assume cs:codesg
codesg segment
    mov ax,4c00h
    int 21h
start: mov ax,0
    s:nop
      nop
      mov di,offset s
      mov si,offset s2
      mov ax,cs:[si]
      mov cs:[di],ax
   s0:jmp short s
   s1:mov ax,0
      int 21h
      mov ax,0
   s2:jmp short s1
      nop
codesg ends
end start