CSM booting in UEFI
CSM or Compatibility Support Module is something that allows booting in legacy BIOS mode on UEFI systems.
From Wikipedia:
To ensure backward compatibility, most UEFI firmware implementations on PC-class machines also support booting in legacy BIOS mode from MBR-partitioned disks, through the Compatibility Support Module (CSM) that provides legacy BIOS compatibility. In this scenario, booting is performed in the same way as on legacy BIOS-based systems.
The Compatibility Support Module allows legacy operating systems and some legacy option ROMs that do not support UEFI to still be used.
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