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 VMS and Windows NT. It was designed (and mainly used) for process control, but was also popular for program development.

Screenshot showing the output of a SHOW MEMORY shell command.
RSX-11M Plus 4.6 under the Ersatz PDP-11 emulator.
DeveloperDigital Equipment Corporation
Written inMACRO-11
OS familyDEC OS family
Working stateDiscontinued
Source modelClosed source; non-commented kernel source code included
Initial release1972; 47 years ago (1972)
Default user interfaceCommand line interface


Name and originsEdit

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.[1] The main architect for RSX-15 (later renamed XVM/RSX) was Dan Brevick.[2]

Commenting on the RSX acronym, Brevik says:[3]

My purpose was to come up with a good acronym and then find some appropriate words to justify it. ... Oh, by the way, the acronym stood for Real-Time System Executive. Years later that was changed to Resource Sharing Executive, which I think is even better.


...At first I called the new system DEX-15. It was an acronym for Digital's Executive - for the PDP-15

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.[4] 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[5] to 1976 the RSX-11M project was spearheaded by noted operating system designer Dave Cutler, then at his first project.[5] Principles first tried in RSX-11M appear also in later designs led by Cutler, DEC's VMS and Microsoft's Windows NT.[6][7][8]

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.[9]

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. 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.

Release datesEdit

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 is expected to be later than the corresponding version of RSX-11M.

Date OS comment
March 1973[10] RSX-11A 1.0
May 1973[11] RSX 11D 1.0
December 1973[12] RSX-11C 7A Final release of RSX-11C
November 1974[13] RSX-11M 1.0
June 1975[14] RSX-11D 6.2 Final version of RSX-11D
September 1975 RSX-11M 2.0
RSX-11S 2.0
RSX-11S 1.0 never existed
December 1975 IAS 1.0
April 1977 RSX-11M 3.0
RSX-11S 3.0
December 1977 RSX-11M 3.1
RSX-11S 3.1
bef. October 1979[15] 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
Micro/RSX 3.0
September 1987 RSX-11M Plus 4.0
Micro/RSX 4.0
Final Micro/RSX version
May 1990[16] IAS 3.4 Final IAS Release
February 1993 RSX-11M 4.7
RSX-11S 4.7
Last release from Digital Equipment
February 1999 RSX-11M Plus 4.6 Released by Mentec

Legal ownership, development model and availabilityEdit

RSX-11 is proprietary software. Copyright 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 was used during the system generation process. The notable exception to this rule is Micro-RSX, which came 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[17] as part of a broader agreement.[18] Mentec Inc. was the US 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,[19] 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.[20]

Legal ownership of RSX-11A, RSX-11B, RSX-11C, RSX-11D, and IAS never changed hands; therefore it passed to Compaq when it acquired Digital in 1998[21] and then to Hewlett-Packard in 2002.[22] In late 2015 Hewlett-Packard split into two separate companies (HP Inc. and Hewlett Packard Enterprise),[23] so the current owner cannot be firmly established. New commercial licenses have not been issued at least since October 1979 (RSX-11A, RSX-11B, RSX-11C)[24] or 1990 (IAS),[16] and no one of these operating systems was ever licensed for hobbyist use.


Main versionsEdit

  • 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, later 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,[25] a computer that was never released, but RSX-11M-Plus was then used widely as a standard operating system on the PDP-11/70.

Hardware-specific variantsEdit

  • 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 and other Eastern Bloc countriesEdit

In 1968,[26] the Soviet Government decided that manufacturing copies of IBM mainframes[27] and DEC minicomputers,[28][29] in cooperation with other COMECON countries,[26][30] was more practical than pursuing original designs. Cloning of DEC designs began in 1974,[30] 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 ES EVM 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, Russian: ОСРВ-СМ, ОСРВM[31] – 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'.[32] 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 bug-fixing done by Soviet engineers. However, the original RSX-11M was more used than its Russian clone ОСРВ,[citation needed] 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.

A clone of the RSX-11M operating system ran on the Romanian-made CORAL family of computers (such as CORAL 2030, a clone of PDP-11).


RSX-11 was often used for general-purpose timeshare computing, even though this was the target market for the competing RSTS/E operating system. RSX-11 provided features to ensure better than a maximum necessary response time to peripheral device input (i.e. real-time processing), its original intended use. These features 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 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.

See alsoEdit


  1. ^ RSX-15 release date taken from:"RSX-15 Real Time Executive reference manual" (PDF). Digital Equipment Corporation. 1971. Retrieved 4 December 2017.
  2. ^ Lacroute, Bernard (3 May 1982). "Reference letter for Dan Brevick" (PDF). Archived from the original (PDF) on 18 January 2005. Retrieved 4 December 2017. Cite uses deprecated parameter |dead-url= (help)
  3. ^ "RSX FAQ, General". Retrieved 4 December 2017.
  4. ^ DIGITAL EQUIPMENT CORPORATION - Nineteen Fifty-Seven To The Present (PDF). Digital Equipment Corporation. 1975.
  5. ^ a b David Cutler (1993). "Foreword to Inside Windows NT, by Helen Custer". Retrieved 4 December 2017.
  6. ^ Mark Russinovich (30 November 1998). "Windows NT and VMS: The Rest of the Story". Retrieved 4 December 2017.
  7. ^ 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 Prism processor. See: Digital Equipment Corporation (10 June 1988). "Internal Memo" (PDF). Retrieved 4 December 2017.
  8. ^ "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.
  9. ^ "TOPS-10/TOPS-20 RSX-20F System Reference Manual" (PDF). November 1980.
  10. ^ Digital Equipment Corporation (March 1973). "RSX 11A Programmer's Reference Manual" (PDF). Retrieved 6 December 2017.
  11. ^ Digital Equipment Corporation (August 1997). "DIGITAL Computing Timeline". Retrieved 10 December 2017. Originally published on CD-ROM, copy hosted on the website of Gordon Bell
  12. ^ Data from the labels of the distribution paper tapes. Digital Equipment Corporation (15 December 1973). "RSX 11C Paper Tape Labels". Retrieved 10 December 2017.
  13. ^ General availability date. See: Digital Equipment Corporation (November 1974). "RSX 11M Task Builder Reference Manual" (PDF). Retrieved 7 December 2017.
    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: Digital Equipment Corporation (June 1974). "RSX 11M Working Design Document" (PDF). Retrieved 7 December 2017.
  14. ^ Digital Equipment Corporation (February 1977). "RSX 11D System Generation Reference Manual" (PDF). Retrieved 6 December 2017.
  15. ^ "PDP-11 System Software Component Catalogue" (PDF). Digital Equipment Corp. October 1979. Retrieved 11 December 2017. p. 64
  16. ^ a b Digital Equipment Corporation (May 1990). "IAS Version 3.4 Software Product Description" (PDF). Retrieved 6 December 2017.
  17. ^ Mentec Inc. (1995). "Notice of Copyright Change for PDP-11 Software Products" (PDF). Retrieved 11 December 2017.
  18. ^ Digital Equipment Corporation (29 June 1994). "Press release: Digital and Mentec announce PDP-11 Software Agreement". Retrieved 11 December 2017.
  19. ^ Mentec Inc. (13 December 2006). "Calyx buys Mentec". Retrieved 11 December 2017.
  20. ^ Mentec Inc. (May 1998). "Mentec Hobbyist license for PDP-11 operating systems". Retrieved 11 December 2017.
  21. ^ Michael Kanellos (26 January 1998). "Compaq to buy Digital for $9.6 billion". Retrieved 11 December 2017.
  22. ^ Michael Kanellos (10 March 2002). "HP to acquire Compag for $25 billion". Retrieved 11 December 2017.
  23. ^ Arik Hesseldahl (2 November 2015). "Hewlett-Packard splits in two today, now what ?". Retrieved 11 December 2017.
  24. ^ Products no longer appeared on Digital PDP-11 System Software Catalogue issued Oct. 1979: "PDP-11 System Software Component Catalogue" (PDF). Digital Equipment Corp. October 1979. Retrieved 11 December 2017.
  25. ^ Eric Postpischil (1990-01-22). "Multiprocessing PDP-11s". alt.folklore.computers newsgroup. Retrieved 2015-08-25.
  26. ^ a b Gábor Képes. "Hungary: Computers behind the Iron Curtain" (PDF). p. 8. Retrieved 12 December 2017.
  27. ^ Boris Nikolaevich Malinovsky (2010). Pioneers of Soviet Computing (PDF). p. 25. Retrieved 12 December 2017.
  28. ^ John Impagliazzo; Eduard Proydakov (2011). "Preface" (PDF). In John Impagliazzo; Eduard Proydakov (eds.). Perspectives on Soviet and Russian Computing. Springer. p. XIV.
  29. ^ Raffai, Maria (2006). "Computing Behind the Iron Curtain and Beyond Hungarian National Perspective" (PDF). In Impagliazzo, John (ed.). History of Computing and Education 2. IFIP International Federation for Information Processing. 215. Springer. p. 157. doi:10.1007/978-0-387-34741-7_11. ISBN 9780-387-34637-3.
  30. ^ a b N. L. Prokhorov; G. A. Egorov (2011). "SM EVM Control Computer Development" (PDF). In John Impagliazzo; Eduard Proydakov (eds.). Perspectives on Soviet and Russian Computing. IFIP Advances in Information and Communication Technology. 357. Springer. pp. 69–73. doi:10.1007/978-3-642-22816-2_9. ISBN 978-3-642-22815-5.
  31. ^ ОСРВМ is the model of ОСРВ-СМ for the SM-1425. See:СМ 1425 (in Russian). 2002-07-19. Archived from the original on 2015-05-02. Retrieved 2015-08-25.
  32. ^ Not surprisingly, the six-character string 'OCPBCM' fits nicely in the same 16-bit RADIX-50 word as 'RSX11M'

External linksEdit