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Comment by zinekeller | Hacker News Reader

I'm pretty sure that even x86 UEFI cannot boot from CDs/DVDs (unless they are in BIOS emulation mode). This is the reason why physical retail Windows 10 images are in a read-only USB.

Update: some enterprise UEFI setups do boot from DVDs but the ISO should be a hybrid type (aka must have EFI-style partitions).

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UEFI supports bootable DVDs, although they use an extension of El Torrito. See section 13.3.2.1 of the UEFI Specification, Version 2.8.

> To boot from a CD-ROM or DVD-ROM in the boot services environment, an EFI System partition is stored in a “no emulation” mode as defined by the “El Torito” specification.

This was chosen specifically to allow polyglot ISO files that can both be booted bh UEFI and legacy BIOS. The implementation of tools to create such polyglot files are, to say the least, interesting. The tools effectively generate both a FAT and ISO partition, each having its own file information table, but sharing the actual file contents.

This sounds fascinating and horrific. Where can I learn more?

https://wiki.syslinux.org/wiki/index.php?title=Isohybrid https://repo.or.cz/syslinux.git/blob/HEAD:/utils/isohybrid.c

Matthew Garrett wrote about this, about a decade ago. It turns out you can create an iso/usb hybrid image that boots correctly on all combinations of BIOS/UEFI, optical disc/usb drive, and PC/Mac.

https://mjg59.dreamwidth.org/4957.html

Dreamcast uses El Torito for booting as well, I bet you could make that work too. Possible issues are that I don't know if the Dreamcast will inspect multiple boot images to find an appropriately labeled one, and I don't know if the other platforms will accept a bootable disc that's prepared for Dreamcast (must have at least two sessions, first session audio, second session data with el torito for booting)

https://pdos.csail.mit.edu/6.828/2014/readings/boot-cdrom.pd...

Phoenix and IBM coming up with a standard. Wonder if that was still a hard pill the swallow in 1995.

Ultimately they had to if they wanted BIOS to be able to boot CD-ROMs. The BIOS boot sequence sticking around for so long is an unfortunate consequence of it becoming a decentralized standard outside of any one vendor's control, and so the only way to make it all work was for vendors to agree, and the solutions wouldn't be obvious or trivial in most cases because that boot sequence is incredibly primitive and full of assumptions coherent in a system with two floppy drives and maybe a disk if you were lucky.

But I'm sure the bad blood was measurable, especially for the higher-ups.

UEFI doesn't much care what sort of storage medium something is on, so long as it is capable of reading it and it has a valid EFI partition. It is much less picky than BIOS, for which the El Torito standard had to be invented, and can be implemented easily on disks that use that format for dual-use. I haven't encountered UEFI implementations that don't support optical disks but I imagine it is possible there are some laptop vendors that might not have implemented optical drive support.

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