Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
All encodings are such that the lead byte of an encoding signifies the type of declaration or reference
being made. The type either has an implicit or explicit length in the stream. All explicit length
declarations take the form shown below, where PkgLength is the length of the inclusive length of the
data for the operation.
LeadByte PkgLength data... LeadByte ...
PkgLength
Figure 5-32 AML Encoding
Encodings of implicit length objects either have fixed length encodings or allow for nested
encodings that, at some point, either result in an explicit or implicit fixed length.
The PkgLength is encoded as a series of 1 to 4 bytes in the stream with the most significant two bits
of byte zero, indicating how many following bytes are in the PkgLength encoding. The next two bits
are only used in one-byte encodings, which allows for one-byte encodings on a length up to 0x3F.
Longer encodings, which do not use these two bits, have a maximum length of the following: twobyte
encodings of 0x0FFF, three-byte encodings of 0x0FFFFF, and four-byte length encodings of
0x0FFFFFFFFF.
It is fatal for a package length to not fall on a logical boundary. For example, if a package is
contained in another package, then by definition its length must be contained within the outer
package, and similarly for a datum of implicit length.
At some point, the system software decides to “load” a Definition Block. Loading is accomplished
when the system makes a pass over the data and populates the ACPI namespace and initializes
objects accordingly. The namespace for which population occurs is either from the current
namespace location, as defined by all nested packages or from the root if the name is preceded with
‘\’.
The first object present in a Definition Block must be a named control method. This is the Definition
Block’s initialization control.
Packages are objects that contain an ordered reference to one or more objects. A package can also be
considered a vertex of an array, and any object contained within a package can be another package.
This permits multidimensional arrays of fixed or dynamic depths and vertices.
Unnamed objects are used to populate the contents of named objects. Unnamed objects cannot be
created in the “root.” Unnamed objects can be used as arguments in control methods.
Control method execution may generate errors when creating objects. This can occur if a Method
that creates named objects blocks and is reentered while blocked. This will happen because all
named objects have an absolute path. This is true even if the object name specified is relative. For
example, the following ASL code segments are functionally identical.
216 April, 2015 Version 6.0