Hardware Reference
In-Depth Information
management functions in the OS. Microsoft Windows 98 and later automatically use ACPI if ACPI
functions are found in the system BIOS. The need to update system BIOSs for ACPI support is one
reason many computer vendors have recommended performing a BIOS update before installing
Windows 98 or later on older systems.
ACPI was initially released in 1996 and first appeared in the Phoenix BIOS around that time. ACPI
became a requirement for the Intel/Microsoft “PC'97” logo certification in 1996, which caused
developers to work on integrating ACPI into system designs around that time. Intel included ACPI
support in chipsets starting with the PIIX4E South Bridge in April 1998, and ACPI support was
included in Windows starting with the release of Windows 98 (June 25, 1998) as part of what
Microsoft called its “OnNow” initiative. By the time Windows 2000 came out (February 17, 2000),
ACPI had universally replaced APM on new systems. ACPI 4.0a was released in April 2010, and the
latest ACPI 5.0 Specification was released in November 2011. The official ACPI specifications can
be downloaded from
www.acpi.info
.
Placing power management under the control of the OS enables a greater interaction with
applications. For example, a program can indicate to the OS which of its activities are crucial,
forcing an immediate activation of the hard drive, and which can be delayed until the next time the
drive is activated for some other reason. For example, a word processor may be set to automatically
save files in the background, which an OS using ACPI can then delay until the drive is activated for
some other reason, resulting in fewer random spin-ups of the drive.
ACPI goes far beyond the previous standard, APM, which consisted mainly of processor, hard disk,
and display control. ACPI controls not only power but also all the Plug and Play (PnP) hardware
configuration throughout the system. With ACPI, system configuration (PnP) and power-management
configuration are no longer controlled via the BIOS Setup; they are instead controlled entirely within
the OS.
ACPI enables the system to automatically turn on and off internal peripherals (such as optical drives,
network cards, hard disk drives, and modems) as well as external devices such as printers, monitors,
or any devices connected to serial, parallel, USB, video, or other ports in the system. ACPI
technology also enables peripherals to turn on or wake up the system. For example, a telephone
answering machine application can request that it be able to respond to answer the telephone within 1
second. Not only is this possible, but if the user subsequently presses the power or sleep button, the
system only goes into the deepest sleep state that is consistent with the ability to meet the telephone
answering application's request.
ACPI enables system designers to implement a range of power-management features that are
compatible with various hardware designs while using the same OS driver. ACPI also uses the Plug
and Play BIOS data structures and takes control over the Plug and Play interface, providing an OS-
independent interface for configuration and control.
ACPI defines several system states and substates. There are four Global System states, labeled from
G0 through G3, with G0 being the fully operational state and G3 being mechanically turned off.
Global System states are immediately obvious to the user of the system and apply to the entire system
as a whole. Within the G0 state, there are four CPU Power states (C0-C3) and four Device Power
states (D0-D3) for each device. Within the C0 CPU Power state, there are up to 16 CPU Performance
states (P0-P15).
Device Power states
are states for individual devices when the system is in the G0 (Working) state.
