814cb4eb80
The GRUB2 bootloader in legacy mode, i.e. non-EFI mode, consists of
several stages. The fist one place in the in the Master Boot Record
of the disk will load and execute the next, second stage, consisting
of core modules and the grub kernel. The first bit is also known as
'boot' and the second as 'core'. When the 'MBR' partition layout is
being used, there is a gap between the Master Boot Record (MBR) and
the first partition (for historical and performance reasons). The
core image normally is placed into this gap (call the MBR gap).
When the partition layout is 'gpt' there is no standard gap that can
be used, instead a special partition ("BIOS boot" [1]) needs to be
created that can store the grub2 core image. Additionally, the 'boot'
image need to modified to point the sector of that partition. The
core image itself also needs to be modified with the information of
the location its own second sector. The location of the pointers
were taken from the grub2 source ([2] at commit [3]). For the 'boot'
image it is 'GRUB_BOOT_MACHINE_KERNEL_SECTOR' (0x5c) from 'pc/boot.h'
and for the core image "0x200 - GRUB_BOOT_MACHINE_LIST_SIZE (12)" to
be found in 'pc/diskboot.S'.
[1] https://en.wikipedia.org/wiki/BIOS_boot_partition
[2] https://github.com/rhboot/grub2
[3] 2a2e10c1b39672de3d5da037a50d5c371f49b40d
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