1. Cyg_RealTimeClokc rtc
   1. Cyg_Clock
      1. Cyg_counter
         1. alarm_list
   2. Interrupt
      1. isr
         1. (Almost do nothing, except all dsr)
      2. dsr
         1. rtc->tick(count)
            1. alarm->alarm(alarm, alarm->data
2. Cyg_Thread