C4 CPU Power State and its importance in CPU voltage
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 and the DPRSTP pin
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.
How does the C4 CPU Power State save energy?
Just for you to have some real examples of how the 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 the C4 state, a 79.32% reduction in consumption. On a mobile Core 2 Duo T9400 or T9600, which has 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.