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Virtual  Memory: Move parts of programs in and out of memory. MMU (Memory Manage Unit): Physical Memory is broken down into fixed sized blocks (frames) (page frames).
Logical Memory (Virtual) is broken down into blocks of same size called pages, same size of disk segment for easy to read/write from data segment.
The purpose of the page table is to map virtual pages onto page frames.
The page number is used as an index into page table, yielding the number of the page frame corresponding to that virtual page. See figure 4-9.
There is a present/absent bit on page table.  From this bit, we can know which page is on physical memory or on disk.
If a page is absent (not in RAM) then a page fault occurs.  OS must go get page from disk and put into physical memory.
Page fault: an attempt to access a page that is not present in physical memory.
Each process has its own view of memory (virtual memory)

Page size is dictated by hardware.

Size of logical address space 2m
Page size 2n
Logical address will be  related to  m-n pages and n offset.
For example:
    page size: 512 bytes (9 bits)  29 = 512    n = 9
    216 = 64 K,   m = 16
    number of pages = 216 / 29 = 27 (128 pages) (Each one point to particular 512 physical memory)
 
Virtual Address
-------------->
Physical Address
1   0
0       1
 -------------->
0       1
0     1
9
 -------------->
5
Multilevel Page Tables      for quickly access and map logical address to physical address.

Associative Memory - TLBs (Translation Look aside Buffer)
            It is a small hardware device inside the MMU for mapping virture addresses to physical addresses without going through the page table.

Inverted Page Tables
            One entry perpage frame in real memory instead of one entry perpage of virtual address space.