I like the following x86 assembly real-mode to protected-mode bootloader a lot:

(Taking a quick glance, it seems to be very well written and robust -- at least as far as "old-skool" (should run on ancient PC's back to 386's!) FAT, non-UEFI, x86 assembly bootloaders go...)

https://github.com/ZystemOS/pluto/tree/develop/src/bootloade...

Last commit was 3 years ago. Would be cool to see this updated to the latest zig! There have been a lot of advancements since then.