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Understanding Your BIOS Power Management Settings


Power Management
This is for Green PCs, or those complying with the EPA Energy Star programs; the intention is to save unnecessary power usage if the system becomes inactive. Power is reduced automatically to the devices and restored as quickly as possible when activity is detected (that's the theory, anyway). This is usually done with idle timing and event monitoring techniques. A Power Management Unit (PMU) monitors interrupt signals through an interrupt events detector. If it hears nothing for a while, the system is put gradually and progressively to sleep, in that the longer the time inactive, the more parts of the system will close down. However, setting all this up in the BIOS only goes so far - you should do it in your operating system as well (not NT) - certainly, ensure that 95/98's compatibility with APM 1.0 is enabled through Control Panel.

Choices available range from simple "dozing" to complete shutdown:

  • Dozing slows the CPU down only, to around half speed.

  • Standby shuts down HD and video, or CPU and SCLK (depends on the chipset).

  • Suspend shuts down all devices except the CPU.

  • Inactive stops the CPU, slows the SCLK and powers down the L2 cache.

  • HDD Power Down just shuts down the hard disk.

As with anything, there are industry standards. For energy saving purposes, these include:

  • APM, or Advanced Power Management
    Devised by Intel/Microsoft. This must be active if you want to keep the time and date when the system is suspended, with power.exe for DOS (try power.drv for Windows) that coordinates BIOS, DOS and program activity. APM is responsible for shutting the system down on quitting the operating system, typically Windows '95, and other useful tricks.

  • ATA, or AT Attachments Specification
    For IDE drives. Some ATA compliant devices provide Spin down facilities.

  • DPMS, or Display Power Management Signaling
    Monitors and cards conforming to this are meant to be matched, as signals are sent between them to put the CRT into various low power states, which need instructions from the BIOS. These are recognized power management states:

  • Run, Standby, Suspend and Off. 
    Suspend is slower to return to the Run state than Standby, which is regarded as being temporarily idle. Disable Standby and Suspend if you don't want PM.

  • ACPI, or Advanced Configuration and Power Interface
    Hashed out mainly by Intel, Microsoft and Toshiba. This will allow desktop PCs to have instant on, and be better for voicemail and household device control, as peripherals can be turned off as well as the main system unit. It will work the other way, too. Only devices with an ACP BIOS later than Jan 1 1999 are guaranteed to work with Windows 2000. Some BIOS's have their own maximum and minimum settings for the times allocated, but you may have a "User Defined" option for your own. More options may be available for SL (low power) CPUs. SM Out, by the way, means the System Management Output control pin.

 

Smart Battery System
This is where circuitry is added to a battery pack to allow better power management, battery life and information for the user, such as time remaining. The battery can talk to the system, and tell it what services are required (some charging systems depend on battery heat as an indication of charge status). All this has been formalized into the SBS system, which actually stems from five documents containing the specifications for the battery itself, host system hardware, BIOS and charging. The SMBus is a separate bus allowing direct communication between the host and the battery. The Smart Charger allows a battery to control its own charge, while a Smart Battery Selector is used in multiple systems to determine which one is in use, which is charging, etc.

PM Control by APM
Or Power Management Control by Advanced Power Management. 
Switches APM on or off; choices are Yes or No. 
If Yes, combine DOS and Windows utilities for Green Mode (only with S-series CPUs). When enabled, an Advanced Power Management device will be activated to enhance the maximum Power Saving mode and stop the CPU internal clock. In other words, the BIOS will wait for a prompt from APM before going into any power management mode. 
If disabled, the BIOS will ignore APM. You need DOS and Windows utilities as well.

IDE Standby Power Down Mode
Also known as Hard Disk Timeout, or HDD Power Down (Award) Allows automatic power down of IDE drives after a specified period of inactivity, but some don't like it (notebook drives are OK). 15 minutes is a suggested minimum, to avoid undue wear and tear on the drive. Probably doesn't affect SCSI drives.

Standby Mode Control
Sets standby clock speed to fractions of CPU speed, and enables/disables the video.

This is an article from  Phil Croucher, author of  Communications and Networks. Phil has a way of explaining in "plain" English. The information is well presented and is well above A+ standard.

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Updated 07/06/04

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