Introduction To eMS The Conversational Monitor system (CMS), the major subsystem of Vft/370, provides a comprehensive set of conversational facilities to the user. Several copies of CMS may run under CP, thus providing several users
with their own time sharing system. CftS is designed specifically for
the Vft/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 CftS user has his own machine configuration and his own files. Debugging is simpler because the files and storage area are protected fro. other users .• Programs can be debugged fro. the terminal. The terminal is used as
a printer to examine limited a.ounts 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. CMS, 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, CftS 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 create and modify job streams,
and to analyze virtual Frinter 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 CBS executes
has virtual storage available to it and managed for it. The CP commands are recognized by CBS. For example, the co •• ands 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. For detailed information on CftS commands, refer to the CMS Figure 4 describes CMS command processing. CMS Introduction 2-3

The File 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 .anagement routines$ The
third is a filemode designator that describes the location and access mode of the file.
The compilers available under eMS 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. User files can be created directly from the terminal with the CftS EDIT facility. EDIT provides extensive context editing services. Pile characteristics such as record length and format, tab locations, and
serialization options can be specified. The system includes standard
definitions for certain filetypes. eMS 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. 1 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 CftS disk files are
written as aOO-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. 1 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. Virtual disks may be shared by eMS users; the facility is provided by YM/370 to all virtual .achines, 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. eMS files can be written onto and restored from unlabeled tapes via CMS commands. Multiple write access under eMS can produce unpredictable
results.
2-4 IBM VM/370 System Logic and Program Determination--Volume 2