将寄存器al中二进制数输出有关问题，请指点

将寄存器al中二进制数输出问题，请指点
data segment
data ends
code segment
assume ds:data,cs:code
start: mov al,0ffh
 mov cx,8
again: shl al,1
jnc disp_0
  jc disp_1
disp_1:mov dl,31h
mov ah,2
int 21h
dec cx
cmp cx,0
jz done
jnz again
disp_0: mov dl,30h
mov ah,2
int 21h
dec cx
cmp cx,0
jz done
jnz again
done: mov ax,4c00h
int 21h
code ends
 end start
该程序的目的是吧al中所存储的二进制数输出，为什么输出的结果总是不对呢？高手指点

[解决办法]
那是因为你程序中
disp_1: mov dl,31h
mov ah,2
int 21h
和
disp_0: mov dl,30h
mov ah,2
int 21h
改变了al的值

下面这样写就可以了：

Assembly code

data segment  n db ?data endscode segmentassume ds:data,cs:codestart: mov al,0ffh    mov n, al    mov cx,8 again: shl n,1    jnc disp_0    jc disp_1disp_1: mov dl,31h    mov ah,2    int 21h    dec cx    cmp cx,0    jz done    jnz againdisp_0: mov dl,30h    mov ah,2    int 21h    dec cx    cmp cx,0    jz done    jnz againdone: mov ax,4c00hint 21hcode ends  end start