ISRO2018-20

Question

Determine the number of page faults when references to pages occur in the order - $1, 2, 4, 5, 2, 1, 2, 4$. Assume that the main memory can accommodate $3$ pages and the main memory already has the pages $1$ and $2$, with page $1$ having brought earlier than page $2$. (assume LRU i.e. Least-Recently-Used algorithm is applied)

1. $3$
2. $4$
3. $5$
4. None of the above

Comments

Ans B) 4

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X: NO FAULT

F: FAULT

Initially, we have: Top 2,1 Bottom

Page Ref

1	2	4	5	2	1	2	4
X	X	F	F	X	F	X	F
		4	5	2	1	2	4
1	2	2	4	5	2	1	2
2	1	1	2	4	5	5	1

Answer 1

Reference String: $1,2,4,5,2,1,2,4$

Frame can contain only $3$ pages, $1$ and $2$ are already present

$1$: Hit, Frame Status: $1,2$

$2$: Hit, Frame Status: $1,2$

$4$: Page fault, Frame Status: $1,2,4$

$5$: Page fault, $1$ will be replaced as per LRU policy, Frame Status: $5,2,4$

$2$: Hit, Frame Status: $5,2,4$

$1$: Page fault, $4$ will be replaced, Frame Status: $5,2,1$

$2$: Hit, Frame Status: $5,2,1$

$4$: Page fault, $5$ will be replaced, Frame Status: $4,2,1$

4 Page faults hence option B) is correct

Answer 2

option b