RSDP
| ACPI |
|---|
| Fixed Tables |
| Differentiated Tables |
| Tools/Libs |
RSDP (Root System Description Pointer) is a data structure used in the ACPI programming interface.
In ACPI Version 1.0 it has this structure:
struct RSDP_t {
char Signature[8];
uint8_t Checksum;
char OEMID[6];
uint8_t Revision;
uint32_t RsdtAddress;
} __attribute__ ((packed));
since Version 2.0 it has been extended:
struct XSDP_t {
char Signature[8];
uint8_t Checksum;
char OEMID[6];
uint8_t Revision;
uint32_t RsdtAddress; // deprecated since version 2.0
uint32_t Length;
uint64_t XsdtAddress;
uint8_t ExtendedChecksum;
uint8_t reserved[3];
} __attribute__ ((packed));
Detecting the RSDP
The RSDP is either located within the first 1 KB of the EBDA (Extended BIOS Data Area) (a 2 byte real mode segment pointer to it is located at 0x40E), or in the memory region from 0x000E0000 to 0x000FFFFF (the main BIOS area below 1 MB). To find the table, the Operating System has to find the "RSD PTR " string (notice the last space character) in one of the two areas. This signature is always on a 16 byte boundary.
If you're using UEFI, you can find it somewhere in EFI_SYSTEM_TABLE. Thus, there's no need for searching the RAM.
Note: The standard methods to find the RSDP may not work on UEFI systems. Because of that, finding it inside EFI_SYSTEM_TABLE is the correct and reliable method (see ACPI 6.2 section 5.2.5.2 'Finding the RSDP on UEFI Enabled Systems').
When booting with a multiboot2 compliant loader, a copy of the RSDP is contained in the ACPI new RSDP or ACPI old RSDP tag respectively.
Validating the RSDP
Once you have found the RSDP Table you have to see what version of ACPI the BIOS is using, then you must check that the checksum is valid.
Detecting ACPI Version
The ACPI Version can be detected using the Revision field in the RSDP. If this field contains 0, then ACPI Version 1.0 is used. For subsequent versions (ACPI version 2.0 to 6.1), the value 2 is used [1]. The exact version of ACPI can be deduced via the FADT table.
Checksum validation
Before the RSDP is relied upon you should check that the checksum is valid. For ACPI 1.0 (the first structure) you add up every byte in the structure and make sure the lowest byte of the result is equal to zero. For ACPI 2.0 and later you'd do exactly the same thing for the original (ACPI 1.0) part of the second structure, and then do it again for the fields that are part of the ACPI 2.0 extension.
Explaining the fields
Always present
Signature
This 8-byte string (not null terminated!) must contain "RSD PTR ". It stands on a 16-byte boundary.
Checksum
The value to add to all the other bytes (of the Version 1.0 table) to calculate the Checksum of the table. If this value added to all the others and casted to byte isn't equal to 0, the table must be ignored.
OEMID
The specification says: "An OEM-supplied string that identifies the OEM".
Revision
The revision of the ACPI. Larger revision numbers are backward compatible to lower revision numbers. See Detecting ACPI Version for further information.
Rsdt Address
32-bit physical (I repeat: physical) address of the RSDT table.
Since Version 2.0
Length
The size of the entire table since offset 0 to the end.
Xsdt Address
64-bit physical address of the XSDT table. If you detect ACPI Version 2.0 you should use this table instead of RSDT even on IA-32, casting the address to uint32_t.
Extended Checksum
This field is used to calculate the checksum of the entire table, including both checksum fields.
Reserved
3 bytes to be ignored in reading and that must not be written.