[TUHS] Unix for PDP11/20 w/o mmu or paging option

[Noel Chiappa]

    > From: Roland Huisman

    > I have a PDP11/20 and I would love to run an early Unix version on
    > it. ... But it seems that the earliest versions of Unix do not need the
    > extra memory.  Does anyone have RK05 disk images for these early Unix
    > versions?

Although the _kernel_ source for V1 is available:

  https://minnie.tuhs.org//cgi-bin/utree.pl?file=V1

most of the rest is missing; only 'init' and 'sh' are available. So one would
have to write almost _everything_ else. Some commands are available in PDP-11
assembler in later versions, and might be movable without _too_ much work -
but one would have to start with the assembler itself, which is luckily in
assembler.


If I were trying to run 'UNIX' on an -11/20, I think the only reasonable
choice would be MINI-UNIX:

  https://gunkies.org/wiki/MINI-UNIX

It's basically V6 UNIX with all use of the PDP-11 memory management
removed. The advantage of going MINI-UNIX is that almost all V6 source
(applications, drivers, etc) will run on it 'as is'.

It does need ~56KB of main memory. If you don't have that much on the -11/20,
LSX (links in the above) would be an option; it's very similar to MINI-UNIX,
but is trimmed down some, to allow its use on systems with less main memory.

I'm not sure if MINI-UNIX has been run on the -11/20, but it _should_ run
there; it runs on the -11/05, and the only differences between the /20 and the
/05 are that the /20 does not have the RTT instruction (and I just checked,
and MINI-UNIX doesn't use RTT), and SWAB doesn't clear the V condition code
bit. (There are other minor differences, such as OP Rn, (Rn)+ are different on
the -11/20, but that shouldn't be an issue.)

Step 1 would be to get MINI-UNIX running on an -11/20 under a simulator; links
in the above to get you there.

	Noel