What is the C-State?


What is the C-State?


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.



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Article ID: QNA41893

Last Date Modified: 06/11/2020 04:24 AM

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