;---------------------
; Spiral.asm - Author unknown. Flushes screen display down the drain.

.model tiny
.code
public  _Spiral

start:
call _Spiral
mov ax,4C00h
int 21h

_Spiral proc far
push    ax bx cx dx es ds si di
push    cs
pop     ds

mov     ax,0B800h
mov     es,ax                           ;Set ES = Screen
mov     word ptr ds:[Spiral_Count],0Ch
mov     word ptr ds:[Starting_Point],0D0h

Spiral_Loop:
mov     ax,ds:[Starting_Point]
mov     ds:[Step_Count],ax

One_Step_Loop:
mov     word ptr ds:[Horiz_Line_Length],39h
mov     word ptr ds:[Vert_Line_Length],1
mov     di,798h
mov     ax,ds:[Spiral_Count]
mov     ds:[Line_Count],ax

Move_Horiz_Line_Left:
mov     cx,ds:[Horiz_Line_Length]
dec     cx

push    ds
push    es
pop     ds
mov     si,di
add     si,2
cld
rep     movsw
pop     ds

mov     cx,ds:[Vert_Line_Length]
push    ds
push    es
pop     ds
mov     si,di
sub     si,0A0h
mov     ax,0A2h
cld

Move_Vert_Line_Up:
movsw
sub     di,ax
sub     si,ax
loop    Move_Vert_Line_Up

Move_Horiz_Line_Right:
pop     ds
mov     cx,ds:[Horiz_Line_Length]
push    ds
push    es
pop     ds
mov     si,di
sub     si,2
std
rep     movsw
pop     ds
mov     cx,ds:[Vert_Line_Length]
inc     cx
push    ds
push    es
pop     ds
mov     si,di
add     si,0a0h
mov     ax,0A2h
std

Move_Vert_Line_Down:
movsw
add     di,ax
add     si,ax
loop Move_Vert_Line_Down

pop     ds

add     word ptr ds:[Horiz_Line_Length],2
add     word ptr ds:[Vert_Line_Length],2
dec     word ptr ds:[Line_Count]
jnz     Move_Horiz_Line_Left
dec     word ptr ds:[Step_Count]
jz      Done_One_Spiral
jmp     One_Step_Loop

Done_One_Spiral:
sub     word ptr ds:[Starting_Point],8
dec     word ptr ds:[Spiral_Count]
jz      Done_Spiral
jmp     Spiral_Loop

Done_Spiral:
pop     di si ds es dx cx bx ax
ret


Starting_Point          dw      ?
Spiral_Count            dw      ?
Horiz_Line_Length       dw      ?
Vert_Line_Length        dw      ?
Step_Count              dw      ?
Line_Count              dw      ?
endp _Spiral

end start