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One of the most important considerations in understanding the performance capabilitiesof a modern processor is the memory hierarchy. Unfortunately, as we haveseen, not all memory is created equal, and some types are far less efficient andthus cheaper than others. To deal with this disparity, today’s computer systemsuse a combination of memory types to provide the best performance at the bestcost. This approach is called hierarchical memory. As a rule, the faster memoryis, the more expensive it is per bit of storage. By using a hierarchy of memories,each with different access speeds and storage capacities, a computer system canexhibit performance above what would be possible without a combination of thevarious types. The base types that normally constitute the hierarchical memorysystem include registers, cache, main memory, and secondary memory.Today’s computers each have a small amount of very high-speed memory,called a cache, where data from frequently used memory locations may be temporarilystored. This cache is connected to a much larger main memory, which istypically a medium-speed memory. This memory is complemented by a verylarge secondary memory, composed of a hard disk and various removable media.By using such a hierarchical scheme, one can improve the effective access speedof the memory, using only a small number of fast (and expensive) chips. Thisallows designers to create a computer with acceptable performance at a reasonablecost.We classify memory based on its “distance” from the processor, with distancemeasured by the number of machine cycles required for access. The closer memoryis to the processor, the faster it should be. As memory gets further from themain processor, we can afford longer access times. Thus, slower technologies areused for these memories, and faster technologies are used for memories closer tothe CPU. The better the technology, the faster and more expensive the memorybecomes. Thus, faster memories tend to be smaller than slower ones, due to cost.The following terminology is used when referring to this memory hierarchy:• Hit—The requested data resides in a given level of memory (typically, we areconcerned with the hit rate only for upper levels of memory).• Miss—The requested data is not found in the given level of memory.• Hit rate—The percentage of memory accesses found in a given level of memory.• Miss rate—The percentage of memory accesses not found in a given level ofmemory. Note: Miss Rate = 1 - Hit Rate.• Hit time—The time required to access the requested information in a givenlevel of memory.
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