Used to contain a mirror of Wolfgang Helbig's material about v6 unix and pdp-11, but it has been removed.
See also chapter 1.0 of his operating system lecture notes at:
http://doc.cat-v.org/unix/v6/operating-systems-lecture-notes/script/
among others, it explains the assembler language, together with the description of the instruction set
http://doc.cat-v.org/unix/v6/operating-systems-lecture-notes/pdp11/doc/cpu
and finally the Unix Assembler Reference Manual(1) at
http://doc.cat-v.org/unix/v6/operating-systems-lecture-notes/v6/doc/index.html

Other parts of the site deal with Plan 9, fonts and more.