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VM/CMS (Virtual Machine/Conversational Monitor System) is an IBM operating system used on System 360, System 370, System 390 and zSeries mainframes. (It has recently been renamed z/VM by IBM.)

VM/CMS has two main components, VM and CMS, of which VM is by far the most interesting. VM is named for its ability to run software in virtual machines; thus VM can run any other operating system that the hardware supports, even itself.

CMS is a simple single-user operating system that runs under VM. Each VM/CMS user is given their own virtual machine to run CMS in. CMS originally stood for Cambridge Monitor System, after the IBM laboratory (Cambridge, Massachusetts) where VM/CMS was developed; it was latter renamed by IBM to Conversational Monitor System.