help with FPU in ASM

category: code [glöplog]
Hi people, I'm trying to learn how to use the FPU but stuff isn't working out here.

This is my code, it's supposed to draw a circumference on the screen:

Code: ; Initialize video mode mov al, 13h int 10h push word 0xA000 pop es ; Initialize FPU finit draw: ;;;;;;;;;CIRCLE ;;;;;;;;; ; x = 160 + R*cos(t) y = 100 + R*sin(t) ; di = 160 + R*cos(t) + 320 * (100 + R*sin(t)) ;;;;;;;;;;;;;;;;;;;;;;;;; ;LOAD T TO FPU fld dword [_t] ;ST(0) = t ;ADD TINC TO T AND SAVE IT FOR THE NEXT POINT fld st0 ;ST(0) = t, ST(1) = t fadd dword [_tinc] ;ST(0) = t+tinc , ST(1) = t fstp dword [_t] ;ST(0) = t ;GET SIN AND COS OF T fsincos ;ST(0) = cos(t), ST(1) = sin(t) ;MULTIPLY COS BY R fmul dword [_R] ;ST(0) = R*cos(t), ST(1) = sin(t) ;GET COS INTO DI fistp dword [_pos] ;ST(0) = sin(t) mov di, [_pos] ;di = R*cos(t) ;ADD 160 TO DI add di,160 ;di = 160 + R*cos(t) ;MULTIPLY SIN BY R fmul dword [_R] ;ST(0) = R*sin(t) ;GET SIN INTO BX fistp dword [_pos] ;FPU Stack is empty mov bx,[_pos] ;BX = R*sin(t) ;ADD 100 TO BX add bx,100 ;BX = 100 + R*sin(t) ;MULTIPLY BX BY 320 imul bx,320 ;BX = 320 * (100 + R*sin(t)) ;ADD BX TO DI add di,bx ;DI = 160 + R*cos(t) + 320*(100 + R*sin(t)) ;Draw the pixel mov al,20h ;color stosb ;Draw the pixel ; Check for ESC keypress and exit in al, 60h dec al jnz draw mov al, 03h ;Select text mode and int 10h ;do the interruption ret ; Values _t: dd 0.0 _tinc: dd 0.01 _R: dd 50.0 _pos dd 0.0

Everytime I get a value from the FPU with fstp or fistp I get a 0 (zero), so the program only draws a pixel in the center of the screen.

Any help is appreciated
added on the 2012-06-19 02:44:27 by gungunner gungunner
Instead of making us read all that code, maybe you can narrow down the problem to the smallest portion that doesn't behave as you expect. (In the process, maybe you'll see the solution yourself!)
added on the 2012-06-19 04:06:36 by yesso yesso
Are you sure everything is treated as the proper data types that they are supposed to? I.e, you are not mixing up floats and doubles I hope?
The code itself is correct.
Your forgot only this:
org 100h
Without it fpu load/save addresses are wrong.
added on the 2012-06-19 06:37:48 by frag frag
what assembler do you use?
Code: . . . fistp dword [_pos] ;ST(0) = sin(t) mov di, [_pos] ;di = R*cos(t) . . . fistp dword [_pos] ;FPU Stack is empty mov bx,[_pos] ;BX = R*sin(t) . . .

as you see you try to drop dwords into words. that won't bring the correct result.

Code: . . . fistp word [tester] ;ST(0) = sin(t) mov di, [tester] ;di = R*cos(t) . . . fistp word [tester] ;FPU Stack is empty mov bx,[tester] ;BX = R*sin(t) . . . ; Values

tester dw ?

here you have your blue circle (=

@frag: really? i did not get it running under fasm. sizes won't match on the parts above in the original code. hm.
@sensenstahl really.
>as you see you try to drop dwords into words. that won't bring the correct result.
It's only address, dosent't matter here, if you load store the same 4-byte value it should work.
added on the 2012-06-19 06:43:42 by frag frag
fix: ... the same 2-byte value...
added on the 2012-06-19 06:44:57 by frag frag
indeed (=
frag, for god sake, if you have an ability to speak asm. Go work for Kaspersky. They need dedicated coder analysts like you are.

And frag, most advanced virus ever made, technically pointi of view, is 1990 made WHALE virus. Even novadays, how it works, what it does, is unknown.

I'll give you 200 euros via paypal, if you can disasseble it and make a paper what it does. This virus was like a stuxnet or flame. Far ahead of its time. It was piece of an art.

added on the 2012-06-19 20:37:32 by moredhel moredhel
oh wow, it was the org 100h

I thought that was done automatically when assembling with nasm and it never gave me a problem with other stuff

thanks a lot frag!

BB Image
added on the 2012-06-19 21:20:43 by gungunner gungunner
You made a circle!

Anyway, just wanted to say the FISTP instruction should have been censored... it's so dirty...
added on the 2012-06-19 23:14:47 by Photon Photon
as kb_ once put it eloquently "fist dword ptr [me]". :)
added on the 2012-06-20 01:24:52 by trc_wm trc_wm
oops.. sorry kb_ .. it was gaffer..
added on the 2012-06-20 01:25:51 by trc_wm trc_wm