The VTOC is a data set that describes the contents of the direct access volume on which it resides. It is a contiguous data set; that is, it resides in a single extent on the volume and starts after cylinder 0, track 0 and before track 65,535. A VTOC's address is located in the VOLVTOC field of the standard volume label (see Figure 1). The volume label is described in z/OS DFSMS Using Data Setsz/OS DFSMS Using Data Setsz/OS DFSMS Using Data Sets. A VTOC consists of complete tracks. Some other System z® operating systems support VTOCs that are compatible with z/OS®. Some of them allow the VTOC to begin in the middle of a track. Normal z/OS operations do not support such a VTOC. However if you do not create or extend data sets from z/OS in such a VTOC, you may be able to read data sets on z/OS that were created on the other operating system.

Figure 1. Locating the volume table of contents

The VTOC is composed of 140-byte¹ data set control blocks (DSCBs) that correspond either to a data set currently residing on the volume, or to contiguous, unassigned tracks on the volume.

A special type of data is in a hierarchical file system (HFS) data set. Each one contains a UNIX type of file system that is compliant with the IEEE POSIX standard and OSF XPG/4.2 standard. Certain linear data sets contain z/OS file system (zFS) file systems. They also meet these standards. Each data set containing an HFS or zFS file system can contain UNIX files and directories belonging to multiple users. You can access the files through z/OS UNIX System Services, BSAM, QSAM, and VSAM. See z/OS DFSMS Using Data Sets. DSCBs for data sets describe their characteristics and location. DSCBs for contiguous, unassigned tracks indicate their location.