Power Management

Windows XP Professional offers enhanced power-management features for desktop and mobile computers. The operating system supports the Advanced Configuration and Power Interface (ACPI) specification, which provides reliable power management and system configuration.

On an ACPI-compliant system, Windows XP Professional manages, directs, and coordinates power so the system is instantly accessible to users when needed, but consumes the least possible power when not actively working. In earlier power management architectures such as APM, the BIOS controlled the power state of system devices without coordinating with the operating system.

By contrast, devices and applications designed in compliance with the ACPI specification work with the operating system to respond to or request a change in the system power state. For example, an active application or input from a device, such as a mouse, indicates to the operating system that the computer or device is in use. The operating system s power policy manager then allocates full power to the system. Otherwise, the operating system attempts to put the computer into a lower power or sleep state. For example, a fax modem can operate while the system is in a low-power state, consuming little energy until the phone rings, at which time the system returns to full power to receive a fax, and transitions back into a low-power state when the system is no longer needed.

For more information about power management, ACPI, and Advanced Power Management (APM), see Supporting Mobile Users in this book.

Power Management Features

Windows XP Professional supports power management features based on ACPI and the OnNow design initiative. The operating system also provides more limited power management support for systems based on the older APM specification. Windows XP Professional includes the following power management features:

• Improved boot and resume performance. Reduced startup times mean the computer is ready for use quickly when powering up from a low-power state; reduced shutdown times allow the computer to quickly enter a low-power state.

• Wake-on support. The computer is in a low-power state when not in use, but can still respond to wake-up events, such as a phone call or a network request.

• Improved power efficiency. Features that improve power efficiency, especially for portable computers, include native support for processor performance control technologies, LCD dimming when on battery power, and turning off the laptop display panel when the lid is closed.

• Power management features in applications. Applications designed to use power management features in Windows tell the operating system what not to put into sleep mode. For example, presentation software, which might be displaying a screen but not actively processing, can tell the operating system not to put the monitor into a sleep state.

• Power policy ownership for individual devices. All devices designed to use power management features in Windows can participate in power management. When they are not in use, they can request that the operating system put them into a low-power state to conserve power.

• User interface for setting power preferences. In Control Panel, Power Options provide an interface through which a user can set preferences by choosing or creating power schemes, specifying battery usage options, and setting low-power alarms. If an Uninterruptible Power Supply (UPS) is present, Power Options can also manage the UPS.

• Decreased thermal output. Power reduction for unused devices results in decreased thermal output, which can prolong the life of hardware components.

• Decreased noise. When the computer is in a low-power state, power can be reduced sufficiently to cause cooling fans to turn off, thereby decreasing noise.

Caution 

Devices or applications that are not ACPI-compliant might prevent the operating system from putting the system into a low-power state, such as standby or hibernation. Non-compliant applications or devices might cause data loss or other failures if the computer wakes up and the application or device is not properly designed to handle a change in the system power state.

Power Policy Overview

The goal of power management is to conserve power while the computer is working and to put the computer into low-power states when it is not working. The power policy manager, in conjunction with applications and devices, implements the decisions that determine how to save energy and when to put the computer into a low-power state. Power policies are based on user preferences, the requirements of applications, and the capabilities of the system hardware. The implementation of power policy is distributed throughout the system, with system components acting as policy owners for the various devices. For example, the operating system is the policy owner responsible for determining when the computer goes into a low-power state, the level of power reduction, and how to operate the processor to reduce power consumption.

Each device in the computer has a power policy owner, which is the component that manages power for that device. Each policy owner works in conjunction with the operating system s policy for putting the computer into low-power states.

Device drivers carry out power policy controlling devices so that when power consumption or capabilities change for each specific device, these changes are shared among the drivers in the stack. Device-specific drivers save and restore device settings across transitions to and from low-power states. When a device power policy owner detects conditions that permit or require a change in the power state of a device, it sends a request to the power policy manager in the operating system to put the device into the desired state. For example, if a network cable is unplugged, the network device driver can notify the operating system that the network adapter does not need full power and can be put into a lower power state until the network cable is plugged back in.

Another instance of the use of power management is Wake on LAN. As a network administrator, you can send information over the network to run an application or configure a system remotely. A remote system in a low-power state powers up when it receives the LAN request, accepts the information, and then returns to a low-power state when the task is complete.