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June 23, 2010
9) set the IA-32 registers according to the hybrid MBR boot code hand over procedure (see A.4) and jump
to 7C00h.
NOTE 4 — In contrast, legacy MBR boot code searches the MBR Partition Records for a partition
with the Boot Indicator field set to 80h, then loads the VBR based on the Starting LBA field and
passes the Size In LBA field to the VBR.
A.4 Hybrid MBR boot code endeavor procedure
Hybrid MBR boot code fills in the IA-32 registers according to table A.2 before jumping to 7C00h.
Register
DL Disk number
ES:DI Pointer to $PnP
EAX 54504721h (i.e., "!GPT"). Indicates that the hybrid
MBR hand over structure is being passed with
DS:SI rather than the legacy MBR Partition Record
DS:SI Pointer to the hybrid MBR hand over structure (see
table A.3)
Table A.3 defines the Hybrid MBR boot code hand over structure.
NOTE 5 — With legacy MBR boot code, this structure included fields from the MBR Partition Record
of the boot partition. Since the GPT partition may be located at an LBA beyond the 32-bit LBA
addressing boundary, additional fields are added to communicate the full information to the VBR.
Mnemonic
Boot Indicator
Starting CHS
OS Type
Ending CHS
Starting LBA
Size In LBA
Size Of Partition Entry
GPT Partition Entry
Working Draft Enhanced Disk Drive - 4 (EDD-4)
Table A.2 — Hybrid MBR endeavor IA-32 register values
Description
Table A.3 — Hybrid MBR boot code hand over structure
Byte
Byte
Offset
Length Description
0
1
Set to 80h (i.e., bootable).
1
3
Set to FF_FFFFh. The VBR boot code
should ignore this field.
4
1
Set to the OS Type of the bootable
partition (i.e., that would have been
assigned had the partition been
installed in an MBR disk layout).
5
3
Set to FF_FFFFh. The VBR boot code
should ignore this field.
8
4
Set to FFFF_FFFFh.
12
4
Set to FFFF_FFFFh.
16
4
Set to the Size Of Partition Entry field of
the GPT Partition Header.
20
Partition
Set to the GPT Partition Entry of the
Entry
boot partition
Size
Differences from legacy MBR hand over
No change
No change
New
New
Differences from legacy
MBR hand over structure
Fixed value
Fixed value
No change
Fixed value
Fixed value
Fixed value
New value
New value
T13/2132-D Revision 3
73
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