ABCs of z/OS System Programming Volume 3 - IBM Redbooks

Table 2-1 DSCBs that can be found in the VTOC
Type Name Function How many
0 Free VTOC
DSCB
Describes unused DSCB records
in the VTOC (contains 140 bytes
of binary zeros). To delete a
DSCB from the VTOC, a format-0
DSCB is written over it.
1 Identifier Describes the first three extents
of a data set or VSAM data space.
2 Index Describes the indexes of an ISAM
data set. This data set
organization is old, and is not
supported anymore.
3 Extension Describes extents after the third
extent of a non-VSAM data set or
a VSAM data space.
4 VTOC Describes the extent and
contents of the VTOC, and
provides volume and device
characteristics. This DSCB
contains a flag indicating whether
the volume is SMS-managed.
5 Free space On a non-indexed VTOC,
describes the space on a volume
that has not been allocated to a
data set (available space). For an
indexed VTOC, a single empty
format-5 DSCB resides in the
VTOC; free space is described in
the index and DS4IVTOC is
normally on.
7 Free space
for certain
device
Only one field in the format-7
DSCB is an intended interface.
This field indicates whether the
DSCB is a format-7 DSCB. You
can reference that field as
DS1FMTID or DS5FMTID. A
character 7 indicates that the
DSCB is a format-7 DSCB, and
your program is not to modify it.
One for every unused 140-byte record
in the VTOC. The DS4DSREC field of
the format-4 DSCB is a count of the
number of format-0 DSCBs in the
VTOC. This field is not maintained for an
indexed VTOC.
One for every data set or data space on
the volume, except the VTOC.
One for each ISAM data set (for a
multivolume ISAM data set, a format-2
DSCB exists only on the first volume).
One for each data set on the volume
that has more than three extents. There
can be as many as 10 for a PDSE, HFS,
extended format data set, or a VSAM
data set component cataloged in an
integrated catalog facility catalog.
PDSEs, HFS, and extended format data
sets can have up to 123 extents per
volume. All other data sets are
restricted to 16 extents per volume. A
VSAM component can have 7257
extents in up to 59 volumes (123 each).
One on each volume.
One for every 26 non-contagious
extents of available space on the
volume for a non- indexed VTOC; for an
indexed VTOC, there is only one.
This DSCB is not used frequently.
Chapter 2. Data set basics 47