I used RSX-11M and I remember a few interesting things about it. First, it had an overlay linker: code would swap in if it used the same space as other unused

I used RSX-11M and I remember a few interesting things about it. First, it had an overlay linker: code would swap in if it used the same space as other unused code. Second, this extended to the operating system. There was a giant system build process where you linked all system programs in with the operating system. To run a command, it first had to be explicitly loaded into memory. So during this system generation process, you decide which commands should not interfere with other commands so that many could be loaded at once. Anyway, I remember it took many hours to run this process. The 'M' means the system had virtual memory, but virtual address space was tiny- smaller than the physical memory space.