In order to save energy when the CPU is idle, the CPU can be commanded to enter a low-power mode. Each CPU has several power modes and they are collectively called “C-states” or “C-modes”. In this tutorial we will explain what these modes are, what they do and the modes supported by each processor.

The lower-power mode was first introduced with the 486DX4 processor, so this concept is far from being new. With time, however, more power modes were introduced and enhancements were made to each mode so the CPU could consume less power when it is one of these low-power modes.

The basic idea of these modes is to cut the clock signal and power from idle units inside the CPU. The more units you stop (by cutting the clock), reduce the voltage or even completely shut down, more energy you save, but more time is required for the CPU to “wake up” and be again 100% operational.

These modes are known as “C-states”. They are numbered starting at C0, which is the normal CPU operating mode, i.e., the CPU is 100% turned on. The higher the C number is, deeper is the CPU sleep mode, i.e., more circuits and signals are turned off and more time the CPU will take to go back to C0 mode, i.e., to wake-up.

Each mode is also known by a name and several of them have sub-modes with different power saving – and thus wake-up time – levels.

In the table below we summarize all C-state modes currently available. Modes C1 to C3 work by basically cutting clock signals used inside the CPU, while modes C4 to C6 work by reducing the CPU voltage. “Enhanced” modes can do both at the same time.

Now we are going to explain each C-state in details.