ACPI

The Advanced Configuration and Power Interface (ACPI) provides standardized, flexible mechanisms for device discovery, operating system configuration and power management (OSPM), thermal management and RAS (reliability, availability and supportability) features. The ACPI standard improves system power distribution and conservation through its communications with the system firmware, operating systems and peripheral devices. ACPI can be applied to all classes of systems and devices and is OS and processor architecture agnostic. It has an open framework that helps reduce market fragmentation and provides a robust interface for managing platform hardware at runtime.

FAQ

Why was the ACPI standard created?

The ACPI specification was created to define a standard model for controlling power state transitions and enabling new platform technologies to evolve independently in the operating system and hardware. Prior to the ACPI standard, custom vendor solutions for power management and system configuration inundated the market. Each proprietary solution required custom OS support, resulting in market fragmentation and influencing the demand for standardized firmware solutions.
Intel, Microsoft, Toshiba, HP and Phoenix set out to unify system power management through the ACPI standard. Over time, the ACPI standard replaced a collection of power management application-programming interfaces (APIs), such as Advanced Power Management (APM), as well as system configuration interfaces such like PNPBIOS APIs and the MultiProcessor Specification (MPS) for x86 architecture. More recently, bindings for ARM architecture and SoC devices were added in the ACPI v5.0 release.

What is the UEFI Forum’s involvement with the ACPI standard?

The ACPI specification was transferred to the UEFI Forum in October 2013, following the ACPI v5.0a release. Uniting ACPI with the Forum’s existing standards portfolio helps synchronize interface definitions and increases participation of open-source developers, which is critical to expanding ACPI adoption onto new classes of platforms and devices. A working group within the UEFI Forum, the ACPI Specification Working Group (ASWG), was created to manage and evolve future ACPI developments.

What are the ACPI features?

The two primary roles of ACPI include device configuration and power management. Under the role of device configuration, ACPI can let the OS know what hardware it contains, including devices that are not readily detectible. As for the role of power management, once the OS is running, there are functions that can move the machine into a low-power state. These mechanisms tend to be platform-specific, each with a different design and management process. Additionally, ACPI provides standard interfaces for RAS (reliability, availability and supportability) features.

What functional areas are covered by the ACPI specification?

Platforms compliant with the ACPI specification provide Operating System-directed configuration and Power Management (OSPM) with direct and exclusive control over the power management and motherboard device configuration functions of a computer. These functional areas include:

System power management
Device power management
Processor power management
Configuration/Plug and Play
System events
Battery management
Thermal management
Embedded controller
SMBus controller

ACPI requires that once an ACPI compliant platform is in ACPI mode, the platform’s hardware, firmware, or other non-OS software must not manipulate the platform’s configuration, power, performance, and thermal control interfaces independently of OSPM. OSPM alone is responsible for coordinating the configuration, power management, performance management, and thermal control policy of the system.

Who is intended to implement the ACPI standard?

The target audiences for the ACPI specification include:

OEMs building hardware containing ACPI-compatible interfaces
Operating system and device driver developers
BIOS and ACPI system firmware developers
CPU and chip set vendors
Peripheral vendors

What computing platforms are compatible with the ACPI standard?

ACPI supports a wide range of platforms including laptops, tablets, smartphones, workstations, desktops and servers. The specification already includes some support for ARM SoC (System-on-Chip) and architecture extensions. Future iterations of the ACPI specification will continue to develop and expand support for the ARM processor core architecture.

What are the benefits to implementing the ACPI standard?

The benefits to using the ACPI standard include enhanced power management functionality and a robust interface for configuring motherboard devices. ACPI offers stability and backward-compatibility for legacy operating systems running on new hardware, without requiring major re-writes or re-builds of the software stack. This provides an open application platform that sits on top of standard compliant firmware. The hardware-discovery and hard-control, separated by the ACPI abstraction layer, allows OSes to condense test-case scenarios to a single image test. This reduces engineering expenses, as the AML (ACPI Machine Language) encoding embedded within the ACPI tables removes the need for the kernel image to contain individual drivers.

Content