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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.
- Spring '14