All the clock hardware does is to generate interrupts at known intervals. Everything
else involving time must be done by the software, the clock driver. The exact duties
of the clock dirver vary among operating systems, but ususally include most of the
following:
1. Maintaining the time of day.
2. Preventing processes from running longer than they are allowed to.
3. Accounting for CPU usage.
4. Handling the alarm system call made by user processes.
5. Providing watchdog timers for parts of the system itself.
6. Doing profiling, monitoring, and statistics garthering.
Modern Operating System --- Chap 5.5.2 Clock Software
原文：http://www.cnblogs.com/miaoyong/p/4855309.html