ACPI Component Architecture (ACPICA)
Overview
ACPI Component Architecture (ACPICA) is a reference implementation of the Advanced Configuration and Power Interface (ACPI) specification. This implementation simplifies complex ACPI operating system implementations by integrating it directly as a kernel-resident subsystem, without changes to the core ACPICA code.
Introduction to ACPI Architecture
Why ACPICA?
With advanced power management (APM), power management is controlled by the BIOS independently of the operating system. When BIOS settings are exceeded, the BIOS turns off the screen or shuts down the hard drive. This creates an imperfect power management policy because it requires reimplementing the policy for each platform. Plug and play BIOS (PNPBIOS) specification is also unreliable.
With ACPI, power management moves from the BIOS to the operating system.
- An application can tell the operating system that the display is in use, and change its power policy accordingly.
- The operating system does not have the size limitation of the BIOS.
- The ACPI system firmware describes the system's characteristics by placing data, organized into tables, into the main memory.
ACPICA code is fairly mature and implements the following:
- An AML (ACPI machine language) interpreter
- A table manager
- A namespace manager
- A resource manager
- Fixed and general purpose event support
- ACPI hardware support
- Support for the ACPI 5.0 specification
How It Works
- ACPICA defines and implements a group of software components for both 32-bit and 64-bit platforms.
- The architecture isolates all operating system dependencies to a relatively small translation or conversion layer (called the OS Services Layer)
- This isolation enables the bulk of the ACPICA code to be independent of any individual operating system.
- Requires no source code changes within the ACPICA code itself for new operating systems.
What's Included
Major kernel-level components:
- AML Interpreter
- ACPI Table Manager
- ACPI Namespace Manager
- ACPI Resource Manager
- ACPI Fixed and General Purpose Event Support
- ACPI Hardware Support
- AML Disassembler (optional)
- AML Debugger (optional)
User-space tools and utilities:
- ACPI Source Code Compiler & Disassembler (iASL)
- ACPI Simulator/Executer (AcpiExec)
- ACPI System Table dump to ASCII utility/ (acpidump)
- ACPI Table Extractor (inverse of acpidump utility)/ (acpixtract)
- ACPI Help Utility (AcpiHelp)
Note The ACPICA components run in both kernel mode and user mode.
System Requirements
- Write operating systems in C (or able to interface to C code).
- Add ACPICA initialization calls into the startup routine.
For more details, see the ACPI specifications.
Environments
ACPICA is written in ANSI C, and can be generated under many different 32-bit and 64-bit operating system development environments. Source code packages are provided for Microsoft Windows* and UNIX*.
- The Windows package includes Visual C++* project files and other ACPI utilities
- The UNIX package has a format and licensing suitable for inclusion by commercial operating system vendors.
There is no stand-alone Linux* source code package since ACPICA updates for Linux are provided periodically in patch form. The ACPICA subsystem is modified to integrate smoothly with the Linux kernel source. This includes conversion of the ACPICA source code to the Linux kernel coding standard and licensing under the GNU General Public License.
News
- August 27, 2024 - The ACPI Reduced Hardware mode now supports waking up from the S3 state fully.
- March 31, 2023 - iASL preprocessor now supports full #define macros.
- March 31, 2022 - All Windows binaries are now digitally signed.
- December 17, 2021 - iASL: Added support for the NHLT, AGDI, and TDEL ACPI tables.
- June 4, 2021 - iASL: Added support for the PRMT, BDAT, RGRT, SVKL, and IVRS ACPI tables.
- April 1, 2021 - ACPI 6.4 is now supported in ACPICA version 20210331.
Support
For all ACPICA developer discussions, issues, and source code patches, subscribe to the discussion list.
To report ACPICA kernel-resident subsystem and ACPICA tools or utility issues, submit your issue on GitHub*.
FAQ
AML stands for ACPI Machine Language. This is a binary pseudo-code format, which the operating system's ACPI AML interpreter parses to discover the machine's properties. Since most BIOS implementers do not like to write in binary, the ACPI specification also includes ASL (ACPI Source Language). System designers describe their system in ASL. It is then compiled into AML, and included in the system's flash BIOS. Then, on boot, the BIOS startup code copies it into RAM, where it can be interpreted by the OS's ACPI AML interpreter.
ACPICA is actively maintained and updated. Releases occur approximately on a monthly to quarterly basis, and contain new features and bug fixes. ACPICA also closely tracks new releases of the ACPI specification.
Except for Windows, almost all operating systems use the ACPICA code base to enable ACPI for power management and configuration, as well as the iASL compiler to write BIOS implementations.
ACPICA can be licensed under the GNU General Public License or through a separate license that may be more favorable to commercial operating system vendors. See the source code license header for specifics.
The AML definition blocks in some systems contain an error, where the AML return operator is not used to return a value from a control method. Some ACPI-defined control methods (for example, _STA) must always return a value. Due to an errata, this bug does not manifest itself on Windows' ACPI implementations, and therefore was not detected when OEMs tested their systems using that OS. ACPICA implements a "slack mode" that enables these implicit returns.
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