8253 通道 0 工作在方式 0（脉冲计数产生中断），由按键产生单脉冲接入 CLK0、令计数初值为 0FH，

手动逐个输入单脉冲，编程在计算机屏幕上显示计数值，并用逻辑笔观察 OUT0 的状态，键盘有键按下时

返回 DOS。-8253 channel 0 0 work in way (pulse counting the interrupt), single pulse generated by the key access CLK0, counting the initial value of 0 fh,

Manually enter them one by one single pulse, programming on a computer screen display count value, using logical pen observation OUT0 state, the keyboard has a key press

Returns the DOS.