On Thu, Dec 8, 2016 at 5:39 AM, Joerg Schilling &lt;schily(a)schily.net&gt; wrote: ​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 did. 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 stepping. Clem