Virtual memory allows processes to execute using both physical RAM and disk storage. When a process references a page not in RAM, demand paging brings that page into a free frame from disk. Page replacement completes the separation of logical and physical memory, allowing a larger virtual memory in a smaller physical RAM. Common page replacement algorithms include FIFO, LRU, and LFU.

2. Virtual memory is a technique that is used to execute
processes that are not in main memory completely. The
remaining part are placed on the backing storage.
Demand paging
It is a technique that is used to bring a page into memory only
when it is needed.

3.  Page is needed  reference to it
 Referenced page not in real memory  Page Fault
 Just not in memory  Demand paging
 Find free frame
 Swap page into frame
 Reset tables to indicate page now in memory
 Restart the instruction that caused the page fault

4. “Page replacement completes separation between logical
memory and physical memory large virtual memory can be
provided on a smaller physical memory”

5.  It terminates the program immediately
Or
 Swaps out program & frees all of its frames
 It reduces the degree of multiprogramming

6. Steps performed by OS while replacing a page upon a page
fault:
1. Find the location of the desired page on disk
2. Find a free frame:
If there is a free frame, use it
If there is no free frame, use a page replacement algorithm
to select a victim frame; if the victim page is modified, write it
back to disk.
3. Bring the desired page into the (new) free frame; update the
page and frame tables
4. Restart the process

7.  Want lowest page-fault rate on both first access and re-
access
 There is particular string of memory references.
 String is just page numbers, not full addresses
 Repeated access to the same page does not cause a
page fault
 Results depend on number of frames available
 In all our examples, the reference string of referenced
page numbers is
7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1

8.  Oldest page in main memory is the one which will be
selected for replacement
 Easy to implement, keep a list, replace pages from the tail
and add new pages at the head.

9.  Adding more frames can cause more page
faults!
 Belady’s Anomaly
15 page faults

11.  Replace page that will not be used for longest period of time
 How do you know this?
 Used for measuring how well your algorithm performs

12.  Use past knowledge rather than future
 Replace page that has not been used in the most amount of time

13.  12 faults – better than FIFO but worse than OPT
 Generally good algorithm and frequently used

14. With each page associate a bit, initially= 0
When page is referenced bit set to 1

16.  This Algorithm is a FIFO replacement algorithm wit
consideration given to the reference bit .when a page
has been selected for replacement , its reference bit is
inspected .
 When a page is given a second chance its reference bit
is cleared and its arrival time reset to the current time

17.  Improve algorithm by using reference bit and modify bit (if
available) in concert
1. (0, 0) neither recently used not modified – best page to
replace
2. (0, 1) not recently used but modified – not quite as good,
must write out before replacement
3. (1, 0) recently used but clean – probably will be used
again soon
4. (1, 1) recently used and modified – probably will be used
again soon and need to write out before replacement

18.  Each process needs minimum number of pages
 Various allocation approaches
› fixed allocation
› global allocation
› local allocation

19.  Equal allocation – For example, if there are 100 frames
and 5 processes, give each process 20 frames.
 Proportional allocation – Allocate according to the size of
process
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20.  When a page fault occurs for a process and we need page
replacement, there are two general approaches:
› Global replacement – select a victim frame from the
set of all frames;
 one process can take a frame from another
› Local replacement – select a victim frame only from
the frames allocated to the process.
 A process uses always its allocated frames

21.  At some time , the processor may be spend most of its
time swapping pages and doing little productive work, this
is known as Thrashing.