Table of Contents (VTOC), and files can be referenced either thru the catalog or by specifying the volume serial number directly. Specifying a dataset name without specifying a serial number in JCL implies a catalog search. The catalog stores only a pointer to the volume. The OS/360 catalog has a number of differences from other hierarchical file systems. The "root directory", a dataset named SYSCTLG, may contain file pointers or links to subdirectories called CVOLS. The path separator is .. All paths are absolute paths, a leading . is always assumed and cannot be coded. path components (between .) are limited to eight characters. The catalog is not limited to disk (DASD) files, but can also catalog magnetic tape datasets. Not all volumes indicated in the catalog have to be online, and the system allows disk and tape volumes to be mounted on request following a catalog search. Unlike most other systems, it is possible to create a file without cataloging it. Cataloging is requested thru JCL or a utility program. A cataloged dataset name might look like PAYROLL.WEEKLY.HOURS. IBM virtual storage operating systems replaced SYSCTLG with a VSAM Catalog. Cataloging is mandatory for VSAM datasets, but, as before, non-VSAM datasets may be cataloged or not cataloged. The program "Access Method Services" (IDCAMS) maintains the VSAM Catalog. Personal computers The CP/M operating system used a flat file system, with a directory containing information on a maximum of 64 files—adequate when a floppy disk held only 128 KB. IBM PC DOS and MS-DOS 1.0 inherited the same structure. DOS 2.0, which supported hard disk drives, introduced a hierarchical file system. The hierarchical file system was used instead of simply expanding the flat directory for performance reasons. "A flat DOS file structure with a single directory and 10 times as many files would logically require 10 times as long to search." OS/2 and Windows also support a hierarchical file system, using the same path syntax as DOS. References Computer file systems Operating system technology Hard disk software