In the late 60s, the Incompatible Timesharing System ran on a PDP-6 without any paging hardware (i.e. an MMU in modern terminology). See the Jargon entry for BLT, for example, which states (my emphasis):

... one resource-intensive memory-shuffling operation done on pre-paged versions of ITS, WAITS, and TOPS-10 was sardonically referred to as "The Big BLT".

It was later moved onto a PDP-10 and, sometime around 1970, a paging system was installed and ITS was modified to use virtual memory.

The MIT Dynamic Modeling group adopted ITS for their PDP-6 and PDP-10 (named MIT-DMS). Initially, there was no disk to provide swapping store, so they ran a non-paged ITS until around 1973.

There is a copy of ITS for the AI lab PDP-10 from 1971, but there is no trace of support for running on a PDP-6 or any possibility of running without virtual memory.

I believe the majority of software development in the non-paging era was done using DECtape so, if an pre-historic ITS was to be found, it would probably be on such a tape. Another possibility is paper tape, since this was often used to load software for booting.

Does anyone know where this (a non-paging ITS) could be found?

  • 3
    I loved ITS. I think the AI ITS system had a passing box by 1969. It was made by the "systems concepts" company and had a small cache of mappings. An indirect reference chain could easily be inevitable. The page table cache was reloaded by system software -- no hardware page table walks. I didn't know that Dynamod didn't have paint hardware.
    – cmm
    Commented Sep 12, 2018 at 2:13
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    I worked in DM during part of that time frame. I remember a KA-10 with a small disk connected. A lot of source code was kept on DECtape, if only for backup. The KA-10 had dual protection and relocation, which is not the same as paging. I have no recollection of DM ever having its own PDP-6. It's more likely that they used the AI PDP-6. Commented Oct 24, 2018 at 15:52
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    Please elaborate. The earliest ITS version I know is 724 from 1972, and the binary file is 44K words. What length paper tape is that, and how long would it take to load? I would assume the PDP-6 ITS would be much smaller. Let's pull a number from thin air: maybe 10K words? Commented Oct 26, 2018 at 7:54
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    FWIW, I bumped into a page which says "You won't find ITS sources to run on a PDP-6 or non-paged KA. Such sources don't exist any more". I don't know whether that's authoritative. Link: inwap.com/pdp10/cpus.txt
    – dave
    Commented Nov 16, 2018 at 23:48
  • 1
    It was written by Mark Crispin, so it is pretty authoritative. But I refuse to give up. Commented Nov 18, 2018 at 20:36

3 Answers 3


This is a frame from MIT AI film #43. I'm not sure what year it's from, but it seems to be from the PDP-6 era. The code is very similar to old ITS source code, so it's likely this is a tiny fragment of PDP-6 ITS. The use of "↓₁₄" identifies this as text displayed by PDP-6 TECO.

MIT AI film #43

Transcription, including a few more lines visible in the film:

        LDB B,[MLU,,MEMBLT(A)]
        DPB B,[MLO,,MEMBLT(E)]
        LDB E,[MLU,,MEMBLT(A)]
        CAIE B,1777
        DPB E,[MLU,,MEMBLT(B)]
        SOS MEMFR
        JRST POPJ1

        DPB B,[MLO,,MEMBLT(A)]
        DPB A,[MLU,,MEMBLT(B)]
        MOVEI B,1777
        DPB B,[MLU,,MEMBLT(A)]
        MOVEI B,75
        DPB B,[MUR,,MEMBLT(A)]
        AOS MEMFR
        POPJ P,
        PUSHJ P,UFLS
        MOVEI B,TIOP(A)
        CAMN B,TIBEP(A)
        SUBI B,TIBL
        HRRM B,TIOP(A)
        ILDB B,TIOP(A)
        MOVEM B,(C)
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    It is customary on StackExchange to transcribe images into text, because image sources disappear, some people have visual impairments, and others want to be able to copy/paste. Can you please do that? Then you may use that text to address Wilson's concern.
    – DrSheldon
    Commented Feb 13, 2019 at 2:02
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    Wilson, first grepped all files I have collected from the ITS systems. The monitor itself was the only good match, so I compared it against ITS 785 which is the oldest source code I know of. Symbol names and labels match, though the code has been rearranged. Commented Feb 13, 2019 at 6:28
  • So you have a copy of the ITS source code? I thought I'd read somewhere that it had been lost to obscurity. Commented Feb 13, 2019 at 10:24
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    Oh, not at all! See here: github.com/PDP-10/its Commented Feb 13, 2019 at 12:25
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    Arguably, ITS was "lost" to obscurity since its inception. :-) But it's very much alive and kicking. Commented Feb 26, 2020 at 7:40

Another piece of tantalizing evidence has surfaced. It's an old ITS file called RG; FD 4/1/68. The name obviously suggests a dating, and it's before a pager was installed. The file contents are listings from DECtapes. The files listed include LISP, DDT, TECO, MIDAS, and ITS versions in the three hundreds.

  • Do you have a link for this?
    – cjs
    Commented Mar 3, 2020 at 15:15
  • @cjs, sorry not yet. Commented Mar 3, 2020 at 16:10

Here's another clue. It seems Gerald Sussman has a printout of ITS from 1967.

The[n] he pulled out a printout that he handled as if it were an original Gutenberg bible. It was the original code for the ITS operating system — the Incompatible Timesharing system. It even seemed awesome to me. Today we just accept the fact that a mainframe computer can be in the basement of a building and desktop terminals can be spread throughout a building, or throughout the world, providing anyone with a terminal, access to the computer. It is hard to realize that as late as the 1960s this wasn't the case. If you wanted computer time, you wrote your program, took it to the computer room, and had it run oftentimes by someone else, even if you wanted to do it yourself.

It is a tribute to the modesty and especially to the irreverence of the early hackers that when they created the world's first timesharing system at MIT, they provided it with a name that was completely opposite from what it really was: the Incompatible Timesharing System.

There was Gerry Sussman holding that half-inch printout, a blessed relic from 1967. The machine has a Moby memory, and as if reciting from the opening words of the Great American novel, Sussman held the printout in his hands and said, "The first line of it is, Moby is One." It was written as Moby = 1, but the way Sussman read it, there was the sound of poetry and philosophy, as existential a piece of haiku as I have ever heard: Moby is One. "These days such an operating system would have maybe one hundred times as much code. But this is what it was like then, filled with some very beautiful code — and some very ugly code. Some of it is humorous, and some of it is sad. Some of it had ideas in it that have been lost. Some historian could go through it someday and read it the way people read the Dead Sea Scrolls and say, 'Aha, The guys who wrote this were real smart guys. And some of the things they knew, we don't even know now.'"

I contacted Sussman who confirms he still have the listing, which is ITS version 138. So the answer to the question is: YES!

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