Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
5.5.2 Control Method Execution
OSPM evaluates control method objects as necessary to either interrogate or adjust the system-level
hardware state. This is called an invocation.
A control method can use other internal, or well defined, control methods to accomplish the task at
hand, which can include defined control methods provided by the operating software. Control
Methods can reference any objects anywhere in the Namespace. Interpretation of a Control Method
is not preemptive, but it can block. When a control method does block, OSPM can initiate or
continue the execution of a different control method. A control method can only assume that access
to global objects is exclusive for any period the control method does not block.
Global objects are those NameSpace objects created at table load time.
5.5.2.1 Arguments
Up to seven arguments can be passed to a control method. Each argument is an object that in turn
could be a “package” style object that refers to other objects. Access to the argument objects is
provided via the ASL ArgTerm (ArgX) language elements. The number of arguments passed to any
control method is fixed and is defined when the control method package is created.
Method arguments can take one of the following forms:
• An ACPI name or namepath that refers to a named object. This includes the LocalX and ArgX
names. In this case, the object associated with the name is passed as the argument.
• An ACPI name or namepath that refers to another control method. In this case, the method is
invoked and the return value of the method is passed as the argument. A fatal error occurs if no
object is returned from the method. If the object is not used after the method invocation it is
automatically deleted.
• A valid ASL expression. In the case, the expression is evaluated and the object that results from
this evaluation is passed as the argument. If this object is not used after the method invocation it
is automatically deleted.
5.5.2.2 Method Calling Convention
The calling convention for control methods can best be described as call-by-reference-constant. In
this convention, objects passed as arguments are passed by “reference”, meaning that they are not
copied to new objects as they are passed to the called control method (A calling convention that
copies objects or object wrappers during a call is known as call-by-value or call-by-copy).
This call-by-reference-constant convention allows internal objects to be shared across each method
invocation, therefore reducing the number of object copies that must be performed as well as the
number of buffers that must be copied. This calling convention is appropriate to the low-level nature
of the ACPI subsystem within the kernel of the host operating system where non-paged dynamic
memory is typically at a premium. The ASL programmer must be aware of the calling convention
and the related side effects.
However, unlike a pure call-by-reference convention, the ability of the called control method to
modify arguments is extremely limited. This reduces aliasing issues such as when a called method
unexpectedly modifies a object or variable that has been passed as an argument by the caller. In
effect, the arguments that are passed to control methods are passed as constants that cannot be
modified except under specific controlled circumstances.
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