8255并行通讯(自发自收),设置8253计数器0工作方式,给8253计数器0送初值,向8251控制端口送3个0,向8251送控制字允许其发送和接收-8255 parallel communication (spontaneous self-closing), set 8253 counter 0 work, to the 8253 counter 0 to send the initial value to the 8251 control port to send 3 0 control word sent to the 8251 to allow it to send and receive