Re: SMP

[Phil Brutsche <pbrutsch@creighton.edu>]

A long time ago, in a galaxy far, far way, someone said...

> On Wed, 16 Feb 2000, aphro wrote:
> 
> > you sure that MB supports dual processors? it seems as if linux doesn't
> > say its a SMP capable MB at all.
> 
> Well, the machine (AcerAltos) has two slots for CPU daughterboards.
>  
> > can linux run on SMP 486s ? never heard of anyone who ever ran a dual cpu
> > 486, although ive heard of their existance..
> 
> What's so "special" about 486s so that they couldn't run in parallel?

In order to run parallel, processors need special wiring and circuitry to
handle the 'bus protocol' for synchronizing bus accesses and what not.  
Not all processors have that; it's possible that 486s don't.  That would
explain why Linux doesn't see the motherboard as dual capable - the
necessary circuitry isn't there.  The necessary circuitry would then be
provided by the second CPU card.  This is why AMDs K6, K6-II, and K6-III
(iirc[1]) don't do SMP - they lack the necessary circuitry.

Another reason why Linux doesn't see the computer as dual capable just
dawned on me: in pre-Pentium systems (and even some early Pentium systems)
there was no unified 'standard' method for the operating system to detect
and initialize the additional CPUs.

In later Pentiums Intel introduced the APIC to their CPUs and motherboard
chipsets - a way to synchronize bus accesses between processors.  AFAIK
it's one of two ways Linux can use Intel-based SMP systems (the other is
IO-APIC used on PIIs on up, and maybe PPros).

To put it succinctly, 486s (and earlier proccesors - I've heard of 8-way
386 "servers") don't have Intel's APIC, so the motherboard manufacturers
most likely had to create the equivalent themselves.  Each manufacturers'
motherboard was incompatible with everyone else.  Linux doesn't support an
'APIC' other than Intel's when run on x86 processors.

Please keep in mind that I'm currently suffering from a head cold, and am
probably not thinking straight.  Therefore take most of that with a grain
of salt.

[1] Actually, some AMD processors do support SMP - the K6 (I think it was
the K6) and the Athlon (most definitely - it uses the bus protocol of
Compaq's Alpha, called EV6).  However, there are no such systems that I've
ever heard of; no one makes a motherboard that supports dual AMD
processors, and there are no operating systems that support dual AMD
processors.

-- 
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Phil Brutsche					pbrutsch@creighton.edu

"There are two things that are infinite; Human stupidity and the
universe. And I'm not sure about the universe." - Albert Einstein