Question

How do I subtract a decimal value in assembly. IA32 (linux)

1:       mov edx, 1/2   
         sub ecx, ebx
         sub ecx, edx
         mov bal2, ecx

I tried this but it some how skips the subtraction with the decimal.
And if I enter .5 it gives me an error.

error:junk `.5′ after expression

Answer 1

As lightbulbone correctly says in his answer, you cannot work on floating point values when using general purpose registers (eax etc.). You could use the FPU, as lightbulbone suggests, but that is comparatively tedious.

Alternatively – unless your code needs to run on ancient CPUs – you can use the SSE instructions for this task, which is much more straightforward. Code similar to what you are showing might look like the following:

.data 
  OneHalf dd 0.5
  bal2 dd ?
.code
  movss xmm0, OneHalf  ; instead of mov edx, 1/2
  subss xmm1, xmm2     ; instead of sub ecx, ebx
  subss xmm1, xmm0     ; instead of sub ecx, edx
  movss bal2, xmm1     ; instead of mov bal2, ecx