IBM System/360 System Programmer's Guide

APPENDIX A: CATALOG BLOCK ENTRIES
This section describes the contents of all catalog entries.
Control Entries
A volume index control entry is always the first entry in a volume
index. The volume index control entry is 22 bytes long and contains
eight fields.
Field 1: Name Field (8 bytes) -- contains only a binary one to ensure
that this entry is the first entry in the first block of the index.
Field 2: Last Block Address (3 bytes) -- contains
address of the last block in the volume index.
form TTR.
the relative track
The address is in the
Field 3: Half-word Count (1 byte) -- contains a binary five to indicate
that five half words follow.
Field 4: Catalog Upper Limit (3 bytes) -- contains the relative track
address of the last block in the catalog data set. The address is in
the form TTR.
Field 5: Zero Field (1 byte> -- contains binary zeros.
Field 6: First Available Block Address (3 bytes) contains the
relative track address of the unused block in the catalog that is
closest to the beginning of the catalog data set.
Field 7:
Zero Field (1 byte) -- contains binary zeros.
Field 8:
count of
index.
Unused Bytes in Last Block (2 bytes) -- contains the binary
the number of unused bytes in the last block of the volume
An index control entry is the first entry in all indexes except
volume indexes. The index control entry is 18 bytes long and contains
six fields.
Field 1: Name Field (8 bytes) -- contains only a binary one to ensure
that this entry, because it has the lowest binary name value, is the
first entry in the first block of the index.
Field 2: Last Block Address (3 bytes) -- contains
address of the last block assigned to the index.
form TTR.
the relative track
The address is in the
Field 3: Half-word count (1 byte)
indicate that three half words follow.
contains a binary three to
Field 4: Index Lower Lin,it (3 bytes) -- contains the relative track
address of the block in which this entry appears. The address is in the
form TTR.
Field 5: Number of Aliases (1 byte) -- contains the binary ccunt of the
number of aliases assigned to the index. If the index is not a hiyh
level index, this field is zero.
Field 6: Unused Bytes in Last Block (2 bytes) -- contains the Linary
count of the number of unused bytes remaining in the last block of the
index.
Maintaining the Catalog and the Volurre Table of Contents 25

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English