Just ran across this document about the University of Michigan Executive System (warning, archive link might be slow to load).
The thing is a really good description, generally, of what computer OS’s actually do, but what was extra-interesting to me was that this was apparently a predecessor of the Michigan Terminal System (MTS), which I used in the 1980s at the University of British Columbia. The UMES is even mentioned in the MTS Wikipedia page.
h/t @[email protected] on Mastodon.