I agree with you that AS/400/i has always been called mid-range. I was actually referring to NonStop as usually being called "mainframe" not mid-range, or mini-computer, at least the early pre-RISC models. It's another interesting computer architecture.
For more on the AS/400, there's a book by the lead architect of it, Frank Soltis on archive.org that goes into a lot of detail. It is really quite different from anything else.
For more on the AS/400, there's a book by the lead architect of it, Frank Soltis on archive.org that goes into a lot of detail. It is really quite different from anything else.
https://archive.org/details/insideas4000000solt/mode/2up
EDIT: bitsavers has a bunch of Tandem (NonStop) stuff as well.