Original source code for UNIX v6 had no comments in it, so its recommended to take a look at J.Lions commentary
Xv6 a simple Unix-like teaching operating system, developed by MIT.
Unix archive of sorts,source code and manuals of various old versions of Unix.
Several systems started with Unix source code, but this was written out over time so that no original Unix code remains. The best known examples are OpenSolaris, FreeBSD, NetBSD and OpenBSD.
|OpenSolaris build 135||2010-03|
There have been many systems which implement the Unix system calls, library APIs and commands, but which did not include any original Unix source code. Here is a small selection.