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In the following code, I am wondering about the role of fld ST(0). I have heard it duplicates the top value in the stack but I don't know where does it bring that value in the following code?

fild    dword ptr [L1000F140]
fstp    qword ptr [esp+20h]
fld ST(0)
fdiv    qword ptr [L1000F148]
fmul    qword ptr [L1000A520]

My interpret is that code above in symbolic way means: 

value[esp+20h]= value[L1000F140]
new stack top value=value[esp+20h]/value[L1000F148]*value[L1000A520]

Am I correct?

I wonder why fstp does not pop the loaded value so fld has duplicated value[L1000F140] it?

Peter Cordes
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barej
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1 Answers1

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There's a good guide to x87 FPU by Raymond Filiatreault. Chapter 1 explains how the FPU register stack works.

Yes, fld st(0) pushes a copy of the top of the stack. The Intel insn ref manual explicitly mentions this special-case use of fld st(n)

I believe you're right that fstp does pop after the fild.

  • The first fild / fstp pair converts a global int (at L1000F140) to a float on the stack.

  • Then fld st(0) duplicates the top of the stack (i.e. the value that was there before the fild/fstp).

  • Then divide that value by another global, and multiply by another.
  • final stack:
st(0)=orig / global1 * global2
st(1)=orig
Peter Cordes
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