assume cs:code
stack segment
db 128 dup (0)
stack ends
data segment
dw 0,0
data ends
code segment
start:mov ax,stack
mov ss,ax
mov sp,128
mov ax,data
mov ds,ax
mov ax,0
mov es,ax
push es:[9*4]
pop ds:[0]
push es:[9*4+2]
pop ds:[2] ；将原来的int9中断例程入口地址保存
mov word ptr es:[9*4],offset int9
mov es:[9*4+2],cs ；设置新的int9 中断例程的入口地址
mov ax,0b800h
mov es,ax
mov ah,'a'
s:mov es;[160*12+40*2],ah
call delay
inc ah
cmp ah,'z'
jna s
mov ax,0
mov es,ax
push ds:[0]
pop es:[9*4]
push ds:[2]
pop es:[9*4+2]
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
int9:push ax ；新的int9中断例程
push bx
push es
in al,60h
pushf
pushf
pop bx
and bh,11111100b
push bx
popf
call dword ptr ds:[0] ；将int指令进行模拟
cmp al,1
jne int9ret
mov ax,0b800h
mov es,ax
inc byte ptr es:[160*12+40*2+1]
int9ret:pop es
pop bx
pop ax
iret
code ends
end start