FADT

FADT (Fixed ACPI Description Table) is a data structure used in the ACPI programming interface. This table contains information about fixed register blocks pertaining to power management.

Finding the FADT

The FADT is pointed to by an entry in the RSDT. The signature is 'FACP'. Even if the pointer was found in another ACPI valid structure, you should anyway do the checksum to check that the table is valid.

Structure

The FADT is a complex structure and contains a lot of data. Here it is:

struct FADT
{
    struct   ACPISDTHeader h;
    uint32_t FirmwareCtrl;
    uint32_t Dsdt;

    // field used in ACPI 1.0; no longer in use, for compatibility only
    uint8_t  Reserved;

    uint8_t  PreferredPowerManagementProfile;
    uint16_t SCI_Interrupt;
    uint32_t SMI_CommandPort;
    uint8_t  AcpiEnable;
    uint8_t  AcpiDisable;
    uint8_t  S4BIOS_REQ;
    uint8_t  PSTATE_Control;
    uint32_t PM1aEventBlock;
    uint32_t PM1bEventBlock;
    uint32_t PM1aControlBlock;
    uint32_t PM1bControlBlock;
    uint32_t PM2ControlBlock;
    uint32_t PMTimerBlock;
    uint32_t GPE0Block;
    uint32_t GPE1Block;
    uint8_t  PM1EventLength;
    uint8_t  PM1ControlLength;
    uint8_t  PM2ControlLength;
    uint8_t  PMTimerLength;
    uint8_t  GPE0Length;
    uint8_t  GPE1Length;
    uint8_t  GPE1Base;
    uint8_t  CStateControl;
    uint16_t WorstC2Latency;
    uint16_t WorstC3Latency;
    uint16_t FlushSize;
    uint16_t FlushStride;
    uint8_t  DutyOffset;
    uint8_t  DutyWidth;
    uint8_t  DayAlarm;
    uint8_t  MonthAlarm;
    uint8_t  Century;

    // reserved in ACPI 1.0; used since ACPI 2.0+
    uint16_t BootArchitectureFlags;

    uint8_t  Reserved2;
    uint32_t Flags;

    // 12 byte structure; see below for details
    GenericAddressStructure ResetReg;

    uint8_t  ResetValue;
    uint8_t  Reserved3[3];
  
    // 64bit pointers - Available on ACPI 2.0+
    uint64_t                X_FirmwareControl;
    uint64_t                X_Dsdt;

    GenericAddressStructure X_PM1aEventBlock;
    GenericAddressStructure X_PM1bEventBlock;
    GenericAddressStructure X_PM1aControlBlock;
    GenericAddressStructure X_PM1bControlBlock;
    GenericAddressStructure X_PM2ControlBlock;
    GenericAddressStructure X_PMTimerBlock;
    GenericAddressStructure X_GPE0Block;
    GenericAddressStructure X_GPE1Block;
};

GenericAddressStructure

Before going to far, keep in mind that the GAS is a structure used by ACPI to describe the position of registers. Its structure is:

struct GenericAddressStructure
{
  uint8_t AddressSpace;
  uint8_t BitWidth;
  uint8_t BitOffset;
  uint8_t AccessSize;
  uint64_t Address;
};

Possible values of AddressSpace

Value	Address Space
0	System Memory
1	System I/O
2	PCI Configuration Space
3	Embedded Controller
4	SMBus
5 to 0x7E	Reserved
0x7F	Functional Fixed Hardware
0x80 to 0xBF	Reserved
0xC0 to 0xFF	OEM Defined

BitWidth and BitOffset are required only when accessing a bit field, and I guess you know how to use them.

AccessSize defines how many bytes at once you can read/write.

Possible values of AccessSize

Value	Access size
0	Undefined (legacy reasons)
1	Byte access
2	16-bit access
3	32-bit access
4	64-bit access

Address is a 64-bit pointer in the defined address space to the data structure.

Fields

FirmwareCtrl

This is a 32-bit pointer to the FACS. Since ACPI 2.0 a new field has been added to the table, X_FirmwareControl of type GAS, which is 64-bits wide. Only one of the two fields is used, the other contains 0. According to the Specs, the X_ field is used only when the FACS is placed above the 4th Gb.

Dsdt

A 32-bit pointer to the DSDT. This has a X_Dsdt brother too.

PreferredPowerManagementProfile

This field contains a value which should address you to a power management profile. For example if it contains 2, the computer is a laptop and you should configure power management in power saving mode.

Values for PreferredPowerManagementProfile

Value	Meaning
0	Unspecified
1	Desktop
2	Mobile
3	Workstation
4	Enterprise Server
5	SOHO Server
6	Aplliance PC
7	Performance Server
>7	Reserved

SCI_Interrupt

The SCI IRQ number when working using the 8259 PIC. The System Control Interrupt is used from the ACPI hardware to notify the OS of particular events, such as the user pressing of the power button.

SMI_CommandPort

This is an I/O Port. This is where the OS writes AcpiEnable or AcpiDisable to get or release the ownership over the ACPI registers. This is 0 on systems where the System Management Mode is not supported.

What's next?

You should preserve the pointer to the FACS (in FirmwareControl (if < 4gb) or in X_FirmwareControl (if >= 4gb)). Then you should parse the DSDT.