我的程序为什么不能返回啊

assume cs:code
data segment
db 'welcome to masm!',0
data ends
code segment
start: mov dh,8
      mov al,3
      mov cl,2
      mov ax,data
      mov ds,ax
      mov si,0
      call show_str
      
      mov ax,4c00h
      int 21h
      
show_str: push cx
       mov ax,dx
       mov al,0
       mov bx,160
       mov cx,[si]
       jcxz s
       mov ax,0b800h
       mov es,ax
       pop cx
       mov bx,0
       mov bh,[si]
       mov bl,cl
       mov es:[dl],bx
       inc si
       add di,2
       jmp word ptr show_str
    s:ret