when to use FWAIT?

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ProphetOfDoom

Joined: 08 Aug 2008
Posts: 120
Location: UK

ProphetOfDoom 07 Dec 2010, 23:52

Hi,

I've been googling but can't find an answer to my question. The Simply FPU tutorial seems a bit vague on the subject. Do I need an FWAIT instruction in this code?

Code:

push 1.5 ; push any old float
fadd dword [esp] ; add [esp] to st
add esp, 4 ; restore the old value of esp

In other words, is there a danger that the third instruction might finish executing before the second one?

Thanks in advance.

07 Dec 2010, 23:52

nop

Joined: 01 Sep 2008
Posts: 165
Location: right here left there

nop 08 Dec 2010, 04:54

afaik the answer to both questions is no

afaik all potential synchronisation conflicts between cpu and fpu are handled automatically in modern processors except where exceptions are raised and flag testing instructions are involved. thats where wait or fwait is needed

here for a conflict to arise the cpu would have to finish the 3rd instruction before the fpu had read the operand data of the 2nd instruction. when the fpu actually finishes the fadd instruction is not critical (except maybe for any subseqent compare instructions handled by the cpu)

but at the end of the day only thorough testing of your code will prove it one way or the other

08 Dec 2010, 04:54

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 08 Dec 2010, 08:24

As nop said, WAIT checks for pending FP exceptions so you don't need it.

WAIT is documented well in offical processor manuals:

http://x86asm.net/links/index.html#Processor-Manuals

08 Dec 2010, 08:24

mindcooler

Joined: 01 Dec 2009
Posts: 423
Location: Västerås, Sweden

mindcooler 08 Dec 2010, 08:48

But you still need it at compares?

Code:

                ficomp  [ecx]
                (implicit wait)
                fstsw   ax
                fwait
                sahf

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08 Dec 2010, 08:48

ProphetOfDoom

Joined: 08 Aug 2008
Posts: 120
Location: UK

ProphetOfDoom 08 Dec 2010, 12:33

Thanks for clarifying guys.

08 Dec 2010, 12:33

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