能解释下为什么这段程序执行时总是显示divide overflow呢？多谢

data segment                                             
mess1 db 0ah,0dh, 'input:$'                              
mess2 db 0ah,0dh,'output:$'                              
x     db ?,?,?                                           
data ends                                                
                                                         
code segment                                             
assume cs:code,ds:data                                   
start:                                                   
mov ax,data                                              
mov ds,ax                                                
let0:                                                    
mov dx,offset mess1    ;显示提示                         
mov ah,9                                                 
int 21h                                                  
mov ah,1               ;输入0~9                          
int 21h                                                  
cmp al,27               ;按ESC退出                       
jz out1                                                  
cmp al,'0'                                               
jb let0