C4E / C5 State
C4E CPU Power Mode
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.
Examples of Power Savings Featuring C5
Let’s give some examples of power savings brought by the 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 has 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. This is an addition to power saving, introduced with the 45-nm version of the mobile Core 2 Duo – i.e., models with numbering starting with 8 or 9 and the 7350 model.
Deep Power Down State
It is important to note that the desktop version of the 45 nm version of the desktop Core 2 Duo does not bring this functionality, which is also known as Deep Power Down. When the CPU enters this state it saves its entire architectural state inside a special static RAM, which is fed from an independent power source.
This allows the CPU internal voltage to be lowered to any value, including 0 V, which would completely turn off the CPU when it is idle. Then when the CPU is waked up it loads the previous state of all internal units from its special static RAM. Of course, waking up the CPU from this state takes a lot longer than the previous states we discussed, but it is faster than turning off the computer and then turning it back on and loading the operating system, etc.
CPU Voltage Line
Notice that there is only one voltage line for the entire CPU (the only component with a different voltage source is the abovementioned special memory) and lowering or turning off the CPU voltage is an all-or-nothing kind of deal: if you turn off the CPU, you have to turn off it entirely when it goes into C6 mode.
Core i7 Nehalem Features
The Core i7 CPU (codenamed Nehalem) has an embedded power control unit that allows the voltage for individual parts of the CPU to be reduced or turned off. For example, if only one processing core of the CPU is idle, it will be able to turn off just one of the cores, putting it on C6 mode. On current 45-nm mobile Core 2 Duo CPUs, you can’t do that.