In the early days (late 60s), the Incompatible Timesharing System ran on a PDP-6 without any paging hardware (i.e. an MMU in modern terminology). (See "The Big BLT".) It was later moved onto a PDP-10, and around 1970 a pager 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 MIT-AI and MIT-ML (but not MIT-DMS) from 1973, 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.

Would you please check your archives? Thanks a lot!

  • 2
    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 Sep 12 at 2:13
  • 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. – Walter Mitty Oct 24 at 15:52
  • ITS was too big to store on paper tape. – Walter Mitty Oct 24 at 15:52
  • 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? – Lars Brinkhoff Oct 26 at 7:54
  • 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 20 hours ago

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