Andrew S. Tanenbaum (OS) Edition 4 Exercise 3 Question 36 (Page No. 258)

Question

A computer has four page frames. The time of loading, time of last access, and the $R$ and $M$ bits for each page are as shown below (the times are in clock ticks):

1. Which page will NRU replace?
2. Which page will FIFO replace?
3. Which page will LRU replace?
4. Which page will second chance replace?

Answer 1

Ans: (a) Page 1 because R M = 0 0
      (b) Page 3 because it is loaded at 110 (First In)
      (c) Page 1 because is referenced at 265 (Least Recently)
      (d) Page 1 because it is loaded at 120 and the reference bit is 0.

Comments

in second chance, first page 3 will be considered for replacement coz it was loaded the earliest. but bcoz its referenced bit is set, it will be given a second chance and pushed to the end of the queue. now page 0 will be examined next, and it too will be given a second chance and wont be replaced. Finally, page 2 is expelled as its referenced bit is 0 and its load time is earlier than page 1.