On Fri, Dec 27, 2024 at 06:39:03PM +0000, g4sra wrote:
>
>
> Ralph
>
> The image devuan_daedalus_5.0.1_amd64_netinstall.iso has two partitions, both with 'boot' and 'EFI' directories in each, but only one of the partitions appears to have correct config files else I am missing something.
> Handing over to someone who knows what they are doing (you :).
>

Well, the ISO is a "hybrid ISO" which means it as a whole is an
iso9660 filesystem and thus the first sector is "unused"; it begins
with some zero's of significance, and then the MBR part (which is not
significant for an iso9660) is prepared to show 2 partitions.

In brief terms: the first parition is the whole data area (from 0 to
end) and the second partition is actually one of the files within the
iso9660 filesystem. It's a raw image file - a contiguous data block
that is formated as a EFI/FAT partition with due content. You may
review the El Torrito specification about this, and digest the
extensive "xorriso" man page for particulars on the preparation of it.

So, when this ISO is presented as a disk image, e.g. via a USB drive,
then that system will see 2 partitions (and ignore the fact that the
second is actually a portion of the first).

when this ISO is presented as a CDROM only the iso9660 will be seen,
and that EFI partition turns up a one of the files.

The iso9660, when mounted, will show the /boot/isolinux directory with
the setup for isolinux.

The FAT, when mounted, will show both the syslinux setup in / and the
UEFI boot setup in /EFI/boot.

The preparation of the EFI boot comes with a "syslinux constraint"
that the kernel and initrd must reside within the same partition,
together with the "hybrid ISO building constraint" that the partition
is less than 32M. Those are the drivers for having the boot-up split
into a preamble initrd that gets loaded and started before the
installer initrd proper, since the latter is much larger than the ~20M
available.

Ralph.

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