Despite saying on libreboot and fedi earlier, also rboot, that 26.11 release and beyond are UEFI-only, I did add this today:
https://libreboot.org/docs/install/legacy.html
I no longer build GRUB/MT86+ in lbmk, but i wrote an easy guide for users to re-add it. FUN FACT:
The EFI version of SeaBIOS (as CSM) here is ReBIOS. I briefly used CSMWRap in 26.10, but ReBIOS is a continuation of it and it's... better in every way. It has full VGA emulation for starters, so legacy operating systems work - graphical mode switching works, and it can switch back to text mode. It's a full, libre VGA BIOS, unlike the SeaVGABIOS code. I'm thinking of how to port this to coreboot framebuffer (libgfxinit) for legacy SeaBIOS setups.
Anyway. here are some photos of ReactOS booting for the first time in Libreboot. Booted using ReBIOS CSM module, added to Libreboot EDK2 using the above guide. Yes, this is amazing:
(could also boot old Linux/BSD with this - e.g. OpenBSD from before it had kernel mode setting / drm support):
To be clear: this is a major milestone in Libreboot. Historically, Libreboot could only work with operating systems that didn't use VGA BIOS, because SeaBIOS's VGA BIOS emulation (SeaVGABIOS) only supported a few higher resolution modes - basically enough to boot Linux bootloaders reliably. Also some text-mode bootloaders. Switching from text mode to graphical modes wasn't possible in VGA (int10h interrupts), and switching back to text mode wasn't possible...
But it worked because Linux/BSD systems had stable KMS (kernel mode setting), setting up their own framebuffers in e.g. Intel driver. Then recently we added EDK2, which allows for full use of EFI fb/gop. E.g. Windows 10/11 work.
But if you wanted to run, say, Windows XP/7/Vista in BIOS mode? Nope. The ReBIOS project wraps SeaBIOS in a hypervisor (IOMMU/vt-d required unfortunately), and emulates a full VGA BIOS. Any old OS that has drivers for your machine will work. It does hw passthrough (hence vt-d) so real drivers work on it.