IBM System/360 System Programmer's Guide

An index link entry is the last entry in all index blocks. The entry
is 12 bytes long and contains three fields.
Field 1: Name Field (8 bytes) -- contains only the hexadecimal number
FF to ensure that this entry, because it has the highest binary name
value, will appear as the last entry in any index block.
Field 2: Link Address (3 bytes) -- contains the relative track address
of the next block of the same index, if there is a next block in the
index. Otherwise, the field contains binary zeros.
Field 3: Half-word count (1 byte)
that no additional fields follow.
Pointer Entries
contains a binary zero to indicate
An index pOinter entry can appear in all indexes except generation
indexes. The entry is 12 bytes long and contains three fields.
Field 1: Name Field (8 bytes) -- contains the name of the index being
pointed to by field 2.
Field 2: Index Address (3 bytes) -- contains the relative track address
of the first block of the index named in field 1. The address is in the
form 'ITR.
Field 3: Half-word count (1 byte) -- contains a binary zero to indicate
that no additional fields follow.
A data set pointer entry can appear in any index. It contains the
simple name of a data set ana from one to five 12-byte fields that each
identify a volume on which the named data set resides. If the data set
resides on more than five volumes, a volume control block n:ust be used
to point to the volumes. The volume control block is identified by a
volume control block pointer entry, not a data set pointer entry.
set pointer entry varies in length. The length is The data
determined
containing
d.ata set
fields.
by the formula (14+12w), where IT is the number of volumes
the data set. The variable m can be from 1 through 5. The
pointer entry can appear in any index, and it contains five
Field 1: Name Field (8 bytes) -- contains the simple name of the data
set whose volurres are identified in field 5.
Field 2: Address Field (3 b}tes) -- contains a binary zero.
Field 3: Half-word count (1 byte) -- contains the binary count of the
numter of half words that follow. The nun:ber is found by the formula
(6m+1), where m is the number of volumes on which the data set resides.
The variable m can be from 1 through 5.
Field 4: Volume count (2 bytes) -- contains the binary count of the
nurrber of volumes identifiea in field 5 of this entry.
Field 5: Volume Entries (12 to 60 bytes) -- contains from one to five
12-byte entries, each of which identifies a volume on which the data set
resides. Each entry contains a 4-byte device code, a 6-byte volume
serial nurrber, and a 2-~yte data set sequence number. The data set
sequence nUITber is zero for direct-access volumes.
A volume control block pointer entry can appear in any index. It can
identify up to 20 volurres. The entry is 14 bytes long and contains four
fields.
26

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English