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 active data. 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 ACPI.
Battery Monitor
ACPI checks the battery charge status and provides information about it. Addition-
ally, it coordinates actions to perform when a critical charge status is reached.
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.
Processor Speed Control
In connection with the CPU, energy can be saved in three different ways: frequency
and voltage scaling (also known as PowerNow! or Speedstep), throttling and putting
the processor to sleep (C-states). Depending on the operating mode of the computer,
these methods can also be combined.
23.2 Advanced Conguration and
Power Interface (ACPI)
ACPI was designed to enable the operating system to set up and control the individual
hardware components. ACPI supersedes both Power Management Plug and Play (PnP)
and Advanced Power Management (APM). It delivers information about the battery,
AC adapter, temperature, fan and system events, like “close lid” or “battery low.”
The BIOS provides tables containing information about the individual components and
hardware access methods. The operating system uses this information for tasks like
assigning interrupts or activating and deactivating components. Because the operating
system executes commands stored in the BIOS, the functionality depends on the BIOS
implementation. The tables ACPI can detect and load are reported in /var/log/boot
422 Reference
