Virtual Memory System

The OpenVMS (Open Virtual Memory System) operating system (OS), also known as VMS, is a multiuser, multiprocessing OS that was designed by Digital (now owned by Hewlett-Packard) in conjunction with their 32-bit VAX processor for use in time sharing, batch processing, and transaction processing. OpenVMS also runs on the 64-bit Alpha processor, and an upcoming port has been demonstrated on the Itanium processor.

OpenVMS also supports clustering (called VAXcluster or later VMScluster), where multiple systems share processing, job queues, print queues, and disk storage, connected either by specialized hardware or Ethernet. An Ethernet-based cluster is called a LAVC, for local area network VMScluster. OpenVMS supports up to 96 nodes in a single cluster.

OpenVMS can be divided into three layers

• the kernel, made up of input/output, memory management, and process/time management subsystems
• core services, made up of DCL, RMS, DECwindows (OpenVMS's X11 compliant windowing system), and the RTLs
• utility programs for support, system management, and programming

OpenVMS was originally designed by Dave Cutler, who had earlier developed Digital's RSX-11 operating systems. Cutler was hired in 1988 by Microsoft to build the team that developed Windows NT. The original name of the operating system was VMS, but it was renamed OpenVMS in the 1990s in a bid by Digital to position the product as an alternative to Unix.

OpenVMS-related acronyms include:

DCL - Digital Command Language - command line interface
DECwindows - Digital's implementation of the X Window System
RMS - Record Management Services - high-level, language/device-independent IO
RTL - Runtime Libraries - shared routines and functions, callable from any language
FDL - File Description Language - defines file record/field structure
DECnet - Digital's proprietary networking architecture