Intel Extensible Firmware Interface - Manual (Page 373)

Protocols — Bootable Image Support
Version 1.10 12/01/02 11-13
DVD-ROM images formatted as required by the UDF 2.00 specification (OSTA Universal Disk
Format Specification, Revision 2.00) can be booted by EFI. EFI supports booting from an
ISO-9660 file system that conforms to the “El Torito” Bootable CD-ROM Format Specification on
a DVD-ROM. A DVD-ROM that contains an ISO-9660 file system is defined as a “UDF Bridge”
disk. Booting from CD-ROM and DVD-ROM is accomplished using the same methods.
Since the EFI file system definition does not use the same Initial/Default entry as a legacy
CD-ROM it is possible to boot Intel architecture personal computers using an EFI CD-ROM or
DVD-ROM. The inclusion of boot code for Intel architecture personal computers is optional and
not required by EFI.
11.2.2.3 Legacy Master Boot Record
The legacy master boot record is the first block (sector) on the disk media. The boot code on the
MBR is not executed by EFI firmware. The MBR may optionally contain a signature located as
defined in Table 11-5. The MBR signature must be maintained by operating systems, and is never
maintained by EFI firmware. The unique signature in the MBR is only 4 bytes in length, so it is
not a GUID. EFI does not specify the algorithm that is used to generate the unique signature.
The uniqueness of the signature is defined as all disks in a given system having a unique value
in this field.
Table 11-5. Legacy Master Boot Record
Mnemonic Byte
Offset Byte
Length
Description
BootCode 0 440 Code used on legacy Intel architecture system to select
a partition record and load the first block (sector) of the
partition pointed to by the partition record. This code is
not executed on EFI systems.
UniqueMBRSignature 440 4 Unique Disk Signature, this is an optional feature and
not on all hard drives. This value is always written by
the OS and is never written by EFI firmware.
Unknown 444 2 Unknown
PartitionRecord 446 16*4 Array of four MBR partition records.
Signature 510 2 Must be 0xaa55.
The MBR contains four partition records that define the beginning and ending LBA addresses that a
partition consumes on a hard disk. The partition record contains a legacy Cylinder Head Sector
(CHS) address that is not used in EFI. EFI utilizes the starting LBA entry to define the starting
LBA of the partition on the disk. The size of the partition is defined by the size in LBA field.
The boot indicator field is not used by EFI firmware. The operating system indicator value of 0xEF
defines a partition that contains an EFI file system. The other values of the system indicator are not
defined by this specification. If an MBR partition has an operating system indicator value of 0xEF,
then the firmware must add the EFI System Partition GUID to the handle for the MBR partition
using
InstallProtocolInterface() . This will allow drivers and applications, including
OS loaders, to easily search for handles that represent EFI System Partitions.