Editing CP/CMS (section)

==Overview==
{{FamilyTree CP/CMS|float=middle}}

CP/CMS was built by IBM's [[Cambridge Scientific Center]] (CSC), a [[research and development]] lab with ties to [[MIT]], under the leadership of [[Robert Creasy]]. The system's goals, development process, release, and legacy of breakthrough technology, set this system apart from other operating systems of its day and from other large IBM projects. It was an [[open-source software|open-source]] system, made available in source code form to all IBM customers at no charge &ndash; as part of the unsupported [[IBM Type-III Library]]. CP/CMS users supported themselves and each other. Unusual circumstances, described in the [[CP/CMS#History|History]] section below, led to this situation.

CP/CMS consisted of two main components:

* '''CP''', the ''Control Program'', created the virtual machine environment. The widely used version was CP-67, ran on the S/360-67. (The research system CP-40 established the architecture. A third version, CP-370, became VM/370.) Instead of explicitly dividing up memory and other resources among users, which had been the traditional approach, CP provided each user with a simulated stand-alone [[System/360]] computer. Each system was able to run any S/360 software that ran on the [[bare machine]] and in effect gave each user a private computer system.
* '''CMS''', the ''Cambridge Monitor System'' (and also ''Console Monitor System''<ref>Creasy, ''op. cit., p.'' 488 &ndash; "Console Monitor System"</ref> &ndash; but renamed [[Conversational Monitor System]] in VM) was a lightweight single-user operating system, for interactive time-sharing use. By running many copies of CMS in CP's virtual machines &ndash; instead of multiple copies of large, traditional multi-tasking OS &ndash; the overhead per user was less. This allowed a great number of simultaneous users to share a single S/360.

The CP/CMS virtual machine concept was an important step forward in operating system design.

* By isolating users from each other, CP/CMS greatly improved system reliability and security.
* By simulating a full, stand-alone computer for each user, CP/CMS could run ''any'' S/360 software in a time-sharing environment, not just applications specifically designed for time-sharing.
* By using lightweight CMS as the primary user interface, CP/CMS achieved unprecedented time-sharing performance. In addition, the simplicity of CMS made it easier to implement user interface enhancements than in traditional OS.

IBM reimplemented CP/CMS as its VM/370 product line, released in 1972 when [[virtual memory]] was added to the [[S/370]] series. VM/370's successors (such as [[z/VM]]) remain in wide use today. (IBM reimplemented CP-67, as it had CP-40, and did not simply rename and repackage it. VM coexisted with CP/CMS and its successors for many years. It is thus appropriate to view CP/CMS as an independent OS, distinct from the VM family.)