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Re: simple message under BIOS/CSM/LEGACY

From: Pascal Hambourg
Subject: Re: simple message under BIOS/CSM/LEGACY
Date: Wed, 3 Aug 2022 20:25:43 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:91.0) Gecko/20100101 Thunderbird/91.10.0

Le 03/08/2022 à 16:11, Pascal a écrit :
*this is python biting its tail ;-)*

here is what I plan to test :
leave the protective partition in place (1), mark it as active (2) and
change its first sector to zero (3).

1) its absence seems to be a problem (at least with qemu/ovmf),

The GPT scheme is usually not recognized without a protective partition.

2) gdisk does not activate it by default,

As expected. Setting the boot flag on the protective partition is against the EFI specification.

Also, some UEFI firmware refuse to boot in EFI mode if the protective partition has the boot flag set. Setting the boot flag on another (even empty) partition entry has given good results with both legacy and EFI boot.

the only point that seems contentious to me is whether a partition can have
its first sector set to zero ?

In the MBR/DOS partition scheme, it should not because the first sector is reserved for the MBR (but in Sun/BSD disklabel, partition 'c' starts at sector 0 and covers the whole drive). But Debian installation images for x86 are set up this way :

Device Boot Start     End Sectors  Size Id Type
sdb1   *        0 1320959 1320960  645M  0 Empty
sdb2         4288   13343    9056  4,4M ef EFI (FAT-12/16/32)

does the BIOS check this kind of thing ?

Not in my experience, but I haven't used the Debian installer with all existing firmware.

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