27.1 Power Saving Functions
Power saving functions are not only significant for the mobile
use of laptops, but also for desktop systems. The main functions and their
use in the power management systems APM and ACPI are:
This operating mode turns off the display. On some computers,
the processor performance is throttled. This function is not
available in all APM implementations. This function corresponds
to the ACPI state S1 or S2.
- Suspend (to memory)
This mode writes the entire system state to the RAM. Subsequently,
the entire system except the RAM is put to sleep. In this state,
the computer consumes very little power.
The advantage of this state is the possibility of resuming work
at the same point within a few seconds without having to boot
and restart applications. Devices using APM can usually be suspended
by closing the lid and activated by opening it. This function
corresponds to the ACPI state S3. The support of this state
is still under development and therefore largely depends on the hardware.
- Hibernation (suspend to disk)
In this operating mode, the entire
system state is written to the hard disk and the system is powered off.
There must be a swap partition at least as big as the RAM to write all the
Reactivation from this state takes about 30 to 90
seconds. The state prior to the suspend is restored. Some
manufacturers offer useful hybrid variants of this mode,
such as RediSafe in IBM Thinkpads. The corresponding
ACPI state is S4. In Linux, suspend to disk
is performed by kernel routines that are independent from APM
- Battery Monitor
ACPI and APM check the battery charge status
and provide information about it.
Additionally, both systems coordinate actions
to perform when a critical charge status
- Automatic Power-Off
Following a shutdown, the computer is powered off.
This is especially important when an automatic shutdown
is performed shortly before the battery is empty.
- Shutdown of System Components
Switching off the hard disk is the greatest
single aspect of the power saving potential of the overall system.
Depending on the reliability of the overall system, the hard disk
can be put to sleep for some time. However, the risk of losing data
increases with the duration of the sleep periods. Other components,
like PCI devices that can be put into a special power saving mode, can
be deactivated with ACPI (at least theoretically) or permanently
disabled in the BIOS setup.
- Processor Speed Control
In connection with the CPU, energy can be
saved in three different ways: frequency
and voltage scaling (also known as
throttling, and putting the processor to
sleep (C states). Depending on the operating mode
of the computer, these methods can also be