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Protocols — Bootable Image Support
Version 1.10 12/01/02 11-11
If the primary GUID Partition Table is invalid the backup GUID Partition Table is located on the
last logical block on the disk. If the backup GUID Partition Table is valid it must be used to restore
the primary GUID Partition Table. If the primary GUID Partition Table is valid and the backup
GUID Partition Table is invalid software must restore the backup GUID Partition Table. If both the
primary and backup GUID Partition Table is corrupted this block device is defined as not having a
valid GUID Partition Header.
The primary and backup GUID Partition Tables must be valid before an attempt is made to grow
the size of a physical volume. This is due to the GUID Partition Table recovery scheme depending
on locating the backup GUID Partition Table at the end of the physical device. A volume may
grow in size when disks are added to a RAID device. As soon as the volume size is increased the
backup GUID Partition Table must be moved to the end of the volume and the primary and backup
GUID Partition Table Headers must be updated to reflect the new volume size.
Table 11-2. GUID Partition Entry
Mnemonic
Byte
Offset
Byte
Length
Description
Partition Type Guid 0 16 Unique ID that defines the purpose and type of this
Partition. A value of zero defines that this partition
record is not being used.
Unique Partition Guid 16 16 GUID that is unique for every partition record. Every
partition ever created will have a unique GUID. This
GUID must be assigned when the GUID Partition Entry
is created. The GUID Partition Entry is created when
ever the
NumberOfPartitionEntries in the
GUID Partition Table Header is increased to include a
larger range of addresses.
StartingLBA 32 8 Starting LBA of the partition defined by this record.
EndingLBA 40 8 Ending LBA of the partition defined by this record.
Attributes 48 8 Attribute bits, all bits reserved by EFI.
Partition Name 56 72 Unicode string.
The SizeOfPartitionEntry variable in the GUID Partition Table Header defines the size of
a GUID Partition Entry. The GUID Partition Entry starts in the first byte of the GUID Partition
Entry and any unused space at the end of the defined partition entry is reserved space and must be
set to zero.
Each partition record contains a Unique Partition GUID variable that uniquely identifies every
partition that will ever be created. Any time a new partition record is created a new GUID must be
generated for that partition, and every partition is guaranteed to have a unique GUID. The partition
record also contains 64-bit logical block addresses for the starting and ending block of the partition.
The partition is defined as all the logical blocks inclusive of the starting and ending usable LBA
defined in the GUID Partition Table Header. The partition record contains a partition type GUID
that identifies the contents of the partition. This GUID is similar to the OS type field in the legacy
MBR. Each file system must publish its unique GUID. The partition record also contains
Attributes that can be used by utilities to make broad inferences about the usage of a partition. A