Link: The Unix Heritage Society
The Unix Heritage Society has a nice statement of its aims on its front page, but let me skip straight to the neat bits: complete source code for early Unix versions, such as V7 and V6. You can browse things online, or get your own personal mirror. For a long time, having this sort of thing was a Unix geek dream, and now I have my own (legal!) copy of it all.
One of the neat things I like doing with TUHS
is browsing to see the original full versions of such famous Unix bits
as the 'you are not expected to understand this' kernel source comment.
Here it is in full, from the
swtch() routine in /usr/sys/ken/slp.c in the Sixth
/* * If the new process paused because it was * swapped out, set the stack level to the last call * to savu(u_ssav). This means that the return * which is executed immediately after the call to aretu * actually returns from the last routine which did * the savu. * * You are not expected to understand this. */
While I'm in the area, I'd be remiss if I didn't link to the Wikipedia entry on Lions' Commentary on UNIX 6th Edition, with Source Code. This is a famous work for old Unix geeks, and the Wikipedia entry even has links to a PDF version.
(TUHS also has links to PDP-11 simulators and disk images, so you can actually run V7 et al. Maybe even faster than it ran on a real PDP-11/70, back in the days.)