Volume Table of Contents

A VTOC is added to a DASD volume when it is initialized using the Device Support Facilities[note 5] program, ICKDSF, in current systems.

When in OS/360 and successors allocates a data set, it generally searches the catalog to determine the volumes on which it resides.

[note 6] When a program opens a Direct Access Storage Device (DASD) dataset, the OPEN routine searches the VTOC index (VTOCIX) if there is one, or directly searches the VTOC if there is no VTOCIX.

The VTOC consists of a sequence of 140-byte records known as Data Set Control Blocks[3][4] (DSCBs).

[5] For example, if the dataset is created with the DD card specification SPACE=(CYL,(4,2)) the dataset is initially allocated four cylinders, and an additional two cylinders, not necessary contiguous, will be allocated each time more space is needed, up to fifteen additional extents.