System Control Blocks

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VTOC Data Set User Label Extent* or Data Set Extent Standard Volume Label VOLVTOC NOTES: S - User labels may only be used with physical sequential or direct organization data sets. * - Not a I ways present. Figure 31. Volume Table of Contents 384 OS System Control Blocks (Release 21.7) Legend: EJ Data Set Control Block } 44 Byte Key Area )96 Byte Data A,ea Label - } 80 Byte Data Area
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 39.) On secondary stqrage, 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 fOL1US a 140 byte block. Each 140 byte block makes up a data set control block (DSCB). Seven DseE formats exist to accommodate various kinds of information about a volume and the data' sets that reside on it. DSCB formats 1, 2, and 3 provide data set information; OSCB format 4 describes the VTOC data set itself, its size and the characteristics of the device it resides on; DSCB formats 5 and 6 describe the available or shared space on the volume. The 140-byte records that are not occupied by one of these DSCBs are called format 0 OSCBs and contain binary zeros. (For additional information about each DSCB format, refer to the OSCB 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 OSCB. If there are any more format 5 OSCBs, they are chained from the first format 5 DSCB. If there are any format 6 DSCBs, they are chained from the format 4 OSCB. 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 OSCBs are found by using a Search (Equal) command with an argument of the OSNAME operand; they are not chained to one another nor to the format 4 DSCB. If the data set has more than three extents, a format 3 OSCB 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 385
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Systems Reference Library IBM Syste
Page 6 and 7:
Figures Figure 1A. Pointer Diagram
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System Control Blocks System contro
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. ..J MFT Figure lA. Pointer Diagra
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Contents Directory Entry (MVT Only)
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20 OS System Control Blocks (Releas
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Offset -8 (-8) 2 Bytes and Field Al
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Offset 124 (7C) 4 128 (80) 4 132 (8
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0(0) 4 (4) 8 (8) 12 (c) 16 (10) 20
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DATA CONTROL BLOCK -- SAM Continued
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Offset 53 (35) • 3 56 (38) 1 57 (
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Device Interface 16 (10) Common Int
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Offset 16 (10) 1 11 (11) • 1 18 (
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Offset 40 (28) 8 48 (30) 1 49 (31)
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Offset Bytes and Field Alignment Na
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Device Interface 16 (10) Common Int
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WTTA Interface 16 (10) DCBBQFLG Dat
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Offset 20 (14) 1 21 (15) • 3 24 (
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Offset Bytes and Field Alignment N
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DATA CONTROL BLOCK -- TCAM The form
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Offset 33 (21) • 3 36 (24) 1 37 (
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Offset 52 (34) 1 52 (34) 4 56 (38)
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112 OS System Control Blocks (Relea
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Appendage Table DATA EXTENT BLOCK -
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offset -36 (-24) 4 -32 (-20) 4 -28
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Offset +0 +0 +0 +1 +0 +4 +8 +0 +4 +
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DATA EXTENT BLOCK -- QTAM MESSAGE P
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Offset 48 (30) 4 52 (34) 1 53 (35)
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- 36 (- 24) -32 (-20) -28 (+ lC) -2
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DATA EVENT CONTROL BLOCK -- BSAM Th
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Offset 12 (e) 4 16 (10) 4 20 (14) 4
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DATA EVENT CONTROL BLOCK -- BlSAM T
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o (0) DATA EVENT CONTROL BLOCK -- B
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DATA EVENT CONTROL BLOCK -- QTAM Th
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DATA EVENT CONTROL BLOCK -- BTAM Th
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Offset 28 (lC) 29 (lD) • 1 Bytes
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Device Name Table The device name t
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Page 166 and 167:
166 05 System Control Blocks (Relea
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10 (0) ....,r- 44 (2C) DS1FMTID For
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Page 177 and 178:
72 (48) FORMAT 2 -- INDEX DATA SET
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Offset 64 (40) 2 Bytes and Alignmen
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FORMAT 3 -- EXTENSION DATA SET CONT
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Offset o (OJ 44 44 (2C) 1 45 (2D)
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...... ---r 0(0) 4 (4) 44 (2C) DS5F
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FORMAT 6 -- SHARED EXTENT DATA SET
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Data Set Labels -- Magnetic Tape Th
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o (0) :.::: 4 (4) .,r- Continued Co
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198 OS system Control Blocks (Relea
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0(0) 4 (4) 16 (10) 36 (24) FL2RECFM
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Event Control Block The event contr
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INTERRUPTION CONTROL BLOCR 0(0) ICB
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Input/Output Block The input/output
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Offset 68 (44) 1 70 (46) •• 2 7
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Offset 101 (65) • 1 101 (65) •
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JOB STEP CONTROL BLOCK 240 (FO) JSC
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240 os System Control Blocks (Relea
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Offset 40 (28) 2 40 (28) 41 (29) 42
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Offset 96 (60) 1 91 (61) • 3 100
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Offset I o (0) 8 8 (8) 3 *tes and ,
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Offset Bytes and Alignment I 24 (18
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Offset Bytes and Field A1ignment Na
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REQUEST BLOCKS -- MVT CONFIGURATION
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Interruption Request Block -- MVT T
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proqram Request Block -- MVT The pr
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Offset o (0) B 8 (8) 1 9 (9) • 1
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Supervisor Request Blocks -- MVT Th
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Supervisor Request Block -- MVT --
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SUPERVISOR REQUEST BLOCK -- MVT --
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302 OS system Control Blocks (Relea
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Offset o 1 2 4 8 (0) 1 Bytes and Fi
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Bytes and Offset Alignment 105 (69)
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Page 314 and 315:
1-32 (20) r- L,. 0(0) 4 (4) 8 (8) 1
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Bytes and Field Offset Alignment Na
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Page 325 and 326:
1116 (74) 120 (78) 124 (7C) 128 (80
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Offset 31 (IF) 32 (20) Byte and Fie
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Offset 168 (AB) 4 172 (AC) 1 173 (A
Page 334 and 335: TIMING CONTROL TABLE 0(0) TCTQA 3 (
Page 337 and 338: Offset o (0) 3 Bytes and Field Alig
Page 339 and 340: Offset o 4 6 8 10 +0 (0) 4 (4) 2 By
Page 344: Offset +3 +4 +12 +15 +0 Bytes and F
Page 352: UCS 1403 Printer Extension 0(0) 4 (
Page 357: Offset Bytes and Field Alignment Na
Page 360: Offset 45 (2D) • 3 48 (30) 8 56 (
Page 364: Offset Bytes and Alignment 10 (A)
Page 369 and 370: MAGNETIC TAPE DEVICE CLASS UCBTYP B
Page 371: Byte 1 lOS Flogs I Model Code I UCB
Page 374: UCB + 16 (10) Offset 16 (10) 1 Byte
Page 378 and 379: 378 OS System Control Blocks (Relea
Page 381 and 382: ASCII Volume Header Label I 0 (0) 1
Page 383: Volume Table of Contents The volume
Page 387 and 388: Time-Sharing Option Control Blocks
Page 389: Driver Parameter Area The driver pa
Page 392: ENVIRONMENT CONTROL TABLE 0(0) ECTR
Page 395 and 396: Protected Step Control Block The pr
Page 397 and 398: Offset o (0) 7 Byte and Field Align
Page 399: Time-Sharing Job Block The time sha
Page 415 and 416: Indexes to system reference library
Page 417: liO coo.nter table 339 I/O lookup t
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