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Whats the best way to nop the FSTP instruction?

 
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ExVault
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PostPosted: Sun Jul 21, 2013 6:58 am    Post subject: Whats the best way to nop the FSTP instruction? Reply with quote
Hello there, I am new to assembly and have one question.

Lets say you have something like this:
Code:
fstp dword ptr [some_mem]

As Intel developer manual states, the fstp instruction marks the ST(0) register as empty and increments the stack pointer (TOP) by 1.
So, as I can assume one cannot just replace this instruction with nops.
What I am doing now is:
Code:
alloc(stub,4)
...
fstp dword ptr [stub]
...
dealloc(stub)


It works fine, but I am curious is there any better way to do that?
Thank you.
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mgr.inz.Player
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PostPosted: Sun Jul 21, 2013 10:06 am    Post subject: Reply with quote
Code:
fstp st(0)

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ExVault
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PostPosted: Mon Jul 22, 2013 1:28 pm    Post subject: Reply with quote
Did not think that its possible to load a register into itself. Thx for the tip.
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mgr.inz.Player
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PostPosted: Mon Jul 22, 2013 3:53 pm    Post subject: Reply with quote
fst instruction copies the value in the ST(0) register to the destination operand. If we choose ST(0) as destination ( which means ST(0) is source and ST(0) is destination ) we do not change anything.

fstp instruction performs the same operation and then pops the register stack.


You can NOP almost any FPU instruction.
But you can not simply NOP those: fstp, fsubp, fmulp, fdivp, ...........


Sometimes it is better to leave original instruction, and then overwrite memory with desired value. You have two options:

-
Code:
newmem:
(...)
fstp dword ptr [XXXXXXXX]
mov dword ptr [XXXXXXXX],(float)5.0    // or whatever value you want
(...)



-
Code:
newmem:
(...)
fstp ST(0)
mov dword ptr [XXXXXXXX],(float)5.0    // or whatever value you want
(...)

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