Advanced Configuration and Power Interface. In a computer, the Advanced Configuration and Power Interface (ACPI) provides an open standard that operating systems can use to discover and configure computer hardware components, to perform power management by (for example) putting unused components to sleep, and to perform status monitoring. First released in December 1.
ACPI aims to replace Advanced Power Management (APM), the Multi. Processor Specification, and the Plug and Play BIOS (Pn. P) Specification.[1] ACPI brings the 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.[2] The specification is central to the Operating System- directed configuration and Power Management (OSPM) system, an implementation for ACPI which removes device management responsibilities from legacy firmware interfaces via a UI. Internally, ACPI advertises the available components and their functions to the operating system kernel using instruction lists ("methods") provided through the system firmware (Unified Extensible Firmware Interface (UEFI) or BIOS), which the kernel parses. ACPI then executes the desired operations (such as the initialization of hardware components) using an embedded minimal virtual machine. Intel, Microsoft and Toshiba originally developed the standard, while HP, Huawei and Phoenix also participated later.
In October 2. 01. ACPI standard agreed to transfer all assets to the UEFI Forum, in which all future development will take place.[3]The UEFI Forum published the latest version[update] of the standard, "Revision 6. May 2. 01. 7.[4]Architecture[edit]The firmware- level ACPI has three main components: the ACPI tables, the ACPI BIOS, and the ACPI registers. Unlike its predecessors, such as the APM or Pn.
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P BIOS, the ACPI implements little of its functionality in the ACPI BIOS code, whose main role is to load the ACPI tables in system memory. Instead, most of the firmware ACPI functionality is provided in ACPI Machine Language (AML) bytecode stored in the ACPI tables. To make use of these 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.[5][6]As ACPI also replaces Pn. P BIOS, it also provides a hardware enumerator, mostly implemented in the Differentiated System Description Table (DSDT) ACPI table.
The advantage of a bytecode approach is that unlike Pn. P BIOS code (which was 1.
ACPI bytecode may be used in any operating system, even in 6. Overall design decision was not without criticism. In November 2. 00. Linus Torvalds—author of the Linux kernel—described ACPI as "a complete design disaster in every way".[7][8] In 2. Linux software developers like Alan Cox expressed concerns about the requirements that bytecode from an external source must be run by the kernel with full privileges, as well as the overall complexity of the ACPI specification.[9] In 2. Mark Shuttleworth, founder of the Ubuntu.
Linux distribution, compared ACPI with Trojan horses.[1. ACPI Component Architecture (ACPICA)[edit]The ACPI Component Architecture (ACPICA), mainly written by Intel's engineers, provides an open- source platform- independent reference implementation of the operating system–related ACPI code.[1. The ACPICA code is used by Linux, Haiku and Free. BSD,[5] which supplement it with their operating- system specific code. History[edit]The first revision of the ACPI specification was released in December 1. It was not until August 2.
ACPI received 6. 4- bit address support as well as support for multiprocessor workstations and servers with revision 2. In September 2. 00. ACPI specification support for SATA controllers, PCI Express bus, multiprocessor support for more than 2. Released in June 2. ACPI specification added various new features to the design; most notable are the USB 3. APIC support. Revision 5. ACPI specification was released in December 2.
July 2. 01. 4.[1. The latest specification revision is 6. May 2. 01. 7.[4]Operating systems[edit]Microsoft's Windows 9. ACPI,[1. 4][1. 5] but its implementation was somewhat buggy or incomplete,[1. ACPI hardware.[1. Windows 9. 8 first edition disabled ACPI by default except on a whitelist of systems. Other operating systems, including later versions of Windows, e.
Com. Station, Free. BSD, Net. BSD, Open. BSD, HP- UX, Open. VMS, Linux, and PC versions of Solaris, have at least some support for ACPI.[1. Some newer operating systems like Windows Vista require ACPI- compliant BIOS to work at all[2. 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.
Old ACPI BIOS implementations tend to be quite buggy, and consequently are not supported by later operating systems. For example, Windows 2. Windows XP, and Windows Server 2. ACPI if the BIOS date is after January 1, 1. Windows 9. 8 Second Edition this date is December 1, 1.
Similarly, Linux kernel 2. ACPI BIOS from before January 1, 2. OSPM responsibilities[edit]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[edit]Global states[edit]The ACPI specification defines the following four global "Gx" states and six sleep "Sx" states for an ACPI- compliant computer system: [2. G0 (S0), Working: The computer is running and the CPU executes instructions. Awaymode" is a subset of S0, where monitor is off but background tasks are running.
G1, Sleeping: Divided into four states, S1 through S4. S1, Power on Suspend (POS): Processor caches are flushed, and the CPU(s) stops executing instructions.
The power to the CPU(s) and RAM is maintained. Devices that do not indicate they must remain on may be powered off. S2: CPU powered off. Dirty cache is flushed to RAM. S3, commonly referred to as Standby, Sleep, or Suspend to RAM (STR): RAM remains powered. 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 (S5), Soft Off: G2/S5 is almost the same as G3 Mechanical Off, except that the power supply unit (PSU) still supplies power, at a minimum, to the power button to allow return to S0. A full reboot is required. No previous content is retained. Other components may remain powered so the computer can "wake" on input from the keyboard, clock, modem, LAN, or USB device. G3, Mechanical Off: The computer's power has been totally removed via a mechanical switch (as on the rear of a PSU). The power cord can be removed and the system is safe for disassembly (typically, only the real- time clock continues to run using its own small battery). The specification also defines a Legacy state: the state on 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[edit]The device states D0–D3 are device dependent: D0 or Fully On is the operating state. D1 and D2 are intermediate power- states whose definition varies by device. D3: The D3 state is further divided into D3 Hot (has aux 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[edit]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, also support an Enhanced C1 state (C1. E or Enhanced Halt State) for lower power consumption.[2.
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.