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From: ebergman@isis.cs.du.edu (Eric Bergman-Terrell)
Newsgroups: comp.windows.ms.programmer
Subject: Windows and Floating Point
Message-ID: <1991Jun3.045252.5431@mnemosyne.cs.du.edu>
Date: 3 Jun 91 04:52:52 GMT
Sender: usenet@mnemosyne.cs.du.edu (netnews admin account)
Reply-To: ebergman@isis.cs.du.edu (Eric Bergman-Terrell)
Organization: Nyx, Public Access Unix (sponsored by U. of Denver Math/CS dept.)
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Disclaimer1: Nyx is a public access Unix system run by the University of
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I'm working on a Windows program that does a lot of floating point 
calculations.  I'm using BC++ 2.0 on my 386SX which does have a 387SX
math coprocessor.

The Problem:  If I leave the program running and shell out to DOS and then
come back, the Windows program usually crashes, sometimes with a UAE, and
sometimes with a message like "coprocessor stack underflow".

What's even stranger is that I didn't do any floating point while in DOS-land.

So I tried the following:  in my main event loop (switch statement from HELL)
I call a routine to reset the 8x87 upon receipt of WM_KILLFOCUS and
WM_SETFOCUS messages.  The routine is just:

asm fwait
asm finit
asm fwait

Result:  I haven't been able to make the problem recur.  Even if I shell out
to DOS in the middle of a computation, the program doesn't crash when I get
back.

SO:  is Windows forgetting to save/restore the coprocessor state when the
DOS shell is started?  Or is this the responsibility of the programmer?
Furthermore, is it safe to simply call the finit instruction?  I'm assuming
that on a machine lacking a math chip the instruction will cause the 
corresponding emulation routine to be called.  I'll find out tomorrow when
I try the prog. on a coprocessorless machine...

Furthermore, is the Windows floating point emulation library documented 
ANYWHERE?  I have Petzold, Prog. Ref., Guide to Programming, and the DOCS
for BC++ 2.0.  Haven't found any coverage of the emul library anywhere
...


Terrell