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Extensible Firmware Interface Specification
11-10 12/01/02 Version 1.10
Table 11-1. GUID Partition Table Header (continued)
Mnemonic
Byte
Offset
Byte
Length
Description
MyLBA 24 8 The LBA that contains this data structure.
AlternateLBA 32 8 LBA address of the alternate GUID Partition Table
Header.
FirstUsableLBA 40 8 The first usable logical block that may be
contained in a GUID Partition Entry.
LastUsableLBA 48 8 The last usable logical block that may be
contained in a GUID Partition Entry.
DiskGUID 56 16 GUID that can be used to uniquely identify the
disk.
PartitionEntryLBA 72 8 The starting LBA of the GUID Partition Entry array.
NumberOfPartitionEntries 80 4 The number of Partition Entries in the GUID
Partition Entry array.
SizeOfPartitionEntry 84 4 The size, in bytes, of each the GUID Partition
Entry structures in the GUID Partition Entry array.
Must be a multiple of 8.
PartitionEntryArrayCRC32 88 4 The CRC32 of the GUID Partition Entry array.
Starts at Partition Entry LBA and is
NumberOfPartitionEntries *
SizeOfPartitionEntry
in byte length.
Reserved 92 BlockSize
– 92
The rest of the block is reserved by EFI and must
be zero.
The following test must be performed to determine if a GUID Partition Table is valid:
• Check the GUID Partition Table Signature
• Check the GUID Partition Table CRC
• Check that the MyLBA entry points to the LBA that contains the GUID Partition Table
• Check the CRC of the GUID Partition Entry Array
If the GUID Partition Table is the primary table, stored at LBA 1:
• Check the AlternateLBA to see if it is a valid GUID Partition Table
If the primary GUID Partition Table is corrupt:
• Check the last LBA of the device to see if it has a valid GUID Partition Table.
• If valid backup GUID Partition Table found, restore primary GUID Partition Table.
Any software that updates the primary GUID Partition Table Header must also update the backup
GUID Partition Table Header. The order of the update of the GUID Partition Table Header and its
associated GUID Partition Entry array is not important, since all the CRCs are stored in the GUID
Partition Table Header. However, the primary GUID Partition Table Header and GUID Partition
Entry array must always be updated before the backup.