[syslinux] Isohybrid wiki page and UEFI

Thomas Schmitt scdbackup at gmx.net
Wed Oct 28 01:51:57 PDT 2015


obviously the SYSLINUX problem with EFI-in-ISO is not bound to
the size of the FAT filesystem or the way it is advertised in
El Torito.

More or less off topic for this thread:

> xorriso : FAILURE : -as mkisofs: Unrecognized option '-efi-boot'

Ah yes. A near miss.
xorrisofs has an option --efi-boot with two dashes. Introduced for
grub-mkrescue with automatic -eltorito-alt-boot -no-emul-boot .
And it has -e, requested by a Fedora user.

I see that Fedora's -e is an alternative for Fedora's -efi-boot,
so your changed script should work with Fedora's genisoimage, too:
I mention Fedora, because El Torito EFI images are not supported
by Debian's genisoimage.

> >  xorriso -indev ....iso  -report_el_torito as_mkisofs \

Saved you a lot of searching in the man page. :))

In the native command set of xorriso 1.4.1 it has a new companion

  -boot_image any replay

which after ISO image manipulations re-applies the boot options
as told by the native command reporter
  -report_el_torito cmd
with really ugly output, i confess.

The job of mkisofs emulation was not the original goal of xorriso.
It rather was founded as a backup tool for creating and updating
ISO filesystems on optical media.
Users forced me to learn the other stuff. The scattered semantics
of command -boot_image reflect this history.

> >           -report_el_torito plain \

> Drive current: -indev '/tmp/uefi.8274/testuefi.iso'
> ...
> El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
> El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4       15035
> El Torito boot img :   2  UEFI  y   none  0x0000  0x00  60000          35

This ISO (i assume from Fedora's genisoimage under the name
"mkisofs") has an EFI boot image smaller than 32 MB. All its
El Torito parameters are appropriate for EFI.

(Note that LBA (Logical Block Address) is told in 2048 byte blocks
 whereas Ldsiz (Load Size) is in blocks of 512 bytes. Not very
 consistent, but that's how El Torito expresses position and size.)

> >           -report_system_area plain \
> xorriso : NOTE : No System Area was loaded

No isohybrid, no MBR partitions, no GPT. I.e. no boot from USB stick.

Most bootable Linux ISOs have equipment which xorriso could reproduce.
In any position among the -as mkisofs arguments:

  -isohybrid-mbr $path_to_isohdpfx.bin_or_to_isohybrid_ISO_image \

and after option -e:

There is also a HFS+ image in Fedora Live which gets advertised
by GPT and Apple Partition Map. But your ISO has no such file.

Fedora Live CD is obviously the reason why SYSLINUX program
isohybrid learned EFI and APM. See
and older articles from that series by Matthew Garrett.
But it does not use SYSLINUX for EFI.

Have a nice day :)


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