division 8086/8087 1/3 =3?

Question

I´m having a huge problem using fdiv!!!! I'm trying to divide 1/3 so this is what I do

.model small
.stack 100h 
.data 

var1 dd 1
var2 dd 3 
var3 dd 2
resultado dt 0.0

.code 

mov ax,@data 
mov ds,ax 
finit 
fild var1
fild var2
fdiv
fstp resultado
ffree 

.exit 

As you can see I move the result to resultado because I can't see the result in st(0) (I'm using tasm so I just can see variables in the debugger) the result should be 3EAAAA3A but I'm getting 40400000... that's 3!!! how is it possible?? I tried exchanging variables and nothing!!! Can you help me?? =(

Answer 1

If the FPU is as old as a 8087 you have to put a fwait instruction after the fdiv. Otherwise you may read a result from the FPU while the FPU is still executing the division and the result you write back is undefined.

This 'feature' has been removed in the 80287 FPU.

Answer 2

The presented code does not exhibit the stated problem. It calculates 0.3(3) with FDIV (it's actually FDIVP ST(1), ST, so it divides ST(1)=1.0 by ST(0)=3.0) and then stores it as as a sequence of 10 bytes (AB AA AA AA AA AA AA AA FD 3F) into resultado.