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How many page faults occur in LRU page replacement algorithm for the given reference


string, with four-page frames


7, 0, 1, 2, 0, 3, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1

  • a)
    6

  • b)
    8

  • c)
    9

  • d)
    7

Correct answer is option 'D'. Can you explain this answer?
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Solution:
To understand why the least recently used (LRU) page will be 3, let's go through the page reference sequence and the page frames.

Page Reference Sequence:
0 2 1 3 2 1 0 1 2 1

Page Frames:
Page frames are the available slots to store pages in memory. In this case, we have four page frames.

Initially, all page frames are empty:
- Frame 1: Empty
- Frame 2: Empty
- Frame 3: Empty
- Frame 4: Empty

Page Reference 0:
Since the page frame is empty, we can directly allocate page 0 to the first frame.
- Frame 1: 0
- Frame 2: Empty
- Frame 3: Empty
- Frame 4: Empty

Page Reference 2:
Page 2 is not present in any of the frames, so we allocate it to the next available frame.
- Frame 1: 0
- Frame 2: 2
- Frame 3: Empty
- Frame 4: Empty

Page Reference 1:
Page 1 is not present in any of the frames, so we allocate it to the next available frame.
- Frame 1: 0
- Frame 2: 2
- Frame 3: 1
- Frame 4: Empty

Page Reference 3:
Page 3 is not present in any of the frames, but all frames are already occupied. We need to replace a page using the LRU algorithm, which means we need to select the page that was least recently used.

The least recently used page is 0 because the page reference for 0 is the earliest among all the pages currently in the frames. So, we replace 0 with 3.
- Frame 1: 3
- Frame 2: 2
- Frame 3: 1
- Frame 4: Empty

Page Reference 2:
Page 2 is already present in frame 2, so there is no need to make any changes.
- Frame 1: 3
- Frame 2: 2
- Frame 3: 1
- Frame 4: Empty

Page Reference 1:
Page 1 is already present in frame 3, so there is no need to make any changes.
- Frame 1: 3
- Frame 2: 2
- Frame 3: 1
- Frame 4: Empty

Page Reference 0:
Page 0 is not present in any of the frames, but all frames are already occupied. We need to replace a page using the LRU algorithm, which means we need to select the page that was least recently used.

The least recently used page is 3 because the page reference for 3 is the earliest among all the pages currently in the frames. So, we replace 3 with 0.
- Frame 1: 0
- Frame 2: 2
- Frame 3: 1
- Frame 4: Empty

Page Reference 1:
Page 1 is already present in frame 3, so there is no need
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