Page Replacement Strategy
When the Valid = 0, a page fault is signaled.
Some page must be replaced -- pick the page that will be used furthest into the future: Opt
Replace the least recently used (LRU) page.
LRU strategies are effective, but expensive.
"Use" bits can be a decent approximation.
Space required for page tables can be substantial:
4K pages imply 20 bits of virtual address.
At x bytes per entry implies xMB for full page table.
Represent only the prefix of the table using base + extent.
Grow low addresses and high addresses separately by using the msb to indicate which part of the VM space is being used.