42 IBM VM/370 eMS User's Guide
Pg. of GC20-1819-2 Rev March 30, 1979 by Supp. SD23-9024-1 for 5748-XX8 Section 4. The eMS File System
The file is the essential unit of data in the CMS system. CMS disk
files are unique to the CMS system and cannot be read or written using
other operating systems. When you create a file 1n CMS, you name it
using a file identifier. The file identifier consists of three fields: Filename (fn) Filetype (ft) Filemode (fm) When you use CMS commands and programs to modify, update, or
reference files, you must identify the file by using these fields. Some CMS commands require you to enter only the filename, or the filename and
filetype; others require you to enter the filemode field as well. This
section contains information about the things you must consider when you
give your CMS files their identifiers, notes on the file system commands
that create and modify eMS files, and additional notes on using CMS disks. eMS File Formats
The eMS file management routines write eMS files on disk in fixed
physical blocks; regardless of whether they have fixed-or
variable-length records. For most of your CMS applications, you never
need to specify either a logical record length and record format or
block size when you create a eMS file. When you create a file with the eMS editor, the file has certain
default characteristics, based on its filetype. The special filetypes
recognized by the editor, and their applications, are discussed under "What are Reserved Filetypes1" VSAM files written by eMS are in
written by OS/VS or DOS/VS and are
systems. you cannot, however, use any
and write VSAM files, because VSAM virtual storage access method.
the same format as VSAM files
recognized by those operating eMS file system commands to read
file formats are unique to the
For a mini disk formatted in 800-byte physical blocks, a single CMS file can contain up to 12,848,000 bytes of data grouped into as many as
65,533 logical records, all of which must be on the same minidisk. If
the file is a source program, the file size limit may be smaller. The
maximum number of files per real disk in the SOO-byte physical block
format is 3400 for a 3330, 3333, 3340, or 3350 disk, or 3500 for a 2314
or 2319.
For a minidisk formatted in 1024-, 2048-, or 4096-byte logical
blocks, a single CMS file can contain up to about (2
31
- 132,000) disk
blocks of data, grouped into as many as 2
31
-1 logical records, all of
which must be on the same minidisk. The approximate limits to the
number of files per disk, expressed in thousands, are: Section 4. The eMS File System 43
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