ITPEC FE Subject A October 2025 Question 15
The page capacity of main memory is three pages, and the pages are referenced in the order {1, 2, 3, 2, 1, 4, 5}. Which of the following is the page that will be replaced from main memory for the last page reference if LRU is used as the page replacement technique?
Explanation
Page 2 — track when each page was last used, hits included, and replace the one unused for longest.
At the reference to 4, page 3 was last used at step 3, earlier than 2 (step 4) and 1 (step 5). At the reference to 5, page 2 was last used at step 4, earlier than 1 (step 5) and 4 (step 6).
Answer (b)
Why not others:
- •(c) 3 — is the page replaced at the reference to 4, one step earlier; by the last reference it is no longer in memory
- •(a) 1 — was used at step 5, more recently than page 2
- •(d) 4 — is the most recently used page; it would go under LFU, having been used only once, but LRU looks at recency
Key rule: LRU ranks pages by their last use, so every hit refreshes a page; FIFO ranks by load time and ignores hits, and LFU counts uses.
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