[TUHS] Any oldtimers remember anything about the KS11 on the -11/20?

[Noel Chiappa]

This is an appeal to the few really-old-timers (i.e. who used the PDP-11/20
version of Unix) on the list to see if they remember _anything_ of the KS11
memory mapping unit used on that machine.

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.

I don't expect documentation, but I am hoping someone will remember
_basically_ what it did. My original guess as to its functionality, from that
page, was that it's not part of the CPU, but a UNIBUS device, placed between
the UNIBUS leaving the CPU, and the rest of the the bus, which perhaps mapped
addresses around (and definitely limited 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 UNIBUS higher addresses, where 'extra'
memory is configured - but that's just a guess; but it is an example of the
kind of info I'd like to find out about it - just the briefest of high-level
descriptions would be an improvement on what little we have now!

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.)

Even extremely dim memories will be an improvement on the blank canvas we
have now!

	Noel