Conversational Monitor System

The Conversational Monitor System (CMS – originally: "Cambridge Monitor System") is a simple interactive single-user operating system. CMS was originally developed as part of IBM's CP/CMS operating system, which went into production use in 1967. CMS is part of IBM's VM family, which runs on IBM mainframe computers. VM was first announced in 1972, and is still in use today as z/VM.

CMS runs as a "guest" operating system in a private virtual machine created by the VM control program. The control program plus CMS together create a multi-user time-sharing operating system.

CMS was originally developed as part of IBM's CP/CMS operating system. At the time, the acronym meant "Cambridge Monitor System" (but also: "Console Monitor System").

In 1972, IBM released its VM/370 operating system, a re-implementation of CP/CMS for the System/370, in an announcement that also added virtual memory hardware to the System/370 series. Unlike CP/CMS, VM/370 was supported by IBM. VM went through a series of versions, and is still in use today as z/VM.

Through all its distinct versions and releases, the CMS platform remained still quite recognizable as a close descendant of the original CMS version running under CP-40. Many key user interface decisions familiar to today's users had already been made in 1965, as part of the CP-40 effort. See CMS under CP-40 for examples.

Both VM and CP/CMS had checkered histories at IBM. VM was not one of IBM's "strategic" operating systems, which were primarily the OS and DOS families, and it suffered from IBM political infighting over time-sharing versus batch processing goals. This conflict is why CP/CMS was originally released as an unsupported system, and why VM often had limited development and support resources within IBM. An exceptionally strong user community, first established in the self-support days of CP/CMS but remaining active after the launch of VM, made substantial contributions to the operating system, and mitigated the difficulties of running IBM's "other operating system".

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