Advanced Configuration and Power Interface Specification

ACPI Hardware Specification
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ACPI Hardware Specification
ACPI defines standard interface mechanisms that allow an ACPI-compatible OS to control and
communicate with an ACPI-compatible hardware platform. These interface mechanisms are
optional (See "Hardware-Reduced ACPI", below).However, if the ACPI Hardware Specification is
implemented, platforms must comply with the requirements in this section.
This section describes the hardware aspects of ACPI.
ACPI defines “hardware” as a programming model and its behavior. ACPI strives to keep much of
the existing legacy programming model the same; however, to meet certain feature goals, designated
features conform to a specific addressing and programming scheme. Hardware that falls within this
category is referred to as “fixed.”
Although ACPI strives to minimize these changes, hardware engineers should read this section
carefully to understand the changes needed to convert a legacy-only hardware model to an ACPI/
Legacy hardware model or an ACPI-only hardware model.
ACPI classifies hardware into two categories: Fixed or Generic. Hardware that falls within the fixed
category meets the programming and behavior specifications of ACPI. Hardware that falls within
the generic category has a wide degree of flexibility in its implementation.
4.1 Hardware-Reduced ACPI
For certain classes of systems the ACPI Hardware Specification may not be adequate. Examples
include legacy-free, UEFI-based platforms with recent processors, and those implementing mobile
platform architectures. For such platforms, a Hardware-reduced ACPI mode is defined. Under this
definition, the ACPI Fixed Hardware interface is not implemented, and software alternatives for
many of the features it supports are used instead. Note, though, that Hardware-reduced ACPI is not
intended to support every possible ACPI system that can be built today. Rather, it is intended to
introduce new systems that are designed to be HW-reduced from the start. The ACPI HW
Specification should be used if the platform cannot be designed to work without it. Specifically, the
following features are not supported under the HW-reduced definition:
• The Global Lock, SMI_CMD, ACPI Enable and ACPI Disable. Hardware-reduced ACPI
systems always boot in ACPI mode, and do not support hardware resource sharing between
OSPM and other asynchronous operating environments, such as UEFI Runtime Services or
System Management Mode.
• Bus Master Reload and Arbiter Disable. Systems that depend on OS use of these bits to maintain
cache coherency across processor sleep states are not supported.
• GPE block devices are not supported.
Platforms that require the above features must implement the ACPI Hardware Specification.
Platforms that are designed for the Hardware-reduced ACPI definition must implement Revision 5
or greater of the Fixed ACPI Descriptor Table, and must set the HW_REDUCED_ACPI flag in the
Flags field.
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