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Power Management Features
Power management controls a computer's use and conservation of power. Windows 2000 Professional implements a system-wide, integrated approach to power management. In compliance with the OnNow design initiative, the operating system's power management features conserve energy while the computer is working and put the computer to sleep when it is not working. Power management features in Windows 2000 Professional include the following:System Power Management The ACPI defines mechanisms for controlling the computer's sleep and wake states, allowing any device to wake up the computer.Device Power Management Windows 2000 Professional can put a device into a low power state based on the level of usage of the device. The ACPI design separates the decision process from the actual implementation, so decisions regarding a device's power state are made by the component best able to do so.Processor Power Management Windows 2000 Professional controls the processor's power state, enabling it to meet conservation, thermal, and audible noise goals.System Events The ACPI defines a mechanism for handling thermal events, docking, device insertion and removal, and system events.Battery Management On an ACPI-compliant computer running Windows 2000 Professional, battery management is provided by the operating system. For example, it is the operating system that determines battery-warning thresholds and calculates remaining battery capacity.Power management provides for:Minimal Startup and Shutdown Delays The system can sleep in a low power state from which it can "wake up" without rebooting.Greater Overall Power Efficiency and Prolonged Battery Life Power is only applied to devices that are being used. When a device is not being used, it can be powered down and then powered up later, on demand.Quieter Operation Powering down unused devices reduces noise.Unlike previous approaches to power management, OnNow manages power for the entire system including all system devices and peripherals. To make this possible, the operating system must direct power to the computer.With legacy power management architectures, the BIOS controls the power state of system devices. However, OnNow makes it possible for the operating system to coordinate power management activities at all levels and define the power-state transitions for the system.Computer power management means there is control over how the computer consumes energy and integrates its components. For example, a program that is active or input from a device such as a mouse, keyboard, or joystick indicates to the power management system that the computer is in use. As a result, the power management system allocates full power to the computer. Otherwise, the power management system puts the computer into a sleep state. Another example is a fax modem, which does not need to use full power all the time. The fax modem can operate in a "standby" state, consuming less energy until it needs to receive an incoming fax, at which time it is given full power. This section describes how Windows 2000 Professional supports power management.
How Power Management Works
By using Windows 2000 Professional power management, a computer can do the following:Turn On Instantly It can be ready for immediate use upon awakening from a sleep state when the user presses the power button.Respond to Wake Up Events It appears to be shut down when not in use, but it can still respond to wakeup events. Wakeup events might be triggered by a device receiving some input (such as modem receiving a call) or by software requesting that the computer wake up at a predetermined time (for example, an e-mail client that automatically downloads your e-mail in the morning).Adjust Software To Changing Power States Software adjusts its behavior when the power state of the computer changes. The operating system and applications work together to operate the computer, to deliver effective power management according to the user's current needs and expectations. For example, applications do not keep the computer busy unnecessarily; instead they proactively participate in shutting down the computer to conserve energy and reduce noise.Incorporate New Devices Into Power Management All devices, whether originally installed in the computer or added later by the user, participate in the power management scheme. Any new device can have its power state changed as system use dictates.Figure 20.1 shows the components of power management.
Figure 20.1 Power management components
NOTE
Applications developed before the advent of OnNow power management assume that the computer is always fully powered while the application is running. Such applications can inadvertently cause the system to enter a lower power state. In addition, these applications might stop responding when the computer wakes up, either when enough time passes or when a device is removed. In such cases, you can disable power management in Control Panel, or you can shut down the applications before the system goes to sleep.
Though Windows 2000 Professional performs most of the work for power management, applications must be designed for OnNow power management to make progression through various power states a seamless activity.
Understanding Power Policy
The goal of power management is to conserve power while the computer is working and to put the computer to sleep when it is not working. The decisions that determine how to save energy and when to go to sleep are implemented by the power policy. The power policy of OnNow is based on the preferences of the end user, the requirements of applications, and the capabilities of the system hardware. The implementation of power policy is distributed throughout the system, with different system components acting as policy owners for different aspects. For example, the operating system is the policy owner that is responsible for determining when the computer should go to sleep, determining the level of sleep into which the computer should go, and knowing how to operate the processor to reduce power consumption, heat, and noise.Each device in the computer has a policy owner. The policy owner is the component that knows how end users and applications use the device. Each policy owner must make appropriate decisions about power management for its device and work consistently with the operating system's policy for putting the computer to sleep.Carrying out power policy—actually controlling devices so that power consumption or capabilities change—is the responsibility of the device drivers for the affected device and is shared among the drivers in the stack. Device-specific drivers, known as minidrivers, are responsible for saving and restoring the device's settings across transitions to and from low-power states. When a policy owner makes the decision to put a device into a low-power state and communicates that to the device driver through the system, the device driver saves the device's settings and sends the request to the bus driver. The bus driver gives the command to the device to enter the low-power state. When the device is turned on, the bus driver powers up the device, and the device-specific driver restores the saved context.
