Device Path Protocol
Version 1.02 12/12/00 123
5.3.3 ACPI Device Path
This Device Path contains ACPI Device IDs that represent a device’s Plug and Play Hardware ID
and its corresponding unique persistent ID. The ACPI IDs are stored in the ACPI _HID and _UID
device identification objects that are associated with a device. The ACPI Device Path contains
values that must match exactly the ACPI name space that is provided by the platform firmware to
the operating system. Refer to the ACPI specification for a complete description of the _HID and
_UID device identification objects.
The _HID value is an optional device identification object that appears in the ACPI name space.
The _HID must be used to describe any device that will be enumerated by the ACPI driver. The
ACPI bus driver only enumerates a device when no standard bus enumerator exists for a device.
The _UID object provides the OS with a serial number-style ID for a device that does not change
across reboots. The object is optional, but is required when a system contains two devices that
report the same _HID. The _UID only needs to be unique among all device objects with the same
_HID value. If no _UID exists in the APCI name space for a _HID the value of zero must be stored
in the _UID field of the ACPI Device Path.
The ACPI Device Path is only used to describe devices that are not defined by a Hardware Device
Path. An _HID is required to represent a PCI root bridge, since the PCI specification does not
define the programming model for a PCI root bridge.
Table 5-8. ACPI Device Path
Mnemonic
Byte
Offset
Byte
Length Description
Type 0 1 Type 2 – ACPI Device Path.
Sub-Type 1 1 Sub-Type 1 ACPI Device Path.
Length 2 2 Length of this structure in bytes. Length is 12 bytes.
_HID 4 4 Device’s PnP hardware ID stored in a numeric 32-bit
compressed EISA-type ID. This value must match the
corresponding _HID in the ACPI name space.
_UID 8 4 Unique ID that is required by ACPI if two devices have the
same _HID. This value must also match the corresponding
_UID/_HID pair in the ACPI name space. Only the 32-bit
numeric value type of _UID is supported; thus strings must
not be used for the _UID in the ACPI name space.
5.3.4 Messaging Device Path
This Device Path is used to describe the connection of devices outside the resource domain of the
system. This Device Path can describe physical messaging information like SCSI ID or abstract
information like networking protocol IP addresses.