Power management
PC
Power management features are built into a part of the chipset called South Bridge. Early desktop power management used the BIOS to determine whether a device had been idle long enough to reduce power. With the introduction of Advanced Power Management (APM) the OS began giving suggestions as to the power settings and timings. With the Advanced Configuration and Power Interface (ACPI) specification all power management moved from the BIOS to the hardware and operating system. New chipsets support ACPI power and thermal management functions to control various system-level and processor-level power and sleep states, and they also still support APM. Tests of mobile PCs in the 'on mode', using the Business Winstone 2001 BatteryMark, show that the various ACPI-versions save a factor 2 to 3 on energy (battery) use.
How ACPI --or IAPC (Instantly Available PC) as Intel would like to call the improved versions-will work in practice depends very much on the operating system and of course its users. The fact that 80-90% of users turn off ENERGY STAR features and that the OS is also not fully dedicated in power saving are important issues in this respect.
'on' mode power management
In 2001, when a desktop PC was actually being used, the power consumption would range from anywhere between 'Windows desktop' (waiting for input) of around 90-100W to 'full load' of 130-150W and higher. With the introduction of better power management (ACPI, PowerNow!/SpeedStep/ LongRun) in 2001-2002 models, power use will not be so easy to tell in the future.
The ZD BatteryMark test, conceived to measure energy use with real-life office behaviour, features a multitude of micro- and mini- pauses that take up 85% of 'working time', between more or less demanding processing. The new power management features are capable to follow these micro-pauses dynamically and almost immediately switch the CPU back to a active standby mode (2-3 W). Moreover, as long as processing is not very demanding, the CPU can switch back to a lower clock speed (usually in the 500 MHz range) and voltage, consuming 40% less than at 'full load' without the user noticing.
There are a few snags: First of all, not all vendors are convinced desktop users care, so there are still considerable (though decreasing) numbers of PCs shipped without these power management features. Secondly, IT system managers in businesses seem to be allergic to power management because of bad experiences in the past. And some of this allergy has surely rubbed of on home users, also given recent research results showing that more than 80% of users turn e.g. the ENERGY STAR power management features off. Thirdly, environmental policy makers are still sceptical whether it really will work. For example, it has not been incorporated in ENERGY STAR. Fourthly, the BatteryMark test shows that these power management features do wonders for the CPU energy use, but far less for the rest of the platform (graphics, chipset, etc.).
PC 'standby' power management
Standby energy use has been the focus of environmental policy makers from the beginning of the '90s. Annual consumption in 'standby', i.e. not being used, was more than twice as high as the 'on' mode use. ENERGY STAR was instrumental to bring this to the attention of the manufacturers, who created a rather crude form of power management. In fact, it was so crude that 80-90% of users turned the power management feature off, as was recently revealed. But now the industry hopes to change all that by introducing PC power management that the user does not even notice. With the introduction of ACPI and the power management software like PowerNow! (AMD), SpeedStep (Intel) and LongRun(Transmeta), the reduction of standby power is starting to become a serious issue with sleep-mode power as low as 5-6 W (in desktop PCs, lower in laptops).
For further reading download PC Power Management.pdf
Courtesy of the French Energy Agency ADEME, Future Electronics project.
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