In the preceding section, we were concerned about the power used when the CPU is busy. In this section, we will look at how to save power when the CPU is idle.
When a processor has no more work to do, it executes a halt instruction and enters an idle state. While idle, the CPU uses less power. It exits the idle state when an event such as a hardware interruption occurs. Most CPUs have multiple idle states that use varying amounts of power. Usually, there is a trade-off between the power usage and the latency, or the length of time, it takes to exit the state. In the ACPI specification, they are called C-states.
In the deeper C-states, more circuitry is turned off at the expense of losing some state, and so it takes longer to return to normal operation. For example, in some C-states the CPU caches may be powered off, and so when the CPU runs again...
