This is pretty standard virtual memory stuff. They made a few off design decisions. Also to note is how little has changed since 1978.
Pages are minimum unit of protection, etc.
Top two bits of virtual address have special meaning, dividing between system and process space. The page table for system space is located via a physical address stored in a register. Page tables for process address spaces are located via a virtual address in system space. This virtual address is held in registers which are updated at context switch time. Thus, it is a two-step process to xlate a process' virtual address to a phyical address.