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Mobile computers today exhibit a variety of shapes, formats and features, but they all have one thing in common. Their design must cater to a major issue users care about most-long battery life. Perhaps no one aspect of portable-computer system design affects-and is affected by-the architecture, ultimate configuration and desired performance as power management in these systems. Consequently, a number of strategies have been developed to husband the power of today's batteries to the maximum extent while maintaining full performance of the system. This must be accomplished in a manner that is transparent to the user. As a consequence, power management is now a major aspect of mobile-computer system design, and commands the efforts of chip makers, system makers, software developers, packaging designers and even the battery suppliers themselves.
There are two kinds of power management that designers bring to bear on the problem-passive and active. Passive power-management techniques are brought to bear when there is no user activity and the system is idling. In this case, the system goes into various levels of "sleep" mode. A more complex and sophisticated approach is active power management, by which power consumed by the computer is reduced while it is actively in use. The accompanying figure describes all the typical power-management modes currently in use in notebook computers.
One of the major power consumers in a mobile computer is the CPU. According to Rick Bergman, marketing manager for Texas Instruments' PC System products, (Dallas) which produces a 486DX4 processor aimed at portable applications, the processes for fabricating microprocessors and other logic are rapidly closing the gap with processes for producing memory chips, resulting in a constantly improving overall system performance.
At the same time, as processes move to smaller and smaller geometries, supply voltages go down as well. As Bergman points out, according to the simple power equation P= CV2, just reducing the voltage from 5 V to 3 V, to which most portables are moving today, gives a power saving of 64 percent, and moving to 0.9 V with a 1-micron process will yield a 97 percent saving.
Another critical issue, according to Bergman, is how much memory to integrate on the microprocessor. "This is critical from a performance point of view," he said, "but...