In FIFO page replacement algorithm, when a page must be replaced

Question

BlackTom AI · Accepted Answer

FIFO always replaces the page that has been in memory the longest — the oldest loaded page.

1. Relevant knowledge points and concepts
- Page replacement algorithms decide which memory page to evict on a page fault when no free frames remain.
- FIFO (First-In, First-Out) treats frames like a queue: pages enter at the rear and the one at the front (the earliest loaded) is removed first.
- FIFO does not consider usage frequency or recency (unlike LRU); it only uses arrival order.

2. Complete reasoning process (step by step)
- When a page fault occurs and frames are full, FIFO looks at the order in which pages were brought into memory.
- The page that was loaded earliest (the “first in”) is chosen for replacement.
- This ensures the oldest resident page is evicted regardless of whether it has been recently used.

3. Example showing intermediate steps
Assume 3 frames and reference string: 7, 0, 1, 2, 0, 3, 0, 4.
- Load 7 -> frames: [7]
- Load 0 -> frames: [7,0]
- Load 1 -> frames: [7,0,1]
- Reference 2 (fault, frames full): FIFO evicts oldest (7) -> frames: [2,0,1]
- Reference 0 (hit) -> frames unchanged [2,0,1]
- Reference 3 (fault): evict next oldest (0) -> frames: [2,3,1]
- Reference 0 (fault): evict next oldest (1) -> frames: [2,3,0]
- Reference 4 (fault): evict next oldest (2) -> frames: [4,3,0]
Each replacement removed the oldest-resident page.

4. Why the correct answer is correct
FIFO’s defining rule is chronological eviction: the oldest page in memory (first loaded) is the one replaced. This matches the option “oldest page is chosen.”

5. Why other options are incorrect
- “newest page is chosen”: Opposite of FIFO; that would be LIFO-like behavior, not FIFO.
- “random page is chosen”: Describes Random replacement, not FIFO.
- “none of the mentioned”: Incorrect because a specific, well-defined choice exists for FIFO.

6. Conclusion
The correct answer is “oldest page is chosen,” which matches the provided answer.

In FIFO page replacement algorithm, when a page must be replaced单项选择题

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