L19 - Cache Review ECS 154B Computer Architecture II Winter...

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ECS 154B Computer Architecture II Winter 2008 Caches 7.2-7.3 2 Cache Review • Caches attempt to hide delay from using large, slow memory by using small, fast memory – Data used frequently is copied from slow memory and stored in fast memory, the cache – Utilizes locality of data • Temporal locality: recently referenced data tends to be used again soon • Spatial locality: data near recently referenced data tends to be used soon • A hit occurs when requested data is in the cache • A miss occurs when requested data is not in the cache and must be retrieved from slow memory • Forms part of memory hierarchy 3 Cache Performance • Before we ignored cache misses • Now consider miss penalties • Variables: – I: Number of instructions – P: Clock period (cycle time) – C: Number of clock cycles • Before: T = I • CPI • P = C • P • Now: T = (C Instructions + C Memory ) • P 4 Cache Performance • Example: – 100 cycle miss penalty – CPI = 2 without stalls (misses) – 2% instruction cache miss rate – 4% data cache miss rate – 36% of instructions are memory reference • What is the CPI counting cache misses? – CPI = Execution CPI + I Cache CPI + D Cache CPI
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L19 - Cache Review ECS 154B Computer Architecture II Winter...

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