Power management for computer systems has traditionally focused on regulating the power consumption in static modes such as sleep and suspend. These are de-activating states, often requiring a user action to re-activate the system. There are usually significant latencies and overheads for entering and exiting these states, and in desktop and server systems a firmware layer typically supports these modes.
Dynamic power management refers to power management schemes implemented while programs are running. Many architectures provide the equivalent of a halt instruction that reduces CPU power during idle periods. The operating system and device drivers may also manage power of peripheral devices, for example spinning down disks during periods of inactivity. Highly integrated processors with on-board peripherals often include software-controlled clock management capabilities to reduce power consumed by inactive peripherals and peripheral controllers. The memory subsystem also provides a profitable area for dynamic power management, either through the memory controller implementation or through software-based schemes.
Recent advances in processor design techniques have led to the development of systems that support very dynamic power management strategies based on dynamic voltage and frequency scaling. Since CPU power consumption typically decreases with the cube of voltage while frequencies scale linearly with voltage, significant opportunities exist for tuning the power-performance tradeoff to the needs of the application. Processors such as the TransmetaTM CrusoeTM, Intel® StrongARMTM and XScaleTM processors, and the recently announced IBM® PowerPCTM 405LP allow dynamic voltage and frequency scaling of the processor core in support of these dynamic power management strategies. Aside from the Transmeta system, all of the processors named above are highly integrated system-on-a-chip (SOC) processors designed for embedded applications. The applications of these processors typically do not include a traditional BIOS, therefore control of the dynamic power state of the system must be implemented in the operating system.
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