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Newbie
Join Date: Dec 2004
Posts: 7
Rep Power: 0 
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I'm not new to x86 ASM, but I am new to the square-root-related instructions that Intel has provided. I don't know what I'm doing wrong in my program, but I get a divide overflow. Perhaps someone could take a look at my code and see what is causing this.
This program isn't finished yet, but it's going to draw a sphere. I am doing it purely out of experimental interest. Right now I am just trying to get it to draw pixels to the screen, but I won't know if my algorithms and such are right or not if I can't see anything but an error. I wonder if it's the way I've used the square-root. This is the first time I've made use of it in ASM, so I don't even know if I did it right or not.
Anyway, any help would be appreciated.
BTW: If you find the below code to be hard to read due to the lack of 'tabs' on these forums, you can download the source file -- http://shadowsoft-games.com/saxman/3D.ASM
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; This program is designed to draw a sphere using the following:
;
; Ymax = (Ry/Rx) SQRT(Rx^2 - X^2)
; Zmax = (Rz/Ry) SQRT(Ry^2 - Y^2)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
jmp start
xpos* dw ?*; X position (0-319)
ypos* db ?*; Y position (0-199)
zpos* db ?*; Z position (0-255)
xrad* db 64*; X radius (0-255)
yrad* db 64*; Y radius (0-255)
zrad* db 64*; Z radius (0-255)
store1 dd ?*; Used to store a floating-point
ymax* db ?*; Ymax = (Ry/Rx) SQRT(Rx^2 - X^2)
zmax* db ?*; Zmax = (Rz/Ry) SQRT(Ry^2 - Y^2)
palette db 00, 00, 00, 01, 01, 01, 02, 02, 02, 03, 03, 03
* db 04, 04, 04, 05, 05, 05, 06, 06, 06, 07, 07, 07
* db 08, 08, 08, 09, 09, 09, 10, 10, 10, 11, 11, 11
* db 12, 12, 12, 13, 13, 13, 14, 14, 14, 15, 15, 15
* db 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19
* db 20, 20, 20, 21, 21, 21, 22, 22, 22, 23, 23, 23
* db 24, 24, 24, 25, 25, 25, 26, 26, 26, 27, 27, 27
* db 28, 28, 28, 29, 29, 29, 30, 30, 30, 31, 31, 31
* db 32, 32, 32, 33, 33, 33, 34, 34, 34, 35, 35, 35
* db 36, 36, 36, 37, 37, 37, 38, 38, 38, 39, 39, 39
* db 40, 40, 40, 41, 41, 41, 42, 42, 42, 43, 43, 43
* db 44, 44, 44, 45, 45, 45, 46, 46, 46, 47, 47, 47
* db 48, 48, 48, 49, 49, 49, 50, 50, 50, 51, 51, 51
* db 52, 52, 52, 53, 53, 53, 54, 54, 54, 55, 55, 55
* db 56, 56, 56, 57, 57, 57, 58, 58, 58, 59, 59, 59
* db 60, 60, 60, 61, 61, 61, 62, 62, 62, 63, 63, 63
start:
mov ax, 13h* *; Setup 320x200 video mode
int 10h
mov ax, 0a000h* *; Set the video address
mov es, ax
write_palette:
mov si, offset palette
mov cl, 192* *; Read 192 color values (64 total colors)
mov dx, 03c8h* *; Set the palette address
xor** al, al
out dx, al
cld
inc dx
rep outsb
draw_setup:
mov di, 16000* *; Start from the center of the screen
mov dx, 63* *; DX is our color selection
call calculate_ymax
draw_y:
;mov dx, 63
mov es:[di], dx
inc ypos
sub di, 320
mov al, ymax
cmp ypos, al
jl draw_y
inc xpos
mov di, 16000
add di, xpos
call calculate_ymax
mov ax, word xrad
cmp xpos, ax
jl draw_y
calculate_ymax:
; Ymax = (Ry/Rx) SQRT(Rx^2 - X^2);
push ax
push dx
mov ax, word yrad
div xrad
mov bx, ax* *; BX acts as (Ry/Rx)
mov ax, word xrad
mul xrad
push ax
mov ax, xpos
mul xrad
pop dx
sub ax, dx
mov word store1, ax
; (Ry/Rx)
fld store1* *; Load X
fld st(0)* **; Duplicate X on TOS
fsqrt* * *; Computer SQRT(X**2 + Y**2)
fst store1* *; Store away result in Z
mov ax, word store1
mul bx
mov ymax, al
pop dx
pop ax
ret
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