[syslinux] Is efiboot.img required?

Thomas Schmitt
Sun, 05 Apr 2015 13:17:19 -0000

Hi,

i wrote:
> > Would it be desirable to explain how Fedora et.al.
> > created their FAT boot images by help of old or new
> > GRUB ?
> > (If developers of EFI bootable ISOs are reading this,
> >  please give a short sketch of the procedure.)

piranna at gmail.com wrote:
> I did it by including the Linux kernel and the initramfs

None of the ISOs i know has this stuff in the FAT image.
There must be some way for GRUB/GRUB2 to escape from FAT
into the ISO.
debian-7.7.0-amd64-netinst.iso does the job by a
bootx64.efi binary of 421888 bytes. It's the only file
in the FAT (of 458752 bytes).

It might be a workaround for ISOLINUX to bring ISOLINUX
configuration and a minimal Linux into the FAT which is
then able to start the desired operating system from
the ISO.


> but that's sub-optimal since I need to have them in two places.

Especially since the size of an El Torito boot image
is restricted to 32 MiB (65536 x 512 bytes).


> That's why I
> asked about re-using the same MBR entry so no need for an EFI image

EFI ignores the boot code in the MBR and only follows
the partition table entry of type 0xef, if there is any.

In any case there is no other boot path specified than
to look up /efi/boot/bootx64.efi in the FAT filesystem
and to execute it.
Only the methods of finding the FAT differ, depending
on the medium type (El Torito vs. partition table) and
on the choice of the partition table (MBR vs. GPT).