Figure 98 shows the results of this experiment

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Unformatted text preview: for a particular combination of program and data, the machine will execute a fixed sequence of instructions. Instruction execution is controlled by a processor clock that is regulated by a precision oscillator. There are many factors, however, that can vary from one execution of a program to another. Computers do not simply execute one program at a time. They continually switch from one process to another, executing some code on behalf of one process before moving on to the next. The exact scheduling of processor resources for one program depends on such factors as the number of users sharing the system, the network traffic, and the timing of disk operations. The access patterns to the caches depend not just on the references made by the program we are trying to measure, but on those of other processes executing concurrently. Finally, the branch prediction logic tries to guess whether branches will be taken or not based on past history. This history can vary from one execution of a program to another. In this chapter, we describe two basic...
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