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Extensible Firmware Interface Specification
11-14 12/01/02 Version 1.10
Table 11-6. Legacy Master Boot Record Partition Record
Mnemonic
Byte
Offset
Byte
Length
Description
Boot Indicator 0 1 Not used by EFI firmware. Set to 0x80 to indicate that this is
the bootable legacy partition.
Start Head 1 1 Start of partition in CHS address, not used by EFI firmware.
Start Sector 2 1 Start of partition in CHS address, not used by EFI firmware.
Start Track 3 1 Start of partition in CHS address, not used by EFI firmware.
OS Type 4 1 OS type. A value of 0xEF defines an EFI system partition.
Other values are reserved for legacy operating systems, and
allocated independently of the EFI specification.
End head 5 1 End of partition in CHS address, not used by EFI firmware.
End Sector 6 1 End of partition in CHS address, not used by EFI firmware.
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.
11.2.2.4 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 11-7. If the GPT partition is larger than a partition that can be
represented by a legacy MBR, values of all Fs must be used to signify that all space that can be
possibly reserved by the MBR is being reserved.