12.3.2.1 ISO-9660 and El Torito

IS0-9660 is the industry standard low level format used on CD-ROM and DVD-ROM. The 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 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 2.0 specification (OSTA Universal Disk 
Format Specification, Revision 2.0) 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 personal computers using an EFI CD-ROM or DVD-ROM. The inclusion of 
boot code for personal computers is optional and not required by EFI. 


12.3.3 Number and Location of System Partitions

UEFI does not impose a restriction on the number or location of System Partitions that can exist on a 
system.  System Partitions are discovered when required by UEFI firmware by examining the 
partition GUID and verifying that the contents of the partition conform to the FAT file system as 
defined in Section 12.3.1.1.  Further, UEFI implementations may allow the use of conforming FAT 
partitions which do not use the ESP GUID.  Partition creators may prevent UEFI firmware from 
examining and using a specific partition by setting bit 1 of the Partition Attributes (see 5.3.3) which 
will exclude the partition as a potential ESP.
Software installation may choose to create and locate an ESP on each target OS boot disk, or may 
choose to create a single ESP independent of the location of OS boot disks and OS partitions.  It is 
outside of the scope of this specification to attempt to coordinate the specification of size and 
location of an ESP that can be shared by multiple OS or Diagnostics installations, or to manage 
potential namespace collisions in directory naming in a single (central) ESP.


Table 9. UEFI Image Types

EFI ARCH                    Filename Convention      PE Executable Machine Type *
32-bit                      BOOTIA32.EFI             0x14c
x64                         BOOTx64.EFI              0x8664
Itanium architecture        BOOTIA64.EFI             0x200
ARM architechture           BOOTARM.EFI              0x01c2

Note:  * The PE Executable machine type is contained in the machine field of the COFF file header as 
defined in the Microsoft Portable Executable and Common Object File Format Specification, Revision 6.0