Programming Forums - Divide Overflow?

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

;