REDUCED POWER MODES

3/10/2001

Reduced Power Modes: Special PC operating modes that attempt to reduce PC power consumption when the PC is not in use. They allow the PC to be brought back to full power without loss of work in progress. These modes are generally pretty successful at saving power. They are less successful at restoring the PC's state. In particular, they often have trouble restoring I/O devices to the state they were in prior to entering the power saving mode. This sometimes results in the computer hanging up or in a peripheral being unusable until the PC is rebooted.

Control over power management is provided by the PC BIOS and/or the PC Operating system -- often under the acronym ACPI.

Four rather imprecisely defined power-down states are identified. Many PCs support several of these states either as options or as progressive stages in powering down. The state names may overlap or may be used differently by some vendors:

Standby: Displays may be blanked, Some peripherals may be shut down. Power savings are small. Recovery from Standby to Normal will usually take no more than a second or two. Standby is similar to sleep, but less dramatic. The intent of standby is to obtain whatever power savings can be obtained while permitting quick recovery.

Sleep: Sleep is a reduced power mode where peripherals may be shut down and the CPU may be slowed. Displays are often blanked in sleep mode. Power savings are moderate. Recovery time is typically only a few seconds. Sleep extends the power savings of standby but retains reasonable quick recovery.

Suspend: Peripherals are shut down and the CPU is slowed to the slowest feasible speed. The information necessary to restore normal operation is saved in memory, which remains active. Power savings can be very substantial. Recovery time is generally a number of seconds. Suspend attempts to achieve major power savings, but still allow fairly rapid recovery.

Hibernation: All state information is copied to non-volatile storage -- typically the hard drive -- and the PC is powered off. Recovery requires rebooting the PC. The power savings are very great, But the time to bring the PC back into service can be measured in minutes on some PCs. Hibernate yields infinite power savings. The PC is turned off. But recovery times are very long.

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