OpenVMS

The OpenVMS operating system (Virtual Memory System) is a multi-user, multi-threaded system designed by DEC, now part of VMS Software Inc. (VSI), for use in timesharing environments, real time, batch processing and transaction processing. Initially known as VMS, it ran on VAX systems, the name changed to OpenVMS in 1990. It was later ported to DEC Alpha (1992), Intel Itanium (2005) and x86-64 (2019).

Since 2014, OpenVMS continues to be developed and supported by a company called VMS Software Inc. (VSI).

OpenVMS enables high availability through clustering and the ability to distribute the system across multiple physical machines. VMS clusters allow the creation of applications that can withstand planned or unplanned outages of parts of the cluster. Applications and clustered data remain continuously available while operating system software and hardware upgrades and maintenance are performed, or when an entire data center is destroyed. VMS cluster uptimes of 17 years have been recorded. Customers using OpenVMS include banks and financial services, hospitals and healthcare, telecommunication operators, network information services, and industrial manufacturers. During the 1990s and 2000s, there were approximately half a million VMS systems in operation worldwide.

Architecture

The OpenVMS operating system has a layered architecture, consisting of a privileged Executive, a Command Line Interpreter (CLI) that runs at an intermediate level of privilege, and Runtime Libraries (RTLs) and utilities that run in a non-privileged mode, but can potentially run at a higher level of privilege if allowed to do so. Unprivileged code typically invokes the functionality of the Executive through system services (equivalent to system calls on other operating systems).

OpenVMS layers and mechanisms are built around certain features of the VAX architecture, including:

• The availability of four modes of access to the processor (called Kernel, Executive, Supervisor and User, in order of decreasing privilege). Each mode has its own stack, and each memory page can have memory protections specified by mode.
• A virtual address space that is divided into private process space sections and system space sections that are common to all processes.
• 32 interrupt priority levels used for synchronization.
• Hardware support to deliver asynchronous system traps to processes.

These VAX architecture mechanisms are implemented on Alpha, Itanium, and x86-64 by mapping to the corresponding hardware mechanisms on those architectures, or through emulation (via PALcode on Alpha, or in software on Itanium and x86-64).

Influence

During the 1980s, the MICA operating system for the PRISM architecture was intended to be the eventual successor to VMS. MICA was designed to maintain backwards compatibility with VMS applications, while also supporting Ultrix applications on top of the same kernel. MICA was eventually canceled along with the rest of the PRISM platform, leading Dave Cutler to leave. Digital by Microsoft. At Microsoft, Cutler led the creation of the Windows NT operating system, which was largely inspired by the MICA architecture. As such, VMS is considered an ancestor of Windows NT, along with RSX-11, VAXELN, and MICA, and there are many similarities between VMS and NT. This lineage is made clear in Cutler's foreword to Helen Custer's "Inside Windows NT".

A now defunct project called FreeVMS attempted to develop an open source operating system following VMS conventions. FreeVMS was built on the L4 Microkernel and supported the x86-64 architecture. Previous work investigating the implementation of VMS using a microkernel-based architecture had previously been performed as a prototyping exercise by DEC employees with the assistance of Carnegie Mellon University using the Mach 3.0 microkernel ported to the VAXstation 3100 hardware, adopting a multi-server architectural model.

An unofficial derivative of VAX/VMS called MOS VP (Russian: Многофункциональная операционная система с виртуальной памятью, МОС ВП, Multifunctional Operating System with virtual memory}}) was created in the Soviet Union during the 1980s for the SM 1700 line of VAX clone hardware. The main difference between MOS VP and the official Digital versions was the translation of commands, messages, and documentation into Russian, and the support of Cyrillic script using the KOI-8 encoding. Modified derivatives of MicroVMS known as MicroMOS VP (Russian МикроМОС ВП) or MOS-32M (Russian МОС-32М) were also created..

Hobbyist programs

In 1997, OpenVMS and a number of layered products were made available to hobbyists free of charge and non-commercially as part of the OpenVMS Hobbyist Program. ). Since then, several companies that produce software for OpenVMS have made their products available under the same terms, such as Process Software. Prior to the x86-64 port, the age and cost of hardware capable of running OpenVMS made emulators such as SIMH a common choice for hobbyist installations.

In March 2020, HPE announced the end of the OpenVMS Hobbyist Program. This was followed by VSI's announcement of the Community Licensing Program (CLP) in April 2020, which was intended to replace the HPE Hobbyist Program. The CLP was released in July 2020, and provides licenses for VSI OpenVMS versions on Alpha and Integrity systems. OpenVMS x86-64 licenses will be available when a stable version for this architecture is released. OpenVMS for VAX is not covered by the CLP, as there are no VSI versions of OpenVMS VAX, and older versions remain the property of HPE.