Date:	Fri, 22 Jan 93 08:34:46 MET
From:	MAHB AT IBM DOT RZ DOT TU-CLAUSTHAL DOT DE
Subject:	coprocessor exception
To:	djgpp AT sun DOT soe DOT clarkson DOT edu

Consider the following program:

       /* this is test.c */
       int main(){
         double x,y;
         x = 0.0;
         y = 1.0/x;
         return 0;
       }

Obviously this program should produce an exception. But this does *not*
happen.
Running the code under debug32 gives the following output.

      c:/djgpp/bin/debug32.exe version 1.09 Copyright (C) 1991 DJ Delorie
      4064+4096+0 = 8160
      XMS driver detected
      356 Kb conventional, 7360 Kb extended - 7716 Kb total RAM available
      Reading symbols...
      Building symbol trees... DONE
      >> n
      _main() (test.c#1): int main(){
      00001070: 55             push    ebp                 ebp=00000000
      00001071: 89e5           mov     ebp,esp             esp=7ffffd30
      00001073: 83ec10         sub     esp,0x10            esp=7ffffd30
      00001076: e821000000     call    ___main
      #3:   x = 0.0;
      0000107b: c745f800000000 mov     [ebp-8],0           ebp=7ffffd30
      00001082: c745fc00000000 mov     [ebp-4],0           ebp=7ffffd30
      #4:   y = 1.0/x;
      00001089: d9e8           fld1
      0000108b: dc75f8         fdivq   [ebp-8]             ebp=7ffffd30
      >> n
      0000108e: dd5df0         fstpq   [ebp-16]            ebp=7ffffd30
      >> npx
      Control: 0xffff037a  Status: 0xffffb884  Tag: 0xffff3fff
      st(0)  + 8000 0000 0000 0000 e 3fff    Valid  1
      st(1)  + 0080 0200 4000 8000 e 4140    Empty
      st(2)  - 488c 0001 4a20 8408 e 48c4    Empty
      st(3)  + c018 0100 0110 0110 e 41a0    Empty
      st(4)  - 0400 4400 2400 0100 e 4015    Empty
      st(5)  + 0000 0000 0000 0000 e 0000    Empty
      st(6)  + a000 0000 0000 0000 e 4002    Empty
      st(7)  - 8000 0000 0000 0000 e 7fff    Empty
      #5:   return 0;
      00001091: 31c0           xor     eax,eax             eax=00001174
      00001093: eb03           jmp     _main+40 (0x1098 )
      #6: }
      00001098: c9             leave
      00001099: c3             ret
      0000105a: 83c40c         add     esp,0xc             esp=7ffffd38
      0000105d: 50             push    eax                 eax=00000000
      0000105e: e865010000     call    _exit
      Program exited normally, return code 0 (0x0)
      00001542: cd21           int     0x21
      XMS memory freed

As you can see, the fdiv instruction sets the coprocessor
status word correctly (division by zero is indicated), the zero
division flag in the coprocessor control word is *not* masked!
So, why isn't there an exception? Any suggestions?

Henning Behnke

- Raw text -