From Wikipedia, the free encyclopedia - View original article
|This article needs additional citations for verification. (July 2010)|
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 system implementing ACPI, which removes device management responsibilities from legacy firmware interfaces.
The standard was originally developed by Intel, Microsoft and Toshiba, and was later joined by HP and Phoenix. The latest version is "Revision 5.0", which was published on 6 December 2011. As the ACPI technology gained wider adoption with many operating systems and processor architectures, the desire to improve governance model of the specification has increased significantly. In October 2013, original developers of the ACPI standard agreed to transfer all assets to the UEFI Forum, where all future development will be taking place.
ACPI aims to consolidate, check and improve upon existing power and configuration standards for hardware devices. It provides a transition from earlier standards to entirely ACPI-compliant hardware, with some ACPI operating systems already[when?] removing support for legacy hardware. With the intention of replacing Advanced Power Management, the MultiProcessor Specification and the Plug and Play BIOS Specification, 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.
The ACPI specification contains numerous related components for hardware and software programming, as well as a unified standard for device/power interaction and for bus configuration. 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". 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.
Microsoft's Windows 98 became the first operating-system that fully implemented 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.
Once an OSPM-compatible operating system activates ACPI, it 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.
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. Though, 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.
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, 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:
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.
A specification for reporting of hardware errors e.g. from the chipset, to the operating system.
The first revision of the ACPI specification was released in December 1996, supporting 16 and 32-bit addressing spaces. It was not 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. The latest of the major publications is revision 5.0, released in November 2011.