[expanded. 2004.03.25]
Lately, I looked through sources of RSX executive at bitsavers.org. RSX (which stands for Realtime System eXecutive or Resource Sharing eXecitive) is a collective name for the series of operating systems DEC developed for its PDP processors. Initial version was RSX-15 developed for PDP-15 processor (a member of 18b family of DEC processors---poorer and ultimately abandoned cousins of another branch that got us PDP-11) by Sam Reese, Dan Brevik, Hank Krejci and Bernard LaCroute. See: a bit of history, original design, official DEC reference manual. Later on, RSX was ported to PDP-11 (yes, version numbers can very well go down), resulting in succession of RSX-11A, RSX-11B, RSX-11C, and RSX-11D, the latter being full-blown operating system, multi-user and multi-tasking. You think it's quite an achievement for a platform with 32K bytes of primary storage? Obviously David Cutler wasn't impressed, as he re-wrote it from scratch to consume less resources giving birth to RSX-11M (and later to VAX VMS, and Windows NT). Unfortunately, no RSX-11 kernel sources seem available (neither A-D, nor M and beyond versions), except for little witty pieces here and there (bitsavers, however sports materials from DEC's classes on RSX-11 internals). XVM/RSX-15 sources are available at bitsavers:
  • Executive proper: P1 and P2, and
  • MCR (Monitor Console Routine)
Why would one waste time looking through the obscure non-portable kernel that ran on a hardware decommissioned quarter of century ago? A lot of reasons:
  • It is radically different from UNIX, which is, basically, the only publicly available kernel source base to-day.
  • It was written at the times when computing was still young and engineers didn't take a lot of things for granted, so one can see how and why they come to their designs.
  • It's rather small by nowadays standards and easy to understand.
  • It has surprisingly good comments.
  • It was one of the first and probably (for quite a long time) largest open-source projects: hundreds of megabytes of software for RSX were available in the source form (DECUS tapes)
  • I agree with the sentiments R. Pike, esq. expressed in Systems Software Research is Irrelevant article (also see Rik van Riel's note): modern operating system research revolves around (or, rather, inside of) a cluster of well-known old ideas. By looking at the roots of operating system development, one sees that these ideas are but choices.
  • Nostalgia, of course. At the end of 80s I worked on some PDP clones: Электроника-100/25, СМ-4.

At the center of RSX-15 is its executive, also known as monitor or kernel. This is small program written in PDP-15 assembly language, that maintains following abstractions:
memory partitioning
rudimentary memory management scheme. Memory is partitioned during system generation into partitions and common blocks.
task and scheduling.
Task is a machine code that can be executed under control of executive. Task is installed when it is known to the executive (specifically, registered in STL---global System Task List). Tasks are created by task builder---special task that builds task by linking relocatable code with libraries and storing task on disk in the same format it will be stored in the primary storage (i.e., task loading is just copying it from disk to the core, which is important for real-time operating system). Some tasks are resident, which means they are always in core. Installed task may be requested for execution (placed into ATL---Active Task List), and get its share of processor cycles subject to priority and memory constraints. Simple fixed priority scheme is used. Memory is managed on partition granularity: each task has a partition assigned to it during build, and will be loaded into core when said partition is free. Partition can be used for one task only.
interrupt management
executive itself handles clock interrupt only. All other interrupts are handled by special IO Handler Tasks (IOHTs), any installed task may become IOHT by connecting to interrupt line.
device management.
list of physical devices and mapping between physical and logical units (LUNs). Per-device prioritized IO queues. Asynchronous IO as a default mechanism. LUNs are global.
interface to space management for disks
bitmap of free blocks. Implemented by IO handler task, not executive.
interface to simple flat file system.
Implemented by IO handler task.

[...To be continued...]
Here are some interesting features:
  • usage of API (Automatic Priority Interrupts) to implement concurrency control.

    In RSX-11M this was replaced with FORK processing: interrupts never directly access kernel tables, hence, no synchronization is required on the uniprocessor. Similar to soft interrupts.

  • separation of STL (system task list) and ATL (active task list)---a form of two level scheduling: long level scheduling decisions are made by installing task into STL, and assigning resources (devices and core partitions) to it. Short term scheduling is done by scanning ATL which is sorted by task priority.
  • interrupt is viewed (and used) more like asynchronous goto, rather than asynchronous procedure call. For example, there is a procedure that scans ATL (active task list, sorted by task priority). Interrupt may add task to the ATL, and so scan has to be restarted if interrupt happens during it. To achieve this, interrupt handlers do a jump to the address stored in L6TV ("Level Six Transfer Vector"). When normal task runs, L6TV points to the routine that saves registers in the task's "core partition". When ATL scan is started, L6TV is modified to point to the ATL scan procedure itself. As a result, if interrupt happens, scan will be restarted automatically. Note, that interrupt handlers do _not_ save registers in the stack (they do not create stack frame).
  • "pseudo partitions" that overlay IO memory used to control devices from non-privileged tasks.
  • RSX is a radical micro-kernel: the only thing executive handles is processor scheduling. Everything else (including processing of interrupts other than clock) is handled by tasks. One consequence of this is that executive interface is fundamentally asynchronous: most calls take event variable as an argument, and calling task may wait for call completion by doing
            WAITFOR eventvar
    This is impossible to do in UNIX where system calls are served on the kernel stack permanently attached to the process. In UNIX, kernel is a library executing in protected mode, while RSX is a collection of autonomous tasks each running with its own stack (this architecture, of course, requires very fast context switches).
  • file system as special device driver layered on top of driver for underlying stable storage device: file oriented device. Note, that this also unifies the notion of file and device, but... other way around.
  • SEEK call is used to open particular file on a devcie. After that LUN is effectively narrowed (in EMACS sense) to mean opened file.
  • all executables are known to the executive (listed in STL).
  • system image can be stored on the permanent storage (by SAVE) and booted from (warm start), like in Smalltalk or LISP system.
  • instead of having separate run-queues and wait-queues, tasks are kept in single ATL. As the only synchronization mechanism available is event variable, "scheduler" simply checks whether waiting tasks can be run while scanning ATL.
  • tasks are so light-weight that clock queue (callouts in UNIX terms) is a list of tasks rather than functions.
  • each task provides its own procedure to save its context on preempt
  • no security of any sort. Give them enough rope.


  1. Thank you for your interest in the
    archive. I'd like to include your
    commentary on the rsx-15 section
    of bitsavers, if that would be ok.

    -al kossow (bitsavers.org)

  2. Hello, Al,

    yes, it's a honour to be linked from bitsavers. :-)

    By the way, am I right, that RSX11 sources are not available publicly?

  3. > RSX11 sources are not available

    Unfortunately, DEC sold the rights to
    the PDP11, and RT11 RSX11 and RSTS
    to a company called MENTEC,
    so the sources cannot be put on line.