System Control Blocks

Offset
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+1 . 3
Bytes and Field
Alignment Name
(m-1)x1 DEBEXTNM
8 DEBDSNAM
1 B/T
. 3 B/E
4 T/P
4 B/P
4 B/E
4
4
Load Mode Section
4 n times
DEBUCBAD
+0 (0) 4 DEBPUT
DATA EXTENT BLOCK -- ORDINARY
Field Description, contents, Meaning
Only one of the following fields is present:
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.
For a partitioned data set opened for output for a
member name, this field is the member name.
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.
Number of blocks per track.
Number of blocks per extent.
Only Eresent for fixed-length records with the option
of relative block addressing and track overflow
These fields occur only once within a DEB:
Number of tracks per period.
Number of blocks per period.
The following field occurs once for each extent:
Number of blocks per extent.
BTAM
This segment is always present for BTAM. It is used
when a buffer pool or dynamic buffering is used; else
the fields are zero.
Address of the buffer routine.
Address of the first CCW on the queue.
Address of following CCWs on the queue.
GAM
Reserved.
Address of UCB •
124 OS System Control Blocks (Release 21.7)
ISAM ACCESS METHOD DEPENDENT SECTION
Address of the PUT processing module.