EENG-155Computer ArchChapter 5 Large and Fast Exploiting Memory Hierarchy.ppt

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EENG-155Computer ArchChapter 5 Large and Fast Exploiting Memory Hierarchy.ppt

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EENG-155Computer ArchChapter 5 Large and Fast Exploiting Memory Hierarchy

Chapter 5 — Large and Fast: Exploiting Memory Hierarchy Morgan Kaufmann Publishers Chapter 5 — Large and Fast: Exploiting Memory Hierarchy Chapter 5 Large and Fast: Exploiting Memory Hierarchy Memory Technology Static RAM (SRAM) 0.5ns – 2.5ns, $2000 – $5000 per GB Dynamic RAM (DRAM) 50ns – 70ns, $20 – $75 per GB Magnetic disk 5ms – 20ms, $0.20 – $2 per GB Ideal memory Access time of SRAM Capacity and cost/GB of disk Principle of Locality Programs access a small proportion of their address space at any time Temporal locality Items accessed recently are likely to be accessed again soon e.g., instructions in a loop, induction variables Spatial locality Items near those accessed recently are likely to be accessed soon E.g., sequential instruction access, array data Taking Advantage of Locality Memory hierarchy Store everything on disk Copy recently accessed (and nearby) items from disk to smaller DRAM memory Main memory Copy more recently accessed (and nearby) items from DRAM to smaller SRAM memory Cache memory attached to CPU Memory Hierarchy Levels Block (aka line): unit of copying May be multiple words If accessed data is present in upper level Hit: access satisfied by upper level Hit ratio: hits/accesses If accessed data is absent Miss: block copied from lower level Time taken: miss penalty Miss ratio: misses/accesses= 1 – hit ratio Then accessed data supplied from upper level Cache Memory Cache memory The level of the memory hierarchy closest to the CPU Given accesses X1, …, Xn–1, Xn Direct Mapped Cache Location determined by address Direct mapped: only one choice (Block address) modulo (#Blocks in cache) Tags and Valid Bits How do we know which particular block is stored in a cache location? Store block address as well as the data Actually, only need the high-order bits Called the tag What if there is no data in a location? Valid bit: 1 = present, 0 = not present Initially 0 Cache Example 8-blocks, 1 word/block, direct mapped Initial state Cache Example Cache Exa