FPU Questions.

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Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 22 Nov 2011, 21:10

hello everyone, recently I've started learning some FPU. I have some questions about it.
1) is there any way to convert unsigned to signed integer ? example what I mean:

Code:

;check if sqrt will be int..
x dd 90
fild dword[x]
fsqrt
fistp dword[x] ;here
fild dword[x] ;and here.
fmul st0,st0
sub esp,4
fistp dword[esp]
mov eax,dword[x]
mov ecx,dword[esp]
add esp,4
cmp eax,ecx
jne ...

2) is there any instruction that will allow me to write near integer instead of float ? for example:

Code:

;sin 90
x dd 90
fild dword[x]
fsin ;st0 = 0.8939966636005578905
fistp dword[x] ;here, x should be 1 instead of 0 and float number, because 0.8 ~= 1

3) which instruction clears fpu stack without storing it somewhere ? (fld/fild)
Thank you Wink

22 Nov 2011, 21:10

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 22 Nov 2011, 21:57

The FPU has 4 possible rounding modes that can be configured using the control word (fldcw/fstcw) and applied using the frndint instruction. In some cases you will need to adjust the value before rounding e.g. ceiling/floor add/sub 0.5 before rounding.

The FPU only has signed representation but you can load a 32bit unsigned as a 64bit signed integer by padding the upper dword with 0; otherwise you have to explicitly convert the unsigned value in the FPU.

The instructions ending with "-p" pop from the FPU stack and in some cases double pop (fcompp); there is also the fdecstp and finstp instructions.

22 Nov 2011, 21:57

AsmGuru62

Joined: 28 Jan 2004
Posts: 1620
Location: Toronto, Canada

AsmGuru62 22 Nov 2011, 22:10

FSIN parameter is in Radians (as per Intel Manual), so SIN 90 in your case is a sin(90 rads). That is why it is 0.8993...
Smile

A proper SIN(90) would be this one:

Code:

FLDPI
PUSH 2
FIDIV DWORD [ESP]
ADD ESP,4
FSIN
;
; NOW ST0 = SIN(90 degrees)
;

To pop the stack without storing is this:

Code:

FSTP ST0     ; Sweet! We're still storing!

22 Nov 2011, 22:10

Overflowz

Joined: 03 Sep 2010
Posts: 1046

Overflowz 23 Nov 2011, 07:38

Thank you guys! Wink

23 Nov 2011, 07:38

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