.
Physical sequential organization .•
Direct organization•
(Reserved bits)
Partitioned organization •
Unmovable - the data contains
location dependent informationo

Byte 2
27

(lB)

GS

1 •••

.xxx xxxx
28

(lC)

4

DCBIOBAD

Graphics organization.
(Reserved bits)

Address of the lOB when chained scheduling is used.
FOUNDATION EXTENSION

32

(20)

1

DCBHIARC, DCBBFTEK, DCBBFALN
Code

x... • x ••
0 •••• 0 ••

o. ..

.1 •.

1. •. .0 .•

.x.x
.. 1.0
• 0.1

Buffer pool location.
coded in the DCB macro instruction:
Before Open none No choice made in the DCB macro instruction.
After Open If no choice is made in the DD statement either
(as shown by the JFCBHIAR field),
the Open routine resets these two bits from 00
to 01.
o Hierarchy 0 main storage.
(See also: After Open. above.)
1
Hierarchy 1 main storage.
S
E

•• x. x .••

33

(21)

• 3

• .xx
• .10
•• 01

D

• .11

F

DCBEODAD

F

Buffering Technique:
Simple buffering •
Exchange buffering •
(Reserved bits.)
Buffer alignment:
Doubleword boundary.
Fullword not a doubleword boundary, coded in the
DCB macro instruction.
FUllword not a doubleword boundary, coded in the
DD statement.

End-of-data address. Address of a user-provided routine to handle end-of-data conditions.

DCB - SAM

31

DATA CONTROL BLOCK -- SAM
Offset

Bytes and Field
Alignment Name

Field Description, Contents, Meaning
FOUNDATION EXTENSION (Continued)

36

(24)

1

DCBRECFM

Record Format
Code

10 ••
01 ••
11 ••

•• 1.
••• 1
1 •••

• 10.
• 01.
• 00.
o ••

31

(25)

• 3

Fixed record length.
Variable record length.
Undefined record length.
U
T
Track overflow •
B
Blocked records •
May not occur with undefined (U).
Fixed length record format:
Standard blocks.
S
(No truncated blocks or unfilled tracks are
embedded in the data set.)
Variable length record format: Spanned records.
A
ASA control character •
M
Machine control character •
No control character •
Key length (KEYLEN) was specified in the DCB macro
instruction. This bit is inspected by the Open routine to prevent overriding a specification of KEYLEN=
o by a nonzero specification in the JFCB or data set
label.
F
V

1

DCBEXLST

Exit List.

Address of a user-provided list.
FOUNDATION SEGMENT

FOUNDATION SEGMENT -- BEFORE OPEN
40

(28)

8

DCBDDNAM

This 8 byte name must be the same as that appearing
in the name field of the data definition statement
which defines the data set associated with this DCB.
It is used by the Open routine to locate the JFCB.

48

(30)

1

DCBOFLGS

Flags used by the Open routine.

1 •••

0 •••
. 1 •.

•• 1.
•.• 1
1 •.•
• 1 ••

.. o.

.• 1.
••. 1

Last I/O operation was a WRITE.
Last I/O operation was a READ or POINT.
For direct-access devices, this means that the track
balance field is invalid.
Last I/O operation was in READ backward mode •
Set to ! by EOV when it calls the Close routine for
concatenation of data sets with unlike attributes.
An OPEN has been successfully completed •
Set to ! by a problem program to indicate a
concatenation of unlike attributes.
Tape mark has been read •
Set to Q by an I/O support function when that
function takes a user exit. It is set to 0 to inhibit other I/O support functions from processing this
particular DCB.
Set to 1 on return from the user exit to the I/O support function which took the exit.
Set to 1 by an I/O support function if the DCB is to
be processed by that function.

(
32

OS System Control Blocks

DATA CONTROL BLOCK -- SAM
Offset

Bytes and
Address

Field
Name

Field Description, Contents, Meaning
FOUNDATION SEGMENT -- BEFORE OPEN (Continued)

49

(31)

• 1

DCBIFLG
00 ••
01 ••
11 ••

•• 10
• • 01
00 ••
11 ••

01 ••
10 ••
• .xx
50

(32)

•• 2

DCBMACR

Used by I/O supervisor in communicating error
conditions and in determining corrective procedures.
Not in error procedure.
Error correction in process.
Permanent error condition.
Channel 9 printer carriage tape punch sensed •
Channel 12 printer carriage tape punch sensed •
Always use I/O supervisor error routine.
Never use I/O supervisor error routine.
Never use I/O supervisor error routine.
Never use I/O supervisor error routine.
(Reserved bits.)
Macro instruction reference.
Major macro instructions and various options
associated with them. Used by the Open routine to
determine access method. Used by the access method
executors in conjunction with other parameters to
determine which load modules are required.
EXCP ACCESS METHOD

Byte 1
50

(32)

Execute Channel Program (EXCP).
Foundation extension is present with EXCP •
Appendages are required with EXCP •
Common interface is present with EXCP •
(Reserved bits.)

1 •..
. 1 ..
•• 1.
... 1

xxxx
51

(33)

Byte 2
xxx x

(Reserved bits.)
Five word device interface is present with EXCP.
Four word device interface is present with EXCP •
Three word device interface present with EXCP.
One word device interface is present with EXCP.

1 •..
. 1 ..
o .1.
· .. 1

BSAM - Input

Byte 1
50

(32)

00 ••
R

•. 1.

••• x x •• x
.1 ..
· .1.
51

(33)

Byte 2
00 ••
. . 1.

P
C

w

••• x • •• x
1 ....
• 1 ••

•• 1.

L

P
C

Always zero for BSAM •
READ
(Reserved bits.)
POINT (which implies NOTE) •
CNTRL
BSAM - output
Always zero for BSAM •
WRITE
(Reserved bits)
Load mode BSAM (create BDAM data set).
POINT (which implies NOTE) •
CNTRL

)
DCB - SAM

33

DATA CONTROL BLOCK -- SAM
Offset

Bytes and
Alignment

Field
Name
DC Bl-1'..ACR
(Cont'd. )

Field Description, Contents, Meaning
FOUNDATION SEGMENT -- BEFORE OPEN (Continued)
Code

Byte 1
50

o•••

(32)

• 1 ••

G

•• 0.
• .• 1
1 ••.
• 1 ..
•• 1.
•.• 1
51

Byte 2
0 •••
• 1 ..
•• 0.

(33)

M
L
T
C

D

P
M

• •• 1

1 ••.
. 1 ..

•• 1.

L
T
C

.•. 1

D

(32)

00 ••

•• 1.
• 1 .•

R

READ

P

POINT (which implies NOTE) •
(Reserved bits)

••• x x.xx
Byte 2
51

(33)

000 •

w

•• 1.
• 1 .•

• •• x x. xx

QSAM - Output
Always zero for QSAM.
PUT
Always zero for QSAM •
Move mode.
Locate mode.
Substitute mode •
CNTRL
Data mode •
BPAM - Input
Always zero for BPAM.

Byte 1
50

QSAM - Input
Always zero for QSAM.
GET
Always zero for QSAM •
Move mode.
Locate mode.
Substitute mode •
CNTRL
Data mode •

p

BPAM - Output
Always zero for BPAM.
WRITE
POINT (which implies NOTE) •
(Reserved bits)

FOUNDATION SEGMENT -- AFTER OPEN
40

(28)

2

DCBTIOT

Offset from the TIOT origin to the TIOELNGH field in
the TIOT entry for the DD statement associated with
this DCB.

42

(2A)

•• 2

DCBMACRF

Contents and meaning are the same as those of the
DCBMACR field in the foundation segment before OPEN.

44

(2C)

1

DCBIFLGS

Contents and meaning are the same as those of the
DCBIFLG field in foundation segment before OPEN.

45

(2D)

•

DCBDEBAD

Address of the associated DEB.

3

Note: The above fields are overlaid on the DCBDDNM
field during OPEN and are restored to their original
form at CLOSE.
48

(30)

1

DCBOFLGS

Contents and meaning are the same as those of the
DCBOFLGS field in the foundation segment before OPEN.

(
34

OS System Control Blocks

DATA CONTROL BLOCK -- SAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
ACCESS METHOD INTERFACE SEGMENTS
EXCP ACCESS METHOD INTERFACE

49

(31)

• 3

52

(34)

1

Reserved
DCBOPTCD
xxxx x.xx
.1 ••

Option Codes
Code
(Reserved bits)
z Magnetic tape devices:
Use reduced error recovery procedure.

53

(35)

• 7

60

(3C)

2

DCBEOEA

End-of-Extent Appendage ID (Identification)
note).

62

(3E)

•• 2

DCBPCIA

Program-Controlled-Interruption Appendage ID (Identification) (See note).

64

(40)

2

DCBSIOA

Start-I/O Appendage ID (Identification)

66

(42)

•• 2

DCBCENDA

Channel-End Appendage ID (Identification)

68

(44)

2

DCBXENDA

Abnormal-End Appendage ID (Identification)
note).

70

(46)

•• 2

Reserved
(See

(See note).
(See note).
(See

Reserved
Note: The ID is that of an executable load module in
the SVC Library. The module is loaded by the Open
routine. Its address is placed into the appropriate
slot in an appendage vector table constructed by the
Open routine. If the ID is blank, its slot in the
appendage vector table will contain the address of a
return-point in I/O supervisor.
BSAM, BPAM INTERFACE

49

(31)

• 3

DCBREAD, DCBWRITE
Address of READ or WRITE module .•

52

(34)

1

DCBOPTCD
1 .••

.1 ••

•• 1.
••• x x. xx
.1 ••

»

Option codes
Code
W
Write validity check.
U
Allow a data check caused by an invalid character (1403 printer with UCS feature).
C
Chained scheduling using the Program Controlled
Interruption.
(Reserved bits)
z Magnetic Tape Devices: use reduced error recovery procedure.

53

(35)

• 3

DCBCHECK

Address of the CHECK module.

56

(38)

1

DCBIOBL

lOB length in double words.

57

(39)

• 3

DCBSYNAD

Address of user's synchronous error routine to
be entered when a permanent error occurs.

60

(3C)

1

DCBCINDl
x •• x xxx.
• 1 ••

•• 1.
•.• 1

Condition Indicators.
(Reserved bits>
Search direct •
Volume full or paper tape EOV •
Exchange buffering supported •
DCB - SAM

35

DATA CONTROL BLOCK -- SAM
Offset
61

(3D)

Bytes and
Alignment
• 1

Field
Name

Field Description, Contents, Meaning

DCBCIND2

Condition Indicators

1 ....
.. 1 ...
•• 1 ..

••.1
1 ... ..
• 1 .. ..
•• 1 ..

.... 0

Partitioned data set: STOW has been performed ..
Sequential data set: Update.
Direct organization data set:
Last I/O was a write record zero.
Sequential data set: UPDATE EOF is indicated ..
PUT entered from Close while in Update mode (QSAM
only) ..
Permanent I/O error ..
OPEN acquired buffer pool.
Chained scheduling being supported ..
FEOV bit (QSAM only) •
Always set to Q for BSAM/BPAM ..

62

(3E)

•• 2

DCBBLKSI

Maximum block size. Maximum value:
32,764.
For fixed-length blocked record format, it must be a
multiple of the length given in DCBLRECL.
For variable-length records, this must include the 4
byte block length field.

64

(40)

1

DCBWCPO

write channel program offset.. Offset of write channel program from the start of the lOB.

65

(41)

•

DC BWCPL

Length of Write channel program.

66

(42)

•• 1

DCBOFFSR

Offset of the Read CCW from the BSAM/BPAM prefix of
the lOB.

67

(43)

••• 1

DCBOFFSW

Offset of the Write CCW from the BSAM/BPAM prefix of
the lOB.

68

(44)

4

DCBIOBA

Normal scheduling: Address of BSAM/BPAM prefix of
lOB ..
Chained Scheduling: Address of ICB ..

72

(48)

1

DCBNCP

Number of channel programs. Number of READ or WRITE
requests which may be issued prior to a CHECK; the
number of lOBs generated.. Maximum number: 99 ..

73

(49)

• 3

DCBEOBR

Address of the end-of-block module fo.r read ..

76

(4C)

4

DCBEOBW

Address of the end-of-block module for write.

80

(50)

2

DCBDIRCI'

For BPAM only:
Directory count.
Number of bytes used in last directory block..
range from 0 to a maximum of 254 ..

1

82

(52)

•• 2

84

(54)

4

DCBLRECL

May

Logical record length.
For fixed-length blocked record format, the presence
of DCBLRECL allows BSAM to read truncated records ..

DCBCNTRL, DCBNOTE, DCBPOINT
Address of the CNTRL module or of the NOTE/POINT
module ..

(
36

OS System Control Blocks

DATA CONTROL BLOCK -- SAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
QSAM INTERFACE

49

(31)

• 3

DCBGET, DCBPUT
Address of GET module or PUT module.

52

(34)

1

DCBOPI'CD

Option codes.
Code

1 •• '.

.1 ••

W
U

•• 1.

C

••• x x. xx
.1 ••

z

Validity check.
Allow a data check for an invalid character
(1403 with UCS).
Chained scheduling using the program controlled
interruption.
(Reserved bits)
Magnetic tape devices.
Use reduced error ~ecov
ery procedure.

53

(35)

• 3

DCBGERR, DCBPERR
Address of the synchronizing routine for GET or of
the synchronizing routine for PUT.

56

(38)

1

DCBIOBL

lOB length in double words.

57

(39)

•

DCBSYNAD

Address of the user's synchronous error routine to be
entered when a permanent error occurs.

60

(3C)

1

DCBCINDl

Condition Indicators.

3

1,. '••

• 1 ••

•• 1.
••• x xxxx

61

(3D)

• 1

DCBCIND2
1 •••
• 1 ...

•• x •
••• 1

1 •••
• • 1,.

•.• 1

2540 Card Punch:
Data set was opened but no data was written.
Search direct •
Volume full· •
(Reserved bits.)
Condition Indicators.
STOW has been performed.
Last I/O was a write record zero •
(Reserved bit)
Permanent I/O error.
OPEN acquired buffer pool.
Track overflow in use •
This is a QSAM DCB •

62

(3E)

•• 2

DCBBLKSI

Maximum block size. Maximum value:
32,764.
For fixed-length blocked record format, it must be a
multiple of DCBLRECL.
For variable-length records this must include, 4 byte
block length field provided by the access method .•

64

(40)

1

DCBWCPO

Write channel program offset; offset of Write channel
program from the start of the lOB..

65

(41)

• 1

DCBWCPL

Length of Write channel program.

66

(42)

... 1

DCBOFFSR

Offset of the Read CCW from the QSAM prefix of the
lOB.

67

(43)

'. '• • 1

DCBOFFSW

Offset of the Write CCW from the QSAM prefix of the
lOB .•

»
DCB - SAM

37

DATA CONTROL BLOCK -- SAM

Offset

Bytes and
Alignment

Field
Name

68

( 44)

4

DCBIOBA

Normal Scheduling: Address of QSAM prefix of lOB.
Chained Scheduling: Address of ICB.

72

(48)

4

DCBEOBAD

End-of-buffer address.
Address of last byte of the current buffer.

76

(4C)

4

DCBRECAD

Address of the current or next logical record.

80

(50)

2

82

(52)

•• 2

DCBLRECL

Format F records:
Record length.
Format U records:
Record length.
Format V records Unspanned record format Record length.
GET, PUTX:
PUT:
Actual or maximum rec0rd length.
Spanned record format Locate mode Segment length.
GET:
PUT:
Actual or minimum segment
length.
Move mode GET:
Record length.
PUT:
Actual or maximum record length.
Data mode, GET Data records up to 32,752 bytes:
Data length.
Data records exceeding 32,752 bytes:
Before Open:
X'8000'.
After Open:
Data Length.
output mode" PUTX (output data set):
Segment 1 ength.

84

(54)

1

DCBEROPT

Error option.
Disposition of permanent errors if the user returns
from a synchronous error exit (DCBSYNAD), or if the
user has no synchonous error exit.

Reserved

1 •• '.
.1. ,.

•• 1.
••• x xxxx

38

Field Description, Contents, Meaning

Code
ACC Accept
SKP Skip
ABE Abnormal end of task •
(Reserved bits)

85

(55)

• 3

88

(58)

2

90

(SA)

•• 2

DCBPRECL

Format F records:
Block length.
Maximum block length.
Format U records:
Format V records Unspanned record format:
Maximum block length.
Spanned record format other than data mode, PUT:
Maximum block length.
Data mode, PUT:
Data length.

92

(5C)

4

DCBEOB

Address of the end of block module.

DCBCNTRL

Address of the CNTRL module.
Reserved

os System Control Blocks

(

Data Control Block-ISAM

This data control block (DCB> is used by
the indexed sequential access-method (ISAM)
routines and holds data pertinent to the
use of a data set that is maintained by the
ISAM routines. The common interface and
the foundation sections serve the same purpose in all DCBs although the formats may
vary slightly for different access method
routines.
Figure 3 illustrates the format
of this DCB. Descriptions of the fields
follow the illustration.

)
DCB -- ISAM

39

DATA CONTROL BLOCK -- ISAM
Device Interface
16 (10)

DCBKEYLE

17 (11)

DCBDEVT

Key Length

18 (12)

DCBTRBAL
Track Balance

Device Type

19 (13)

Common Interface
21 (15)

20 (14)

DCBBUFCB
Address of the Buffer Pool

DCBBUFNO
Buffer Required

26 (1A)

24 (18)

DCBDSORG
Data Set Organization

DCBBUFL
Buffer Length
28 (lC)
Reserved

31 (1F)

Foundation Extension
32 (20)
DCBHIARC, DCBBFTEK, DCBBFALN

33 (21)

36 (24)

37 (25)

DCBRECFM

DCBEODAD
Address of EODAD Routine

DCBEXLST
Address of the Exit List

Record Format

39 (27)

Foundation
40 (28)
DCBDDNAM
DD Statement Data Set Name

50 (32)

49 (31)

48 (30)
DCBOFLGS
Open Routine Flags

I/o

DCBMACR
Macro Instruction Code

DCBIFLG
Supervisor Flags

51 (33)

After OPEN
40 (28)

DCBMACRF
Macro Instruction Code

45 (2D)

44 (2C)

I/o
48 (30)

42 (2A)

DCBTIOT
Offset in TIOT Table to DD Entry

DCBDEBAD
Address of the DEB

DCBIFLGS
Supervisor Flags

DCBOFLGS
Open Routine Flags

,

,

!... -- ...... -----------------------~------------------------------------------------------------------------------------------------------------_ ... _------ ... ------ ..... - ... ------------------------- ... _----------------:

Access Method Interface - ISAM
49 (31)
DCBGET, DCBPUT
Address of GET or PUT Module
52 (34)

DCBOPTCD

53 (35)

Option Code

DCBMAC
DCBMACRF Overflow

56 (38)

54 (36)

DCBNTM

55 (37)

Index Size

DCBCYLOF
No. of Overflow Tracks

DCBSYNAD
Address of User's Synchronous Error Routine
62 (3E)

60 (3C)
DCBRKP
Relative Key Position

DCBBLKSI
Block Size
Continued

-Figure

40

3.

Data Control Block - ISAM (Part 1 of 4)

as System Control Blocks

(

DATA CONTROL BLOCK -- ISAM
Continued
64 (40)

DCBMSWA
Address of Work Area
70 (46)

68 (44)

DCBSMSW
Size of Work Area

DCBSMSI
Size of Area for Highest Level Index
72 (48)

DCBNCP
No. of Channel Programs

73 (49)

DCBMSHI
Address of Area for Highest Level Index

76 (4C)
DCBSETL
Address of SETL Module
80 (50)

DCBEXCD1

81 (51)

Condition Flags

DCBEXCD2

82 (52)

Condition Flags

DCBLRECL
Logical Record Length

84 (54)
DCBESETL
Address of ESETL Routine
88 (58)
Address of READ

K or

DCBLRAN
WRITE K or Read Exclusive Module

92 (5C)
DCBLWKN
Address of WRITE KN Module
96 (60)

DCBRELSE
Work Area for Register Contents

100 (64)
DCBPUTX
Work Area for Register Contents
104 (68)

DCBRELX
Address of Read Exclusive Module

108 (6C)

DCBFREED
Address of Dynamic Buffering Module

112 (70)
DCBDBUFN
Reserved

113 (71)

DCBFTMI2
Direct-Access Address of Second-Level Master Index

120 (78)
DCBLEMI2
Direct-Access Address of Last Entry in Second-Level Master Index
125 (7D)
DCBFTMI3
Direct-Access Address of Third-Level Master Index

128 (80)

Continued

Figure

3.

Data Control Block - ISAM (Part 2 of 4)

»
DCB -- ISAM

41

DATA CONTROL BLOCK -- ISAM
Continued
132 (84)
DCBLEMI3
Direct-Access Address of Last Entry in Third-Level Master Index
137 (89)

DCBNLEV
No. of Index Level s

141 (8D)

DCBHMASK
2301, not 2301

Continued
144 (90)

DCBHIRCM
Highest R for Indexes

148 (94)

145 (91)

DCBHIRPD
Highest R for Prime Data

DCBTDC
Tag Deletion Count

152 (98)

138 (8A)

DCBFIRSH
HHR of First Prime Data Record

142 (8E)

DCBLDT
HH of Last Prime Data Track

146 (92)

DCBHIROV
Highest R for Overflow

150 (96)

147 (93)

DCBHIRSH
Last R of Shared Track

DCBNCRHI
Bytes Needed for Highest-Level Index

DCBRDRG3
Count of Access to Overflow Records Other than the First

156 (9C)

DCBNREC
No. of Logical Records in Prime Data Area

160 (AO)

DCBST

161 (A1)

Status Indicators
DCBFTCI
Direct-Access Address of First Track of Cylinder Index

168 (A8)

169 (A9)
Reserved
DCBFTMl1
Direct-Access Address of First Track of

176 (BO)

DCBNTHI
Size of Highest Index

First-L~vel

Master Index

177 (B1)
DCBFTHI
Direct-Access Address of First Track of Highest-Level Index

184 (B8)
DCBLPDA
Direct-Access Address of Last Prime Data Record in Prime Data Area

Figure

3,.

Data Control Block - ISAM (Part 3 of 4)

Continued

(
42

OS System Control Blocks

DATA CONTROL BLOCK -- ISAM
Continued
192 (CO)
DCBLETI
Direct-Access Address of Last Active Normal Entry of Track Index on Last Cylinder
198 (C6)

197 (C5)

DCBNBOV
No. of Bytes Left on Overflow Track

Reserved
200 (C8)

DCBLECI
Direct-Access Address of Last Active Entry in Cylinder Index
205 (CD)

206 (CE)
DCBRORG2
No. of Tracks Left in Overflow Area

Reserved
208 (DO)

DCBLEM'I1'
Direct-Access Address of Last Active Entry in First-Level Master Index
213 (D5)

214 (D6)
DCBNOREC
No. of Logical Records in Overflow Area

Reserved
216 (D8)

DCBLlOV
Direct-Access Address of Last Record in Overflow Area

224 (EO)

DCBRORG1
No. of Full Cylinder Overflow Areas

226 (E2)
Reserved

228 (E4)
DCBWKPTl
Pointer to Work Area or Channel Program
232 (E8)

DCBWKPT2
Pointer to Work Area or Channel Program

236 (EC)
DCBWKPT3
Pointer to Work Area or Channel Program
240 (FO)

DCBWKPT4
Pointer to Work Area or Channel Program

244 (F4)

DCBWKPT5
Pointer to Work Area or Channel Program

248 (F8)

DCBWKPT6
Pointer to Work Area or Channel Program
251 (FB)

Figure

3.

Data Control Block - ISAM 
Disk Drive.
Parallel Drum.
Serial Drum.
Disk Storage •
Data Cell Drive •
Disk Storage Facility.

2311
2301
2303
2302
2321
2314

Track balance.
Number of bytes remaining on current track.
COMMON INTERFACE

20

(14)

1

DCBBUFNO

Number of buffers required for this data set:

21

(15)

• 3

DCBBUFCB

Address of buffer pool control block.

24

(18)

2

DCBBUFL

Length of buffer:

26

(lA)

DCBDSORG

Before OPEN:
Data set organization to be used.
After OPEN: Data set organization in use.

Byte 1
26

(lA)

0

•

••• x xx ••
•• 1.
... 1

32,767.

Code
IS
PS
DA

1.
. 1 ..
. . 1.

0 -

0-255.

PO
U

Indexed sequential organization.
Physical sequential organization •
Direct organization •
(Reserved bits>
Partitioned organization •
Unmovable - the data contains location dependent
information.

Byte 2
27

(lB>

28

(lC>

1 ...
.xxx xxxx
4

GS

Graphics Organization.
(Reserved bits)

Reserved

(
44

OS System Control Blocks

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning
FOUNDATION EXTENSION

32

(20)

1

DCBHIARC, DCBBFTEK, DCBBFALN
x •• '.

• x ••

0 •••• 0 ••

0 •••• 1 ••
1 •••. 0 ••

Buffer pool location, coded in the DCB macro
instruction:
Before Open none No choice made in the DCB macro instruction.
After Open If no choice is made in the DD statement either
(as shown by the JFCBHIAR field), the Open routine resets these two bits from 00 to 01.
o Hierarchy 0 main storage.
(See also: After Open, above.)
1 Hierarchy 1 main storage,.
(Reserved bits)

.xxx x •••
• .xx
• .10
• .01

D
F

• .,11

F

Buffer alignment:
Doubleword boundary.
FUllword not a doubleword boundary, coded in the
DCB macro instruction.
FUllword not a doubleword boundary, coded in the
DD statement.

33

(21)

• 3

DCBEODAD

Address of a user-provided routine to handle end-ofdata conditions.

36

(24)

1

DCBRECFM

Record Format.
Code

10 ••
01 ••
11..

F
V
U
T
B

•• 1.
•. .1
1 ..•

S

• 10.
,.01.

M

• 00.
• .. 1

37

(25)

• 3

DCBEXLST

A

Fixed length records.
Variable length records.
Undefined length records.
Track overflow •
Blocked records •
may not occur with undefined (U).
Standard records. No truncated blocks or
unfilled tracks are embedded in the data set.
ASA control character •
Machine control character.
No control character •
Key length (KEYLEN) was specified in the DCB
macro instruction; this bit is inspected by the
open routine to prevent overriding a specification of KEYLEN=O by a non-zero specification in
the JFCB or data set label.

Exit List.

Address of a user-provided list.

»
DCB -- ISAM

45

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning
FOUNDATION SEGMENT
FOUNDATION SEGMENT -- BEFORE OPEN

40

(28)

8

DCBDDNAM

This 8 byte name must be the same as that appearing
in the name field of the data definition statement
which defines the data set associated with this DCB.
It is used by the open routine to locate the JFCB.

48

(30)

1

DCBOFLGS

Flags used by the Open routine.

1 •••

o•• '.
• 1,••

•• 1.
•• • 1

1 •••
. 1 ..
•• 0.

'•• 1.
•.• 1
49

(31)

• 1

DCBIFLG
00 ••
01 ••

110.
• • 10
•• 01
00 ••
11 ••
01 ••
10 ••

•• xx

Last I/O operation was a WRITE.
Last I/O operation was a READ or POINT. For direct
access devices, this means that the track balance
field is invalid.
Last I/O operation was in READ backward mode •
Set to 1. by EOV
routine when it calls the Close routine for concatenation of data sets with unlike attributes.
An OPEN has been successfully completed •
Set to 1. by a problem program to indicate a
concatenation of unlike attributes.
Tape mark has been read •
Set to Q by an I/O support
function when that function takes a user exit. It is
set to Q to inhibit other I/O support functions from
processing this particular DCB.
Set to 1 on return from the user exit to the I/O
support-function which took the exit.
Set to 1. by an I/O support function if the DCB is to
be processed by that function.
Used by I/O supervisor in communicating error
conditions and in determining corrective procedures.
Not in error procedure.
Error correction in process.
Permanent error condition.
Channel 9 printer carriage tape punch sensed •
Channel 12 printer carriage tape punch sensed.
Always use I/O supervisor error routine.
Never use I/O supervisor error routine.
Never use I/O supervisor error routine.
Never use I/O supervisor error routine.
(Reserved bits)

(
46

OS System Control Blocks

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning
FOUNDATION SEGMENT -- BEFORE OPEN (Continued)

50

(32)

50

(32)

•• 2

DCBMACR

Byte 1
00.0 0 •••

• .• x

BISAM - Input
Always zero for BISAM.
READ
Dynamic buffering •
CHECK
(Reserved bit)

Byte 2
00.0 0000
•• 1.

BISAM - output
Always zero for BISAM.
WRITE

•. 1.
• 1 ••

· . 1.

51

50

Macro instruction reference: specifies the major
macro instructions and various options associated
with them. Used by the Open routine to determine
access method. Used by the access method executors
in conjunction with other parameters to determine
which load modules are required.

(33)

R
S
C

w

Byte 1
0.0 • • 0 ••

(32)

• 1 ••

••.1
1 •••
• .. xx

G
M
L

Byte 2
51

(33)

1 •••
.1 •.

S
P

•• 0.
••• 1

1 ....
• 1 ••

•• 1.
•.. 1

M
L
U
K
I

QISAM - Input
Always zero for QISAM •
GET
Move mode of GET •
Locate mode for GET.
(Reserved bits)
QISAM - output
SETL
PUT or PUTX.
Always zero for QISAM •
Move mode of PUT •
Loca te mode of PUT .•
Update in place (PUTX) •
SETL by key •
SETL by ID •

FOUNDATION SEGMENT -- AFTER OPEN
40

(28)

2

DCBTIOT

A two byte field containing the offset from the TIOT
origin to the TIOELNGB field in the TIOT entry for
the DD statement associated with this DCB.

42

(2A)

•• 2

DC BMACRF

Contents and meaning are the same as those of the
DCBMACR field in the foundation before Open.

44

(2C)

1

DCBIFLGS

Contents and meanings are the same as those of the
DCBIFLG field in the foundation before Open.

45

(2D)

.. 3

DCBDEBAD

Address of the associated DEB .•
Note: The above fields overlay the DCBDDNAM field
during OPEN and are restored to their original form
at CLOSE.

48

(30)

1

DC BOFLGS

Contents and meanings are the same as those of the
DCBOFLGS field in the foundation before Open.

)
DCB -- ISAM

47

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning
ACCESS METHOD INTERFACE -- ISAM

49

(31)

• 3

DCBGET, DCBPUT
Address of GET module or of PUT module.

52

(34)

1

DCBOPTCD
1 ..•

.x.. • x .•
•• 1.
• . •1
1 ....
• . 1.
•.. 1
53

(35)

• 1

DCBMAC
xxxx ••• x
1 •.•

. 1 ..
• . 1.

option codes:
Code
W
Write Validity check.
(Reserved bits)
Master Indexes •
M
Independent overflow area •
I
Cylinder overflow areaa
Y
L
Delete option •
Reorganization criteria •
R
Extension of the DCBMACRF field for ISAM.
Code
(Reserved bits)
U
Update for read.
U
Update type of write •
A
Add type of write •

54

(36)

'.. 1

DCBNTM

Number of tracks that determine the development of a
Master Index. Maximum permissible value:
99.

55

(37)

••• 1

DCBCYLOF

Number of tracks to be reserved on each cylinder to
hold records that overflow from other tracks on that
cylinder. Maximum permissible value: 99.

56

(38)

4

DCBSYNAD

Address of user's synchronous error routine to be
entered when uncorrectable errors are detected in
processing data records.

60

(3C)

2

DCBRKP

Relative position of the first byte of the key within
each logical record. Maximum permissible value:
logical record length minus key length.

62

(3E)

•• 2

DCBBLKSI

Block size.

64

(40)

4

DCBMSWA

Address of a main storage work area for use by the
control program when new records are being added to
an existing data set.

68

(44)

2

DCBSMSI

Number of bytes in area reserved to hold the highest
level index.

70

(46)

'.. 2

DCBSMSW

Number of bytes in work area used by control program
when new records are being added to the data set.

72

(48)

1

DCBNCP

Number of copies of the READ-WRITE (type K) channel
programs that are to be established for this data
control block ( 99 maximum).

73

(49)

• 3

DCBMSHI

Address of a main storage area to hold the highest
level index.

76

(4C)

4

DCBSETL

Address of SETL module.

(
48

OS System Control Blocks

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning
ACCESS METHOD INTERFACE -- ISAM (Continued)
I

80

(50)

DCBEXCDl

1

1 ...
. 1 •.
· .1 .•
... 1

1 ...
• 1 ••

• • 1.
.•. 1
81

(51)

.

DCBEXCD2

1

1 •••
. 1 ••

· .1.
• .• 1

xxxx

»

First byte in which exceptional conditions detected
in processing data records are reported to the user.
Lower key limit not found.
Invalid device address for lower limit •
Space not found.
Invalid request •
Uncorrectable input error.
Uncorrectable output error •
Unreachable block •
Overflow record •
Second byte in which exceptional conditions detected
in processing data records are reported to the user.
Sequence check.
Duplicate record •
DCB closed when error was detected.
Overflow record.
(Reserved bits)

82

(52)

•• 2

DCBLRECL

Logical record length for Fixed-length record
formats.
Variable-length record formats:
Maximum logical record length or an actual logical
record length changed dynamically by the user when
creating the data set.

84

(54)

4

DCBESETL

Address of the ESETL routine in the GET module.

88

(58)

4

DCBLRAN

Address of READ-WRITE K module or exclusive module.

92

(5C)

4

DCBLWKN

Address of WRITE KN module.

96

(60)

4

DCBRELSE

Work area for temporary storage of register contents.

100

(64)

4

DCBPUTX

Work area for temporary storage of register contents.

104

(68)

4

DCBRELX

Address of read exclusive module.

108

(6C)

4

DCBFREED

Address of dynamic buffering module.

112

(70)

1

DCBDBUFN

Reserved

113

(71)

• 7

DCBFTMI2

Direct access device address of the first track of
the second level master index (in the form MBBCCHH).

120

(78)

5

DCBLEMI2

Direct access device address of the last active entry
in the second level master index (in the form CCHHR).

125

(7D)

. 7

DCBFTMI3

Direct access device address of the first track of
the third level master index (in the form MBBCCHH) •

132

(84)

5

DCBLIMI3

Direct access device address of the last active entry
in the third level master index (in the form CCHHR).

137

(89)

• 1

DCBNLEV

Number of levels of index.

138

(8A)

•• 3

DCBFIRSH

HHR of the first data record on each cylinder.

•

•

DeB -- ISAM

49

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning

141

(8D)

• 1

DCBHMASK

142

(8E)

•• 2 •

DCBLOT

HH is the last prime data track on each cylinder.

144

(90)

1

DCBHIRCM

Highest possible R for tracks of the cylinder and
master indices.

145

(91)

• 1

DCBHIRPD

Fixed-length record format:
Highest possible R for prime data tracks.

146

(92)

•• 1

DCBHIROV

Fixed-length record format:
Highest possible R for overflow data tracks.

147

(93)

••• 1

DCBHIRSH

R of the last data record on a shared track, if
applicable.

148

(94)

2

DCBTDC

Tag deletion count.

150

(96)

•• 2

DCBNCRHI

Number of storage locations needed to hold the highest level index.

152

(98)

4

DCBRORG3

For each use of the data set, the number of READ or
WRITE accesses to an overflow record which is not the
first in a chain of such records.

156

(9C)

4

DCBNREC

Number of logical records in the prime data area.

160

(AO)

1

DCBST

Status indicators.

•

1 ••.
• 1 ••

•• 1.
••• 1
x •••
• 1 ••
• • 1.
••• 1

07
FF

Device is a 2301 drum.
Device is other than a 2301 drum.

Single schedule mode.
Key sequence checking is to be performed •
Loading has completed. Set to 1 by the Close routine
and to 0 by the first execution of the Put routine.
The extension of the data set will begin on a new
cylinder.
Reserved
First macro instruction not yet received •
Last block full •
Last track full •

161

(Al)

• 7

168

(A8)

1

169

(A9)

• 7

DCBFTMIl

Direct access device address of the first track of
the first level master index (in the form MBBCCHH).

176

(BO)

1

DCBNTHI

Number of tracks of high-level index.

177

(Bl)

• 7

DCBFTHI

Direct access device address of the first track of
the highest level index (in the form MBBCCHH).

184

(B8)

8

•

DCBLPDA

Direct access device address of the last prime data
record in the prime data area (in the form MBBCCHHR).

192

(CO)

5

•

DCBLETI

Direct access debice address of the last active normal entry of the track index on the last active
cylinder (in the form CCHHR).

197

(C5)

• 1

50

DCBFTCI

Direct access device address of the first track of
the cylinder index (in the form MBBCCHH).
Reserved

OS System Control Blocks

Reserved

(

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- ISAM
Hex.
Dig. Field Description, Contents, Meaning
ACCESS METHOD INTERFACE - ISAM (Continued)

198

(C6)

•• 2

•

OCBNBOV

Number of bytes left on the current track of the
independent overflow area.

200

(C8)

5

•

OCBLECI

Direct access device address of the last active entry
in the cylinder index (in the form CCHHR).

20S

(CO)

• 1

206

(CE).. 2

208

(00)

S

213

(OS)

• 1

214

(06)

•• 2

0

DCBNOREC

Number of logical records in an overflow area.

216

(D8)

8

0

DCBLIOV

Direct access device address of the last record written in the independent overflow area (in the form
MBBCCHHR) •

224

(EO)

2

•

DCBRORG1

Number of cylinder overflow areas that are full.

226

(E2)

•• 2

228

(E4)

4

OCBWKPT1

A pointer to a work area or to a constructed channel
program for which space is obtained by GETMAIN macro
instructions issued by Open executors.

232

(E8)

4

DC BWKPT2

Additional pointer as in OCBWKPT1.

236

(EC)

4

DCBWKPT3

Additional pointer as in DCBWKPT1.

240

(FO)

4

DCBWKPT4

Additional pointer as in DCBWKPT1.

244

(F4)

4

DCBWKPTS

Additional pointer as in DCBWKPT1.

248

(F8)

4

DCBWKPT6

Additional pointer as in DCBWKPT1.

Reserved
•
•

DCBRORG2

Number of tracks (partially or wholly) remaining in
the independent overflow area.

OCBLEMl1

Direct access device address of the last active entry
in the first level master index (in the form CCHHR).
Reserved

Reserved

)
DCB -- ISAM

Sl

(

Data Control Block-BDAM

The data control block for BDAM is given
below. The common interface and foundation
sections are the same for all DeBs. The
direct access storage device and BDAM
interface sections complete this format of
the DCB.
Figure 4 illustrates the format of the
data control block used in BDAM. Descriptions of the fields follow the
illustration.

DCB -- BDAM

53

DATA CONTROL BLOCK -- BDAM
Device Interface
17 (11)

16 (10)

DCBKEYLE

DCBREL
No. of Tracks or Blocks

Key Length

19 (13)

Common Interface
21 (15)

20 (14)

DCBBUFCB
Address of Buffer Pool Control Block

DCBBUFNO
No. of Buffers Required

26 (IA)

24 (18)

DCBDSORG
Data Set Organization

DCBBUFL
Buffer Length

28 (IC)

DCBIOBAD
First lOB Address

31 (1F)

Foundation Extension
32 (20)
DCBHIARC, DCBBTFEK, DCBFALN

33 (21)

36 (24)

37 (25)

Reserved

DCBEXLST
Exit List Address

DCBRECFM
Record Format

39 (27)

Foundation

'-----Sef-o-re--OPE-i,r--------------------------------------------------.---------------------.---_.-----------------------------------------------.---------------------------.. -----.... -.--.--.--.
40 (28)
DCBDDNAM
DD Statement Name

48 (30)

DCBOFLGS

49 (31)

DCBIFLG

50 (32)
Type of

lOS Flags

Open Flags

(

DCBMACR
Instruction and Options

I/o Macro

51 (33)

After OPEN
42 (2A)

40 (28)

DCBTlOT
Offset to DD Entry in TlOT

44 (2C)

DCBIFLGS

45 (2D)

105 Flags
48 (30)

Type of

DCBMACRF
Instruction and Options

I/o Macro

DCBDEBAD
Address of DEB

DCBOFLGS
Open Flags

-Figure

4.

Data Control Block - BDAM (Part 1 of 2)

c
54

OS system Control Blocks

DATA CONTROL BLOCK -- BDAM
BDAM Interface
49 (31)

52 (34)
DCBOPTCD

DCBREAD, DCBWRITE
Address of Read or Write Module

53 (35)
DCBCHECK
Address of Check Module

Option Codes
56 (38)

DCBSYNAD
Address of SYNAD Routi ne
60 (3C)

62 (3E)
Reserved

DCBBLKSI
Maximum Block Size

64 (40)
DCBIOBSQ
Address of First lOB on Unscheduled Queue
68 (44)
DCBSQND
Address of Lost lOB on Unscheduled Queue
72 (48)

DCBIOBUQ
Address of First lOB on Un posted Queue

76 (4C)
DCBUQND
Address of Last lOB on Unposted Queue
80 (50)

81 (51)
DCBLlMCT
No. of Tracks/No. of Relative Blocks to be Searched

Reserved
84 (54)

DCBXCNT

85 (55)

DCBXARG
Address of Read Exclusive List

88 (58)
DCBDRDX
Address of Read Exclusive Module
92 (5C)

DCBDFOR
Address of Format Module

96 (60)
DCBDFBK
Address of Feedback Module
100 (64)
DCBDYNB
Address of Dynamic Buffer Module

Figure

4.

Data control Block - BDAM (Part 2 of 2)

)
DCB -- BDAM

55

DATA CONTROL BLOCK -- BDAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
DEVICE INTERFACE

16

(10)

1

DCBKEYLE

Key length.

17

(11)

• 3

DCBREL

Number of relative tracks or blocks in this data set.
COMMON INTERFACE

20

(14)

p

DCBBUFNO

Number of buffers required for this data set.
May range from 0 to 255.

21

(15)

• 3

DCBBUFCB

Address of buffer pool control block.

24

(18)

2

DCBBUFL

Length of buffer.

26

(lA)

•• 2

DCBDSORG

Data set organization being used.

26

(lA)

Byte 1
• •. 1
27

(lB)

28

(lC)

Code

xx.x xxx.
•• 1.

Byte 2
4

DCBIOBAD

May range from 0 to 32,767.

DA
U

(Reserved bits)
Direct organization •
Unmovable - the data contains location dependent
information.

Reserved
Address of the standard fields of the first lOB in
the pool of lOBS.
FOUNDATION EXTENSION

32

(20)

1

DCBHIARC, DCBBFTEK, DCBBFALN

x. •. .x ..
0 •••• 0 ••

o. ..

.1 ..

1. .• .0 ..

Buffer pool location, coded in the DCB macro
instruction:
Before Open none No choice made in the DCB macro instruction.
After Open If no choice is made in the DD statement either
(as shown by the JFCBHIAR field), the Open routine resets these two bits from 00 to 01.
o
Hierarchy 0 main storage.
(See also: After Open, above.)
1
Hierarchy 1 main storage.

.xxx x •••

(Reserved bits.)

• .xx

33

(21)

• .10
• .01

D

• .11

F

• 3

F

Buffer alignment:
Doubleword boundary.
Fullword not a doubleword boundary, coded in the
DCB macro instruction,.
Fullword not a doubleword boundary, coded in the
DD statement.
Reserved

(
56

OS System Control Blocks

DATA CONTROL BLOCK -- BDAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
FOUNDATION EXTENSION (Continued)

36

(24)

1

DCBRECFM

Record Format.
Code

10 ••
01 ••
11. '.
• .• 1.
••• 0 000,.
'••• 1

37

(25)

•

3

DCBEXLST

F
V
U
T

Fixed record length.
Variable record length.
Undefined record length.
Track overflow,.
Always zeros •
Key length (KEYLEN) was specified in the DCB
macro instruction,. This bit is inspected by the
Open routine to prevent overriding a specification of KEYLEN=O by a nonzero specification in
the JFCB or data set label.

Exit list.
Address of a user-provided exit list.
FOUNDATION SEGMENT
FOUNDATION SEGMENT -- BEFORE OPEN

40

(28)

8

DCBDDNAM

This name must be the same as that appearing in the
name field of the data definition statement which
defines the data set associated with this DCB.

48

(30)

1

DCBOFLGS

Flags used by the Open routine.

1 .•••

• x'.. .x ••
• .0.. 0 ••••
.•. 1

•• 0.

•. 1.
... 1
49

(31)

• 1

DCBIFLG

00 ••
01 ••

11 ••
0.00
00 ••
11 ••
10 ••
01 ••

•• xx

The data set is being opened for INOUT or
OUTPUT.
(Reserved bits)
Always set to Q.
An OPEN has been successfully completed •
Set to Q by an I/O support function when that
function takes a user exit. It is set to 0 to
inhibit other I/O support functions from processing this particular DCB.
Set to 1 on return from user exit to the I/O
support-function which took the exit.
Set to ! by an I/O support function if the DCB
is to be processed by that function.
Used by I/O Supervisor in communicating error
conditions and in determining corrective
procedures.
Not in error procedure.
Error correction in process.
Permanent error condition,.
Always zeros.
Always use I/O supervisor error routine.
Never use I/O supervisor error routine.
Never use I/O supervisor error routine.
Never use I/O supervisor error routine.
(Reserved bits)

)
DCB -- BDAM

57

DATA CONTROL BLOCK -- BDAM
Offset

Bytes and
Alignment

Field
Name

Field Description.. Contents.. M'eaninq
FOUNDATION SEGMENT -- BEFORE OPEN (Continued)

50

(32)

'.. 2

DCBMACR

Macro instru~tion reference.
Major macro instructions and various options associated with them that will be used.

Byte 1
50

(32)

00. '•
• ,.1.
•• • 1
1,•••
,.1 ••
'•• 1,.
'. '•• 1

R
K
I
S

x
C

Always zero for BDAM.
READ
Key segment with READ •
ID argument with READ.
System provides area for READ (dynamic
buffering).
Read exclusive.
CHECK macro instruction.

Byte 2
51

(33)

000 •
• • 1,.
1 •• '.

W
K
I

'•• 1,.

A

'••• 1
.x.x

Always zero for BDAM.
WRITE
Key segment with WRITE.
ID argument with WRITE.
(Reserved bits)
Add type of WRITE.

FUNDATION SEGMENT -- AFTER OPEN
40

(28)

2

DCBTIOT

Offset from the TIOT origin to the TIOELNGH field in
the TIOT entry for the DD statement associated with
this DeB.

42

(2A)

•• 2

DCBMACRF

contents and meaning are the same as DCBMACR in the
foundation before OPEN.

44

(2C)

1

DeBIFLGS

Contents and meaning are the same as DCBIFLG in the
foundation before OPEN.

45

(2D)

•

DCBDEBAD

Address of the associated DEB.
Note: The above fields overlay the DCBDDNM field
during OPEN and are restored to their original form
at CLOSE.

48

(30)

1

DCBOFLGS

contents and meaning are the same as DCBOFLGS in the
foundation before OPEN.

3

(
58

OS system Control Blocks

DATA CONTROL BLOCK -- BDAM
Offset

Bytes and Field
Alignment Name

Field Description, Contents, Meaning
ACCESS METHOD INTERFACE -- BDAM

49

(31)

• 3

DCBREAD, DCBWRITE
Address of the READ/WRITE module.

52

(34)

1

DCBOPTCD
1 •••
• 1. '.

•• 1.

.• • • 1

1 .••
. 1 ..

•• 1.
.•. 1

Option Codes:
Code
W
Write validity check.
Track overflow •
E
Extended search •
F
Feedback •
A
Actual addressing,.
Dynamic buffering •
Read exclusive,•
R
Relative block addressing •

53

(35)

• 3

DCBCHECK

Address of the CHECK module, IGG019LI.

56

(38)

4

DCBSYNAD

Address of SYNAD (synchronous error) routine.

60

(3C)

2

62

(3E)

•• 2

DCBBLKSI

Maximum block size.

64

(40)

4

DCBIOBSQ

Address of first IOB on unscheduled queue for either;
A WRITE-add request when another WRITE-add is in progress, or a READ-exclusive request when the READexclusive list is full.

68

(44)

4

DCBSQND

Address of last IOB on unscheduled queue.

72

(48)

4

DCBIOBUQ

Address of the first IOB on the unposted queue.
This queue is for IOBs requesting a record already
under READ exclusive control.

76

(4C)

4

DCBUQND

Address of the last job on the unposted queue that is
maintained by the READ exclusive module.

80

(50)

1

81

(51)

'. 3

DCBLIMCT

Number of tracks or number of relative blocks to be
searched (extended search option).

84

(54)

1

OCB XC NT

Number of entries in the READ exclusive list.

85

(55)

'. 3

DCBXARG

Address of the READ exclusive list.

88

(58)

4

DCBDRDX

Address of the READ exclusive module.

92

(SC)

4

DCBDFOR

Address of a FORMAT module.

96

(60)

4

DCBDFBK

Address of a FEEDBACK module.

100

(64)

4

DCBOYNB

Address of dynamic buffer module.

Reserved

Reserved

)
DCB -- BDAM

59

(

Data Control Block-QTAM

The format of a data control block (DeB) in
QTAM is determined by the character of the
data set and is shown by variations in the
block segments. Figure 5 shows the format
of the block by segments; descriptions of
the fields follow the illustration.
QTAM Data Sets
QTAM message processing programs and QTAM
message control programs use a data control
block (DeB) to describe their respective
data sets:
• processing Program Message Queues.
• Direct-Access Storage Device (DASD)
Message Queues.
• Line Groups.
• Checkpoint Data Set.
The checkpoint DeB is identical in format to that used for the DASD message
queue; it is distinguished by the entry
TPCHKPNT in the DCBDDNAM field in the foundation segment before Open.
QTAM DCB Segments
The three segments of a DCB, and their uses
in QTAM, shown and described here, are:
Prefix Segment
• Line Group Interface.
• Processing Program Message Queue
Interface.
• DASD Message Queue Interface, Checkpoint Data Set Interface.
Foundation Segment
• Before Open.
• After Open.
Extension Segment
• Line Group Extension (Polling List
Origin).
• Processing Program Message Queue
Extension.

)
DeB -- QTAM

61

DATA CONTROL BLOCK -- QTAM
WTTA Interface
16 (10)

17(11)
DCBBQFLG
WTTA Flags

18 (12)

19 (13)

DCBWTEOM
EOM Character

DCBWTEOT
EOT Character

DCBWTPAD
No. of Padding Characters

Data Set Interface

---I.Tiie---Gro.u-p----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------1
21 (IS)

20 (14)

DCBCLPS
Address of the LPS Routine

DCBBUFRQ
Buffers Requested
24 (18)

DCBINTVL
Intentional Interval

25 (19)

DCBACLOC

26 (1A)

DCBDSORG
Data Set Organization

Offset

,
28 (IC)

DCBDEVTP

29 (lD)

DCBIOBAD
Address of First lOB

Device Type
33 (21)

32 (20)

DCBCPRI
Priority

DCBLCBAD
Base for Addressing LCBs

37 (25)

36 (24)

DCBEXLST
Address of the Exit List

DCBEIOBX
Size of the LCB

38 (27)

i

Processing Program Message Queue
21 (15)

20 (14)

DCBTRMAD
Address of the Terminal Name

DCBBUFRQ
Buffers to be Filled

26 (IA)

24 (18)

28 (lC)

DCBSEGAD
Address of Current Segment

32 (20)

DCBEODAD
Address of the EODAD Routine
37 (25)

36 (24)

DCBRECFM
Record Format

DASD Message

DCBEXLST
Address of the Exit List

DCBBUFNO

21 (15)

Reserved

DCBBUFCB
Address of Terminal Table
26 (lA)

24 (18)

DCBBUFL
Length of the Data

28 (lC)

DCBIOBAD
Address of the lOB

5.

39 (27)

Queue I Checkpoint

20 (14)

Figure

DCBDSORG
Data Set Organization

DCBSOWA
Size of the Work Area

DCBDSORG
Data Set Organization

31 (IF)

Data Control Block - QTAM (Part 1 of 2)

(
62

OS System Control Blocks

DATA CONTROL BLOCK -- QTAM

j····.Seiiire··.OPffr.·.......··.·............·..···............................................................................................................................................. .

Foundation
40 (28)

DCBDDNAM
DD Statement Data Set Name

48 (30)

50 (32)

49 (31)
DCBOFLGS
Open Routine Flags

I/o

DCBIFLGS
Supervisor Flags

DCBMACR
Macro Instructi ons

51 (33)

After OPEN
40 (28)

42 (2A)
DCBTlOT
Offset in TlOT Table to DD Entry

44 (2C)

45 (2D)

I/o
48 (30)

DCBMACRF
Macro Instruction

DCBIFLGS
Supervisor Flags

DCBOFLGS
Open Routine Flags

DCBDRBAD
Address of the DEB
49 (31)

..
.. --- .. --- .. --------- ..... ----- .. -... -- .... ----------- ...
Extension
------~

,

DCBREAD I DCBWRITE I DCBGET I DCBPUT
Address of the Access Modules

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

51 (33)

.. --- .... ----- ... .. -- .... - ........ _------_ ..... --------- .. - .... --_ .. ------ ............. - ..... - .. --_ .. - .................... --_ ........... ---_ ........ --........ -_:

·····Cfne-·Gj.·o·u·p···································.. --...... -... -... -.........-.. ------... ---..... ----------.... -------.. -.......... -.-..-..... -.-...... -... -.........--.. -... -.. -...... --...... -52 (34)

DCBKSTAT
Error Threshold

56 (38)
DCBCPOLL
Polling List Origin

I

DCBCPOLL

l

r-59 (3B)

r-f--

DCBCPOLL

I

DCBCPOLL

Processing Program Message Queue
52 (34)
DCBRECRD
Not Used
56 (38)
DCBSYNAD
Address of the SYNAD Routine

l. . . . ~._.~. _.~_ _ . . . . . . . . . . . . . ._. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
60 (3C)

Figure

DCBEOBLK
Not Used

5.

63 (3F)

Data Control Block - QTAM (Part 2 of 2)

)
DCB -- QTAM

63

DATA CONTROL BLOCK -- QTAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
WTTA Interface

16

(10)

OCBBQFLG

1

x •• '. .xxx
• 1 ••
,. ,.1.
'. '. ,.1
1 •••

·1

17

(11)

18

(12)

1

19

(13)

.

1

WTTA flag byte.
(Reserved bits)
WRU feature is to
lAM feature is to
WRU feature to be
WRU feature is to

be used •
be used.
used in the Send Header subgroup.
be used in the End Send subgroup..

OCBWTEOM

The EOM character.

DC BWTEOT

The EOT character.

OCBWTPAD

Number of padding characters required for motor-on
delay,.
LINE GROUP INTERFACE

20

(14)

1

DCBBUFRQ

Number of buffers requested for a read or write
operation.

21

(15)

• 3

DCBCLPS

Address of the line procedure specification routine .•

24

(18)

1

DCBlNTVL

Number of seconds of intentional delay between passes
through a polling list for nonswitched lines.

25

(19)

• 1

DCBACLOC

Offset, relative to zero, of the device access field
for each terminal table entry.

26

(lA)

'.. 2

DCBDSORG

Data set organization.

26

(lA)

Byte 1
xx •• xxxx
CX

•• 01
27

(lB)

28

(lC)

1

29

(10)

•

32

(20)

1

Byte 2

3

Device type pointer.

DCBlOBAD

Address of first lOB.

DCBCPRl

Communication priority,. Relati ve priority to be
given to sending and receiving operations.

,.1,••
• ,.1.
• '•• 1

(21)

·

36

(24)

37

(25)

Reserved

OCBOEVTP

xxx x x·•• '.

33

(Reserved bits)
Line group •

R
E
S

(Reserved bits)
Receiving has priority.
Receiving and sending have equal priority,•
Sending has priority •

DCBLCBAD

Base for addressing LCBs.
(Base = Address of first LeB -- length of one LCB).

1

DCBEIOBX

Extended lOB index.
(LCB) '.

·3

DCBEXLST

Address of the exit list,.

3

Size of a line control block

(
64

OS System Control Blocks

DATA CONTROL BLOCK -- QTAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
PROCESSING PROGRAM MESSAGE QUEUE INTERFACE

20

(14)

1

DCBBUFRQ

Number of buffers to be filled from the direct access
queue.

21

(15)

• 3

DCBTRMAD

Address of a user-provided area in which the terminal
name is stored.

24

(18)

2

DCBSOWA

Size of the user-provided work area.

26

(lA)

•• 2

26

(lA)

DCBDSORG
Byte 1
xxxx x.xx

Data set organization.
Code
(Reserved bits)
MQ Problem program message queue •

• 1 ••
27

(lB)

28

(lC)

4

DCBSEGAD

Address of current segment.

32

(20)

4

DCBEODAD

Address of a user-provided routine.

36

(24)

1

DCBRECFM

Record Format.
Code
(Reserved bits)
R
Record
G
Message
S
Segment

Byte 2

0000 ••• 0
'•.• 1.
.1,••
1 •••

37

(25)

• 3

DCBEXLST

Reserved

Address of the exit list.
DIRECT ACCESS STORAGE DEVICE (DASD) MESSAGE QUEUE
INTERFACE, CHECKPOINT DATA SET INTERFACE

20

(14)

1

DCBBUFNO

Reserved

21

(15)

• 3

DCBBUFCB

Address of the terminal table.

24

(18)

2

DCBBUFL

Size of the data in the buffer equated to IECKBUFL.

26

(lA)

•• 2

26 (llA)
27

(lB)

28

(lC)

4

DCBDSORG
Byte 1
xxxx .xxx
10 ••

Data set organization.
Code
(Reserved bits)
CQ Direct-access message queue

Byte 2

Reserved

DCBIOBAD

Address of input/output block.
FOUNDATION SEGMENT
FOUNDATION SEGMENT -- BEFORE OPEN

40

(28)

8

DCBDDNAM

Data set name as used in data definition statement.
Used by Open routine to locate address of job file
control block (JFCB).
Note: If the DD name is TPCHKPNT, this DCB is used
for the checkpoint data set.

)
DCB -- QTfu'vl

65

DATA CONTROL BLOCK -- QTAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
FOUNDATION SEGMENT

48

(30)

9

DCBOFLGS
xxx. xxx.
,. ,. ,.1

• '•• 1

49

(31)

• 1

DCBIFLGS
00. '.
01 ••
11 ...

• .10
• .. 01
00 ••
11 ••
10 ••
01 ••

•• xx
50

(32)

50

(32)

•• 2

DCBMACR

Byte 1
x,•• x xxxx
. 1 ••
• .1,.

51

Byte 2
x,•• x xxxx

(33)

,.1,. '.
• .1,.

BEFORE OPEN (Continued)

Flags used by OPEN.
(Reserved bits)
Opening has been successfully completed.
This bit is set to 1 by an I/O support routine if the
DCB is to be processed by that routine.
Used by lOS in communicating error conditions and in
determining error procedures.
Not in error procedure.
Error correction in process.
Permanent error conditi 00..
Channel 9 printer carriage punch.
Channel 12 printer carriage punch.
Always use lOS error routine.
Never use lOS error routine.
Never use lOS error routine.
Never use lOS error routine.
(Reserved bits)
Macro instruction reference -- specifies the major
macro instructions and various options associated
with them. Used by open routine to determine the
access method.
(Reserved bits)
PUT for message queue •
WRITE for line group.
(Reserved bits>
GET for message queue.
READ for line group.
FOUNDATION SEGMENT -- AFTER OPEN

40

(28)

2

DCBTIOT

Points to the DO entry in the task I/O table for this
DCB. It is the offset of the DO entry from the
beginning of the task I/O table.

42

(2A)

•• 2

DCBMACRF

Contents and meaning are the same as DCBMACR in the
foundation before execution of open.

44

(2C)

1

DCBIFLGS

Contents and meaning are the same as DCBIFLGS in the
foundation before execution of open.

45

(2D)

•

DCBDEBAD

Address of the DEB associated with this DCB.

48

(30)

1

DCBOFLGS

contents and meaning are the same as DCBOFLGS in the
foundation before execution of open,.

49

(31)

• 3

DCBREAD, DCBGET, DCBPUT, DCBWRITE
Address of the READ, GET, PUT, and WRITE module.

3

(
66

OS System Control Blocks

DATA CONTROL BLOCK -- QTAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
LINE GROUP EXTENSION (POLLING LIST ORIGIN)

52

(34)

4

52

(34)

Byte 1

Threshold value for number of transmissions.

53

(35)

Byte 2

Threshold value for number of data checks.

54

(36)

Byte 3

Threshold value for number of interventions required.

55

(37)

Byte 4

Threshold value for number of timeouts.

56

(38)

56

(-38)

n times

DCBKSTAT

DCBCPOLL
Byte 1

xxxx ,.xxx
i ....

57

(39)

Byte 2-4

Four threshold values for error counts.

A 4-byte fie1d for each (n) po11ing 1ist.
Adapter type
(Reserved bits)
WTTA
Address of the po1ling 1ist.
PROCESSING PROGRAM MESSAGE QUEUE EXTENSION

52

(34)

4

DCBRECRD

Not used by QTAM.

56

(38)

4

DCBSYNAD

Address of the user provided routine to be entered if
a work unit is 10nger than the work area provided for
input.

60

(3C)

4

DCBEOBLK

Not used by QTAM .•

)
DCB -- QTAM

67

(

Data Control Block-BTAM

The data control block (DeB> used in BTAM
is described in the following. The common
interface and foundation extension exist
for all DeBs. Figure 6 illustrates the
format of this DeB; a description of the
fields follows the illustration.

)
DeB -- BTAM

69

DATA CONTROL BLOCK -- BTAM

WTTA Interface
16 (10)

17 (11)

DCBBQFLG
WTTA Flags

18 (12)

DCBWTEOM
EOM Character

19 (13)
DCBWTPAD
Number of Padding Characters

DCBWTEOT
EOT Character

Common Interface
20 (14)

DCBBUFNO
Number of Buffers

24 (18)

28 (Ie)

DCBBUFCB
Address of Buffer Pool Control Block

21 (15)

DCBBUFL
Buffer Length
DCBDEVTP
Index to Directory
I/O Directory

DCBDSORG
Data Set Organization

26 (I A)

29 (ID)

DCBIOBAD
Base for Addressing lOBs

Foundation Extension
32 (20) DCBHIARC, DCBBFTEK

36 (24)

33 (21)

DCBERROP
Error Recovery Procedures

37 (25)

DCBEIOBX
Size of lOB

34 (22)
Reserved

DCBEXLST
Address of User-Provided List

r-.Foundation
. . . . . ·. -..-.. . . -----.. . . . ·---.. .-----·---.. -.. . ---..-.. . -.. . . . .--.. .----------------.. . ----.. --.. . . . --.. . . . . . . . . . --.. -.--.. . . .-.. . --.. --.. . . . -.. -.. . . . . . . . -.. -.. . . --------.. ---.. . . -.. ----.. . -.. . -----.. . . . . . -.. . . . . . ----.. . ----.. . . . . . . . :
~

! i:::)dation
i!
iI

!

Before OPEN

Jl i

DCBDDNAM
Name From DD Statement

~-

48 (30)

DCBOFLGS
Open Flags

49 (31)

DCBIFLG
lOS Error Flags

"'1-'

DCBMACR
Type of I/O Macro Instruction and Options

50 (32)

I

, Foundation After OPEN
40 (28)

DCBTlOT
Offset to DD Entry in TIOT

44 (2C)

DCBIFLGS
lOS Error Flags

48 (30)

DCBOFLGS
Open Flags

:

!:

45 (2D)

DCBMACRF
Type of I/O Macro Instruction and Options

42 (2A)

1

!

DCBDEBAD
Address of DEB

S-TA-,Vi--inte-rfcice---------------------------------------------------------------------------------------------------------------------------------------------------------------------------,

r-

49 (31)

148 (30)

DCBLERB
Address of Line Error Block

52 (34)

-Figure

DCBREAD, DCBWRITE
Address of Read or Write Module

6.

Data Control Block - BTAM (Part 1 of 2)

(
70

OS System Control Blocks

DATA CONTROL BLOCK -- BTAM

sse

Interface - Before Open
57 (39)

56 (38)
Reserved

58 (3A)
Reserved

DCBBSTSX
Address of the Interface Resolution Routine

60 (3C)

-

DCBXCODE
PTOP Flag

64 (40 )

-i..-

Tsse Interface - After Open
56 (38)

DCBXMODE
BSC Transmission Mode

57 (39)

60 (3C)

DCBBSTSX
DLE

61 (3D)

Reserved
99 (63)T

DCBXCODE
Control Station Flag
Transmission Code
DCBBSSTX
STX

DCBBSAKO
ACK-O

64 (40)

68 (44)

DCBBSENQ
ENQ

72 (48)

DCBBSEOT
EaT

69 (45)

58 (3A)

DCBBSRSV
DLF

59 (3B)

DCBBSWBT
WBT

62 (3E)

DCBBSTEX
DLE

63 (3F)

DCBBSETX
ETX

DCBBSAKI
ACK-l

66 (42)

DCBBSNAK
NAK

70 (46)

73 (49)

DCBBSETB
ETB

71 (47)

DCBBSDLE
DLE

DCBBSSYN
SYN, SYN, SYN

76 (4C)

DCBBSONL

78 (4E)

80 (50)

DCBBSRVI

82 (52)

DCBBSSAK

-'-

~

Reserved

T
-Figure

6.

Data Control Block - BTAM (Part 2 of 2)

)
DCB -- BTAM

71

DATA CONTROL BLOCK -- BTAM
Offset

Bytes and Field
Alignment Name

Field Description, Contents, Meaning
WTTA Interface

16

(10)

17

(11)

18

(12)

19

(13)

1

• 1
1

1

DCBBQFLG

WTTA flag byte.

x •• x xxxx
. 1 ••
.. 1.

(Reserved bits.)
WRU feature to be used •
lAM feature to be used,•

DCBWTEOM

The EOM character.

DCBWTEOT

The EOT character.

DCBWTPAD

Number of padding characters required for motor-on
delay.
COMMON INTERFACE

20

(14)

1

DCBBUFNO

Number of buffers obtained by the Open routine.
Range: 0-255

21

(15)

• 3

DCBBUFCB

Address of the buffer pool control block.

24

(18)

2

DCBBUFL

Buffer length,.

26

(lA)

••

DCBDSORG

Data set organization being used:

26

(lA)

2

Byte 1
xxx. xxxx
CX

• • ,.1

27

(lB)

28

(lC)

29

(lD)

Range:

0 - 32,760 bytes

(Reserved bits)
Telecommunications -- line group •

Byte 2

Reserved

1

DCBDEVTP

Index to the device entry in the device I/O
directory.

• 3

DCBIOBAD

Base for addressing lOBs.
(Base = Address of first lOB -- length of an lOB)
FOUNDATION EXTENSION

32

(20)

1

DCBHIARC, DCBBFTEK
Code

x. •• • x ••

o•• '.

.0 ••

o. ..

.1 ..

1 ••• ,.0 ••

.xxx •• xx
x •••
1 •••

Buffer pool location, coded in the DCB macro
instruction:
Before Open none No choice made in the DCB macro instruction.
After Open If no choice is made in the DD statement either
(as shown by the JFCBHIAR field), the Open routine resets these two bits from 00 to 01.
o Hierarchy 0 main storage.
(See also: After Open, above.)
1
Hierarchy 1 main storage.
(Reserved bits.)
Buffering Technique:
D
Dynamic buffering.

(
72

OS System Control Blocks

DATA CONTROL BLOCK -- BTAM
Offset

Bytes and Field
Alignment Name

Field Description, Contents, Meaning
FOUNDATION EXTENSION (Continued}

33

(21)

• 1

DCBERROP
xxx.
••• 1
1 ••.

. 1 ..

•• 1.
••• 0
..• 1

Error recovery procedure:
Code
(Reserved bits)
T
On-line test facilities to be used •
Threshold and cumulative error counts to be
C
maintained.
W
Text-write errors to be retried •
R
Text-read errors to be retried •
E
Basic error procedures to be followed •
N
No error recovery procedures to be followed •

(22)

•

35

(23)

••• 1

36

(24)

1

DCBEIOBX

Size of extended lOB.
Size of an lOB associated with this DCB.

37

(25)

• 3

DCBEXLST

Address of (a user-provided) exit list.

• 1

DCBBUFCT

Contains maximum number of buffers to be obtained by
BTAM for a Read or write operation (dynamic buffering
only).
Reserved

FOUNDATION SEGMENT
FOUNDATION SEGMENT -- BEFORE OPEN
40

(28)

8

DCBDDNAM

DD name of the data set. This name is matched to the
name field of the data definition (DD) statement.
(It is used by the Open routine to locate the appropriate DD entry in the task input/output table
(TIOT).)

48

(30)

1

DCBOFLGS

Flags used by the Open routine:

xxx. xx.x

(Reserved bits)
OPEN has been successfully completed •
Set to Q by an I/O support function when that function takes a user exit.
(It is set to 0 to inhibit
other I/O support functions from processing this particular DCB.)
Set to 1 on return from the user exit to the I/O support function which took the exit.

•• • 1

•• 0.

• • •• • .1.
49

(31)

• 1

DCBIFLG

Flags used by I/O supervisor to communicate error
conditions and to determine corrective procedures:

00 ••
01 ••
11 ••
• • 10
•• 01

Not in error procedure.
Error correction in process.
Permanent error condition.
Channel 9 printer carriage tape punch sensed •
Channel 12 printer carriage tape punch sensed •
Always use I/O supervisor error routine.
Test lOS mask (IMSK) for error procedure.
Never use I/O supervisor error routine.
(OPEN sets
these bits.)
(Reserved bits)

00 ••
01 ••
11 ••
10 ••
01 ••
•• xx

)
DCB -- BTAM

73

DATA CONTROL BLOCK -- BTAM
Offset

Bytes and Field
Alignment Name

Field Description, Contents, Meaning
FOUNDATION SEGMENT -- BEFORE OPEN (Continued)

50

(32)

50

(32)

'.. 2

DCBMACR

Macro instruction reference:

Byte 1

xx.x xxxx
•• 1.

(Reserved bits)
READ

Byte 2
51

xx.x xxxx

(33)

,•. 1.

(Reserved bits)
WRITE
FOUNDATION SEGMENT -- AFTER OPEN

40

(28)

2

DCBTIOT

Offset of the TIOELNGH field to the TIOT reference
point. The TIOELNGH field is located in the DD entry
of the TIOT that was created from the DD statement
associated with this DCB.

42

(2A)

•• 2

DCBMACRF

Contents and meaning are the same as in DCBMACR field
before Open.

44

(2C)

1

DCBIFLGS

contents and meaning are the same as in DCBIFLG field
before Open.

45

(2D)

•

DCBDEBAD

Address of the associated DEB

3

Note: The above fields overlay the DCBDDNAM field
during OPEN and are restored to their original form
at CLOSE.
48

(30)

1

DCBOFLGS

Contents and meaning are the same as in DCBOFLGS
field before OPEN.
ACCESS METHOD INTERFACE -- BTAN

49

(31)

• 3

DCBREAD,
DCBWRITE

Address of READ/WRITE routine.

52

(34)

4

DCBLERB

Address of line error block.
BSC Interface -- Before Open

56

(38)

1

57

(39)

• 1

Reserved
DCBXCODE

PTOP flag.

x.xx xxxx

(Reserved bits)
If PTOP is specified in the SYSGEN procedure:
Schedule an asynchronous exit to the interface resolution routine.

.1. '.

58

(3A)

•• 2

60

(3C)

4

64

(40)

36

Reserved
DCBBSTSX

If PTOP is specified in the SYSGEN procedure:
Address of the interface resolution routine.
Reserved

(
74

OS System control Blocks

DATA CONTROL BLOCK -- BTAM
Offset

Bytes and Field
Alignment Name

Field Description, Contents, Meaning
BSC Interface -- After Open

56

(38)

1

DCBXMODE

Mode of transmission for binary synchronous communication (BSC).

• 1 ••
• ,.1.

x •• x .xxx

Intermediate block checking is to be performed •
Transmission is through a 2701 Data Adapter Unit Dual
Communication Interface B.
Transmission is in code B for a 2701 Data Adapter
Unit Dual Code Feature.
Reserved

DCBXCODE

BSC control station flag# transmission code.

x •• '.

BSC control station flag.
This is the control station a
This is the remote station.

1 .••

57

(39)

1

0,•••

1,•• '.

»

• x ••
.1,••

If PTOP is specified in the SYSGEN procedure:
Schedule an asynchronous exit to the interface
resolution routine.

,•• 1.1 •••
• .,.1 .1 ••

00 ••

6-bit Transcode is being used.
USASCII transmission code is being used.
EBCDIC transmission code is being used.

•• xx

(Reserved bits)

58

(3A)

1

DCBBSRSV

DLE control character.

59

(3B)

1

DCBBSWBT

WBT control character.

60

(3C)

1

DCBBSTSX

DLE control character.

61

(3D)

1

DCBBSSTX

STX control character.

62

(3E)

1

DCBBSTEX

DLE control character.

63

(3F)

1

DCBBSETX

ETX control character.

64

(40)

2

DCBBSAKO

ACK-O control character.

66

(42)

2

DCBBSAKl

ACK-l control character.

68

(44)

1

DCBBSENQ

ENQ control character.

69

(45)

1

DCBBSNAK

NAK control character.

70

(46)

1

DCBBSETB

ETB control character.

71

(47)

1

DCBBSDLE

DLE control character.

72

(48)

1

DCBBSEOT

EOT control character..

73

(49)

3

DCBBSSYN

SYN# SYN, SYN control characters.

76

(4C)

2

DCBBSONL

SOH % control characters.

78

(4E)

2

DCBBSSAK

DLE SAK control characters.

80

(50)

2

DCBBSRVI

DLE

82

(52)

18

~

control characters.

Reserved
DCB -- BTAM

75

(

Data Control Block-GAM

This data control block (DeB) is used by
the graphics access method (GAM) routines.
It has the common interface and foundation
sections. which serve the same purposes for
all access method routines, although the
format may vary slightly among them. An
interface section that contains information
about a particular graphic device precedes
the common section. Figure 7 illustrates
the format of the DeB used in GAM.
Descriptions of the fields follow the
illustration.

)
DeB -- GAM

77

DATA CONTROL BLOCK -- GAM

Graphic Device Interface

teo)
~I""

1
-r--

Reserved

12 (C)

14 (E)

DCBBRSA
Buffer Restart Address

16 (10)

15 (F)

DCBGTYPE
Basic/Express

Reserved

18 (12)

DCBBFRST
Buffer Start Address

DCBBFRSZ
Buffer Size

19 (13)

Common Interface
20 (14)
Reserved
26 (IA)

28 (1C)

DCBDSORG
Data Set Organization

DCBIOBAD
Address of First lOB

31 (1 F)

Foundation Extension
32 (20)

33 (21)

36 (24)

37 (25)

DCBGNCP
No. of I/o Instructions Before WAIT

DCBPOLST
Address of DCB List for Polling
DCBEXLST
Address of User's Exit List

Reserved

Foundation
r---B.for.--Optf.j----------------------------------------------------------------------.-------------~---------------------------

39 (27)

.

--------~-~---.----.--.----.-------------'1

40 (28)
DCBDDNAM
Name from DD Statement

48 (30)

DCBOFLG

49 (31)

DCBIFLG

50 (32)

105 Error Flags

Open Flags

DCBMACR
Type of Macro Instruction and Options

51 (33)

After OPEN
40 (28)

44 (2C)

DCBTIOT
Offset to DD Entry in T/OT
DCBIFLGS
105 Error Flags

48 (30)

DCBOFLGS

45 (2D) .,..

49 (31)

Open Flags

Figure

7.

42 (2A)

DCBMACRF
Type of I/O Macro Instruction ond Options
DCBDEBAD
Address of DEB

DCBGIOCR
Address of I/o Control Routine

51 (33)

Data Control Block - GAM

(
78

OS System Control Blocks

Offset

Bytes and
Alignment

Field
Name

DATA CONTROL BLOCK -- GAM
Hex.
Dig. Field Description, Contents, Meaning
DEVICE-DEPENDENT INTERFACE

o

(0)

12

12

(c)

2

14

(E)

••

Reserved
Blank before execution of the second I/O operation.
Last buffer start address.

DCBBRSA
DCBGTYPE

1

00
01

Type of buffer management and attention handling.
Express
Basic
Reserved

15

(F)

16

(10)

2

DCBBFRST

Blank before execution of Open routine.
starting address for the buffer after execution of
Open routine.

18

(12)

•• 2

DCBBFRSZ

Blank before execution of Open routine.
Size of buffer after execution of Open routine.

•••

1

COMMON INTERFACE
20

(14)

6

26

(lA)

•• 2

26

(lA)

27

(lB)

Reserved
DCBDSORG
Byte 1

All zeros.

Byte 2

Code
GS Graphics organization.
(Reserved bits)

1 •.•

.xxx xxxx
28

(lC)

4

Data set organization.

DCBIOBAD

Blank before execution of Open routine.
Address of the standard fields of the first input/
output block (lOB) after execution of Open routine.
FOUNDATION EXTENSION

32

(20)

1

DCBGNCP

Number of I/O instructions to be issued before a WAIT
macro instruction.

33

(21)

• 3

DCBPOLST

Address of area where a DCB list is to be constructed
for polling purposes.

36

(24)

1

37

(25)

• 3

Reserved
DCBEXLST

Address of user's exit list.
FOUNDATION SEGMENT
FOUNDATION SEGMENT -- BEFORE OPEN

40

(28)

8

DCBDDNAM

Eight byte name from the data definition statement
that defines the data set associated with this DCB.

)
DCB -- GAM

79

Offset
48

(30)

Bytes and
Alignment
1

Field
Name
DCBOFLG
1. '. ,.
0,•• '.

.x ••
• ,.1,.
• '. ,.1

1. '.•
• 1 ••

. '. o.
'•• 1.
•.. 1

DATA CONTROL BLOCK -- GAM
Hex.
Dig. Field Description, Contents, Meaning
Flags used by the Open routine.
Last I/O operation was a GWRITE.
Last I/O operation was a GREAD.
(Reserved bit)
set to ! by EOV when it calls the Close routine for
concatenation of data sets with unlike attributes.
An OPEN has been successfully completed.
Set to ! by a problem program to indicate a
concatenation of unlike attributes.
Tape mark has been read •
set to Q by an I/O support
function when that function takes a user exit. It is
set to Q to inhibit other I/O support functions from
processing this particular DCB.
set to 1 on return from the user exit to the I/O
support-function which took the exit.
set to ! by an I/O support function if the DCB is to
be processed by that function.

49

(31)

• 1

DCBIFLG

set to zero by the graphics routines but used by I/O
supervisor in communicating error conditions and in
determining corrective procedures.

50

(32)

•• 2

DCBMACR

Major macro instructions and their associated
options.

50

(32)

xx.x xx.x
Byte 1

• .1,.

'•• 1.
51

xx.x xx.x

(33)

Byte 2

• .1,.

•• 1.

(Reserved bits)
Read operation to be performed.
Control operation to be performed with the read
operation.
(Reserved bits)
Control operation to be performed with the write operation. •
FOUNDATION SEGMENT -- AFTER OPEN

40

(28)

2

DCBTIOT

Offset from the TIOT origin to the DD entry associated with this DCB.

42

(2A)

'.. 2

DCBMACRF

Contents and meaning are the same as DCBMACR field in
the foundation segment before OPEN.

44

(2C)

1

DCBIFLGS

Contents and meaning are the same as DCBIFLG field in
the foundation segment before OPEN.

45

(2D)

• 3

DCBDEBAD

Address of the associated DEB.
Note: The above fields are overlayed on the DCBDDNAM
field during OPEN and are restored to their original
form at CLOSE.

48

(30)

1

DCBOFLGS

contents and meaning are the same as DCBOFLG field in
the foundation segment before OPEN,.

49

(31)

• 3

DCBGIOCR

Address of the graphics input/output control routine.

(
80

OS system control, Blocks

Data Extent Blocks

There are two kinds of data extent blocks
-- an ordinary one, used in all access
methods (including BTAM and the message
control portion of QTAM). and another one
used in the message processing portion of
QTAM.

Accordingly, separate diagrams and
descriptions are presented for the following uses of DEBs:
• Ordinary
• QTAM Message Processing Program

»
DEB

81

(

Data Extent Block-Ordinary

The data extent block (DEB) contains an
extension of information in the DCB.
Each
DEB is associated with a DCB, and the two
point to each other. The DEB contains
information concerning the physical characteristics of the data set and other information that is used by the control program.
Figure 8 illustrates the format of the DEB.
Descriptions of the fields follow the
illustrations.
This data extent block is used in all
access methods and is used in QTAM by a
message control program to describe a queue
which is on a direct access storage device
or to describe a line group.
(QTAM processing programs use a DEB described
separately.)

DEB

83

DATA EXTENT BLOCK -- ORDINARY

Appendage Table
-36 (-24)

DEBEOEA (I)
Address of End-of-Extent Appendage

-32 (-20)

DEBSIOA (I)
Address of Start I/o Appendage

-28 (-IC)

DEBPCIA(I)
Address of PCI Appendage

-24 (-18)

DEBCEA (I)
Address of Channel-End Appendage

-14 (-20)

DEBXCEA (I)
Address of Abnormal-End Appendage

-17(-11)

DEB Prefix
-16 (-10)
DEBWKARA
I/o Support Work Area

-15 (-F)
DEBDSCBA
Address of DSCB

-8 (-8)

DEBDCBMK
DCB Modification Mask

-4 (-4)

DEBLNGTH
Length of DEB

-3 (-3)
Reserved

-1 (-1)

Basic Section
0(0)

DEBNMSUB
No. of Subroutines

1 (1)

DEBTCBAD
Address of TCB

4 (4)

DEBAMLNG
Acc M S'n Length

5 (5)

DEBDEBAD
Address of Next DEB

8 (8)

DEBOFLGS
Data Set Status

9 (9)

DEBIRBAD
Address of IRB

12 (C)

DEBOPATB
Type of I/O

13 (D)

16 (10)

DEBNMEXT
No. of Extents

17 (11)

DEBUSRPG
Address of First lOB in User Purge Chain

20 (14)

DEBPRIOR
Priority

21 (15)

DEBECBAD
Address of Parameter List to Find Purge ECB

24 (18) DEBPROTG, DEBDEBID
Protection Key, DEB Id

25 (19)

28 (lC)

29 (lD)

DEBEXSCL
Extent Scale

DEBQSCNT
PURGE - Quiesce Count

14 (E)
Reserved

DEBDCBAD
Address of DCB
DEBAPPAD
Address of I/o Appendage Vector Table

31 (IF)

(I) Field names used only in BTAM and parts of QTAM.

Figure

8.

Data Extent Block -- Ordinary (Part 1 of 4)

(
84

OS System Control Blocks

DATA EXTENT BLOCK -- ORDINARY

32 (20)

DEBNIEE
No. of Extents

33 (21)

DEBFIEAD
Address of Fi rst Index Extent

36 ('24)

DEBNPEE
No. of Extents

37 (25)

DEBFPEAD
Address of First Prime Data Area Extent

40 (28)

DEBNOEE
No. of Extents

41 (29)

DEBFOEAD
Address of First Overflow Extent

44 (2C)

DEBDISAD
Address of Privileged Module

47 (2F)

5e-,,-ice--oep-e-n(j-en-f--se-ct"i"on-----------------------------------------------------------------------------"----------------------------------------------------____________________ J
---[jnlf-R-e-cord-~-M-ag-rietic--ta-pe--Ijev-fces--se-c"ffon------- --------------------------------------------------------------------------------------------------r

32 (20)

DEBDVMOD
Device Modifier

32 (20)

DEBUCBAD
Address of UCB

33 (21)

DEBUCBAD
Address of UCB

Reserved

I

35 (23)

I-

35 (23)

I-i--

I

DIrect

I

- Access
DEBDVMOD
File Mask

+0

St ora< e DeVlce

DEBUCBAD

Sec t'Ion

DEBUCBAD
Address of UCB

+1

II--

+4

DEBBINUM
Bin Number

+6

DEBSTRCC
Cylinder Start Address
DEBENDCC
Cylinder and Address

+8

DEBSTRHH
Read or Write Track Start Address

+10

+12

DEBENDHH
Read or Write Track End Address

+14

I

I

-

No. of Tracks Allocated in this Extent

+15

I
I

I

I

EXCP Access Method, BSAM t OSAM Dependent Section
+0

+4

DEBVOLSQ
Volume Sequence Number

+2

DEBVOLNM
No. of Volumes

DEBDSNM
(Only present when OPEN for a Member Name)

+11

~- ------- ........ -........ -- ... -----_ .... - ........ ------ ........................... --_ .............. --- .. -_ .............. -- .. -_ .. -.-- .. _-_ ........... --_ .... -_ ........ -_ ...... -_ ........ -- -- -_ .. -- . - ---- ... _--- ...... -- .. ---- .......... -- -- --.- ---------_ .. -- --_ -_ ........ -_ ........ --- ----- ---_ .. ----- --~
Figure 8. Data Extent Block -- Ordinary (Part 2 of 4)
....

DEB

85

DATA EXTENT BLOCK -- ORDINARY

--·BPA-M·-O-ep-e·nde·rlt--Secti 0 n5-·--------------·-··--·---·--·-····-··········-····-..-.-..-....-..--.-..-.-.....-.-.--.... -...-......--............................-.-...-.-j
-·~~f--~···-

+0

.--..-_. -.

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

.-----.-. -. -. -. -. -. -. - --- - -- -- -- -- -- -- -- -- -- -- .

DEBEXTNM

I

I

I

Output
+0

DEBDSNAM
Member Name
(Only present when OPEN for a Member Name)

,

I
+7
I
L~eDA~§!.~tJ.~1!.. ____ . . _~ ___ ~ _____________ . ______________________________________

j

i

Fixed-Length Records, Relative Block Addressing (No Track Overflow)
+0

B/T
No. of Blocks Per Track

I

+1

r--

B/E:
No. of Blocks Per Extent

+3

l~

I

I
Fixed -Length Records, Relative Block Addressing Track Overflow
+0

TIP
No. of Tracks Per Period

+4

B/P
No. of Blocks Per Period

+8

B/E
No. of Blocks Per Extent

+11

I

I

-

l

I

L -__._. _____ .__

.. ______ .___._________ ._____________________________ . __________ .J

~

BTAM Section
+0
Address of Buffer Routine

-

+4
Address of First CCWon Queue

,

I

I

I

GAM Section
Reserved

I

I--

r--

I

+0

I

I

Figure

-

DEBUCBAD
Address of UCB

+3

~

I

1.. - - _ - __ .. ____ .. _____ ..... __________ ............. _______ .. _______ ... ____ ..... _ .......... ____ ....... ___ ........ ____ ............ ___ ...... _ ....... ______ .. _ .. _ .. __ ...... ___ .... _ .. ___ ............. __ .. ___ oo

86

+7

8.

Data Extent Block -- Ordinary --

OS System Control Blocks

.. _ _ _ _ .. _ . . . . . . . . . . . . . . . . . . . . . . . _

(Part 3 of 4)

. . . . . . . . . . . . _ _ . . . . . . _ _ ................ _ _ _ _ . . . . _ _ . . . . _ _ .. _ .. _ _ .. _ _ _ oo .. _ _ _ _ ..

(

DATA EXTENT BLOCK -- ORDINARY

Subroutine Name Section
+0

DEBSUBID
Subroutine ID

--+1

~I--------------

I

-

I

Figure

8.

Data Extent

Block -- Ordinary (Part 4 of 4)

)
DEB

87

DATA EXTENT BLOCK -- ORDINARY
Offset

Bytes and
Aliqrunent

Field
Name

Field Description, Contents, Meaning
APPENDAGE TABLE
BSAM, QSAM, BPAM, BDAM, GAM

-36 (-24)

4

(End of
Extent)

Address of the end-of-extent appendage routine.

-32 (-20)

4

(start I/O)

Address of the start I/O appendage routine.

-28 (-lC)

4

(PCI)

Address of the program-controlled-interruption appendage routine.

-24 (-18)

4

(Channel
End)

Address of the channel-end appendage routine.

-20 (-14)

4

(Abnormal
End)

Address of the abnormal-end appendage routine.
BTAM, QTAM MESSAGE CONTROL

-36 (-24)

4

DEBEOEA

Address of the end-of-extent appendage routine.

-32 (-20)

4

DEBS lOA

Address of the start I/O appendage routine.

-28 (-lC)

4

DEBPCIA

Address of the program-controlled-interruption appendage routine.

-24 (-18)

4

DEBCEA

Address of the channel-end appendage routine.

-20 (-14)

4

DEBXCEA

Address of the abnormal-end appendage routine.
END OF APPENDAGE TABLE
PREFIX SECTION
DIRECT-ACCESS STORAGE DEVICES

-16 (-10)

1

DEBWKARA

I/O Support work area.

-15

• 7

DEBDSCBA

DSCB address (BBCCHHR) used by I/O support.

(-9)

ALL DEVICES
-8

(-8)

4

DEBDCBMK

DCB modification mask used by I/O support .•

-4

(-4)

1

DEBLNGTH

Length of DEB in double words.

-3

(-3)

• 3

Reserved

(
88

OS System Control Blocks

DATA EXTENT BLOCK -- ORDINARY
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
DEB PROPER
BASIC SECTION

o

(0)

1

1

(1)

•

4

(4)

1

5

(5)

•

8

(8)

1

3

3

DEBNMSUB

Number of subroutines loaded by the open executor
routines.

DEBTCBAD

Address of the TCB for this DEB.

DEBAMLNG

Number of bytes in the access method dependent section.
For BDAM this field contains the length expressed in number of words.

DEBDEBAD

Address of the next DEB in the same task.

DEBOFLGS

Data Set status flags.

01 ••
10 ••
11 ••

•• 1.
••• 1
1 ...
.1 ..
•• 1.
•.• 1

9

(9)

•

12

(C)

1

3

Disposition is OLD
Disposition is MOD
Disposition is NEW
EOV or EOF
Release unused external storage
DCB modification
Split cylinder
Nonstandard labels
Magnetic Tape Devices:
Use reduced error recovery procedure.

DEBIRBAD

IRB storage address used for appendage asynchronous
exits.

DEBOPATB

The method of input/output processing and the disposition that is to be performed when an end of volume
condition occurs.

00 ••
•• 01
•• 11

0000
1111
0011
0111
0001
0100
DEBQSCNT

(always zero)
REREAD
LEAVE
INPUT
OUTPUT
INOUT
OUTIN
RDBACK
UPDAT
PURGE (SVC 16) - Quiesce count. Number of devices
executing user's channel programs, as shown by bits 5
and 6 of UCBFLl fields.

13

(D)

• 1

14

(E)

•• 2

16

(10)

1

DEBNMEXT

Number of extents specified in the DSCBs.

17

(11)

• 3

DEBUSRPG

Address of first lOB in the user purge chain.

20

(14)

1

DEBPRIOR

priority of the task.

21

(15)

• 3

DEBECBAD

Address of a parameter list used to locate the purge
ECB for an SVC purge request.

Reserved

DEB

89

DATA EXTENT BLOCK -- ORDINARY
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
BASIC SECTION (Continued)

24

(18)

1

DEBPROTG, DEBDEBID
xxxx
1111

Protection key.
A hex nF" to identify this block as a DEB.

25

(19)

• 3

DEBDCBAD

Address of DCB associated with this DEB.

28

(lC)

1

DEBEXSCL

Extent scale:
4 for direct access device and 2 for
nondirect access device and communication device.
This field is used to determine the size of the
Device Dependent Section.

29

(lD)

• 3

DEBAPPAD

Address of the I/O appendage vector table.
ISAM DEPENDENT SECTION
Present only if ISAM is used.
Follows the basic section. Precedes the device dependent section.

32

(20)

1

DEBNIEE

Number of extents of independent index area.

33

(21)

• 3

DEBFIEAD

Address of first index extent.

36

(24)

1

DEBNPEE

Number of extents of prime data area.

31

(25)

• 3

DEBFPEAD

Address of the first prime data extent.

40

(28)

1

DEBNOEE

Number of extents of independent overflow area.

41

(29)

• 3

DEBFOEAD

Address of the first overflow extent.

44

(2C)

4

DEBDISAD

Address of privileged module entered when a BISAM
macro instruction is executed.
DEVICE DEPENDENT SECTION
Follows the basic section, except in ISAM.
follows the ISAM dependent section.

In ISAM,

UNIT RECORD AND MAGNETIC TAPE
32

(20)

1

DEBDVMOD

Device Modifier.
Magnetic Tape -- SET MODE operation code.
Unit record -- Not used.

33

(21)

• 3

DEBUCBAD

Address of a UCB associated with a given data set.
TELECOMMUNICATIONS DEVICES

32

(20)

n times

List of addresses (n) of UCBs for lines (n).

1

Reserved

• 3

DEBUCBAD

Address of the UCB for the line.

(
90

OS System Control Blocks

DATA EXTENT BLOCK -- ORDINARY
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
DIRECT-ACCESS STORAGE DEVICES
EXTENT DESCRIPTION SEGMENTS:
For each extent there is a 16-byte segment as
follows.

+0

1

+1

·

+4

2

+6

'. ·

+8

2

3

+10

'. ·

+12

2

+14

··

2

2

2

DEBDVMOD

Device modifier:

file mask.

DEBUCBAD

Address of UCB associated with this data extent.

DEBBINUM

Bin number.

DEBSTRCC

Cylinder address for the start of an extent limit,.

DEBSTRHH

Read/Write track address for the start of an extent
limit.

DEBENDCC

Cylinder address for the end of an extent limit.

DEBENDHH

Read/Write track address for the end of an extent
limit.

DEBNMTRK

Number of tracks allocated to a given extent.
ACCESS METHOD DEPENDENT SECTION
Follows the device dependent section, except for
ISAM. The ISAM dependent section precedes the device
dependent section.
BSAM, QSAM, EXCP Access Method

+0

2

DEBVOLSQ

Volume sequence number for multivolume sequential
data sets.

+2

• • 2

DEBVOLNM

Total number of volumes in a multivolume sequential
data set.

+4

8

DEBDSNM

Member name.
This field appears only when an output data set has
been opened for a member name and the DSCB specifies
a partitioned data set.

Only one of the following fields is present:
+0

(m-l)x1

DEBEXTNM

For a partitioned data set opened for input, each one
byte field contains the extent number of the first
extent entry for each data set except the first, if
two or more data sets (m) are concatenated. The
number of bytes in the field is equal to one less
than the number of data sets concatenated.

+0

8

DEBDSNAM

For a partitioned data set opened for output for a
member name, this field is the member name.

DEB

91

DATA EXTENT BLOCK -- ORDINARY
Offset

Bytes and
Alignment

Field
Name

Field Description., Contents, Meaning

Only present for fixed-length records with the option
of relative block addressing (but not track overflow)
There is one of these four byte fields for each
extent described in the device dependent section.
+0

1

B/T

Number of blocks per track,.

+1

'. 3

B/E

Number of blocks per extent.
Only present for fixed-length records with the option
of relative block addressing and track overflow
These fields occur only once within a DEB:

+0

4

T/P

Number of tracks per period.

+4

4

B/P

Number of blocks per period.
The following field occurs once for each extent:

+8

4

B/E

Number of blocks per extent.

This segment is always present for BTAM. It is used
when a buffer pool or dynamic buffering is used; else
the fields are zero.
+0

4

Address of the buffer routine.

+4

4

Address of the first

4

n times

ccw

on the queue.

Address of following CCWs on the queue.
GAM

+0

1

+1

• 3

Reserved.
DEBUCBAD1.

Reserved

1

'. 3

Pointer to first UCB.

DEBUCBADn

Pointer to last UCB.
SUBROUTINE NAME SECTION
Follows the access method dependent section, or the
device dependent section if there is no access method
dependent section..

n times
+0

DEBSUBID

2

Subroutine Identification.
Each access method subroutine. appendage subroutine,
and IRB routine will have a unique eight-byte name.
The low-order two bytes of each routine name will be
in this field if the subroutine is loaded by the Open
routines.

(
92

OS system Control Blocks

Data Extent Block-QTAM
Message Processing Program

This data extent block is used in QTAM by a
message processing program to describe message process queues and destination queues,
which are in main storage.
(BTAM, and QTAM
message control programs, use the ordinary
DEB described previously.)
It is an extension of the information in the DeB concerning the physical characteristics of the
data set and other information that is used
by the control program. Figure 9A shows
the format of the DEB proper used for message processing queues; Figure 9B shows the
DEB for destination queues.

DEB -- QTAM

93

DATA EXTENT BLOCK -- QTAM

Prefix
-16 (-10)
Work area

1-

15 (-F)
DSCB Address

-8 (-8)
DCB Mask
-4 (-4)

1-3 (-3)
Reserved

Length

-1 (-1)

Basic Section
1 (1)

0(0)

Address of TCB

Reserved
5 (5)

4 (4)

Address of Next DEB

Reserved
~

1...8 (8)

~

-r-

~r-

Reserved
17 (11)
Address of Next Record
21 (15)

20 (14)

Address of Next DEB

Reserved
25 (19)

24 (18)
ID

Address of DCB
29 (10)

28 (lC)

Address of DEB + 48

Reserved
32 (20)

35 (22)

1st Word of Dummy LCB

Queue Control Block
36 (24)

137 (25)
Address of Dummy Entry

Reserved
40 (28)
Reserved
145 (2D)

Address of QPRIRITY Subtask

47 (2F)

Buffer Request Block
48 (30)
Reserved
53 (35)

52 (34)

Reserved

Priority
57 (39)

56 (38)

Address of QCB

Op Code
61 (3D)

60 (3C)

Address of DEB

Hex Code

+ 32

63 (3F)

66 (42)

64 (40)
Size of Work Area

Reserved

T

87 (57)T

Figure 9A.

Data Extent Block -- QTAM -- Message Process Queue

(
94

OS System Control Blocks

DATA EXTENT BLOCK -- QTAM

Prefix
-16 (-10)

1-15 (-F)
Work Area
DSCB Address

-8 (-8)
DCB Mask

-4 (-4)

1-3 (-3)
Length

-1 (-1)

Reserved

Basic Section
0(0)

1 (1)
Address of TCB

Reserved

4 (4)

5 (5)
Address of Next DEB

Reserved

8 (8)

~'-

-~

~r-

Reserved

~~

21 (15)
Address of Next DEB

24 (18)

25 (19)
Address of DC B

ID

28 (lC)
31 (IF)

Reserved

Queue Control Block

---I~!!t~~:~!~£::~!~~E:::::::=:::::::::::::::::::::::::::::::::::::::::::R~,.e~.~:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::}:l-! f""""
Line
Block
-_ . . . .Control
. . . . . ------..............
-_ . --.. . ---------.. --_. . ----_ . -_ . --........ ----.. . -----.. ---------_ . ------.. . ---.. ---.. . ----.. . -----...................... -_ . --.. . -_ . -_ . ---.. --............ ------............ -----_ . --................ ----roOi .... !i--.. -:
~

_~

, i!

28
40 ( bondHion Cod.

41 (29)

Adde... of QPRIRITY S,bto,k

43 (2B)

I

I

j

l

-----r--r-------------------------------------------------------------------~-------------------------------------------------------------------------------------------------------------------,

11144 (2C)

l~~)

I
.....-

-~

47 (2F)1

Reserved

1

I

Reserved

69

1

(45)

Adde... of OCB

72 (48)

- ~

Save Area

84 (54)

185 (55)
Reserved

186 (56)
Message Priority

Reserved

87 (57)

:

'

1. _ _ _ _ _ - - - - - - - - - - - - - - - - _ ... _ - _ ...... _ - _ ... - - - - - - - - - - - - - - - - - - - - - - - - ... - - - .. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ... _ _ _ _ _ _ _ _ ... _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ .. '

Figure 9B.

Data Extent Block -- QTAM -- Destination Queue

DEB -- QTAM

95

DATA EXTENT BLOCK -- QTAM
Offset

Bytes and
Alignment

Field Description, Contents, Meaning
MESSAGE PROCESS QUEUE
PREFIX

-16 (-10)

1

Work area used by I/O support routines.

-15

(-F)

• 7

Direct access device address used by I/O support routines.
(BBCCHHR)

-8

(-8)

4

DCB modification mask used by I/O support routines.

-4

(-4)

1

Length of this DEB.

-3

(-3)

• 3

Reserved

Format

DEB PROPER
BASIC SECTION
0

(0)

1

Reserved

1

(1)

• 3

Address of the TCB.

4

(4)

1

Reserved

5

(5)

• 3

Address of the next DEB in the same task.

8

(8)

9

Reserved

17

(11)

• 3

Address of the next available record of the process queue on the
direct access device.

20

(14)

1

Reserved

21

(15)

·

24

(18)

1

X'OF'; identifies this block as a DEB.

25

(19)

• 3

Address of the DCB.

28

(lC>

1

Reserved

29

(lD)

• 3

Address of the beginning of the buffer request block (BRB) portion
of this DEB.

32

(20)

4

Address of a dummy LCB.

3

Address of the next DEB on the chain of the process program's DEBs.

QUEUE CONTROL BLOCK
36

(24)

1

Reserved

37

(25)

• 3

Address of dummy last entry in queue.

40

(28)

5

Reserved

45

(2D)

• 3

Address of QPRIRITY subtask.

c
96

OS System Control Blocks

DATA EXTENT BLOCK -- QTAM
Offset

Bytes and
Aligrunent

Field Description, Contents, Meaning
BUFFER REQUEST BLOCK

48

(30)

4

Reserved

52

(34)

1

Priority

53

(35)

• 3

Reserved

56

(38)

1

X'08'; identifies the operation code for a TIC command.

57

(39)

• 3

Address of the process queue control block on the direct access
device.

60

(3C)

1

X'07'; indicates a dummy buffer request block.

61

(3D)

'. 3

Address of the beginning of the line control block portion of this
DEB.
END OF BUFFER REQUEST BLOCK

64

(40)

2

Size of work area necessary for GET.

66

(42)

'• • 22

Reserved
DESTINATION QUEUE
PREFIX

-16 (-10)

1

Work area used by I/O support routines.

-15

(-F)

• 7

Direct access device address used by I/O support routines.
(BBCCHHR)

-8

(-8)

4

DCB modification mask used by I/O support routines.

-4

(-4)

1

Length of this DEB.

-3

(-3)

• 3

Reserved

Format

DEB PROPER
BASIC.SECTION

o

(0)

1

1

(1)

.•

4

(4)

1

Reserved

5

(5)

.. 3

Address of the next DEB in the same task.

8

(8)

13

Reserved

21

(15)

• 3

Address of the next DEB on the chain of proceSSing program's DEBs

24

(18)

1

X'OF' identifies this block as a DEB.

25

(19)

'. 3

Address of the DCB.

28

(lC)

4

Reserved

Reserved
3

Address of TCB.

DEB -- QTAM

97

DATA EXTENT BLOCK -- QTAM
Offset

Bytes and
Alignment

Field Description, Contents, Meaning
BUFFER REQUEST BLOCK, QUEUE CONTROL BLOCK

32

(20)

8

Reserved
LINE CONTROL BLOCK

40

(28)

1

Condition code from the line control block.

41

(29)

• 3

Address of QPRIRITY subtask.
END OF QUEUE CONTROL BLOCK

44

(2C)

4

Reserved
END OF BUFFER REQUEST BLOCK

48

(30)

21

Reserved

69

(45)

• 3

Address of the queue control block for the destination queue.

72

(48)

12

Save area.

84

(54)

1

Reserved

85

(55)

• 1

Temporary location for the message priority code.

86

(56)

2

Reserved
END OF LINE CONTROL BLOCK
END OF QTAM DEB

98

OS System Control Blocks

Data Event Control Blocks

Data event control blocks (DECB) contain
information about an input or output operation requested by a READ or WRITE macro
instruction.
separate diagrams and descriptions are
presented for the following uses of DECBs:
• BSAM
• ISAM

• BDAM
•

QTAM

• BTAM

DECB

99

Data Event Control Block-BSAM

The data event control block (DECB) is
created when a READ or WRITE macro instruction is expanded. It contains information
about the input or output operation that is
requested by the macro instruction. Figure
10 shows the format of the DECB used in
BSAM. Descriptions of the fields follow
the illustration.

DECB -- BSAM

101

DATA EVENT CONTROL BLOCK -- BSAM

0(0)

DECSDECB
Event Control Block

6 (6)

4 (4)

DECLNGTH
Length of Key and Data

DECTYPE
Type of I/o Request

8 (8)

DECDCBAD
Address of DCB

-----'-"""'-"---

12 (C)
DECAREA
Address of Key and Data

16 (10)
DECIOBPT
Address of lOB

Figure 10.

Data Event Control Block -- BSAM
DATA EVENT CONTROL BLOCK -- BSAM

Offset

Bytes and
Alignment

Name
Field

Field Description, Contents, Meaning

o

(0)

4

DECSDECB

Event control block.

4

(4)

2

DECTYPE

Type of I/O request.

4

(4)

Byte 1
1 •••

.xxx xxxx
Byte 2
5

(5)

1 •••

Type of length operand:
S coded for length.
(Reserved bits)
Type of operation:
SF
READ SB
WRITE SF
WRITE SD
(Reserved bits.)
WRITE SZ
READ

.1,••

•• 1.
• ,•• 1

x.xx
.1 •.
6

(6)

•• 2

DECLNGTH

Length of key and data.

8

(8)

4

DECDCBAD

Address of the DCB to which this I/O request is
related.

12

(C)

4

DECAREA

Address of the key and data.

16

(10)

4

DECIOBPT

Address of the lOB.

102

OS System Control Blocks

19 (13)

Data Event Control Block-ISAM

The data event control block (DECB) is
created when a READ or WRITE macro instruction is expanded. It contains information
about the input or output operation that is
requested by the macro instruction. Figure
11 shows the format of the DECB used in
ISAM. Descriptions of the fields follow
the illustration.

DECB -- ISAM

103

DATA EVENT CONTROL BLOCK -- ISAM
0(0)
DECBECB
Event Control Block
5 (5)

4 (4)
DECBTYPI
Options

6 (6)
DECBLGTH
No. of Bytes Read or Written

DECBTYP2
Type of I/o

8 (8)
DECBDCBA
Address of DCB
12 (C)
DECBAREA
Storage Address for Record
16 (10)
DECBLOGR
Address of Logical Record
20 (14)
DECBKEY
Address of Key Portion of Record
24 (18)

DECBEXCI
Exceptional Condition Codes

Figure 11.

104

25 (19)

DECBEX2
Exceptional Condition Codes

Data Event Control Block -- ISAM

OS System Control Blocks

DATA EVENT CONTROL BLOCK -- BISAM
Offset

o

(0)

o

(0)

Bytes and
Alignment

Field
Field Description, Contents. Meaning
DECBECB

4

.xxx xxxx

Awaiting completion of the event.
Flag field.
Awaiting completion of the event.
(Reserved bits)
Address of the RB for the program awaiting the event.

Byte 1
1 ••.

1

(1)

Byte 2-4

o

(0)

Byte 1
x.xx xxxx
.1

1

(1)

4

(4)

G

•

Byte 2-4
DECBTYPl

1

(5)

•

1

After completion of the event:
Flag field.
(Reserved bits)
Event has completed (normally or abnormally). If the
event completed abnormally. fields DECBEXCl and
DECBEXC2 will show the reason.
Reserved
Options:

••• 1

(Reserved bits)
Length coded as '5' •
Area coded as '5' •

DECBTYP2

Type of I/O request.

xxxx xx ••
•• 1.
5

Event control block.

1 •••

READ K

.x.x •• xx
•• 1.

(Reserved)
READ KU.
WRITE K•
WRITE KN.

1 •.•
• 1 ••

6

(6)

•• 2

DECBLGTH

Number of bytes read or written.

8

(8)

4

DECBDCBA

Address of the data control block.

12

(C)

4

DECBAREA

Address of the area in storage for the record.

16

(10)

4

DECBLOGR

Address of the logical record.

20

(14)

4

DECBKEY

Address of the key portion of the record.

24

(18)

1

DECBEXCl

Exceptional condition code.

1 •••
• 1 ••

•• 1.
.•. 1
1 •••
• 1 •.

•• 1.
••• 1

25

(19)

• 1

DECBEXC2
xxx x xxx.
••• 1

Record not found.
Record length check •
Space not found in which to add a record •
Invalid request •
Uncorrectable I/O error.
Unreachable block •
Overflow record •
Duplicate record presented for inclusion in the data
set.
Exceptional condition code.
(Reserved bits)
Previous macro instruction was READ KU •

)
DECB -- ISAM

105

(

Data Event Control Block-BDAM

The data event control block (DECB) is
created when a READ or WRITE macro instruction is expanded. It contains information
about the input or output operation that is
requested by the macro instruction. Figure
12 shows the format of the DECB used in
BDAM. Descriptions of the fields follow
the illustration.

DECB -- BDAM

101

DATA EVENT CONTROL BLOCK -- BDAM
0(0)
DECSDECB
Event Control Block
4 (4)

6 (6)
DECTYPE
Type of I/O Request

8 (8)

DECLNGTH
Length of Data
DECDCBAD
Address of DCB

12 (C)
DECAREA
Address of the Data
16 (10)

DECIOBPT
Address of the lOB

20 (14)
DECKYADR
Address of the Key
24 (18)
DECRECPT
Address of Block Reference Field

Figure 12.

108

Data Event Control Block -- BDAM

as System Control Blocks

27 (1 B)

DATA EVENT CONTROL BLOCK -- BDAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning

DECSDECB

Event control block.

o

(0)

o

(0)

Byte 1
1 •••
• xxx xxxx

1

(1)

Byte 2-4

o

(0)

Byte 1
x.xx xxxx

4

• 1,••

1

Byte 2
1 •••
• 1 ••
• .1.
••• 1

(1)

1 •••
• 1 ••
•• 1.
••• 1

2

Byte 3
x •••

( 2)

.1 ..

•• 1.
• •• 1

1 •••
.1 ••
• .• 1.
••. 1

3

(3)

4

(4)

4

(4)

Byte 4
DECTYPE
1

1 •••
.1 ••
• .1 •
•• • 1
1 •••
• 1 ..
• .1.
••• 1

5

(5)

• 1

1 ••.•
• 1 •.

• .xx • '. ,.x
x •••
0 •••
1 •••
.x ••
• 0 ••
• 1 .•

•• 1.

Awaiting event completion.
waiting for completion of event.
(Reserved bits)
Address of the request block for the program waiting
for completion of the event.
After event completion:
(Reserved bits)
Event has completed •
Record not found.
Record length check •
Space not found,.
Invalid request.
(This condition also sets a bit in
the next byte.)
Uncorrectable I/O error.
End of data •
Uncorrectable error other than an I/O error •
A READ with exclusive control was not preceded by a
WRITE with exclusive control.
(A reserved bit)
A WRITE macro instruction was addressed to an input
data set.
An extended search was specified with the DCBLIMCT
field set to zero.
The block requested is not within the data set.
A write-by-identification (DI) addressed record zero.
A search-on-key (DK) was specified with the DCBKEYLE
field set to zero or without an address for the key.
A macro instruction used an option not set in the
DCB.
The key for the fixed-length record to be added
begins with hex. FF.
Reserved
Type of I/O request.
Verify.
Overflow.
Extended search.
Feedback.
Actual addressing.
Dynamic buffering •
Read exclusive.
Relative block addressing •
S coded for key address.
S coded for block length •
(Reserved bits.)
Type of operation WRITE
READ
Type of search argument Id •
Key.
Add option of WRITE operation •

DECB -- BDAM

109

DATA EVENT CONTROL BLOCK -- BDAM
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning

DECLNGTH

Length of the data.

6

(6)

..

8

( 8)

4

DECDCBAD

Address of the DCB to which this I/O request is
related.

12

(C)

4

DECAREA

Address of the data.

16

(10)

4

DECIOBBT

Address of the IOB.

20

(14)

4

DECKYADR

Address of the key.

24

(18)

4

DECRECPT

Address of the Block Reference Field.

110

2

OS System Control Blocks

Data Event Control Block-QTAM

The data event control block (DECB) is
created when a READ or WRITE macro instruction is expanded. It contains information
about the input or output operation that is
requested by the macro instruction. Figure
13 shows the format of the DECB used in
QTAM. Descriptions of the fields follow
the illustration.

DECB -- QTAM

111

DATA EVENT CONTROL BLOCK -- QTAM
0(0)

LlNEDECB
Always Zero

4 (4)

5 (5)

16 (6)

Reserved

Op Code

Length of Input Area

8 (8)
Address of DCB
12 (C)
Address of Data in Buffer
16 (10)
Reserved
20 (14)

21 (15)
No. Messages Received

24 (18)

Address of Active Entry in Polling List
25 (19)

Reserved

126 (lA)
Index, in DEB, to UCB

Reserved

28 (1C)
Reserved
32 (20)
Address of Addressing Characters in Terminal Entry
36 (24)

37 (25)
Reserved

eFigure 13.
Offset

Address of Poll i ng List

39 (27)

Data Event Control Block -- QTAM
Bytes and
Alignment

Field
Name

Field DescriQtion, Contents, Meaning

LINEDECB

Always zero.

0

(0)

4

4

(4)

1

5

(5)

·

6

(6)

8

(8)

4

Address of the DCB.

12

(C)

4

starting address for data in a buffer.

16

(10)

4

Reserved

20

(14)

1

Number of messages received.

21

(15)

24

(18)

1

Reserved

25

(19)

·1

Index to the UCB address in the DEB.

26

(lA)

28

(lC)

4

Reserved

32

(20)

4

Address of the addressing characters in the terminal
entry.

36

(24)

1

Reserved

37

(25)

·3

Address of the polling list.

Reserved
Operation code for the current segment.

1
2

3

Length of input area for the initial read.

Address of currently active entry in the polling
list.

2

Reserved

c
112

OS System Control Blocks

Data Event Control Block-BTAM

The data event control block (DECB) is used
in the execution of a READ or WRITE macro
instruction. It contains information about
the input or output operation that is
requested by the macro instruction. Figure
14 shows the format of the DECB. Descriptions of the fields follow the
illustration.

DECB -- BTAM

113

DATA EVENT CONTROL BLOCK -- BTAM
0(0)

DECSDECB
Event Control Block

4 (4)

6 (6)

DECTYPE
Programming, Indicators, Code

8 (8)

DECBUFCT
Buffer Count

9 (9)

DECBLNGTH
Buffer Length, Message Area Length

DECDCBAD
DCB Address

12 (C)

DECAREA
Buffer Address, Message Area Address
17(11)

16 (10)

18 (12)

DECSENSO
Sense Byte

DECSENS 1
Reserved

20 (14)

DECCOUNT
CSW Residual Count

DECCMCOD, DECENTRY
Error Command, Terminal List Address

24 (18)

DECFLAGS
Operations Status

28 (lC)

DECTPCOD
Operation

25 (19)

26 (1A)

DECRLN
Relative Line No.

29 (lD)

DECRESPN
Addressing Response, VRC/LRC Response

30 (1 E)

DECERRST
Error Status

DECCSWST
CSW Status

I/o

32 (20)

DECADRPT
Address of Previous Entry in Addressing List

36 (24)

DECPOLPT
Contents Depend on Use of Autopoll, Programmed Polling, or BSC

B SC Extension
42 (2A)

40 (28)
Reserved
44 (2C)

• Figure 14.

114

DECWAREA
Data Area Address

Data Event Control Block -- BTAM

OS System Control Blocks

DECWLNG
Data Area Length

47 (2F)

Offset

Bytes and
Alignment

Field
Name

DATA EVENT CONTROL BLOCK -- BTAM
Hex.
Dig. Field Description, Contents, Meaning

o

(0)

4

DECSDECB

Event control block.

4

(4)

2

DECTYPE

Programming indicators.

4

(4)

Byte 1
xx ••

Except during BSC on-line test:
READ, using Autopoll
(Reserved bit)
During BSC on-line test:
Qn-line test, requested by RFT message.
on-line test, initiated by ONLTST macro
instruction.
Sending text messages •
Receiving text messages •
(Reserved bits)
'Sf coded for terminal entry •
'Sf coded for area •
'Sf coded for length •

1 •••

.• x ...

xx ••
0 •••
1 •••

• 0 ••
• 1 ••

•• xx x •••
. 1 ••
• • 1.
••• 1

Command
Code
5

(5)

Byte 2

00
01
02
03
04
05
06
07
07
08
09
OA
OB
OC
OD
OE
OF
10
11
12
13
14
15
16
17
19
lA
lD
lF
82
83
84
85
86
87
89
8C
92
94

write break.
Read initial .•
Write initial.
Read continue.
Write continue.
Read conversational.
write conversational.
Read repeat (other than WTTA).
WTTA: Read continue with identification change.
Write acknowledge.
Read skip.
Write negative acknowledge,
Write reset (BSC).
Read buffer.
Write at line address.
Write initial conversational.
Write erase.
Write continue conversational.
Write disconnect.
Read stop.
Write initial transparent.
Read continue with leading graphics.
Write continue transparent.
Read inquiry.
Write inquiry .•
Read repeat with leading graphics.
Read initial inquiry.
Write wait before transmitting.
Write initial conversational transparent.
Write continue conversational transparent.
Write initial with reset.
Read continue with reset.
Write continue with reset.
Read conversation with reset.
TVR Write conversation with reset.
TPR Read repeat with reset.
TSR Read skip with reset.
TLR Write at line address with reset.
TIXR Write initial transparent with reset.
TTXR write continue transparent with reset.
TB

TI
TI
TT
TT
TV
TV
TP
TE
TA
TS
TN
TR
TB
TL
TIV
TS
TTV
TD
TTS
TIX
TTL
TTX
TQ
TQ
TPL
TIQ
TW
TIVX
TTVX
TIR
TTR
TTR
TVR

DECB -- BTAM

115

Offset

Bytes and
Alignment

Field
Name

DATA EVENT CONTROL BLOCK -- BTAM
Hex.
Dig. Field Description, Contents, Meaning

6

(6)

•• 2

DECLNGTH

Length of buffer or message area.

8

(8)

1

DECBUFCT

contains a running count of buffers obtained by BTAM
for the current Read or Write operation.
(Dynamic
buffering only.)

9

(9)

·

DECDCBAD

Address of associated DCB.

12

(C)

4

DECAREA

Address of buffer or message area.

16

(10)

1

DECSENSO

Sense information.

17

(11)

• 1

DECSENSl

Reserved

18

( 12)

·

DECCOUNT

Residual count from CSW for last CCW executed.

20

(14)

1

21

(15)

·

24

(18)

1

3

• 2

DECCMCOD, DECENTRY
Command for which the error occurred.
3

DECENTRY

Address of the terminal list.

DECFLAGS

Operation status.

xxx •••••

1 •••
• 1 ••

•• 1.
••• 1

1 •.•
.• 1 ••

•• x •
••• 1

116

OS System Control Blocks

One of These:
Start-Stop Operations
(Reserved Bits)
BSC Operations:
SAK received.
Incorrect acknowledgement received •
Acknowledgment alteration incorrect •
One of These:
TWX 33/35 terminal, BSC terminal:
Incorrect ID received.
Autopoll:
Index byte received does not match an active one.
BSC network:
Contention occurred.
WTTA:
Contention occurred.
READ, dynamic buffering:
No buffer was available.
(Message lost.)
One of These:
OPENLST, POLLING:
Negative response to polling received.
WRAPLST:
All entries are inactive.
Addressing:
Negative response to addressing received.
WTTA:
Last message received ended with EOT or time-out.
(A reserved bit.)
WTTA:
Last message received ended with WRU.

Offset

Bytes and
Alignment

Field
Name

DATA EVENT CONTROL BLOCK -- BTAM
Hex.
Dig. Field Description, Contents, Meaning

25

(19)

• 1

DECRLN

Relative line number.

26

(lA)

•• 2

DECRESPN

Response indicators (One of these).

26

(lA)

Stop - start Operations:
Response from a terminal to addressing,.
vertical redundancy character and longitudinal redundancy character (VRC/LRC) response.

Byte 1
Byte 2
26

(lA)

28

(lC)

1

BSC Operations:
Response from a terminal to addressing.
Type of
Terminal

DECTPCOD
00

on-line test,.

01

Disable when disable is the first command
of a channel program.
Dial.
Enable.
Prepare.
Write pad character.
Write wait before transmitting.

02

WTTA

2740,
Basic
03
TWX
TWX,BSC

Sense
Write control characters @ © ©
before selection.
Write EOT sequence before polling or
addressing.
Write response to text.
Write @ and 15 idle characters.

©

Write polling, addressing, or broadcast
characters.
Poll write inquiry.
Write turn around sequence.
Write CPU-ID sequence.

04

2740
w/st.c
2260R
83B3
1030
WTTA

05

Read response to polling.

(DECTPCOD 06
Continues)

Write space
(w/st.c. - with station control).
Write 2848 command.
Write FIGS shift.
Write 1.
Write WRU.
Write Identification.
Write padding characters.
Write letter shift characters.

Read response to addressing.

DECB -- BTAM

117

Offset

Bytes and
Alignment

Fi el d
Name

DATA EVENT CONTROL BLOCK -- BTAM
Hex,.
Dig,. Field Description, Contents, Meaning
Type of
Terminal

28

(lC)

(DECTPCOD 07
Continued) 08

TWX,BSC
1030
1050
2740
1060
2260
BSC

Read ID response .•
write end-of-addressing character
after addressing.

write response to Inquiry.
write response to text.
write EOT, SYN, SYN, SYN, before polling or
addressing.

09

NOP or TIC after Poll in a READ with
SSALST, SSAWLST, AUTOLST, or AUTOWLST.

OA

Read Index (auto poll).
Read response to polling (Programmed
polling).

OB

BSC

Read inquiry.

OC

BSC

Read response to inquiry.

10

2260R

write at line address.

11

Read or write text.

12

Read skip or TIC for dynamic buffering.

13

BSC

write end-of-transparent-text characters.

20

start - Stop read response to text.

21

All reset commands.

22

Read skip.

23

write break.

24

V open, LOPEN or close routine operation.

25

BSC

Read response to text.

40-4C
50-53
61-65

,The last ccw executed was the first Read
or Write Text CCW to be executed in a
channel program using dynamic buffering.

80-8C
90-93
Al-A5

The last CCW in a channel program was
executed.

c
118

OS System Control Blocks

Offset
29

(lD)

Bytes and
Alignment
• 1

Field
Name

DATA EVENT CONTROL BLOCK -- BTAM
Hex.
Dig. Field Description, Contents, Meaning

DECERRST
1 ....
• 1 ••
'•. 1 ..
... 1

1 •.•

••••• xxx

I/O error status flags.
SIO resulted in a condition code of 3.
Undefined error condition •
An error condition occurred during an I/O operation
initiated by the error recovery routines.
Diagnostic Write/Read operation ended because of
error, (2701 only).
Disable command issued to a switched-connected line
by error recovery routine because of permanent error
on that line.
(Reserved bits)

30

(lE)

•• 2

DECCSWST

status bits from CSW for last CCW executed.

32

(20)

4

DECADRPT

Address of addressing list entry used in previous
operation.

36

(24)

4

DECPOLPT

One of these:
Programmed Polling:
Address of the current entry in the polling list.
Autopoll:
Byte 1: Indexed to current entry in polling list.
Bytes 2-4: Address of polliqg list.
BSC On-Line test:
Address of text data.
BSC Extension
Fields are present only if BSC is specified in the
OPEN macro instruction.

40

(28)

2

42

(2A)

•• 2

DECWLNG

Length, in bytes, of the data area in leadinggraphics and conversational type operations.

44

(2C)

4

DECWAREA

Address of the data area in leading-graphics and conversational operations.

Reserved

DECB -- BTAM

119

(

Data Set Control Blocks

The data set label for a data set residing
on a direct access volume is called a data
set control block (DSCB). One or more
DSCBs are used to describe the data set.
Each DSCB is 140 bytes, consisting of a 44
byte key and a 96 byte data portion.
The DSCBs describing all data sets on a
volume make up the Volume Table of Contents
(VTOC> .
separate diagrams and descriptions are
presented for the following uses of DSCBs:
•
•
•
•
•
•

DSCB
DSCB
DSCB
DSCB
DSCB
DSCB

Format
Format
Format
Format
Format
Format

1
2
3
4

5
6

In addition, there is a format 0 DSCB.
It
has the same format as other DSCBsi however, it contains all binary zeros.

DSCB

121

c

Data Set Control Block-Format 1

This data set control block (DSCB)
describes the characteristics and up to
three extents of a data set. For data sets
having indexed sequential (IS> organization, additional characteristics are specified in a format 2 DSCB pointed to by the
format 1 DSCB. Additional extents are
described in a format 3 DSCB pointed to by
the format 1 DSCB (or format 2 when the
data set has IS organization). A data set
can have a maximum of 16 extents on one
volume.
Figure 15 shows the format of the
format 1 DSCB. Descriptions of the fields
follow the illustration.

DSCB

123

DATA SET CONTROL BLOCK -- FORMAT 1

1
..,

10 (0)
DSlDSNAM
Data Set Name

............-r-

44 (2C)

DS1FMTlD
Format Identifier

45 (2D)

53 (35)

Continued

56 (38)

60 (3C)

DS1DSSN
Data Set Serial Number

DS1VOLSQ
Volume Sequence No.

59 (3B)

DS1NOEPV
No. of Extents

DS1CREDT
Creation Date
DS1EXPDT
Expiration Date

DS1NOBDB

51(33)

61 (3D)

62 (3E)
Reserved

--. -

DS1SYSCD
Programming System Code ID

-'-

-r-

75 (4B)

Reserved

82 (52)

Figure 15.

DSlDSORG
Data Set Organization

Data set Control Block -- Format 1 (Part 1 of 2)

(
124

OS System Control Blocks

DATA SET CONTROL BLOCK -- FORMAT 1
84 (54)

DS1RECFM
Record Format

88 (58)

85 (55)

86 (56)

DS10PTCD
Option Codes

90 (5A)

DS1LRECL .
Logical Record Length
93 (5D)

Continued

Continued

101 (65)

Continued

104 (68)

DSlDSIND
Data Set Indicators

DS1BLKL
Block Length

91 (5B)

DS1 KEYL
Key Length

DS1RKP
Relative Key Position

94 (5E)

DS1SCALO
Secondary Allocation

98 (62)

DS1LSTAR
Pointer to Last Written Block

DSlTRBAL
LL Part of Disk Address

103 (67)

Reserved

105 (69)
Reserved

DS1 EXTl
First Extent Description
115 (73)

_....

--

DS1EXT2
Second Extent Description

_I-..

-r-

125 (70)

DS1EXT3
Third Extent Description
135 (87)

DS1PTRDS
Direct-Access Address of Next DSCB
139 (8B)

Figure 15.

Data Set Control Block -- Format 1 (Part 2 of 2)

DSCB

125

Offset

o

(O)

44

Bytes and
Alignment

Field
~

DATA SET CONTROL BLOCK -- FORMAT 1
Hex.
Dig. Field Description, Contents, Meaning

44

DSIDSNAM

Data set name.

(2C)

1

DSIFMTID

Format identifier - Hex Fl.

45

(2D)

• 6

DSIDSSN

Data set serial number.

51

(33)

DSIVOLSQ

Volume sequence number.

53

(35)

• 3

DSICREDT

Creation date, in the form ydd.
y - year: 00 - 99, dd - day: 1- 366

56

(38)

3

DS1EXPDT

Expiration date, in the form ydd (as above).
If neither a retention period nor an expiration date
has been specified, ydd is zero.

59

(3B)

.... 1

DSINOEPV

Number of separate extents in which the data set
resides on this volume.

60

(3C)

1

DSINOBDB

Number of bytes used in the last PDS directory block.

61

(3D)

• 1

62

(3E)

'• • 13

75

(4B)

'... 7

82

(52)

• 2

Reserved

2

DS1SYSCD

System code to identify the programming system.
Reserved

DSIDSORG
Byte 1
11. ".Ie
. 1 .. '.
• _.1,.

Data
Code
IS
PS
DA

••• x xx ••
• ,.1,.
'.... 1

83

(53)

84

(54)

Byte 2
xxxx xxxx
1

DSIRECFM

10. '.
01..••

11,••
.. 1.
• '. 1.1
1 •••

• 10.
• 01.
.. 00.
.. ,•.• 0

85

(55)

86

(56)

'. 1
2

PO
U

set organization.
Indexed sequential organization.
Physical sequential organization,•
Direct organization •
(Reserved bits)
Partitioned organization •
Unmovable - the data contains location dependent
information.

Reserved
Record format,.
Code
-FFixed length record format.
V
Variable length record format,.
U
Undefined length record format.
T
Track overflow •
B
Blocked: may not occur with undefined (U) •
S
Fixed length record format: Standard blocks no
truncated blocks or unfilled tracks are embedded
in the dataset.
Variable length record format: Spanned records.
A
ASA control character •
M
Machine control character •
No control character •
Always zero •

DSI0PI'CD

Option code - same as DCBOPTCD field in DCB.

DSIBLKL

Block length for fixed length records or maximum
block size for variable or undefined length records,.

c
126

OS System Control Blocks

Offset
88

(58)

90

(SA)

91

(5B)

93

(5D)

Bytes and
Alignment
2

·.1
.
·1

2

Field
Name

DATA SET CONTROL BLOCK -- FORMAT 1
Hex.
Dig. Field Description. Contents, Meaning

DS1LRECL

Format F records:
Record length.
Format U records: Zero.
Format V records Unspanned record format:
Maximum record length.
Spanned record format Records up to 32,756 bytes:
Maximum record length,..
Records exceeding 32,756 bytes:
X'8000'.

DS1KEYL

Key length.

DSlRKP

Relative key position in the data block ..

DS1DSIND
1 •• '.
•• 1 ..
.... 1
.. x ... xxxx

94

(5E)

94

(5E)

·

..

4

DS1SCALO

Byte 1
01. '.
01 ••
10 ••
11 ••
,.,.xx
1 •••
• 1,••
... 1.
'. .• 1

Data set indicators.
This is the last volume on which this data set normally resides
Block length must always be a
multiple of 8 bytes.
Data set is security protected by a password ..
(Reserved bits)
secondary Allocation ..
Allocation parameters.
Type of request issued for the initial allocation and
to be used for subsequent extensions ..
Original request was:
In tracks relative to a specific location. No secondary allocation will be allowed.
In blocks (physical records).
In tracks ..
In cylinders.
(Reserved bits)
For a contiguous extent,.
For the maximum contiguous extent on the volume •
For the five (or less) largest extents that are
greater than or equal to a specified minimum ..
In records, to be rounded up to a cylinder boundary ..

95

(SF)

98

(62)

•• 3

DS1KLSTAR

The last-block pointer indentifies the last block
written in a sequential or partitioned organization
data set. It is in the format TTRLL (LL is defined
under the next field name):
TT - Relative address of track containing the last
block ..
R - Block number on that track.

101

(65)

• 2

DS1TRBAL

LL portion of the format given in DS1LSTAR.
LL - Number of bytes remaining on track following the
block.
Note: If both fields contain binary zeros, the last
block pointer does not apply.

Byte 2-4

secondary allocation quantity.
Number of blocks, tracks, or cylinders to be
requested at end of data set when processing a
sequential data set.

)
DSCB

127

Offset
103

(67)

105

(69)

Bytes and
Alignment

Field
Name

DATA SET CONTROL BLOCK -- FORMAT 1
Hex.
Dig. Field Description. Contents. Meaning
Reserved

2

• 10

DS1EXT1

Extent description for the first extent.
This extent description is also used in format 3 and
4 DSCBs.
Data set extent type indicator.

105

(69)

Byte 1

00
01
02
04
40
80
81

Following 9 bytes do not indicate any extent.
The extent contains the data blocks (user's blocks),
or is a prime area (for IS data sets)
The extent is an overflow area (for IS data sets
only).
The extent is an index area (for IS data sets only).
The first extent description describes the user label
extent.
The extent described is sharing one or more cylinders
with one or more data sets
The extent described begins and ends on cylinder
boundaries, i.e., the extent is composed of one or
more cylinders.

106

(6A)

Byte 2

Extent sequence number (M)

107

(6B)

Byte 3-6

Lower limit of this extent (CCHH).

111

(6F)

Byte 7-10

Upper limit of the extent (CCHH).

115

(73)

.• '. • 10

DS1EXT2

Extent description for the second extent.
mat as DS1EXTl field.

125

(7D)

• 10

DS1EXT3

Extent description for the third extent.
as DS1EXTl field.

135

(87)

••• 5

DS1PTRDS

Pointer to a format 2 DSCB. if data set has IS
organization, or pointer to a format 3 DSCB if data
set has sequential, direct, or IS organization and
more than 3 extents. This pointer has the format
CCHHR.

Same forSame format

(
128

OS System Control Blocks

Data Set Con.trol Block-Format 2

This data set control block (DSCB)
describes characteristics of a data set
having indexed sequential organization. It
is pointed to by a format 1 DSCB which contains additional data set characteristics
and up to three extents descriptions,.
Additional extents are described in a format 3 DSCB pointed to by the format 2 DSCB.
Figure 16 shows the format of the format 2
DSCB. Descriptions of the fields follow
the illustration.

DSCB

129

DATA SET CONTROL BLOCK-- FORMAT 2
0(0)

1 (1)
Hex Code
DS22MIND
Starting Address of Second-Level Master Index

8 (8)

DS2L2MEN
Ending Address of Second-Level Master Index
13 (D)
DS23MIND
Starting Address of Third-Level Master Index

20 (14)

DS2L3MIN
Ending Address of Third-Level Master Index
25 (19)

Reserved

-

_J...

-r

1

44 (2C)

DS2FMTID
Format Identifier

45 (2D)

DS2NOLEV
No. of Index Levels

46 (2E)
DS2DVIND
Master Index for these many tracks
50 (32)

Continued

52 (34)
DS2CYLOV
No. of Tracks in Overflow

53 (35)
DS2HIRIN
Highest R of High-Level Index

54 (36)

56 (38)
DS2RSHTR
Last Data Record R on Shared Track

57 (39)

58 (3A)

Continued

64 (40)

DS2HIRTI
Highest R of Track Index

61 (3D)

DS2NOBYT
No. of Bytes for Highest-Level Index

DS21RCYL
HHR of First Data Record
On Each Cylinder

DS2LTCYL
HH of Last Data Record on Each Cylinder
DS2HIRPR
Highest R of Prime Data

55 (37)
DS2HIROV
Highest R of Overflow Tracks
59 (3B)

Reserved

DS2TAGDT
No. of Delete Records

DS2RORG3
No. of References to Succeeding Overflow Records
66 (42)

DS2NOTRK
No. of Bytes

67 (43)
DS2PRCTR
No. of Records in Prime Data Area
71 (47)

Continued

Figure 16.

47 (2F)

Data Set Control Block -- Format 2

DS2STIND
Indicators

(Part 1 of 2)

c
130

OS System Control Blocks

DATA SET CONTROL BLOCK -- FORMAT 2
72 (48)
DS2CYLAD
Address of First Track of Cylinder Index

79 (4F)

DS2ADLIN
Address of First Track of Lowest-Level Master Index

86 (56)

DS2ADHIN
Address of First Track of Highest-Level Master Index
93 (5D)

DS2LPRAD
Address of Last Record in Prime Data Area

DS2LTRAD
Address of Last Entry in Track Index on Last Cylinder

101 (65)

106 (6A)

111 (6F)

DS2LCYAD
Address of Last Entry in Cyl inder Index
DS2LMSAD
Address of Last Entry in Master Index

~ -116 (74)

DS2LOVAD
Address of Last Record Written in Independent Overflow Area

--

::~

124 (7C)

DS2BYOVL
No. of Bytes Left on Independent Overflow Track

126 (7E)

DS2RORG2
No •. of Tracks Left on Independent Overflow Area

128 (80)

DS20VRCT
No. of Records in Overflow Area

130 (82)

DS2RORG1
No. of Full Cylinder Overflow Areas
135 (87)

132 (84)
Reserved
DS2PTRDS
Direct-Access Address of Next DSCB

Figure 16.

139 (8B)

Data Set Control Block -- Format 2 (Part 2 of 2)

DSCB

131

Offset

Bytes and
Alignment

Field
Name

DATA SET CONTROL BLOCK -- FORMAT 2
Hex.
Dig. Field Description, Contents, Meaning

o

(0)

1

1

(1)

• 7

DS22MIND

Address of the first track of the second level master
index in the form MBBCCHH.

8

(8)

5

DS2L2MEN

CCHHR of the last active index entry in the second
level master index.

13

(D)

• 7

DS23MIND

Address of the first track of the third level master
index in the form MBBCCHH.

20

(14)

5

DS2L3MIN

CCHHR-of the last active entry in the third level
master index.

25

(19)

• 19

44

(2C)

1

DS2FMTID

Format identification for format 2 DSCB.
"2". )

45

(2D)

• 1

DS2NOLEV

Number of index levels.

46

(2E)

'• • 1

DS2DVIND

Number of tracks determining development of the master index.

47

(29)

DS21RCYL

HHR of the first data record on each cylinder.

50

(32)

•• 2

DS2LTCYL

HH of the last data track on each cylinder.

52

(34)

1

DS2CYLOV

Number of tracks of cylinder overflow area on each
cylinder.

53

(35)

• 1

DS2HIRIN

Highest possible R on a track containing high level
index entries.

54

(36)

'.. 1

DS2HIRPR

Highest possible R on prime data tracks for form F
records.

55

(7)

••• 1

DS2HIROV

Highest possible R on overflow data tracks for form F
records.

56

(38)

1

DS2RSHTR

R of the last data record on a shared track.

57

(39)

• 1

DS2HIRTI

Highest possible R on an unshared track of the track
index.

58

(3A)

•• 1

59

(3B)

61

(3D)

64

02

Hex code 02 to avoid conflict with a data set name.

Reserved

• 3

• 2

(EBCDIC

Reserved
BS2TAGDT

Number of records that have been tagged for deletion.

• 3

DS2RORG3

Number of random references to overflow records other
than the first overflow record.

(40)

2

BS2NOBYT

Number of bytes needed to hold the nighest-level
index in main storage .•

66

(42)

,• • 1

BS2NOTRK

Number of tracks occapied by the highest level index.

67

(43)

••• 4

DS2PRCTR

Number of records in the Frime data area.

(
132

OS System Control Blocks

DATA SET CONTROL BLOCK -- FORMAT 2
Offset
11

(47)

Bytes and
Alignment
••• 1

Field
Field Description, Contents, Meaning
DS2STIND
x •• x xx ••
. 1 ••

•• 1.
• . 1.
• • •1

Status indicators.
Reserved
Key sequence checking is to be performed •
An initial load has been completed •
Last block full •
Last track full •

72

(48)

7

DS2CYLAD

Address of the first track of the cylinder index in
the form MBBCCHH.

79

(4F)

••• 7

DS2ADLIN

Address of the first track of the lowest level master
index in the form MBBCCHH.

86

(56)

•• 7

DS2ADHIN

Address of the first track of the highest level index
in the form MBBCCHH.

93

(5D)

• 8

DS2LPRAD

Address of the last record in the prime data area, in
the form MBBCCHHR.

101

(65)

• 5

DS2LTRAD

CCHHR of the last normal entry in the track index on
the last cylinder.

106

(6A)

•• 5

DS2LCYAD

CCHHR of the last index entry in the cylinder index.

111

(6F)

DS2LMSAD

CCHHR of the last index entry in the master index.

116

(74)

8

DS2LOVAD

Address of the last record written in the current
independent overflow area, in the form MBBCCHHR.

124

(7C)

2

DS2BYOVL

Number of bytes remaining on the current independent
overflow track.

126

(7E)

•• 2

DS2RORG2

Number of tracks remaining in the independent overflow area.

128

(80)

2

DS20VRCT

Number of records in the overflow area.

130

(82)

•• 2

DS2RORG1

Number of cylinder overflow areas that are full.

132

(84)

3

• 5

Reserved
136

(88)

••• 5

DS2PTRDS

Pointer to format 3 DSCB if a continuation is needed
to describe this data set. This pointer has the format CCHHR.

)
DSCB

133

(

Data Set Control Block-Format 3

This data set control block (DSCB)
describes up to thirteen additional extents
that cannot be described in a format 1
DSCB. It is pointed to by a format 1 or
format 2 DSCB. Figure 17 shows the format
of the format 3 DSCB. Descriptions of the
fields follow the illustration.

DSCB

135

DATA SET CONTROL BLOCK -- FORMAT 3

Key Identifier

DS3EXTNT
4 Extent Descriptions

44 (2C)

DS3FMTlD
Format Identifier

45 (2D)

DS3ADEXT
9 Extent Descriptions

135 (87)

DS3PTRDS
Reserved

Figure 17.
Offset

139 (8B)

Data set Control Block -- Format 3
Bytes and
Alignment

Field
Name

Hex.
Dig. Field Description. Contents, Meaning

0

(0)

4

(Key
03
identifier)

A hexadecimal 03 in each byte.

4

(4)

40

DS3EXTNT

Extent (in key) - four ten-byte fields indentical to
the DS1EXT1 field in the format 1 DSCB.

44

(2C)

1

DS3FMTID

45

(2D)

135

(87)

90
• '. • 5

F3

Format identifier - Hex F3.

DS3ADEXT

Additional extent - nine ten-byte fields identical to
the DS1EXT1 field in the format 1 DSCB.

DS3PTRDS

Reserved - contains binary zeros.

c
136

OS System Control Blocks

Data Set Control Block-Format 4

This data set control block (DSCB)
describes the volume table of contents
(VTOC) data set. It is always the first
DSCB in the VTOC. Figure 18 shows the format of a format 4 DSCB. Descriptions of
the fields follow the illustration.

DSCB

137

DATA SET CONTROL BLOCK -- FORMAT 4

-

1

Padding Bytes

44 (2C)

DS41DEMT
Format Identifier

45 (2D)

~I'

DS4HPCHR
Highest Disk Address of Format 1 DSCB
50 (32)

52 (34)

DS4DSREC
No. of Available Format 0 DSCBs in VTOC

DS4HCCHH
CCHH of Next Alternate Track

56 (38)

DS4NOATK
No. of Alternate Tracks Available

58 (3A)

59 (3B)

DS4VTOCI
VTOC Indicators

DS4NOEXT
VTOC Constant

Device Constants
60 (3C)

62 (3E)

DS4DEVSZ
No. of Logical Cylinders or No. of Tracks

66 (42)

DS4DEVTK
Device Track Length

Reserved

Continued

68 (44)

DS4DEVI
Constant for Keyed Block

72 (48)

69 (45)

DS4DEVL
Constant for Last Block

DS4DEVTL
Device Tolerance

70 (46)
DS4DEVK
Constant for no Key in Block

71 (47)
DS4DEVFG
No. of Directory Blocks Per Track

74 (4A)
DS4DEVDT
No. of DSCBs on a Track

75 (4B)
DS4DEVDB
No. of Directory Blocks Per Track

1-

Reserve d

.... r-

100 (64)

DS4F6PTR
Direct-Access Address of First Format 6 DSCB
105 (69)

DS4VTOCE
Extent Description of the VTOC
115 (73)

~L.

Reserved

T

-"-

139 (8B)1

Figure 18,.

Data Set Control Block -- Format 4

(
138

OS System Control Blocks

Offset

o

(0)

44

Bytes and
Alignment

Field
Name

DATA SET CONTROL BLOCK -- FORMAT 4
Hex.
Dig. Field Description, Contents, Meaning

44

(Padding 04
Bytes)

Hex 04 in each byte.

(2C)

1

DS4IDFMT F4

Format identifier,
Hex F4.

(2D)

• 5

DS4HPCHR

Highest address previously used for a format 1 DSCB.
The address is in the format CCHHR.

50

(32)

•• 2

DS4DSREC

Number of available format 0 DSCBs in the VTOC.

52

(34)

4

DS4HCCHH

CCHH of next alternate track available.

56

(38)

2

DS4NOATK

Number of alternate tracks available.

58

(3A)

•• 1

DS4VTOCI

VTOC Indicators.
Either no format 5 DSCBs exist or they do not reflect
the true status of the volume.
(Reserved bits)

1 •••

.xxx xxxx
59

(3B)

••• 1

60

(3C)

2

DS4NOEXT 01

Hexadecimal constant '01' to indicate the VTOC is one
extent.
Reserved
Device constants (DS4DEVxx)
The following fields describe the device on which
this volume was mounted when the VTOC was created.

62

(3E)

•• 4

62

(3E)

Bytes 1-2

Number of logical cylinders.
A logical cylinder is the smallest collection of two
or more tracks that can be processed by a set file
mask CCW (hex IF).

64

(40)

Bytes 3-4

Number of tracks per logical cylinder.

66

(42)

•• 2

DS4DEVTK

Device track length.
Number of available bytes on a track exclusive of
home address and record zero.

68

(44)

1

DS4DEVI

Overhead bytes required for a keyed block that is not
the last block on a track (see note).
Note: Overhead bytes are the number of bytes
required for gaps, check bits, and count field for
each block.

69

(45)

• 1

DS4DEVL

Overhead bytes required for a keyed block that is the
last block on a track (see preceding note).

70

(46)

•• 1

DS4DEVK

Overhead bytes to be subtracted from DS4DEVI or
DS4DEVL if block has no key field.

71

(47)

••• 1

DS4DEVFG

Flag byte.

DS4DEVSZ

xxxx xxx.
••• 1

Device size.

(Reserved bits)
A tolerance factor must be applied to all but the
last block of the track.

DSCB

139

DATA SET CONTROL BLOCK -- FORMAT 4
Offset

Bytes and
Alignment

Field
Field Description. Contents. Meaning

72

(48)

2

DS4DEVTL

Device tolerance.
Value which when divided by 512 is used to determine
effective length of a block on a track.

74

(4A)

•• 1

DS4DEVDT

Number of full DSCBs that can be contained on one
track (44 byte key plus 96 byte data length).

75

(4B)

••• 1

DS4DEVDB

Number of full PDS directory blocks that can be contained on one track (8 byte key plus 256 byte data
length) •

76

(4C)

24

100

(64)

5

DS4F6PTR

Pointer to the first format 6 DSCB.
This pointer has the form CCHHR. It contains binary
zeros when not in use.

105

(69)

• 10

DS4VTOCE

VTOC extent.
contents and meaning are the same as DS1EXTl in the
format 1 DSCB.

115

(73)

• • • 25

Reserved

Reserved

c
140

OS System Control Blocks

Data Set Control Block-Format 5

This data set control block (DSCB)
describes the amount of available space on
the volume that can be allocated to a data
set. Up to 26 available extents can be
recorded in one format 5 DSCB. Additional
extents are described in other format 5
DSCBs. The first format 5 DSCB follows the
format 4 DSCB. Figure 19 shows the format
of the format 5 DSCB. Descriptions of the
fields follow the illustration.

OSCB

141

DATA SET CONTROL BLOCK -- FORMAT 5

0(0)

DS5KEYID
Key ID

4 (4)

DS5AVEXT
An Avai lable Extent
9 (9)

-"-

-'""

DS5AVEXT
7 More Available Extents

44 (2C)

DS5FMTlD
Format Identifier

-r

45 (2D)

DS5MAVET
18 More Available Extents

135 (87)

DS5PTRDS
Direct-Access Address of Next Format 5 DSCS

Figure 19.
Offset

139 (8S)

Data Set Control Block -- Format 5
Bytes and
Alignment

Field
Name

Hex.
Dig. Field Description, Contents, Meaning

o

(0)

4

DS5KEYID 05

Key identification - Hex 05 in each byte.

4

(4)

5

DS5AVEXT

Available extent,
space available for allocation of a data set.

4

(4)

Bytes 1-2

Relative track address of the first track in the
extent.

6

(6)

Bytes 3-4

Number of full cylinders in the extent.

8

(8)

Byte 5

Number of tracks in the extent in addition to full
cylinders.

9

(9)

44

• 35

DS5EXTAV

Available extents.
7 five-byte fields identical in format to the
DS5AVEXT field.

(2C)

1

DS5FMTID

Format identifier

45

(2D)

• 90

DS5MAVET

Available extents.
18 five-byte fields identical in format to the
DS5AVEXT field.

135

(87)

••• 5

DS5PTRDS

Pointer to next format 5 DSCB.
This pointer has the form CCHHR.

142

OS System Control Blocks

Hex F5.

c

Data Set Control Block-Format 6

This data set control block (OSCB) is used
for shared cylinder allocation. It
describes the extent of space (one or more
contiguous cylinders) that are being shared
by two or more data sets. Up to 26 extents
can be described in one format 6 OSCB.
Additional extents are described in other
format 6 OSCBs. The format 6 OSCB is
pointed to by the format 4 OSCB. Figure 20
shows the format of the format 6 DSCB.
Descriptions of the fields follow the
illustration.

)
DSCB

143

DATA SET CONTROL BLOCK -- FORMAT 6

9 (0)

DS6KEYID
Key ID

4 (4)

DS6AVEXT
Address and Size of a Shared Extent

9 (9)

DS6EXTAV
Address and Size of Seven More Shared Extents

_L..

44 (2C)

_L..

-

45 (2D)

DS6FMTID
Format Identifier

DS6MAVET
Address and Size of Eighteen More Shared Extents

"135 (87)

DS6PTRDS
Direct-Address Address of Next Format 6 DSCB

Figure 20.
Offset

139 (8B)

Data Set Control Block -- Format 6
Bytes and
Alignment

Field
Name

Hex.
Dig. Field Description. Contents, Meaning

0

(0)

4

DS6KEYID 06

Key identification - Hex 06 in each byte.

4

(4)

5

DS6AVEXT

Extent of space (one or more contiguous cylinders)
that is being shared by one or more data sets.

4

(4)

Bytes 1-2

Relative track address of the first cylinder.

6

(6)

Bytes 3-4

Number of full cylinders being shared.

8

(8)

Byte 5

Number of data sets sharing the extent.

9

(9)

·

44

(2C)

1

45

(2D)

·

135

(87)

·..

35

90
5

DS6EXTAV

Shared extents.
7 five-byte fields identical in format to DS6AVEXT.
The fields are in relative track address sequence.

DS6FMTID F6

Format identifier

DS6MAVET

Shared extents.
18 five-byte fields identical in format to DS6AVEXT.

DS6PTRDS

Pointer to next format 6 DSCB.
This pointer has the form CCHHR.

-

Hex F6.

(
144

OS System Control Blocks

Data Set Labels-Magnetic Tape

The blocks of information that serve as
labels for data sets residing on magnetic
tape are the data set label 1 and the data
set label 2. These blocks are 80 bytes
long and are in EBCDIC characters in main
storage and on nine-track tape, and in BCD
characters on seven-track tape.

I

A set of a data set label 1 and a data
set label 2, together with user labels (if
used), is used to make up header labels,
end-of-volume trailer labels, and end-ofdata-set trailer labels. Separate diagrams
and descriptions are presented for these
different formats:
• Data Set Labell (FL1) •
• Data Set Label 2 (FL2).

DSL

145

c

Data Set Label I-FLI

Data set label 1 is 80 characters in length
and describes the associated data set.
This format is used for header labels, endof-volume trailer labels, and end-of-data
set trailer labels. It is followed by data
set label 2. All header labels, end-ofvolume trailer labels, and end-of-data set
trailer labels must consist of both of
these labels. In main storage these labels
are recorded in EBCDIC. They are written
in extended binary coded decimal interchange code (EBCDIC) on nine track tape
units and in binary coded decimal (BCD) on
seven track tape units.
Figure 21 shows
the format of data set label 1. Descriptions of the fields follow the
illustration.

DSL

147

DATA SET LABEL 1 -- FL1
0(0)

3 (3)

FLlLABI
Label ID

FL1NO
Label No.

FLlID
D00
t Se t ID

:;:: 4 (4)

...,r-

...,r

21 (15)
FLlFILSR
Data Set Serial No.

Continued

Continued

Continued

Continued

27 (lB)

FL 1VOLSQ
Volume Sequence No.

31 (IF)

FL 1FILSQ
Data Set Sequence No.

35 (23)

FL 1GNO
Generation No.

39 (27)

FL1VNG
Version No.

41 (29)
FL1CREDT
Creation Date
47 (2F)

FL 1EXPDT
Expiration Date
53 (35)

FL1FSEC
Security

54 (36)

FL1BLKCT
Trailer Block Count

~h

FL 1SY SC D
System Code

r60 (3C)

~

73 (49)
Reserved
79 (4F)

Figure 21.

Data Set Label 1

(
148

OS System Control Blocks

Offset

o

(0)

3

(3)

4

(4)

21

Bytes and
Alignment

Field
Name

DATA SET LABEL 1 -- FLl
Hex.
Dig. Field Description. Contents, Meaning

FL1LABI

Label identifier.
HDR - header label.
EOV - end-of-volume trailer label.
EOF - end-of-data set trailer label.

FL1NO

Data set label number

17

FL1ID

Data set identifier.

(15)

• 6

FL1FILSR

Data set serial number.
Same as the code that appears in the VOLSERNO field
of the initial volume label of the first or only
volume of the data set or multi-data set aggregate.

27

(lB)

••• 4

FL1VOLSQ

Volume sequence number.
Indicates the volume on which the data set is recorded in relation to the volume on which the data set
begins.

31

(IF)

••• 4

FL1FILSQ

Data set sequence number.
Indicates the position of the data set relative to
the first data set in a multi-data set aggregate.

35

(23)

••• 4

FL1GNO

Generation number of the data set.

39

(27)

• 2

FL1VNG

version number of a generation of the data set.

41

(29)

FL1CREDT

Creation date.
year and day - in format byyddd.

3

• 1

• 6

b
yy
ddd

47

(2F)

••• 6

FL1EXPDT

53

(35)

• 1

FL1FSEC

= 1.

blank
year (00-99)
day (001-366)

Expiration date.
Expressed in the same format as creation date.
FO
Fl

Data set security indicator.
Data set is not security protected.
Data set is security protected.

54

(36)

•• 6

FL1BLKCT

Unused in header labels - zero. In trailer labels,
the number of blocks in the data set or on the current volume of a mUlti-volume data set.

6

(3C)

13

FL1SYSCD

System code identifying the programming system.

73

(49)

• 7

Reserved - must be recorded as blanks.

DSL

149

(

Data Set Labe12-FL2

Data Set label 2 immediately follows data
set label 1. It is 80 characters in length
and contains information about the data
set, in addition to that in data set label
1. Figure 22 shows the format of data set
label 2. Descriptions of the fields follow
the illustration.

DSL

151

DATA SET LABEL 2 -- FL2
0(0)

4 (4)

FL2LABI
Label ID
FL2RECFM
Record Format

3 (3)

5 (5)
FL2BLKL
Block Length

10 (A)
FL2LRECL
Record Length

16 (10)

FL2FILP
Volume Switch

FL2NO
Label No.

15 (F)

FL2DEN
Density

17 (11)

FL2JOBD
Job ID

25 (19)

FL2JSSP
/(Slash)

26 (lA)

FL2STEPD
Step ID

34 (22)

36 (24)

FL2CNTRL
Printer Control

37 (25)

-I-

Reserved

38 (26)

FL2TRTCH
Recording Technique

FL2BLKA
Block Attribute

39 (27)

.,JI-...

Reserved

1

79

• Figure 22.

(4F)J

Data Set Label 2

(
152

OS System Control Blocks

DATA SET LABEL 2 -- FL2
Offset

Bytes and
Aligrunent

Field
Name

Field Description, Contents, Meaning

o

(0)

3

FL2LABI

Label identifier.
HDR - Header label.
EOV - End-of-volume trailer label.
EOF - End-of-data set trailer label.

3

(3)

••• 1

FL2NO

Data set label number = 2.

4

(4)

1

FL2RECFM

Record
F V U -

5

(5)

• 5

FL2BLKL

Block length. Depends on the record format.
Form F - Block length
Form V - Maximum block length
Form U - Maximum block length

10

(A)

•• 5

FL2LRECL

Format F records: Record length.
Format U records:
Zero.
Format V records Unspanned record format:
Maximum record length.
Spanned record format Records up to 32,756 bytes:
Maximum record length.
Records exceeding 32,756 bytes:
99999.

15

(F)

••• 1

FL2DEN

Tape density.
2400 Series Magnetic Tape Devices.
Field Value
in EBCDIC 7-track 9-track
o
200 bpi
1
556 bpi
2
800 bpi 800
3
1600

16

(10)

1

FL2FILP

Data set position.
Field Value
in EBCDIC
1
Volume switch previously occurred
o
No volume switch has occurred

17

(11)

8

FL2JOBD

Job Identification.

25

(19)

·
·

1

FL2JSSP

Slash

26

(lA)

FL2STEPD

step identification.

34

(22)

FL2TRTCH

These characters denote the tape recording technique
used to create this dataset (7-track tape only) •

·.8
·.2

Cb
Eb
Tb
ET

format.
Fixed length.
Variable length.
Undefined length.

(/)

.

-

Data conversion feature used.
Even parity used.
BCD to EBCDIC translation required.
Even parity and BCD to EBCDIC translation
required.
bb - Odd parity and no translation required.

DSL

153

DATA SET LABEL 2 -- FL2
Offset
36

(24)

Bytes and
Alignment
1

Field
Name
FL2CNTRL

Field Description, Contents, Meaning
Printer control
This character denotes whether a printer carriage
control set was used to create the data set and the
type of carriage control specified.
A - ASA control characters.
M - Machine control characters .•
b - Records do not contain control characters.

37

(25)

• 1

38

(26)

•• 1

39

(27)

• • • 41

Reserved
FL2BLKA

Block attribute.
B - Blocked records.
S
Spanned records.
R
Records are both blocked and spanned.
b
Records are neither blocked nor spanned.
(b - blank)
Reserved
Must be recorded as blanks.

(
154

OS system Control Blocks

Device Name Table

The device name table (DNT) contains all of
the device names that are in use. This
table is a part of the job management
initiator/terminator routine. The information in this table and the UCBs is used in
allocation of devices as specified in DD
cards. Figure 23 shows the format of the
device name table.
Descriptions of the
fields follow the illustration.

DNT

155

DEVICE NAME TABLE

:-------------------------------------------------------------------------------l
I
o

I~
_

(0)

Number of Entries

3 (3)

I

0

Device Entries

l----oevyce--Entry-------------------------------------------------------------------------------------------------------------.. ----------------------------------------------------------

ll~
-

1
-r-

Device Name

r1

+8
Device Type

+11

Device Entry

Devi ce Entry

Device Entry

_______________________________________________________________________________ J

Figure 23.

Device Name Table

(
156

os System Control Blocks

DEVICE NAME TABLE
Offset

Bytes and
Alignment

Field Description, Contents, Meaning
Number of Entries.

o

(0)

4

The number of 12 byte entries in the table.
one entry.

Each device name has

THE FOLLOWING 12 BYTE FIELD IS REPEATED FOR EACH DEVICE
+0

8

Device name.
A device name which is in one of three classes: specific, generic,
or user assigned. The name is left justified and padded with
blanks to the right.
Specific name:
IBM generated 3 character or 5 character (if the device is a 2321)
name. The first byte indicates the channel, the second byte indicates the control unit, and the third byte indicates the device.
If the device is a 2321, the fourth byte is a slash (/) and the
fifth byte indicates the bin.
Examples:
180 (tape)
190 (direct access)
193/5 (2311)
Generic name:
IBM generated name up to 8 characters in length.
Examples:
2400 (2400 series 9-track Magnetic Tape Drive)
2311 (disk drive)
User assigned name:
User assigned name up to 8 characters in length.
Examples:
MAGTAPE

+8

Device type.

+8

4

Generic name:
The contents of the field are the same as those of the UCBTYP field
in the UCB.

+8

2

Specific name, user assigned name:
A digit one higher than the digit for the preceding specific or
user assigned name. The first entry for a specific or user
assigned name will contain a 1 in this field.

• . 1

xxxxxx •• If one device is associated with the device name, these bits will
be the same as bits 0-5 of the device class field (byte 3) of the
UCBTYP field.
If more than one device is associated with the name,
these bits will indicate the result of ORing the device class field
of the UCBTYP field for each device •
•••• •• 00 Always zero.
• • ~ 1
Zero

DNT

157

c

Event Control Block

The event control block (ECB) is used for communication between various components of the
control program, as well as between processing programs and the control program. An ECB
is the subject of WAIT and POST macro instructions. Figure 24 shows the format of the
event control block. A description of its fields follows the illustration.

+1

Figure 24.
Offset
+0

Event Control Block
Bytes and
Alignment

Field
Name

Hex.
Dig. Field Description, Contents, Meaning
Awaiting completion of an event:
W - waiting for completion of an event.
After completion of an event:
C - The event has completed •
completion code •

1
1 •••
• 1 •.

• • xx xxxx

One of the following completion codes will appear at
the completion of a channel program:
Access Methods Other Than BTAM
Channel program has terminated without error.
contents useful.)

41

Channel program has terminated with permanent error.
(CSW contents useful.)

42

Channel program has terminated because a direct
access extent address has been violated.
(CSW contents do not apply.)

44

Channel program has been intercepted because of permanent error associated with device end for previous
request. You may reissue the intercepted request.
(CSW contents do not apply.)

48

Request element for channel program has been made
available after it has been purged.
(CSW contents do
not apply.)

4F

Error recovery routines have been entered because of
direct access error but are unable to read home
address or record o.
(CSW contents do not apply.)
Completed normally
completed with an I/O error
Enable command halted, or, I/O operation purged.
Completed with an I/O error
Enable command halted, or, I/O operation purged.

7F
41
48
41
48
+1

• 3

(CSW

7F

Awaiting completion of an event:
Request block address.
After completion of the event:
Zeroes, or remainder of completion code.

ECB

159

(

Interruption Control Block

The interruption control block (ICB) is
created by the Open routines when chained
channel-program scheduling has been specified. The lCB is used by the access method
routines and is always pointed to by an
lOB. Figure 25 shows the format of the
lCB. Descriptions of the fields follow the
illustration.

lCB

161

INTERRUPTION CONTROL BLOCK
0(0)
Link Address

4 (4)
ECB
8 (8)

Flag 1

9 (9)

Flag 2

I/o flags

10 (A)

I/o flags

Sense 1
First Sense Byte

11 (B)

Sense 2
Second Sense Byte

12 (C)
ECB Address

16 (10)

Flag 3
lOS error flags

17 (11)

CSW
Low-Order Bytes of Last CSW

24 (18)

Channel Program Pointer
Address of Channel Program to be Executed

28 (Ie)

Increment Amount
Block Count Constant

30 (IE)
Indicators

31 (IF)

Direct - Access Storage Devices

1

Seek Information
No. of DEB Extent and Seek Address
(This field is present only for direct-access storage devices)

132 (20)

39 (28)

Channel Program

Channel Program

Figure 25.

1
-......

J

Interruption Control Block

(
162

OS System Control Blocks

INTERRUPTION CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
Link Address.

o

(0)

4

Address of the next ICB.
first ICB.

4

(4)

4

Event control block.
Shows status of an I/O operation.

8

(8)

1

Flag byte 1.
00 ••
01 ••
10 ••
11 ••

•• 1.
••• 1

1 .••
.1 .•
•.1.
••• 0
• . •1

The last ICB points to the

No chaining (see note).
Command chaining (see note).
Data chaining (see note).
Both command and data chaining (see note).
Error routine in control •
Device is to be repositioned •
Cyclic redundancy check (CRC) needed - tape only.
Exceptional condition. If this bit is on after control has been returned from the error routine, the
error is considered permanent.
lOB unreleated flag (i.e., nonsequential) •
START •
RESTART.
Note: Chained channel-program scheduling does NOT
depend on these bits to perform its chaining.

9

(9)

.

1

Flag byte 2 •

1 •••
• 1 ••
•• 1.
••• 1
xxx.
••• 1

10

(A)

11

(B)

.

12

(C)

4

1

..

Halt I/O has been issued.
Sense will not be performed until the device is free •
lOB has been purged •
Home address (RO) record is to be read •
Internal I/O supervisor error correction flags.
QSAM error recovery routine in control for a 2540
Punch with three buffers.
First sense byte (device dependent).

1

Second sense byte (device dependent).
Address of the ECB to be posted upon completion of an
I/O event.
ECB address.
EXCP - Address of the ECB to be posted upon the completion of an I/O event.
BSAM/BPAM - Address of the ECB in the DECB to be
posted upon the completion of an I/O event.
QSAM - Address of the ECB in the QSAM prefix to the
lOB to be posted upon the completion of an I/O
event.

ICB

163

Offset

Bytes and
Alignment

Field
Name

INTERRUPTION CONTROL BLOCK
Hex.
Dig. Field Description, Contents, Meaning
Flag byte 3.

16

(10)

Flags for I/O supervisor error routine (device
dependent) •

7

CSW
17

(11)

Low order seven bytes of the last CSW.
status for this request.

• 7

Shows channel

Channel Program Pointer
24

(18)

Address of the channel program to be executed.

4

Increment Amount
Magnetic tape
28

(lC)

Constant that is used to increment the block count.

2

Always zero for direct access.
30

(lE).. 2

30

(lE)

•• 1

Indicators.
1. •.• • •••

• xxx xxxx

Special volume full indicator signifying end-of-tape
mark or reflective spot sensed along with a read or
write erroro
(Reserved Bits) Always zero •

31

(IF)

••• 1

Reserved

32

(20)

8

Seek Information
This field is present for direct access devices only.

32

(20)

Byte 1

The number of the DEB extent to be used for this
request. The first extent is number zero.

33

(21)

Bytes 2-8

The Seek address for this I/O request.

40

(28)

Channel program.

(
164

OS System Control Blocks

Input/Output Block

The input/output block (IOB) is the communication medium between a routine that
requests an I/O operation and the I/O
supervisor. All the information required
by the I/O supervisor to execute an I/O
operation is contained in the IOB, or is
pointed to by the IOB. Figure 26 shows the
format of the IOB. Descriptions of the
fields follow the illustration.
The IOB format falls into three segments
whose use varies mainly by access method:
Prefix -• GAM. QISAM.
• BSAM. QSAM, BPAM
• BSAM. QSAM, BPAM

Normal scheduling.
Chained scheduling.

standard Fields
• Displacements 0-31 (decimal), 0-lF
(hexadecimal) •
Extension
•
•
•
•
•
•
•

BTAM.
GAM.
Direct-access storage devices.
BSAM. QSAM. BPAM.
QISAM, Scan Mode.
BISAM.
BDAM.

The following illustrates the relationship of these segments.

Prefix Segment

0(0)
Standard Fields Segment

32 (20)
Extension Segment

IOB

165

INPUT/OUTPUT BLOCK

PREFIX

----G-A-M-;-a-isA-M----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1-4 (-41

Event Control Block

-1 (-1)

QSAM, BSAM, BPAM - Normal Scheduling
-8 (-8)

-7 (-7)

I/o

Flags

Address of Next lOB

-4 (-4)
Event Control Block

-1 (-1)

QSAM, BSAM - Chained Scheduling
-16 (-10)

FLAG 1
Indicators

-15 (-F)

I/o

-14 (-E)
INNOP
Offset to Last I/O for Input

Reserved

-13 (-D)
OUTNOP
Offset to Last I/o for Output

-12 (-C)
Event Control Block
-8 (-8)

FIRSnCB
Address of First ICB

-4 (-4)
Last NOP Address

-1 (-1)

Standard Fields
0(0)

IOBFLAG1
I/o Flags

1 (1)

4 (4)

10BECBCC
Completion Code

5 (5)

8 (8)

IOBFLAG3
Error Flags

IOBFLAG2
I/o Flags

2 (2)

3 (3)

10SENSO
First Sense Byte

IOBSENS1
Second Sense Byte

10BECBPT
Address of ECB

9 (9)

I/o

10BCSW
Seven Low-Order Bytes of Last CSW

16 (10)

10BSIOCC
SIO Condition Code

20 (14)

17 (11)

10BSTART
Address of Channel Program

21 (15)

10BDCBPT
Address of DC B

Reserved
24 (18)

10BRESTR
PURGE Chain/CCHH/Command, Channel Program

28 (1C)

10BINCAM
(use vari es)

-Figure 26.

30 (lC)

10BERRCT
No. of Error Retries

31 (1F,

Input/Output Block (Part 1 of 3)

c
166

OS System Control Blocks

INPUT/OUTPUT BLOCK

Extension
r---STAM---------------------------------------------------------------------------------------------------------------------------------------.---------------------------------------------.----~

32 (20)

10BUCBX
UCB Index

33 (21)
10BWORK
Error Routine and ONL TT Routine Work Area

_~O (28)

1.

10BERCCW
Error Routine CCW

(30)

-L..

I
I

10BERINF
Error Routine Data

L(40)

""'-

10BCPA
Channel Programs

I

""'-

""'-

J

GAM
32 (20)

10BUCBX
UCB Index

36 (24)

33 (21)
Reserved

37 (25)

10BNXTPT
Address of Next Available lOB

Status Indicators

..J ... 40

(28)

10BCCW
List of CCWs

T
132

-a..

71 (47)}

10BSEEK
No_ of DEB Extent and Seek Address
(This field may be present only for direct-access storage devices)

(20)

1
39 (27)

BSAM, QSAM, BPAM

L
""'-

I

1
""'""'-

Channel Program

J

Addi tiona I Search Addresses
(This field may be present only for direct-access storage devices)

Continued
Figure 26.

Input/Output Block (Part 2 of 3)

lOB

167

INPUT/OUTPUT BLOCK

Extension (Continued)

r---os-iAM----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------,

40 (28)

W1IEXTEN,W10EXTEN
Appendage Codes

41 (29)

BISAM
40 (28)

10BCCWAD
Address of First CCW

44 (2C)

10BINDCT
Queue Indicators

48 (30)

10BCOUNT
Write Check Count

45 (2D)
10BUNSOR
Reason Queue Unscheduled

46 (2E)

49 (31)

52 (34)

10BAPP
Appendage Codes

47 (2F)

10BASYN
Asynchronous Code

10BCHAD
Forward Chain Address
10BBCHAD
Backward Chain Address
55 (37)

BDAM
40 (28)

44 (2C)

10BDBYTR
No. of Unused Track Bytes
10BDAVLI
Availability Indicator

48 (30)

45 (2D)

50 (32)

10BDSTAT
Status of Request

IOBDCPND
Address of Channel Program End
10BDBYTN
No. of Bytes Per Block

60 (3C)

58 (3A)
Reserved
10BDQPTR
Address of Next lOB

-'... 64 (40)

-L...

Reserved

L(481

10BDNCRF
Count Field for Next Block

t(SOI
Channel Program

I

1
T

T
Figure 26.

168

10BDIOBS
Size of lOB

10BDPLAD
Address of Next lOB in Pool

10BDTYPE
Type of I/o and Options

52 (34)

56 (38)

42 (2A)

Input/Output Block (Part 3 of 3)

OS System Control Blocks

INPUT/OUTPUT BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
PREFIXES
GAM, QISAM PREFIX

-4

(-4)

4

GAM: Event control block that is within first lOB
only.
QISAM: Event control block used to indicate status
of an I/O event.
BSAM, QSAM, BPAM -- NORMAL SCHEDULING PREFIX

-8

(-8)

Flag byte.

1

1 •••
• 1 ••

•• 1.
.•. 1
1 •..

• xx.
••• 1

PRTOV has occurred.
A WRITE operation is in process •
A READ operation is in process •
Update flag.
Set on together with bit 1 of this byte
to show that the block is to be updated. Can only
occur if the OPEN parameter is UPDAT.
lOB being used for backspace, control, or note/point
operation.
(Reserved bits) •
This is the first lOB •

-7

(-7)

• 3

Address of the next lOB associated with one particular DCB. The lOBs are chained in sequential order.

-4

(-4)

4

An ECB used by QSAM to indicate the status of the I/O
event.
BSAM, QSAM, BPAM -- CHAINED SCHEDULING PREFIX

-16 (-10)

1

FLAGl
xxxx x •••
.1 ••
• • 1.
••• 1

.1
'. . 1

-15

(-F)

-14

(-E)

-13

(-D)

...1

-12

(-C)

1

-8

(-8)

4

-4

(-4)

4

I/O Indicators.
(Reserved bits)
Error has been processed once by abnormal-end appendage routine.
Restart channel •
Set when a program-controlled interruption (PCI)
occurs.
Reserved

INNOP

Offset of the last I/O instruction for input operation (NOP CCW) from the origin of the ICB.

OUTNOP

Offset of the last I/O instruction for an output
operation (NOP CCW) from the origin of the ICB.

An ECB used by BSAM or QSAM.
I/O operation.
FIRSTICB

Shows the status of the

Address of the first interrupt control block (ICB) on
the ICB queue.
Address of the NOP instruction at the end of the
queue.

)
lOB

169

INPUT/OUTPUT BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description. Contents. Meaning
STANDARD FIELDS

o

(0)

1

IOBFLAGl
00 ••
01 ••
10 ••
11 ••

•• 1.
•••1
1 •.•
.1 ..

•• 1.
••• 0
••• 1
1

(1)

•

IOBFLAG2

1

1. '•.
• 1 ••

•• 1.
.•. 1

xxx.
• '•• 1

2

(2)

1

3

(3)

•

4

(4)

1

5

(5)

•

8

(8)

1

9

(9)

•

3

7

1

Flag byte 1
No chaining.
Command chaining.
Data chaining.
Both command and data chaining.
Error routine in control •
Device is to be repositioned •
Cyclic redundancy check (CRC) needed - tape only.
Exceptional condition. After the error routine
returns and this bit is on. the error is considered
permanent •
lOB unrelated flag (i.e., nonsequential) •
START
RESTART
Flag byte 2
Halt I/O has been issued.
Sense will not be performed until the device is free •
lOB has been purged •
Home address (RO) record is to be read •
Internal I/O supervisor error correction flags.
QSAM -- error recovery in control for a 2540 Punch
with three buffers.
BTAM -- RESETPL macro instruction was used.

IOBSENSO

First sense byte (device dependent).

IOBSENSl

Second sense byte (device dependent).

IOBECBCC

completion code for an I/O event. This code will
appear in the first byte of an ECB.
(For specific
codes see ECB.)

IOBECBPT

EXCP - Address of the ECB to be posted upon the completion of an I/O event.
BSAM/BPAM - Address of the ECB in the DECB to be
posted upon the completion of an I/O event.
QSAM - Address of the ECB in the QSAM prefix to the
lOB to be posted upon the completion of an I/O
event.

IOBFLAG3

I/O supervisor error routine flag byte (device
dependent) •

IOBCSW

Low order seven bytes of the last csw that reflects
the status for this request.

(
170

OS System Control Blocks

INPUT/OUTPUT BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
STANDARD FIELDS  •
89

(S9)

'. 1

JFCBBIAR"

x. '••• x ••
0.,• • • 0.,.

0 •• ,• • 1 ••

JFCBFTEK, JFCBFALN
Code
Access methods other than QTAM
Buffer pool location, coded in the DD statement
0, Hierarchy 0 main storage.
none
1 Hierarchy 1 main storage.

.x.x x •••
• 1 ••
.•• 1
1 •••

S
E
D

(Reserved bit)

• x ••
'•• xx

• • 10
•• 01

)

90

(SA)

... 2

JFCBUFL

Buffering technique:
Simple buffering •
Exchange buffering •
Dynamic buffering.

D
F

Buffer alignment:
Doubleword boundary •
Fullword nota doubleword boundary •

Buffer length

JFCB

181

JOB FILE CONTROL BLOCK
Offset
92

(SC)

Bytes and
Alignment
1

Field
Name
JFCEROPT
1 •• '.

.1 ••
•• 1.
••• x xxxx

Field Description, Contents, Meaning
Error option.
Disposition of permanent errors if user returns from
a synchronous error exit.
(QSAM)
Accept
Skip
Abnormal end of task
(Reserved bits)
Device Characteristics Field

93

(SD)

The content of this one-byte field depends upon the
device in use.
MAGNETIC TAPE

93

(SD)

• 1

JFCTRTCH
0010
0011
0001
0010

0011
1011
0011
1011

Tape recording technique for seven track tape.
Code
Even parity.
E
T BCD/EBCDIC translation.
C
Data conversion.
ET Even parity and translation.
DIRECT-ACCESS STORAGE

93

(SD)

• 1

JFCKEYLE

Direct access key length.
CARD READER" CARD PUNCH

93

(SD)

'. 1

JFCMODE
1000
0100
JFCSTACK
0001
0010

Mode of operation.
Code
C Column binary mode.
E EBCDIC mode.
Stacker Selection.
Code
--1- Stacker 1
2 Stacker 2
PRINTER

93

(SD)

• 1

JFCPRTSP
0000
0000
0001
0001

0001
1001
0001
100i

Normal printer spacing.
Code
- 0 - No spacing,.
1 Space one line,.
2 Space two lines.
3 Space three lines.
PAPER TAPE

93

(SD)

• 1

JFCCODE
1000
0100
0010
0001
0000
0000
0000

0000
0000
0000
0000
1000
0100
0010

Conversion code.
Code
N No conversion.
IBM BCD.
I
F
Friden.
B
Burroughs.
C National Cash Register.
A ASCII (8-track).
T Teletype.
End of Device Characteristics Field.

(
182

OS System Control Blocks

JOB FILE CONTROL BLOCK
Offset
94

(5E)

Bytes and
Alignment
•• 1

Field
Name
JFCDEN

Field Description, Contents, Meaning
Tape density.
2400 series magnetic tape units ..
Code 7-track

0000
0100
1000
1100
95

(SF)

••• 3

96

(60)

2

98

(62)

98

(62)

0011
0011
0011
0011

JFCLIMCT

. 1
1. '...
. 1 ...

•• 1.
••• x xx ••
•• 1 ..

.•. 1

99

(63)

.. • • 1

(64)

1

JFCRECFM
10 ...
01 ...
11 ••
. . 1 ..
.•. 1
1 ....

.. 10.
• 01.
• 00.

.... 0
101

101

(65)

(65)

556 bpi
800 bpi

3

800 bpi
1600 bpi

BDAM:
Search limit.

.. 1

.. 1

JFCOPTCD

1 •..
.1 ..

... 1.
••• x

Data set organization being used.
Code
IS Indexed sequential organization.
PS Physical sequential organization •
DA Direct organization •
(Reserved bits)
PO Partitioned organization •
U UDnlovable - the data contains
location dependent information ..
GS

0 .. '...

.xxx xxxx
100

1
2

9-track

Data set opened for MOD:
If automatic step restart was requested - Track
balance existing when the data set was first opened
during the original execution of the current step.
JFCDSORG

..

- 0 - 200 bpi

xx
.. 1 ..

X ..

Graphics organization ..
(Reserved bits)

Record format.
Code
F Fixed.
V Variable.
U Undefined ..
T Track overflow.
B Blocked: may not occur with
undefined (U) ..
S Fixed length record format: Standard blocks no
truncated blocks or unfilled tracks are embedded
in the set.
Variable length record format:
Spanned records ..
A ASA control character •
M Machine code control character ..
No control character ..
Always zero ..
Option codes.
QSAM, BSAM, BPAM
Code
W Write validity check.
U
Allow a data check caused by an invalid character (1403 printer with UCS feature.)
C
Chained scheduling using the Program Controlled
Interruption ..
(Reserved bits)
z Magnetic Tape Devices: Used reduced error recovery procedure.

JFCB

183

JOB FILE CONTROL BLOCK
Offset

101

(65)

Bytes and
Alignment

•

1

Field
Name

1 •• '.

.x. I. ,.x. '.

• .• 1.
• '•• 1
1 •••
• • 1 .•
•.. . 1

Field Description, Contents, Meaning
BISAM, QISAM
Code
---W- Write validity check.
(Reserved bits)
M Master Indexes.
I Independent overflow area.
Y Cylinder overflow area.
L Delete option •
R Reorganization criteria .•
BDAM

101

(65)

•

1

--w- Write

1 .•••
• 1 ••
• ,.1.

.• • • 1
1 •••

E
F
A

.xx.
•.. . 1

102

(66)

••

104

(68)

106

107

2

R

validity check.
Track overflow •
Extended search.
Feedback •
Actual addressing,.
(Reserved bits)
Relative block addressing •

JFCBLI directory
entry. Figure 30A shows the general format
of an entry in a POS directory. Following
the illustration is a general description
of the fields of an entry.

POS

197

1

10(0)
Member or Alias Name

-r-

-~

8 (8)
Relative Address of Named Member

Indicators

:.: 12 (C)

User Data

T
Figure 30A.

T

PDS Directory Entry -- General Format
PDS DIRECTORY ENTRY

Offset

o

(0)

Bytes and
Alignment

8

Field Description, Contents, Meaning

Member name of alias name.
TTR

8

(8)

3

TTR of the first block of the named member.

c
11

(B)

••• 1
1. • •
• xx.

Indicators.
Name is an alias,•
Number of TTR' s in the user data field. A maximum of three is
allowed •
• ,•• x xxxx Length of the user data field in half words.
User Data

12

(C)

n

variable user data as provided as input to the STOW macro
instruction.
Up to three pointers to locations within the member may be provided. The pointers must be four bytes long and must appear at the
beginning of the user data field. Their format is as follows:
TT - 2 bytes - Relative track from the beginning of the data set.
R - 1 byte - Block number on that track.
N - 1 byte - If the TTR points to a note list, this byte indicates
the number of entries in the note list.
If the TTR does not point to a note list, this byte
contains zeros.
The remaining bytes in the user data area are optional in their
format and use.

198

OS System Control Blocks

Partitioned Data Set Directory
Entry-Format 1

This format appears in the partitioned data
set (POS) directory and is the format produced by linkage editor for a load module.
Figure 30B shows the format of an entry in
a FOS directory for a load module.
The difference between format 1 and format 2 of linkage editor POS directory
entries lies in two fields inserted into
format 1 at offsets +11 and +12 (decimal)
by the BLOL routine when it places the
entry into a BLDL list.

POS

199

PDS DIRECTORY ENTRY

A II Load Modu les

10(0)
-....

1
'

Member of Alias Name

8 (8)

....

11 (B)
Re lative Address of First Block (TTR- P)

Indicators

12 (C)

15 (F)
Relative Address of First Block of Text (TTR-T)

Zeros

16 (l0)

19 (13)
Relative Address of Note List or Scat/Trans Table

20 (l4)

No. of List Entries

22 (l6)
Module Attributes

Main Storage Needed for Module

25 (19)

Continued

27 (lB)
Length of First Text Block

Entry Point Address

30 (lE)

Continued

First Text Block Origin

Continued
32 (20)

33 (21)

35 (23)
Scatter List Size

37 (25)

Continued

Translation Table Size
39 (27)

ID of ESD for First Text Block Control Section

ID of ESD

Continued
40 (28)

Load Modules With Alias Names and RENT or REUS Attributes
33 (21)
Entry Point for Member Name

136

(24)

-L.

T
Load Modules - Scatter, With Alias Names and RENT or REUS Attributes
Member Name of a Load Module

43 (2B)

T

40 (28)
Entry Point for Member Name

144
T

(2C)
Member Name of a Load Module

Figure 30B.

200

51 (33)T

PDS Directory Entry -- Format 1

OS System Control Blocks

PDS DIRECTORY ENTRY
Offset

Bytes and
Aligrunent

Field Description, Contents, Meaning
standard Field

o

(0)

8

Load module member name or alias name ..

8

(8)

3

TTR of the first block of the named member (load module).
Indicators

11

(B)

•••

1

Bit State

Meaning
Name is an alias in the first field.
1-2 (variable) Number of TTRs in the user data field.
3-7 (variable) Length of user data field in half words.

o

-----1

User Data Field
TTR-T
12

(C)

3

15

(F)

•••

TTR of the first block of text ..
1

Zeros.
TTR-N/S

16

(10)

3

TTR of the Note List or Scatter/Translation Table. Used for
modules in scatter load format or overlay structure only.
NL

19

(13)

...

20

(14)

2

1

The number of entries in the note list for modules in overlay
structure; otherwise zero.
Attributes

Byte 1
1 .•.

• 1 ••
•• 1.
••. 1

1 •••
• 1 ..

•• 1.
•..1
••• 0

Reenterable ..
Reusable ..
In overlay structure •
Module to be tested - TESTRAN •
Only loadable.
Scatter format •
Executable ..
Module contains no RLD items and only one block of text •
Module contains multiple records with at least one block of text ..

Byte 2
1. '..
0 •••

• 1. '.
• • 0.

•• 1 ..
.... 1

1 ....
• 1 •.
.• 1.

• • •1

Module can be processed only by F level of linkage editor.
Module can be processed by all levels of linkage editor.
Linkage editor assigned origin of first block of text is zero •
Linkage editor assigned origin of first block of text is not zero ..
Entry point assigned by linkage editor is zero •
Module contains no RLD items •
Module cannot be reprocessed by linkage editor ..
Module contains TESTRAN symbol cards •
Module created by linkage editor F •
Refreshable module ..

PDS

201

PDS DIRECTORY ENTRY
Offset

Bytes and
Alignment

Field Description, Contents, Meaning
User Data Field (Continued)
Main Storage

22

(16)

•• 3

Total contiguous main storage requirement of module.
First Text Block Length

25

(19)

•

2

Length of the first block of text.
EP Address

27

(lB)

_ _ • 3

Entry point address associated with member name or with alias name
if the alias indicator is on.
First Text Block Origin

30

(lE)

•• 3

Linkage editor assigned origin of the first block of text.
LOAD MODULE -- SCATTER
Scatter List Size

33

(21)

• 2

Number of bytes in the scatter list.
Transl-Table Size

35

(23)

••• 2

Number of bytes in the translation table.
ESDID-T

37

(25)

_ 2

Identification of the ESD item (ESDID) of the control section to
which the first block of text belongs.
ESDID-CSECT

39

(27)

••• 2

Identification of the ESD item (ESDID) of the control section containing the entry point.
LOAD MODULE -- ALIAS NAME AND RENT OR REUS ATTRIBUTE
EP-Member Name

41

(29)

• 3

The entry point associated with the member name when the first
field is an alias name and the load module has reenterable or reusable attributes.
Member Name

44

(2C)

8

The member name of the load module when the first field is an alias
name and the load module has reenterable or reusable attributes.

c
202

OS System Control Blocks

Partitioned Data Set Directory EntryFormat 2

This format is received upon issuance of a
BLDL macro instruction where the specified
members are load modules produced by linkage editor. The user data field for a load
module is described here. Figure 30e shows
the format of PDS directory entries for
linkage editor load modules in a BLDL list.
Descriptions of the fields follow the
illustration.
The difference between format 2 and format 1 of linkage editor PDS directory
entries lies in the concatenation number
and library flag fields inserted in format
2 by the BLDL routine at offsets +11 and
+12 (decimal).

PDS

203

PDS DIRECTORY ENTRY

All Load Modules

1

10(0)
~,...

~

Module Member Name or Alias

8 (8)

...

11 (B)
Concatenation No.

Relative Address of First Block
12 (C)

13 (D)

14 (E)

Type of Library

Indicators

Relative Address of First Text Block

17 (1 I)

Continued

18 (12)
Relative Address of Note List or Scat/Trans Table

Zeros
21 (15)
No. of Note List Entries

Continued

22 (16)
Module Attributes
27 (1 B)

24 (18)
Main Storage Needed for Module

Length of First Text Block

29 (ID)

Continued

Entry-Point Address

32 (20)
First Text Block Origin
34 (22)

--------------------------- ... ------------------------------------- ... -------------------------------------------------------------------------------------------------------------------------------------,

Load Module - Scatter
35 (23)
Sea tter Li st Size

Continued

Continued

37 (25)
Translation Table Size

39 (27)
ID of ESD for First Text Control Section

41 (29)
ID of ESD for Entry-Point Control Section

Load Modules With Alios Names and RENT or REUS Attributes
35 (23)
Entry-Point for Member Name

38(26)

Continued

Load Module Member Name

45 (2D)

Load Modules -Scatter, With Alios Names and RENT or REUS Attributes
43 (2B)
Entry-Point for Member Name
43(2B)
Continued

Load Module Member Name

53(35)1

Figure 30C.
204

PDS Directory Entry -- Format 2

OS System Control Blocks

PDS DIRECTORY ENTRY
Offset

Bytes and
Alignment

Field Description, Contents, Meaning
Standard Field

0

( 0)

8

Name
Load module member name or alias name.

8

(8)

3

TTR-P
TTR of the f-irst block of the named member (load module).

11

(B)

...

12

(C)

1

13

(D)

• 1

1

Concatenation number of the data set.
Library
This byte is normally zeros.
If the DCB operand in the BLDL macro
instruction was specified as zero, this byte will contain a 1 if
the name was found in the link library, and a 2 if the name was
found in the job library.
Indicators
Bit Setting

Meaning

o
1
Name is an alias in the first field.
1-2 (variable) Number of TTRs in the user data field.
3-7 (variable) Length of user data field in half words.
User Data Field
TTR-T
14

(E)

17

(11)

3

TTR of the first block of text.

• 1

Zeros.
TTR-N/S

18

(12)

•• 3

TTR of the Note List or Scatter/Translation Table. Used for
modules in scatter load format or overlay structure only.
NL

21

(15)

·

22

(16)

·.

The number of entries in the note list for modules in overlay
structure.

1

Attributes

2

Byte 1
1. '••
• 1 ••
• .1,•
•• ,.1
1,•••
• 1 ••
•• 1.

••• 1
••• 0

Reenterable.
Reusable •
In overlay structure.
Module to be tested - TESTRAN.
Only loadable.
Scatter format •
Executable.
Module contains no RLD items and only one block of text •
Module contains multiple records with at least one block of text •

Byte 2
1 •••
0,•• '.

,.1 ••
,.0 ••

• ,.1.
••• 1
1 •••

• 1,••
•• 1.
'••• 1

Module can be processed only by F level of linkage editor.
Module can be processed by all levels of linkage editor.
Linkage editor assigned origin of first block of text is zero.
Linkage editor assigned origin of first block of text is not zero.
Entry point assigned by linkage editor is zero.
Module contains no RLD items •
Module cannot be reprocessed by linkage editor.
Module contains TESTRAN symbol cards •
Module created by linkage editor F •
Refreshable module.
PDS

205

PDS DIRECTORY ENTRY
Offset

Bytes and
Alignment

Field Description, Contents, Meaning
User Data Field (Continued)
Ma in storage

25

(19)

• 2

Total contiguous main storage requirement of module.
First Text Block Length

27

(lB)

••• 2

Length of the first block of text.
EP Address

29

(lD)

• 3

Entry point address associated with member name or with alias name
if the alias indicator is on.
First Text Block Origin

32

(20)

3

Linkage editor assigned origin of the first block of text.
LOAD MODULE -- SCATTER
scatter List Size

35

(23)

••• 2

Number of bytes in the scatter list.
Transl-Table Size

37

(25)

• 2

Number of bytes in the translation table.
ESDID-T

39

(27)

••• 2

Identification of the ESD item (ESDID) of the control section to
which the first block of text belongs .•
ESDID-CSECT

41

(29)

• 2

Identification of the ESD item (ESDID) of the control section containing the entry point.
LOAD MODULE -- ALIAS NAME AND RENT OF REUS ATTRIBUTE
EP-Member Name

43

(2B)

••• 3

The entry point associated with the member name when the first
field is an alias name and the load module has reenterable or reusable attributes.
Member Name

46

(2E)

•• 8

The member name of the load module when the first field is an alias
name and the load module has reenterable or reusable attributes.

(
206

os System Control Blocks

Request Blocks

Request blocks are used by the supervisor
for maintaining information concerning programs and routines (logically distinct sections of code>. Other components of the
control program may create request blocks
and/or refer to information in them.
The various request blocks are shown and
described separately as follows:
• PCP, MFT Configuration -IRB, LRB, LPRB, PRB, SIRB, SVRB:
Figure 3iA and following text
• MVT Configuration --

IRB:

Figure 3iB and following text

PRB:

Figure 3iC and following text

SIRB:

Figure 3iD and following text

SVRB --

Resident SVC Routines:
Figure 3iE and following text
Transient SVC Routines:
Figure 3iF and following text

RB

207

c

Request Block-PCP, MFT Configurations

Request blocks used by the PCP and MFT configurations of System/360 Operating System
are described and illustrated here. Figure
31A shows the different formats; field
descriptions follow the illustration,.

RB -- PCP, MFT

209

REQUEST BLOCK -- PCP, MFT

LPRB LRB
-8 (-8)

XRBSUC
Load List Pointer to Previous RB

-4 (-4)

XRBPRE
Load List Pointer to Next RB

IRB PRB, SIRB SVRB

~--~---------~-------~------------------------------------------------------~-l

I

,-

! 10 (0)

1 :

XRBNM
Program Name

8 (8)

XRBSZ

12 (C)

XRBUSE
Use Count

10 (A)

13 (D)

End of LRB

-...

XSTAB
Flag Bytes
XRBEP
Entry-Point Address

-----------------------------._----------------------------------------------------~---

116(10)

-r""

..

--- ,_.--------------------------- ------------------------_.-.--.-._----------._------------------------

1

XRBPSW
Save Area for PSW

-I'"

,
24 (18)

28 (1C)

XRBQ

XRBWT
Wait Count

29 (10)

XRBLNK
Address of Previous RB or TCB

, End of LPRB

---1-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

~-~~Q-~-fR~------------------------------------------- _________ ._._. ___ . ___ ._._

1
1

::::::32 (20)

~ _~
SYNC macro instruction requested,•
XCTL macro instruction requested •
Load macro instruction requested •

• 3

RBCDE

Address to contents directory entry for the modules
that this request block is associated with.

(10)

8

RBOPSW

User's old PSW.

24

(18)

1

25

(19)

• 3

RBPGMQ

Address of a request block indicating a request to
use the same serially reusable program..

28

(le)

1

RBWCF

Number of requests waiting (wait count).

29

(lD). 3

RBLINK

Address of next request block, or
Address of the TCB, when this is the first request
block.

32

(20)

64

RB GR SAVE

General register save area used by the supervisor:
register sequence 0 to 15.

96

(60)

48

RBEXSAVE

Extended save area for SVC routine.

Zeros.

SVRB -- MVT

233

(

Supervisor Request Block-MVTTransient SVC Routines

This supervisor request block (SVRB) is
used by the supervisor to hold information
for type 3 or 4 (transient) svc routines.
Figure 31F shows its format used in MVT;
field descriptions follow the illustration.

SVRB -- MVT

235

SUPERVISOR REQUEST BLOCK -- MVT -- TRANSIENT SVC ROUTINES
0(0)

4 (4)

8 (8)

2 (2)

RBTABNO
Displ for TACT Entry

RBRTLNTH
SVC Routine Length

RBABOPSW
4 Low Order Bytes of Routine Name or Right-Half of User's Old PSW
RBWCSA
Wait-Count Save-Area

9 (9)

RBSIZE
Size of This RB

12 (C)

10 (A)

RBSTAB
Status and Attribute Bits

RBSVTQN
Address of Next RB on Transient User Queue

_1,..16 (10)

RBOPSW
Old PSW

-~

24 (18)
RBTAWCSA
Wait Count Overlay Save Area

25 (19)

28 (1C)

29 (1C)

RBWCF
Wait Count

-,-,
-r-

RBSVTTR
TTR for SVC Routine
RBLINK
Address of Next RB or TCB

-L..

.,.JL.,.

:::::32 (20)

RBGRSAVE
General Register Save Area

.......,
----

I

:::::96 (60)

----

.......,

I

RBEXSAVE
Extended Save Area for SVC Routines

.......,

.......,

T

143 (8F)

Figure 31F.

~

.......,
----

J

Supervisor Request Block -- MVT -- Transient SVC Routines

(
236

OS system Control Blocks

Offset

SUPERVISOR REQUEST BLOCK -- MVT TRANSIENT SVC ROUTINE
Bytes and Field
Alignment Name
Field Description, Contents, Meaning

o

(0)

2

RBTABNO

Displacement from beginning of transient area control
table (TACT) to entry for module represented by this
SVRB.

2

(2)

.... 2

RBRTLNTH

Length of SVC routine in bytes.

4

(4)

4

RBABOPSW

After execution of transient a.rea handler routine:
Four low-order bytes of name of requested routine.
After execution of ABTERM routine:
PSW ..

Right-half of old

8

(8)

1

RBWCSA

Number of requests waiting at time of termination
(wait-count save-area) ..

9

(0)

.. 1

RBSIZE

Size of request block in doublewords.

10

(A)

••

RBSTAB

Status and attribute bits.

2

Byte 1
00 ••

01 ••
10 ....
11 ••

•.• x.

X ..

xx

..... • 1

.1 •.•

Program request block (PRfr).
Interruption request block (IRB) ..
System interruption reques~ block (SIRB).
Supervisor request block (SVRB) ..
(Reserved bits)
SVRB for transient SVC routines •
A checkpoint may be taken in a user exit from this
SVC routine.

Byte 2
1 •••
• 1 ••

•.• xx
00 ••
01 ••
11 ••

•• 1.
'••• 0
.••. 1

)

RBLINK field points to TCB.
Program is active (applies to IRB or SIRB) •
(Reserved bits)
Request queue element is not to be returned.
IRB has queue elements for asynchronously executed
routines that are RQEs ..
IRB has queue elements for asynchronously executed
routines that are IQEs,.
Request block storage can be freed at exit •
Wait for a single event or all of a number of events.
Wait for a number of events that is less than the
total number of events waiting '.

12

(C)

4

RBSVTQN

Address of next request block on queue of transient
routines.

16

(10)

8

RBOPSW

User's old PSW.

24

(18)

1

RBTAWCSA

Save area for number of requests field used if transient routine is overlaid.

25

(19)

• 3

RBSVTTR

Relative direct access device address in the format
of TTR for the SVC routine.

28

(lC)

1

RBWCF

Number of requests waiting (wait count).

29

(lD)

• 3

RBLINK

Address of the next request block, or Address of the
TCB, when this is the first request block.

32

(20)

64

RBGRSAVE

General register save area used by the supervisor;
register sequence 0 to 15 ..

96

(60)

RBEXSAVE

Extended save area for SVC routines.
SVRB -- MVT

231

(

Task Control Block

The task control block (TCB) serves as a
repository for information and pointers
associated with the task in progress.
Various components of the control program
place information in the TCB, and obtain
information (or its location) by reference
to it. Figure 32 shows the format of the
TCB. Descriptions of the fields follow the
illustration.

»
TCB

239

TASK CONTROL BLOCK

1

-.....-32 (-20)

0(0)

~

....

TCBRBP
Address of RB

4 (4)

TCBPIE
Address of Program Interrupt Element

8 (8)

TCBDEB
Address of DEB Queue

12 (C)

TCBTIO
Address of Task I/o Table

16 (10)

TCBCMP
Task Completion Code

20 (14)

TCBTRN
Flag, Address of Control Core Table (TESTRAN)

24 (18)
TCBNROC
MVT: Roll-Out Eligibility
28 (1C)

1

TCBFRS
Floating - Point Register Save Area

"'11""

TCBPKE
Protection Key

25 (19)

TCBMSS
PCP, MFT: Address of Boundary Box
MVT: Address of Last SPQE

29 (18)

TCBFLGS
Task End, Miscellaneous, and Dispatchability Flags
(PCP and MFT use differs from MVT use)
34 (22)
TCBLMP
PCP, MFT: Enqueue Count
MVT: Limit Priority

35 (23)

TCBDSP
Dispatching Priority

TCBLLS
PCR, MFT: Address of Last RB for Program Loaded by LOAD
MVT: Address of Load List Element for Program Loaded by LOAD

36 (24)

40 (28)

TCBJLB
Address of JOBLIB DCB

44(2C)
PCP, MFT: Reserved
MVT: TCBJ PQ - (Job Step TCB) Address of CDE for JPA

-::::48 (30)

......

...

TCBGRS
General Register Save Area

112 (70)

PCP ,MFT: TCBIDF - TCB Identifier
MVT: TCBQE L- Enqueue Count

-Figure 32 ..

113 (71)

TCBFSA
Address of First Program Save Area

Task control Block (Part 1 of 2)

(
240

OS System Control Blocks

TASK CONTROL BLOCK

116 (74)

TCBTCB
Address of Next Lower Priority TCB

120 (78)

TCBTME
Address of Timer Element

124 (7C)

PCP: Reserved
MFT: TCBPIB, Partition Type Address of PIB
MVT: TCBJSTCB-Address of 1st TCB for Job Step

128 (80)

PCP, MFT: Reserved(a)
MVT: TCBNTC - Address of Previous TCB on Subtask Queue

132 (84)

PCP, MFT: Reserved(a)
MVT: TCBOTC - Address of Originating TCB

136 (88)

PCP, MFT: Reserved(a)
MVT: TCBLTC - Address of Last TCB on Subtask Queue

140 (8C)

PCP, MFT: Reserved(a)
MVT: TCBIQE - Address of IQE for ETXR Routine

144 (90)

PCP, MFT: Reserved(a)
MVT: TCBECB - Address of ECB Posted on Task Completion

148 (94)

PCP, MFT, MVT: Reserved (a)

152 (98)

PCP, MFT: Reserved (a)
MVT: TCBPQE - Address of Region Dummy PQE - 8

156 (9C)

PCP, MFT: Reserved(a)
MVT: TCBAQE - Address of Allocated Queue Element

160 (AO)

TCBNSTAE
STAE Flags Address of Current STAE Control Block

164 (A4)

PCP, MVT: Reserved
MFT: TCBTCT - CPU Time

168 (A8)

TCBUSER
User Field

171 (AB)

(a) - Bytes 128-159 are overlayed by other blocks in PCP and MFT.

• Figure 32 ..

Task Control Block (Part 2 of 2)

)
TCB

241

Offset

-32 (-20)

Bytes and
Alignment

Field
Name

TASK CONTROL BLOCK
Hex.
Dig. Field Description, Contents, Meaning

32

TCBFRS

Floating point register save area .•

o

(0)

4

TCBRBP

Address of the RB for executing program.

4

(4)

4

TCBPIE

Address of the Program Interrupt Element (PIE).

8

(8)

4

TCBDEB

Address of the queue of DEBs.

12

(C)

4

TCBTIO

Address of the Task I/O Table (TIOT).

16

(10)

4

TCBCMP

Task completion code.

20

(14)

TCBTRN

A byte used for flags as described:

20

(14)

1

(Reserved Bits)
Suppress taking checkpoints for this step •
Job step TCB-MFT,

••• x xxxx
. 1 •.

•• 1.

MVT:

This is a graphics foreground job or the graphic job
processor.
Both TESTRAN and decimal simulator programs being
used on a Mod 91 machine.

1 .• '. . ...

21

(15)

• 3

24

(18)

1

TESTRAN:
TCBNROC

MVT Job Step TCB:
00
nz

(19)

• 3

TCBMSS

Address of control core table.
Roll-out Eligibility.

This job step may be rolled out.
This job step may not be rolled out.
(nz - A non-zero digit.)
PCP, MFT:
Address of the boundary box.
MVT:
Address of last subpool queue element (SPQE)

28

(lC)

1

TCBPKF
xxxx

0000
29

(lD)

• 5

TCBFLGS

Storage
storage
Storage
Must be

protection key for this task. If there is no
protection, all bits are zero.
protection key Must be zeros.
zeros.

Flag byte fields.
The use of this field in PCPandMFT differs from its
use in MVT.
PCP, MFT Use:

29

(lD)

Byte 1
Abnormal termination in progress.
Normal termination in progress •
ABEND was initiated by the resident abnormal termination routine.
Recursion through ABEND is permitted •

1 •••
• 1 ••

•• 1.
.•• • 1
1 •••

• 1 .•

•• 1.
••• 1

242

OS System Control Blocks

MFT:
Graphics Abnormal Termination routine has been
entered for this task~
CLOSE initiated by ABEND •
Problem program storage has been overlaid to process
ABEND.
Prohibit queuing of asynchronous exits for this task •

(

Offset

Bytes and
Alignment

TASK CONTROL BLOCK
Hex.
Dig. Field Description, Contents, Meaning

Field
Name

TCBFLGS
(Continued)
PCP, MFT Use (Continued):
30

(lE)

Byte 2
1,. '. '.

.xx. • '. x.
•.• 1
1 ••.
,.1 ••

•• .1
31

(IF)

Byte 3
x'•• x ••• x
,.1 ••
'. ,.1.

1,•• '.
• 1 •.
•• 1.

32

(20)

Bytes 4-5

system task:
ABEND prohibited for this task.
(Reserved bits)
Task has issued a system-must-complete and set all
other tasks in the system non-dispatchable.
Task has issued a step-must-complete and turned off
all other tasks in the step.
Dump processing has been initiated in ABEND.
MFT:
This task is a member of a time-sliced group,.
(Reserved bits)
MFT: Res erved.
Exit Effector:
system error routines already operating for this
task.
Floating pofnt registers exist.
Job scheduler routines in process •
XCTL routine is changing the storage protection key
in the PSW from zero to the one used by the problem
program.
Reserved
MVT Use:

29

(10)

Byte 1
1 ..•
• 1 ••
•• 1,.
•• ,.1
1 •••

• 1 ••

•• 1.
o ••

1

Abnormal termination in progress.
Normal termination in progress •
Enter Erase routine in ABEND when ABEND in
control again.
Enter Purge routine in ABEND when ABEND in
control again.
Graphics Abnormal Termination routine is in control
of this task.
(Bit 7 of byte 3 must also be on.)
Top task in tree being abnormally terminated •
Abnormal termination dump has been completed •
Asynchronous exits cannot be scheduled.

TCB

243

Offset

Bytes and
Alignment

Field
Name

TASK CONTROL BLOCK
Hex.
Dig. Field Description. Contents. Meaning

TCBFLGS
(Continued)
MVT Use (Continued):
30

(lE)

Byte 2
1. '••
,.1 ••
'. ,.1.
'. ,. ,.1
1 •••
• 1 ••
•• 1.
• •• 1

31

(IF)

Byte 3
1. '. '.
.1 ••

• ,.1.
• • ,.1 • •• x
1 •• x
,.1 ••

• .1x

'••• x x.xl
32

(20)

Byte 4
1 •••
.1. '.
• ,.1.

••• x xx ••
• ,.1.

•

33

(21)

<4

••

• '•• 1

Byte 5
1 •••
• 1 ••
'. ,.1,.

• • ,.1
1 •••
.1 ••
• ,.1.

• •• x

244

Operands of ABEND macro instruction have been saved
in TCBCMP field.
Initiator TCB:
Second job step interval has expired.
Job Step TCB:
Job step can cause rollout,.
System must complete. Current task can be performed;
other tasks in system cannot.
Step must complete; other tasks in job step cannot be
performed.
Job step TCB: SYSABEND already open •
ETXR exit requested by attaching task •
Task is a member of a time-sliced group.

OS System Control Blocks

All PSWs for this task in supervisor state.
Job step TCB:
Job step has invoked rollouts that are still in
effect.
Prevent multiple ABEND.
OPEN issued for SYSABEND.
(See also bit 7.)
ABDUMP in process for this task.
(See also bit 7.)
Job step TCB:
No abnormal termination dumps can be provided within
this job step.
CLOSE has been issued during ABEND processing.
(See
also bit 7.)
Valid reentry to ABEND indicated if bits 3. 4. or 6
of this byte or bit 4 of byte 29 is also on.
(If any bit in this byte is 1, the task is
non-dispatchable.)
Set by ABDUMP.
Machine check occurs. All tasks except current task
placed in wait state.
Supply of I/O request queue elements exhausted.
(Reserved bits)
M65 Multiprocessing:
Task has been set non-dispatchable by one CPU to prevent any CPU from working on it.
ABEND routine was entered by this task while DCB for
SYSABEND was being opened for another task.
(If any bit in this byte is 1. the task is
non-dispatchable.)
Terminated.
To be terminated by ABEND •
A routine of this task has issued an unconditional
GETMAIN which must be satisfied by rollout of another
job step.
The job step has been rolled out.
Another task is in system-must-complete status.
Another task in this job step is in step-mustcomplete status.
Initiator task: request for a region could not be
satisfied.
Reserved

(

Offset
34

(22)

Bytes and
Alignment
•• 1

Field
Name
TCBLMP

TASK CONTROL BLOCK
Hex.
Dig. Field Description, Contents, Meaning
PCP, MFT:
Number of resources for which this task is enqueued .•
MVT:

Limit priority.
30

(23)

••• 1

TCBDSP

Dispatching priority for this task.

36

(24)

4

TCBLLS

PCP, MFT:
Address of the most recently added RB on the list of
programs loaded via the LOAD macro instruction.
MVT:
Address of load list element (LLE) for program loaded
via the LOAD macro instruction,.

40

(28)

4

44

(2C)

4

44

(2C)

4

44

(2C)

TCBJLB

Address of a JOBLIB DCB.
PCP, MFT:
Reserved

TCBJPQ

MVT:
Job step TCB:

Byte 1
1,•••

.xxx xxxx
45

(2D)

Bytes 2-4

44

(2C)

4

48

(30)

64

112

(70)

1

Purge flag.
(Reserved bits)
Address of last entry in contents directory for
job pack area (JPA) control queue.
Other TCBs:
Zero.

TCBGRS

General register save area.

TCBIDF

(One of these:)
PCP, MFT:
TCB identifier field.

TCBQEL

MVT:

Number of resources for which this task is enqueued.
113

(71)

• 3

TCBFSA

Address of the first problem program save area.

116

(74)

4

TCBTCB

Address of next TCB of lower priority on the ready
queue.

120

(78)

4

TCBTME

Address of the timer element.

124

(7C)

4

PCP:

Reserved

TCB

245

TASK CONTROL BLOCK
Offset

124

Bytes and
Alignment

(7C>

Field
Name

TCBPIB

Hex.
Dig. Field Description, Contents, Meaning

A field used for two items of information.
Partition type.

124

(7C)

1

00 ••
01 ••
10 ••
11. '.
•• 1.
• '. 0,.
• ,•• x xxxx

125

(70)

•

System task partition.
Reader partition.
Writer partition.
Processing program partition.
Large partition •
Small partition •
(Reserved bits>
Address of the partition information block (PIB).

3

MVT:
124

(7C)

4

TCBJSTCB

Address of the first TCB for a job step. For tasks
with a protection key of zero, this field contains
the address of this TCB.
PCP, MFT:
Reserved (See Note A>

128

(80)

4

TCBNTC

MVT: Address of the previous TCB on the originating
task's subtask queue. The TCB for the last subtask
has zeros in this field.
PCP, MFT:
Reserved (See Note A)

132

(84)

4

TCBOTC

MVT:

Address of the TCB for the originating task.

PCP, MFT:
Reserved (See Note A)
136

(88)

4

TCBLTC

MVT: Address of the ·last TCB on the subtask queue
for this task. The TCB for the last subtask has zero
in this field.
PCP, MFT:
Reserved (See Note A)

140

(8C)

4

TCBIQE

MVT: Address of an interruption queue element (IQE)
for scheduling the ETXR routine on the originating
task.
PCP, MFT:
Reserved (see Note A)

144

(90)

4

TCBECB

MVT: Address of the ECB that will be posted by the
supervisor' $ task termination routines when normal or
abnormal termination occurs.
PCP, MFT:
Reserved (See Note A)

148

(94)

4

TCBSTAE

Reserved (See Note A)
PCP, MFT:
Reserved (See Note A)

(
246

OS System Control Blocks

Offset
152

(98)

Bytes and
Alignment
4

Field
Name
TCBPQE

TASK CONTROL BLOCK
Hex.
Dig. Field Description, Contents, Meaning
MFT: Address of the region dummy partition queue
element minus 8 (DPQE-8).
PCP, MFT:
Reserved (See Note A)

156

(9C)

4

TCBAQE

MVT:

Address of an allocated queue element (AQE).

PCP, MFT:
Reserved (See Note A)
160

(AO)

1

TCBNSTAE

• 3

164

168

(A4)

(A8)

4

4

Flags internal to STAE routine.
Address of the current STAE control block.
PCP, MVT:

TCBTCT

Reserved.
MFT:
CPU time used by the step, in 26 micro-second time
units.

TCBUSER

A field available to the user.
Note A: Bytes 128-159 are overlayed by other system
control blocks in PCP and MFT to conserve main
storage space.

TCB

247

(

Task Input/Output Table

The task input/output table (TIOT) is constructed by job management routines.
It
resides in the higher portion of the dynamic area of main storage during step execution. The TIOT provides the I/O support
routines (OPEN,CLOSE,EOV) with pointers to
JFCBs and allocated devices. Figure 33
shows the format of a TIOT. Descriptions
of the fields follow the illustration.

TIOT

249

TASK INPUT/OUTPUT TABLE

1

TIOCNJOB
Job Name

10(0)

1.(81

TIOCSTP
Job Step Name, Procedure Step Name

1'6(101

For a Procedure Step: Job Step Name

T

23 (17)

I
IT

i::.:.£>'['--':::~!~l ..........................................................................----------------------------------------------. -------------------------------------------------" ___,
+0

+1

TlOELNGH
Length of DD Entry

+2
+2

TlOESTTA
Status Byte A

_~4

TIOERLOC
TIOEWTCT
1+3
TlEOLINK
No. of Devices Requested
Allocation: Link - Close: Flag

-

TlOEDDNM
DD Name

+12

TIOEJFCB
Relative Address of JFCB, or of SlOT

roo,

+15

TlOESTTC
Status Byte C

Device Entry
TlOESTTB
Status Byte B

+0

I

+1

TIOEFSRT
During Allocation: 2 Offsets
During Problem Program: Address

+3

rI--

I

I
-------------------------------------------- .. _--------_ .. _-_ ....

-----------------------------,
... - ........... -- -- .. -- .. -- -- ........ -- ......

-~

Zero-End-of-TI OT Indicator

Figure 33.

Task Input/Output Table

(
250

OS System Control Blocks

TASK INPUT/OUTPUT TABLE
Offset

Bytes and
Alignment

Field
Name

Field Description.. Contents. Meaning

TIOCNJOB

Job name.

o

(0)

8

(8)

8

(8)

8

For a job step that is not a procedure step:
Job step name.
For a job step that is a procedure step:
Procedure step name.

16

(10)

8

For a job step that is not a procedure step:
(Field not used.)
For a job step that is a procedure step:
Job step name of the job step that called the
procedure.

8

TIOCSTEP

DD Entries:
There is a DD entry for each DD statement in the Job
step or procedure step,.
(References to GDG (all)
data sets. the JOBLIB data set or PGM=*.ddname create
still other DD entries,.)
DD Entry:
A DD entry includes a device entry. Before allocation. there may be several device entries in each DD
entry.
+0

1

TIOELNGH

Length, in bytes. of this DD entry (including all
device entries).

+1

. 1

TIOESTTA

Status Byte A.

x... .x ..
O. •• .0. ,.
0,. •• ..1 ••
1 .... ,.. 0 ••
,.1. '.

• • 1. • •••

••• 1
1 ....
'•• 1.
.... 1

+2

•

• 1

+3

..

• . 1

Tape label processing to be performed:
NL, BLP.
SL, SUL.
NSL.
During allocation: Split cylinder primary.
(This is the first DD entry for a split cylinder.)
During step termination:
No unallocation necessary.
During allocation: Split cylinder secondary •
(This is not the first DD entry for a split
cylinder. )
During step termination:
Rewind but no unloading.
JOBLIB indicator •
DADSM allocation necessary.
Keep. catalog, uncatalog.
Data set to be deleted •

TIOERLOC

(A 2 byte field consisting of:)

TIOEWTCT

During allocation:
Number of devices requested for this data set.

TIOELINK

x ... ..
1 ... .

During allocation:
Link to the appropriate prime split. unit affinity.
volume affinity or suballocate TIOT entry.
After CLOSE:
This is a SYSOUT data set that contains data.

.. xxx xxxx

(Reserved bits)

x .... .
1 ... '.

TIOT

251

TASK INPUT/OUTPUT TABLE
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
DD Entry (Continued)

+4

8

TIOEDDNM

DD name.

+12

3

TIOEJFCB

Relative track address (TTR) of the JFCB.
(During
allocation, TTR of the SlOT if suballocate was
requested. )

+15

..

TIOESTTC

Status Byte C. Used during allocation only.
zeros at end of allocation..

• .. 1

1,•••
,.1. '.

'. ·.1.

'••• 1
1 ...
• 1,. ,.

•• 1.
••• 1

Set to

Secondary suballocate .•
Deferred mount.
Primary unit affinity.
Secondary unit affinity.
Primary volume affinity.
Secondary volume affinity,•
Primary suballocate •
Data set to be passed •
DEVICE ENTRIES

During allocation:

1 device entry for each device required, or for each
public device eligible.
During problem program: 1 device entry for each allocated device.
+0

1

TIOESTTB
1 •• '.
,.1 ••
• ,.1.
'•• ,.1
1 •.•
.x,••
• 1 ...
• 0 ••

•• 1.
• .. .. 1

+1

• 3
TIOEFSRT
During allocation

Status byte B During allocation and during problem program:
Data set is on device.
Data set will use device.
Device violates separation •
Volume serial present.
Setup message required.
Disposition:
Retain unloaded volume if unload required •
Delete unloaded volume if unload required •
Unload required •
Verification required •
Bits 0 - 11:
Offset, in the UCB look-up table, to an address for a
device required or eligible for this data set.. For
other than a 2321, the UCB look-up table has
addresses of UCBs .. For a 2321 data cell drive, its
addresses are those of the descriptions in the UCB of
cells in bins ..

Bits 12 - 23:
Offset, in the step volume table (VOLT), to the
volume serial number for the volume required or eligible for this data set.
During problem program: Devices other than 2321: Address of the UCB.
2321 data cell drive:
Address of the description in the UCB of the cell in
the bin.
(The description of the cell in hin 0
begins at UCB + 56.)
END-OF-TIOT INDICATOR
4

252

OS System Control Blocks

Binary Zeros.

(

Unit Control Block

There is a unit control block (UCB) for
each device attached to the system. It
describes the characteristics of the device
to the I/O supervisor and is used by the
job scheduler during allocation of the
device. Figure 34 shows the format of the
UCB. Descriptions of the fields follow the
illustrations. The field descriptions of
the UCB are followed by a diagram and
detailed descriptions of the UCBTYP field
of the UCB.
The unit control block consists of two segments: a segment common to all devices
(common segment) and a segment that varies
with different devices (device-variable
segment). The following illustrates the
relationship of these segments.

0(0)
Common Segment

24 (18)

Device-Variable Segment:
-

UCS
Graphic Devices
Magnetic Tape
Direct-Access Storage
Data Cell Drive

UCB

253

UNIT CONTROL BLOCK

M65MP Prefix

I~

UCBFL3
M65MP Flags

1-

3
Reserved
-1

Common Segment
0(0)

SRTEJBNR
Internal Job No.

1 (1)

SRTECHAN
Allocation Channel Mask

2 (2)

UCBID
Identifier

3 (3)

SRTESTAT
Status Byte A

4 (4)

UCBCHA
Channel Address

5 (5)

UCBUA
Unit Address

6 (6)

UCBFL1
Flag Byte 1

7 (7)

UCBDTI
Index to Device Table

8 (8)

UCBETI
Error Routine Key

9 (9)

UCBSTI
Statistics Table Index

11 (B)

UCBATI
Attention Table Index

12 (C)

10 (A)

UCBLCI
Channel Table Index

13 (D)

UCBWGT
Flags and Mask

UCBNAME
Unit Name

16 (10)

UCBTYP
Device Type

20 (14)

UCBLTS
Last Request Element

~_~yice

-

___

Var_~~~le ~_~_9~_~~! ~_~_~

22 (16)

__________________________________________________________________________________________________________________________________________ _

24 (18)

28 (lC)

UCBSNS
Sense Information

UCBUCSID
UCS Image Name
UCBUCSOP
UCS Image Format

29 (lD)
Reserved
31 (IF)

Graphic Device
24 (18)

26 (lA)

27 (lB)

Additional Sense Information

Use Count

(GCB)
Control Byte

28 (lC)
Task Entry Address
32 (20)
Restart Address
36 (24)

37 (25)
Device Index

Buffer Tabl e Address

39 (27)

l ............ ________ ....... __ ....... _____ .... _ ........... __ .. ___ ... __ .. __ .............. _ .. __ ......... __ ... __ ............... ______ ........ __ .... ________________ ,. _________ ........... ________ .. _____ ....... ____ .. ___________________ .. ______________________________ .• ___________ ,)

-Figure 34.

Unit Control Block (Part 1 of 3)

(
254

OS System Control Blocks

UNIT CONTROL BLOCK

Magnetic Tape
24 (18)
Additional Sense Information
28 (lC)

SRTEVOLI
Volume Serial No.
34 (22)

36 (24)

SRTESTAB
Status Byte B

38 (26)

SRTEFSCT
Sequence Count

35 (23)

SRTEDMCT
Vol M Sw I DCB Count

SRTEFSEQ
Sequence No.

40 (28)
Data Set Serial Number
46 (2E)
Reserved

_,,,48 (30)

T

Irec t

UCBROR
Read-Opposite Recovery CCW

-~

55 (37)T

.
Access St orage DeVlce (E xcep t Da tell
a e Drive

24 (18)
Additional Sense Information
28 (lC)

SRTEVOLI
Volume Serial No.
34(22)

36 (24)

40 (28)

35 (23)

SRTEDMCT
No. of DCBs Open

SRTEFSCT
Relative Address of VTOC
UCBSQC
RESERVE Count

41 (29)
UCBDVRES
Device Reservation Indicator

--

-.A8 (30)

56 (38)

SRTESTAB
Status Byte B

42 (2A)
Reserved

-...

UCBSKA
Direct-Access Address of Lost Seek

SRTEUSER
No. of Users

57 (39)

SRTEECBA
Direct Access ECB Address

~60 (3C)

-...

Error Recovery Work Area

I

lOO(MI

)

1

Overflow Work Area (Present When Overflow Specified)
139

(8811

:
Continued :
,--------------------------------------------------------------------------------------------------------_ .... --------------------------.,-------------- .. ---------------------------------_ ....... __ .... _--------- ...

eFigure 34.

Unit Control Block (Part 2 of 3)
UCB

255

UNIT CONTROL BLOCK

!

r----2j2;---Oo-ta---ce-ff--orlv-e----------------------------------------------------------------------------------------------------------------------------------------------------------1

':.;, 124
_

Additional Sense Data

(18)

:T
lL~

_

J
1

28 (lC)

:

Error Routine Work Area A

1
~8

I

Re~'Ved

(30)

!

UCBSKA
Seek Address Last Used

T

Description of Cell in Bin 0
56 (38)

DCELBBNR
Bin Number

59 (3B)

58 (3A) DCELSTAB
Status Byte B

DCELSTAT
Cell/Bin Status

I-I--

60 (3C)

I--

DCELVOLI
Volume Serial Number

-

66 (42) DCELJ BNR
Internal Job Numbers

r-

67 (43)
DCELDMCT
No. of DCBs Open

~

68 (44)

DCELVTOC
Relative Address of VTOC

-

71 (47) DCELUSER
Allocated Data Sets

I--

I

Cell in Bin 1

I

87 (57)

Cell in Bin 2

I

103 (67)

Cell in Bin 3

I

119 (77)

Cell in Bin 4

I

135 (87)
151 (97)

Cell in Bin 5

I

167 (A7)

Cell in Bin 6

I

Cell in Bin 7

I

Cell in Bin 8

I

Cell in Bin 9

,1

215 (D7)

-__

Error Routine Work Area B

124B (FB)
Error Routine Work Area for Track Overflow

,

199 (C7)

1

",,-216 (D8)

T

.... - - . . _ _ . . . . . . . . . . . . . . - ' ' ' ' ' _

183 (B7)

. . _ _ _ _ . . . . . . _ _ _ ... _

Figure 34.

.......... __ .... _

.... ____ .. __ .. _

. . 00 . . . . . . . . . . . . . . . . . . _

. . . . _ _ _ _ ".. _ _ . . _ _ _ _ . . . . . . . _ _ . . _ _ _ ...... _ _ _ _ . . _ _ . . . . . . . . . . . . . . . _ . . _ _ _ ........... _

......... _

.. _

....... -

. . - - ..... _ _ _ _ . . . . _ _ _ _ _ _ ... _ _ _ _ _ _ . . . . . . _ _ . . . . _

I
,

..... _ _ _ . . _ _ _ _ _ . . . . . . . . . . . _ _ _ _ . . . . . _ _ _ . . . . _ _ .. _ _ _ _ _ . . . . . . . . . . . . . . . . . . . . . . . _ - _ . . . .

unit Control Block (Part 3 of 3)

(
256

OS System Control Blocks

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
PREFIX SEGMENT
Present only if M65MP was specified in the system
generation (SYSGEN) process.

-4

(-4)

UCBFL3
Byte 1
xx ••• x ••
· '.• 1
• .1,.

0 .....
1 •• '.
• ,.1-.
'••• 1

-3

(-3)

Bytes 2-4

Model 65 Multiprocessing flags,.
(Reserved bits)
CPU A is to use an HIO instruction
CPU B is to use as HIO instruction
CPU A last used an SIO instruction
CPU B last used an SIO instruction
CPU B has no path to this device •
CPU A has no path to this device.

for
for
for
for

this
this
this
this

device.
device,.
device.
device.

Reserved
SEGMENT COMMON TO ALL DEVICES

o

(0)

1

SRTEJBNR

Internal job identification.

1

(1)

• 1

SRTECHAN

Allocation channel mask.

2

(2)

'.. 1

UCBID

UCB identification - Hex FF.

3

(3)

SRTESTAT

Status byte A.
Device status flag:
Device is offline.
Device is online.
Device is to be changed from online to offline
status.
Reserved, that is, the mount status of the volume on
this device is reserved.
(See note A.)
UNLOAD operator command has been addressed to this
device; the device is. not yet unloaded.
Device is allocated.
Permanently resident, that is, the mount status of
the volume on this device, is permanently resident.
(See note A.)
One of these:
This is the systems residence device.
This is the primary console.
One of these:
Standard labels have been verified for this tape
volume.
This is the alternate console.

• 1

x. '. '.

0,•••
1,•• '.

.1 ••
• ,.1,.
• '•• 1
1 •••

.1 •.
.,.'.'• • ,.1.

'.'.-.'. . . . 1

Note A: If the mount status is neither reserved nor
permanently resident, then i t is removable.
4

(4)

UCBCHA

1

1 •••
• 1 .•.

.-.xx x •• '.

,.xxx

5

(5)

•

1

UCBUA

Channel address.
Halt I/O.
Status modifier,•
(Reserved bits)
Channel address - binary number.
Unit address.

»
UCB

257

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
SEGMENT COMMON TO ALL DEVICES (Continued)

6

(6)

•• 1

UCBFLl
1,......
. 1 •.
'.I.X.

'.... 0,.

'. ,.. 1,.
,•.• ,.1

1 .. '••

.. xx ..
.01.
.11.

.01 ..
'••. 1

7

(7)

..oo

8

(8)

9

Flag byte 1.
Busy - Device status.
Not-ready - Device status ..
Post flag.
No channel program is being executed using this
device,.
A channel program using this device has not yet been
posted as having completed.
After a channel end status a separate device end status occurred with an error indication.
(IOBIntercept flag.)
Busy - Control unit status.
Direct access storage devices:
Stand-alone channel program of I/O supervisor is
being or was executed.
(Arm seeking.)
User's channel program is being executed.
(Data
transfer. )
Telecommunications devices:
Inhibit HIO instruction because the line is in
receive status.
I/O error routine is in control of this device.
No
other I/O operations are permitted on this device.

UCBDTI

Index to the Device Table.

1

UCBETI

A binary number used by the exit effector routine to
complete the 8 byte name of an IBM-supplied error
routine for this device.

(9)

• 1

UCBSTI

Increment which, when multiplied by 10, becomes an
index to the statistics Table (STATAB).

10

(A)

'OO.

UCBLCI

Increment which, when multiplied by 8, becomes an
index to the Logical Channel Table (LCHTAB).

11

(B)

•

UCBATI

Index to the Attention Table (ANTAB).

12

(C)

1

UCBWGT

Flags and channel mask.

'OO

1

1

•

1

1 .... ..
,.1 ....
'. ,.. 1,.
'. '•.• 1
xxxx

1,. '....
• 1 .••

'•.• 1.
'. '... 1

SYSIN.
SYSOUT.
Assumed that this device will be allocated for a
public volume request.
Rewind command has been addressed to this magnetic
device by I/O support.
I/O Supervisor path mask.
(Used where there are two
or more paths to a device):
Primary path to the device is inoperative.
Optional path 1 to the device is inoperative •
Optional path 2 to the device is inoperative.
Optional path 3 to the device is inoperative.

(
258

OS System Control Blocks

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
SEGMENT COMMON TO ALL DEVICES (Continued)

13

(D)

16

(10)

• 3

UCBNAME

Unit name (EBCDIC).

4

UCBTYP

Device type.
THIS FIELD IS DESCRIBED SEPARATELY AND IN DETAIL AT
THE END OF THE DESCRIPTION OF THE OTHER UCB FIELDS.

20

(14)

2

UCBLTS

Last Request Element.

22

(16)

•• 2

UCBSNS

Sense information for all devices.
DEVICE-VARIABLE SEGMENT
UCS Seqment
This segment is present if the system generation
process specifies the universal character set (UCS)
feature. Else, no device-variable segment is present
for unit record devices.

24

(18)

4

UCBUCSID

Name of the UCS image in the buffer.

28

(lC)

1

UCBUCSOP

Format of the UCS image in the buffer.

1 .• '.
. 1 ••

•• xx xxxx
29

(10)

•

3

UCS image is the default image.
UCS image is in the FOLD mode •
(Reserved bits)
Reserved
GRAPHIC DEVICES

24

(18)

2

Additional Sense information.
Use Count

26

(lA)

•• 1

Number of DCBs that are currently open for this
device.
GCB

27

(lB)

••• 1

Graphic control byte used for attention handling.
Task Entry Address

28

(lC)

4

Address of task entry (TE) block.
Restart Address

32

(20)

4

Last start address.
Device Index

36

(24)

1

Device or devices on a control unit to which buffer
sections are assigned.
Buffer Table Address

37

(25)

• 3

Address of buffer table.
UCB

259

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
MAGNETIC TAPE DEVICES

24

(18)

4

Additional Sense information.

28

(lC)

6

SRTEVOLI

Volume serial number.

34

(22)

•• 1

SRTESTAB

Status byte B - Volume status.

x •• '.
O•• '.
1 •••

.x ••• xxx
• ,.1.

. '... 1
1 ..•

35

(23)

••. 1

SRTEDMCT

1 •••

0 •••••••

0 •••
O•• '.
• xxx xxxx

Volume sharability:
Sharable.
Not sharable.
(Reserved bits)
Additional volume label processing,•
Private - Volume use status •
Public - Volume use status.
Volume mount switch.
This switch shows whether a volume has been mounted
and whether the volume label found on the volume has
been verified to be the type specified by the DD
statement parameter..
(DD Stat. Parm.)
DD
Stat.
Parma
---- Any Scheduler:
No volume has been mounted.
A volume has been mounted but no volume label
processing has been performed.
(Normal scheduler processing, effective with
release 11.)
SL Open routine:
Volume label is not standard format or serial
number is not correct.
(A mount message has
been issued.)
Standard volume label and correct serial number
has been verified.
NSL Open routine:
Volume label is not standard format.
(Control passes to the processing program's nonstandard label processing routine.)
Volume label is standard format.
(Control
remains with the Open routine. A mount message
has been issued.)
Processing program:
Non-standard volume label has been verified.
NL Open routine:
Standard volume label has been found.
(A mount message has been issued.)
No standard volume label has been found.
BLP Open routine:
Volume label has not been processed.
Number of DCBs open for this volume •

36

(24)

2

SRTEFSCT

Data set sequence count.

38

(26)

•• 2

SRTEFSEQ

Data set sequence number.

40

(28)

6

Data set serial number.

46

(2E)

•• 2

Reserved for future use.

48

(30)

8

260

UCBROR

OS System Control Blocks

CCW for opposite-direction recovery.

(

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description, Contents, Meaning
DIRECT ACCESS STORAGE DEVICE (EXCEPT DATA CELL DRIVE)

24

(18)

4

28

(lC)

6

SRTEVOLI

Volume serial number.

34

(22)

•• 1

SRTESTAB

status byte B - Volume status.

Additional Sense information.

x •••

0 •••
1 •.•
• xx .•
. • .• 1
1 •••
• 1 ••

•• 1.
... 1

Volume sharability:
Sharable.
Not sharable.
(Reserved bits)
Private - Volume use status •
Public - Volume use status.
Storage - Volume use status •
Joblib data set is on this volume •
Control volume - A catalog data set is on this
volume.

35

(23)

••. 1

SRTEDMCT

Number of DCBs open for this volume.

36

(24)

4

SRTEFSCT

Relative address of VTOC for this volume, in form
TTRO.

40

(28)

1

UCBSQC

Number of RESERVE macro instructions issued.

41

(29)

• 1

UCBDVRES

Device reservation indicator.
In a system that includes the shared DASD option,
this indicator is set equal to the contents of the
UCBSQC field after a successful completion of an SIO
instruction for a direct access storage device
(DASD).

42

(2A)

•• 6

48

(30)

8

UCBSKA

Disk address (MBBCCHHR) for last seek.

56

(38)

1

SRTEUSER

Number of current users.

57

(39)

• 3

SRTEECBA

Direct access ECB address.

60

(3C)

40

Error recovery storage and work area.

100

(64)

40

Track overflow work area.
If track overflow is installed these additional bytes
are always a part of a UCB.

Reserved.

2321 DATA CELL DRIVE
24

(18)

4

Additional Sense information.
Error Routine Work Area

28

(19)

12

A

40

(28)

8

Reserved

48

(30)

8

UCBSKA

work area for the error routine.

Address for last Seek, in the form MBBCCHHR.
(2321 DATA CELL DRIVE SEGMENT CONTIIWES)

)
UCB

261

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field Description l

Contents, Meaning

2321 DATA CELL DRIVE (Continued)
Description of cell in bin 0
56

(38)

2

DCELBBNR

Bin number ..

58

(3A)

.... 1

DCELSTAB

Status byte B - volume status.

X'• • •

0,.. '. '.
1 .. '••

.xx ..
'.•• 1

1. '...
,.1. ,.

.. 1.
I• • •

59

(3B)

..... 1

1

DCELSTAT
1,... ,.

o.... '.

.. x. '... -.xx

'. ,.1,.

••• 1
1 .••
.• 1 ....

Volume sharability:
Sharable.
Not sharable ..
(Reserved bits)
Private - Volume use status ..
Public - Volume user status ..
Storage - Volume use status.
Joblib data set is on this volume ..
Control volume - A catalog data set is on this
volume.
Cell/Bin status.
Bin is online and a normal cell is mounted in it,..
Bin is offline or a ballast cell is mounted in it.
(Reserved bits)
Reserved.. - Mount status of the cell in this bin,.
(See note A.)
UNLOAD operator command has been addressed to this
bin; the bin has not yet been unloaded.
Bin is allocated.
Permanently resident,. - The mount status of this
cell.
(See note A.)
Note A:
If the mount status is neither reserved nor permanently resident, then i t is removable.

60

(3C)

66

(42)

67

(43)

68

(44)

71

(47)

DC EVOL I

Volume serial number.

1

DCELJBNR

Internal job number.

• 1

DCELDMCT

Number of data sets opened for this cell.

3

DCELVTOC

Address of VTOC, in form TTR ..

'• • • 1

DCELUSER

Number of data sets allocated to this cell.

• 6

End of Description of cell in bin

o.

(
262

OS System Control Blocks

UNIT CONTROL BLOCK
Offset

Bytes and
Alignment

Field
Name

Field DescriptioD 1 Contents, Meaning
2321 DATA CELL DRIVE (Continued)

72

(48)

16

Description of Cell in Bin 1
(Same format as description of cell in bin 0).

88

(58)

16

Description of Cell in Bin 2
(Same format as description of cell in bin 0).

104

(68)

16

Description of Cell in Bin 3
(Same format as description of cell in bin 0).

120

(78)

16

Description of Cell in Bin 4
(Same format as description of cell in bin 0).

136

(88)

16

Description of Cell in Bin 5
(Same format as description of cell in bin 0).

152

(98)

16

Description of Cell in Bin 6
(Same format as description of cell in bin 0).

168

(A8)

16

Description of cell in Bin 7
(Same format as description of cell in bin 0).

184

(B8)

16

Description of Cell in Bin 8
(Same format as description of cell in bin 0).

200

(C8)

16

Description of Cell in Bin 9
(Same format as description of cell in bin 0).
Work Areas
Error Routine Work Area

216

(08)

32

A work area used by the error routine.
Error Routine Work Area for Track Overflow

288 (120)

40

A work area for the error routine, present and used
only if track overflow has been specified.

UCB

263

(

The UCBTYP Field in the UCB
The UCBTYP field completely describes the device type.
It is the exact analog of the
full device name, except that it includes terminal adapters and similar units when they
are part of the necessary description. Figure 35 shows the type of entries in the field.
Following the illustration, the field is described separately by type of entry and by
type of device.

UCBTYP

I

lOS Flags

Model Code

I

UCB + 16 (10)

Figure 35.
Offset

Byte 3

Byte 2

Byte 1

Optional Features

I

Device Class

18 (12)

17 (11)

Byte 4

I

Unit Type

19 (13)

The UCBTYP Field
Bytes and
Alignment

Bit and
state

Hex.
Pig. Field Description., Contents, Meaning
ENTRY
Devices Other Than Graphic Devices

16

(10)

1

xxxx

I/O Supervisor flags.

x .•• '.

'.... 1

(Reserved bit)
Overrunable device •
Burst mode.
Byte mode •
Data chaining.

xxxx

Graphic Devices
Device class.

Byte 1
• 1,••
'•.• 1 .•

• '. O.

13-

0001
0011

Model code,.
See following description of UCBTYP field by device
class.

xxxx

17

18

(11)

(12)

1053, 2260
2250

• 1

Byte 2

Optional features.
See following description of UCBTYP field by device
class.

•• 1

Device class.

Byte 3
08
10
20
40
80
19

(13)

••• 1

Byte 4

Unit Record.
Graphics.
Direct Access Storage,.
Communication Equipment.
Magnetic Tape.
Unit type.
See following description of UCBTYP field by device
class.

UCB

265

DESCRIPTION OF THE UCBTYP FIELD BY DEVICE CLASS:
UNIT RECORD DEVICE CLASS
UCBTYP
Byte 2

Byte 1
lOS Flags

I

Model Code

UCB + 16 (10)

Offset
16

(10)

I

1
Byte 1

Bit and
State
xxxx

I/O Supervisor flags,.

x .•••
.• 1,•.•
.• • 1,.
• ,.0.

(Reserved bit)
Overrunable device,•
Burst mode •
Byte mode •
Data chaining.
Model Code.
With 1442, 2520
Read Punch.
Punch only.

0000
0001

-1

.

Universal character set (UCS).
(Reserved bits)
Card image (binary mode) '.

I. ,••

(12)

..

Device Class.

2
Byte 3
08

19

(13)

'. '. '.

Unit Type

19 (13)

Optional Features.

1
Byte 2
1 •• '.
• xxx xxx •
1

18

I

Hex.
Dig. Field DescriEtion, Contents, Meaning

xxxx

(11)

Device Class

18 (12)

'•.• ,.1

17

I

Optional Features

17 (11)

Bytes and
Alignment

Byte 4

Byte 3

3

Unit record.
Unit Type.

Byte 4
01
02
03
04
05
08
OA
10
20
21

2540
2540
1442
2501
2520
1403
1404
1443
2671
1052
2150

Card Reader.
Card Punch.
Card Read Punch.
Card Reader .•
Card Read Punch,.
Printer (models Nl., 2, 3, 7) and
Printer (continuous form support only).
Printer (model Nl only).
Paper Tape Reader.
Printer-Keyboard.
Console.

(
266

OS System Control Blocks

MAGNETIC TAPE DEVICE CLASS
UCBTYP
Byte 2

Byte 1
lOS Flags
UCB

I

Model Code

+ 16 (10)

offset
(10)
16
Byte 1

I

Optiona I Features

17 (11)

Bytes and
Alignment

Bit and
State

18
19

(11)

xxxx
x.xx
.1 ••

I/O Supervisor flags.
(Reserved bit)
Overrunable device •
Burst mode •
Byte mode •
Data chaining •
Model Code.
(Reserved bits)
Phase-encode code (Models 4.,

••• x xxxx

Optional Features.
7-track compatibility ( 2400)
Data conversion (2400)
Dual-density (2400)
(Reserved bits)

o••.•

(12)
(13)

.

1
Byte 2

'. .

Device Class

Byte 4

I

1. '. ,.
.1 ..
.•.• 1,.

5, 6, 7)

Byte 3

80

Device Class.
Magnetic Tape.

. '. . 1

01

Unit Type •
2400 Series Magnetic Tape Device.

1

Byte 4

Unit Type

19 (13)

Hex.
Dig. Field DescriEtion, Contents, Meaning

• 1 ••
.• • 1 .•
.•.• 0 .•
•• ,.1

11

I
18 (12)

xxxx

1

Byte 3

)
UCB

267

DIRECT ACCESS STORAGE DEVICE CLASS
UCBTYP
Byte 2

Byte 1
lOS Flags

I

Model Code

UCB + 16 (10)

Offset
16

(10)

I

Optiona I Features

17 (11)

Bytes and
Aligrunent
Byte 1
1

Bit and
state

19

(12)

Optional Features.
Scan feature.
Track overflow.
This device can be shared between two or more CPUs,•
(Reserved bits)

• 1
1 •••
,.1.

I.

'.. 1
Byte 3

20

Device class.
Direct access storage device.

01
02
03
04
05
08

Unit
2311
2301
2303
2302
2321
2314

(13).. 1

Byte 4

Unit Type

19 (13)

I/O Supervisor flags.
(Reserved bit)
Overrunable device •
Burst mode.
Byte mode •
Data chaining •
Model Code.

xxxx

.. 1.
•·•.• x xxxx

18

I

Dig. Field Description, Contents, Meaning

0000
Byte 2

Device Class

Byte 4

Hex.

. 1. '.
'. ,.1 .•
• • 0,.
• '. ,.1
(11)

I
18 (12)

x •••

17

Byte 3

type.
Disk Storage Drive.
Parallel Drum.
Serial Drum,.
Disk Storage.
Data Cell Drive.
Disk Storage Facility_

c

(
268

OS System Control Blocks

GRAPHIC DEVICE CLASS - MODEL 2250 DISPLAY UNIT
UCBTYP
Byte 1
lOS Flags

I

Byte 2
Model Code

UCB + 16 (10)

Offset
16

(10)

I

Optional Features

17 (11)

Bytes and
Alignment
1
Byte 1

Device Class

Byte 4

I

Unit !ype

19 (13)

Hex .•
Dig. Field DescriEtion* Contents, Meaning
J13-K
-1

(11)

I
18 (12)

-2
-3
11

Byte 3

• 1
Byte 2
01234561-

89-

AB-

CDEF-

Device Class
1056,2260
2250
Model Code
Model 1.
Model 2.
Model 3.
Optional Features
Model 0Etional Features
1,2,3 No optional features.
1.2,3 programmed Function Keyboard only.
1,2
Light Pen only.
1,2
programmed Function Keyboard, and Light Pen.
1.2,3 Alphameric Keyboard only.
1,2,3 Programmed Function Keyboard and Alphameric
Keyboard.
1, 2
Alphameric Keyboard and Light Pen .•
1,2
Alphameric Keyboard, Light Pen and programmed
Function Keyboard.
1,2
Absolute Vector Graphics only.
1,2
Absolute vector Graphics and Programmed Function Keyboard.
1,2
Absolute Vector Graphics and Light Pen.
1,2
Absolute vector Graphics, Programmed Function
Keyboard and Light Pen.
1,2
Absolute Vector Graphics and Alphameric
Keyboard.
1, 2
Absolute vector Graphics., Programmed Function
Keyboard and Alphameric Keyboard.
1,2
Absolute vector Graphic, Alphameric Keyboard,
and Light Pen.
1,2
Absolute vector Graphics, Alphameric Keyboard,
Light Pen and Programmed FUnction Keyboard.

UCB

269

GRAPHIC DEVICE CLASS - MODEL 2250 DISPLAY UNIT

Offset

Bytes and
Alignment

Hex.
Dig. Field Description, Contents, Meaning
Model

17

18

(11)

(12)

(cont)
Byte 2

(13)

-1--

-1

-2
-3
-4
-5
-6

1
1
1
1
1
1

-7

1

-8
-9

1
1

-A

1

-B

1

•• 1

Byte 3
19

-0

10

••• 1

Byte 4

02

Optional Features
No optional features.
4K Buffer only .•
8K Buffer only.
Character Generator only,.
4K Buffer and Character Generator.
8K Buffer and Character Generator.
Graphic Design Feature only.
Graphic Design Feature and 4K Buffer .•
Graphic Design Feature and 8K Buffer.
Graphic Design Feature and Character
Generator.
Graphic Design Feature. 4K Buffer, and Character Generator.
Graphic Design Feature, 8K Buffer, and Character Generator.

Device Class
Graphics
Unit Type
2250 Graphic Display Unit.

c
270

OS System Control Blocks

GRAPHICS DEVICE CLASS
2260 Display Station
UCBTYP

lOS Flags

I

Model Code

UCB + 16 (10)

Offset
16

17

(10)

(11)

I
17 (11)

Bytes and
Alignment
1
Byte 1

.

J13-

Device Class.
1053, 2260
2250

-K
-1
-2

Model Code
Modell.
Model 2.
Optional Features

23-

456789ABCDEF-

-B
-C
-D
-E

18

(12)

'.. 1
Byte 3

Byte 4

I

Unit Type

19 (13)

Hex.
Dig. Field Description, Contents, Meaning

1
Byte 2

(Cont)

Device Class

18 (12)

1-

(11)

I

Optional Features

0-

17

Byte 3

Byte 2

Byte 1

10

Optional Features
No optional features.
Line Addressing only.
Numeric Keyboard only.
Line Addressing and Numeric Keyboard.
Alphameric Keyboard only.
Line Addressing and Alphameric Keyboard.
Non-destructive cursor only.
Line Addressing and Non-destructive cursor.
Numeric Keyboard, and Non-destructive cursor,.
Optional Features
Line Addressing, Numeric Keyboard and Nondestructive cursor.
Alphameric Keyboard and Non-destructive
Cursor.
Line Addressing, Alphameric Keyboard and Nondestructive cursor.
Data Entry Keyboard only,.
Data Entry Keyboard and Line Addressing.
Data Entry Keyboard and Non-destructive
Cursor.
Data Entry Keyboard, Line Addressing, and Nondestructive Cursor.
2848 Display Control, Model 1 with 240 character display capability.
2848 Display Control, Model 2 with 480 character display capability.
2848 Display Control, Model 3 with 960 character display capability.
2848 Display Control, Model 21 with 240 character display capability.
2848 Display Control, Model 22 with 480 character display capability.
Device Class
Graphics
Unit Type

19

(13)

••• 1
Byte 4

03

2260 Graphic Display Unit.

UCB

271

GRAPHICS DEVICE CLASS
Other Than 2250 or 2260
UCBTYP
Byte 1
lOS Flags

I

Byte 2
Model Code

UCB + 16 (10)

Offset

Bytes and
Alignment

(10)

1
Byte 1

17

(11)

• 1
Byte 2

19

(12)
(13)

Optional Features

17 (11)

16

18

I

Byte 3

•• 1
Byte 3
••• 1
Byte 4

I

Device Class

18 (12)

Byte 4

I

Unit Type

19 (13)

Hex.
Dig. Field Description, Contents, Meaning

J13-K
-0
-0
-4

Device Class.
1053, 2260
2250
Model Code
2280 Film Recorder.
2282 Film Recorder Scanner.
1053 Printer, Model 4.
Optional Features

00
00
00

Device
1053
2280
2282

10

Device Class
Graphics

04
05
06

Unit
1053
2280
2282

No optional features.
No optional features,.
No optional features.

Type
Printer.
Film Recorder.
Film Recorder/Scanner.

(
272

OS system Control Blocks

COMMUNICATION EQUIPMENT DEVICE CLASS

UCBTYP
Byte 2

Byte 1
lOS Flags

I

Model Code

UCB + 16 (10)

Offset
16

(10)

I

Optional Features

17 (11)

Bytes and
Alignment
1
Byte 1

Bit and
State

(11)

I

Device Class

18 (12)

Byte 4

I

Unit Type

19 (13)

Hex.
Dig. Field Description, Contents, Meaning

xxxx

I/O Supervisor flags

x...
.1..
•• 1.
•• 0.
••• 1

(Reserved bit)
Overrunable device •
Burst mode •
Byte mode •
Data chaining.
Model Code
The value in this field and the value in the adapter
type field (byte 4, bits 0-3) together identify the
model,.
Adapter
~
Model
11050
21030
31050
483B3
5TWX
82260
9S/360
11060
4115A
91130
92780
12740
1WTTA

xxxx

17

Byte 3

•••• 0001

-1

.....

0010

-2

0011
0100
0110

-3
-4
-6

.

optional features.

1
Byte 2
1,•••

• 1 ••
•• 1.
•• • 1
1 .•.

• 1 ••

.... '.

•• xx

Automatic calling.
Automatic polling •
Checking •
Automatic answering •
station control.
Transmit control •
Binary
Value
- 0 - SADZER
SAD ONE
1
SAD TWO
2
SAD THREE
3

UCB

273

Offset
18
19

(12)
(13)

Bytes and
A1ignment

COMMUNICATION EQUIPMENT DEVICE CLASS
Bit and Hex.
State
Dig. Field Description, Contents, Meaning

..
'. . .4 1
Byte
1
Byte 3

40
123456789-1
-2
-3

Device Class.
communication equipment.
Adapter Type.
IBM Terminal Adapter, Type I.
IBM Terminal Adapter, Type II.
IBM Telegraph Adapter.
Telegraph Adapter, Type I.
Telegraph Adapter, Type II.
World Trade Telegraph Adapter,.
Synchronous Adapter, Type I.
IBM Terminal Adapter, Type III.
Synchronous Adapter, Type II.
Control unit
2702
2101
2703

c
274

OS System Control Blocks

V olume Label

A volume label is 80 characters long and
identifies the volume and its owner.
Figure 36 shows the volume label format.
Description of the fields follow the
ill ustration,.
Magnetic tape volumes, the volume label
is the first record on the tape. On ninetrack tape it is written in EBCDIC, on
seven-track tape in BCD.
On direct access volumes, i t is record
number three, following the two IPL records
if they are present" and is recorded in
EBCDIC.

Volume Label

275

VOLUME LABEL

a (0)

3 (3)

VOllABI
label ID

VOlNO
label No.

4 (4)
VOlSERNO
Volume No.

10 (A)

11 (B)
Reserv€d

-"

Ta p e: Reserved
DASD: VOlVTOC - Address of the VTOC DSCB

-c-

21 (15)

........

..,,...

Reserve d

"'r-

41 (29)

VOlOWNER
Owner Name and Code

51 (33)

........

........
Reserved

T

79 (4F)

Figure 36.
Offset

Volume Label
Bytes and
Alignment

0

(0)

3

3

(3)

··

4

(4)

6

10

(A)

··

11

(B)

11

(B)

16

(10)

21

(15)

20

41

(29)

· 10

51

(33)

· · · 29

276

J

• 1

Field
Name

Field Description, Contents, Meaning

VOLLABI

Label identifier - VOL.

VOLNO

Volume label sequence number.

VOLS ER NO

Volume serial number.

1

Reserved - must be recorded as EBCDIC zero.
VOLVTOC

Magnetic Tape:
Reserved - must be recorded as blanks.

···5

Direct Access Storage:
The CCHHR address of the VTOC DSCB on this volume.

5

Reserved - must be recorded as blanks.
Reserved - must be recorded as blanks.
VOLOWNER

OS system Control Blocks

Owner name and address code for the owning
installation.
Reserved - must be recorded as blanks.

(

Volume Table of Contents

The volume table of contents (VTOC) is a
data set consisting of data set control
blocks (DSCB) '. The format of the VTOC" and
its relation to user labels, is shown in
Figure 37. A description follows the illustration.
(The DSCBs are describes separately, under that heading, in this
publication .• )

VTOC

277

I
I

Standard Volume Label

VTOC Data Set
Description of
Device and Volume
'--~

Description of Up to
26 Shared Cyl inders

--

Description of Up to
26 Available Extents

Format 4 DSCB

--

-

Format 6 DSCB
I--

-Format 5 DSCB

I

5

I

5

- r--

I

--

rr-

6

I

6

Avai lable VTOC Space
I-I--

-Format 0 DSCB

I

0

I

0

Physical Sequential, Partitioned, Direct Organization

- ...-

Description
of Data Set

I

Descriptions of
4th-16th Extents

--

---

Format 1 DSCB

Format 3* DSCB

,

_I

1

I

3*

"'-I

1

-- ...-

3*

Index Sequential Organization Data Sets

-

Description
of Data Set

Description of
Indexing Elements
I--

--

I

User Label*

,
f-----I

!1l

User Label Extent*

-

3*

I

Legend:

,

Volume Table of Contents

OS system Control Blocks

3*
"I

~

NOTES:
S - User labels may only be used with physical
sequential or direct organization data sets.
* - Not a Iways present.

218

Format 3* DSCB

,

_I

2

.J

• Figure 31.

~

2
1

...-

---

Format 2 DSCB

1

S

r-

Descriptions of
4th-16th Extents

I--

--

Format 1 DSCB

I

...-

EJ}

Data Set Control Block
44 Byte Key Area
96 Byte Data Area

Label -

I} 80 Byte Data Area

" - -_ _oJ

c

VOLUME TABLE OF CONTENTS

The volume table of contents (VTOC) is a data set consisting of control blocks that
describe the contents of a direct access storage device volume.
(The data set has a
single extent; its address is found in the standard volume label. See figure 36.) On
secondary storage, the control blocks that make up this data set consist of a 44 byte key
segment and a 96 byte data segment; in main storage, each forms a 140 byte block.
Each
140 byte block makes up a data set control block (DSCB).
To accomodate various categories of information about the volume and the data sets on it,
the 140 byte blocks are formatted in different ways. DSCB formats 1, 2, 3, and 4 are
designed for data set information; DSCB formats 5 and 6 describe the available or shared
space. Space in the VTOC not occupied by one of these DSCBs is filled with format 0
DSCBs, which contain binary zeros.
(For a detailed description of each format, refer to
the DSCB section of this publication.)
At the beginning of the VTOC is a single format 4 DSCB.
It is followed by at least one
format 5 DSCB. If there are any more format 5 DSCBs, they are chained from the first
format 5 OSCB.
If there are any format 6 DSCBs, they are chained from the format 4 DSCB. For every data
set on the volume there is a format 1 OSCB, and also a format 2 OSCB if the data set has
index sequential organization. Format 1 DSCBs are found by using a Search (Equal) command with an argument of the DSNAME operand; they are not chained to one another nor to
the format 4 OSCB. If the data set has more than three extents, a format 3 DSCB is
chained from the format 1 DSCB, or the format 2 OSCB in the case of index sequential
organization,. Any space remaining in the VTOC extent carries format 0 OSCBs.
User labels, if used, occupy the first extent described by a format 1 OSCB. This extent,
a separate one for each data set~ is one track long; the labels form 80 byte data
segments.

VTOC

279

(

Index:

Access method blocks and segments
data control block ••••••••• 35,48,59,74
data event control
block •••••••••••••• 101,103,107,111,113
data extent block ••••••••••••••••••• 91
input/output block ••••••••••••• 171,174
Appendage table block segment
data extent block ••••••••.•••.•••••• 88

Common block segments
data control block •••••• 31,44,56,72,79
data extent block •.••••••••.•• 89,96,97
Communication vector table ••••••••••••• 11
CVT
(see communication vector table)
CVT user field ••••••.•••••••••••••••••• 19

Basic block segments
(see common block segments)
BBCCHH (see MBBCCHHR)
BDAM
(see access method block segments)
Binary Synchronous communication
DC B- BT AM ••••••• _ • • ... • • • • • . • • • • • • • • •• 74
DECB-BTAM ••••••••••
117
BISAM
(see access method block segments)
Block segments (see the following:)
access method block segments
appendage table block segment
basic block segment
buffer request block segment
common block segments
device-dependent block segments
DASD message queue block segments
foundation extension segment
foundation prefix segment
foundation segment
line control block segment
line error block segment
line group block segment
MFT block segments, field uses
MVT block segments, field uses
polling list origin block segment
processing message queue block segment
queue control block segment
receive scheduler sub-TCB block segment
scheduling-dependent block segment
subroutine identification block segment
BPAM
(see access method block segments)
BSAM
(see access method block segments)
BSC
(see binary synchronous communication)
BTAM
(see access method block segments)
Buffer request block segment
data extent block-QTAM •••••••••••••• 97

DADSM Interlock - UCB
Deleted
Data cell drive segment, field, flag
data control block . . . . . . . . . . . . . . . 28,44
unit control block ••••••.•.•.•••••• 265
UCBTYP field ••••••••••••••••••••••• 265
DASD (direct access storage device) message
queue block segment (extension, interface)
data control block •••••••••••••••••• 65
data extent block (destination queue) 91
Data control block ••••••••••••••••••••• 21
BDAM •••••••••••••••••••••••••••••••• 53
BISAM •••••••• _ ••••••• '••.•••••••••• Q. 39
BPAM ................................. 23
BS]I,M .................................. 23
BTAM ••••••••••• '. • • • • • • • • • • • • • • • • • • •• 69
EXCP access method ••••••.••••••••••• 23
GAM •••• '. • • • • • • • • • • • • • • • • • • • • • • • • • • •• 77
QI SAM ................ ,........... '••• '. •• 39
QSAM. ...................................
23
QTAM ............................ '. • • ... 61
Data event control block ................ 99
BDAM ................................ 107
BSAM ......................... '. • • • .. • •• 101
BTAM ••••••••••••••••••••••••••••••• 113
I SAM ............ _ . • • • • • • • • • • • • • . • • • • •• 103
QT AM ..................
111
Data extent block •••••••••••••••••••••• 81
appendage table ••••••••••••••••••••• 88
ordinary •..•...•.....•.............. 83
QTAM ................................. 93
Data set control blocks
formats 0, 1, 2, 3, 4, 5, 6 •••••••• 121
Da ta set labe 1
direct access storage
(see data set control block)
magnetic tape •••••••••••••••••••••• 145
label 1 (FL1) •••••••••••••••••••••• 147
label 2 (FL2) ....................... 151
DCB
(see data control block)
DD entry
(see task input/output table)
DEB
(see data extent block)
DEC
(see data event control block)
DECB
(see data event control block)
Destination queue block segment
(see DASD message queue block segment)
Device classes
unit control block (UCBTYP field) ._ 265

d

•••••••••••••••

CCHH
(see MBBCCHHR)
CCHH of defective track
input/output block
173
Chained channel-program scheduling
(see scheduling-dependent block segment)
Checkpoint data set (QTAM)
data control block ••••••••••••• n. 61,65
n

••••••••••••••••

Q

0

•

•

•

•

•

•

•

•

•

•

•

...

••

Index

281

Device-dependent block segments
data control bloc k .......... 28,44,56, 79
data extent block ..................... 90
data set label •••.••••.....•... 121,145
input/output block ••••••••••••• ~ ••• 174
UCBTYP field ............................ 267
unit control block ••••••••••••••••• 259
Device entry
(see task input/output table)
Device name table ••••
155
Device optional path status
unit control block .................... 259
Directory entry
(see partitioned data set directory
entry)
DNT
(see device names table)
DSCB
(s ee data set control block)
DS1, DS2, DS3, DS4. DS5, DS6
(s ee data set control block)
0

••••••••••••••••

Error routine key
unit control block
258
ECB
(see event control block)
Event control block ................ ., ••• 159
EXCP access method
(see access method block segments)

FLl
(see data set label)
FL2
(see data set label)
Foundation extension, prefix
data control block ....... 31,45,57,72,79
Foundation segment, before OPEN, after OPEN
data control block ••• 32,46.57,65,73,79
(see also common block segments)

GAM
(see access method block segments)
Graphic job processing
communication vector table •••••••••• 18
task control block ••••••••••••• 244,245

JFC
(see job file
JFCB
(see job file
Job file control
Job file control

control block)
control block)
block •••••••••••••••• 177
block extension ....... 187

Label processing
task input/output table •••••••••••• 251
LCB
(see line control block)
LERB
(see line error block segment)
Line control block •••••••••••••••••••• 189
Line control block segment
data extent block ••••••••••••••••••• 98
Line DECB
(see data event control block -- QTAM)
Line error block segment
line control block .............. 189,194
Line group block segment (extension,
interface)
data control block-QTAM ••••••• 61,64,67
LPRB
(see request blocks (PCP, MFT»
LRB
(see request blocks (PCP, MFT»
MBBCCHHR ................................. 7
Message queue block segment
(see processing program message queue;
DASD message queue)
MFT ....................................... 7
MFT segments, field uses
communication vector ta.ble •••••••••• 16
task control block ••••••••••••••••• 242
Multiprogramming with a fixed number of
tasks (MFT)
(see MFT)
Multiprogramming with a variable number of
tasks (MVT)
(see MVT)
MVT ........................................ 7
MVT segments, field uses
communication vector table •••••••••• 16
ta sk control block ••••••••••••••.• '. • •• 242
Normal channel-program scheduling
(see scheduling dependent block segment)

HH
(see MBBCCHHR)

Partition information
task control block ••••••••••••.•••• 246
Partitioned data set directory entry •• 195

ICB

PCP •••••.•• ,•••••••••••••••••••••••.••••••

(see interruption control block)
Input/output block .......................
Interruption control block .............
lOB
(see input/output block)
IQE flag
IRB .................................
SIRB •••••••••••••••••••••••••••••••
IRB
(see request blocks)

PDS

282

as System Control Blocks

165
161

(see partitioned data set directory
entry)
Polling list origin block segment
(see line group block segment entension)

220
227

PRB

(see request blocks)
Primary control program
(see PCP)

7

(

Processing program message queue block
segment (extension, interface)
data control block ••••••••••••••• 65,67
data extent block ••••••••••••••••••• 93
PURGE (SVC16) - Quiesce count
data extent block •• _ •••••••••••••••• 89

QISAM
(see access method block segment)
QSAM
(see access method block segments)
QTAM
(see access method block segments)
Queue control block segment
data extent block ••••••••••••••••••• 96

RB
(s ee request block (MVT»
Receive scheduler sub-TCB block segment
line control block •••••••••••••••••
Refreshable module
PDS directory entry ••••••••••••••••
request block ••••••••••••••••••••••
Request blocks ••••••••••••••••••••••••
MVT (IRB, PRB, SIRB, SVRB) •••••••••
PCP, MFT (IRB, LPRB, LRB, PRB, SIRB,
SVRB) •••••••••••••••••••••••••••••
Rollout flags
task control block •••••••••••••••••

194
203
213
207
215
209
242

Shared DASD
communication vector table •••••••••• 17
task control block ••••••••••••••••• 244
unit control block ••••••••••••• 261,269
Scheduling dependent block and segment
input/output block •••
172
interruption control block ••••••••• 161
Seek queue - UCB Deleted
SIRB
(see request blocks)
SRT
(see unit control block)
Subroutine identification block segment
data extent block ••••••••••••••••••• 92
SVRB
(see request blocks)
System resource table -- SRT
(see unit control block)
SYS1. LOGREC
DCB •••••••••••••••• _ •••••••••••••••• 28

Tape volume serial number
communication vector table •••••••••• 17
TCB

(see task control block)
TIOT
(see task input/output table)
Task control block •••••••••••••••••••• 239
Task input/output table ••••••••••••••• 249
Terminal models
UCBTYP field ••••••••••••• '•••••••••• 266
Time-slicing
communication vector table •••••••••• 19
task control block ••••••••••••••••• 242
UCB
(see unit control block)
UCBTYP field, detail description ••••••
UCS segment
job file control block •••••••••••••
unit control block •••••••••••••••••
Unit control block ••••••••••••••••••••
User field
communication vector table •••••••••

265
184
259
253
d

17

VOL
(see volume labe 1)
Volume label •••••••••••••••••••••••••• 275
Volume label processing
unit control block ••••••••••••••••• 262
Volume mount switch
unit control block ••••••••••••••••• 262
Volume serial number
communication vector table •••••••••• 17
Volume table of contents •••••••••••••• 211
relative address - UCB ••••••••••••• 262
VTOC
(see volume table of contents)

ft • • • • • • • • • • • • •

Wi

(see input/output block)
XRB
(see request block (PCP, MFT»
2260
UCBTYP field
281+8
UCBTYP field
2780
UCBTYP field

211
211
273

Index

283

(

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