Source code was extracted from this archive (v6root/usr/sys).

Lions' Commentary on UNIX 6th Edition and PDP-11 Handbook are helpful for understanding the source code, especially the handbook. After I skimmed the handbook, it became easier to follow Lions' book and source code. Before starting with Lions' book, I went through Operating Systems: Three Easy Pieces and MIT 6.S081 and its labs to grab foundamental knowledge on operating system. It is like a journey by reading Lions' book.