要点
1：外设读
2.3个分支 则需2个转移

CODE SEGMENT
	ASSUME CS:CODE
	  START:IN AL,71H
			CMP AL,20
			JAE BIG
			CMP AL,10
			JB SMA
			MOV BL,88H
			JMP EXIT
		BIG:MOV BL,0FFH
				JMP EXIT
		SMA:MOV BL,00
	   EXIT:MOV AL,BL
	   			OUT  73H,AL
	   			MOV AH,4CH
	   		    INT 21H
CODE ENDS
	START END