Other complex instructions need not be supported in

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Unformatted text preview: er in this chapter during the discussion of memory). CPUs with large instruction set, variable-length instructions, and a variety of addressing modes are said to employ CISC (Complex Instruction Set Computer) architecture. Since CISC processors possess so many processing features, they make the job of machine language programmers easier. However, they are complex and expensive to produce. Most of the personal computers of today use CISC processors. In early 1980s, some CPU designers discovered that several of the instructions supported by a CISC-based CPU are rarely used. Hence they came out with an idea that the complexity of CPU design can be greatly reduced by implementing only a bare minimum basic set of instructions plus some of the more frequently used instructions in the hardware circuitry of the CPU. Other complex instructions need not be supported in the instruction set of the CPU because they can always be implemented in software by using the basic set of instructions. While working on simpler CPU design, these designers also came up with the idea of making all the instructions of uniform length so that the decoding and execution of all instructions becomes simple and fast. Furthermore, to speed up computation and to reduce the complexity of handling a number of addressing modes, they decided to design al the instructions in such a way that they retrieve operands stored in registers in CPU rather than from memory These design ideas resulted in producing faster and less expensive processors. CPUs with a small instruction set fixed-length instructions, and reduced references to memory to retrieve operands are said to employ RISC (Reduced Instruction Set Computer) architecture. Since RISC processors have a small instruction set, they place extra demand on programmers, who must consider how to implement complex computations by combining simple instructions. However, due to simpler design, RISC processors are faster for most applications, less complex, and less expensive to produce than CISC processors. RISC processors are used extensively in medium size computers such as the IBM RS/6000 and high...
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This document was uploaded on 04/07/2014.

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