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Processor references that are found in the cache are called cache hits. References not found in the cache are called cache misses. On a cache miss, the cache control mechanism must fetch the missing data from memory and place it in the cache. Usually the cache fetches a spatial locality called the line from memory. The line consists of one or more physical words accessed from main memory. The physical word is the basic unit of access in the memory. |
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The processor-cache interface can be characterized by a number of parameters. Those that directly affect processor performance (Figure 5.3) include: |
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1. Access time for a reference found in the cache (a hit) property of the cache size and organization. |
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2. Access time for a reference not found in the cache (a miss) property of the memory organization. |
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3. Time to initially compute a real address given a virtual address (not-inTLB time) property of the address translation facility, which, though strictly speaking, is not part of the cache, resembles the cache in most aspects and is discussed in this chapter. |
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From the cache's point of view, the processor behavior is integral to an effective design. This includes such issues as: |
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1. The number of requests per cycleproperty of the instruction set and the CPU organization. |
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