汇编 “error A2031”

assume cs:code
code segment
    main:
         mov ax,0020h
         mov ds,ax     ;指定段地址0200h
         mov dx,0h
         mov cx,003fh 
    s:   
         mov [dx],dx
         inc dx
         loop s

         mov ax,4c00h
         int 21h
    code ends
end main

 原因：16 Bit CPU中，不能用DX间接寻址，必须用BX、BP、SI、DI