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.
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