Transistors inside the CPU work as a switch, with two possible states: conductive (a.k.a. “saturation mode”), working as a closed switch, and non-conductive (a.k.a. “cut-off” mode), working as an open switch. The problem is when they are on their non-conductive state in theory they shouldn’t allow any current to flow, but a small amount of current still flows. This current is called leakage and if you add up all leakage currents you have a significant amount of current (and thus power) being wasted and unnecessary heat being generated. One of the challenges in designing CPUs in recent years has been trying to eliminate leakage current.

Nehalem brings a power control unit inside the CPU in order to better manage power (see Figure 8). This unit reduces leakage current and also allows the new “Turbo Mode”, which we will discuss on next page. Basically, the CPU can now have different voltages and frequencies for each core, for the units outside the cores, for the memory controller, for the cache and for the I/O units. On previous CPUs, all cores had to run at the same clock rate but on Nehalem-based CPUs each core can be programmed to run at different clock rates to save power.

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Figure 8: Power control unit.

The embedded power control unit can now switch off any of the CPU cores, feature not available on mobile Core 2 CPUs. In fact now the CPU can put any core into the C6 (“deep power down”) power state independently of the state under the remaining cores are running. This allows energy savings when you are running your PC normally but one or more cores are idle and thus can be shut down.