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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
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
Post 22 Nov 2011, 21:10
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cod3b453



Joined: 25 Aug 2004
Posts: 619
cod3b453
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.
Post 22 Nov 2011, 21:57
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
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!
    
Post 22 Nov 2011, 22:10
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Overflowz



Joined: 03 Sep 2010
Posts: 1046
Overflowz
Thank you guys! Wink
Post 23 Nov 2011, 07:38
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