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El Torito (CD-ROM standard)
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Louis Ohland
May 26, 2020, 11:03:33 AM5/26/20
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https://en.wikipedia.org/wiki/El_Torito_(CD-ROM_standard)
Boot modes
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 80hex (hard disk emulation),
00hex (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.
Boot modes
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 80hex (hard disk emulation),
00hex (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.
Louis Ohland
May 26, 2020, 11:08:00 AM5/26/20
to
http://www.tldp.org/HOWTO/Bootdisk-HOWTO/cd-roms.html
11.2. How it Works
The El Torito standard works by making the CD drive appear, through BIOS
calls, to be a normal floppy drive. This way you simply put any floppy
size image (exactly 1440k for a 1.44 meg floppy) somewhere in the ISO
filesystem. In the headers of the ISO fs you place a pointer to this
image. The BIOS will then grab this image from the CD and for all
purposes it acts as if it were booting from the floppy drive. This
allows a working LILO boot disk, for example, to simply be used as is.
Roughly speaking, the first 1.44 (or 2.88 if supported) Mbytes of the
CD-ROM contains a floppy-disk image supplied by you. This image is
treated like a floppy by the BIOS and booted from. (As a consequence,
while booting from this virtual floppy, your original drive A:
(/dev/fd0) may not be accessible, but you can try with /dev/fd1).
11.2. How it Works
The El Torito standard works by making the CD drive appear, through BIOS
calls, to be a normal floppy drive. This way you simply put any floppy
size image (exactly 1440k for a 1.44 meg floppy) somewhere in the ISO
filesystem. In the headers of the ISO fs you place a pointer to this
image. The BIOS will then grab this image from the CD and for all
purposes it acts as if it were booting from the floppy drive. This
allows a working LILO boot disk, for example, to simply be used as is.
Roughly speaking, the first 1.44 (or 2.88 if supported) Mbytes of the
CD-ROM contains a floppy-disk image supplied by you. This image is
treated like a floppy by the BIOS and booted from. (As a consequence,
while booting from this virtual floppy, your original drive A:
(/dev/fd0) may not be accessible, but you can try with /dev/fd1).
Louis Ohland
May 26, 2020, 11:13:15 AM5/26/20
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Louis Ohland
May 26, 2020, 11:14:23 AM5/26/20
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