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From: feg@cbnewsb.cb.att.com (forrest.e.gehrke)
Newsgroups: comp.os.msdos.desqview
Subject: Re: Qemm slows floating point
Message-ID: <1991May15.133355.20265@cbfsb.att.com>
Date: 15 May 91 13:33:55 GMT
References: <22671@shlump.lkg.dec.com>
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In article <22671@shlump.lkg.dec.com> reisert@mast.enet.dec.com (Jim Reisert) writes:
>
>In article <1991May14.142323.1929@maytag.waterloo.edu>, dmurdoch@watstat.waterloo.edu (Duncan Murdoch) writes...
>>In article <1991May14.123233.17734@cbfsb.att.com> feg@cbnewsb.cb.att.com (forrest.e.gehrke) writes:
>>>I have a Gateway 386/33 with Micronics Asic motherboard
>>>and no coprocessor.  4MB ram, 32 ram cache.
>>>
>>>When I load qemm.sys and run a Whetstone benchmark (or any
>>>floating point operations program), I find that there is
>>>about a 30% reduction in speed.
>>
>>So it appears the only solution is to buy a 387 - then your program will
>>be slower on the first pass through each instruction, but will go full
>>speed after that.
>
>This doesn't seem right.  I have a Cyrix coprocessor in my 386 box, and I
>suffer similar speed penalties as Forrest, when using programs that make
>heavy use of the coprocessor.  It must be something else.
>
>- Jim

Several people have responded directly and on this net to the effect
that QEMM is trapping a floating point exception interrupt with
each instruction, causing this slowdown.

My results using a whetstone benchmark were 202 whetstones/second
without QEMM and 154 with QEMM.  However, one of the people here at
BTL with a machine identical to mine except with an Intel 387
installed reports 2650 whetstones/second with or without QEMM.
Apparently the CPU only generates one interrupt at the beginning
and then the 387 goes its merry way without anymore interrupts
for QEMM to handle.

One person suggested compiling the source with MSC using the
parameter /Fpa which emulates the 387.  He speculates that
this will operate in the same way as having a 387 installed.
Of course, this is only useful if one has the C source for
the program.

Forrest Gehrke feg\@dodger.att.com