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Module 5: Memory Management

20227m

Consider the virtual page reference string 0, 4, 8, 20, 24, 36, 44, 12, 68, 72, 80, 84, 28, 32, 88, 92 on a demand paged virtual memory system running on computer system that has main memory of size 4 page frames which one initially empty. The CPU uses LRU page replacement policy.
(i) How many page faults does this sequence cause?
(ii) What are the page numbers of the pages present in the main memory at the end of the sequence?

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LRU with 4 frames, ref: 0,4,8,20,24,36,44,12,68,72,80,84,28,32,88,92

Simulating LRU: each new page faults until 4 distinct, then replace LRU.
Sequence: 0(F),4(F),8(F),20(F)=4,24(F replace 0),36(F replace4),44(F replace8),12(F replace20),68(F replace24),72(F replace36),80(F replace44),84(F replace12),28(F replace68),32(F replace72),88(F replace80),92(F replace84) => 16 faults (all references distinct except maybe? Actually all distinct, so 16 faults). Final pages in memory (order LRU→MRU): 28,32,88,92. If any repeated, faults reduce. Explanation shows LRU tracks recency stack.

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