This technique is known as the delayed branch

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Unformatted text preview: sonably simple processors, there are significant benefits in introducing pipelines from three to five stages long, but beyond this the law of diminishing returns begins to apply and the added costs and complexity outweigh the benefits. Pipelines clearly benefit from all instructions going through a similar sequence of steps. Processors with very complex instructions where every instruction behaves differently from the next are hard to pipeline. In 1980 the complex instruction set microprocessor of the day was not pipelined due to the limited silicon resource, the limited design resource and the high complexity of designing a pipeline for a complex instruction set. 1.6 The Reduced Instruction Set Computer In 1980 Patterson and Ditzel published a paper entitled 'The Case for the Reduced Instruction Set Computer' (a full reference is given in the bibliography on page 410). In this seminal work they expounded the view that the optimal architecture for a single-chip processor need not be the same as the optimal architecture for a multi-chip processor. Their argument was subsequently supported by the results of a processor design project undertaken by a postgraduate class at Berkeley which incorporated a Reduced Instruction Set Computer (RISC) architecture. This design, the Berkeley RISC I, was much simpler than the commercial CISC processors of the day and had taken an order of magnitude less design effort to develop, but nevertheless delivered a very similar performance. The RISC I instruction set differed from the minicomputer-like CISC instruction sets used on commercial microprocessors in a number of ways. It had the following key features: RISC architecture A fixed (32-bit) instruction size with few formats; CISC processors typically had variable length instruction sets with many formats. A load-store architecture where instructions that process data operate only on registers and are separate from instructions that access memory; CISC processors typically allowed values in memory to be used as operands in data processing instructions. The Reduced Instruction Set Computer 25 A large register...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.

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