A shorter development time a simple processor should

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Unformatted text preview: e! The previous two advantages are easy to accept, but in a world where higher performance had been sought through ever-increasing complexity, this was a bit hard to swallow. The argument goes something like this: smaller things have higher natural frequencies (insects flap their wings faster than small birds, small birds faster than 26 An Introduction to Processor Design large birds, and so on) so a simple processor ought to allow a high clock rate. So let's design our complex processor by starting with a simple one, then add complex instructions one at a time. When we add a complex instruction it will make some high-level function more efficient, but it will also slow the clock down a bit for all instructions. We can measure the overall benefit on typical programs, and when we do, all complex instructions make the program run slower. Hence we stick to the simple processor we started with. These arguments were backed up by experimental results and the prototype processors (the Berkeley RISC II came shortly after RISC I). The commercial processor companies were sceptical at first, but most new companies designing processors for their own purposes saw an opportunity to reduce development costs and get ahead of the game. These commercial RISC designs, of which the ARM was the first, showed that the idea worked, and since 1980 all new general-purpose processor architectures have embraced the concepts of the RISC to a greater or lesser degree. RISC in retrospect Since the RISC is now well established in commercial use it is possible to look back and see more clearly what its contribution to the evolution of the microprocessor really was. Early RISCs achieved their performance through: Pipelining. Pipelining is the simplest form of concurrency to implement in a processor and delivers around two to three times speed-up. A simple instruction set greatly simplifies the design of the pipeline. A high clock rate with single-cycle execution. In 1980 standard semiconductor memories (DRAMs - Dynamic Random Access Memories) could...
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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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