Windows Power ManagementInstant PC availability and energy savingsUpdated: December 4, 2001 
 On This PageOnNow design ensures that PCs are instantly accessible to users when needed, while remaining silent and consuming the least possible power when not actively working. In earlier laptop designs, designers tried to provide power management through BIOS control of devices. This resulted in problems such as delays while hard disks spin up, screens going blank during presentations, and low battery life due to inappropriate device usage. And previously, the only power management for desktop PCs was the power switch. Advances in Microsoft� Windows� operating systems are built on technologies defined in the Advanced Configuration and Power Interface (ACPI) Specification and other industry standards. With PCs designed to work with Windows power management, including good BIOS and device implementations, users can realize great power savings and take advantage of new PC usage scenarios that werent previously possible. Home and business PCs that run Windows�XP should be evaluated against new baselines for robust performance, fast startup, and maximum power savings, as described here. The OnNow Power-Managed PCMost users prefer to leave PCs turned off when not in use to reduce noise and electricity use. However, for new uses at home and in business, the PC must be instantly available to answer the phone, display e-mail, or browse the Web, but be silent and use minimal energy when not in use. To achieve this, the OnNow-capable hardware, operating system, and applications must work together in these ways:
Operating System-centered Power ManagementOnNow capabilities are only possible when the operating system controls power management across the entire PC system. For example, only the operating system can know that an application might be playing back video, even though the processor is idle. The operating system knows that a USB modem was added after BIOS POST completed, or that an IEEE�1394 camera was attached. The operating system can provide a command center, and it can know system parameters and states that applications and BIOS cannot know, such as:
In an OnNow system, Windows directs power management, integrates the activities of other components, and administers power policy. The power policiesthat is, the collection of decisions that determine how to save energy and when to go to sleepare based on end-user preferences, application needs, and system hardware capabilities. The operating system is responsible for the policy of when to go to sleep and how deep a sleep to go into, as well as how to operate the processor to obtain energy conservation and to meet thermal goals. Other components in the system are responsible for other aspects of power policy. Power Management and Energy ConservationEnergy shortages in the Western U.S. have focused public, government, and industry attention on the amount of power consumed by computers and related equipment. It is clear that further efforts are required to reduce the power consumption of PCs. Windows and Power ManagementSystem manufacturers, firmware developers, chipset designers, and many other members of the PC industry worked with Microsoft to develop the combination of hardware, firmware, and operating system support for a new power management model:
Power States To the user, the PC is either on or off, and other conditions are not visible. However, the ACPI specification defines a number of different power states that are supported by Windows operating systems.
System Wakeup and Devices The OnNow power management architecture in Windows supports a wide range of wake-up devices. However, wake-up features are enabled for a device only when requested by an application or when an application performs an action that implies wake-up, such as using the Telephony API (TAPI) to await an incoming phone call. When a device is enabled for wake-up, the combined capabilities of that device and the operating system dictate which sleeping states (S1S4) are available. The operating system policy is to put the system into the lowest power sleeping state that can support all enabled wake-up devices. Wake-up signals from devices on peripheral buses, USB devices, PCI add-on cards, and CardBus devices come through their host bus. Device Support for Power Management Each device in the system must implement power management in its driver and as required for the bus that it uses to attach to the PC. Each bus and each device class has a specification for how power management is to be enabled, and that defines the degree to which the device can participate in system-wide power management. USB devices, for example, can be especially good members of a power-managed PC. The USB specifications define device-specific power management, and the Windows�XP operating system can selectively suspend USB devices and the USB host controller, which allows the system to switch more often to a lower CPU power state. Windows Drivers and Power Management All driver sample and driver models for Windows�XP, Windows�2000, and Windows�Me implement power management support, without the driver developer having to apply special concern for the details of power control. Windows driver support provides an interface for drivers to perform these power management functions:
Applications and Power Management Windows provides an architecture for applications to participate in power management. Although Windows performs most power management tasks, applications can be designed specifically to handle power management functions. Application developers can integrate certain power management functions through system messages and APIs that provide mechanisms for:
For information, application developers can refer to the "Power Management" topic in the Microsoft Platform SDK. Win32 APIs for Power Management Windows�XP OnNow ImprovementsMicrosoft first delivered OnNow power management and ACPI support in Windows�98 and extended that support in Windows�2000 and Windows�Me. Windows�XP delivers new benefits and usage scenarios for end users, in addition to cost and environmental savings:
The Windows development team took bold steps in making fast resume times a reality for PCs running Windows�XP. Heres a summary of key changes.
Technical details about these changes and evaluation tools are provided on the FastBoot/Fast Resume web site. Fast Resume Tools and Guidelines Windows�XP and Mobile PC PowerWindows�XP Optimizations for Mobile PCs. The most radical behavior enhancement that mobile PC users will recognize when running Windows�XP come from the optimizations for resume state performances described in the previous section. The mobile PC user can close the lid while working, then open it hours later and begin working in the same place immediately. Windows�XP also includes optimizations for C-state performancethat is, the ability to run the processor in lower power states when the PC is idle. To do this, Windows developers made it possible for system manufacturers to program the C-state behavior for the specific processor used in a new mobile unit. Windows�XP and Processor Control. Today’s mobile PC users want to play DVD titles and games. This introduces new high-CPU-utilization demands to add to the problems that designers have struggled with in working to conserve battery life. To solve some of this problem, CPU manufacturers have introduced microprocessors that employ different performance states:
To support these new processors, Windows�XP has implemented native processor performance control and new processor performance control policies.
Windows�XP supports processor performance control using the Processor Objects defined in the ACPI Version 2.0 specification and the legacy SMI interface defined by Intel. Microsoft defines functionality, policy, and implementation details for each CPU vendor in a white paper. Windows�XP Native Processor Performance Control The Windows�XP developers implemented support for the individual technologies from different CPU vendors to provide key advances for processor performance control:
Recommendations to Mobile PC Manufacturers.
Guidelines for ManufacturersPrepare for the "Designed for Windows" Logo: Support S3 State.Windows Logo Program System and Device Requirements, Version 2.0, requires support for power management for x86-based PC client systems and devices, based on ACPI and other industry specifications. In particular, the baseline requirement for the "Designed for Windows" logo is: System ensures optimal user experience for suspend and hibernate, including correct BIOS support for the supported sleep states plus a Fast POST implementation. Target goals for fast system startup and resume times are required for x86-based client systems by January 1, 2002. (Note, however, that the initial release of the 64-bit Windows operating system will not include support for power management sleep states.) Windows Logo Program Requirements BIOS Setup. For PCs designed to run Windows 2000 and Windows XP:
Chip Support. Take advantage of CPU and chipset power management features:
Design for Fast Resume. The recommended performance guidelines for a consumer PC running Windows XP are:
Boot and resume times are measured from the time the power switch is pressed to being able to start a program from a desktop shortcut. Test and Evaluate. Thorough power management testing should begin with Windows XP. Microsoft provides the Bootvis tool for manufacturers who want to improve boot and resume times for new PCs.
Windows HCT 10.0 from WHQL Industry SpecificationsACPI Specification, Revisions 1.0b and 2.0�This specification defines an interface to the system board that enables the operating system to implement directed power management and system configuration. ACPI allows system manufacturers to build OnNow-capable systems. See Device Class Power Management Reference Specifications 1394 Trade Association Power Specification, Part 1: Cable Power Distribution; Part 3:�Power State Management PCI Bus Power Management Interface (PCI-PM) Specification, 1.1
This is the only specification that ensures PCI compatibility with Windows�2000/Windows�XP power management, which use PME# as the wake-up signal. See the PCI SIG web site at Simple Boot Flag Specification
This specification defines a mechanism for Windows, the system BIOS, and option ROM BIOSes to work together to optimize the boot process. See USB 1.1 Specification and USB device class specifications
These specifications define bus class and device class power management implementations. See the USB Implementers Forum web site at Microsoft ResourcesOnNow InitiativeImplementation Guidelines Fast startup/fast resume and Bootvis tool for Windows�XP Windows Logo Program for Hardware Windows�XP HCT 10.0 from WHQL | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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