Page fault handler


### It is kind of interrupt
if required PTE (entry) has invalid bit in page table than cause an exception to throw so go to OS(kernel mode), bring in missing page into physical frame, update PTE and restart instruction.
-**Page fault service time: **service the page-fault interrupt + Read in the page + Restart the process
- **Effective access time: **memory access time * (1 - p) + disk access time * p (p = probability of page fault)
**degradation: **effective access time / memory access time (low p than low degradation)

> **Locality**: dependent on page replacement policy
> page change occur by not only by PFH, since predict page usage by machine learning 





