1. Module IV
Memory Organization

2. Optimal Page Replacement
Replace the page that will not be used for the longest period of time.
This algorithm is difficult to implement, because it requires future knowledge of page usage.
It is mainly used in comparison studies

3. Optimal Page Replacement
Example
Consider the reference string :
7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
for a memory with three frames.

4. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7

5. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7

6. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1

7. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2

8. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2

9. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3

10. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3

11. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4

12. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4

13. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4

14. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4

15. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1

16. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1

17. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1

18. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1

19. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1

20. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1

21. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1 7

22. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1 7

23. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1 7

24. 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
Page
Frame
Hit -1 7 1 2 3 4
Page
Frame
Hit 3 2 1 7
Hit Ratio = 11/20
No. of Page Faults = 9