--- title: "Memory characteristics and LRU replacement" subject: "Computer Organization and Architecture" chapter: 6 key: "semester_5/computer_organization_and_architecture:memory-char-lru" source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/memory-char-lru site: BCT Engineering --- # Memory characteristics and LRU replacement Computer Organization and Architecture · Chapter 6: Memory System - Asked in: 2073 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/memory-char-lru - Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-6 ## Question Explain the major characteristics of memory. Explain the LRU (Least Recently Used) replacement policy with example. ## Short answer Memory characteristics: location, capacity, unit of transfer, access method, performance, physical type, volatility, and organization. LRU replacement evicts the block unused for the longest time by tracking usage order. In a 4-block cache with accesses 2,3,2,1,5,2,4,5,3,2,5,2, hits=6, misses=6, hit ratio=0.5. ## Answer #### Characteristics of memory | Characteristic | Meaning | Examples | |---|---|---| | **Location** | Where the memory is: internal (registers, cache, main memory), external (disk, tape) | CPU registers, RAM, HDD | | **Capacity** | Size: words or bytes for internal, bytes for external | 8 GB RAM, 1 TB disk | | **Unit of transfer** | Data moved in one access: word for internal, block for external | Word, block, record | | **Access method** | Sequential (tape), direct (disk), random (RAM), associative (cache tag search) | Tape, disk, RAM, cache | | **Performance** | Access time (latency), memory cycle time, transfer rate = 1 / cycle time | DRAM access: tens of ns | | **Physical type** | Semiconductor, magnetic, optical, magneto-optical | RAM, HDD, CD | | **Physical characteristics** | Volatile or non-volatile; erasable or non-erasable | RAM volatile, ROM non-volatile | | **Organisation** | Physical arrangement of bits into words | Interleaved memory banks | #### LRU replacement policy **LRU (Least Recently Used)** replaces the cache block that has not been used for the longest time. It uses temporal locality: a block used recently is likely to be used again soon. The hardware keeps an age (or use) counter for each line; on each access the age of the used line is reset and others are increased. - **Hit:** the block is in the cache; no replacement. The block becomes the most recently used. - **Miss with free line:** the block is loaded into a free line and becomes the most recently used. - **Miss with cache full:** the least recently used block is removed and the new block is loaded. #### Example: cache of 4 blocks, reference string 2, 3, 2, 1, 5, 2, 4, 5, 3, 2, 5, 2 ```text Ref Result Cache (most recent first) 2 Miss [2] 3 Miss [3, 2] 2 Hit [2, 3] 1 Miss [1, 2, 3] 5 Miss [5, 1, 2, 3] 2 Hit [2, 5, 1, 3] 4 Miss (evict 3) [4, 2, 5, 1] 5 Hit [5, 4, 2, 1] 3 Miss (evict 1) [3, 5, 4, 2] 2 Hit [2, 3, 5, 4] 5 Hit [5, 2, 3, 4] 2 Hit [2, 5, 3, 4] ``` Hits = 6, misses = 6, hit ratio = 6/12 = 0.50. When 4 was referenced the cache was full and the least recently used block was 3, so 3 was evicted; when 3 came back, block 1 was the least recently used, so 1 was evicted. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which pairing of memory and access method is correct? A. Tape - random B. RAM - sequential C. Disk - direct D. Cache - sequential Answer: C. Tape is sequential, disk direct, RAM random and cache tag search associative. (Key point: Access methods) 2. How is transfer rate related to memory cycle time? A. Transfer rate = cycle time B. Transfer rate = 1 / cycle time C. Transfer rate = cycle time^2 D. They are unrelated Answer: B. One word can be transferred per cycle, so rate is the reciprocal of cycle time. (Key point: Performance) 3. Which block does LRU replace when the cache is full? A. The block loaded first B. The block used fewest times C. The block unused for the longest time D. A random block Answer: C. LRU uses temporal locality and removes the least recently used block. (Key point: LRU principle) 4. In the 4-block example, which block is evicted when 4 is referenced and why? A. Block 1, loaded earliest B. Block 3, least recently used C. Block 5, used most D. Block 2, used most recently Answer: B. Before 4 the order is [2,5,1,3] most recent first, so 3 is least recently used. (Key point: LRU example) 5. The example string has 6 hits in 12 references. What is the hit ratio? A. 0.25 B. 0.33 C. 0.50 D. 0.75 Answer: C. 6/12 = 0.50. 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