Definition

RISC (reduced instruction set computer)

RISC (reduced instruction set computer) is a microprocessor that is designed to perform a smaller number of types of computer instructions so that it can operate at a higher speed (perform more millions of instructions per second, or MIPS). Since each instruction type that a computer must perform requires additional transistors and circuitry, a larger list or set of computer instructions tends to make the microprocessor more complicated and slower in operation.

John Cocke of IBM Research in Yorktown, New York, originated the RISC concept in 1974 by proving that about 20% of the instructions in a computer did 80% of the work. The first computer to benefit from this discovery was IBM's PC/XT in 1980. Later, IBM's RISC System/6000, made use of the idea. The term itself (RISC) is credited to David Patterson, a teacher at the University of California in Berkeley. The concept was used in Sun Microsystems' SPARC microprocessors and led to the founding of what is now MIPS Technologies, part of Silicon Graphics. A number of current microchips now use the RISC concept.

The RISC concept has led to a more thoughtful design of the microprocessor. Among design considerations are how well an instruction can be mapped to the clock speed of the microprocessor (ideally, an instruction can be performed in one clock cycle); how "simple" an architecture is required; and how much work can be done by the microchip itself without resorting to software help.

Besides performance improvement, some advantages of RISC and related design improvements are:

  • A new microprocessor can be developed and tested more quickly if one of its aims is to be less complicated.
  • Operating system and application programmers who use the microprocessor's instructions will find it easier to develop code with a smaller instruction set.
  • The simplicity of RISC allows more freedom to choose how to use the space on a microprocessor.
  • Higher-level language compilers produce more efficient code than formerly because they have always tended to use the smaller set of instructions to be found in a RISC computer.

After the introduction of RISC, any "full-set" instruction computer was said to use complex instruction set computing (CISC).

Contributor(s): Steve Brazier, Hrvoje Cekolj, and Marcus Gillette
This was last updated in September 2005
Posted by: Margaret Rouse

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