What is APM Mode?

What is APM Mode?
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 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.

Ready
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.

Stand-by
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. Only supported with Windows98 SE.

Suspended
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.

Hibernation
     Windows Millennium Edition 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.
     Of all the possible power saving modes hibernation is by far the most confusing. In APM hibernation is considered as a "special implementation of the APM Suspend state". APM did not require hibernation support. APM BIOS manufacturers had a hard time implementing this because it required knowledge of the file system (most APM BIOS can only provide this support if the boot partition is FAT (not FAT32 or NTFS). APM 1.2 specifies a special reserved disk partition for this use. This partition is simply blown away by most disk-partitioning software and there appears to be no way for and end user to re-create it.
     To further complicate things Windows 2000 implements hibernation itself without using the support of either APM or ACPI.

Off
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.

How APM Works
     In contrast to ACPI, there is the APM BIOS version 1.2 specification. With APM, the BIOS controls system power management. The BIOS has timers that monitor most interrupts and the data that is being transmitted through the input/output (I/O) port. When the timer for a device exceeds a value set in the BIOS setup, the BIOS turns off the device. When the system-wide timer exceeds some value set in the BIOS, the BIOS sends a message to Windows 2000 to put the entire computer in a low-power state. Windows 2000 then verifies that the computer is ready to be placed into standby mode or hibernate mode, and it tells the BIOS to do so. The APM BIOS is also responsible for monitoring the battery status and requesting a low power state if the battery is getting low. In general, APM has the following limitations:
     Inconsistent user interfaces. Each BIOS has its own user interface and its own power management behavior. This means every computer operates differently - users have to be retrained on each computer.
     Reasons for suspend are not known. Because of the architecture of the APM BIOS interface, the APM BIOS cannot inform Windows 2000 that a request is a response to the user pushing a sleep button, to the BIOS sensing that the system is idle, or to the battery running out of power. As a result, Windows 2000 must always honor this suspend request and attempt to put the computer into low-power state - even if the computer is not idle. For this reason, it is recommended that you set the BIOS time-out settings to a very large value or turn them off.
     Devices might be turned off at inappropriate times. By monitoring I/O ports and interrupts, the BIOS is essentially trying to determine what the user and the applications are doing. Although this often works, there are many scenarios in which the response of the BIOS is incorrect - for example, the BIOS turns off or slows down a computer when it is in use (such as a screen saver turning on in the middle of a presentation), or the BIOS does not turn off a truly idle computer.
     BIOS detects activity only on devices that are residing on the motherboard. The BIOS cannot detect devices that are not on the motherboard, such as USB devices and IEEE 1394 devices. As a result, the system might appear to the BIOS as if it were not in use, even if one or more of these off-motherboard devices actually is in use.

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