What if you have a very modern machine that is entirely UEFI-only, meaning it has no compatibility support module and thus no way of enabling a legacy BIOS mode? Well, install a CSM as an EFI application, of course!
CSMWrap is an EFI application designed to be a drop-in solution to enable legacy BIOS booting on modern UEFI-only (class 3) systems. It achieves this by wrapping a Compatibility Support Module (CSM) build of the SeaBIOS project as an out-of-firmware EFI application, effectively creating a compatibility layer for traditional PC BIOS operation.
↫ CSMWrap’s GitHub page
The need for this may not be immediately obvious, but here’s the problem: if you want to run an older operating system that absolutely requires a traditional BIOS on a modern machine that only has UEFI without any CSM options (a class 3-machine), you won’t be able to boot said operating system. CSMWrap is a possible solution, as it leverages innate EFI capabilities to run a CSM as an EFI application, thereby adding the CSM functionality back in. All you need to do is drop CSMWrap into /efi/boot on the same drive the operating system that needs BIOS to boot is on, and UEFI will list it as a bootable operating system.
It does come with some limitations, however. For instance, one logical core of your processor will be taken up by CSMWrap and will be entirely unavailable to the booted BIOS-based operating system. In other words, this means you’re going to need a processor with at least more than one logical processor (e.g., even a single-core machine with hyperthreading will work). It’s also suggested to add a legacy-capable video card if you’re using an operating system that doesn’t support VESA BIOS extensions (e.g. anything older than NT).
This is an incredibly neat idea, and even comes with advantages over built-in CSMs, since many of those are untested and riddled with issues. CSMWrap uses SeaBIOS, which is properly tested and generally a much better BIOS than whatever native CSMs contain. All in all, a great project.
