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 common prerequisite that open standards use an open license that provides for extensibility. Typically, anybody can participate in their development due to ...

that

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

s can use to discover and configure

computer hardware Computer hardware includes the physical parts of a computer, such as the central processing unit (CPU), random-access memory (RAM), motherboard, computer data storage, graphics card, sound card, and computer case. It includes external devices ...

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 reso ...

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 only the addition of components to a running computer system. Components which ha ...

), and status monitoring. It was first released in December 1996. ACPI aims to replace

Advanced Power Management Advanced power management (APM) is a technical standard for power management developed by Intel and Microsoft and released in 1992 which enables an operating system running an IBM-compatible personal computer to work with the BIOS (part of the com ...

(APM), the MultiProcessor Specification, and the Plug and Play BIOS (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 Hardware abstractions are sets of routines in software that provide programs with access to hardware resources through programming interfaces. The programming interface allows all devices in a particular class ''C'' of hardware devices to be acc ...

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 a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization d ...

UEFI Unified Extensible Firmware Interface (UEFI, as an acronym) is a Specification (technical standard), specification for the firmware Software architecture, architecture of a computing platform. When a computer booting, is powered on, the UEFI ...

), the

components, and the

s. Internally, ACPI advertises the available components and their functions to the operating system kernel using instruction lists (" methods") provided through the system

firmware In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, h ...

(

), 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 or emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve ...

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...

Microsoft Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...

and

Toshiba is a Japanese multinational electronics company headquartered in Minato, Tokyo. Its diversified products and services include power, industrial and social infrastructure systems, elevators and escalators, electronic components, semiconductors ...

originally developed the standard, while HP,

Huawei Huawei Technologies Co., Ltd. ("Huawei" sometimes stylized as "HUAWEI"; ; zh, c=华为, p= ) is a Chinese multinational corporationtechnology company in Longgang, Shenzhen, Longgang, Shenzhen, Guangdong. Its main product lines include teleco ...

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 Business alliance, alliance between technology companies to coordinate the development of the UEFI specifications. The board of directors includes representatives from twelve ''promoter'' companies: Advanced Micro Devices, ...

, in which all future development will take place. of the standard 6.5 was released 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 or digital 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 processin ...

. 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 (normal ...

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

Turing-complete In computability theory, a system of data-manipulation rules (such as a model of computation, 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 ...

domain-specific Domain specificity is a theoretical position in cognitive science (especially modern cognitive development) that argues that many aspects of cognition are supported by specialized, presumably evolutionarily specified, learning devices. The posit ...

low-level language, stored in the ACPI tables. To make use of the ACPI tables, the operating system must have an

interpreter Interpreting is translation from a spoken or signed language into another language, usually in real time to facilitate live communication. It is distinguished from the translation of a written text, which can be more deliberative and make use o ...

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

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 and view the source code, design documents, or content of the product. The open source model is a decentrali ...

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

Haiku is a type of short form poetry that originated in Japan. Traditional Japanese haiku consist of three phrases composed of 17 Mora (linguistics), morae (called ''On (Japanese prosody), on'' in Japanese) in a 5, 7, 5 pattern; that include a ''kire ...

ArcaOS ArcaOS is a Proprietary software, proprietary operating system based on OS/2, developed and marketed by Arca Noae, LLC under license from IBM. It was first released in 2017 and builds on OS/2 Warp 4.52 by adding support for new hardware, fixing ...

and

FreeBSD FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...

,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 a maximum of 32- bit units. Compared to smaller bit widths, 32-bit computers can perform la ...

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 central processing units (CPU) and arithmetic logic units (ALU) are those that are based on processor registers, a ...

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 businessman and philanthropist. A pioneer of the microcomputer revolution of the 1970s and 1980s, he co-founded the software company Microsoft in 1975 with his childhood friend ...

stated in an e-mail that

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

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 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) standard ...

interfaces,

PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe, is a high-speed standard used to connect hardware components inside computers. It is designed to replace older expansion bus standards such as Peripher ...

bus,

multiprocessor Multiprocessing (MP) 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. The ...

support for more than 256 processors, ambient light sensors 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. Initially ACPI was exclusive to

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

architecture; 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 (ISAs) for central processing unit, com ...

support. The revision 5.1 was released in July 2014. The revision 6.6, which was released in May 2025, added the

RISC-V RISC-V (pronounced "risk-five") is an open standard instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. The project commenced in 2010 at the University of California, Berkeley. It transfer ...

support. The latest specification revision is 6.6, which was released in May 2025.

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. It was the second operating system in the 9x line, as the successor to Windows 95. It was Software ...

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 Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

(x86 macOS only), eComStation,

(since FreeBSD 5.0),

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

(since NetBSD 1.6),

OpenBSD OpenBSD is a security-focused operating system, security-focused, free software, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by fork (software development), forking NetBSD ...

(since OpenBSD 3.8),

HP-UX HP-UX (from "Hewlett Packard Unix") is a proprietary software, proprietary implementation of the Unix operating system developed by Hewlett Packard Enterprise; current versions support HPE Integrity Servers, based on Intel's Itanium architect ...

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 Op ...

, GNU/Hurd 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, released five years earlier, which was then the longest time span between successive releases of Microsoft W ...

, 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 Software release life cycle#Release to manufacturing (RTM), released to manufacturing on August 1, 2012, made available for download via Microsoft ...

, 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, targeting the server and business markets. It is the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RT ...

Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct successor to Windows 2000 for high-end and business users a ...

, and

Windows Server 2003 Windows Server 2003, codenamed "Whistler Server", is the sixth major version of the Windows NT operating system produced by Microsoft and the first server version to be released under the Windows Server brand name. It is part of the Windows NT ...

only use ACPI if the BIOS date is after January 1, 1999. Similarly, Linux kernel 2.6 may not use ACPI if the BIOS date is 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 central processing unit, CPUs for Desktop computer, desktops, laptops and entry-level Server (computing), servers manufactured by Intel. The processors were shipped from November 20, 2000 until August 8, 20 ...

and AMD Athlon, 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. The total number of states is device or processor dependent, but can be no greater than 16. P-states have become known as

SpeedStep Enhanced SpeedStep is a series of dynamic frequency scaling technologies (codenamed Geyserville and including SpeedStep, SpeedStep II, and SpeedStep III) built into some Intel's microprocessors that allow the clock speed of the processor to be ...

processors, as PowerNow! or

Cool'n'Quiet AMD Cool'n'Quiet is a CPU dynamic frequency scaling and power saving technology introduced by AMD with its Athlon XP processor line. It works by reducing the processor's clock rate and voltage when the processor is idle. The aim of this techn ...

AMD Advanced Micro Devices, Inc. (AMD) is an American multinational corporation and technology company headquartered in Santa Clara, California and maintains significant operations in Austin, Texas. AMD is a hardware and fabless company that de ...

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

-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 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. 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, trading as Lenovo ( , zh, c=联想, p=Liánxiǎng), is a Chinese multinational technology company specializing in designing, manufacturing, and marketing consumer electronics, personal computers, software, servers, conv ...

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

Samsung Samsung Group (; stylised as SΛMSUNG) is a South Korean Multinational corporation, multinational manufacturing Conglomerate (company), conglomerate headquartered in the Samsung Town office complex in Seoul. The group consists of numerous a ...

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 exist ...

Criticism

In November 2003,

Linus Torvalds Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and lead developer of the Linux kernel. He also created the distributed version control system Git. He was honored, along with Shinya Yam ...

—author of the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

—described ACPI as "a complete design disaster in every way".Linux Magazine issue 162, May 2014, page 9

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 VirtualBox (formerly Sun VirtualBox, Sun xVM VirtualBox and InnoTek VirtualBox) is a hosted hypervisor for x86 virtualization developed by Oracle Corporation. VirtualBox was originally created by InnoTek Systemberatung GmbH, which was ac ...

; 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