安裝一個新的int 7ch 中斷例程，爲顯示輸出提供如下的功能子程序
（1）清屏
（2）設置前景色
（3）設置背景色
（4）向上滾動一行
入口參數：ah傳遞功能號-0清屏，1-設置前景色，2-設置背景色，3-向上滾動一行
對於2,3號功能：al 傳送顏色值，al=(0,1,2,3,4,5,6,7)
assume cs:code
code segment
start:
mov ax,cs ;安裝中斷程序
mov ds,ax
mov si,offset setscreen
mov ax,0
mov es,ax
mov di,200h
mov cx,offset setend-offset setscreen
cld
rep movsb
mov ax,0
mov es,ax
mov word ptr es:[7ch*4],200h ;修改中斷向量表
mov word ptr es:[7ch*4+2],0
mov ax,4c00h
int 21h
setscreen:jmp short set
table dw sub1-setscreen+200h,sub2-setscreen+200h,sub3-setscreen+200h,sub4-setscreen+200h
set:push bx
cmp ah,3
ja sret
mov bl,ah
mov bh,0
add bx,bx
call word ptr cs:[202h+bx]
sret:pop bx
iret ;此處爲中斷，應該用iret 返回
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
sub2: push bx
push cx
push es
mov bx,0b800h
mov es,bx
mov bx,1
mov cx,2000
sub2s:and byte ptr es:[bx],11111000b
or es:[bx],al
add bx,2
loop sub2s
pop es
pop cx
pop bx
ret
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:and 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
setend:nop
code ends
end start
測試用小程序：
assume cs:code
code segment
start:mov ah,1
mov al,4
int 7ch
mov ax,4c00h
int 21h
code ends
end start