Advanced Power Management

Overview

APM operates through a layered system. Applications and device drivers that are APM-aware interact with an operating system-specific APM driver. This driver then communicates with the APM-aware BIOS, which directly manages the hardware. Individual devices can be excluded from APM control, allowing a driver to handle that hardware directly.

Communication is bidirectional: the BIOS sends power management events to the APM driver, while the driver uses function calls to send requests and information back to the BIOS, acting as a middleman. Power management happens both through these function calls, which request state changes, and automatically based on device activity. In versions 1.0 and 1.1, the BIOS held nearly all control; version 1.2 gave the operating system the ability to manage suspend timeout. In 1997, Phoenix Technologies introduced "APM 2.0," a kernel device driver designed to work with an APM 1.2 BIOS.

Power states

The APM specification defines five system power states. Full On means the computer is powered on with no devices in power saving mode. APM Enabled means the computer is on and APM controls device power management as needed. APM Standby puts most devices in low-power state, slows or stops the CPU, and saves system state for quick return. APM Suspend powers off most devices but saves system state, taking longer to resume; hibernation is a special form of this state. Off means the computer is turned off. APM also defines four device power states: Device On (full power), Device Power Managed (some functions reduced), Device Low Power (not working but can wake), and Device Off (powered off). Devices are not required to implement all states.