This document discusses storage management and page replacement algorithms. It begins with an introduction to page replacement and mentions FIFO as a page replacement algorithm. It then provides more details on concepts related to page replacement such as page faults, free frames, and the need for page replacement algorithms. The rest of the document focuses on explaining the FIFO page replacement algorithm through examples and diagrams. It also discusses disadvantages of FIFO and the concept of Belady's Anomaly where increasing the number of page frames can paradoxically increase page faults.

1. Storage Management
• Page replacement- FIFO
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2. Recap
In the last class, you have learnt
Demand Paging
Steps to handle page faults
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3

4. Objective
On completion of this class, you will be able to
know
• Concepts of Page Replacement
• FIFO page replacement algorithm
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5. Page Replacement
• When a user process is executing, a page fault
occurs
• The OS determines where the desired page is
residing on the disk
• Finds that there are no free frames on the free-
frame list
• All memory is in use Figure:1
5

6. Need For Page Replacement
Figure: 1
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7. Procedure to find free Frame:
• Page replacement – find some page in memory,
which is not really in use & swap it out
– In order to swap use one of the Page replacement
algorithm
– Select an algorithm which will result in minimum number
of page faults & improves the performance
• Same page may be brought into memory several
times
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8. Basic Page Replacement
• Find the location of the desired page on disk
• 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
• Read the desired page into the (newly) free
frame as shown in Fig: 2
• Update the page and frame tables
• Restart the process
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9. Page Replacement
Fig: 2
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10. Page Replacement
• Prevent over-allocation of memory
• Include page replacement
• Use modify (dirty) bit to reduce overhead of
page transfers
• Each page or a frame may have a modify bit
associated with it in the hardware
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11. Page Replacement
• Modify bit for a page is set by the hardware when a
word or byte is written to page
• Only modified pages are written to disk
• Page replacement completes separation between
logical memory and physical memory
• Large virtual memory can be provided on a smaller
physical memory
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12. Page Replacement Algorithm
How to select the page replacement
algorithm?
• The string of memory references is called
reference string
• Evaluate the algorithm by running on a
reference string and compute the number of
page faults
• Select the one with lowest page-fault rate
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13. Page Replacement Algorithm
How to determine the no. of page faults?
• Need to have the knowledge of number of
page frames available for a reference string
• As number of page frames increases the
page faults will decrease
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14. FIFO Replacement Algorithms
FIFO - First In First Out
• Simplest page replacement algorithm
• Replace the page that has been in primary
memory the longest
• FIFO queue holds the pages in memory
• Replace the page at the head of the queue
• The new page is inserted at the tail of the queue
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15. FIFO Page Replacement
• Page fault’s at 7,0,1,2,3,0,4,2,3,0,1,2,7,0,1 of
reference string
Fig 3
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16. FIFO
Example : 12
references, 9
faults
Fig 4 16

18. FIFO
Disadvantages of FIFO algorithm
 Performance is not always good
 The page in active use is replaced for the new one
 This bad replacement choice increases the page
faults
 To illustrate the problem consider the
 Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
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19. FIFO
 3 frames (3 pages can be in memory at
a time per process)
1 1 4 55
4
2 2 1 33
1 9 page faults
3 3 2 44
2
Fig 5
 4 frames
1 1 5 44
5
2 2 1 55
1 10 page faults
3 3 2
2
4 4 3
3
Fig 6 19

20. Belady's Anomaly
• FIFO with 3 page frames Fig : 7 (a)
• FIFO with 4 page frames Fig : 7 (b)
• P's show which page references show page
faults
Fig 7
Fig 7
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21. FIFO
 The result is unexpected and is known as
Belady’s Anomaly
 For some page replacement algorithm
Page-fault rate may increase as the no. of
allocated frames increase
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22. Paging Behavior with Increasing
Number of Page Frames
Fig 8
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23. FIFO Illustrating Belady’s Anomaly
Fig 9
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24. Summary
 In this class, you have learnt
 Concepts of Page replacement
 FIFO algorithm
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25. Frequently Asked Questions
What is the need for page replacement?
How do you find a free frame?
What is dirty bit?
What is meant by page replacement?
Explain FIFO page replacement algorithm
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26. Quiz
1. Use _____________to reduce overhead of page
transfers
a) Segmentation
b) Modify (dirty) bit
c) None
2. Modify bit for a page is _____by the hardware when a
word or byte is written to page
a) Reset
b) Hole
c) set
d) None
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27. Quiz
3. ___________completes separation between logical
memory and physical memory
a) Page number
b) MULTICS
c) Page replacement
d) None
4. Each page or a frame may have a
___________associated with it in the hardware
a) External fragmentation
b) modify bit
c) Compaction
d) None
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28. Quiz
5. What is a Page?
a) Blocks of logical memory
b) Blocks of physical memory
c) Blocks of virtual memory
d) None
6. FIFO queue replaces the pages at
a) Head of queue
b) Tail of queue
c) Both
d) None
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29. Quiz
7. ___________completes separation between logical
memory and physical memory
a) Page number
b) MULTICS
c) Page replacement
d) None
8. Each page or a frame may have a ___________associated
with it in the hardware
a) External fragmentation
b) modify bit
c) Compaction
d) None
29

30. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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