Crucial early evolutionary step found, imaged, and ... amazingly ... works

Computer History Museum software curator Al Kossow has successfully retrieved the contents of the over-half-a-century old tape found at the University of Utah last month.

UNIX V4, the first ever version of the UNIX operating system in which the kernel was written in the then-new C programming language, has been successfully recovered from a 1970s nine-track tape drive. You can download it from the Internet Archive, and run it in SimH. On Mastodon, "Flexion" posted a screenshot of it running under SGI IRIX.

https://archive.org/details/utah_unix_v4_raw //

The very first version of Unix, later known as the "Zeroth edition", was hand-coded in assembly language by Thompson in 1969. He wrote it for a spare PDP-7 at Bell Labs, a Digital Equipment Corporation minicomputer from 1965. The PDP-7 was an 18-bit machine: it handled memory in 18-bit words. This was so long ago that things like the eight-bit byte had not yet been standardized. PDP-7 UNIX was reconstructed from printouts between 2016 and 2019.

It did well enough that a few years later, Thompson got his hands on a PDP-11. Thompson rewrote his OS for this 16-bit machine – still in assembly language – to create UNIX First Edition. At first, the machine had a single RS11 hard disk, for a grand total of half a megabyte of storage, although the rebuilt source code is from a later machine with a second hard disk.