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June 23, 2010 T13/2132-D Revision 3
Working Draft Enhanced Disk Drive - 4 (EDD-4) 71
Annex A
(informative)
Hybrid MBR boot
A.1 Hybrid MBR boot overview
This annex describes how MBR boot code called hybrid MBR boot code may be constructed to support a GPT
disk layout (see UEFI-2.3) in a legacy BIOS system.
A.2 GPT support for hybrid MBR boot code
When using hybrid MBR boot code, the GPT disk layout includes one GPT partition with the Legacy BIOS
Bootable bit set to one in the Attributes field (see UEFI-2.3).
The Protective MBR defined in UEFI-2.3 is modified as shown in table A.1 to include hybrid MBR boot code.
Figure A.1 shows an example of a GPT disk layout with hybrid MBR boot code.
Figure A.1 — GPT disk layout with hybrid MBR boot code example
Table A.1 — Protective MBR with hybrid MBR boot code
Mnemonic Byte Offset Byte Length Contents
Boot Code 0 440 Hybrid MBR boot code (see A.3)
Unique MBR Disk Signature 440 4 See UEFI-2.3
Unknown 444 2 See UEFI-2.3
Partition Record 446 16 * 4 See UEFI-2.3
Signature 510 2 See UEFI-2.3
Reserved 512 Logical Block Size - 512 See UEFI-2.3
Protective
MBR with
hybrid
MBR boot
code
Primary
GPT
UEFI
system
partition
Partition Partition
Backup
GPT
Partition
GPT Protective partition
LBA 0 LBA 1 End of disk