Talk:El-Torito

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2k limitation somewhere?

Could someone please explain where the "2k" comes from under "What's Next?" in the sentence: "you can start reading the file system, in the 2k that you have available in your boot sector."

Is the El-Torito standard limited to loading boot sectors no larger than 2KiB? --MessiahAndrw 10:48, 27 July 2011 (UTC)

If you follow the link to Bootable El-Torito CD with GRUB Legacy, you will find the `genisoimage` option `-boot-load-size 4` (also used in this article) explained as follows: "Specifies the number of 512-bytes sectors to load. Four 512-byte sectors (2048 bytes) is one CD sector and is the number supported by most BIOS." (Had to look it up myself, but there. ;-) ) -- Solar 12:13, 27 July 2011 (UTC)

I discovered this through experimentation (I'm trying to boot directly into my kernel using El-Torito rather than via a bootloader). I'm editing the article now to explain this. --MessiahAndrw 14:20, 27 July 2011 (UTC)

2K sectors come from the ISO9660 specification - see http://en.wikipedia.org/wiki/Iso9660#Specifications.

The basic of storage is a Frame of 24 bytes, and a Sector is defined as 98 Frames. This gives 2352 usable bytes per Sector. Because CDs store their data in 1 single spiral, it is impossible to calculate the exact start of every single sector given the technology at the time (It is better now). As a result, all Sectors must have alignment data so the CD reader can precicely locate the start of the sector given a rough approximation from a calculation. In addition, CDs damage very easily, so they have always had error correcting data written into the sector so the sector can still be recovered correctly if damaged a little. As a result, ISO9660 splits the 2352 usable bytes per sector into 12 bytes of alignment/synchronisation data, followed by 2048 bytes of regular data, followed by 288 bytes of error detecting and correcting codes. Andyhhp 00:20, 29 July 2011 (UTC)

Proposals for enhancements

I am the developer of xorriso, which produces bootable ISO 9660 images. Here are some proposals after reading this wiki page. I am ready to implement them here. But first i ask whether such changes would be appreciated.


The examples about floppy emulation and Legacy GRUB are quite antique, meanwhile.

GRUB2 script grub-mkrescue should be mentioned: http://www.gnu.org/software/grub/manual/html_node/Making-a-GRUB-bootable-CD_002dROM.html#Making-a-GRUB-bootable-CD_002dROM

Bootloader ISOLINUX should be mentioned: http://www.syslinux.org/wiki/index.php/ISOLINUX Probably http://wiki.osdev.org/SysLinux should get a sibling for the ISOLINUX aspects of SYSLINUX.


"Writing an El-Torito Boot Sector"

This headline does not match the object which is described. "El-Torito Boot Sector" would normally be the El-Torito Boot Record, which is a ECMA-119 Volume Descriptor. The description rather applies to the boot image, which is the blob pointed to by an entry of the El-Torito Boot Catalog. With BIOS, that blob is the first stage of a boot loader. With EFI, the blob is a FAT filesystem image (but not an emulated floppy).

The structure of El-Torito Volume Descriptor, Boot Catalog, and boot images should be described, in order to clarify the possibilities of multiple boot images for e.g. BIOS and EFI. I.e. Boot Record points to Boot Catalog which lists the available boot images.

Further one should mention that ISO images can carry MBR and GPT for booting the image from hard-disk-like devices (e.g. USB sticks).

Example of combined BIOS and EFI via El-Torito, MBR, and GPT as was done for debian-7.1.0-amd64-netinst.iso:

 xorriso -as mkisofs
 ...
 -c isolinux/boot.cat
 -b isolinux/isolinux.bin
 -no-emul-boot -boot-load-size 4 -boot-info-table
 -isohybrid-mbr syslinux/usr/lib/syslinux/isohdpfx.bin
 -eltorito-alt-boot
 -e boot/grub/efi.img
 -no-emul-boot -isohybrid-gpt-basdat
 ...
 boot1
 ...

Option -eltorito-alt-boot separates the settings for the BIOS boot image from those for the EFI boot image. The files isolinux/isolinux.bin and boot/grub/efi.img get into the ISO image from source directory ./boot1. File syslinux/usr/lib/syslinux/isohdpfx.bin is copied directly from the SYSLINUX installation into the ISO image. It contains the MBR template. Option -isohybrid-gpt-basdat announces the FAT image boot/grub/efi.img as GPT partition.

See http://www.gnu.org/software/xorriso/man_1_xorrisofs.html

Description on byte level: http://bazaar.launchpad.net/~libburnia-team/libisofs/scdbackup/view/head:/doc/boot_sectors.txt


The boot information table does not belong to El Torito. It is rather part of boot loader specifications, possibly invented for ISOLINUX.


 -boot-info-table-o [IsoFile.iso]

must rather be

 -boot-info-table [-o IsoFile.iso]

or more safely for the terminal

 -boot-info-table -o IsoFile.iso

Maybe one should explain the relations of mkisofs, genisoimage, other clones, and the mkisofs emulation of xorriso. I would volunteer to try this as unbiasedly as possible for one of the competitors.

Scdbackup 02:36, 15 October 2013 (CDT)

After trying to apply smaller changes i decided to propose a re-write of large
parts of the text. See my user page User:Scdbackup.
If no objections arise within a few days, i will put it on the article page.
Scdbackup 13:03, 18 October 2013 (CDT)
The re-written parts are now in the article. Review would be appreciated.
Scdbackup 05:51, 20 October 2013 (CDT)
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