Path: utzoo!attcan!uunet!mailrus!iuvax!maytag!watstat.waterloo.edu!dmurdoch
From: dmurdoch@watstat.waterloo.edu (Duncan Murdoch)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Faster 387s and 486 timing (was: Cyrix CX-803D87-20 Coprocessor)
Message-ID: <1990Oct1.163030.3394@maytag.waterloo.edu>
Date: 1 Oct 90 16:30:30 GMT
References: <14244@shlump.nac.dec.com> <1990Aug4.220844.7349@water.waterloo.edu> <128@thor.UUCP> <1990Sep27.060417.23408@agate.berkeley.edu> <1969@sixhub.UUCP>
Sender: daemon@maytag.waterloo.edu (Admin)
Organization: University of Waterloo
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In article <1969@sixhub.UUCP> davidsen@sixhub.UUCP (bill davidsen) writes:
>
>  I believe that was at 25MHz, but not 33MHz. Intel seems to have
>changed the mask in the 33MHz version, so that it is faster by a good
>bit. I believe several tests have shown this, including the recent one
>in either _PC Week_ or _Info World_. I have been told that putting the
>33MHz version in a 25MHz system will be about 40% faster, due to some
>instructions taking fewer cycles. The people who told me this believed
>it enough to buy the fa$ter part.

Do you know the old rated time for a 387?  I've noticed huge drops in cycles
going from an 8087 to an 80486 (e.g. FLD, FADD and FMUL are 80-100 cycles
on the 8087, and are 3, 10, and 16 cycles on the 486).  When did these
happen?

A somewhat related question:  the 486 manuals I've got (which were the source
for the above) have a column called "Concurrent Execution" in the table
giving instruction timings, but only for the floating point instructions,
and as far as I can tell, it's mentioned nowhere in the text.  I'd guess
it's a bit of sloppy merging of the 386 and 387 manuals that lost the
description.  

Does anyone know what the numbers mean?  (Hint:  not all
instructions get entries:  e.g. FLD, FST, FIST, FXAM and FLD1 don't, while 
FILD, FCOM, and FLDPI do.  Those that do generally get a number somewhat
less than the main clock count entry.)

Duncan Murdoch
dmurdoch@watstat.waterloo.edu