Path: utzoo!mnetor!uunet!mcvax!cernvax!ethz!forty2!vogel
From: vogel@forty2.UUCP (Stefan Vogel)
Newsgroups: comp.sys.ibm.pc
Subject: A Problem with the 8087 from Turbo-Pascal
Message-ID: <259@forty2.UUCP>
Date: 5 May 88 15:50:42 GMT
Reply-To: vogel@forty2.UUCP (Stefan Vogel)
Organization: Exp. Physics University Zuerich
Lines: 35
Keywords: 8087 Turbo-Pascal

While  experimentally    determining   the smallest   positive  number
representable  on the 8087,  we stepped  over a  problem with the 8087
"denormalized  operand" exception. The  following little  Turbo-Pascal
program  will  crash in the WriteLn  statement  at the  the expression
"r*10" when the value of r gets denormalized:

Var r : Real;
    Count : Integer;

Begin
   r := 1.0;
   Count := 0;
   While r > 0 Do Begin
      r := r / 2;
      Count := Succ(Count);
      WriteLn(Count,' ',r,' ',r*10);
   End
End.

Now the problem: If we replace the multiplication by  any of the other
arithmetic operations,  the program  will  terminate  normally. Why is
this happening?  Why won't the Turbo-Pascal output routines crash (the
denormalized  values of   r are   printed!)?  What   is  special  with
multiplication? By the way, this happens in TP3 and TP4! I looked into
what I think  is the initialization code for  the 8087  in the TP4 run
time  library.  There  the "denormalized operand"  exception is masked
off!

Are we just stupid, or is this a real problem? Of Turbo-Pascal? Or the
8087 chip?
-- 
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