利用8253和8259芯片实现实时电子时钟的功能；利用8255控制七段数码管完成定时扫描显示。显示格式为xx时xx分xx秒，每隔1s，时间值改一次。按键SHIFT可选择时、分、秒，按键CTRL可对所选的时、分、秒进行加1，小时加至23后清0，分、秒加至59后清0。(Use 8253 and 8259 chip to realize the function of real-time electronic clock; use 8255 control seven segment digital tube to complete the timing scanning display. When the display format is XX, XX minutes XX seconds, every 1s, the time value changed once. Button SHIFT can choose time, minutes, seconds, button CTRL can be selected time, minutes, seconds added 1, hours added to 23 after 0, minutes, seconds to 59 after 0)