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From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: comp.sys.m68k,comp.sys.intel
Subject: Re: small instruction cache (was: Recent Motorola ad seen in Byte)
Message-ID: <556@desint.UUCP>
Date: Thu, 16-Apr-87 14:43:02 EST
Article-I.D.: desint.556
Posted: Thu Apr 16 14:43:02 1987
Date-Received: Sun, 19-Apr-87 12:58:05 EST
References: <362@sbcs.UUCP> <1466@ncr-sd.SanDiego.NCR.COM> <580@plx.UUCP> <513@omen.UUCP> <285@winchester.mips.UUCP> <518@omen.UUCP>
Reply-To: geoff@desint.UUCP (Geoff Kuenning)
Organization: Interrupt Technology Corp., Manhattan Beach, CA
Lines: 28
Keywords: Sieve, 68020 vs. 80386, Cray, CDC 6600, CDC 7600, cache
Summary: the cdc 6600 had a TINY but useful cache
Xref: mnetor comp.sys.m68k:365 comp.sys.intel:161

In article <518@omen.UUCP> caf@.UUCP (PUT YOUR NAME HERE) writes:

> Does somebody have real data on how the 68020's cache improves performance
> on sieve, troff, sort, and other Unix CPU hogs?

Well, I don't know about UNIX codes, but data from the old days of
supercomputers is quite relevant.

The CDC 6600 had what was called an "instruction stack" of about 8 words,
or a maximum of 32 instructions.  The 7600 expanded this to about 25 words
(I'm sure I'm off on the exact numbers), or about 100 instructions.  These
were not true caches;  they were only useful on loops that had backwards
branches.

The "stacks" worked by saving the last n words of instructions executed.
As long as you ran straight-line code, you didn't get any advantage.  However,
if you branched backwards (looped), the machine would try to find the
branch target in the "stack" (cache) and would then run out of cache for
the duration of the loop.

There is absolutely NO question that these caches improved performance on
many useful Fortran applications programs.  In fact, the 7600's instruction
stack was one of the causes of the downfall of the CDC STAR-100, because
it turned out that an in-stack hand-coded vector loop on the 7600 could
outrun the Star's special vector instructions.
-- 

	Geoff Kuenning   geoff@ITcorp.com   {hplabs,ihnp4}!trwrb!desint!geoff