My hobby OS is x86_64 only, straight C. It's GPLv3. I wrote it with the intention of writing a tutorial, partly because of this void, but still have not written the tutorial :)

https://github.com/nwg/nanos

x86_64 bootup / structures

https://github.com/nwg/nanos/blob/master/src/boot.asm

https://github.com/nwg/nanos/blob/master/src/kernel_init.asm