APM Support for Windows 2000 Professional

This article provides background information and technical implementation details for Advanced Power Management (APM) support under Microsoft Windows 2000.

Windows 2000 includes an APM solution based on the APM 1.2 specification. Windows 2000 native support includes only Suspend, Resume, Battery Indicator, and Off (that is, the operating system will power off the system when shut down or hibernate completes). Support is not implemented for other APM BIOS calls.

NOTE: The OnNow power management architecture in the Windows 2000 operating system is designed to provide the optimum user experience and system performance on systems with Advanced Configuration and Power Interface (ACPI) system board and BIOS support to ensure that the operating system controls both device and system power states. The OnNow/ACPI architecture also provides an integrated system-wide user interface that allows the user to conveniently and simply set preferences for power usage.

Windows 2000 support for APM as described in this article is provided solely to ensure compatibility for well-designed legacy APM systems. System designers are strongly encouraged to implement ACPI solutions on both mobile and desktop systems that run either Windows 2000 or Windows 98 operating systems. Power management support in future versions of Windows operating systems will be based solely on ACPI.

On This Page

	APM Capabilities in Windows 2000
	Windows 2000 Setup and APM
	APM in the Registry
	Cautions and Tips for APM Support and Windows 2000
	How Microsoft Tested for the AutoEnable APM List

APM Capabilities in Windows 2000

APM will not work with every system running Windows 2000. Microsoft has tested APM-capable systems to determine how well each system and BIOS combination supports APM.

APM support is being implemented to support mobile systems that meet the following criteria:

AutoEnable APM List. In a similar manner to that used for ACPI BIOS support under Windows 2000, systems that passed testing at Microsoft have been placed on the "AutoEnable APM List." This list is used to let Windows 2000 Setup know whether it is safe to install APM support. This AutoEnable APM List includes the APM system/BIOS combinations that Microsoft believes are stable and ready to support APM while running Windows 2000. Windows 2000 Setup will install and enable APM support automatically on such systems.

Disable APM List. Microsoft placed entries for specific system/BIOS combinations on the Disable APM List that malfunctioned in testing when APM was turned on. For system/BIOS combinations on the Disable APM List, Windows 2000 Setup will not install or enable APM support.

Neutral Systems. Systems that are not on the AutoEnable APM List or the Disable APM List are considered "neutral." In such cases, the user can manually install the APM support in Windows 2000 by using the installation method provided with the operating system:

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Windows 2000 Setup and APM

During installation, Windows 2000 Setup Loader uses values in Biosinfo.inf to determine whether the system is on the AutoEnable APM List or the Disable APM List. Then Setup acts on the system as follows:

Users and OEMs must take care in choosing to enable APM support on "neutral" systems, because there is no guarantee of system behavior when running the NTAPM service.

CAUTION: End-users and OEMs must not attempt to circumvent the Disable APM List. Forcing APM support to run on a system that is on the Disable APM List will generally produce erratic, undesirable results on an otherwise stable system.

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APM in the Registry

The following registry key can contain value entries that indicate whether Windows 2000 Setup found the system to be on the AutoEnable or Disable APM lists:

HKEY_LOCAL_MACHINE\ SYSTEM\ CurrentControlSet\ Control\ BiosInfo\ APM

A value of 1 means AutoEnable; a value of 2 means Disable. If there is no entry, the system is considered to be neutral.

The following registry key contains the MultifunctionAdapter entries for the system:

HKEY_LOCAL_MACHINE\ HARDWARE\ Description\ System\ MultifunctionAdapter

If the Windows 2000 Ntdetect module finds an APM BIOS on the system, it creates a MultifunctionAdapter subkey with an entry for APM. Such subkeys are numbered on each system, starting with 1. To find the MultifunctionAdapter subkey for APM on a specific system, search for a value of Identifier=APM.

In the following example, the MultifunctionAdapter subkey for APM is 22, but it could be any number:

HARDWARE\DESCRIPTION\System\MultifunctionAdapter\22
Class Name:	Adapter
Last Write Time:	12/4/98 - 3:30 PM
Value 0
	Name:	Component Information
Value 1
	Name:	Configuration Data
	Type:	REG_FULL_RESOURCE_DESCRIPTOR
Value 2
	Name:	Identifier
	Type:	REG_SZ
	Data:	APM

If there is no such value of Identifier=APM under a MultifunctionAdapter subkey in the registry, then Ntdetect didn't find an APM BIOS for the system; therefore, the system's class is set to neutral. If such a MultifunctionAdapter subkey exists, it means that APM is present; however, this does not necessarily mean that APM is usable under Windows 2000. Setup checks Biosinfo.inf for that information.

The APM registry information stored by Ntdetect is defined in the following structure:

typedef struct _APM_REGISTRY_INFO {
// To ensure dual-boot compatibility 
// with Windows NT 4.0:
	UCHAR		ApmRevMajor;
	UCHAR		ApmRevMinor;
	USHORT		ApmInstallFlags;
	// Defines for 16-bit interface connect:
	USHORT		Code16BitSegment;
	USHORT		Code16BitOffset;
	USHORT		Data16BitSegment;
	// New part of the structure for Windows 2000:
	UCHAR		Signature[3];
	UCHAR		Valid;
	// Detection Log Space:
	UCHAR		DetectLog[16]; 
} APM_REGISTRY_INFO, *PAPM_REGISTRY_INFO;

When the NTAPM service (Ntapm.sys) starts, it sets the value of ApmActive to Active=1 in the following registry key:

HKEY_LOCAL_MACHINE\ CurrentControlSet\ Control\ ApmActive

A system with an ACPI motherboard that doesn't support APM will install and function as an ACPI system. The following registry key will be present, with a value of Start=0:

HKEY_LOCAL_MACHINE\ SYSTEM\ CurrentControlSet\ Sevices\ ACPI

Not all computers have either APM or ACPI, such as legacy mobile systems with no power management method. In these cases, no power management support is provided by the operating system.

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Cautions and Tips for APM Support and Windows 2000

Disable APM on Multiprocessor Systems. Be certain that APM is turned off in the BIOS of a multiprocessor system, because such a system can seriously malfunction when Ntdetect tries to call APM. Microsoft will not support APM on multiprocessor systems in Windows 2000. However, ACPI works well with multiprocessor systems.

Disable APM on Desktops Systems. APM BIOS and APM support in the operating system must be turned off for all desktop systems. The APM BIOS can cause problems even if the system isn't running operating system support for APM.

For example, a PC with a PCI add-on graphics adapter might have Ntapm.sys turned off and the APM BIOS turned on, which will time out and suspend the system at some point. The APM BIOS cannot correctly save and restore the video display, so upon resume, the video display is gone.

Microsoft encountered another example in testing: a system employed as a server had Ntapm.sys turned off in the operating system, but the APM BIOS was turned on. The BIOS would time out and suspend the system, because it did not appear busy, even though the server was heavily loaded.

Disable APM on an Incompatible System. If APM support has been enabled, but Windows 2000 and APM are not compatible on a specific system, try the following options:

Caution for Graphics Adapter Changes. End users should be cautioned that using a graphics adapter other than the one included in their system could result in problems, because changing the graphics adapter will change the system's APM configuration and function. The APM BIOS might not detect a new graphics adapter added to a system or a graphics adapter in a docking station. If the adapter is not discovered by the APM BIOS, Suspend will not work.

Turn Off Screen Blanking. If a video timeout is set in the BIOS and the screen blanks, moving a built-in pointing device can wake the system, but a USB or other external mouse cannot. To prevent this from occurring, turn off screen blanking in the BIOS, and use the Blank (default) screen saver.

Pre-set Single Power Button. On some systems, there is a single button that can be set in the BIOS to serve as either the Off or Suspend/Resume button. The setting for the button's functionality must be the same for the BIOS and in the user interface setting in the operating system. The most common setting for button functionality is Suspend/Resume.

Ensure Operating System Control. The system BIOS should be configured to allow the operating system to turn off the screen, disk, and so on, rather than letting the APM BIOS control these actions. If APM performs these tasks (rather than the operating system), the components won't always be restored to the original settings upon wake-up. If the operating system controls these devices, there is a more reliable chance of the device being restored to its previous state.

In particular, timeouts in the BIOS should be set to NONE or for as long as possible. Then be sure that the operating system timeouts set in the Control Panel Power applet are shorter than those set in the BIOS. (Some APM BIOSes will refuse to function if the timeouts are set to NONE, so use the longest possible value in such cases.)

APM and User Interface Options. The user can determine whether APM is running on a system by selecting Shutdown from the Start menu--if APM support is running, the Stand-By entry appears in the list of choices.

Two options must be set manually using the Control Panel Power applet:

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How Microsoft Tested for the AutoEnable APM List

In testing APM capabilities to determine which systems would be added to the AutoEnable APM list, the APM test team at Microsoft focused on systems supported in previous versions of Windows, testing systems already available at Microsoft. After those systems were categorized as listed earlier, the testing coverage was expanded to include the systems in the Microsoft test base until all available systems were tested.

NOTE: There is no program for OEMs to submit APM systems for testing, nor will APM systems be accepted if submitted to Microsoft for testing.

Call to action for APM BIOS:

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