Memory protection is a way to control memory access rights on a computer.
It is a part of most modern instruction set architectures and operating systems.
The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it.
Memory protection is implemented by associating protection bit with each frame.
Valid-invalid bit attached to each entry in the page table where 'valid' indicates that the associated page is in the process logical address space,and is thus a legal page and 'invalid' indicates that the page is not in the process logical address space.
If an attempt is made to generate an address for pages having invalid bit a trap is sent to OS.