Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
The hardware register definition was expanded, in ACPI 2.0, to allow registers to exist in address
spaces other than the System I/O address space. This is accomplished through the specification of an
address space ID in the register definition (see Section 5.2.3.2, “Generic Address Structure,” for
more information). When specifically directed by the CPU manufacturer, the system firmware
may define an interface as functional fixed hardware by supplying a special address space identifier,
FfixedHW (0x7F), in the address space ID field for register definitions. It is emphasized that
functional fixed hardware definitions may be declared in the ACPI system firmware only as
indicated by the CPU Manufacturer for specific interfaces as the use of functional fixed hardware
requires specific coordination with the OS vendor.
Only certain ACPI-defined interfaces may be implemented using functional fixed hardware and only
when the interfaces are common across machine designs for example, systems sharing a common
CPU architecture that does not support fixed hardware implementation of an ACPI-defined
interface. OEMs are cautioned not to anticipate that functional fixed hardware support will be
provided by OSPM differently on a system-by-system basis. The use of functional fixed hardware
carries with it a reliance on OS specific software that must be considered. OEMs should consult OS
vendors to ensure that specific functional fixed hardware interfaces are supported by specific
operating systems.
Note: FFH is permitted and applicable to both full and HW-reduced ACPI implementations.
5.2.3.2 Generic Address Structure
The Generic Address Structure (GAS) provides the platform with a robust means to describe register
locations. This structure, described below (Table 5-26), is used to express register addresses within
tables defined by ACPI .
Table 5-26 Generic Address Structure (GAS)
Field
Address Space
ID
Register Bit
Width
Register Bit
Offset
Byte
Length
Byte
Offset
Description
1 0 The address space where the data structure or register exists.
Defined values are:
0 System Memory
1 System I/O
2 PCI Configuration Space
3 Embedded Controller
4 SMBus
5 to 0x09 Reserved
0x0A
Platform Communications Channel (PCC)
0x0B to 0x7E Reserved
0x7F
Functional Fixed Hardware
0x80 to 0xBF Reserved
0xC0 to 0xFF OEM Defined
1 1 The size in bits of the given register. When addressing a data
structure, this field must be zero.
1 2 The bit offset of the given register at the given address. When
addressing a data structure, this field must be zero.
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