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From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: comp.sys.m68k,comp.sys.intel
Subject: Re: 386 vs 020 and big benchmarks (sieve)
Message-ID: <650@desint.UUCP>
Date: Sun, 19-Apr-87 21:17:25 EST
Article-I.D.: desint.650
Posted: Sun Apr 19 21:17:25 1987
Date-Received: Tue, 21-Apr-87 00:24:23 EST
References: <930@intsc.UUCP> <513@omen.UUCP> <933@intsc.UUCP> <866@oakhill.UUCP>
Reply-To: geoff@desint.UUCP (Geoff Kuenning)
Distribution: comp
Organization: Interrupt Technology Corp., Manhattan Beach, CA
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Xref: mnetor comp.sys.m68k:378 comp.sys.intel:170

In article <866@oakhill.UUCP> davet@oakhill.UUCP (Dave Trissel) writes:

> If the 386 only supports three register variables and in this tiny benchmark

As usual, Dave is right on target.  Pcc for the 386 does indeed only allow
three register variables.  Furthermore, I am currently involved in writing
an extremely carefully hand-coded assembly loop (bitblt, if you care) for
the 386.  No matter what you do, you can't get more than seven registers
for your use on the 386.  And even disregarding that, the code on the 386
is *much* uglier than the equivalent 68k (not '020) code.  For example,
the 386 only has one register (esi) that can do autoincrement loads, and
one that can do autoincrement stores.  If you want to mix autoincrements
with autodecrements (fortunately something I don't have to do), you have
to execute a special instruction (std) to switch directions.  However, in
practice, it turns out that the standard 386 instructions are often a faster
way to accomplish things than the "special" ones.  My favorite is the
special "loop" instruction.  It decrements a register and then does a
conditional branch.  This takes 11 clocks, plus pipeline-reloading time.
But if you issue a subtract-immediate instruction followed by a jump,
you will only spend 9 clocks plus pipeline-reloading.

On the other hand, the built-in memory management on the 386 is clearly a
really big plus.  Frankly, Motorola, you dropped the ball badly on that
one, going clear back to the 68k itself.  I would *much* rather have
the MMU built into the '020 than have the extra instructions you guys added
(not that I don't like them too, but I think the MMU is much more important).
And yes I realize that there are many applications that don't need an MMU.
-- 
	Geoff Kuenning   geoff@ITcorp.com   {hplabs,ihnp4}!trwrb!desint!geoff