El Torito (CD-ROM standard)
|This article relies too much on references to primary sources. (September 2013)|
The El Torito Bootable CD Specification is an extension to the ISO 9660 CD-ROM specification. It is designed to allow a computer to boot from a CD-ROM. It was announced in November 1994 and first issued in January 1995 as a joint proposal by IBM and BIOS manufacturer Phoenix Technologies.
According to the El Torito specification, a 32-bit CPU PC BIOS will search for boot code on an ISO 9660 CD-ROM. The standard allows for booting in two different modes. Either in hard disk emulation when the boot information can be accessed directly from the CD media, or in floppy emulation mode where the boot information is stored in an image file of a floppy disk, which is loaded from the CD and then behaves as a virtual floppy disk. This is useful for computers built before about 1999, which were designed to boot only from floppy drive. For modern computers the "no emulation" mode is generally the more reliable method.
The BIOS will assign a BIOS drive number to the CD drive. The drive number (for INT 13h) assigned is any of 80 (hard disk emulation), 00 (floppy disk emulation) or an arbitrary number if the BIOS should not provide emulation. Emulation allows older operating systems to be booted from a CD, by making it appear to them as if they were booted from a hard or floppy disk.
While the El Torito spec alludes to a "Mac" platform ID, Apple computers don't use it.
According to legend, the El Torito CD/DVD extension to ISO 9660 gained its name, because its design originated in an El Torito restaurant in Irvine, California (33.684722,-117.852547). The initial two authors were Curtis Stevens, of Phoenix Technologies, and Stan Merkin, of IBM.
- Boot disk
- Live CD
- Preboot Execution Environment (PXE)
- SYSLINUX ISOLINUX can be booted via El Torito from CD images
- GRUB2 can be booted via El Torito from CD images
- "Phoenix announces bootable CD-ROM specification; Specification developed jointly by Phoenix and IBM" (Press release). Phoenix Technologies Ltd. 1994-11-11. Retrieved 2008-01-31.
- "El-Torito". OSDev. Retrieved 2015-01-03.
- "Bootable hybrid (ISO/HFS) CD-ROMs". Retrieved 2014-01-03.
- Parker, Dana J. "Fresh Tortillas and CD-ROM Standards: The El Torito Bootable CD-ROM Specification". CD-ROM Professional 8 (7). Archived from the original on 1999-10-08. Retrieved 2008-01-31.
- The "El Torito" Bootable CD-ROM Format Specification, Version 1.0 (PDF)
- ISOLINUX source code (see isolinux.asm line 294 onward)
- Ralf Brown's interrupt list (see int 13h in interrupt.b, esp. functions 4a to 4d)
- EL Torito Specification Supplement, discusses shortcomings of the standard