I wrote:
>> The question, though, is how useful would it be.  The KA630 was
>> designed to support a multiprocessor system, ***EXCEPT*** that the
>> multiple processors do NOT share main memory.

der Mouse <mouse@Rodents.Montreal.QC.CA> wrote:
> They do if they want...with some restrictions.
>
> It's not fully symmetric sharing; the processor that owns the memory has
> to explicitly export it to the Qbus, and other processors pay a speed
> penalty (because they access it via Qbus cycles rather than
> through their own memory interconnect).
>
> It's also limited by the address space of the Qbus, which as I recall
> imposes a 22-bit (= 4MB) limit, which limit applies to the total amount
> of memory exported by all processors.  And in practice the limit is even
> less than that because you have to allow space for devices in that 4MB.

My recollection was that there was only a *small* window that could
be shared.  Substantially smaller than 4 MB, IIRC.  But I don't have the
KA630 manual handy, so perhaps I'm mistaken.

> Am I missing something?  I sure don't see anything there that makes it
> incapable of symmetric multiprocessing.  Each CPU even has its own
> console serial port, which a lot of supposedly-SMP systems don't.

That's because supposedly-SMP systems typically have shared I/O, just
like they have shared memory.  If they have independent I/O, that's
actually *less* symmetric, because if a process wants to talk to an
I/O device it must be running on a particular processor.