[syslinux] syslinux mkisofs hard-disk-boot isohybrid

Ady Ady ady-sf at hotmail.com
Thu Dec 22 18:49:24 PST 2016

> Hi,
> Ady wrote:
> > [hard disk emulation boot image works with VirtualBox]
I would say "In VirtualBox, syslinux.efi can boot optical media when 
using the ESP as El Torito hard disk emulation boot image". I have not 
tested (at least, not recently) the floppy-emulation modes.
> For a reader of the UEFI specs this comes very unexpected.

Yes. In spite of my own comments in this Syslinux Mailing List a few 
years ago about the ESP in optical media being some kind of (or having 
more similarity to) El Torito *emulation* mode (as opposed to being set 
as no-emulation mode), the UEFI specs do state that the ESP is supposed 
to be set/used as no-emulation mode. In other words, despite of having 
some logical sense (IMHO), using the emulation modes for UEFI boot is 
still not compliant to UEFI specs. To be clear, in my ignorant opinion, 
the UEFI specs should tolerate / accept this situation, rather than 
restricting the ESP to no-emulation mode.
> Together with the report of Pascal that a hard-disk-emulation works
> even with mkisofs option -b (i.e. Platform Id x86 BIOS) it appears that
> the UEFI implementation treats the El Torito boot image like the whole
> device, regardless of Platform Id.
> On (virtual) CD-ROM this makes some sense. But on virtual hard disk
> (as reported by Pascal) it deviates from all tradition:
> El Torito is not to be interpreted on hard disk. Not by BIOS, not by UEFI.
> I am in doubt whether other implementations of UEFI do this too.
Please also read the side note at the end of this email.

> Have a nice day :)
> Thomas
Considering that this is the Syslinux mailing list, I'd rather focus 
this conversation not that much on what firmware implementations are 
doing, but on why we cannot successfully use syslinux.efi in 
no-emulation mode while other UEFI bootloaders can. What exactly is 
syslinux.efi doing that fails to accept the no-emulation mode? Should 
syslinux.efi care about which exact El Torito mode is being used?

My guess is that unless Peter actually tries to find out (and to solve 
it), the situation for syslinux.efi on optical media will most probably 
remain as it has been.


PS: As a side note, ISO-building tools should also accept building ISO 
images with floppy-emulation modes when the floppy image is not exactly 
of the 1.2/1.44/2.88 format. Some firmware implementations would accept 
booting in floppy emulation mode when the floppy is, for example, 
82x2x36 (or 'n'x2x36) sectors, instead of only accepting 80x2x36 for 
the "2.88" mode. Unfortunately, most ISO-building tools would error out 
(or reject to operate) when the floppy image is not _exactly_ the type 
of El Torito emulation mode that has been set in the command.

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