save FPU ST0 to GPR32

[blackoil]

Hi,

I want to save FPU ST0 content to GPR32

int i;
float f;

fld dword [ f ]
push eax ;placeholder
fistp dword [ esp ]
pop eax
mov [ i ], eax

is it ok?

[Troy Martin]

There are no "int" and "float" datatypes in assembly AFAIK. And I'm pretty sure ST0 is an 80-bit register, not a 32-bit one.

[Craze Frog]

Code: Select all

fld dword [ f ]
push eax ;placeholder
fistp dword [ esp ]
pop eax 
mov [ i ], eax

You're not saving the contents, you're saving the value. And it can be done better.

Code: Select all

fld dword [f]
fistp dword [esp-4] ; placeholder
mov eax, dword [esp-4]
mov dword [i], eax

I am assuming you really need to get it to the register before putting it into i.

[ru2aqare]

Code: Select all

fld dword [f]
fistp dword [esp-4] ; placeholder
mov eax, dword [esp-4]
mov dword [i], eax

I am assuming you really need to get it to the register before putting it into i.

What if there is an interrupt between fistp and mov? Then [esp-4] is thrashed. A better solution would be

Code: Select all

push eax ; push dummy value
fistp dword ptr [esp+0]
pop dword ptr [i]

[Craze Frog]

A better solution would be

Nice, except that it's slower and also doesn't save FPU ST0 to GPR32 (which was vital to the question). If all he wanted to was to get ST0 into an integer memory location he would fistp it directly to the memory location like this:

Code: Select all

fld dword [v_f]
fistp dword [v_i]

[blackoil]

well, it's part code of my compiler for something like

int i;
flloat f;

i = (int) f;

I use it to draw bezier curve, it works well on real machine.
I don't get any stack corruption, as the placeholder ensure the correct value.