Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
• UpdateRule is not applicable to GenericSerialBus operation regions since each virtual register is
accessed in its entirety. This field is ignored for all GenericSerialBus field definitions.
GenericSerialBus operation regions require that all field elements be declared at command value
granularity. This means that each virtual register cannot be broken down to its individual bits within
the field definition.
Access to sub-portions of virtual registers can be done only outside of the field definition. This
limitation is imposed to simplify the GenericSerialBus interface.
GenericSerialBus protocols are assigned to field elements using the AccessAs term within the field
definition. The syntax for this term (from Section 19.2.3, “ASL Root and SecondaryTerms”) is
described below.
AccessAs(
AccessType,
AccessAttribute
)
Where:
//AccessTypeKeyword
//Nothing | ByteConst | AccessAttribKeyword
• AccessType must be set to BufferAcc.
• AccessAttribute indicates the GenericSerialBus protocol to assign to command values that
follow this term. SeeSection 5.5.2.4.5.3, “Using the GenericSerialBus Protocols,” for a listing of
the GenericSerialBus protocols.
An AccessAs term must appear in a field definition to set the initial GenericSerialBus protocol for
the field elements that follow. A maximum of one GenericSerialBus protocol may be defined for
each field element. Devices supporting multiple protocols for a single command value can be
modeled by specifying multiple field elements with the same offset (command value), where each
field element is preceded by an AccessAs term specifying an alternate protocol.
For GenericSerialBus operation regions, connection attributes must be defined for each set of field
elements. GenericSerialBus resources are assigned to field elements using the Connection term
within the field definition. The syntax for this term (from Section 19.6.15 “Connection (Declare
Field Connection Attributes)”) is described below.
Connection (ConnectionResourceObj)
Where:
• ConnectionResourceObj points to a Serial Bus Resource Connection Descriptor (see
Section 6.4.3.8.2, “Serial Bus Connection Resource Descriptors” for valid types), or a named
object that specifies a buffer field containing the connection resource information.
Each Field definition references the initial command offset specified in the operation region
definition. The offset is iterated for each subsequent field element defined in that respective Field. If
a new connection is described in the same Field definition, the offset will not be returned to its initial
value and a new Field must be defined to inherit the initial command value offset from the operation
region definition. The following example illustrates this point.
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