TempleOS expects legacy BIOS booting, which modern firmware has quietly stopped offering, so Terry Davis’s operating system has been drifting out of reach of actual hardware. UwUEFI is the shim: a UEFI bootloader written in C that gets it started on a machine made this decade. The author’s own summary of the project is “a 萌え萌えキュン bootloader for TempleOS”, which tells you the register this was built in.