include irvine16.inc
video equ 0b800h

.data
save_di dw ?
incr dw 1
.code
.386
start proc near
	mov ax, @data
	mov ds, ax
	mov ax, video
	mov es, ax
	mov ax, 4
	int 10h         ;sets at medium resolution
        mov ah, 0ch
	int 21h
	
	xor si, si
	xor dx, dx


call paint_back
mov di, 4    ;to begin the figure at the first even line

redraw:	
	mov save_di,di
	mov al,055h

	call drawball
        
	mov ah,11h;;;check for kbd input
	int 16h
	
	jnz dosret
	

	call mypause
	mov al,0aah
	call drawball;;;erase just the ball with background color
	;call paint_back;;;not the whose screen
	mov di,save_di

	;inc di
	add di,incr
	cmp di,76
	jg negline
	cmp di,4
	jg redraw
negline:	neg incr


	jmp redraw

done:
	mov ah,1
	int 21h
dosRet:
	
	mov ax, 2
	int 10h
	mov ax, 4c00h
	int 21h
	
start endp

mypause proc

	push cx
	mov cx,32767
bup:     pusha
	popa
	loop bup
	pop cx
	ret
mypause endp

paint_back proc
	xor di, di     ;start di at first even line
	mov cx,1f40h   
	mov al, 0aah
	rep stosb
	mov di, 2000h
	mov cx,1f40h
	rep stosb
	
ret
paint_back endp

drawball proc

	mov di,save_di          ;restore the starting position
	        
	mov cx, 1
	rep stosb       ;second even line
	add di, 78
	mov cx, 3
	rep stosb
	add di, 76   ;third even line
	mov cx, 5
	rep stosb
	add di, 76
	mov cx, 3
	rep stosb
	add di, 78
	mov cx, 1
	rep stosb

	mov di,save_di
	add di, 2000h  ;first odd line
	mov cx, 1
	rep stosb
	add di, 78 ;second odd line
	mov cx, 3
	rep stosb
	add di, 76   ;third odd line
	mov cx, 5
	rep stosb
	add di, 76
	mov cx, 3
	rep stosb
	add di, 78
	mov cx, 1
	rep stosb
	ret
	drawball endp
end start