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that will accept being shut off. These devices are usually called Energy Star devices, which signals their compliance with the EPA’s Energy Star standard. To be an Energy Star device, a peripheral must be able to shut down without actually turning off and show that they use much less power than the non–Energy Star equivalent. Last, the system’s OS must know how to request that a particular device be shut down, and the CPU’s clock must be slowed down or stopped.

ACPI goes beyond the APM standard by supplying support for hot-swappable devices—always a huge problem with APM. This feature aside, it is a challenge to tell the difference between an APM system and an ACPI system at first glance.

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NOTE Don’t limit your perception of APM, ACPI, and Energy Star just to laptops. Virtually all desktop systems and many appliances also use the power management functions.

APM/ACPI Levels

APM defined four power-usage operating levels for a system. These levels are intentionally fuzzy to give manufacturers considerable leeway in their use; the only real difference among them is the amount of time each takes to return to normal usage. These levels are as follows:

Full On Everything in the system is running at full power. There is no power management.

APM Enabled CPU and RAM are running at full power. Power management is enabled. An unused device may or may not be shut down.

APM Standby CPU is stopped. RAM still stores all programs. All peripherals are shut down, although configuration options are still stored. (In other words, to get back to APM Enabled, you won’t have to reinitialize the devices.)

APM Suspend Everything in the PC is shut down or at its lowest power-consumption setting. Many systems use a special type of Suspend called hibernation, where critical configuration information is written to the hard drive. Upon a wake-up event, the system is reinitialized, and the data is read from the drive to return the system to the APM Enabled mode. Clearly, the recovery time between Suspend and Enabled will be much longer than the time between Standby and Enabled.

ACPI, the successor to APM, handles all these levels plus a few more, such as “soft power on/off,” that enables you to define the function of the power button. You should familiarize yourself with the following ACPI global (G) and sleeping (S) system power state specifications for both the A+ exams and your own practical application:

G0 (S0) Working state.

G1 Sleeping state mode. Further subdivided into four S states.

S1 CPU stops processing. Power to CPU and memory (RAM) is maintained.

S2 CPU is powered down.

S3 Sleep or Standby mode. Power to RAM still on.

S4 Hibernation mode. Information in RAM is stored to nonvolatile memory or drive and powered off.

G2 (S5) Soft power off mode. Certain devices used to wake a system—such as keyboard, LAN, USB, and other devices—remain on, while most other components are powered to a mechanical off state (G3).

G3 Mechanical off mode. The system and all components, with the exception of the real-time clock (RTC), are completely powered down.

Configuration of APM/ACPI

You configure APM/ACPI via CMOS settings or through Windows. Windows settings override CMOS settings. Although the APM/ACPI standards permit a great deal of flexibility, which can create some confusion among different implementations, certain settings apply generally to CMOS configuration. First is the ability to initialize power management; this enables the system to enter the APM Enabled mode. Often CMOS then presents time frames for entering Standby and Suspend modes, as well as settings to determine which events take place in each of these modes.

Many CMOS versions present settings to determine wake-up events, such as directing the system to monitor a modem or a NIC (Figure 21-29). You’ll see this feature as Wake on LAN or something similar. A true ACPI-compliant CMOS provides an ACPI setup option. Figure 21-30 shows a typical modern BIOS that provides this setting.

Figure 21-29 Setting a wake-up event in CMOS

APM/ACPI