unix developed on 11/20 with 20 on panel or machine that just said pdp/11?

[Noel Chiappa]

    > From: Steve Malikoff

    >> According to this page that Dennis Ritchie wrote ...
    >> https://www.bell-labs.com/usr/dmr/www/picture.html

    > It states that their 11/20 had a KS-11 memory management unit, was
    > that mandatory for running v1 Unix on an 11/20?

Well, the page does say they had two -11/20's, apparently one with and one
without the KS11.

Also, ISTR that the source for the -11/20 system has been recovered from a
listing and run, and IIRC that didn't have the KS11 stuff in it (but it
might be worth checking).


Next to nothing is known of the KS11. Dennis' page "Odd Comments and
Strange Doings in Unix":

  https://www.bell-labs.com/usr/dmr/www/odd.html

has a story involving it (at the end), and that is all I've ever been able
to find out about it (if anyone has anything more, please let me know).

My original guess as to its functionality, from that, was that it's not
part of the CPU, but a UNIBUS device, which perhaps maps addresses around
(and definitely limits user access to I/O page addresses). It might also
have mapped part of the UNIBUS space which the -11/20 CPU _can_ see (i.e.
in the 0-56KB range) up to higher addresses, where 'extra' memory is
configured.

However, on re-reading that page, I see it apparently supported some sort
of user/kernel mode distinction, which might have require a tie-in to the
CPU. (But not necessarily; if there was a flop in the KS11 which stored
the 'CPU mode' bit, it might be automatically cleared on all interrupts.
Not sure how it would have handled traps, though.

I'll have to enquire on the TUHS list.

	Noel