C administering machine for each port is assigned an address (called a port number), is formed in a separate memory space, I / O address space.
Please enter: a traditional PC, I / O address space range
CPU operation is accomplished by peripheral ports dedicated read and write commands. With IN instruction read port, write port with the OUT instruction.
IN AL,21H；表示从21H端口读取一字节数据到AL
IN AX,21H；表示从端口地址21H读取1字节数据到AL，从端口地址22H读取1字节到AH
OUT 21H,AL；将AL的值写入21H端口
OUT 21H,AX；将AX的值写入端口地址21H开始的连续两个字节。（port[21H]=AL,port[22h]=AH）
