Advanced Power Management

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Advanced power management (APM) is an API developed by Intel and Microsoft and released in 1992[1] which enables an operating system running an IBM-compatible personal computer to work with the BIOS (part of the computer's firmware) to achieve power management.

Revision 1.2 was the last version of the APM specification, released in 1996. ACPI is intended as the successor to APM. Microsoft dropped support for APM in Windows Vista. The Linux kernel still mostly supports APM, with the last fully functional APM support shipping in 3.3.


The layers in APM

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.

Power management events

There are 12 power events (such as standby, suspend and resume requests, and low battery notifications), plus OEM-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:[1]

Name Code Comment
System Standby Request Notification 0x0001
System Suspend Request Notification 0x0002
Normal Resume System Notification 0x0003
Critical Resume System Notification 0x0004
Battery Low Notification 0x0005
Power Status Change Notification 0x0006
Update Time Notification 0x0007
Critical System Suspend Notification 0x0008
User System Standby Request Notification 0x0009
User System Suspend Request Notification 0x000A
System Standby Resume Notification 0x000B
Capabilities Change Notification 0x000C Due to setup or device insertion/removal

Power management functions:[1]

Name Code Comment
APM Installation Check 0x00
APM Real Mode Interface Connect 0x01
APM Protected Mode 16-bit Interface Connect 0x02 Avoids real or virtual86 mode.
APM Protected Mode 32-bit Interface Connect 0x03 Avoids real or virtual86 mode.
APM Interface Disconnect 0x04
CPU Idle 0x05 Requests system suspend.
0) Clock halted until timer tick interrupt.
1) Slow clock[1]
CPU Busy 0x06 Driver tells system APM to restore clock speed of the CPU.
Set Power State 0x07 Set system or device into Suspend/Standby/Off state.
Enable/Disable Power Management 0x08
Restore APM BIOS Power-On Defaults 0x09
Get Power Status 0x0A Supports AC status "On backup power". And battery status.
Get PM Event 0x0B Checks for APM events. Shall be called once per second.
Get Power State 0x0C
Enable/Disable Device Power Management 0x0D
APM Driver Version 0x0E
Engage/Disengage Power Management 0x0F APM management for a specific device.
Get Capabilities 0x10
Get/Set/Disable Resume Timer 0x11
Enable/Disable Resume on Ring Indicator 0x12
Enable/Disable Timer Based Requests 0x13
OEM APM Installation Check 0x80 Tells if APM BIOS supports OEM hardware dependent functions.
OEM APM Function 0x80 Access to OEM specific functions.

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.


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

See also


  1. 1.0 1.1 1.2 1.3 "Advanced Power Management (APM) BIOS Interface Specification Revision 1.2 February 1996 Intel/Microsoft".<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles> 090429

External links