wikipedia article

Johnny Billquist bqt at
Tue Jun 13 22:51:04 UTC 2006

Per Fogelström wrote:
> On Tuesday 13 June 2006 14:23, Rick Kelly wrote:
>>Johnny Billquist said:
>>>>There's actually a cheesy way to do demand paging with microprocessors
>>>>that don't support demand paging (such as the original 68000--another
>>>>"16 bit" machine).  The way to do this is to run two processors in
>>>>parallel but skewed by one instruction.  If the first one does a bad
>>>>memory fetch, then the second one will not have fetched the instruction
>>>>causing the fault so contains restartable machine state.  Masscomp sold
>>>>a machine like this once.
>>>Didn't the first Apollos do this?
>>And also the Sun 1.
> IIRC it was simpler than that. When the first cpu caused a 'miss' it was put
> in wait and cpu 2 handled the pagein and then released cpu 1. Keeping the two
> cpus synched, one instruction apart would have been too complicated if not
> impossible...

Your idea will not work, as far as I can tell.
If the first CPU instruction execution causes a miss, the end result in 
the CPU will be pretty undefined, and you cannot restart. That's the 
whole point in why you'd have a second CPU shadowing the first one. So 
that you'd be able to restore the state as it were before the illegal 
memory access.
And that was the problem with the original 68000. On an illegal memory 
reference, you would not know what state the CPU was in before the 
instruction, so you could not back it up, and re-execute the instruction 
after a page fault.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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