Memory Hierarchy Motivation, Definitions, Four Questions about Memory Hierarchy, Improving Performance

Memory Hierarchy— Motivation, Definitions, Four Questions about Memory Hierarchy, Improving Performance Professor Alvin R. Lebeck Computer Science 220...
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Memory Hierarchy— Motivation, Definitions, Four Questions about Memory Hierarchy, Improving Performance Professor Alvin R. Lebeck Computer Science 220 ECE 252 Fall 2006

Admin • • • • •

Some stuff will be review…some will not Homework #4 Work on your projects Costi will lecture next Tuesday There will be two papers to read

© Alvin R. Lebeck 2006

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Outline of Today’s Lecture • • • • • • • • •

Most of today should be review…later stuff will not The Memory Hierarchy Direct Mapped Cache. Two-Way Set Associative Cache Fully Associative cache Replacement Policies Write Strategies Memory Hierarchy Performance Improving Performance

© Alvin R. Lebeck 2006

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The Big Picture: Where are We Now? • The Five Classic Components of a Computer Processor Input Control Memory Datapath

Output

• Today’s Topic: Memory System

© Alvin R. Lebeck 2006

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Who Cares About the Memory Hierarchy? • Processor Only Thus Far in Course: – CPU cost/performance, ISA, Pipelined Execution 1000

CPU

CPU-DRAM Gap 100

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DRAM

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• 1980: no cache in µproc; 1995 2-level cache, 60% trans. on Alpha 21164 µproc (150 clock cycles for a miss!) © Alvin R. Lebeck 2006

CPS 220

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The Motivation for Caches

Memory System

Processor

Cache

DRAM

• Motivation: – Large memories (DRAM) are slow – Small memories (SRAM) are fast

• Make the average access time small by: – Servicing most accesses from a small, fast memory.

• Reduce the bandwidth required of the large memory © Alvin R. Lebeck 2006

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Levels of the Memory Hierarchy Upper Level

Capacity Access Time Cost

Staging Xfer Unit

CPU Registers 100s Bytes 1 / DRAM write cycle – If this condition exist for a long period of time (CPU cycle time too quick and/or too many store instructions in a row): » Store buffer will overflow no matter how big you make it » The CPU Cycle Time

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