Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
5.2.10 Firmware ACPI Control Structure (FACS)
The Firmware ACPI Control Structure (FACS) is a structure in read/write memory that the BIOS
reserves for ACPI usage. This structure is passed to an ACPI-compatible OS using the FADT. For
more information about the FADT FIRMWARE_CTRL field, see Table 5.2.9, “Fixed ACPI
Description Table (FADT).”
The BIOS aligns the FACS on a 64-byte boundary anywhere within the system’s memory address
space. The memory where the FACS structure resides must not be reported as system
AddressRangeMemory in the system address map. For example, the E820 address map reporting
interface would report the region as AddressRangeReserved. For more information about system
address map reporting interfaces, see Section 15, “System Address Map Interfaces.”
Table 5-37 Firmware ACPI Control Structure (FACS)
Byte Byte
Field
Length Offset Description
Signature 4 0 ‘FACS’
Length 4 4 Length, in bytes, of the entire Firmware ACPI Control Structure.
This value is 64 bytes or larger.
Hardware Signature 4 8 The value of the system’s “hardware signature” at last boot.
This value is calculated by the BIOS on a best effort basis to
indicate the base hardware configuration of the system such
that different base hardware configurations can have different
hardware signature values. OSPM uses this information in
waking from an S4 state, by comparing the current hardware
signature to the signature values saved in the non-volatile sleep
image. If the values are not the same, OSPM assumes that the
saved non-volatile image is from a different hardware
configuration and cannot be restored.
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