The thing I find interesting about OS/400 (or whatever it's name is this month) is the single-level store. When your program needs access to the contents of a file, it's just a memory offset. The OS relies on the swapping mechanism to bring those pages into available memory. Which, when you have flat 64 (one could argue 65) bit addressing … why not?
> 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).
