Sarasota PC Monitor
Tech Talk (11/02)
Power Management
by Brian K. Lewis, Ph.D.*
Member of the Sarasota Personal Computer Users Group, Inc.In 1996 Microsoft and a group of hardware manufacturers announced that "OnNow" would be built-in to all future PCs. This is a power management scheme which has resulted in the current Advanced Configuration and Power Interface (ACPI) specification.
Microsoft's stated purpose for this new proposal was: "OnNow is specifically designed to benefit consumers and corporate users with PCs that turn on instantly like VCRs or TVs, without rebooting, and that boot and respond automatically to incoming faxes, voice mail and e-mail even when they appear to be turned off. Users won't have to wait for the computer to boot because it never really turns off, and they save money in reduced energy use because the PC isn't fully powered when idle."
In order for this system to operate correctly, the computer BIOS, the operating system, the system hardware, device drivers and software applications must all participate in the power management scheme. Before the OnNow system was established power management was controlled by the computer BIOS. With OnNow, the operating system (OS) controls the power management and defines the power-state transitions for the system. The computer can be placed in a low-power state which shuts down the monitor and hard drive (S1) or a standby mode (S3) which reduces power to just slightly more than that needed to refresh RAM. There is also a hibernation mode (S4) which puts the computer in an "off" condition, but before shutting down it stores the contents of RAM in a file on the hard drive. In this case, when the power switch is turned on, the computer completes the initial power-on testing (POST), then reads the stored file and restores itself to the condition it was in when it powered down. This can occur much more quickly than a complete reboot. It is not as quick as recovering from standby mode, but it is not susceptible to data loss due to a power outage.
The current power policies have been developed based on "user" preferences, the requirements of applications, and the capabilities of the system hardware. The power policy manager, combined with the applications and devices, makes the decisions that determine how to save energy and when to put the computer into a low-power state. Every device that is designed to use power management can request a low-power state from Windows. Thus power savings can occur while the computer is working by reducing power to unused devices. Applications can also request specific power states, if they are ACPI compliant. There is also a user interface in "Power Options" for setting specific user preferences with regard to the timing of the onset of low-power states.
However, non-compliant devices or applications may prevent the system from entering a low-power state. And, if the system does power down, the non-compliant application or device may cause data loss or a failure of the system to return to full power. In theory, computers with a BIOS date of January 1, 1999 or later should support the OnNow specification. During the boot process, the operating system, Windows 98 or later, checks the BIOS and if it has an acceptable date or is on a "Good BIOS" list, the ACPI hardware abstraction layer (HAL) is loaded. If this is not done, then power management reverts to the older Advanced Power Management (APM) specification.
So, when do you use these modes and how do you set them? If you are running Windows XP, the first step is to determine if you have an ACPI compliant system. Go to Start, Control Panel, Performance and Maintenance, and then System. In the System Properties window, click on Hardware, then on Device Manager. In the hardware list, click on Computer and if "ACPI" is listed then your computer is running in ACPI mode. You should be able to successfully use any of the power saving modes. To access the power settings, either right-click an open space on your screen and select "Properties" then Screen Saver and Advanced, or click Start, Control Panel, Performance and Maintenance, and then click Power Options. Either way takes you to the full range of available power management options. If you are unable to set the Hibernate mode that indicates that your system does not support this function.
If you are going to be away from your computer for a short period of time, then you could set your computer to turn off the monitor and/or hard drives after a specified length of inactivity. This reduces the energy drain caused by the major power users.
The other option is to put your system into "standby" mode which reduces power to all the components of the computer. This is the lowest power setting in which the computer is still powered on. The hibernate mode is something that could be used instead of turning your computer off at night. That would save your current working setup so you could get right back to it the next day. However, be very cautious the first time you use standby or hibernate. Until you've used these modes, you can't be certain your system or applications fully support the latest ACPI functions.
If you are using an earlier version of Windows than XP, I'm not sure you would want to use any power saving mode other than shutting down the monitor and the hard drives. It has been my experience that too many of the Win98/98SE systems do not work well with standby or hibernate. Part of the problem is that many computers of this vintage have legacy applications and hardware that were designed to assume that the computer is always fully powered. Secondly, the newest ACPI specification is just over two years old and much hardware/software may not yet be redesigned to match these specifications. Windows XP is really the only OS which is written to take full advantage of the ACPI 2.0. Earlier versions of Windows were written with the first ACPI specification and much of the hardware/software on earlier computers is not really compatible with all the newer specifications. So care in setting power settings is required. If you have trouble with a specific setting, just realize that your system may not be fully compatible and try something a little less power saving. If nothing else works, just turn off the power switch on your monitor when you walk away for a break. That is certainly the simplest of all the energy saving schemes!
There is another facet of this power management scheme. On many computers manufactured in the past 3-4 years, the power switch does not shut off power to your computer. In fact, the switch is no longer connected to the 120v power source as it used to be. Instead, when the computer is off, this switch is passing a current of more than 2v (5v max) through the motherboard to the Power Supply. When the switch is pressed, the voltage drops, due to grounding, to less than 0.8v. This is the signal that activates the power supply and the motherboard. So even when the system is "powered off' by the operating system, a low voltage supply is being continuously provided to the motherboard. This is somewhat in the manner of the low voltage supply to your TV set that provides the "Instant On" when you press the power switch.
Another point about this switch is that it doesn't turn the computer off instantaneously as a true power switch would. Instead, you have to hold it in for at least 4 seconds while the ACPI module notifies the OS which then attempts a clean shut-down of the system. On some systems, the BIOS allows the switch to either do a "soft-off" or to switch the system into stand-by mode.
There is a true 120v power switch on most new computers, but it is located on the back of the case and is part of the power supply. Turning off this switch isolates the entire computer from a power source. I would suggest using this as a vacation switch if you plan on being gone for several weeks or more.
If your system is a year old or less, it should support the latest ACPI 2.0 specifications. In that case you should be able to use any of the various power saving modes. The only catch being the software applications. So try out the various settings, carefully so you don't lose any data if things go wrong. With Win98/SE, I would suggest sticking to the monitor and hard drive power down functions. Standby is less likely to work on an older system.
*Dr. Lewis is a former university and medical school professor. He has been working with personal computers for more than thirty years. He can be reached via email at brian_klewis@hotmail.com or voice mail at (941) 925-3047. :
Copyright 2002. This article is from the November 2002 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication. We would appreciate receiving a copy of the publication the reprint appears in, please send to above address, Attn: Editor. For further information about our group, email: admin@spcug.org/ Web: http://www.spcug.org/The Sarasota Personal Computer Users Group, Inc. has 1,300+ members and was established in 1982. We are members of the Assoc. of PC User Groups (APCUG), the Florida Assoc. of PC Users Groups, Inc., and we are members of the America Online Ambassador Program.
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