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ACPI (Advanced Configuration and Power Interface)

Today’s computer user needs to leave a computer on for extended periods of time in order to receive faxes, run computer maintenance tasks, automatically answer phone calls, and download software upgrades and patches. Network managers want control of computers so they can push out software upgrades, perform backups, download software upgrades and patches, and perform tests. Laptop users have always been plagued by power management problems, such as short battery life, inconsistent handling of screen blanking, and screen blanking in the middle of presentations. Such problems occurred because originally the BIOS controlled power. Power management has changed.

ACPI (Advanced Configuration and Power Interface) gives the BIOS and operating system control over various devices’ power and modes of operation, as shown in Figure 4.28.

Figure 4.28

Figure 4.28. ACPI actions

With ACPI, the user can control how the power switch operates and when power to specific devices, such as the hard drive and monitor, is lowered. For example, the Instant On/Off BIOS setting can control how long the power switch is held in before the power supply turns on or off. Case temperatures, CPU temperatures, and CPU fans can be monitored. The power supply can be adjusted for power requirements. The CPU clock can be throttled or slowed down to keep the temperature lower and prolong the life of the CPU and reduce power requirements especially in portable devices when activity is low or nonexistent. ACPI has various operating states, as shown in Table 4.3.

Table 4.3. ACPI operating states

Global system state

Sleep state


G0 Working


The computer is fully functional. Software, such as the autosave function used with Microsoft products, can be optimized for performance or lower battery usage.

G1 Sleeping

Requires less power than the G0 state and has multiple sleeping states: S1, S2, S3, and S4.


CPU is still powered, and unused devices are powered down. RAM is still being refreshed. Hard disks are not running.


CPU is not powered. RAM is still being refreshed. System is restored instantly upon user intervention.


Power supply output is reduced. RAM is still being refreshed. Some info in RAM is restored to CPU and cache.


Lowest-power sleep mode and takes the longest to come up. Info in RAM is saved to hard disk. Some manufacturers call this the hibernate state.



Also called soft off. Power consumption is almost zero. Requires the operating system to reboot. No information is saved anywhere.


Also called off, or mechanical off. This is the only state where the computer can be disassembled. You must power on the computer to use it again.

Two common BIOS and adapter features that take advantage of ACPI are Wake on LAN and Wake on Ring. The Wake on LAN feature allows a network administrator to control the power to a workstation remotely and directs the computer to come out of sleep mode. Software applications can also use the Wake on LAN feature to perform updates, upgrades, and maintenance tasks. The feature can also be used to bring up computers immediately before the business day starts. Wake on LAN can be used with Web or network cameras to start recording when motion is detected or to bring up a network printer so that it can be used when needed. Wake on Ringallows a computer to come out of sleep mode when the telephone line has an incoming call. This lets the computer receive phone calls, faxes, and emails when the user is not present. Common BIOS settings related to ACPI are listed in Table 4.4.

Table 4.4. Common BIOS power settings



Delay Prior to Thermal

Defines the number of minutes the system waits to shut down the system once an overheating situation occurs.

CPU Warning Temperatures

Specifies the CPU temperature at which a warning message is displayed on the screen.

ACPI Function

Enables or disables ACPI. This is the preferred method for disabling ACPI in the event of a problem.


Specifies the length of time a user must press the power button to turn off the computer.

Deep S4/S5

Uses less power and only wakes from S4/S5 states with the power button or a RTC (real time clock) alarm, such as waking the computer to complete a task.

Power on by Ring, Resume by Ring, or Wakeup

Allows the computer to wake when an adapter or an external device supports Wake on Ring.

Resume by Alarm

Allows a date and time to be set when the system is awakened from Suspend mode. Commonly used to update the system during nonpeak periods.

Wake Up on LAN

Allows the computer to wake when a Wake on LAN signal is received across the network.

CPU THRM Throttling

Allows a reduction in CPU speed when the system reaches a specific temperature.

Power on Function

Specifies which key (or key combination) will activate the system’s power.

Hot Key Power On

Defines what keystrokes will reactivate system power.

Doze Mode

When the system is in a reduced activity state, the CPU clock is throttled (slowed down). All other devices operate at full speed.

After Power Failure

Sets power mode after a power loss.

Windows 7 has three power plans available, and you can customize these power plans. You might want to customize a power plan when there is a problem with poor video quality when playing a movie. Use the Change plan settings link followed by the Change advanced power settings link to expand a section such as the Multimedia settings option. Table 4.5 shows the three main power plans you can just click and select.

Table 4.5. Windows 7 power plans

Power plan



The most common plan because it provides full power when you need it and saves power when the computer is not being used.

Power saver

Saves power by running the CPU more slowly and reducing screen brightness.

High performance

Select the Show additional plans link to see this option. This provides the maximum performance possible.

Sometimes, when a computer comes out of Sleep mode, not all devices respond, and the computer’s power or reset button has to be pressed to reboot the computer. The following situations can cause this to happen:

  • A screen saver conflicts with ACPI
  • All adapters/devices are not ACPI compliant
  • An adapter/device has an outdated driver
  • The system BIOS or an installed adapter BIOS needs to be updated

To see if the screen saver causes a problem, use the Display Control Panel and set the screen saver option to None. Identifying a problem adapter, device, or driver will take Internet research. Check each adapter, device, and driver one by one. Use the Power Options Control Panel to change the power scheme. Also check all devices for a Power Management tab on the Properties dialog box. Changes can be made there.

Links on the left of the Power Options Control Panel provide access to advanced settings such as requiring a password to come out of sleep mode. The power options for a Windows 7 laptop are shown in Figure 4.29.

Figure 4.29

Figure 4.29. Windows 7 power settings

Other laptop Power Options Control Panel settings include the following links: Require a password on wakeup, Choose what the power button does (as shown in Figure 4.29), Choose what closing the lid does, Create a power plan, Choose when to turn off the display, and Change when the computer sleeps. Laptop power settings affect battery life. Users and technicians should adjust these settings to best fit how the laptop or mobile device is used.

In Windows Vista and 7, use the Power Options Control Panel to edit the power settings. Select the Change advanced power settings link to configure passwords, standby power behavior, and other power-related settings. If the computer does not go into the Sleep mode, check the following:

  • Determine if ACPI is enabled in BIOS.
  • Try disabling the antivirus program to see if it is causing the problem.
  • Set the screen saver to None to see if it is causing the problem.
  • Determine if all device drivers are ACPI compliant.
  • Determine if power management is enabled through the operating system (use the Power Options Control Panel).
  • Disconnect USB devices to see if they are causing problems.
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