Advanced Configuration and Power Interface (ACPI) is an

open standard An open standard is a standard that is openly accessible and usable by anyone. It is also a prerequisite to use open license, non-discrimination and extensibility. Typically, anybody can participate in the development. There is no single definitio ...

that

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s can use to discover and configure computer hardware components, to perform

power management Power management is a feature of some electrical appliances, especially copiers, computers, computer CPUs, computer GPUs and computer peripherals such as monitors and printers, that turns off the power or switches the system to a low-power ...

(e.g. putting unused hardware components to sleep), auto configuration (e.g.

Plug and Play In computing, a plug and play (PnP) device or computer bus is one with a specification that facilitates the recognition of a hardware component in a system without the need for physical device configuration or user intervention in resolving resou ...

and

hot swapping Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are sai ...

), and status monitoring. First released in December 1996, ACPI aims to replace Advanced Power Management (APM), the MultiProcessor Specification, and the

Plug and Play BIOS The term Legacy Plug and Play, also shortened to Legacy PnP, describes a series of specifications and Microsoft Windows features geared towards operating system configuration of devices, and some device IDs are assigned by UEFI Forum. The stan ...

(PnP) Specification. ACPI brings power management under the control of the operating system, as opposed to the previous BIOS-centric system that relied on platform-specific firmware to determine power management and configuration policies. The specification is central to the Operating System-directed configuration and Power Management (OSPM) system. ACPI defines hardware abstraction interfaces between the device's firmware (e.g.

BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...

UEFI UEFI (Unified Extensible Firmware Interface) is a set of specifications written by the UEFI Forum. They define the architecture of the platform firmware used for booting and its interface for interaction with the operating system. Examples ...

), the computer hardware components, and the

s. Internally, ACPI advertises the available components and their functions to the

operating system kernel The kernel is a computer program at the core of a computer's operating system and generally has complete control over everything in the system. It is the portion of the operating system code that is always resident in memory and facilitates in ...

using instruction lists (" methods") provided through the system

firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide ...

(

), which the kernel parses. ACPI then executes the desired operations written in ''ACPI Machine Language'' (such as the initialization of hardware components) using an embedded minimal

virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized har ...

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...

Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, personal computers, and related services headquartered at th ...

and

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originally developed the standard, while HP,

Huawei Huawei Technologies Co., Ltd. ( ; ) is a Chinese multinational technology corporation headquartered in Shenzhen, Guangdong, China. It designs, develops, produces and sells telecommunications equipment, consumer electronics and various smart ...

and Phoenix also participated later. In October 2013, ACPI Special Interest Group (ACPI SIG), the original developers of the ACPI standard, agreed to transfer all assets to the

UEFI Forum UEFI Forum, Inc. is an alliance between technology companies to coordinate the development of the UEFI specifications. The board of directors includes representatives from twelve ''promoter'' companies: AMD, American Megatrends, ARM, Apple, De ...

, in which all future development will take place. The UEFI Forum published of the standard, "Release 6.5" in August 2022.

Architecture

The firmware-level ACPI has three main components: the ACPI tables, the ACPI BIOS, and the ACPI registers. The ACPI BIOS generates ACPI tables and loads ACPI tables into

main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a comput ...

. Much of the firmware ACPI functionality is provided in

bytecode Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (norma ...

of ''ACPI Machine Language'' (AML), a

Turing-complete In computability theory, a system of data-manipulation rules (such as a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be used to simulate any Tu ...

, domain-specific low-level language, stored in the ACPI tables. To make use of the ACPI tables, the operating system must have an interpreter for the AML bytecode. A reference AML interpreter implementation is provided by the ACPI Component Architecture (ACPICA). At the BIOS development time, AML bytecode is compiled from the ASL (ACPI Source Language) code.ACPI in Linux
2005 Overall design decision was not without criticism. In November 2003,

Linus Torvalds Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...

—author of the

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...

—described ACPI as "a complete design disaster in every way".Linux Magazine issue 162, May 2014, page 9 In 2001, other senior Linux software developers like Alan Cox expressed concerns about the requirements that

from an external source must be run by the kernel with full privileges, as well as the overall complexity of the ACPI specification. In 2014, Mark Shuttleworth, founder of the Ubuntu

Linux distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading on ...

, compared ACPI with Trojan horses.

ACPI Component Architecture (ACPICA)

The ACPI Component Architecture (ACPICA), mainly written by Intel's engineers, provides an

open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

platform-independent reference implementation of the operating system–related ACPI code. The ACPICA code is used by Linux,

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ArcaOS ArcaOS is an operating system based on OS/2, developed and marketed by Arca Noae, LLC under license from IBM. It was codenamed Blue Lion during its development. It builds on OS/2 Warp 4.52 by adding support for new hardware, fixing defects and l ...

and

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

,ACPI implementation on FreeBSD - Usenix
/ref> which supplement it with their operating-system specific code.

History

The first revision of the ACPI specification was released in December 1996, supporting 16, 24 and

32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calcula ...

addressing spaces. It was not until August 2000 that ACPI received

64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A comp ...

address support as well as support for multiprocessor workstations and servers with revision 2.0. In 1999, then

CEO

Bill Gates William Henry Gates III (born October 28, 1955) is an American business magnate and philanthropist. He is a co-founder of Microsoft, along with his late childhood friend Paul Allen. During his career at Microsoft, Gates held the positions ...

stated in an e-mail that

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...

would benefit from ACPI without them having to do work and suggested to make it Windows-only. In September 2004, revision 3.0 was released, bringing to the ACPI specification support for

SATA SATA (Serial AT Attachment) is a computer bus interface that connects host adapter, host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) ...

interfaces,

PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common m ...

bus,

multiprocessor Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There ar ...

support for more than 256 processors,

ambient light sensors An ambient light sensor is a component in smartphones, notebooks, other mobile devices, automotive displays and LCD TVs. It is a photodetector that is used to sense the amount of ambient light present, and appropriately dim the device's screen to ...

and user-presence devices, as well as extending the thermal model beyond the previous processor-centric support. Released in June 2009, revision 4.0 of the ACPI specification added various new features to the design; most notable are the USB 3.0 support, logical processor idling support, and x2APIC support. Revision 5.0 of the ACPI specification was released in December 2011, which added the

ARM architecture ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) instruction set architectures for computer processors, configure ...

support. The revision 5.1 was released in July 2014. The latest specification revision is 6.5, which was released in August 2022.

Operating systems

Microsoft's

Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released ...

was the first operating system to implement ACPI, but its implementation was somewhat buggy or incomplete, although some of the problems associated with it were caused by the first-generation ACPI hardware. Other operating systems, including later versions of

Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...

eComStation eComStation or eCS is an operating system based on OS/2 Warp for the 32-bit x86 architecture. It was originally developed by Serenity Systems and Mensys BV under license from IBM. It includes additional applications, and support for new hard ...

(since FreeBSD 5.0),

NetBSD NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...

(since NetBSD 1.6),

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(since OpenBSD 3.8),

HP-UX HP-UX (from "Hewlett Packard Unix") is Hewlett Packard Enterprise's proprietary implementation of the Unix operating system, based on Unix System V (initially System III) and first released in 1984. Current versions support HPE Integrit ...

OpenVMS OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Ope ...

GNU Hurd GNU Hurd is a collection of microkernel servers written as part of GNU, for the GNU Mach microkernel. It has been under development since 1990 by the GNU Project of the Free Software Foundation, designed as a replacement for the Unix kernel, ...

and PC versions of Solaris, have at least some support for ACPI. Some newer operating systems, like

Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...

, require the computer to have an ACPI-compliant BIOS, and since

Windows 8 Windows 8 is a major release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on August 1, 2012; it was subsequently made available for download via MSDN and TechNet on August 15, 2012, and later to ...

, the S0ix/Modern Standby state was implemented. Windows operating systems use acpi.sys to access ACPI events. The 2.4 series of the Linux kernel had only minimal support for ACPI, with better support implemented (and enabled by default) from kernel version 2.6.0 onwards.The State of ACPI in the Linux Kernel
/ref> Old ACPI BIOS implementations tend to be quite buggy, and consequently are not supported by later operating systems. For example,

Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was released to manufacturing on December 15, 1999, and was official ...

Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was release to manufacturing, released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Wind ...

, and

Windows Server 2003 Windows Server 2003 is the sixth version of Windows Server operating system produced by Microsoft. It is part of the Windows NT family of operating systems and was released to manufacturing on March 28, 2003 and generally available on April 24, ...

only use ACPI if the BIOS date is after January 1, 1999. Similarly, Linux kernel 2.6 blacklisted any ACPI BIOS from before January 1, 2001. Linux-based operating systems can provide handling of ACPI events via acpid.

OSPM responsibilities

Once an OSPM-compatible operating system activates ACPI, it takes 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 four global "Gx" states and six sleep "Sx" states for an ACPI-compliant computer system: The specification also defines a ''Legacy'' state: the state of 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'' or ''Fully On'' is the operating state. ** As with S0ix, Intel has ''D0ix'' states for intermediate levels on the SoC. * ''D1'' and ''D2'' are intermediate power-states whose definition varies by device. * ''D3'': The D3 state is further divided into ''D3 Hot'' (has auxiliary power), and ''D3 Cold'' (no power provided): ** ''Hot'': A device can assert power management requests to transition to higher power states. ** ''Cold'' or ''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 Pentium 4 is a series of single-core CPUs for desktops, laptops and entry-level servers manufactured by Intel. The processors were shipped from November 20, 2000 until August 8, 2008. The production of Netburst processors was active from 2000 ...

and

AMD Athlon Athlon is the brand name applied to a series of x86-compatible microprocessors designed and manufactured by Advanced Micro Devices (AMD). The original Athlon (now called Athlon Classic) was the first seventh-generation x86 processor and the fi ...

, also support an Enhanced C1 state (''C1E'' or Enhanced Halt State) for lower power consumption, however this proved to be buggy on some systems. * ''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. * ''Additional states'' are defined by manufacturers for some processors. For example,

's Haswell platform has states up to ''C10'', where it distinguishes ''core'' states and ''package'' states.

Performance state

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. 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

processors, as PowerNow! or Cool'n'Quiet in AMD processors, and as PowerSaver in VIA processors. * ''P0'' maximum power and frequency * ''P1'' less than ''P0'', voltage and frequency scaled * ''P2'' less than ''P1'', voltage and frequency scaled * ''Pn'' less than ''P(n–1)'', voltage and frequency scaled

Interfaces

Hardware

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 An original equipment manufacturer (OEM) is generally perceived as a company that produces non-aftermarket parts and equipment that may be marketed by another manufacturer. It is a common industry term recognized and used by many professional or ...

(OEM). Function Fixed Hardware interfaces are platform-specific features, provided by platform manufacturers for the purposes of performance and failure recovery. Standard

-based

PCs A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or techn ...

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 Platform Error Interface (APEI) is a specification for reporting of hardware errors, e.g. chipset, RAM to the operating system.

Firmware

ACPI defines many tables that provide the interface between an ACPI-compliant

and system firmware (

). This includes RSDP, RSDT, XSDT, FADT, FACS, DSDT, SSDT, MADT, and MCFG, 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 AML can be decompiled by tools like Intel's iASL (open-source, part of ACPICA) for purposes like patching the tables for expanding OS compatibility. The Root System Description Pointer (RSDP) is located in a platform-dependent manner, and describes the rest of the tables.

Security risks

A custom ACPI table called the Windows Platform Binary Table (WPBT) is used by Microsoft to allow vendors to add software into the Windows OS automatically. Some vendors, such as

Lenovo Lenovo Group Limited, often shortened to Lenovo ( , ), is a Chinese multinational technology company specializing in designing, manufacturing, and marketing consumer electronics, personal computers, software, business solutions, and related se ...

, have been caught using this feature to install harmful software such as Superfish.

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shipped PCs with Windows Update disabled. Windows versions older than Windows 7 do not support this feature, but alternative techniques can be used. This behavior has been compared to

rootkit A rootkit is a collection of computer software, typically malicious, designed to enable access to a computer or an area of its software that is not otherwise allowed (for example, to an unauthorized user) and often masks its existence or the exis ...

References

External links

* (UEFI and ACPI specifications)
Everything You Need to Know About the CPU C-States Power Saving ModesSample EFI ASL code
used by

VirtualBox Oracle VM VirtualBox (formerly Sun VirtualBox, Sun xVM VirtualBox and Innotek VirtualBox) is a type-2 hypervisor for x86 virtualization developed by Oracle Corporation. VirtualBox was originally created by Innotek GmbH, which was acquired by ...

; EFI/ASL code itself is from the open source Intel EFI Development Kit II ( TianoCore)
ACPICA
{{DEFAULTSORT:Advanced Configuration And Power Interface BIOS Unified Extensible Firmware Interface Application programming interfaces Computer hardware standards Open standards Electric power System administration