floating point asm

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2 total messages Started by Spine Wed, 05 Sep 2007 13:37

#31

Author: Spine
Date: Wed, 05 Sep 2007 13:37

26 lines
1411 bytes

Hi,

I am just trying a bit around using assembler, and run into problems.

just multiplying integers a and b to res as result. this works fine.
 int a = 2,b = 3,res;
 for (int i=0;i<100;i++) {
  __asm{
   mov eax, a
   mov ecx, b
   mul ecx
   mov res, eax
  } }

then why does this not work? that is, it works fine for 4 iterations or so, 
then it comes up with illegal flaoting point operation. even if I only have 
"fld d_a" it throws the error.
 double d_a = 1.1, d_b = 1.2, d_res;
 for (int i=0;i<100;i++) {
  __asm{
   fld d_a
   fmul d_b
   fst d_res
  } }

Sipke

Re: floating point asm

#40

Author: Michael
Date: Sat, 12 Jan 2008 09:50

35 lines
1995 bytes

Spine wrote:
> Hi,
> 
> I am just trying a bit around using assembler, and run into problems.
> 
> just multiplying integers a and b to res as result. this works fine.
>  int a = 2,b = 3,res;
>  for (int i=0;i<100;i++) {
>   __asm{
>    mov eax, a
>    mov ecx, b
>    mul ecx
>    mov res, eax
>   } }
> 
> then why does this not work? that is, it works fine for 4 iterations or so, 
> then it comes up with illegal flaoting point operation. even if I only have 
> "fld d_a" it throws the error.
>  double d_a = 1.1, d_b = 1.2, d_res;
>  for (int i=0;i<100;i++) {
>   __asm{
>    fld d_a
>    fmul d_b
>    fst d_res
>   } }
> 
> Sipke 


Remember that x86 floating point assembler works on a stack, and stacks 
can overflow.  The code you have is pushing values onto the floating 
point stack, but never popping them back off.  Try changing the "fst" to 
an "fstp", and see if it works.

- Michael

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