Be carefully and do it correctly. Thank you very much. Question 4 6.25 pts Fill

Question

Be carefully and do it correctly. Thank you very much.

Question 4 6.25 pts Fill in multiple blanks. Answer this question using the Second-chance page replacement algorithm. Seven pages are stored in a circular buffer as shown below: Page # Reference bit 16 2 0 The algorithm is currently pointing to Page 2 as the next victim and scanning to the right. The page selected for replacement is Page . Fill in the reference bit for each page after the algorithm selected the "victim" 16 Page # 5 Reference bit

Explanation / Answer

In Second chance page replacement algorithm, when a page has to be replaced, we examine its reference bit. If the reference bit is 0 we proceed to replace the page. However if the reference bit is 1,we give that page a second chance and move on to select the next FIFO page. When a page gets a second chance its reference bit is cleared and arrival time is reset to current time. Thus a page that is given a second chance will not be replaced until all other pages are replaced (or given a second chance).Additionally, if a page is used enough to have its reference bit set all the time, it will never be replaced.

A pointer indicates which page is to be replaced next. When a frame is needed, the pointer advances until it finds a page with a 0 reference bit. As it advances it clears the reference bits. Once a victim page is selected, the page is replaced and a new page is inserted in the circular queue in that position.

When all the bits are set the pointer cycles through the whole queue giving each page a second chance. It clears all the reference bits before selecting the next page for replacement.

So in the figure the algorithm is pointing to page 2,it will cycle through the queue and will select page 16 for replacement as it has a reference bit of ).

So the page selected for replacement is Page 16.

Reference bit for each page after the algorithm selected the victim is

Page #

5

16(Victim page                               to be replaced)

1

7

2

6

3

Reference

0

   0

1

1

0

0

0

                                     

Page #

5

16(Victim page                               to be replaced)

1

7

2

6

3

Reference

0

   0

1

1

0

0

0

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