Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: A3000 CPU Wars!!
Message-ID: <22301@cbmvax.commodore.com>
Date: 10 Jun 91 17:14:57 GMT
References: <16888@helios.TAMU.EDU> <22190@cbmvax.commodore.com> <25302@well.sf.ca.us>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 49

In article <25302@well.sf.ca.us> pwappy@well.sf.ca.us (Jeff Walkup) writes:

>I thought the whole idea of having the Fast Slot in the A3000 was that
>you could add co-processor boards like an '040 board, and they would use
>the motherboard's RAM and bus.  Thereby making it cheap relative to an
>A2000 co-pro. board that would need its own RAM, bus, and SCSI host.

That's correct.  To get any sort of noticable performance increase with a 
fast 32 bit coprocessor board, an A2000 coprocessor device needs its own
memory.  It can, of course, still use the motherboard memory, but only at
that memory's speed, and only 16 bits wide.  Since most 32 bit processors are
optimized for 32 bit wide data transfers, they can actually go slower with 
only 16 bit memory than the 16 bit 68000.

>SO,  the realistic speed limit for an '040 board in an A3000 sounds like
>25MHz, since going any faster means you wouldn't be able to use the
>motherboard's resources.  And likewise, if you have a 16MHz A3000, then
>16Mhz would be the limit.

>Not that you *couldn't* go faster, just that the card would be more
>expensive as it would need its own RAM, etc...

>Am I right?

No, you're confused.  But that's OK...

First of all, as mentioned about a quadrillion times in this very group, a 
coprocessor board can, if designed properly, up the motherboard clock rate of
a 16MHz A3000 to 25MHz.  This works because it was designed to work that way.
That's only speaking to how fast the motherboard clocks go.  The clock on the
coprocessor board's processor can theoretically be any rate the designer
chooses.  However, if the clock is unrelated to the motherboard clock (which,
as mentioned, can be either 16MHz or 25MHz, pick one), the designer will have
to solve the synchronization problems such a setup will inherently create.
The motherboard clock fixes the speed of the motherboard memory system, no 
matter what speed your coprocessor board's processor runs at.  A faster 
coprocessor can talk just fine to the A3000's motherboard memory (in fact,
it's required to).  And if the processor is faster, it's very likely to go
faster than a 25MHz system, regardless of the fact that the memory system
isn't going faster.  To use such a faster CPU to its fullest ability, though,
the designer can stick a full speed cache or some faster 32 bit wide memory
on the coprocessor board.



-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
	"This is my mistake.  Let me make it good." -R.E.M.