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The content of doc/isolinux.txt (release 3.72):


A bootloader for Linux using ISO 9660/El Torito CD-ROMs
Copyright 1994-2008 H. Peter Anvin - All Rights Reserved

This program is provided under the terms of the GNU General Public License, version 2 or, at your option, any later version. There is no warranty, neither expressed nor implied, to the function of this program. Please see the included file COPYING for details.

ISOLINUX is a boot loader for Linux/i386 that operates off ISO 9660/El Torito CD-ROMs in "no emulation" mode. This avoids the need to create an "emulation disk image" with limited space (for "floppy emulation") or compatibility problems (for "hard disk emulation".)

This documentation isn't here yet, but here is enough that you should be able to test it out:

Make sure you have a recent enough version of mkisofs. I recommend mkisofs 1.13 (distributed with cdrecord 1.9), but 1.12 might work as well (not tested.)

To create an image, create a directory called "isolinux" (or, if you prefer, "boot/isolinux") underneath the root directory of your ISO image master file tree. Copy isolinux.bin, a config file called "isolinux.cfg" (see syslinux.txt for details on the configuration file), and all necessary files (kernels, initrd, display files, etc.) into this directory, then use the following command to create your ISO image (add additional options as appropriate, such as -J or -R):

        mkisofs -o <isoimage> \
                -b isolinux/isolinux.bin -c isolinux/ \
                -no-emul-boot -boot-load-size 4 -boot-info-table \

(If you named the directory boot/isolinux that should of course be

                -b boot/isolinux/isolinux.bin -c boot/isolinux/

ISOLINUX resolves pathnames the following way:

- A pathname consists of names separated by slashes, Unix-style. - A leading / means it searches from the root directory; otherwise the search is from the isolinux directory (think of this as the "current directory".) - . and .. in pathname searches are not supported. - The maximum length of any pathname is 255 characters.

Note that ISOLINUX only uses the "plain" ISO 9660 filenames, i.e. it does not support Rock Ridge or Joliet filenames. It can still be used on a disk which uses Rock Ridge and/or Joliet extensions, of course. Under Linux, you can verify the plain filenames by mounting with the "-o norock,nojoliet" option to the mount command. Note, however, that ISOLINUX does support "long" (level 2) ISO 9660 plain filenames, so if compatibility with short-names-only operating systems like MS-DOS is not an issue, you can use the "-l" or "-iso-level 2" option to mkisofs to generate long (up to 31 characters) plain filenames.

ISOLINUX does not support discontiguous files, interleaved mode, or logical block and sector sizes other than 2048. This should normally not be a problem.

ISOLINUX is by default built in two versions, one version with extra debugging messages enabled. If you are having problems with ISOLINUX, I would greatly appreciate if you could try out the debugging version (isolinux-debug.bin) and let me know what it reports. The debugging version does not include hybrid mode support (see below.)


ISOLINUX will search for the config file directory in the order /boot/isolinux, /isolinux, /. The first directory that exists is used, even if it contains no files. Therefore, please make sure that these directories don't exist if you don't want ISOLINUX to use them.


Starting in version 3.72, ISOLINUX supports a "hybrid mode" which can be booted from either CD-ROM or from a device which BIOS considers a hard disk or ZIP disk, e.g. a USB key or similar.

To enable this mode, the .iso image should be postprocessed with the "isohybrid" script from the utils directory:

        isohybrid filename.iso

This script creates the necessary additional information to be able to boot in hybrid mode. It also pads out the image to an even multiple of 1 MB.

This image can then be copied using any raw disk writing tool (on Unix systems, typically "dd" or "cat") to a USB disk, or written to a CD-ROM using standard CD burning tools.

The ISO 9660 filesystem is encapsulated in a partition (which starts at offset zero, which may confuse some systems.) This makes it possible for the operating system, once booted, to use the remainder of the device for persistent storage by creating a second partition.


WARNING: This feature depends on BIOS functionality which is apparently broken in a very large number of BIOSes. Therefore, this may not work on any particular system. No workaround is possible; if you find that it doesn't work please complain to your vendor and indicate that "BIOS INT 13h AX=4C00h fails."

To boot DOS, or other real-mode operating systems (protected-mode operating systems may or may not work correctly), using ISOLINUX, you need to prepare a disk image (usually a floppy image, but a hard disk image can be used on *most* systems) with the relevant operating system. This file should be included on the CD-ROM in the /isolinux directory, and have a .img extension. The ".img" extension does not have to be specified on the command line, but has to be explicitly specified if used in a "kernel" statement in isolinux.cfg.

For a floppy image, the size of the image should be exactly one of the following:

1,228,800 bytes - For a 1200K floppy image
1,474,560 bytes - For a 1440K floppy image
2,949,120 bytes - For a 2880K floppy image

Any other size is assumed to be a hard disk image. In order to work on as many systems as possible, a hard disk image should have exactly one partition, marked active, that covers the entire size of the disk image file. Even so, hard disk images are not supported on all BIOSes.