RSX-11 is a discontinued family of multi-user real-time operating systems for PDP-11 computers created by Digital Equipment Corporation. In widespread use through the late 1970s and early 1980s, RSX-11 was influential in the development of later operating systems such as OpenVMS and Windows NT. It was designed (and mainly used) for process control, but was also popular for program development.
The porting effort first produced small paper tape based real-time executives (RSX-11A, RSX-11C) which later gained limited support for disks (RSX-11B). RSX-11B then evolved into the fully fledged RSX-11D disk-based operating system, which first appeared on the PDP-11/40 and PDP-11/45 in early 1973. The project leader for RSX-11D up to version 4 was Henry Krejci. While RSX-11D was being completed, Digital set out to adapt it for a small memory footprint giving birth to RSX-11M, first released in 1973. From 1971 to 1976 the RSX-11M project was spearheaded by noted operating system designer Dave Cutler, then at his first project. Principles first tried in RSX-11M appear also in later designs led by Cutler, DEC's OpenVMS and Microsoft's Windows NT.While Windows NT system is in some areas a conceptual descendant of RSX-11M and VMS, its architecture descends directly from the Mica operating system, which Cutler developed for the unreleased DEC Prism processor. See: "RSX was a separate path at DEC and the progenitor more than anything of VMS that went to NT via Dave Cutler." — Gordon Bell, Vice President, Research and Development, Digital Equipment Corporation.
Meanwhile, RSX-11D saw further developments: under the direction of Garth Wolfendale (project leader 1972–1976) the system was redesigned and saw its first commercial release. Support for the 22-bit PDP-11/70 system was added. Dr. Wolfendale, originally from the UK, also set up the team that designed and prototyped the IAS operating system in the UK; IAS was a variant of RSX-11D more suitable for time sharing. Later development and release of IAS was led by Andy Wilson, in Digital's UK facilities.
|March 1973||RSX-11A 1.0|
|May 1973 Originally published on CD-ROM, copy hosted on the website of Gordon Bell||RSX 11D 1.0|
|December 1973Data from the labels of the distribution paper tapes.||RSX-11C 7A||Final release of RSX-11C|
|November 1974General availability date. See: |
Selected customers were testing the system at least since June 1974, because preliminary documentation contains a standard DEC license with a 1973 copyright date. See:
|June 1975||RSX-11D 6.2||Final version of RSX-11D|
|September 1975||RSX-11M 2.0|
|RSX-11S 1.0 never existed|
|December 1975||IAS 1.0|
|April 1977||RSX-11M 3.0|
|December 1977||RSX-11M 3.1|
|bef. October 1979 p. 64||IAS 3.0||Final major release of IAS|
|December 1981||RSX-11M 4.0||as stated by SPD 14.35.17|
|July 1985||RSX-11M Plus 3.0|
|September 1987||RSX-11M Plus 4.0|
|Final Micro/RSX version|
|May 1990||IAS 3.4||Final IAS Release|
|February 1993||RSX-11M 4.7|
|Last release from Digital Equipment|
|February 1999||RSX-11M Plus 4.6||Released by Mentec|
Ownership of RSX-11S, RSX-11M, RSX-11M Plus and Micro/RSX was transferred from Digital to Mentec Inc. in March 1994 as part of a broader agreement.
Mentec Inc. was the U.S. subsidiary of Mentec Limited, an Irish firm specializing in PDP-11 hardware and software support. In December 2006 Mentec Inc. was acquired by Irish firm Calyx, which does not sell PDP-11 related services or goods. New commercial licenses are therefore legally unobtainable. Hobbyists can run RSX-11M (version 4.3 or earlier) and RSX-11M Plus (version 3.0 or earlier) on the SIMH emulator thanks to a free license granted in May 1998 by Mentec Inc.
Legal ownership of RSX-11A, RSX-11B, RSX-11C, RSX-11D and IAS never changed hands, therefore it went to Compaq when it acquired Digital in 1998 and then to Hewlett-Packard in 2002. In late 2015 Hewlett-Packard split into two separate companies (HP Inc. and Hewlett Packard Enterprise), so the current owner cannot be firmly established. New commercial licenses haven't been issued at least since October 1979 (RSX-11A, RSX-11B, RSX-11C)Products don't appear anymore on Digital PDP-11 System Software Catalogue issued Oct. 1979: or 1990 (IAS), and no one of these operating systems was ever licensed for hobbyist use.
In order to support large programs within the PDP-11's relatively small virtual address space of 64 KB, a sophisticated semi-automatic overlay system was used; for any given program, this overlay scheme was produced by RSX's taskbuilder program (called TKB). If the overlay scheme was especially complex, taskbuilding could take a rather long time (hours to days).
The standard RSX prompt is ">" or "MCR>", (for the "Monitor Console Routine". All commands can be shortened to their first three characters when entered and correspondingly all commands are unique in their first three characters. Only the login command of "HELLO" can be executed by a user not yet logged in. "HELLO" was chosen as the login command because only the first three characters, "HEL", are relevant and this allows a non-logged in user to execute a "HELP" command.
When run on certain PDP-11 processors, each DEC operating system displays a characteristic light pattern on the processor console panel when the system is idle. These patterns are created by an idle task running at the lowest level. The RSX-11M light pattern is two sets of lights that sweep outwards to the left and right from the center of the console (inwards if the IND indirect command file processor program was currently running). By contrast, the IAS light pattern was a single bar of lights that swept leftwards. Correspondingly, a jumbled light pattern (reflecting memory fetches) is a visible indication that the computer is under load (and the idle task is not being executed). Other PDP-11 operating systems such as RSTS/E have their own distinctive patterns in the console lights.