Advanced power management (APM) is an

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

developed by

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and

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and released in 1992 which enables an

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

running an IBM-compatible

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to work with the BIOS (part of the computer's firmware) to achieve

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. Revision 1.2 was the last version of the APM specification, released in 1996.

ACPI Advanced Configuration and Power Interface (ACPI) is an open standard that operating systems can use to discover and configure computer hardware components, to perform power management (e.g. putting unused hardware components to sleep), auto c ...

is the successor to APM. Microsoft dropped support for APM in

Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...

. The Linux kernel still mostly supports APM, though support for APM CPU idle was dropped in version 3.0.

Overview

APM uses a layered approach to manage devices. APM-aware applications (which include device drivers) talk to an OS-specific APM driver. This driver communicates to the APM-aware BIOS, which controls the hardware. There is the ability to opt out of APM control on a device-by-device basis, which can be used if a driver wants to communicate directly with a hardware device. Communication occurs both ways; power management events are sent from the BIOS to the APM driver, and the APM driver sends information and requests to the BIOS via function calls. In this way the APM driver is an intermediary between the BIOS and the operating system. Power management happens in two ways; through the above-mentioned function calls from the APM driver to the BIOS requesting power state changes, and automatically based on device activity. In APM 1.0 and APM 1.1, power management is almost fully controlled by the BIOS. In APM 1.2, the operating system can control PM time (e.g. suspend timeout).

Power management events

There are 12 power events (such as standby, suspend and resume requests, and low battery notifications), plus

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-defined events, that can be sent from the APM BIOS to the operating system. The APM driver regularly polls for event change notifications. Power Management Events: 090429

APM functions

There are 21 APM function calls defined that the APM driver can use to query power management statuses, or request power state transitions. Example function calls include letting the BIOS know about current CPU usage (the BIOS may respond to such a call by placing the CPU in a low-power state, or returning it to its full-power state), retrieving the current power state of a device, or requesting a power state change.

Power states

The APM specification defines system power states and device power states.

System power states

APM defines five power states for the computer system: * Full On: The computer is powered on, and no devices are in a power saving mode. * APM Enabled: The computer is powered on, and APM is controlling device power management as needed. * APM Standby: Most devices are in their low-power state, the CPU is slowed or stopped, and the system state is saved. The computer can be returned to its former state quickly (in response to activity such as the user pressing a key on the keyboard). * APM Suspend: Most devices are powered off, but the system state is saved. The computer can be returned to its former state, but takes a relatively long time. (Hibernation is a special form of the APM Suspend state). * Off: The computer is turned off.

Device power states

APM also defines power states that APM-aware hardware can implement. There is no requirement that an APM-aware device implement all states. The four states are: * Device On: The device is in full power mode. * Device Power Managed: The device is still powered on, but some functions may not be available, or may have reduced performance. * Device Low Power: The device is not working. Power is maintained so that the device may be 'woken up'. * Device Off: The device is powered off.

CPU

The CPU core (defined in APM as the CPU clock,

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, system bus and system timers) is treated specially in APM, as it is the last device to be powered down, and the first device to be powered back up. The CPU core is always controlled through the APM BIOS (there is no option to control it through a driver). Drivers can use APM function calls to notify the BIOS about CPU usage, but it is up to the BIOS to act on this information; a driver cannot directly tell the CPU to go into a power saving state.

In ATA drives

The ATA specification and

SATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard t ...

specification defines APM provisions for hard drives, which specifies a trade-off between spin-down frequency and always-on performance. Unlike the BIOS-side APM, the ATA APM and SATA APM has never been deprecated. Aggressive spin-down frequencies may reduce drive lifespan by unnecessarily accumulating load cycles; most modern drives are specified to sustain 300,000 cycles and usually last at least 600,000. On the other hand, not spinning down the drive will cause extra power draw and heat generation; high temperatures also reduce the lifespan of hard drives.

References

External links

*APM V1.
Specification
( RTF file). {{DEFAULTSORT:Advanced Power Management BIOS