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Offset<br />
Bytes and<br />
Alignment<br />
(44) 2 DS4DEVOV<br />
68 (44) 1 DS4DEVI<br />
69 (45) • 1 DS4DEVL<br />
70 (46) •• 1 DS4DEVK<br />
71 (47) ••• 1 DS4DEVFG<br />
xxxx<br />
. . .. 1 ...<br />
FORMAT 4 -- VTOC DATA SET CONTROL BLOCK<br />
Field<br />
Field Description, Contents, Meaning<br />
.1 ..<br />
.. 1.<br />
••• 1<br />
72 (48) 2 DS4DEVTL<br />
74 (4A) •• 1 DS4DEVDT<br />
75 (4B) ••• 1 DS4DEVDB<br />
76 (4C) 24<br />
100 (64) 5 DS4F6Pl'R<br />
105 (69) • 10 DS4VTOCE<br />
115 (73) ••• 25<br />
Overhead bytes for any keyed block on the 2305. If<br />
bit 4 of the device indicators field (bit 71.4 of<br />
DSCB4) is set to one, this field (DS4DEVOV) is used<br />
as a single two byte field containing a binary count<br />
of the number of bytes (overhead bytes) occupied by<br />
the count field, gaps, and check bytes of a keyed<br />
record.<br />
If bit 71.4 is zero, the field (DS4DEVOV) consists of<br />
the following subfields:<br />
Contains a count of the number of bytes (overhead<br />
bytes) occupied by the count field, gaps, and check<br />
bits of a keyed record that is not the last record on<br />
a track.<br />
Contains a count of the number of bytes (overhead<br />
bytes) occupied by the count field, gaps, and check<br />
bits of a keyed record that is the last record on a<br />
track.<br />
The number of overhead bytes to be subtracted from<br />
DS4DEVI, DS4DEVL, or DS4DEVOV if the block has no key<br />
field.<br />
Device indicators.<br />
(Reserved bits).<br />
The keyed record overhead field (DS4DEVI) is used as<br />
a two-byte field to specify the overhead required by<br />
a keyed record as in the case of the 2305.<br />
The CCHH of an absolute address is used as a<br />
continuous binary value as in the case of the 2301.<br />
The CCHH of an absolute address is used as four (4)<br />
separate binary values as in the case of the 2321.<br />
A tolerance factor must be applied to all but the<br />
last record on the track.<br />
Note: If bits 5 and 6 are zero, the CC and HH of an<br />
absolute address (CCHHR) are used as halfword binary<br />
values as in the case of the 2311.<br />
Device tolerance.<br />
Value which when divided by 512 is used to determine<br />
effective length of a block on a track.<br />
Number of full DSCBs that can be contained on one<br />
track (44 byte key plus 96 byte data length).<br />
Number of full PDS directory blocks that can be<br />
contained on one track (8 byte key plus 256 byte data<br />
length) •<br />
Reserved.<br />
Pointer to the first format 6 DSCB.<br />
This pointer has the form CCHHR. It contains binary<br />
zeros when not in use.<br />
VTOC extent.<br />
Contents and meaning are the same as'DS1EXTl in the<br />
format 1 DSCB.<br />
Reserved.<br />
186 OS <strong>System</strong> control <strong>Blocks</strong> (Release 21.7)
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