Do More with Less: Rev up power management with ACPI support in Windows XP

Set power options and control power usage by building on ACPI specifications.

The expression “power to the people” has finally come true—in a very literal sense. Power management and conservation have always been the bane of the portable computer user’s existence, but Windows XP offers the people a way to set power options and control power usage, building on the industry-standard Advanced Configuration and Power Interface (ACPI) specifications. In this Daily Drill Down, I’ll discuss ACPI—what it is, where it came from, how it works, and how to use Windows XP’s support of it to optimize power usage through XP’s Power Options.

Why power management?
What are the other benefits of being able to manage your computer’s power usage, aside from saving energy? With Windows XP and ACPI, you can conserve power by making a few Power Options adjustments without the risk, for example, that the monitor will be turned off in the middle of a presentation. You can also ensure that the computer “wakes up” when it’s needed to perform prescheduled tasks, or you can configure power options so that scheduled tasks will not be run if the computer is using battery power. This flexibility puts the power in your hands, letting you optimize power usage to fit your own preferences and situation.

The ACPI specifications
The ACPI specifications were first copyrighted in 1996; version 1.0 was released in December of that year. Interim revisions, 1.0a and 1.0b, were released in 1998 and 1999. The current version is revision 2.0, published in July 2000; its specs can be downloaded in PDF or Word document format.

Although beneficial for laptop and notebook computers that run on limited battery power, ACPI specifications also apply to desktop, workstation, and server machines.

What is ACPI?
ACPI provides an interface for a power management scheme that can be directed and configured through the computer’s operating system. The operating system can determine how much power is allocated to individual peripherals. This allows devices to be powered down or turned off when not in use to conserve energy. This operating system-directed power management is also called OSPM. ACPI’s predecessor, Advanced Power Management (APM) was BIOS-based, yet the operating system was not aware of its activities.

Who owns ACPI?
A group of major powers in the PC industry—Microsoft, Intel, Compaq, Toshiba, and Phoenix Technologies—developed the ACPI specifications, but ACPI is an open standard that can be used by both hardware and software vendors to create products that work with the interface. To use ACPI’s features, both the hardware and the operating system software must support ACPI.

ACPI hardware compliance
An ACPI-compliant computer has a system BIOS that supports ACPI. You may need to set the BIOS configurations to run in ACPI mode. All the computer’s components (sound card, video card, etc.) must also be capable of supporting power management.

Using ACPI with Windows XP
Windows 2000 supports ACPI, and its implementation is fine-tuned in Windows XP. Fast startup was a design goal for Microsoft in the development of XP, and ACPI is part of the OnNow design initiative, a group of specifications that was developed to allow computers to come on instantly, like televisions.

When you install the Windows XP operating system, Setup will detect whether the computer’s components support power management. If you have legacy components that don’t, ACPI won’t be installed. If ACPI is already installed and you add legacy devices, this can cause erratic behavior.


What’s Plug and Play got to do with it?
Running your computer in ACPI mode lets you take full advantage of Windows XP’s Plug and Play abilities, as long as the peripheral devices are PnP-compliant. You can install PnP devices without having to reboot, and the Power Options settings in Control Panel can be configured to ensure that the computer runs efficiently when you’re in the process of installing or removing peripherals.

Configuring Power Options in Windows XP

Figure A
Use the Power Options applet in the Windows XP Control Panel to configure ACPI settings.

Power Options can only be used if your computer is ACPI-compliant. To configure settings, select the Power Options applet in the Windows XP Control Panel, shown in Figure A.

Power Options may vary from one computer to another, depending on the hardware configuration detected by the Power Options function. Only the options that are available with your hardware will be shown.

On the Power Schemes tab, you can choose a set of preconfigured settings that is optimized for a particular type of computer or situation. For example, you can select a power management scheme that’s appropriate for portable or laptop computers in general use or one that is best when doing presentations. Default power schemes include:
  • Home/Office Desk
  • Portable/Laptop
  • Presentation
  • Always On
  • Minimal Power Management
  • Max Battery

Figure B
You can select a preconfigured power scheme or create one of your own.

The settings control the amount of idle time before the monitor and hard disks are turned off and the time at which the system goes into Standby or Hibernation mode (discussed in the next sections). You can also edit the settings for the default power schemes or create a custom scheme and save it. The Power Schemes configuration sheet is shown in Figure B.

On the Advanced tab, you can choose whether to display the power icon on your taskbar. You can also specify here that a password must be entered when the computer resumes operation from Standby mode. (By default, the password requirement is enabled.)

In the lower section of this tab, you can control the behavior of your computer’s power and sleep buttons. For example, you can choose one of the following actions to occur when you press the power button:
  • Shut Down
  • Hibernate
  • Go Into Standby
  • Prompt For Instructions On What To Do
  • Do Nothing

Figure C
The Advanced tab allows you to configure the behavior of the power buttons.

By default, pressing the power button causes the computer to shut down, and pressing the sleep button causes it to go into Standby mode (see Figure C).

Hibernation mode
Hibernation mode allows all the applications that were running and the documents that were open at the time you shut down to be restored when you turn the computer back on. This is accomplished by saving everything in memory to the hard disk, so you must have enough free disk space to save everything in the system’s RAM.

Figure D
Enable or disable Hibernation with the check box.

To use Hibernation, this mode must be supported by the BIOS, and it must be enabled in the Windows XP Power Options. By default, Hibernation is enabled; you may want to disable it if you’re low on disk space. To enable or disable Hibernation, use the Enable Hibernation check box on the Hibernate tab of the Power Options Properties, as shown in Figure D.

Your computer will automatically go into Hibernation after it has been idle for the time period set on the Power Schemes tab. You can also put the computer into Hibernation mode manually, if you’re logged on as a member of either the Administrators or Power Users group, by following these steps:
  1. Click Start.
  2. Click Shut Down on the Start menu.
  3. In the What Do You Want The Computer To Do dialog box, click the drop-down arrow and select Hibernate.

Group policy settings on the network may prevent you from manually placing the computer in Hibernation mode even if you belong to either the Administrators or Power Users group.

Standby mode
In Standby mode, the computer is not turned off completely; the monitor and hard disks are powered down so the computer uses less power. The desktop is restored quickly when you bring the computer out of Standby by clicking the mouse or pressing a key.

It’s important to understand the differences between Hibernation and Standby modes. Unlike with the Hibernation mode, the desktop state is not saved to the disk when you put the computer in Standby. This means you could lose unsaved data if the power fails or is disconnected while the computer is in Standby mode.

Also realize that a computer in Standby mode is not turned off, although it may appear to be. The operating system can reactivate automatically while in Standby to perform certain tasks.

If you’re asked to turn off your computer when flying on a commercial airliner, placing the computer in Standby mode does not meet this requirement. The FAA can impose penalties for failure to comply with these requirements.

Your computer will automatically enter Standby mode at the time set by your selected power scheme. You can also place the computer in Standby mode manually. The procedure is similar to that of manually placing the computer in Hibernation mode, except that you select Stand By from the drop-down list. You can also configure the computer to go into Standby when you close its lid, by using the Advanced tab of the Power Options Properties.

Setting battery warning alarms
When you’re using a portable computer running on a battery, it’s important to conserve energy as much as possible; it’s also beneficial to know when your battery power is getting low. You can use the Power Options to set an alarm to warn you when the battery is low or when the battery reaches a critical level, or both.

Use the Alarms tab in the Power Options Properties to enable the alarms, configure the power levels at which the alarms will be activated, and select the type of notification. You can also choose to have the computer automatically initiate a shutdown when the alarm level is reached.

The Alarms tab won’t appear if your computer doesn’t support this function. Note that in Figures A through D, the Alarms tab does not appear because the computer is a desktop computer rather than a portable machine.

Using power management for scheduled tasks
The Scheduled Tasks feature in Windows XP allows you to run programs or scripts or perform other tasks automatically at preset times. These prescheduled tasks can use valuable battery power, so Microsoft has provided a way to control power usage for scheduled tasks.

Figure E
You can set power options for scheduled tasks via each task’s properties sheet.

Access the Scheduled Tasks via the Start | Programs | Accessories | System Tools menu, right-click the task for which you want to set power options, as shown in Figure E, and select Properties.

Figure F
Use the Settings tab on the task’s properties sheet to configure power settings.

On the Settings tab of the task’s properties sheet, under the Power Management section, shown in Figure F, you can select or deselect the following options:
  • Don’t Start The Task If The Computer Is Running On Batteries
  • Stop The Task If Battery Mode Begins
  • Wake The Computer To Run This Task

These instructions for managing power options in Windows XP illustrate how Windows XP’s support of ACPI gives users greater flexibility and control in managing the computer’s power usage. If your users' XP computers’ BIOS and hardware peripherals are ACPI-compliant, ACPI support will be installed automatically when you install the operating system, and its functionality can be used to turn off peripherals that are not being used. ACPI can put the computer into a state of Standby or Hibernation so that less energy is consumed; yet the system is ready when you want to resume work quickly. You can also use it to better control scheduled tasks to maximize battery power usage.


Debra Littlejohn Shinder, MCSE, MVP is a technology consultant, trainer, and writer who has authored a number of books on computer operating systems, networking, and security. Deb is a tech editor, developmental editor, and contributor to over 20 add...

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