Table of Contents
Advertisement
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
Advertisement
Table of Contents
