4. About ACPI
In the world of power management ACPI is relatively new to the game.
It was first released in 1996 by
Compaq/Hewlett-Packard, Intel, Microsoft, Phoenix and Toshiba. These
developers aimed to replace the previous industry standard for power
management. Their ACPI.info site
contains the official specifications, a list of companies that support ACPI
and a number of other goodies. This is definitely not required reading, but
may be of some interest to the insanely curious.
ACPI allows control of power management from within the
operating system. The previous industry standard for power management, Advanced
Power Management (APM), is controlled at the BIOS level. APM is activated
when the system becomes idle--the longer the system idles, the less power it
consumes (e.g. screen saver vs. sleep vs. suspend).
In APM, the operating system has no knowledge of when the system will
change power states.
ACPI can typically be configured from within the operating system.
This is unlike APM where configuration often involves rebooting and
entering the BIOS configuration screens to set parameters.
ACPI has several different software components:
a subsystem which controls hardware states and
functions that may have previously been in the BIOS configuration
These states include:
a policy manager, which is software that sits on top of
the operating system and allows user input on the system
policies
the ACPI also has device drivers that control/monitor devices such as a
laptop battery, SMBus (communication/transmission path) and EC (embedded controller).
If you would like more information on power management in laptops, check out
the resources on tuxmobil.org.
Specifically: Power Management
with Linux - APM, ACPI, PMU and the Hardware in Detail section of the
Linux
Mobile Guide.