2. PLANNING › 2.2 Supported MVS Data Structures › 2.2.1 Standard VTOC Support

2.2.1 Standard VTOC Support

VCC supports standard VTOCs.  A standard VTOC is defined as
being a volume table of contents that does not have an index
associated with it.

VCC scans a VTOC using VSAM chained scheduling, reading DSCBs
a track at a time, into an I/O buffer.  All information in
the following DSCBs are placed into the VCC record:

    Format 1 DSCB - identifies a data set on the device
    Format 2 DSCB - ISAM index information
    Format 3 DSCB - data set extent information
    Format 4 DSCB - VTOC information
    Format 5 DSCB - volume free space extent information

The first DSCB in a standard VTOC is the Format 4 DSCB.  It
describes the extents of the VTOC, characteristics of the
device on which it resides, and information about the DSCBs
in the VTOC (number of free DSCBs and a CCHHR pointer to the
last Format 1 DSCB).  It also contains VSAM ownership
timestamps (not applicable to ICF catalogs).  There is only
one Format 4 DSCB in a standard OS VTOC.

VCC uses the number of available DSCBs to calculate the
termination point of its VTOC SCAN (total DSCBs minus
available DSCBs = total used DSCBs).

The next DSCB in a standard VTOC is the Format 5 DSCB.  Each
Format 5 DSCB describes up to 19 free space extents on the
volume.  There is also a CCHHR pointer to the next Format 5
DSCB.

Following the above two DSCBs, the other DSCBs can occur in
any order.  The Format 1 DSCB can either be self-contained
(if it has less than four extents), can point to a Format 3
DSCB (if the data set has more than three extents), or can
point to a Format 2 DSCB (ISAM only) which then can point to
a Format 3 DSCB.