And OS/400 files are libraries that you need to open, as another example of strangeness.

Or the OS/360 which uses virtualization for all OSs, like Hyper V does on Windows.

The first OS to boot from the hypervisor has master rights, but all OSs are virtualized.

> Or the OS/360 which uses virtualization for all OSs, like Hyper V does on Windows

No, the hypervisor was called VM. According to WP it most frequently ran CMS guests, which was a light weight single-app OS. But could also run OS/360 guests (which predated VM).

AS/400 isn't mainframe. It's midrange; smaller than a mainframe, and not as flexible.