The CPU can also be managed to save energy. A notebook CPU can be put to sleep in software, reducing
power usage to almost zero. The only thing it can do in this state is waking up when an interrupt occurs.
Therefor, whenever the CPU goes idle, either waiting for I/O or because there is no work to do, it goes to
sleep.
On mang computers, there is a relationship between CPU voltage, clock cycke, and power usage. The CPU
voltage can often be reduced in software, which saves energy but also reduces the clock cycle (approximately
linearly). Since power comsumed is proportional to the square of the voltage, cutting the voltage in half makes
the CPU about half as fast but as 1/4 the power.
Modern Operating System --- Power Management (CPU)
原文：http://www.cnblogs.com/miaoyong/p/4886842.html