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RSX-11 is a discontinued family of 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 and . It was designed (and mainly used) for , but was also popular for program development.


Name and origins
RSX-11 began as a port to the PDP-11 architecture of the earlier RSX-15 operating system for the PDP-15 minicomputer, first released in 1971.RSX-15 release date taken from: The main architect for RSX-15 (later renamed XVM/RSX) was Dan Brevick.

Commenting on the RSX acronym, Brevik says:


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 . 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 , then at his first project. Principles first tried in RSX-11M appear also in later designs led by Cutler, DEC's and Microsoft's .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 processor. See: "RSX was a separate path at DEC and the progenitor more than anything of VMS that went to NT via Dave Cutler." — , Vice President, Research and Development, Digital Equipment Corporation.

Under the direction of Ron McLean a derivative of RSX-11M, called RSX-20F, was developed to run on the PDP-11/40 front-end processor for the KL10 PDP-10 CPU.

Meanwhile, RSX-11D saw further developments: under the direction of (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 . Later development and release of IAS was led by Andy Wilson, in Digital's UK facilities.

Release dates
Below are estimated release dates for RSX-11 and IAS. Data is taken from the printing date of the associated documentation. General availability date is expected to come closely after. When manuals have different printing dates, the latest date is used. RSX-11S is a proper subset of RSX-11M, so release dates are always assumed to be the same as the corresponding version of RSX-11M. On the other side, RSX-11M Plus is an enhanced version of RSX-11M, so it's expected to be later than the corresponding version of RSX-11M.

March 1973RSX-11A 1.0
May 1973 Originally published on , copy hosted on the website of RSX 11D 1.0
December 1973Data from the labels of the distribution paper tapes. RSX-11C 7AFinal 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:
RSX-11M 1.0
June 1975RSX-11D 6.2Final version of RSX-11D
September 1975RSX-11M 2.0
RSX-11S 2.0
RSX-11S 1.0 never existed
December 1975IAS 1.0
April 1977RSX-11M 3.0
RSX-11S 3.0
December 1977RSX-11M 3.1
RSX-11S 3.1
bef. October 1979 p. 64IAS 3.0Final major release of IAS
December 1981RSX-11M 4.0as stated by SPD 14.35.17
July 1985RSX-11M Plus 3.0
Micro/RSX 3.0
September 1987RSX-11M Plus 4.0
Micro/RSX 4.0
Final Micro/RSX version
May 1990IAS 3.4Final IAS Release
February 1993RSX-11M 4.7
RSX-11S 4.7
Last release from Digital Equipment
February 1999RSX-11M Plus 4.6Released by Mentec

Legal ownership, development model and availability
RSX-11 is proprietary software. is asserted in binary files, source code and documentation alike. It was entirely developed internally by Digital. Therefore, no part of it is open source. However, a copy of the kernel source, with all comments removed, is present in every RSX distribution, because it's used during the system generation process. The notable exception to this rule is Micro-RSX, which comes with a pre-generated autoconfiguring binary kernel. Commented kernel source code was available as a separate product to those who already had a binary license, for reference purposes.

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 , 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 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 when it acquired Digital in 1998 and then to 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.


Main Versions
  • RSX-11A, C – small paper tape real time executives.
  • RSX-11B – small real time executive based on RSX-11C with support for disk I/O. To start up the system, first DOS-11 was booted, and then RSX-11B was started. RSX-11B programs used DOS-11 macros to perform disk I/O.
  • RSX-11D – a multiuser disk-based system. Evolved into IAS.
  • IAS – a timesharing-oriented variant of RSX-11D released at about the same time as the PDP-11/70. The first version of RSX to include DCL (Digital Command Language), which in IAS is known by its original name, PDS (Program Development System).
  • RSX-11M – a multiuser version that was popular on all PDP-11s.
  • RSX-11S – a memory-resident version of RSX-11M used in embedded real-time applications. RSX-11S applications were developed under RSX-11M.
  • RSX-11M-Plus – a much extended version of RSX-11M, originally designed to support the multi-processor PDP-11/74, a computer that was never released, but also used widely as a standard operating system on the PDP-11/70.

Hardware specific versions
  • RSX-20F – Customized version of RSX-11M, to be run on PDP-11/40 front end processor operating system for the DEC KL10 processor.
  • Micro/RSX – a pre-generated full version of RSX-11M-Plus with hardware autoconfiguration, implemented specifically for the Micro/PDP-11s, a low-cost multi-user system in a box, featuring ease of installation, no system generation, and a special documentation set. Later superseded by RSX-11M Plus.
  • P/OS – A version of RSX-11M-Plus that was targeted to the DEC Professional line of PDP-11-based personal computers.

Clones in the USSR
In 1968, the Soviet Government decided that manufacturing copies of and DEC minicomputers,
(2019). 9780387346373, Springer. .
in cooperation with other COMECON countries, was more practical than pursuing original designs. Cloning of DEC designs began in 1974, under the name of SM-EVM (Cyrillic:СМ ЭВМ). СМ ЭВМ is an acronym for 'Система Малых электронно-вычислительных машин' which is Russian for 'System of Small electronic computing machines'. As happened with mainframes based on the System/360 architecture, the Russians and their allies sometimes significantly modified Western designs, and therefore not every SM-EVM machine is compatible with DEC offerings of the time.

  • DOS/RV, ОСРВ-СМ, ОСРВMОСРВМ is the model of ОСРВ-СМ for the SM-1425. See: – Three names for an unauthorised clone of RSX-11M produced in the Socialist bloc. The name ОСРВ is an acronym for 'Операционная Система Реального Времени', which is Russian for 'Real-time Operating System'.Not surprisingly, the six-character string 'OCPBCM' fits nicely in the same 16-bit RADIX-50 word as 'RSX11M' This system appears to be an exact duplicate of RSX-11M except a different header in binary files. Differences between RSX and ОСРВ are due to hardware differences between SM and PDP computers and to -fixing done by Soviet engineers. However, the original RSX-11M was more used than its Russian clone ОСРВ, because the programmers modifying the original RSX-11M code were doing a better job, and patched RSX was more stable than ОСРВ. Other benefits included a faster update cycle for drivers and a larger choice of patches, made possible by a wider user community

RSX-11 was often used for general-purpose timeshare computing, even though this was the target use for the RSTS/E operating system. RSX-11 provided features to ensure less than a maximum necessary response time to peripheral device input (i.e. real-time processing), its intended use. These included the ability to lock a process (called a task under RSX) into memory as part of system boot up and to assign a process a higher priority so that it would execute before any processes with a lower priority.

In order to support large programs within the PDP-11's relatively small 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.

See also

External links

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