CP/CMS (Control Program/Cambridge Monitor System) is a discontinued time-sharing operating system of the late 1960s and early 1970s, known for its excellent performance and advanced features. It had three distinct versions: CP-40/CMS, an important "one-off" research system that established the CP/CMS virtual machine architecture CP-67/CMS, a reimplementation of CP-40/CMS for the IBM System/360-67, and the primary focus of this article CP-370/CMS, a reimplementation of CP-67/CMS for the System/370 – never released as such, but became the foundation of IBM's VM/370 operating system, announced in 1972. Each implementation was a substantial redesign of its predecessor and an evolutionary step forward. CP-67/CMS was the first widely available virtual machine architecture. IBM pioneered this idea with its research systems M44/44X (which used partial virtualization) and CP-40 (which used full virtualization). In addition to its role as the predecessor of the VM family, CP/CMS played an important role in the development of operating system (OS) theory, the design of IBM's System/370, the time-sharing industry, and the creation of a self-supporting user community that anticipated today's free software movement. History Fundamental CP/CMS architectural and strategic parameters were established in CP-40, which began production use at IBM's Cambridge Scientific Center in early 1967. This effort occurred in a complex political and technical milieu, discussed at some length and supported by first-hand quotes in the Wikipedia article History of CP/CMS. In a nutshell: In the early 1960s, IBM sought to maintain dominance over scientific computing, where time-sharing efforts such as CTSS and MIT's Project MAC gained focus. But IBM had committed to a huge project, the System/360, which took the company in a different direction. The time-sharing community was disappointed with the S/360's lack of time-sharing capabilities. This led to key IBM sales losses at Project MAC and Bell Laboratories. IBM's Cambridge Scientific Center (CSC), originally established to support Project MAC, began an effort to regain IBM's credibility in time-sharing, by building a time-sharing operating system for the S/360. This system would eventually become CP/CMS. In the same spirit, IBM designed and released a S/360 model with time-sharing features, the IBM System/360-67, and a time-sharing operating system, TSS/360. TSS failed; but the 360-67 and CP/CMS succeeded, despite internal political battles over time-sharing, and concerted efforts at IBM to scrap the CP/CMS effort. In 1967, CP/CMS production use began, first on CSC's CP-40, then later on CP-67 at Lincoln Laboratories and other sites. It was made available via