CPU Power Consumption

CPU POWER CONSUMPTION:

Power consumption is, in some situations, as important as execution time. In this section we study the characteristics of CPUs that influence power consumption and mechanisms provided by CPUs to control how much power they consume. First, it is important to distinguish between energy and power. Power is, of course, energy consumption per unit time. Heat generation depends on power consumption. Battery life, on the other hand, most directly depends on energy consumption. Generally, we will use the term power as shorthand for energy and power consumption, distinguishing between them only when necessary.

The high-level power consumption characteristics of CPUs and other system components are derived from the circuits used to build those components. Today, virtually all digital systems are built with complementary metal oxide semiconductor (CMOS) circuitry. The detailed circuit characteristics are best left to a study of VLSI design [Wol08], but the basic sources of CMOS power consumption are easily identified and briefly described below.

■ Voltage drops: The dynamic power consumption of a CMOS circuit is proportional to the square of the power supply voltage (V²). Therefore, by reducing the power supply voltage to the lowest level that provides the required performance, we can significantly reduce power consumption. We also may be able to add parallel hardware and even further reduce the power supply voltage while maintaining required performance.

■ Toggling: A CMOS circuit uses most of its power when it is changing its output value. This provides two ways to reduce power consumption. By reducing the speed at which the circuit operates, we can reduce its power consumption (although not the total energy required for the operation, since the result is available later).We can actually reduce energy consumption by eliminating unnecessary changes to the inputs of a CMOS circuit—eliminating unnecessary glitches at the circuit outputs eliminates unnecessary power consumption.