Introduction to eMS The Conversational Monitor System (CMS), the major subsystem of VM/370, provides a comprehensive set of conversational facilities to the user. Several copies of CMS may run under CP, thus Froviding several users
with their own time sharing system. CMS is designed specifically for
the VM/370 virtual machine environment.
Each copy of CMS supports a single user. This means that the storage
area contains only the data pertaining to that user. Likewise, each CMS user has his own machine configuration and his own files. Debugging is
simpler because the files and storage area are protected from other
users. Programs can be debugged from the terminal. The terminal is used as
a printer to examine limited amounts of data. After examining program
data, the terminal user can enter commands on the terminal that will
alter the program. This is the most common method used to debug
programs that run in CMS. eMS, operating with the VM/370 Control Program, is a time sharing
system suitable for problem solving, program development, and general
work. It includes several programming language processors, file
manipulation commands, utilities, and debugging aids. Additionally, ces provides facilities to simplify the operation of other operating systems
in a virtual machine environment when controlled from a remote terminal.
For example, CMS capabilities are used to create and modify job streams,
and to analyze virtual printer output. Part of the CMS environment is related to the virtual machine
environment created by CP. Each user is completely isolated from the
activities of all other users, and each machine in which CMS executes
has virtual storage available to it and managed for it. The CP commands
are recognized by CMS. For example, the commands allow messages to be
sent to the operator or to other users, and virtual devices to be
dynamically detached from the virtual machine configuration.
The CMS Command language
The CMS command language offers terminal users a wide range of
functions. It supports a variety of programming languages, service
functions, file manipulation, program execution control, and general
system control. The CMS commands that are useful in debugging are
discussed in the "Debugging with CMS" section of "Part 1. Debugging with YM/370." For detailed information on all other CMS commands, refer to
the gDg Figure 28 describes CMS command processing. Part 3. Conversational Monitor System (Ces) 233

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