Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
• All names are a fixed 32 bits.
• The first byte of a name is inclusive of: ‘A’–‘Z’, ‘_’, (0x41–0x5A, 0x5F).
• The remaining three bytes of a name are inclusive of: ‘A’–‘Z’, ‘0’–‘9’, ‘_’, (0x41–0x5A, 0x30–
0x39, 0x5F).
• By convention, when an ASL compiler pads a name shorter than 4 characters, it is done so with
trailing underscores (‘_’). See the language definition for AML NameSeg in Section 17, “ACPI
Source Language Reference.”
• Names beginning with ‘_’ are reserved by this specification. Definition Blocks can only use
names beginning with ‘_’ as defined by this specification.
• A name proceeded with ‘\’ causes the name to refer to the root of the namespace (‘\’ is not part
of the 32-bit fixed-length name).
• A name proceeded with ‘^’ causes the name to refer to the parent of the current namespace (‘^’
is not part of the 32-bit fixed-length name).
Except for names preceded with a ‘\’, the current namespace determines where in the namespace
hierarchy a name being created goes and where a name being referenced is found. A name is located
by finding the matching name in the current namespace, and then in the parent namespace. If the
parent namespace does not contain the name, the search continues recursively upwards until either
the name is found or the namespace does not have a parent (the root of the namespace). This
indicates that the name is not found 1 .
An attempt to access names in the parent of the root will result in the name not being found.
There are two types of namespace paths: an absolute namespace path (that is, one that starts with a
‘\’ prefix), and a relative namespace path (that is, one that is relative to the current namespace). The
namespace search rules discussed above, only apply to single NameSeg paths, which is a relative
namespace path. For those relative name paths that contain multiple NameSegs or Parent Prefixes,
‘^’, the search rules do not apply. If the search rules do not apply to a relative namespace path, the
namespace object is looked up relative to the current namespace. For example:
ABCD
^ABCD
XYZ.ABCD
\XYZ.ABCD
//search rules apply
//search rules do not apply
//search rules do not apply
//search rules do not apply
All name references use a 32-bit fixed-length name or use a Name Extension prefix to concatenate
multiple 32-bit fixed-length name components together. This is useful for referring to the name of an
object, such as a control method, that is not in the scope of the current namespace.
The figure below shows a sample of the ACPI namespace after a Differentiated Definition Block has
been loaded.
1. Unless the operation being performed is explicitly prepared for failure in name resolution, this is considered
an error and may cause the system to stop working.
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