White Paper
4
Figure 2. Comparison of MBR and GPT disk layouts
Bootable vs Data Drives and UEFI BIOS
Since the partitions on a data-storage drive are accessed by the OS and not the
BIOS, GPT drives can be used on non-UEFI systems as data-storage drives. These
drives cannot, however, be used as boot drives in such systems. The legacy BIOS in
a non-UEFI system cannot find the boot information on a GPT-formatted drive and
therefore cannot complete the boot process into the OS. In using a GPT drive as a
data-storage drive, the drive must be formatted using tools that can create GPT
partitions.
In order to boot from a GPT-formatted drive, the system must have a UEFI BIOS.
UEFI understands how to find the boot information on a GPT drive. In addition, the
system must also have a “UEFI aware” OS such as Microsoft Windows 7 ™64-bit.
For selected desktops, HP implements the Type 0 BIOS Information data structure
(as described in version 2.7 of the SMBIOS specification) to indicate UEFI BIOS
support. The BIOS Characteristics Extension Bytes field within this structure has
defined a bit that, if set, identifies the system as UEFI compliant. Table 1 describes
BIOS Characteristic Extension Byte 2, with Bit 3 identified as the bit for UEFI
specification support.
Master Boot Code
1
st
Partition Table Entry
2
nd
Partition Table Entry
3
rd
Partition Table Entry
4
th
Partition Table Entry
0x55 AA
Primary GUID Partition
Table Header
GUID Partition Entry 1
GUID Partition Entry 2
GUID Partition Entry n
GUID Partition Entry 128
Primary Partition (C:)
Primary Partition (E:)
Primary Partition n
GUID Partition Entry 1
GUID Partition Entry 2
GUID Partition Entry n
GUID Partition Entry 128
Backup GUID
Partition Table Header
Master Boot Record
Master Boot Code
1
st
Partition Table Entry
2
nd
Partition Table Entry
3
rd
Partition Table Entry
4
th
Partition Table Entry
0x55 AA
Primary Partition (C:)
Primary Partition (E:)
Primary Partition (F:)
Logical Drive (G:)
Logical Drive (H:)
Logical Drive n
Protective MBR
Partition Table
Extended
Partition
Primary GUID
Partition Entry
Array
Backup GUID
Partition Entry
Array
Basic GPT Disk Layout Basic MBR Disk Layout