Conversation Monitor System

CMS is an operating system developed at IBM's Cambridge Scientific Center in 1964. It is still in development and wide use today. CMS runs primarily on IBM mainframe computers on top of the VM/SP[?] (Virtual Machine/System Product).

Each CMS user appears to have their own personal operating system on their own private computer. Users actually log in to VM/SP and then proceed to boot their own virtual machine, usually by executing a file named 'PROFILE EXEC' or by issuing the command 'i cms'.

While CMS started in the era of paper line terminals, by the late 70's most users were connecting with 'full-screen terminals' such as the IBM 3270. Unlike the terminals most Unix users are familiar with, full-screen terminals actually have local storage and minimal processing abilities to deal with an entire screen of information at a time. This approach has many advantages for displaying and editing text and data entry. Because the terminal deals with information in its buffer, users feel that the system is typically more responsive, and overall system performance is enhanced because it doesn't have to deal with every keystroke typed by a user, unlike a typical Unix system.

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.