一个简单的汇编程序，如何总是报错

一个简单的汇编程序，怎么总是报错？

Assembly code

assume cs:code,ss:stack,ds:datadata segment     db 0,2,4,7,8,9     db '// :: 'data endsstack segment      db 0,0,0,0,0,0,0,0,0,0,0,0stack endscode segmentstart:   mov ax,stack         mov ss,ax         mov sp,0ch         mov ax,data         mov ds,ax         mov si,0         mov cx,6         mov ah,0      s1:mov al,[si] ;取时间函数         out 70h,al         in al,71h         push ax          inc si ;debug 调试到si==3时就出问题了，提示NIVDM CPU遇到无效的指令，将push ax指令注释掉则正常         loop s1                   mov bp,sp ;sp = 0         call charToAscii                  ;显示         mov ax,0b800h         mov es,ax         mov ax,ss         mov ds,ax         mov si,0         mov di,0a0h*12+4*2         mov cx,12         rep movsb                  mov ax,4c00h         int 21h         charToAscii: push bp             push cx          s2:mov ah,0             mov bh,0             mov al,ss:[bp+1]             add al,30h             mov cl,4             shr byte ptr ss:[bp+1],cl             mov bl,ss:[bp+1]             add bl,30h               mov ss:[bp],ax             mov ss:[bp+1],bx             add bp,2             pop cx             loop s2             ret             code endsend start