1.  Field of the Invention
The present invention relates to techniques for conserving power usage in computer systems.  More specifically, the present invention relates to a method and an apparatus for reducing power consumption in a processor by reducing voltage supplied to an instruction-processing portion of the processor, while maintaining voltage to other portions of the processor. 
2.  Related Art
Dramatic advances in integrated circuit technology have led to corresponding increases in processor clock speeds.  Unfortunately, these increases in processor clock speeds have been accompanied by increased power consumption.  Increased power consumption is undesirable, particularly in battery-operated devices such as laptop computers, for which there exists a limited supply of power.  Any increase in power consumption decreases the battery life of the computing device. 
Modern processors are typically fabricated using Complementary Metal Oxide Semiconductor (CMOS) circuits.  CMOS circuits typically consume more power while the circuits are switching, and less power while the circuits are idle.  Designers have taken advantage of this fact by reducing the frequency of (or halting) clock signals to certain portions of a processor when the processor is idle.  Note that some portions of the processor must remain active, however.  For example, a cache memory with its associated snoop circuitry will typically remain active, as well as interrupt circuitry and real-time clock circuitry. 
Although reducing the frequency of (or halting) a system clock signal can reduce the dynamic power consumption of a processor, static power consumption is not significantly affected.  This static power consumption is primarily caused by leakage currents through the CMOS devices.  As integration densities of integrated circuits continue to increase, circuit devices are becoming progressively smaller.  This tends to increase leakage currents, and thereby increases static power consumption.  This increased static power consumption results in reduced battery life, and increases cooling system requirements for battery operated computing devices. 
What is needed is a method and an apparatus that reduces static power consumption for a processor in a battery operated computing device.