TheFile System The Conversational Monitor System interfaces with virtual disks, tapes,
and unit record equipment. The CMS residence device is kept as a
read-only, shared, system disk. Permanent user files may be accessed
from up to nine active disks. Logical access to those virtual disks is
controlled by CMS, while CP facilities manage the device sharing and
virtual-to-real mapping. User files in CMS are identified with three designators. The first
is filename. The second is a filetype designator that may imply
specific file characteristics to the CMS file management routines. The
third is a filemode designator that describes the location and access
mode of the file. User files can be created directly from the terminal with the CMS EDIT facility. EDIT provides extensive context editing services. File
chdracteristics such as record length and format, tab locations, and
serialization options can be specified. The system includes standard
definitions for certain filetypes.
A single user file is limited to a maximum of 65533 records and must
reside on one virtual disk. The file management system limits the
number of files on anyone virtual disk to 3400. All CMS disk files are
written as 800-byte records, chained together by a specific file entry
that is stored in a table called the Master File Directory; a separate
Master File Directory is kept for, and on, each virtual disk. The data
records may be discontiguous, and are allocated and deallocated
automatically. A subset of the Master File Directory (called the User File Directory) is made resident in virtual storage when, the disk
directory is made available to CMS; it is updated on the virtual disk at
least once per command if the status of any file on that disk has been
changed.
The compilers available under CMS default to particular input
filetypes, such as ASSEMBLE, but the file manipulation and listing
commands do not. Files of a particular filetype form a logical data
library for a user; for example, the collection of all COBOL source
files, or of all object (TEXT) decks, or of all EXEC procedures. This
allows selective handling of specific groups of files with minimum input
by the user .. CMS automatically allocates compiler work files at the beginning of
command execution on whichever active disk has the greatest amount of
available space, and deallocates them at completion. Compiler object
decks and listing files are normally allocated on the same disk as the
input source file or on the primary read/write disk, and are identified
by combining the input filename with the filetypes TEXT and LISTING. These disk locations may be overridden by the user. Virtual disks may be shared by CMS users; the facility is provided by VM/370 to all virtual machines, although a user interface is directly
available in CMS commands. Specific files may be spooled between
virtual machines to accomplish file transfer between users. Commands
allow such file manipulations as writing from an entire disk or from a
specific disk file to a tape, printer, punch, or the terminal. Other
commands write from a tape or virtual card reader to disk, rename files,
copy files, and erase files. Special macro libraries and text or
program libraries are provided by CMS, and special commands are provided
to update and use them. CMS files can be written onto and restored frem
unlabeled tapes via CMS commands.
234 IBM VM/370 System Programmer's Guide

Caution: Multiple write access under eMS can produce unpredictable results. Problem programs that execute in CMS tape in any record and block size; the
variable, or undefined.
Program Development
can create files on unlabeled
record format can be fixed, Conversational Monitor System includes commands to create and
compile source programs, to modify and correct source programs, to build
test files, to execute test programs and to debug from the terminal.
The commands of CMS are especially useful for as and DOS/VS program
development, but also may be used in combination with other operating
systems to provide a virtual machine program development tool. CMS utilizes the as and DOS/VS compilers via interface modules; the
compilers themselves normally are not changed. In order to provide
suitable interfaces, CMS includes a certain degree of as and DOS/iS simulation. The sequential, direct, and partitioned access methods are
logically simulated; the data records are physically kept in the chained 800-byte blocks that are standard to CMS, and are processed internally
to simulate as data set characteristics. CMS supports VSAM catalogs,
data spaces, and files on as and DOS disks using the DOS/VS Access
Method Services. as Supervisor Call functions such as GETMAIN/FREEMAIN and TIME are simulated. The simulation restrictions concerning what
types of as object programs can be executed under CMS are primarily
related to the as/pcP, MFT, and MiT Indexed Sequential Access Method (ISAM) and the telecommunications access methods, while functions
related to multitasking in as and DOS/VS are ignored by CMS. For more
information, see "aS Macro Simulation under CMS" and "DOS/VS Support under CMS." Part 3. Conversational Monitor System (CMS) 235