Advanced Configuration and Power Interface Specification

ACPI Hardware Specification
would be called to access this address space. This takes a long time and will either adversely affect
the power of the system (when trying to enter a low-power state) or the accuracy of the event (when
trying to get a time stamp value).
Access to fixed hardware by OSPM allows OSPM to control the wake process without having to
load the entire OS. For example, if PCI configuration space access is needed, the bus enumerator is
loaded with all drivers used by the enumerator. Defining these interfaces in fixed hardware at
addresses with which OSPM can communicate without any other driver’s assistance, allows OSPM
to gather information prior to making a decision as to whether it continues loading the entire OS or
puts it back to sleep.
If elements of the OS fail, it may be possible for OSPM to access address spaces that need no driver
support. In such a situation, OSPM will attempt to honor fixed power button requests to transition
the system to the G2 state. In the case where OSPM event handler is no longer able to respond to
power button events, the power button override feature provides a back-up mechanism to
unconditionally transition the system to the soft-off state.
4.3 Generic Hardware Programming Model
Although the fixed hardware programming model requires hardware registers to be defined at
specific address locations, the generic hardware programming model allows hardware registers to
reside in most address spaces and provides system OEMs with a wide degree of flexibility in the
implementation of specific functions in hardware. OSPM directly accesses the fixed hardware
registers, but relies on OEM-provided ACPI Machine Language (AML) code to access generic
hardware registers.
AML code allows the OEM to provide the means for OSPM to control a generic hardware feature’s
control and event logic.
The section entitled “ACPI Source Language Reference” describes the ACPI Source Language
(ASL)—a programming language that OEMs use to create AML. The ASL language provides many
of the operators found in common object-oriented programming languages, but it has been
optimized to enable the description of platform power management and configuration hardware. An
ASL compiler converts ASL source code to AML, which is a very compact machine language that
the ACPI AML code interpreter executes.
AML does two things:
• Abstracts the hardware from OSPM
• Buffers OEM code from the different OS implementations
One goal of ACPI is to allow the OEM “value added” hardware to remain basically unchanged in an
ACPI configuration. One attribute of value-added hardware is that it is all implemented differently.
To enable OSPM to execute properly on different types of value added hardware, ACPI defines
higher level “control methods” that it calls to perform an action. The OEM provides AML code,
which is associated with control methods, to be executed by OSPM. By providing AML code,
generic hardware can take on almost any form.
Another important goal of ACPI is to provide OS independence. To do this, the OEM AML code has
to execute the same under any ACPI-compatible OS. ACPI allows for this by making the AML code
interpreter part of OSPM. This allows OSPM to take care of synchronizing and blocking issues
specific to each particular OS.
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