Unix V6 Source code
This is an open course from MIT (http://pdos.csail.mit.edu/6.828/2012).
From this website you can get the source code of xv6 and many useful materials, including a detail document like "Commentary on Unix Source- J. Lions" to help you understand the code of xv6.
In my opinion xv6 is better than unix v6, because xv6 is written in ANSI C and AT&T assembly language, but unix v6 is designed to run on PDP machine.
I'm also learning OS by my self, and I'm looking xv6 and linux0.11 source code.
I hope these words may help you. ^_^
Nowadays you don't require to make a bootloader by hand but require to use uefi firmware (available for download free) this firmware has the required functions which can be called on demand for loading the is onto RAM from Hard Drive .... Partitioning is also managed .It is written in c/c++ ... I have been learning to try to make a is of my own ..I came across it .. It is a good point to start from in modern UEFI enabled motherboards .