What is the C-State?

Article Number: 000060621

What is the C-State?

Summary: Explanation of C state in relation to your CPU and the various modes.

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Symptoms

In order to save energy when the CPU is idle, you can command the CPU to enter a low-power mode. Each CPU has several power modes, which are collectively called "C-states" or "C-modes." In this article, we explain what these modes are, what they do, and which processors support which modes.

The lower-power mode was first introduced with the 486DX4 processor, so this concept is far from new. Newer modes have been enhanced to allow the CPU to consume even less power.

These modes work by cutting the clock signal and power from idle units inside the CPU. The more units that you stop by cutting the clock, reducing the voltage, or even completely shutting down, the more energy you save. However, it takes more time for the CPU to fully "wake up" from sleep mode.

These modes or "C-states" start at C0, which is the normal CPU operating mode (the CPU is 100% activated). The higher the C number is, the deeper into sleep mode the CPU goes. In other words, more circuits and signals are turned off, and the CPU takes more time to fully wake up into C0 mode.

Each C-state has a name, and several of them have submodes with different power-saving levels and wake-up times.

The table below summarizes all currently available C-state modes. Modes C1 to C3 work by cutting the CPU's clock signals, while modes C4 to C6 work by reducing the CPU's voltage. "Enhanced" modes can do both simultaneously.

Mode	Name	What it does	CPUs
C0	Operating State	CPU fully turned on	All CPUs
C1	Halt	Stops CPU main internal clocks via software; bus interface unit and APIC are kept running at full speed	486DX4 and above
C1E	Enhanced Halt	Stops CPU main internal clocks via software and reduces CPU voltage; bus interface unit and APIC are kept running at full speed	All socket 775 CPUs
C1E	—	Stops all CPU internal clocks	Turion 64, 65-nm Athlon X2 and Phenom CPUs
C2	Stop Grant	Stops CPU main internal clocks via hardware; bus interface unit and APIC are kept running at full speed	486DX4 and above
C2	Stop Clock	Stops CPU internal and external clocks via hardware	Only 486DX4, Pentium, Pentium MMX, K5, K6, K6-2, K6-III
C2E	Extended Stop Grant	Stops CPU main internal clocks via hardware and reduces CPU voltage; bus interface unit and APIC are kept running at full speed	Core 2 Duo and above (Intel only)
C3	Sleep	Stops all CPU internal clocks	Pentium II, Athlon and above, but not on Core 2 Duo E4000 and E6000
C3	Deep Sleep	Stops all CPU internal and external clocks	Pentium II and above, but not on Core 2 Duo E4000 and E6000; Turion 64
C3	AltVID	Stops all CPU internal clocks and reduces CPU voltage	AMD Turion 64
C4	Deeper Sleep	Reduces CPU voltage	Pentium M and above, but not on Core 2 Duo E4000 and E6000 series; AMD Turion 64
C4E/C5	Enhanced Deeper Sleep	Reduces CPU voltage even more and turns off the memory cache	Core Solo, Core Duo and 45-nm mobile Core 2 Duo only
C6	Deep Power Down	Reduces the CPU internal voltage to any value, including 0 Volts	45-nm mobile Core 2 Duo only

Note: Changing the C state in the BIOS settings does not affect performance of the device however it will take more time for the CPU to fully "wake up" from sleep mode.

Note: Changing the C state in the BIOS settings will result in a different signature sound emanating from the CPU. This is due to the different power profile having different cooling needs.