Page table is accessed by the OS as part of execuation of an instruction

The PTBR is loaded by the OS

User process can update it's own page table entries

User process can update it's own PTBR

OS creates the page table for every process

An invalid entry on a page means, either it was illegal memory reference or the page was not present in memory.

Page table is accessed by the MMU as part of execution of an instruction

An invalid entry on a page means, it was an illegal memory reference