ACPI

Advanced Configuration and Power Interface is an open standard that operating systems can use to discover and configure computer hardware components, to perform power management, auto configuration, and status monitoring. It was first released in December 1996. ACPI aims to replace Advanced Power Management, the MultiProcessor Specification, and the Plug and Play BIOS 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 system. ACPI defines hardware abstraction interfaces between the device's firmware, the computer hardware components, and the operating systems.
Internally, ACPI advertises the available components and their functions to the operating system kernel using instruction lists provided through the system firmware, which the kernel parses. ACPI then executes the desired operations written in ACPI Machine Language using an embedded minimal virtual machine.
Intel, Microsoft and Toshiba originally developed the standard, while HP, Huawei and Phoenix also participated later. In October 2013, ACPI Special Interest Group, the original developers of the ACPI standard, agreed to transfer all assets to the UEFI Forum, in which all future development will take place. of the standard 6.6 was released in.

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. Much of the firmware ACPI functionality is provided in bytecode of ACPI Machine Language, a Turing-complete, 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. At the BIOS development time, AML bytecode is compiled from the ASL code.

ACPI Component Architecture (ACPICA)

The ACPI Component Architecture, mainly written by Intel's engineers, provides an open-source platform-independent reference implementation of the operating system-related ACPI code. The ACPICA code is used by Linux, Haiku, ArcaOS and FreeBSD, 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 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 1999, then Microsoft CEO Bill Gates stated in an e-mail that Linux 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 interfaces, PCI Express bus, multiprocessor 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 architecture; Revision 5.0 of the ACPI specification was released in December 2011, which added the ARM architecture support. The revision 5.1 was released in July 2014. The latest revision 6.6, which was released in May 2025, added the RISC-V support.

Operating systems

Microsoft's Windows 98 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, macOS, eComStation, ArcaOS, FreeBSD, NetBSD, OpenBSD, HP-UX, OpenVMS, Linux, GNU/Hurd and PC versions of Solaris, have at least some support for ACPI. Some newer operating systems, like Windows Vista, require the computer to have an ACPI-compliant BIOS, and since Windows 8, 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 from kernel version 2.6.0 onwards. Old ACPI BIOS implementations tend to be quite buggy, and consequently are not supported by later operating systems. For example, Windows 2000, Windows XP, and Windows Server 2003 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:

Gx	Name	Sx	Description
G0	Working	S0	The computer is running and the CPU executes instructions. "Away mode" is a subset of S0, where monitor is off but background tasks are running.
G1	Sleeping	S0ix	Modern Standby, or "Low Power S0 Idle". Partial processor SoC sleep. Sub states include S0i1, S0i2 and S0i3. Known to ARM and x86 devices.
G1	Sleeping	S1	Power on Suspend : Processor is powered off, and the CPU stops executing instructions. The power to the CPU and RAM is maintained. Peripherals such as monitor and hard disk may be turned off.
G1	Sleeping	S2	CPU powered off. CPU cache is flushed to RAM.
G1	Sleeping	S3	Commonly referred to as Standby, Sleep, or Suspend to RAM : RAM remains powered, and RAM enters self refresh mode. Most peripherals are turned off. Fans are usually turned off. Requires GPU drivers on Windows.
G1	Sleeping	S4	Hibernation or Suspend to Disk: All content of the main memory is saved to non-volatile memory such as a hard drive, and the system is powered down.
G2	Soft Off	S5	Shutdown: system is powered down.
G3	Mechanical Off		The computer's power has been totally removed via a mechanical switch. The power cord can be removed and the system is safe for disassembly.

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, and D3 Cold :
* 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 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 and AMD Athlon, also support an Enhanced C1 state for lower power consumption, however this proved to be buggy on some systems.
C2 is a state where the processor maintains all software-visible state, but may take longer to wake up. This processor state is optional.
C3 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 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. They are reported to the system via the method. For example, Intel's Haswell platform has states up to C10, where it distinguishes core states and package states: the difference being that the package'' not only includes the processor cores, but also components such as the L3 cache, memory controller, and other I/O functions. Similarly, AMD Zen 4 CPUs diffentiate between C-states and P-states for the core and the Data Fabric.
For describing the idle states of groupings of components, the method is used. This should not be confused with Intel's private table, used to describe S0ix sleep in package C10 or PCH SLP_S0 state.