学步园

assume cs:code

code segment
start:
	mov ax,cs
	mov ds,ax
	mov si,offset setscreen
	mov ax,0
	mov es,ax
	mov di,200h
	mov cx,offset setscreenend-offset setscreen
	cld
	rep movsb

	mov ax,0
	mov es,ax
	mov word ptr es:[7ch*4],200h
	mov word ptr es:[7ch*4+2],0

	mov ax,4c00h
	int 21h
	
	;伪指令，表示下一条指令从偏移地址200H开始，正好和安装后的偏移地址相同  
	;因为如果没有ORG 200H，此中断例程被安装以后，标号所代表的地址变了，和之前编译器编译有别
	org 200h  

;ah中保存了功能号，0清屏，1改前景，2改背景，3上滚动一行
setscreen:
	jmp short set

	table dw sub1,sub2,sub3,sub4

	set:
	push bx
	cmp ah,3
	ja sret
	mov bl,ah
	mov bh,0
	add bx,bx ;dw占2个字节，乘2来取得对应位置

	
	call word ptr table[bx]

	sret:
	pop bx
	iret

	sub1:
	push bx
	push cx
	push es
	mov bx,0b800h
	mov es,bx
	mov bx,0
	mov cx,2000
	sub1s:
	mov byte ptr es:[bx],' '
	add bx,2
	loop sub1s
	pop es
	pop cx
	pop bx
	ret
	
	;改变前景色，al中保存了颜色值
	sub2:
	push bx
	push cx
	push es
	mov bx,0b800h
	mov es,bx
	mov bx,1
	mov cx,2000
	sub2s:
	add byte ptr es:[bx],11111000b
	or es:[bx],al
	add bx,2
	loop sub2s
	pop es
	pop cx
	pop bx
	ret

	;改变背景色，al中保存了颜色值
	sub3:
	push bx
	push cx
	push es
	mov cl,4
	shl al,cl
	mov bx,0b800h
	mov es,bx
	mov bx,1
	mov cx,2000
	sub3s:
	add byte ptr es:[bx],10001111b
	or es:[bx],al
	add bx,2
	loop sub3s
	pop es
	pop cx
	pop bx
	ret

	sub4:
	push cx
	push si
	push di
	push es
	push ds
	mov si,0b800h
	mov es,si
	mov ds,si
	mov si,160
	mov di,0
	cld
	mov cx,24
	sub4s:
	push cx
	mov cx,160
	rep movsb
	pop cx
	loop sub4s
	mov cx,80
	mov si,0
	sub4s1:
	mov byte ptr [160*24+si],' '
	add si,2
	loop sub4s1
	pop ds
	pop es
	pop di
	pop si
	pop cx
	ret

setscreenend:
	nop
code ends
end start

assume cs:code
code segment
;为了看到效果，我们最后做清屏
start:
	mov ah,1
	mov al,1
	int 7ch
	call delay
	mov ah,2
	mov al,2
	int 7ch
	call delay
	mov ah,3
	int 7ch
	call delay
	mov ah,0
	int 7ch
	
	mov ax,4c00h
	int 21h

delay:
	push ax
	push dx
	mov dx,1000h
	mov ax,0
	s1:
	sub ax,1
	sbb dx,0
	cmp ax,0
	jne s1
	cmp dx,0
	jne s1
	pop dx
	pop ax
	ret

code ends
end start