What are the different power states of Advanced Power Management?


The Advanced Power Management (APM) Specification defines the following power states:

Three of these states apply both to individual computer components and to your computer as a whole. The suspended state is a special low power condition that applies to your computer as a whole, and not the individual components.


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

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Stand-by is an intermediate system-dependent state which attempts to conserve power. Stand-by is entered when the central processing unit (CPU) is idle and no device activity is known to have occurred within a specific period of time. Your computer will not return to ready until one of the following events occur: • A device raises a hardware interrupt

• Any controlled device is accessed

All data and operational parameters are preserved when your computer is in the Stand-by state.


The Suspended state is a computer state which is defined to be the lowest level of power consumption available that preserves operational data and parameters. The suspend state can be initiated by either the system Basic Input Output System (BIOS) or the software above the BIOS. The system BIOS may place your computer into the suspended state without notification if it detects a situation which requires an immediate response such as the battery entering a critically low power state. When your computer is in the Suspended state, computation will not be performed until normal activity is resumed. Resumption of activity does not occur until signaled by 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 and inactive. Data and operational parameters may or may not be preserved in the Off state.