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From: dmurdoch@watstat.waterloo.edu (Duncan Murdoch)
Newsgroups: comp.os.msdos.desqview
Subject: Re: Qemm slows floating point
Message-ID: <1991May15.145054.12574@maytag.waterloo.edu>
Date: 15 May 91 14:50:54 GMT
References: <22671@shlump.lkg.dec.com>
Sender: news@maytag.waterloo.edu (News Owner)
Distribution: na
Organization: University of Waterloo
Lines: 78

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:
>>>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.
>
>Digital Equipment Corp.         UUCP:      ...decwrl!mast.enet!reisert

Very mysterious.  I wrote a little test program in Turbo Pascal (it's attached
below), that just runs loops multiplying 16 bit integers, 32 bit integers, and
64 bit doubles.  I calibrated it under Desqview so that each loop took 5 seconds
on my 486-25 (which has the coprocessor built in).

Times under various conditions are shown below; it sure looks as though QEMM
has no effect when the floating point instructions are used, but causes a big
slowdown when they're emulated.

Duncan Murdoch

                        Time in seconds
 Type           Desqview       QEMM         Clean      Number of cycles

 Integer          5.0           4.0          4.0            4150000
 Longint          4.9           4.0          4.0             990000
 FPU Doubles      5.0           4.0          4.0            2210000
 Emul Doubles     6.4           5.2          3.7              60000

  program timecalc;

  { Times integer and floating point arithmetic }

  uses opdos;  { Object professional unit supplies the timer }

  var
    i,j : integer;
    i1,i2 : integer;
    l1,l2 : longint;
    d1,d2 : double;
    start,stop : longint;
  begin
    start := timems;

    i1 := 5;
    for i:=1 to 10000 do
      for j:=1 to 415 do
        i2 := i1*i1;
    stop := timems;
    writeln('Integers took ',(stop-start)/1000:8:3,' seconds.');

    start := timems;

    l1 := 5;
    for i:=1 to 10000 do
      for j:=1 to 99 do
        l2 := l1*l1;
    stop := timems;
    writeln('Longints took ',(stop-start)/1000:8:3,' seconds.');

    start := timems;

    d1 := 5;
    for i:=1 to 10000 do
      for j:=1 to 6 do  { use 6 for 87=n, 221 for 87=Y }
        d2 := d1*d1;
    stop := timems;
    writeln('Doubles took  ',(stop-start)/1000:8:3,' seconds.');

  end.