[TUHS] Moto and MMU issues -- was Unix & Memory Management Units (MMU)

Clem Cole clemc at ccc.com
Fri Dec 9 02:29:14 AEST 2016

On Thu, Dec 8, 2016 at 5:39 AM, Joerg Schilling <schily at schily.net> wrote:

> Back to the Bourne Shell:
> The original Bourne Shell did not use malloc, but rather had a SIGSEGV
> handler
> that used to extend the "string stack" called "stak" via sbrk() whenever
> the
> code tried to access data beyond the end ot the heap.

​Right although the 68K was not the first or the only system to have that
problem - again IIRC Series/1 and maybe one of the PE systems.  You are
correct then SunOS and >>some<< of the 68000 based system did have the
problem you suggested.  And in fact, Masscomp (and Apollo) which used
68000's (but used two of them so could run full VM) could survive that
style of fault (because it never occurred).

BTW: the "conceptual problem" , you mentioned while true, is being a little
harsh.   As the one of the 68K designers (Les Crudele) said to me once,
when they wrote the microcode, there were not thinking about instruction
restart - so the issue was that Nick did not save enough state to do it.

The model for the 68000 that they were using was the base/limit PDP-11 and
the original PDP-10.  Also at the time, the competition was the 6800, the
8080, Z80, 6502.   They were trying to put a PDP-11/70 on chip (without
getting into trouble like CalData did - which Les, Nick and team were very
aware having been DEC and CalData customers before they were at Moto].
While we think of the 68000 family has being 32 bit, the fact is inside it
is a 16 bit system (the barrel shifter and execution functions are 16).
And it was a skunk works project -- the 6809 was Moto's real processor.
It was an experiment done by a couple of rogue engineers that said - hey we
can do better,   The fact was they made a chip that was good enough to
actually compete with the Vax in the end, because it was fast enough AND
they had the wisdom to define 32 bits of address space and 32 bit
operations (again having been PDP-11 users), but as Les used to say - part
of the thinking was that while DEC was moving to the Vax, they had hoped to
break into the area that they 16 bits minis claimed - which they in-fact

And if you think in terms of a Clay Christensen's disruption theory, the
fact that VM did not work easily (i.e. was a "worse" technology than the
Vax) was ok - a new breed of customer did not care.  68000 was huge
success, despite Moto marketing ;-)

To me the larger issue with the 68010 was that when Nick did add the
restart microcode, the new '10 microcode actually dumped version dependant
state on the external stack (in Intel terminology -different "step" '10 put
different state on the external stack or worse, could not restart an
instruction that had been saved from a different step processor).

This screw up was a huge issue when we did replaced the "executor" with a
68010, because it meant that all cpu boards had to be the same processor
microcode revision. Masscomp was of course the first to make an MP, so was
the the first firm to run into the issue (I remember debugging it - we
could not reproduce the issue because of course tjt and my own machine's by
chance had "MPU" boards as we called them with the same step -- it was one
of the field service guys that realized that the customer system had a
mixed step board -- i.e. when they replaced a single MPU in the field, the
system stopped working).  IIRC:  Moto never fixed the '10, as that
processor was reasonably short lived in the open market.   They did fix the
microcode in the '20 so the state on the external stack was independent of

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://minnie.tuhs.org/pipermail/tuhs/attachments/20161208/a7a937ff/attachment.html>

More information about the TUHS mailing list