利用8253定时器设计一个具有时、分、秒显示的电子时钟，并定义一个启动键，当按下该键时电子时钟从当前设定值（可在显示缓冲区中予置）开始走时-Design using a timer 8253, the minutes and seconds display electronic clock, and define a start button, when you press the key, electronic clock from the current settings (which can be set in the display buffer) for travel time