利用定时器 8253 和外扩展的键盘—显示模块，实现时钟显示及时间设置功能：
1）时钟显示内容为“分：秒”，即“MM.SS”； 2）P 键用来启动时钟设置，通过键盘上的数字修改时间，参数在修改过程中 处于闪烁状态，闪烁频率 2Hz(Using timer 8253 and extended keyboard display module to realize clock display and time setting function:
1) the clock display content is "minute: second", "MM.SS"; 2) P key is used to start the clock setting. Through the digital modification time on the keyboard, the parameters are in the flicker state during the modification process and the scintillation frequency 2Hz)