The current ACPI general specification (version 2.0) is only about 2 years old, and it's still evolving. The last revision--2.0a--was released in March. It's actually a well-thought-out and well-supported spec whose overall intent is to define a way in which all the hardware and software on a system can communicate and interact with regard to their current power states, needs, and abilities. One familiar example of this interaction is the "intelligent" power switch found on most new PCs: When the PC is on, pressing the nominal "power" switch doesn't simply kill the power; rather, it sends a signal through ACPI to the operating system, which then initiates a controlled shutdown. With ACPI, an operating system may even let you assign different functions--sleep, suspend, hibernate, off--to the power switch.
Most newer systems, add-on devices, and operating systems support the current ACPI spec and thus offer excellent power control. They can, for example, shut down to any of several low-power or power-off modes, then resume operation without difficulty.
But older systems may not handle power-management tasks so gracefully, and here's why: ACPI replaced an older, less-versatile Advanced Power Management spec, which basically just put the BIOS in control of power management. This is fundamentally different from ACPI, so hardware and software designed for APM may not work well with ACPI, and vice versa.
If the computer industry had made a clean break from APM, things might have worked more smoothly, but that's not how it went. For several years in the mid- to late 1990s, APM and ACPI were both in wide play. As a result, it was not at all uncommon for a given system to have an operating system, a system BIOS, and hardware drivers that might be designed either for different power-management standards or different versions of the same standard.
What's more, the old APM spec was never very good to begin with, and the initial versions of ACPI also had rough edges that sometimes prevented systems from living up to their power-management potential.
In short, for several years, power management was a mess. This is one reason power-management problems (including Windows' infamous "hang on shutdown" glitch) are so common in systems more than a couple of years old.
(By the way: The permanent fix for that type of problem usually involves getting current drivers for all add-on hardware and updating your system BIOS. See, for example, How To Troubleshoot Windows 98 Shutdown Problems. Or check out the many third-party sites with information on solving shutdown problems here. Microsoft also has archival information on the now-obsolete APM spec that may be useful if you're struggling with power management on older, APM-era hardware.)
Fortunately, the current ACPI spec makes most of this moot: If your operating system, BIOS, and device drivers all support the current ACPI 2.0 spec, odds are power management will work as it should on your system.
Adjusting The Sleep/Power States
If your system supports the current ACPI spec, you may be able to attain very high levels of energy savings by either or both of two methods: First, see what operating-system-level power-management options are available to you. For example, in Windows, open Control Panel, click on the Power applet, and explore your choices.
Your PC's BIOS may also offer secondary adjustments. For example, some BIOS setup utilities let you assign a default ACPI power setting to be used whenever the operating system calls for a sleep mode. I have a system with an Award BIOS, for instance, that lets me specify either the S1 or S3 sleep modes (see above). By choosing S3, I can ensure greater energy savings. Conversely, if my system experiences trouble going into or recovering from the "deep sleep" of S3, I could use S1 to achieve greater stability, albeit with lesser energy savings. To access your BIOS setup, watch your screen at startup for instructions: Usually, you must press a key--the "Del" key is commonly used--before you hear the system beep. If you can't find a way into your BIOS setup, visit your system vendor's Web site for information.
The exact power-management choices open to you will depend on the specifics of your hardware and software, but here are some links to get you started:
A feature article called Power Management In The Corporate Network (Win2K/XP) offers good, practical implementation advice on ACPI systems.
Users running Linux on Intel- or AMD-type hardware can obtain tons of ACPI information here.
Once you know the choices available to you, you can make better use of them, saving energy--and money--in the process!
What are your power-management experiences, tips, and tricks? Join the Listening Post discussion at http://www.informationweek.com/forum/fredlanga