Extensible Firmware Interface Specification
316 12/12/00 Version 1.02
Table 16-6. Legacy Master Boot Record Partition Record (continued)
Mnemonic
Byte
Offset
Byte
Length Description
End Track 7 1 End of partition in CHS address, not used by EFI firmware.
Starting LBA 8 4 Starting LBA address of the partition on the disk. Used by
EFI firmware to define the start of the partition.
Size In LBA 12 4 Size of partition in LBA. Used by EFI firmware to determine
the size of the partition.
EFI defines a valid legacy MBR as follows. The signature at the end of the MBR must be
0xaa55. Each MBR partition record must be checked to make sure that the partition that it
defines physically resides on the disk. Each partition record must be checked to make sure it does
not overlap with other partition records. A partition record that contains an OSIndicator value of
zero, or a SizeInLBA value of zero may be ignored. If any of these checks fail the MBR is not
considered valid.
16.2.4 Legacy Master Boot Record and GPT Partitions
The GPT partition structure does not support nesting of partitions. However it is legal to have a
legacy Master Boot Record nested inside a GPT partition.
On all GUID Partition Table disks a Protective MBR (PMBR) in the first LBA of the disk precedes
the GUID Partition Table Header to maintain compatibility with existing tools that do not
understand GPT partition structures. The Protective MBR has the same format as a legacy MBR,
contains one partition entry of OS type 0xEE and reserves the entire space used on the disk by the
GPT partitions, including all headers. The Protective MBR that precedes a GUID Partition Table
Header is shown in Table 16-7. If the GPT partition is larger than a partition that can be
represented by a legacy MBR, values of all F’s must be used to signify that all space that can be
possibly reserved by the MBR is being reserved.
Table 16-7. PMBR Entry to Precede a GUID Partition Table Header
Mnemonic
Byte
Offset
Byte
Length Description
Boot Indicator 0 1 Must be set to zero to indicate non-bootable partition.
Start Head 1 1
Start Sector 2 1
Start Track 3 1
Set to match the Starting LBA of the EFI Partition
structure. Must be set to 0xFFFFFF if it is not possible
to represent the starting LBA.
OS Type 4 1 Must be 0xEE.
End head 5 1
End Sector 6 1
End Track 7 1
Set to match the Ending LBA of the EFI Partition
structure. Must be set to 0xFFFFFF if it is not possible
to represent the starting LBA.
Starting LBA 8 4 Must be 1 by definition.
Size In LBA 12 4 Length of EFI Partition Head, 0xFFFFFFFF if this value
overflows.