MOV AL,80H 
  MOV DX,303H
  OUT DX,AL
  MOV DX,300H
AGAIN: MOV AH,01
  INT 16H 
  JNZ STOP 
  SUB AL,AL
BACK: OUT DX,AL
  INC AL
  CMP AL,0
  JZ AGAIN
  CALL DELAY 
  JMP BACK
STOP: