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Computer pioneers correctly predicted that programmers would want unlimited amounts of fast memory. An economical solution to that desire is a memory hierarchy, which takes advantage of locality and cost/performance of memory technologies. The principle of locality, presented in the first chapter, says that most programs do not access all code or data uniformly (see section 1.6, page 38). This principle, plus the guideline that smaller hardware is faster, led to the hierarchy based on memories of different speeds and sizes. Since fast memory is expensive, a memory hierarchy is organized into several levels-each smaller, faster, and more expensive per byte than the next level. The goal is to provide a memory system with cost almost as low as the cheapest level of memory and speed almost as fast as the fastest level. The levels of the hierarchy usually subset one another; all data in one level is also found in the level below, and all data in that lower level is found in the one below it, and so on until we reach the bottom of the hierarchy. Note that each level maps addresses from a larger memory to a smaller but faster memory higher in the hierarchy.
