Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
Generally, the objects passed to a control method via the ArgX terms cannot be directly written or
modified by the called method. In other words, when an ArgX term is used as a target operand in an
ASL statement, the existing ArgX object is not modified. Instead, the new object replaces the
existing object and the ArgX term effectively becomes a LocalX term.
The only exception to the read-only argument rule is if an ArgX term contains an Object Reference
created via the RefOf ASL operator. In this case, the use of the ArgX term as a target operand will
cause any existing object stored at the ACPI name referred to by the RefOf operation to be
overwritten.
In some limited cases, a new, writable object may be created that will allow a control method to
change the value of an ArgX object. These cases are limited to Buffer and Package objects where the
“value” of the object is represented indirectly. For Buffers, a writable Index or Field can be created
that refers to the original buffer data and will allow the called method to read or modify the data. For
Packages, a writable Index can be created to allow the called method to modify the contents of
individual elements of the Package.
5.5.2.3 Local Variables and Locally Created Data Objects
Control methods can access up to eight local data objects. Access to the local data objects have
shorthand encodings. On initial control method execution, the local data objects are NULL. Access
to local objects is via the ASL LocalTerm language elements.
Upon control method execution completion, one object can be returned that can be used as the result
of the execution of the method. The “caller” must either use the result or save it to a different object
if it wants to preserve it. See the description of the Return ASL operator for additional details
NameSpace objects created within the scope of a method are dynamic. They exist only for the
duration of the method execution. They are created when specified by the code and are destroyed on
exit. A method may create dynamic objects outside of the current scope in the NameSpace using the
scope operator or using full path names. These objects will still be destroyed on method exit. Objects
created at load time outside of the scope of the method are static. For example:
Scope (\XYZ) {
Name (BAR, 5)
// Creates \XYZ.BAR
Method (FOO, 1) {
Store (BAR, CREG) // same effect as Store (\XYZ.BAR, CREG)
Name (BAR, 7) // Creates \XYZ.FOO.BAR
Store (BAR, DREG) // same effect as Store (\XYZ.FOO.BAR, DREG
Name (\XYZ.FOOB, 3) // Creates \XYZ.FOOB
} // end method
} // end scope
The object \XYZ.BAR is a static object created when the table that contains the above ASL is
loaded. The object \XYZ.FOO.BAR is a dynamic object that is created when the Name (BAR, 7)
statement in the FOO method is executed. The object \XYZ.FOOB is a dynamic object created by
the \XYZ.FOO method when the Name (\XYZ.FOOB, 3) statement is executed. Notice that the
\XYZ.FOOB object is destroyed after the \XYZ.FOO method exits.
5.5.2.4 Access to Operation Regions
Control Methods read and write data to locations in address spaces (for example, System memory
and System I/O) by using the Field operator (see Section 19.6.46 Field (Declare Field Objects)”) to
declare a data element within an entity known as an “Operation Region” and then performing
220 April, 2015 Version 6.0