Description of the Different Advanced Power Management States

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This article describes the different Advanced Power Management (APM)states.
The Advanced Power Management (APM) Specification defines the following power states:
  • Ready
  • Stand-by
  • Suspended
  • Hibernation
  • Off
Three of these states apply both to individual computer components and toyour computer as a whole. The suspended state is a special low powercondition that applies to your computer as a whole, and not the individualcomponents.


In the ready state, your computer or device is fully powered up and readyfor use. The APM definition of Ready only indicates that your computer ordevice is fully powered on, it does not differentiate between active andidle conditions.


Stand-by is an intermediate system-dependent state which attempts toconserve power. Stand-by is entered when the central processing unit (CPU)is idle and no device activity is known to have occurred within a specificperiod of time. Your computer will not return to ready until one of thefollowing events occur:
  • A device raises a hardware interrupt
  • Any controlled device is accessed
All data and operational parameters are preserved when your computer is inthe Stand-by state.


The Suspended state is a computer state which is defined to be the lowestlevel of power consumption available that preserves operational data andparameters. The suspend state can be initiated by either the system BasicInput Output System (BIOS) or the software above the BIOS. The system BIOSmay place your computer into the suspended state without notification ifit detects a situation which requires an immediate response such as thebattery entering a critically low power state. When your computer is inthe Suspended state, computation will not be performed until normalactivity is resumed. Resumption of activity does not occur until signaledby an external event such as a button press, timer alarm, and so on.


Windows XP has built-in support for hibernation (OS-controlled ACPI S4 sleep state). Hibernation saves the complete state of the computer and turns off the power. The computer appears to be off. This is the lowest power sleeping state available and is secure from power outages.

When you resume from a hibernated sleep state, the BIOS performs the normal POST, and then reads the hiberfile that was created to save the computer state. The computer returns to the last state it was in before the computer entered hibernation mode. Hibernate mode reduces start time.

Note that when you service the computer, make sure you shut down the computer instead of using hibernate mode.

Windows XP supports Hibernate capabilities (ACPI S4 sleep state). Windows XP S4OS Hibernate is available on new computers and upgrades that meet the requirements for the correct video drivers and no VXD audio drivers.

S4 is the hibernation state. It is very close to the APM Suspend to Disk state.

Hibernation Requirements

Computer must support APM 1.2, or ACPI.
A paging device that supports D3 (note - certain SCSI configurations do not support this).
WDM audio.
No legacy capture devices connected.
WebTV for Windows is not installed.
Non-ICS Host (client is OK).


When in the Off state, your computer or device is powered down andinactive. Data and operational parameters may or may not be preserved inthe Off state.

Note If you use Windows Vista, you can visit the following Microsoft Web page for help with power consumption and battery life problems:
APM 1.0 Specification, Microsoft Windows 98 Resource Kit

Article ID: 308535 - Last Review: 12/06/2015 05:55:55 - Revision: 2.2

Microsoft Windows XP Professional, Microsoft Windows XP Home Edition

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