Summary of "Computer Organization & Architecture ( COA ) 02 | Cache Memory | CS & IT | GATE 2025 Crash Course"

Summary of "Computer Organization & Architecture (COA) 02 | Cache Memory | CS & IT | GATE 2025 Crash Course"

Main Ideas and Concepts

  1. Introduction to Cache Memory
    • Cache Memory is a crucial topic in Computer Organization & Architecture (COA) and frequently appears in GATE exams.
    • The course aims to cover COA basics and Cache Memory comprehensively in a few days.
  2. Memory Hierarchy
    • Memory in a computer is organized hierarchically to balance speed, cost, and size.
    • Levels include:
      • Registers and Cache Memory (small, very fast, inside CPU)
      • Main Memory (RAM/ROM) (larger but slower)
      • Secondary Storage (HDD, SSD, tape drives for permanent storage)
    • Larger memories are slower and cheaper; smaller memories are faster and costlier.
    • The CPU is very fast and needs quick data access to avoid waiting, hence the need for cache.
  3. Why Cache Memory?
    • The CPU executes instructions sequentially, fetching data from memory.
    • Main Memory is slower compared to CPU speed, causing delays.
    • Cache Memory acts as a small, fast memory between CPU and Main Memory to reduce access time.
    • Only frequently or recently used data/instructions are stored in cache (principle of locality).
  4. Locality of Reference
    • Temporal locality: Recently accessed data/instructions are likely to be accessed again soon.
    • Spatial locality: Data/instructions near recently accessed locations are likely to be accessed soon.
    • Cache uses this principle to anticipate future CPU requests and pre-load data.
  5. Cache Memory Operation
    • Cache stores copies of data from Main Memory.
    • When CPU needs data:
      • It first checks cache (cache hit if found).
      • If not found (cache miss), it fetches from Main Memory and loads a block of data (including surrounding data) into cache.
    • Cache Memory reduces average memory access time.
  6. Cache Hit, Miss, and Hit Ratio
    • Cache Hit: Requested data found in cache; fast access.
    • Cache Miss: Data not in cache; slower access from Main Memory.
    • Hit Ratio: Fraction of accesses that result in cache hits.
    • Miss Ratio: Fraction of accesses that result in cache misses.
    • Formulas:
      • Hit Ratio = Number of Hits / Total Memory References
      • Miss Ratio = Number of Misses / Total Memory References
  7. Average Memory Access Time (AMAT)
    • Without cache: AMAT = Main Memory Access Time.
    • With cache: AMAT = (Hit Ratio × Cache Access Time) + (Miss Ratio × Miss Penalty).
    • Miss penalty includes time to access Main Memory plus any block transfer time.
    • Two access models:
      • Simple (serial) access: Cache checked first; if miss, then Main Memory accessed.
      • Hierarchical (parallel) access: Cache and Main Memory accessed simultaneously; result depends on hit/miss.
  8. Cache Memory Write Policies
    • Write-Through Cache:
      • CPU writes data to both cache and Main Memory simultaneously.
      • Ensures data consistency but is slower due to multiple writes.
    • Write-Back Cache:
      • CPU writes data only to cache initially.
      • Modified cache blocks (dirty blocks) are written back to Main Memory only when replaced.
      • Faster but may cause data inconsistency temporarily.
    • Write Allocate vs. No Write Allocate:
      • On write miss, write allocate loads the block into cache before writing.
      • No write allocate writes directly to Main Memory without loading cache.
  9. Cache Block Replacement
    • When cache is full, new blocks replace existing ones.
    • If replaced block is dirty (modified), it must be written back to Main Memory in write-back policy.
    • Replacement policies affect cache efficiency and consistency.
  10. Practical Examples and Analogies
    • Tea-making analogy to explain caching: small box of jaggery (cache) vs. big storage box (Main Memory).
    • Classroom analogy to explain locality of reference.
  11. Exam Preparation Tips
    • Understand key formulas for hit ratio, miss ratio, and average memory access time.
    • Recognize question cues for hierarchical vs. simple access in exams.
    • Practice problems involving cache hit/miss times and speedup calculations.
    • Revise write policies and their implications thoroughly as questions frequently arise from these topics.

Methodology / Instructions for Cache Memory Concepts

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