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Enabling CONFIG_CSM allows SeaBIOS to be built as a Compatibility Support
Module for use with the OMVF/EDK-II UEFI firmware.
It will provide "legacy" BIOS services for booting non-EFI operating
systems and will also allow OVMF to display on otherwise unsupported
video hardware by using the traditional VGA BIOS.
Windows 2008r2 is known to use INT 10h BIOS calls even when booted via
EFI, and the presence of a CSM makes this work as expected too.
Having built SeaBIOS with CONFIG_CSM, you should be able to drop the
result (out/Csm16.bin) into your OVMF build tree at
OvmfPkg/Csm/Csm16/Csm16.bin and then build OVMF with 'build -D
CSM_ENABLE'. The SeaBIOS binary will be included as a discrete file
within the 'Flash Volume' which is created, and there are tools which
will extract it and allow it to be replaced; satisfying the
requirements of the LGPL licence.
A patch to OVMF is required, to prevent it from marking the region from
0xC0000-0xFFFFF as read-only before invoking our Legacy16Boot method. See