This is the latest addition on 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. 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, what 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.
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.
The forthcoming Core i7 CPU (codename Nehalem) will have 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.
In order to enter C6 state the CPU must enter first into C4 state and from there switch to C6 state.
Let’s give some examples of power savings brought by C6 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 A when the CPU is in C6 state, an 81.35% 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 5.7 A when the CPU is under C6 mode, an impressive 87.87% decrease in consumption.