The PDP-1x is a Digital Equipment Corporation PDP-1 computer modified to support both ones and twos complement arithmetic, and to include a memory management unit. It ran a locally developed operating system with specific facilities to represent virtualized devices and inter-process communications. The top-level element was called a "Sphere". It contained resources (called capabilities) and execution elements. I believe the execution elements were called "processes".

There was a manual for the OS named "INSTR5 ALL". I would like to read more about the OS to see if I was just a youngster amazed by something I was unfamiliar with, or if this operating system was unique and worthy of re-examination today.

Where could I find documentation of the PDP-1x Operating System?

  • 1
    Is this doc related in any way? Probably not, but I thought it wouldn't hurt to post it just in case.
    – dave
    Commented Mar 20, 2019 at 22:10
  • This looks to predate the changes which turned the pdp1 into the pdp1x. Good for history, and I will read it. Thank you!
    – cmm
    Commented Mar 20, 2019 at 23:12
  • I'm still hoping to find the documentation for the PDP-1x OS.
    – cmm
    Commented Jul 17, 2019 at 1:05
  • I'm very interested in this, because the MIT Logo group SITS operating system for their PDP-11/45 also has capabilities and the concept of a "sphere". As far as I can see, they mean exactly the same thing. Commented Jan 23, 2020 at 6:41
  • The basic ideas for spheres, capabilities, etc were laid out in publications.csail.mit.edu/lcs/pubs/pdf/MIT-LCS-TR-023.pdf
    – John Yates
    Commented Oct 18, 2021 at 21:20

1 Answer 1


I believe this is the best place for PDP-1X documentation:

(Linking to a mirror because the main bitsavers.org is offline for the moment.)

The 1975 memo PDP35, part 5A documents spheres and capabilities:

A virtual memory space, any virtual processors (processes) that might be executing inside that memory space, plus the list of associated resources (C-list) comprise a sphere.

Interestingly, these exact concepts - spheres, processes, capabilities - were carried forward to the MIT Logo group timesharing system SITS.

  • Lars, this is exactly what I couldn't find. Thank you. -- Carl
    – cmm
    Commented Jan 23, 2020 at 11:01

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .