[TUHS] First Unix that could run on a PDP-11 with QBUS

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun Jul 27 13:26:39 AEST 2014

    > From: Mark Longridge <cubexyz at gmail.com>

    > I was digging around trying to figure out which Unixes would run on a
    > PDP-11 with QBUS. It seems that the very early stuff like v5 was
    > strictly UNIBUS and that the first version of Unix that supported QBUS
    > was v7m (please correct me if this is wrong).

That may or may not be true; let me explain. The 11/23 is almost
indistinguishable, in programming terms, from an 11/40. There is only one
very minor difference (which UNIX would care about) that I know of - the
11/23 does not have a hardware switch register.

Yes, UNIBUS devices can't be plugged into a QBUS, and vice versa, _but_ i)
there a programming-compatible QBUS versions of many UNIBUS devices, and ii)
there were UNIBUS-QBUS converters which actually allowed a QBUS processor to
have UNIBUS peripherals.

So I don't know which version of Unix was the first run on an 11/23 - but it
could have been almost any.

It is quite possible to run V6 on an 11/23, provided you make a very small
number of very minor changes, to avoid use of the CSWR. I have done this, and
run V6 on a simulated 11/23 (I have a short note explaining what one needs to
do, if anyone is interested.) Admittedly, this is not the same as running it
on a real 11/23, but I see no resons the latter would not be doable.

I had started in on the work needed to get V6 running on a real 11/23, which
was the (likely) need to load Unix into the machine over a serial line. WKT
has done this for V7:


but it needs a little tweaking for V6; I was about to start in on that.

    > I have hopes to eventually run a Unix on real hardware

As do a lot of us... :-)

    > It seems like DEC just didn't make a desktop that could run Bell Labs
    > Unix, e.g. we can't just grab a DEC Pro-350 and stick Unix v7 on it.

I'm not sure about that; I'd have to check into the Pro-350. If it has memory
mapping, it should not be hard.

Also, even if it doesn't have memory mapping, there was a Mini-Unix done for
PDP-11's without memory mapping; I can dig up some URLs if you're interested.
The feeling is, I gather, very similar.

    > it would be nice to eventually run a Unix with all the source code at
    > hand on a real machine.

Having done that 'back in the day', I can assure you that it doesn't feel
that different from the simulated experience (except that the latter are
noticeably faster :-).

In fact, even if/when I do have a real 11, I'll probably still mostly use the
simulator, for a variety of reasons; e.g. the ability to edit source with a
nice modern editor, etc, etc is just too nice to pass up! :-)


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