VMS (Virtual Memory System) is an operating system from the Digital Equipment Corporation (DEC) that runs in its older mid-range computers. VMS originated in 1979 as a new operating system for DEC's new VAX computer, the successor to DEC's PDP-11. VMS is a 32-bit system that exploits the concept of virtual memory.
VMS was renamed OpenVMS when it was redeveloped for the Alpha processor. (OpenVMS is also the name now used on the VAX computer.) The "Open" suggests the added support for the UNIX-like interfaces of the Portable Operating System Interface (POSIX) standard. Programs written to the POSIX standard, which includes a set of standard C language programming functions, can be ported to any POSIX-supporting computer platform.
Among other features, OpenVMS can be used with special software that facilitates its use with Windows NT servers.