RISC is an abbreviation for reduced instruction set computer.The example of RISC machine are DEC's Alpha, powerPC,ULTRASPARC,etc.CISC is an abbreviation for complex instruction set computer.The examples of CISC machines are:Intel 80486,pentium,pentium II,celeron,Pentium III,etc.
The aim of employing RISC design is to increase the computing speed by reducing the execution time of instructions.It is seen that usually 80% of a computers's instructions are used less frequently,only for the 20% of the time.The 20% of the instructions are used more frequently,for the 80% of the time.The philosophy of RISC machine is based on the fact that frequently used 20% of the instructions are implemented in the hardwired circuitry of the chip itself i.e.by hardware technique.The remaining 80% of the instructions are implemented by computer's software. Almost all computations are performed using simple instructions.Usually, a RISC machine is 5 to 10 times faster than a CISC machine.
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