VM (often: VM/CMS) is a family of IBM virtual machine operating systems used on IBM mainframes System/370, System/390, zSeries, System z and compatible systems, including the Hercules emulator for personal computers. The following versions are known: Virtual Machine Facility/370 VM/370, released in 1972, is a System/370 reimplementation of earlier CP/CMS operating system. VM/370 Basic System Extensions Program Product VM/BSE (BSEPP) is an enhancement to VM/370 that adds support for more devices (such as 3370-type fixed-block-architecture DASD drives), improvements to the CMS environment (such as an improved editor), and some stability enhancements to CP. VM/370 System Extensions Program Product VM/SE (SEPP) is an enhancement to VM/370 that includes the facilities of VM/BSE, as well as a few additional fixes and features. Virtual Machine/System Product VM/SP, a milestone version, replaces VM/370, VM/BSE and VM/SE. Release 1 added EXEC2 and XEDIT System Product Editor; Release 3 added REXX; Release 6 added the shared filesystem. Virtual Machine/System Product High Performance Option VM/SP HPO adds additional device support and functionality to VM/SP, and allows certain S/370 machines that can utilize more than 16 MB of real storage to do so, up to 64 MB. This version was intended for users that would be running multiple S/370 guests at once. Virtual Machine/Extended Architecture Migration Aid VM/XA MA is intended to ease the migration from MVS/370 to MVS/XA by allowing both to run concurrently on the same processor complex. Virtual Machine/Extended Architecture System Facility VM/XA SF is an upgraded VM/XA MA with improved functionality and performance. Virtual Machine/Extended Architecture System Product VM/XA SF is an upgraded VM/XA MA with improved functionality and performance, offered as a replacement for VM/SP HPO on machines supporting S/370-XA. It includes a version of CMS that can run in either S/370 or S/370-XA mode. Virtual Machine/Enterprize Systems Architecture VM/ESA provides the facilities of VM/SP, VM/SP HPO and VM/XA SP. VM/ESA version 1 can run in S/370, ESA/370 or ESA/390 mode; it does not support S/370 XA mode. Version 2 only runs in ESA/390 mode. The S/370-capable versions of VM/ESA were actually their own separate version from the ESA/390 versions of VM/ESA, as the S/370 versions are based on the older VM/SP HPO codebase, and the ESA/390 versions are based on the newer VM/XA codebase. z/VM z/VM, the last version still widely used as one of the main full virtualization solutions for the mainframe market. z/VM 4.4 was the last version that could