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Thread: C on PDP-11 questions...

  1. #1

    Default C on PDP-11 questions...

    Oscar has his fine PiDP-11 being sold in kits to many and I've been looking at his page about it. It looks like they have Unix 6 and Unix 7 in it which I am presuming has a C compiler.

    I've done some reading and it sounds like the PDP-11 can access 64KB, but had memory extensions to handle 256KB or possibly 4MB as well.

    My question is, how much could the C compiler in Unix 6/7 handle? Was it limited to a memory model that was 64KB or could it access more? Were their larger memory models for programs that needed to be larger than 64KB?

    Which takes me to question #2 - what was the first plaform that could handle more than 64KB for C?

    CP/M C I would think would have that same 64KB memory model.

    Was it C under MS-DOS? Surely there was computers before this that could do larger than 64KB programs??

  2. #2
    Join Date
    May 2009
    Blog Entries


    Depends on the PDP-11 model. A user process can have up to 128 kB of memory split between code and data; roughly equivalent to MS-DOS small model. Multiple processes would be available to the big memory systems though each process retained smaller limits. The super cheap 11/03 only supported a maximum of 64 kB (32 kW) rather limiting its capabilities. I think there were some that could support more than 64k but limited each process to 64k. I don't remember the model numbers off hand.

    By about 1980, vi was no longer able to fit in the 128 kB address region. Many computers could support larger than 128 kB programs before MS-DOS came around. VAX (PDP-11/780) started in 1977 and that was a relative late comer.
    Last edited by krebizfan; October 10th, 2018 at 05:07 PM.

  3. #3
    Join Date
    Sep 2006
    Silicon Valley


    "what was the first plaform that could handle more than 64KB for C?"

    The Unix port using the Portable C Compiler to the Interdata 8/32 done internally in AT&T
    That code has been lost.
    The port of V6 to the Interdata 7/32 at the University of Wollongong ca. 1976/77 occured at roughly the
    same time.
    Last edited by Al Kossow; October 10th, 2018 at 06:01 PM.

  4. #4


    Code overlays were also supported by some language runtimes. On the later PDP-11 models where kernel/supervisor/user spaces were supported, BSD 2.11 used supervisor space to split off the networking code from the remainder of the kernel.

  5. #5
    Join Date
    Dec 2012
    Portland Oregon


    I know that Ultrix 3.1 includes a complete C compiler and full source code. Ultrix uses overlays and the programming manual explains how to use the included tools to manage the 64kb segments. There is a lot of work needed to bring it up to speed, but it looks like an interesting project.


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