cpu与外设之间的种种不匹配，速度不匹配，缓冲，信号电平不同等等。所以必须有专门的代理来处理IO事件。输入输出控制中心（ICH,I/O controller hub）,也就是南桥芯片。

I/O Controller Hub (ICH) is a family of Intel southbridge microchips used to manage data communications between a CPU and a motherboard, specifically Intel chipsets based on the Intel Hub Architecture. It is designed to be paired with a second support chip known as a northbridge. As with any other southbridge, the ICH is used to connect and control peripheral devices.

南北桥的作用如下图

南北桥

IO端口设计之初，就被设计成要通过寄存器的方式同cpu进行通信，其内部有专门用于数据交换的寄存器，只不过这些寄存器在IO接口之中，为了区别cpu内部的寄存器，IO端口中的寄存器被称作端口。

in al,dx
in ax,dx
//dx寄存器中是端口号，表示从端口号中读取数据

out dx,al;
out dx,ax;
//dx中存储端口号，表示向端口中