Reduced Instruction Set Computer

(processor)

Reduced Instruction Set Computer - (RISC) A processor whose design is based on the rapid execution of a sequence of simple instructions rather than on the provision of a large variety of complex instructions (as in a Complex Instruction Set Computer). Features which are generally found in RISC designs are uniform instruction encoding (e.g. the op-code is always in the same bit positions in each instruction which is always one word long), which allows faster decoding; a homogenous register set, allowing any register to be used in any context and simplifying compiler design; and simple addressing modes with more complex modes replaced by sequences of simple arithmetic instructions. Examples of (more or less) RISC processors are the Berkeley RISC, HP-PA, Clipper, i960, AMD 29000, MIPS R2000 and DEC Alpha. IBM's first RISC computer was the RT/PC (IBM 801), they now produce the RISC-based RISC System/6000 and SP/2 lines. Despite Apple Computer's bogus claims for their PowerPC-based Macintoshes, the first RISC processor used in a personal computer was the Advanced RISC Machine (ARM) used in the Acorn Archimedes.