Conversational Monitor System

From Academic Kids

The Conversational Monitor System (CMS) is a relatively simple interactive computing single-user operating system which was for many years IBM's principal time-sharing product. CMS runs primarily on IBM mainframe computers on top of the VM virtual machine operating system. Each CMS user appears to have their own personal operating system on their own private computer. The pair form a standard product, VM/CMS.

CMS provides users an environment for running applications or batch jobs, manage data files, create and debug applications, and communicate with other systems or users.

The acronym CMS originally stood for Cambridge Monitor System, after the IBM laboratory (the Cambridge Scientific Center) where it was developed; it was later renamed by IBM when it became an official IBM product.

CMS was developed in concert with VM to provide a time-sharing system; its original form was heavily influenced by CTSS, which was built by people at the same location (at MIT) as CSC. CTSS was actually used to develop initial versions of CMS software, using cross assemblers.

Users actually log in to VM and then proceed to boot their own virtual machine, usually by executing a file named 'PROFILE EXEC' or by issuing the command 'IPL CMS'. ("IPL" stands for "initial program load"; standard IBM jargon for booting a machine.)

While CMS started in the era of paper line terminals, by the late '70s most users were connecting with full-screen terminals such as the IBM 3270. Unlike dumb terminals, they have local storage and minimal processing abilities to deal with an entire screen of information at a time. The terminal deals with information in its buffer during editing. This enhances overall system performance because the mainframe doesn't have to deal with every keystroke typed by a user.

Two of the commonly used CMS tools include the editor XEDIT and the REXX programming language. Both of these products have been ported to other platforms and are now widely used outside the mainframe environment.

CMS is still in development and wide use today.

See also

  • VM/CMS for the history of the development of the entire system, including VM

Further reading

  • Melinda Varian, VM and the VM Community: Past, Present, and Future (available here ( is an excellent detailed history, starting with the experimental precursor to VM

External links


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