First released in December 1996, ACPI 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.
ACPI aims 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 MultiProcessor 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 to determine power management and configuration policy.
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. 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".
Microsoft Windows 98 was the first operating system with full support for ACPI, with Windows 2000, Windows XP, Windows Vista, Windows 7, eComStation, FreeBSD, NetBSD, OpenBSD, HP-UX, OpenVMS, Linux and PC versions of SunOS all having at least some support for ACPI.
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.
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 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 dis-assembly (typically, only the real-time clock is running off its own small battery).
Furthermore, the specification 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.
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. All ACPI-conformant processors must support this power state. Some processors, such as the Pentium 4, also support an Enhanced C1 state (C1E or Enhanced Halt State) 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. 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.
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.
- P0 max power and frequency
- P1 less than P0, voltage/frequency scaled
- Pn less than P(n-1), voltage/frequency scaled
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 Intel-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 defines a large number of tables that provide the interface between an ACPI-compliant operating system and system firmware. For example:
- DSDT – Differentiated System Description Table
- SSDT – Secondary System Description Table
- SRAT – System Resource Affinity Table
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 Root System Description Pointer is located in a platform-dependent manner, and describes the rest of the tables.
ACPI Component Architecture (ACPICA)
The ACPI Component Architecture (ACPICA) provides an open-source OS-independent reference implementation of the ACPI specification.
The first revision of the ACPI specification was released in December 1996 supporting 16 and 32-bit addressing spaces. It wasn't until August 2000 that ACPI received 64-bit address support as well as support for multiprocessor workstations and servers with revision 2.0. In September 2004, revision 3.0 gave the ACPI specification support for SATA connectors, PCI Express bus, >256 multiprocessor support, ambient light sensors and user-presence devices, as well as extending the Thermal model beyond the previous processor centric support. The latest of the major publications is that of revision 4.0. Released in June 2009, the 4.0 specification added many new features to the design; most notable are USB 3.0 support, logical processor idling support, and x2APIC support.
- Active State Power Management - hardware power management protocol for PCI Express
- Advanced Power Management (APM)
- Green computing
- Power management keys - Keyboard related
- Simple Firmware Interface (SFI)
- Advanced Configuration and Power Interface Specification
- ACPI home page
- Intel's ACPI page
- Intel's ACPI Component Architecture
- How Linux Suspend and Resume works in the ACPI age
- Everything You Need to Know About the CPU C-States Power Saving Modes