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In computing, the Advanced Configuration and Power Interface (ACPI) specification provides an open standard for unified operating system-centric device configuration and power management. ACPI, first released in December 1996, 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 therefore removes device management responsibilities from legacy firmware interfaces. The standard was originally developed by Intel, Microsoft, and Toshiba, and last published as "Revision 4.0", on June 16, 2009. As of 2009, developers of ACPI also include HP and Phoenix.[1]



ACPI aims to consolidate 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 (APM), the MultiProcessor Specification (MPS) and the Plug and Play (PnP) BIOS Specification[3], the standard brings power management into operating system control (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. As a document that unifies many previous standards it covers many areas, for system and device builders as well as system programmers. Some software developers have trouble[5] implementing ACPI and express concerns about the requirements that bytecode from an external source must be run by the system with full privileges. 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 Windows 98 was the first operating system with full support for ACPI, with Windows 2000, Windows XP, Windows Vista, Windows 7, FreeBSD, NetBSD, OpenBSD, HP-UX, OpenVMS, Linux and PC versions of SunOS all having at least some support for ACPI.

OSPM responsibilities

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.


Power States

Global states

The ACPI specification defines the following seven states (so-called global states) for an ACPI-compliant computer-system:

  • G0 (S0) Working
  • G1 Sleeping (subdivides into the four states S1 through S4)
    • S1: All processor caches are flushed, and the CPU(s) stop executing instructions. Power to the CPU(s) and RAM is maintained; devices that do not indicate they must remain on may be powered down.
    • S2: CPU powered off
    • S3: Commonly referred to as Standby, Sleep, or Suspend to RAM. RAM remains powered
    • S4: Hibernation or Suspend to disk. All content of main memory is saved to non-volatile memory such as a hard drive, and is powered down.
  • G2 (S5) Soft Off. G2, S5, and Soft Off are synonyms. G2 is almost the same as G3 Mechanical Off, but some components remain powered so the computer can "wake" from input from the keyboard, clock, modem, LAN, or USB device.[7]
  • G3 Mechanical Off: The computer's power consumption approaches close to zero, to the point that the power cord can be removed and the system is safe for dis-assembly (typically, only the real-time clock is running off its own small battery).

Furthermore, the specification defines a Legacy state: the state when an operating system runs which does not support ACPI. In this state, the hardware and power are not managed via ACPI, effectively disabling ACPI.

State Description
S0/Working System is on. The CPU is fully up and running; power conservation operates on a per-device basis.
S1 Sleep System appears off. The CPU is stopped; RAM is refreshed; the system runs in a low power mode.
S2 Sleep System appears off. The CPU has no power; RAM is refreshed; the system uses a lower power mode than S1.
S3 Sleep (Standby) System appears off. The CPU has no power; RAM is in slow refresh; the power supply is in a reduced power mode. This mode is also referred to as 'Save To RAM'.
S4 Hibernate System appears off. The hardware is completely off, but system memory has been saved as a temporary file onto the harddisk. This mode is also referred to as 'Save To Disk'.
S5/Off System is off. The hardware is completely off, the operating system has shut down; nothing has been saved. Requires a complete reboot to return to the Working state.

System states

  • S0 Working (G0), Processor in C0-C3, full context save RAM maintained
  • S1 Sleeping with processor context maintained, RAM maintained
  • S2 Sleeping with processor content not necessarily maintained, RAM maintained, most devices in D3
  • S3 Sleeping, lower than S2, RAM maintained, most devices in D3
  • S4 Sleeping, lower than S3, RAM not maintained, most devices in D3
  • S5 Sleeping, lower than S4, no context saved, reboot necessary

Device states

The device states D0-D3 are device-dependent:

  • D0 Fully-On is the operating state.
  • D1 and D2 are intermediate power-states whose definition varies by device.
  • D3 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. Some processors, such as the Pentium 4, also support an Enhanced C1 state (C1E) for lower power consumption, all processors must support this power state.
  • 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 optionally supported by the system.
  • 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 optionally supported by the system.

Performance states

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.

  • P0 max power and frequency
  • P1 less than P0, voltage/frequency scaled
  • Pn less than P(n-1), voltage/frequency scaled

Hardware Interface

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 AML provided by the Original Equipment Manufacturer.

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[8], 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.

Firmware Interface

ACPI defines a large number of tables that provide the interface between an ACPI-compliant operating system and system firmware. These allow description of system hardware in a platform-independent manner, and are presented as either fixed formatted data structures or in ACPI Machine Language (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.

ACPI Component Architecture (ACPICA)

Stable release 20090730 / 2009-07-30; 5 months ago
Written in C
Type ACPI implementation
License GNU General Public License

The ACPI Component Architecture (ACPICA) provides an open-source OS-independent reference implementation of the ACPI specification.[9] ACPICA is written in ANSI C and released as free software under the terms of the GNU General Public License.

See also


External links

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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