Programming Forums > Application Development > Assembly

saxman · Dec 8th, 2004, 10:45 PM

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

(Toggle Plain Text)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 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
;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; 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
;

kurifu · Dec 9th, 2004, 12:09 PM

When you divide with asm this is what happens.

You take usually a 32bit value (though it can be 16bit, or 8bit) perform the division and store both the result and the remainder (no decimal values here) into the same block of data we performed the operation on.

Which means dividing a 32bit value results in that 32bit value being broken into a 16bit result and a 16bit remainder. I am sure you can see where this goes wrong now...

What happens if you take a 32bit value, which requires all 32 bits and divide that value by 1? Well as you can imagine the result WILL NOT fit in the 16 bit register in which it is supposed to fit. One of your flags should be raised for this exception, probably OF, you will need to verify that and start checking that flag to make sure that the division occured without error and deal with the error when it occures appropriatly.

Dec 9th, 2004, 12:09 PM	#2
kurifu Expert Programmer Join Date: Jul 2004 Location: Halifax, Nova Scotia (Canada) Posts: 784 Rep Power: 5	When you divide with asm this is what happens. You take usually a 32bit value (though it can be 16bit, or 8bit) perform the division and store both the result and the remainder (no decimal values here) into the same block of data we performed the operation on. Which means dividing a 32bit value results in that 32bit value being broken into a 16bit result and a 16bit remainder. I am sure you can see where this goes wrong now... What happens if you take a 32bit value, which requires all 32 bits and divide that value by 1? Well as you can imagine the result WILL NOT fit in the 16 bit register in which it is supposed to fit. One of your flags should be raised for this exception, probably OF, you will need to verify that and start checking that flag to make sure that the division occured without error and deal with the error when it occures appropriatly. __________________ Clifford Matthew Roche <geek@cliffordroche.com> Web Hosting: http://www.crd-hosting.com Consulting: http://www.crdev-consulting.com

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Dec 8th, 2004, 10:45 PM	#1
saxman Newbie Join Date: Dec 2004 Posts: 7 Rep Power: 0	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 (Toggle Plain Text) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; 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(X2 + Y2) fst store1 ; Store away result in Z mov ax, word store1 mul bx mov ymax, al pop dx pop ax ret ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; 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(X2 + Y2) 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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Divide Overflow?