title: How to know if an adress is valid within a page
body: <p>Vistual and physical memory are divided into pages. To check if a page is valid there is a valid bit in the page table entry. But within a page how can the MMU know if an adress is valid? For example, a page that belongs to a part of the heap. not all the adresses of that page are valid, so how can the MMU check the addr to see if its valid?</p>