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CISC (Complex Instruction Set Computer)

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Computer processors contain instructions or microcode to carry out various functions. The richer the instruction set, the easier it is to write programs for the microprocessor, but a rich set of microcode can affect performance. This trade-off differentiates the two categories of microprocessors:

  • CISC (Complex Instruction Set Computer) designs include a rich set of microcode that simplifies the creation of programs that run on the processor.

  • RISC (Reduced Instruction Set Computer) designs, as the name implies, have a reduced set of instructions that improves the efficiency of the processor but requires more complex external programs.

RISC designs are based on work performed at IBM by John Cocke, who found that about 20 percent of a computer's instructions did about 80 percent of the work. Thus, RISC-designed systems are generally faster than CISC systems. His 80/20 rule spawned the development of RISC architecture.

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