Power management is an important function in several classes of computer systems. In particular, mobile computer systems, such as personal digital assistants (PDAs), laptops, cell phones, watch-computers, MP3 players, and other portable computer systems have limited capacity power supplies, such as batteries, that directly influence the usability and marketability of these computer systems. Using a larger capacity battery in these systems is often not feasible because of size and cost constraints and hence system-level power management is imperative for these devices.
Power management is also important for tethered computer systems, such as computer systems connected to an AC power source. Examples of tethered computer systems include personal desktop computers and servers. For these types of systems, the cost of electricity is a driving factor for power management. For example, ratings like EnergyStar in the United States and TopRunner in Japan encourage manufacturing and using energy efficient computer systems that are typically more environmentally friendly. Also, power supplies for computer systems tend to generate a significant portion of the amount of heat dissipated by these computer systems. By reducing power consumption, less costly cooling systems may be used with these computer systems. Also, a cooler running computer system may minimize system failures caused by thermal redlining.
Conventional computer system designs have traditionally viewed input and output (I/O) devices as human-computer interfaces that are optimized for maximizing user acceptance instead of maximizing for energy conservation. For example, a system designer of a PDA may provide a user with a choice of using a keyboard, a speech recognition interface or a handwriting recognition interface. Then the user, typically based on the user's personal preference, chooses between these various I/O devices without considering the power consumption of a selected I/O device, which can impact the available battery life of the PDA.