The Advanced Configuration and Power Interface (ACPI)
specification is 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 and therefore removing device management from legacy firmware interfaces. The standard was originally developed by Intel
, and Toshiba
, and last published as "Revision 3.0b", on October 10, 2006. Current developers also include, HP
, and Dell
ACPI was an attempt to consolidate and improve upon existing power and configuration standards for hardware devices. It provides a transition from existing standards to entirely ACPI compliant hardware, with some ACPI operating systems already removing support for legacy hardware. With the intention of replacing Advanced Power Management, the Multi-processor Specification and the Plug and Play BIOS Specification, the standard brings Power Management into operating system control (OSPM), as opposed to the previous BIOS central system, which relied on platform specific firmware.
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 implementing ACPI and express concerns about the requirements that bytecode from an external source must be run by the system with full privileges.
Microsoft Windows 98 was the first operating system with full support for ACPI, with FreeBSD, the Linux kernel, NetBSD, OpenBSD and PC versions of Solaris all having at least some support for ACPI.
In ACPI, it is required that once an OSPM compatible operating system activates ACPI on a computer, that it takes over, and has exclusive control of, all aspects of power management and device configuration responsibilities. The OSPM implementation must define an ACPI compatible environment to Device Drivers, which exposes certain System, Device and Processor states.
The ACPI specification defines the following seven states (so-called global states) which an ACPI-compliant computer system can be in:
- 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: The CPU is powered off.
- S3: Commonly referred to as Standby or Sleep. RAM is still powered.
- S4: Hibernation. 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.
- 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 disassembly (typically, only the real-time clock is running off its own small battery).
Furthermore, a state Legacy is defined as 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.
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.
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.
- C2 (often known as Stop-Clock) is a state where the processor maintains all software-visible state, but may take longer to wake up.
- 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.
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, PowerNow! or Cool'n'Quiet in AMD processors and PowerSaver in VIA processors.
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, which provides a set of core functionality that reduces a ACPI-compliant systems 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. 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
The Root System Description Pointer is located in a platform-dependent manner, and goes on to describe the rest of the tables.
ACPI Component Architecture (ACPICA)
The ACPI Component Architecture (ACPICA) is an open source OS-independent reference implementation of the ACPI specification. ACPICA is written in ANSI C.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.