I/O and Memory Organization Overview

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Memory Hierarchy (8 Levels)

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Principle of Locality

The memory hierarchy works because of two observed properties of program execution:

Temporal Locality: If a memory location is accessed, it is likely to be accessed again soon.

• Example: Loop variable i is accessed every iteration
• Solution: Keep recently used data in cache

Spatial Locality: If a location is accessed, nearby locations are likely accessed soon.

• Example: Sequential array traversal, program instructions execute in order
• Solution: Cache fetches entire cache lines (typically 64 bytes) not just one word

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Study Deep: The Cost-Speed-Capacity Triangle

The fundamental trade-off in memory design — you can only optimize two of three:

The memory hierarchy is the engineering solution: use fast-expensive memory for frequently accessed data, slow-cheap memory for bulk storage, and rely on locality to make this effective.

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Unit IV Lessons Overview

📝 Exam Tip: The memory hierarchy is a classic exam topic. Remember the access time order: Registers (~0.3 ns) → L1 (~1 ns) → L2 (~5 ns) → L3 (~20 ns) → RAM (~60 ns) → SSD (~100 µs) → HDD (~10 ms). Each level is 5–100× slower than the previous.