Path: utzoo!attcan!uunet!cbmvax!jesup
From: jesup@cbmvax.UUCP (Randell Jesup)
Newsgroups: comp.arch
Subject: Re: dedicated vs general-purpose CPUs
Message-ID: <4438@cbmvax.UUCP>
Date: 8 Aug 88 18:24:32 GMT
References: <5254@june.cs.washington.edu> <76700032@p.cs.uiuc.edu> <1988Aug3.180947.12070@utzoo.uucp> <1221@ficc.UUCP> <1216@nud.UUCP>
Reply-To: jesup@cbmvax.UUCP (Randell Jesup)
Organization: Commodore Technology, West Chester, PA
Lines: 34

In article <1216@nud.UUCP> df@nud.UUCP (Dale Farnsworth) writes:
>Yes, that does work, but what Henry has been saying is that when you
>run out of MIPS, stick in another GP processor as an additional main CPU.
>That way you can dynamically assign functions to processors rather than
>having them "hard" partitioned.

	This can work, but is not always optimal.  It depends heavily on
either seperate address spaces or very fast shared memory.

>Modern processors typically contain a superset of the performance and
>functionality of typical graphics accelerators chips.  I believe that
>we are near the point (if we haven't already crossed it) where the GP
>chip beats the special purpose chip in price as well as performance.

	What is a typical "graphics accelerator chip"?  Very few people make
such things, since it requires a foundry and silicon expertise.  You're
right, add-on accelerators rarely produce big improvements, for a number of
reasons.  One is that often the software architecture isn't suited for
hardware assist, or the chip was seen as a stopgap, and is thus often very
simple, and doesn't help with much.  An example is the (still not released,
I believe) Atari blitter.  It was just a rectangle-copy chip, no special ops,
nothing else.  Compare this to the amiga chips, where the blitter was part of
the original design.  The blitter has 256 operations (3-source,1 dest), can
also do line-draw and fills, and is only a small part of chip it's on, and
shares hardware with other special purpose functions of the chips, like
dma channel addressing and arbitration.

	Custom hardware can do operations in a cycle that even the best risc
cpus will take several to do, since they are general purpose.  In some cases
a general purpose CPU is fine (memory transfer is usually reasonable), but
not for all uses can it keep up with custom harware.

-- 
Randell Jesup, Commodore Engineering {uunet|rutgers|allegra}!cbmvax!jesup