Extensible Firmware Interface Specification
314 12/12/00 Version 1.02
16.2.2 ISO-9660 and El Torito
IS0-9660 is the industry standard low level format used on CD-ROM and DVD-ROM. CD-ROM
format is completely described by the “El Torito” Bootable CD-ROM Format Specification
Version 1.0. To boot from a CD-ROM or DVD-ROM in the boot services environment, an EFI
System partition is stored in a “no emulation” mode as defined by the “El Torito” specification. A
Platform ID of 0xEF hex indicates an EFI System Partition. The Platform ID is in either the
Section Header Entry or the Validation Entry of the Booting Catalog as defined by the “El Torito”
specification. EFI differs from "El Torito" "no emulation" mode in that it does not load the "no
emulation" image into memory and jump to it. EFI interprets the "no emulation" image as an EFI
system partition. EFI interprets the Sector Count in the Initial/Default Entry or the Section Header
Entry to be the size of the EFI system partition. If the value of Sector Count is set to 0 or 1, EFI
will assume the system partition consumes the space from the beginning of the "no emulation"
image to the end of the CD-ROM.
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.
16.2.3 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 16-5. 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.