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17th sector of ISO 9660 file system may contain so called Boot Record. It is a special Volume Descriptor with type = 0x00. The only Boot Record I know is Boot Record for El Torito and it contains LBA of Booting catalog. The questions is

Does any other booting system exist, for ISO 9660 except El Torito? As I understood specification of ISO 9660 allows that, but I could not find any other except Eltorito.

Art Spasky
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  • UEFI also has something for optical drives I think. – Daniel B Dec 17 '22 at 09:31
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    @DanielB It's not specified by ISO 9669 though. UEFI will consider anything bootable if it contains a supported FS (ISO 9660 included) and a file `efi/boot/bootx64.efi` (or variations for different architectures). So it's not based on the built-in booting provisions in ISO 9660 – gronostaj Dec 17 '22 at 09:57
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    @gronostaj: UEFI does specifically [reference El Torito](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_10_Aug29.pdf#page=530), however: "[...] an EFI System partition is stored in a “no emulation” mode as defined by the “El Torito” specification. A Platform ID of 0xEF indicates an EFI System Partition." – u1686_grawity Dec 17 '22 at 10:08

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The Wikipedia article ISO 9660 says this:

There are several extensions to ISO 9660 that relax some of its limitations. Notable examples include Rock Ridge (Unix-style permissions and longer names), Joliet (Unicode, allowing non-Latin scripts to be used), El Torito (enables CDs to be bootable) and the Apple ISO 9660 Extensions (file characteristics specific to the classic Mac OS and macOS, such as resource forks, file backup date and more).

As El Torito is the only listed extension to the standard for creating bootable CDs, it is probably the only one. I don't think that any reason ever existed for creating a competing standard, especially since the CD was pretty much deprecated after a few years.

harrymc
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    I created a question bacause https://wiki.osdev.org/ISO_9660 article states the follownig: The most common Boot System Use specification is El Torito. It records at bytes 71 to 74 as little-endian 32-bit number the block address of the El Torito Boot Catalog. This catalog lists the available boot images, which serve as starting points of booting systems. – Art Spasky Dec 18 '22 at 02:21