Modes C1, C2 and C3 deals basically with the clock signal. Since on C3 mode all clock signals inside the CPU can be stopped, there is no other way to save power by playing with the CPU clock signals. The next step on reducing the CPU idle power is to reduce the CPU voltage. Since power is directly proportional to voltage (P = V x I) if we reduce the CPU voltage we also reduce the amount of power being wasted.

The first mode to implement voltage reduction is called C4 or Deeper Sleep State and was first implemented on the Pentium M processor (i.e. Centrino platform), which was a CPU targeted to the mobile market, where power saving is extremely important to extend battery life. This mode is also available on Core Solo, Core Duo (the dual-core version of Pentium M), on all mobile versions of Core 2 Duo and desktop Core 2 Duo models with model numbers starting with “7” or “8” (i.e. 45-nm CPUs). This mode isn’t available on 65 nm desktop Core 2 Duo CPUs (models starting with “4” or “6”). This mode is also available on AMD’s Turion 64 processor.

Deeper Sleep State is achieved from Deep Sleep state, i.e. the CPU must first enter Deep Sleep State (C3) and then, from there, it can reduce its internal voltage. On Core Duo CPUs this must be done by activating a pin on the CPU called DPRSTP.

Just for you to have some real examples of how C4 state can save energy, let’s consider mobile Core 2 Extreme X9100. In normal (C0) mode working at its full clock this CPU has a maximum current consumption of 59 A, which drops to 12.2 A when the CPU is in C4 state, a 79.32% reduction in consumption. On a mobile Core 2 Duo T9400 or T9600, which have a maximum current consumption of 47 A, maximum current consumption drops to 11.7 A when the CPU is under C4 mode, a 75.11% decrease in consumption.

Core Solo, Core Duo (the dual-core version of the Pentium M targeted to the mobile market and codenamed Yonah) and the 45-nm version of the mobile Core 2 Duo (i.e. models with numbering starting with 8 or 9 and the 7350 model) have another C4 mode, called Enhanced Deeper Sleep or simply C4E, which allows the CPU voltage to be reduced even more after the L2 memory cache has been disabled. This mode isn’t available on other CPUs. This mode is also called C5 by some people, even though this isn’t the real name of this mode.

Let’s give some examples of power savings brought by C4E state. First let’s consider mobile Core 2 Extreme X9100. In normal (C0) mode working at its full clock this CPU has a maximum current consumption of 59 A, which drops to 11.7 A when the CPU is in C4E state, an 80.17% reduction in consumption. On a mobile Core 2 Duo T9400 or T9600, which have a maximum current consumption of 47 A, maximum current consumption drops to 10.5 A when the CPU is under C4E mode, a 77.66% decrease in consumption.