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In [[computing]], the '''Advanced Configuration and Power Interface''' ('''ACPI''') specification provides an [[open standard]] for unified [[operating system]]-centric device configuration and [[power management]]. 

First released in December 1996, ACPI defines [[Cross-platform|platform-independent]] interfaces for hardware discovery, configuration, power management and monitoring. The specification is central to ''Operating System-directed configuration and Power Management'' (OSPM); a term used to describe a system implementing ACPI, which therefore removes device management responsibilities from legacy firmware interfaces. 

The standard was originally developed by [[Intel]], [[Microsoft]], and [[Toshiba]] - later joined by [[Hewlett-Packard|HP]] and [[Phoenix Technologies|Phoenix]]. - and latest iteration is "Revision 4.0a", published on April 5, 2010.

== Overview ==
ACPI aims to consolidate and improve upon existing power and configuration standards for hardware devices. It provides a transition from existing standards to entirely ACPI-compliant hardware, with some ACPI operating systems already removing support for legacy hardware. With the intention of replacing [[Advanced Power Management]], the [[MultiProcessor Specification]] and the [[Legacy Plug and Play|Plug and Play BIOS]] Specification, the standard brings power management into operating system control (OSPM), as opposed to the previous BIOS central system, which relied on platform-specific firmware to determine power management and configuration policy.

The ACPI specification contains numerous related components for hardware and software programming, as well as a unified standard for device/power interaction and bus configuration. As a document that unifies many previous standards it covers many areas, for system and device builders as well as system programmers. Some software developers have trouble  implementing ACPI and express concerns about the requirements that [[bytecode]] from an external source must be run by the system with full privileges. Linus Torvalds]], creator of the [[Linux kernel]], once described it as "a complete design disaster in every way", in relation to his view that "modern PCs are horrible".

Microsoft [[Windows 98]] was the first operating system with full support for ACPI{{Citation needed|date=July 2010}}, with [[Windows 2000]], [[Windows XP]], [[Windows Vista]], [[Windows 7]], [[eComStation]], [[FreeBSD]], [[NetBSD]], [[OpenBSD]], [[HP-UX]], [[OpenVMS]], [[Linux kernel|Linux]] and [[IBM PC compatible|PC]] versions of [[SunOS]] all having at least some support for ACPI.

== OSPM responsibilities ==
ACPI requires that, once an OSPM-compatible operating system has activated ACPI on a computer, it then takes over and has exclusive control of all aspects of power management and device configuration. The OSPM implementation must expose an ACPI-compatible environment to device drivers, which exposes certain system, device and processor states.

=== Power States ===
==== Global states ====
The ACPI specification defines the following seven states (so-called global states) for an ACPI-compliant computer-system:

* '''G0''' ('''S0'''): ''Working''
* '''G1''', ''Sleeping'' subdivides into the four states S1 through S4:
** '''S1''': All processor caches are flushed, and the CPU(s) stop executing instructions. Power to the CPU(s) and RAM is maintained; devices that do not indicate they must remain on may be powered down.
** '''S2''': CPU powered off
** '''S3''': Commonly referred to as  ''Standby'', ''[[Sleep mode|Sleep]]'', or ''Suspend to RAM''. [[RAM]] remains powered
** '''S4''': ''[[Hibernation (computing)|Hibernation]]'' or ''Suspend to Disk''. All content of [[RAM|main memory]] is saved to [[non-volatile memory]] such as a [[hard drive]], and is powered down.
* '''G2''' ('''S5'''), ''Soft Off'': G2 is almost the same as G3 ''Mechanical Off'', but some components remain powered so the computer can "wake" from input from the keyboard, clock, [[Wake-on-Ring|modem]], [[Wake on LAN|LAN]], or [[Universal Serial Bus|USB]] device.
* '''G3''', ''Mechanical Off'': The computer's power consumption approaches close to zero, to the point that the power cord can be removed and the system is safe for dis-assembly (typically, only the [[real-time clock]] is running off its own small battery).

Furthermore, the specification defines a ''Legacy'' state: the state on an operating system which does not support ACPI. In this state, the hardware and power are not managed via ACPI, effectively disabling ACPI.

==== Device states ====
The device states ''D0''-''D3'' are device-dependent:

* '''D0''' ''Fully On'' is the operating state.
* '''D1''' and '''D2''' are intermediate power-states whose definition varies by device.
* '''D3''' ''Off'' has the device powered off and unresponsive to its bus.

=== Processor states ===
The CPU power states ''C0''-''C3'' are defined as follows:

* '''C0''' is the operating state.
* '''C1''' (often known as ''Halt'') is a state where the processor is not executing instructions, but can return to an executing state essentially instantaneously. All ACPI-conformant processors must support this power state. Some processors, such as the [[Pentium 4]], also support an Enhanced C1 state ('''C1E''' or Enhanced Halt State) for lower power consumption.
* '''C2''' (often known as ''Stop-Clock'') is a state where the processor maintains all software-visible state, but may take longer to wake up. This processor state is optional.
* '''C3''' (often known as ''Sleep'') is a state where the processor does not need to keep its [[cache]] coherent, but maintains other state. Some processors have variations on the C3 state (Deep Sleep, Deeper Sleep, etc.) that differ in how long it takes to wake the processor. This processor state is optional.

=== Performance states ===
While a device or processor operates (D0 and C0, respectively), it can be in one of several [[Voltage and frequency scaling|power-performance states]]. These states are implementation-dependent, but P0 is always the highest-performance state, with P1 to P''n'' being successively lower-performance states, up to an implementation-specific limit of ''n'' no greater than 16.

P-states have become known as [[SpeedStep]] in [[Intel]] processors, as [[PowerNow!]] or [[Cool'n'Quiet]] in [[Advanced Micro Devices|AMD]] processors, and as [[PowerSaver]] in [[VIA Technologies|VIA]] processors.

* '''P0''' max power and frequency
* '''P1''' less than P0, voltage/frequency scaled
* '''Pn''' less than P(n-1), voltage/frequency scaled

== Hardware Interface ==
ACPI-compliant systems interact with hardware through either a "Function Fixed Hardware (FFH) Interface" or a platform-independent hardware programming model which relies on platform-specific ACPI Machine Language (AML) provided by the [[original equipment manufacturer]] (OEM).

Function Fixed Hardware interfaces are platform-specific features, provided by platform manufacturers for the purposes of performance and failure recovery. Standard [[Intel]]-based [[IBM PC Compatible|PCs]] have a fixed function interface defined by Intel, which provides a set of core functionality that reduces an ACPI-compliant system's need for full driver stacks for providing basic functionality during boot time or in the case of major system failure.

== Firmware interface ==
ACPI defines a large number of tables that provide the interface between an ACPI-compliant operating system and system firmware. For example:
* DSDT – Differentiated System Description Table
* SSDT – Secondary System Description Table
* SRAT – System Resource Affinity Table

The tables allow description of system hardware in a platform-independent manner, and are presented as either fixed-formatted data structures or in AML. The main AML table is the DSDT (differentiated system description table).

The Root System Description Pointer is located in a platform-dependent manner, and describes the rest of the tables.

== ACPI Component Architecture (ACPICA) ==
The ACPI Component Architecture (ACPICA) provides an [[open source|open-source]] OS-independent reference implementation of the ACPI specification.

== History ==

The first revision of the ACPI specification was released in December 1996 supporting 16 and 32-bit addressing spaces.  It wasn't until August 2000 that ACPI received 64-bit address support as well as support for multiprocessor workstations and servers with revision 2.0.  In September 2004, revision 3.0 gave the ACPI specification support for [[SATA]] connectors, [[PCI Express]] bus, >256 multiprocessor support, ambient light sensors and user-presence devices, as well as extending the Thermal model beyond the previous processor centric support.  The latest of the major publications is that of revision 4.0.  Released in June 2009, the 4.0 specification added many new features to the design; most notable are [[USB 3.0]] support, logical processor idling support, and [[x2APIC]] support.

== See also ==
* [[Active State Power Management]] - hardware power management protocol for [[PCI Express]]
* [[Advanced Power Management]] (APM)
* [[Green computing]]
* [[Power management keys]] - Keyboard related
* [[Wake-on-LAN]]
* [[Wake-on-Ring]]
* [[Simple Firmware Interface]] (SFI)

== External links ==
* [http://www.acpi.info/DOWNLOADS/ACPIspec40a.pdf Advanced Configuration and Power Interface Specification]
* [http://www.acpi.info/ ACPI home page]
* [http://www.intel.com/technology/iapc/acpi/ Intel's ACPI page]
* [http://www.acpica.org/ Intel's ACPI Component Architecture]
* [http://www.advogato.org/article/913.html How Linux Suspend and Resume works in the ACPI age]
* [http://www.hardwaresecrets.com/article/611 Everything You Need to Know About the CPU C-States Power Saving Modes]