Everything You Need to Know About the CPU C-States Power Saving Modes

Introduction

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.

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 LGA775 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 V 45-nm mobile Core 2 Duo only

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

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Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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