RSX-11: A family of Real-time operating systems mainly for PDP-11 computers created by Digital Equipment Corporation (DEC), common in the late 1970s and early 1980s, designed for and much used in process control, but also popular for program development.

It existed in many versions:

  • RSX-11/D -- the original, used as the operating system on the small PDP-11 used as a boot device on the giant Decsystem 10 and DECsystem 20.
  • RSX-11M -- a multiuser version that was popular on all PDP-11s.
  • 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. The first to include DCL (Digital Command Language).
  • RSX-11-S -- a memory-resident version used in real-time applications.
  • Micro/RSX -- a stripped-down version 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.

Dave Cutler was the chief designer of all these systems. Principles first tried here later appeared in DEC's VMS and Microsoft's Windows NT systems. 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.

Al Kossow posted some further notes on RSX-11 in alt.sys.pdp11.


  • "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." -- David Cutler, foreword to Inside Windows NT