APM

5/8/2004

Advanced [occasionally Automated] Power Management (APM). APM was the first formalized approach to power management on Personal Computers. Power Management is a term for automatic power on or off of a PC or of PC components. For the most part, APM bases its decisions on time since last usage -- idle time -- and on device activity. That is to say that a device such as a monitor or disk drive can be shut down if it or something else is unused for a specified period of time. When it is used again, the device will (hopefully) be powered back on . Typical Power Management functions include putting monitors, hard drives, and/or CPUs into reduced power modes during periods of inactivity. APM also includes restoring the PC to full operation when needed based on user activity, OS activity, or activity of a communications device such as a serial line (e.g. a phone connection via a modem) or Ethernet connection. On many PCs, APM can reprogram the power switch to Suspend the system rather than shutting down.

APM put power management into the BIOS where timeouts for devices were configured in BIOS Setup, and the device was powered down after it was idle for the specified time. APM allowed the Operating System to reset many of the power management parameters. The APM Power Management scheme was simple but was subject to difficulties, the most important of which is that the fairly simple timed actions on idle devices by the BIOS sometimes caused trouble for the Operating System which had no knowledge of what the BIOS was going to do or when it was going to do it.

APM was replaced in the 1999 timeframe with the Advanced Configuration and Power Interface (ACPI) which moved control of most of the power management functions from the BIOS to the Operating System. The BIOS interrogates the hardware about Power Management features, does basic configuration, then passes the information on to the Operating System which was free to implement sophisticated Power Management schema.

In general, Power Management is a mixed bag in practice. It often works well and reliably on laptop/notebook computers using the vendor's OS configuration. It is important to extending battery life on those PCs. Power Management frequently does not work especially well on desktop PCs. The increased complexity of ACPI sometimes makes the effort of configuring it -- which can be considerable if either the OS or BIOS has flaws or documentation issues -- uneconomic. Thus, some ACPI capable PCs are run using APM instead or with no power management at all. Both APM and ACPI can run into trouble with devices that do not fully restore their state when repowered. For example, a PC returning to service after being suspended may come up with an inoperative sound card or network interface.

Windows will attempt to configure Power Management to default settings when it is installed, selecting none, APM, or ACPI based on the Windows Version; Windows internal motherboard information; the information available from the BIOS; and the result of power management capability tests. In some cases it will not install power management but will leave the Control Panel option to install it available. Some BIOSes and power management drivers are imperfect and can result in instability or the inability to configure or use some hardware. In some cases, there are alternative Power Management drivers available that can be used to overcome otherwise intractable resource management problems caused by Power Management.

Linux supports both APM and ACPI, but installing it may require recompilation and may not be supported in SMP mode. Problems are reported.

See Microsoft Knowledge Base: Q307525: How Windows XP Installs Advanced Power Management (APM) support.

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