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Creator: DEC
Architecture: PDP-11

RSX-11 is a family of real-time operating systems for PDP-11 computers, created by DEC; it was common in the late 1970s and early 1980s. It was designed for and much used in process control, but was also popular for program development.


RSX-11 existed in many 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 multi-user disk-based system. Evolved into IAS.
  • IAS -- a time-sharing-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 was originally known as PDS (Program Development System).
  • RSX-11M -- a multi-user version that was an adaptation of the earlier RSX-11D for a smaller memory footprint, it was popular on all PDP-11s; Dave Cutler was the project leader.
  • RSX-11S -- a memory-resident version of RSX-11M used in embedded real-time applications.
  • 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.
  • RSX-20F -- PDP-11/40 front end processsor operating system for the KL10 processor. Derived from RSX-11D and RSX-11M.
  • Micro/RSX -- a stripped-down version of RSX-11M-Plus implemented specifically for the Micro/PDP-11, a low-cost multi-user system in a box, featuring ease of installation, no system generation, and a special documentation set.
  • P/OS -- A version of RSX-11M-Plus that was targeted to DEC's PRO-325, PRO-350, and PRO-380 line of PDP-11-based personal computers.
  • DOS/RV, OSRV-SM -- Two names for the clandestine clone of RSX-11M that was produced in the Socialist bloc. This system appeared to be an exact duplicate of RSX-11M save that the prompt was changed in the binary files. According to other sources, RSX-11M source code might have been stolen by the KGB. If read as Cyrillic, the name OSRV is an abbreviation for 'Operatsionnaya Sistema Realnogo(Razdelenija) Vremeni' -- Russian for 'Real Time(Time dividing) Operating System'. Not surprisingly, the six-character string 'OSRVSM' fits nicely in the same 16-bit RADIX-50 word as 'RSX11M'. But, there are differences between RSX and OSRV because of differences between SM and PDP' hardware and recognised by Soviet engineers bugs in RSX. (OSRVM is the next model of OSRV-SM for the SM-1425.[1]) But RSX11M 'patched' for the SM's processor was used more often than rewritten OSRV. That happened because of better work by the RSX-11' re-coders, stability of patched RSX, and a faster update cycle for SM-RSX drivers & patches, made possible by the SM users community. OSRV and RSX driver interfaces are different & incompatible.


Principles first tried in RSX-11M later appeared in DEC's VMS. Microsoft's Windows NT system is a distant descendant of RSX-11M but is more directly descended from an object-oriented operating system Cutler developed for a RISC processor (PRISM) which was never released. This lineage is made clear in Cutler's foreword to Inside Windows NT, quoted on Neil Rieck's "Windows-NT" is "VMS re-implemented" page.


  • "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.
  • "My first operating system project was to build a real-time system called RSX-11M that ran on Digital's PDP-11 16-bit series of minicomputers. ... a multitasking operating system that would run in 32 KB of memory with a hierarchical file system, application swapping, real-time scheduling, and a set of development utilities. The operating system and utilities were to run on the entire line of PDP-11 platforms, from the very small systems up through the PDP-11/70 which had memory-mapping hardware and supported up to 4 MB of memory." -- Dave Cutler, foreword to Inside Windows NT

RSX-11 trivia

  • 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. Outside the office of the engineer in charge of ongoing maintenance of the taskbuilder was a whiteboard labeled "Taskbuilder wishlist". For several years, the top item on the wishlist was "same day service".
  • Before DCL, the usual RSX prompt was ">" or "MCR>", standing for the 'Monitor Console Routine'.
  • When run on certain PDP-11 processors, each version of RSX displayed a characteristic light pattern on the front of the processor any time the system was idle (and the processor was executing the PDP-11 WAIT instruction). The RSX-11M light pattern was two sets of lights that swept outwards to the left and right from the center of the light display (or inwards if the IND indirect command file processor program was currently running in older versions of -11M). The IAS light pattern was a single bar of lights that swept leftwards.

See also

External links

  • Dan's RSX-11 prehistory contain documents which trace RSX-11 back through RSX-15 and the real time executive written by John Neblett in the late 50's for the RW-300 process control computer. (Via, version fetched April 4, 2005)