Looking at weird systems is not only fun but also challenges a lot of assumptions we make on how all computers are supposed to work. See TempleOS and the way it's C all the way up. Or Lisp Machines.
Yet another such parallel, alien universe is IBM's AS/400 computers. Note that these are not mainframes; it's a newer, smaller kind of system. Nevertheless, they're still alien enough to be really interesting, with many conventions coming from mainframes.
So: now I'm planning a mini-series on them here on this blog, with this post being an intro one.
(Also: no, I'm not an expert; I've been playing around with one recently, plus there is this really interesting long PDF, in German. No actual experience in production. But then, yet again, this is #100DaysToOffload.)
Meanwhile, for completeness' sake... they eventually renamed AS/400 "IBM i", in their infinite marketing wisdom. We're going to ignore this here, as "AS/400" is way easier to search for, and, accordingly, half of the world still calls it that.
Why are they weird?
Just look at this. It's me playing Battleship on one.
... or this UI:
This... most definitely doesn't look like your standard UNIX server.
Better yet: these things don't really even have a file system. Or a distinction between disk and RAM. Its terminals operate on entire screens; you can fill out an entire form without any interaction from the server side! Plus, they've been doing "write once, run anywhere", with JIT compilation, since the 80s or so.
We'll take a look at some of these later; stay tuned!