Release Date: November 1973
Released By: Bell Labs Research
Source Code: a kernel which predates 4th Edition is nsys.tar.gz in the Unix Archive
Documentation: 4th Edition man pages, browsable in the Unix Tree

The fourth edition of Unix was the first version to have a kernel written in a high level language, C, along with some of the commands. A full and complete copy of Fourth Edition no longer exists. We have:

• The manuals for Fourth Edition in machine-readable format, and
• A copy of the earliest extant kernel in C, the “nsys” kernel, which pre-dates the actual Fourth Edition by only a few months.

The “nsys” kernel was donated by Dennis Ritchie. This is a version of the kernel quite close to that released in Fourth Edition, but without pipes. Dennis Ritchie writes:

This is a tar archive derived from a DECtape labelled “nsys”. What is contains is just the kernel source, written in the pre-K&R dialect of C. It is intended only for PDP-11/45, and has setup and memory-handling code that will not work on other models (it's missing things special to the later, smaller models, and the larger physical address space of the still later 11/70.) It appears that it is intended to be loaded into memory at physical address 0, and transferred to at location 0.

For more information about Fourth Edition Unix, see The Evolution of the Unix Time-sharing System by Dennis Ritchie.