Re: UEFI/"BIOS" booting

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

i wrote:
> > And as said previously, BIOS does not expect any partitions.

Pascal Hambourg wrote:
> experience taught me that many BIOS implementations wrongly
> expect an MBR partition entry with the boot flag set in order to boot from
> the disk.

You are right (and those firmwares are wrong).

I had to introduce a special option in xorriso in order to cater for this:
  https://savannah.gnu.org/bugs/?46716
  http://lists.gnu.org/archive/html/bug-xorriso/2015-12/threads.html
with the final conclusion described in
  https://lists.gnu.org/archive/html/bug-grub/2016-03/msg00087.html

Summary:
- Boot flag on MBR partition of type 0xEE is bad on several EFI
  implementations.
- No MBR partition with boot flag is bad on some very few BIOS
  implementations.
- Compromise is to set the boot flag on a dummy partition of type 0x00.
  This is barely UEFI-compliant because the specs say that a partition of
  type 0x00 shall be regarded as non-existent.

Debian isohybrids for BIOS do not have this problem because MBR partition 1
has the boot flag set, anyway. But grub-mkrescue ISOs might be in need of
xorrisofs option
  --mbr-force-bootable
if the owner suffers from one of those rare BIOSes.
(It was decided that from the view of GRUB this is a workaround for a
 specs violation by the firmware. iSo --mbr-force-bootable is not applied
 by default.)


Have a nice day :)

Thomas