There's absolutely a void when it comes to a full amd64 assembly (and associated low level) code. It's a shame, because amd64 is actually rather plesant (as opposed to x86) -- and now rather ubiqutous.

I haven't really looked at the code, but I guess the following at least contains a bit of example code:

http://www.returninfinity.com/baremetal.html

Menuetos also looks rather interesting, unfortunately, as I recall the license for the 64bit version puts the code in a bit of a limbo as far how it's actually useful. There's a fork, but not for amd64 afaik..

http://menuetos.net/

http://kolibrios.org/en/