;程序功能：按A键，当松开的时候在屏幕的80*25的里面全部显示字母A，不松开则不作任何处理，其他的键照常处理。

assume cs:code

data segment

db  dup()

data ends

code segment

start:

    mov ax,

    mov es,ax

    mov di,200h

    mov ds,ax

    mov si,[*]

    mov cx,

    cld

    rep movsb

    mov ax,offset int9

    mov ds:[*],ax

    mov ax,cs

    mov ds:[*+],ax

    call breakPoint

    ;还原int9中断向量表

    mov ax,

    mov ds,ax

    mov si,200h

    mov di,*

    mov cx,

    cld

    rep movsb

    mov ax,4c00h

    int 21h

;实现int9中断例程

int9:

    cli

    push ax

    mov ax,0b800h

    push es

    mov es,ax

    push bx

    mov bx,

    push cx

    mov cx,

    in al,60h

    cmp al,2eh+80h

    jne int9ret

    mov ah,'B'

    s0:

    mov byte ptr es:[bx],ah

    add bx,

    loop s0

    int9ret:

    int 80h

    pop cx

    pop bx

    pop es

    pop ax

    iret

breakPoint:

    push ax

    pushf

    pop ax

    or ah,        ;这里可以增加判断，例：当cx为100的时候再设置断点

    push ax

    popf

    pop ax

    ret

code ends

end start