In computing, the Advanced Configuration and Power Interface (ACPI) specification provides an open standard for device configuration and power management by the operating system.
First released in December 1996, ACPI defines 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 removes device management responsibilities from legacy firmware interfaces.
The standard was originally developed by Intel, Microsoft, and Toshiba - later joined by HP and Phoenix[1]. The latest version is "Revision 5.0," published on November 23, 2011.
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ACPI aims to consolidate, check and improve upon existing power and configuration standards for hardware devices.[1] It provides a transition from existing standards to entirely ACPI-compliant hardware, with some ACPI operating systems already removing support for legacy hardware.[2] With the intention of replacing Advanced Power Management, the MultiProcessor Specification and the Plug and Play BIOS Specification,[3] the standard brings power management under the control of the operating system (OSPM), as opposed to the previous BIOS-central system, which relied on platform-specific firmware to determine power management and configuration policy.[4]
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. 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.[5] 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".[6]
Microsoft's Windows 98 became the first operating system with full support for ACPI, with Windows 2000, Windows XP, Windows Vista, Windows 7, eComStation, FreeBSD, NetBSD, OpenBSD, HP-UX, OpenVMS, Linux and PC versions of SunOS all having at least some support for ACPI[7].
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.
The ACPI specification defines the following seven states (so-called global states) for an ACPI-compliant computer-system:
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.
The device states D0-D3 are device-dependent:
The CPU power states C0-C3 are defined as follows:
While a device or processor operates (D0 and C0, respectively), it can be in one of several power-performance states. These states are implementation-dependent, but P0 is always the highest-performance state, with P1 to Pn 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 AMD processors, and as PowerSaver in VIA processors.
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 PCs have a fixed function interface defined by Intel,[9] 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.
ACPI defines a large number of tables that provide the interface between an ACPI-compliant operating system and system firmware. For example:[10]
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.
The ACPI Component Architecture (ACPICA) provides an open-source OS-independent reference implementation of the ACPI specification.[11]
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. 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.[1] The latest of the major publications is revision 5.0, released in November 2011. [1]
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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