El Torito (CD-ROM standard)
Encyclopedia
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
.
CPU PC
BIOS
will search for boot code on a ISO 9660 CD-ROM. The standard allows for booting in two different modes. Either 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. It is sometimes useful to have this choice because booting directly from CD-ROM is vulnerable to BIOS bugs, in which case it is better to boot from the floppy image. This mostly affects computers built before about 1999, which were designed to boot only from floppy drive. In fact, for modern computers "no emulation" mode is generally the more reliable method.
The BIOS will assign a BIOS drive number to the CD drive. The drive number assigned is either 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. Newer operating systems do not require emulation to boot; all that is needed is an appropriate boot loader such as ISOLINUX from the SYSLINUX
project.
restaurant in Irvine, California
. The initial two authors were Curtis Stevens, of Phoenix Technologies, and Stan Merkin, of IBM.
ISO 9660
ISO 9660, also referred to as CDFS by some hardware and software providers, is a file system standard published by the International Organization for Standardization for optical disc media....
CD-ROM
CD-ROM
A CD-ROM is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data....
specification. It is designed to allow a computer to boot
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
from a CD-ROM. It was announced in November 1994 and first issued in January 1995 as a joint proposal by IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
and BIOS manufacturer Phoenix Technologies
Phoenix Technologies
Phoenix Technologies Ltd designs, develops and supports core system software for personal computers and other computing devices. Phoenix's products — commonly referred to as BIOS or firmware — support and enable the compatibility, connectivity, security and management of the various components and...
.
Boot modes
According to the El Torito specification, a 32-bit32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
CPU PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
will search for boot code on a ISO 9660 CD-ROM. The standard allows for booting in two different modes. Either 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
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
, which is loaded from the CD and then behaves as a virtual floppy disk. It is sometimes useful to have this choice because booting directly from CD-ROM is vulnerable to BIOS bugs, in which case it is better to boot from the floppy image. This mostly affects computers built before about 1999, which were designed to boot only from floppy drive. In fact, for modern computers "no emulation" mode is generally the more reliable method.
The BIOS will assign a BIOS drive number to the CD drive. The drive number assigned is either 80 (hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
emulation), 00 (floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
emulation) or an arbitrary number if the BIOS should not provide emulation.
Emulation allows older operating systems
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
to be booted from a CD, by making it appear to them as if they were booted from a hard or floppy disk. Newer operating systems do not require emulation to boot; all that is needed is an appropriate boot loader such as ISOLINUX from the SYSLINUX
SYSLINUX
The SYSLINUX Project is a suite of lightweight IBM PC MBR bootloaders for starting up computers with the Linux kernel. It is the work of H. Peter Anvin, and consists of several separate systems, the best-known of which is ISOLINUX.-List:...
project.
Etymology
According to legend, the El Torito CD/DVD extension to ISO 9660 gained its name because its design originated in an El ToritoEl Torito
El Torito is a Mexican restaurant chain, consisting of 69 restaurants which are located primarily in California. Some of the restaurants are located in Oregon, Arizona, and overseas. El Torito is one of several Mexican cuisine restaurant chains operated by Real Mex Restaurants...
restaurant in Irvine, California
Irvine, California
Irvine is a suburban incorporated city in Orange County, California, United States. It is a planned city, mainly developed by the Irvine Company since the 1960s. Formally incorporated on December 28, 1971, the city has a population of 212,375 as of the 2010 census. However, the California...
. The initial two authors were Curtis Stevens, of Phoenix Technologies, and Stan Merkin, of IBM.
External links
- The "El Torito" Bootable CD-ROM Format Specification, Version 1.0 (PDFPortable Document FormatPortable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....
) - ISOLINUX source code (see isolinux.asm line 294 onward)
- Ralf Brown's interrupt list (see int 13h in interrup.b, esp. functions 4a to 4d)
- EL Torito Specification Supplement, discusses shortcomings of the standard