RSX-11M-Plus 4.6 running on the SIMH emulator.
DeveloperDigital Equipment Corporation
Written inMACRO-11, BLISS
Working stateDiscontinued
Source modelClosed source; kernel source code included
Initial release1972; 52 years ago (1972)
Influenced byRSX-15
user interface
DCL and MCR Command-line interface

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.

As the original Real-Time System Executive[1][2] name suggests, RSX was designed (and commonly used) for real time use, with process control a major use.[3][4] It was also popular for program development[5] and general computing.


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,[6] first released in 1971.[7] The main architect for RSX-15 (later renamed XVM/RSX) was Dennis “Dan” Brevik.[8][9]

Commenting on the RSX acronym, Brevik says:[10]

"At first I called the new system DEX-15.  It was an acronym for 'Digital's Executive - for the PDP-15.'  The homonymic relation between DEC, DEX and deques (used as the primary linkage mechanism in the kernel) appealed to my sense of whimsy.  People readily adopted the acronym without question.

But in a short time I was asked to submit the choice to the corporate legal department for a trademark search and registration.  They sent me a memo that DEX was already trademarked by some paper company and I would have to rename the product.  I pointed out to them that software and paper mills didn't seem to have a hell of a lot of connection, but they wouldn't budge.

So I sat down with pencil and paper, and in a few moments came up with better than a dozen candidate acronyms and names.  My purpose was to come up with a good acronym and then find some appropriate words to justify it.  For example, X always appealed to me as part of an acronym because it is pronounced so forcefully, inferring (at least to me) some power and drama.  I used a lot of X's.  These potential acronyms were submitted back to the legal department.  At the time I had no favorite.

In a week or so they came back with a subset of my list that they could accept as trademarks.  It was left to me to make the final choice.

Bob Decker and I met in my office one afternoon to discuss the choice.  Bob was a marketeer who worked for me.  I chalked all the candidates on the blackboard and we started going through them one by one, pronouncing each out loud, savoring the sound, trying to get the feel of each one.  After ten minutes or so we had narrowed down the selection to three.

Bob sat back in silence as I kept looking at each acronym, seeing how it flowed off my tongue, what impression it gave me, and most importantly, the overall feeling about it.  After three or four minutes a strong feeling came over me about one of them.  It really felt right.  I looked at Bob and announced, "It's RSX".  I went to the board and erased all the rest until the only writing left was RSX.  It even looked right.

I have absolutely no memory about the other candidate acronyms.  They are lost forever, I suppose.  Well, maybe there's just a chance that the legal department kept copies of the correspondence - after all they are lawyers and they seem to hold on to everything (especially my money).

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.

...And that is how RSX got its name, on the 3rd floor of building 5 in the old mill."

RSX-11D and IAS

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).[6] 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.[11] 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[12] to 1976, the RSX-11M project was spearheaded by noted operating system designer Dave Cutler, then at his first project.[12] Principles first tried in RSX-11M appear also in later designs led by Cutler, DEC's VMS and MICA and Microsoft's Windows NT.[13][14][15]

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.[16]

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 Interactive Application System (IAS)[11] 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 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 is expected to be later than the corresponding version of RSX-11M.

Date RSX-11A, C RSX-11D IAS RSX-11M, S RSX-11M Plus Micro/RSX Comment
March 1973[17] RSX-11A 1.0
May 1973[18] RSX 11D 1.0
December 1973[19] RSX-11C 7A Final release of RSX-11C
November 1974[20] RSX-11M 1.0
June 1975[21] 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
May 1979 RSX-11M 3.2
RSX-11S 3.2
RSX-11M Plus 1.0
bef. October 1979[22] IAS 3.0 Final major release of IAS
November 1981 RSX-11M 4.0

RSX-11S 4.0

RSX-11M Plus 2.0
April 1983 RSX-11M 4.1
RSX-11S 4.1
RSX-11M Plus 2.1
July 1985 RSX-11M 4.2
RSX-11S 4.2
RSX-11M Plus 3.0 Micro/RSX 3.0
September 1987 RSX-11M 4.3
RSX-11S 4.3
RSX-11M Plus 4.0 Micro/RSX 4.0 Final Micro/RSX version
May 1988 RSX-11M 4.4
RSX-11S 4.4
RSX-11M Plus 4.1
January 1989 RSX-11M 4.5
RSX-11S 4.5
RSX-11M Plus 4.2
January 1990 RSX-11M 4.6
RSX-11S 4.6
RSX-11M Plus 4.3
May 1990[23] IAS 3.4 Final IAS Release
February 1993 RSX-11M 4.7
RSX-11S 4.7
RSX-11M Plus 4.4 Last release from Digital Equipment
March 1995 RSX-11M Plus 4.5
November 1998 RSX-11M 4.8
RSX-11S 4.8
Released by Mentec
February 1999 RSX-11M Plus 4.6 Micro/RSX 4.6 Released by Mentec

Legal ownership, development model and availability

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 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. Full sources 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[24] as part of a broader agreement.[25] Mentec Inc. was the US subsidiary of Mentec Limited, an Irish firm specializing in PDP-11 hardware and software support. In 2006 Mentec Inc. was declared bankrupt while Mentec Ltd. was acquired by Irish firm Calyx in December 2006.[26] The PDP-11 software, which was owned by Mentec Inc. was then bought by XX2247 LLC, which is the owner of the software today.[citation needed] It is unclear if new commercial licenses are possible to buy at this time. 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.[27]

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[28] and then to Hewlett-Packard in 2002.[29] In late 2015 Hewlett-Packard split into two separate companies (HP Inc. and Hewlett Packard Enterprise),[30] so the current owner cannot be firmly established.[dubious ] No new commercial licenses have been issued since at least October 1979 (RSX-11A, RSX-11B, RSX-11C)[31] or 1990 (IAS),[23] and none of these operating systems have ever been licensed for hobbyist use.


Main versions

RSX-11M-Plus also ran on PDP-11/44, PDP-11/84, PDP-11/94 (Unibus machines), as well as PDP-11/73, PDP-11/83, and PDP-11/93 (Qbus machines). One of the advantages of RSX-11M-Plus over RSX-11M was that larger programs could be created. This was achieved by having the task builder (the linker) build the program to use the separate instruction and data space feature of some PDP-11 models to put executable code and data into separate address spaces. This also allowed programs to run faster, as it reduced the need for "overlays", in which you could overlay object modules at task build time, for very large programs. Overlays were specified in a task build command file.

Hardware-specific variants

Clones in the USSR and other Eastern Bloc countries

In 1968,[34] the Soviet Government decided that manufacturing copies of IBM mainframes[35] and DEC minicomputers,[36][37] in cooperation with other COMECON countries,[34][38] was more practical than pursuing original designs. Cloning of DEC designs began in 1974,[38] under the name of SM EVM (Russian: СМ ЭВМ or Russian: Система Малых Электронно-Вычислительных Машин, lit.'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 many SM EVM machines were binary-incompatible with DEC offerings at the time.

A clone of the RSX-11M operating system ran on the Romanian-made CORAL series 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 on older versions of RSX). 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


  1. ^ "Real Time System Executive".
  2. ^ "User's Handbook Vol.1 Processor" (PDF). BitSavers.
  3. ^ William Y. Young; Donald P. Svrcek; Brent R. Mahoney (2014). "1: A Brief History of Control and Simulation". A Real Time Approach to Process Control (3 ed.). Chichester, West Sussex, United Kingdom: John Wiley & Sons Inc. pp. 1–2. ISBN 978-1-1199-9387-2.
  4. ^ "COMPAQ Software Product Description".
  5. ^ "AA-J302B-TC_Introduction_to_FEP_and_FRP_Aug82.pdf" (PDF). RSX-11 M/FEP and the FRP (FORTRAN Real-Time Package) ...
  6. ^ a b Cutler, Dave (2016-02-25). "Dave Cutler Oral History". (Interview). Interviewed by Grant Saviers. Computer History Museum. Archived from the original on 2021-12-11. Retrieved 2021-02-26.
  7. ^ RSX-15 release date taken from:"RSX-15 Real Time Executive reference manual" (PDF). Digital Equipment Corporation. 1971. Retrieved 4 December 2017.
  8. ^ "General FAQ". Retrieved 2019-12-10.
  9. ^ Lacroute, Bernard (3 May 1982). "Reference letter for Dan Brevick" (PDF). Archived from the original (PDF) on 18 January 2005. Retrieved 4 December 2017.
  10. ^ "RSX FAQ, General". Retrieved 4 December 2017.
  11. ^ a b c DIGITAL EQUIPMENT CORPORATION - Nineteen Fifty-Seven To The Present (PDF). Digital Equipment Corporation. 1975.
  12. ^ a b David Cutler (1993). "Foreword to Inside Windows NT, by Helen Custer". Retrieved 4 December 2017.
  13. ^ Mark Russinovich (30 November 1998). "Windows NT and VMS: The Rest of the Story". Retrieved 4 December 2017.
  14. ^ 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: "Internal Memo" (PDF). Digital Equipment Corporation. 10 June 1988. Retrieved 4 December 2017.
  15. ^ "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.
  16. ^ a b "TOPS-10/TOPS-20 RSX-20F System Reference Manual" (PDF). November 1980. p. 1-6. For these reasons, RSX-11M was chosen as the basis for RSX-20F.
  17. ^ Digital Equipment Corporation (March 1973). "RSX 11A Programmer's Reference Manual" (PDF). Retrieved 15 November 2022.
  18. ^ 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
  19. ^ Data from the labels of the distribution paper tapes. Digital Equipment Corporation (15 December 1973). "RSX 11C Paper Tape Labels". Retrieved 10 December 2017.
  20. ^ General availability date. See: Digital Equipment Corporation (November 1974). "RSX 11M Task Builder Reference Manual" (PDF). Retrieved 15 November 2022.
    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 15 November 2022.
  21. ^ Digital Equipment Corporation (February 1977). "RSX-11D System Generation Reference Manual" (PDF). Retrieved 15 November 2022.
  22. ^ "PDP-11 System Software Component Catalogue" (PDF). Digital Equipment Corp. October 1979. Retrieved 11 December 2017. p. 64
  23. ^ a b Digital Equipment Corporation (May 1990). "IAS Version 3.4 Software Product Description" (PDF). Retrieved 6 December 2017.
  24. ^ Mentec Inc. (1995). "Notice of Copyright Change for PDP-11 Software Products" (PDF). Retrieved 15 November 2022.
  25. ^ Digital Equipment Corporation (29 June 1994). "Press release: Digital and Mentec announce PDP-11 Software Agreement". Retrieved 11 December 2017.
  26. ^ Mentec Inc. (13 December 2006). "Calyx buys Mentec". The Register. Retrieved 11 December 2017.
  27. ^ Mentec Inc. (May 1998). "Mentec Hobbyist license for PDP-11 operating systems". Retrieved 11 December 2017.
  28. ^ Michael Kanellos (26 January 1998). "Compaq to buy Digital for $9.6 billion". Retrieved 11 December 2017.
  29. ^ Michael Kanellos (10 March 2002). "HP to acquire Compag for $25 billion". Retrieved 11 December 2017.
  30. ^ Arik Hesseldahl (2 November 2015). "Hewlett-Packard splits in two today, now what ?". Retrieved 11 December 2017.
  31. ^ 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.
  32. ^ Eric Postpischil (1990-01-22). "Multiprocessing PDP-11s". alt.folklore.computers newsgroup. Retrieved 2015-08-25.
  33. ^ Wesley Melling (June 1983). "Digital's Professional 300 Series / A Minicomputer Goes Micro". Byte. pp. 96–106. Retrieved 5 February 2015.
  34. ^ a b Gábor Képes. "Hungary: Computers behind the Iron Curtain" (PDF). p. 8. Retrieved 12 December 2017.
  35. ^ Boris Nikolaevich Malinovsky (2010). Pioneers of Soviet Computing (PDF). p. 25. Retrieved 12 December 2017.
  36. ^ John Impagliazzo; Eduard Proydakov (2011). "Preface" (PDF). In John Impagliazzo; Eduard Proydakov (eds.). Perspectives on Soviet and Russian Computing. Springer. p. XIV.
  37. ^ 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. Vol. 215. Springer. p. 157. doi:10.1007/978-0-387-34741-7_11. ISBN 9780-387-34637-3.
  38. ^ 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. Vol. 357. Springer. pp. 69–73. doi:10.1007/978-3-642-22816-2_9. ISBN 978-3-642-22815-5.
  39. ^ 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.
  40. ^ Not surprisingly, the six-character string 'OCPBCM' fits nicely in the same 16-bit RADIX-50 word as 'RSX11M'