Reduced instruction set computer

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Reduced instruction set computing, or RISC (pronounced /ˈrɪsk/), is a CPU design strategy based on the insight that simplified (as opposed to complex) instructions can provide higher performance if this simplicity enables much faster execution of each instruction. A computer based on this strategy is a reduced instruction set computer (also RISC). There are many proposals for precise definitions,[1] but the term is slowly being replaced by the more descriptive load-store architecture. Well known RISC families include DEC Alpha, AMD 29k, ARC, ARM, Atmel AVR, MIPS, PA-RISC, Power (including PowerPC), SuperH, and SPARC.

Some aspects attributed to the first RISC-labeled designs around 1975 include the observations that the memory-restricted compilers of the time were often unable to take advantage of features intended to facilitate manual assembly coding, and that complex addressing modes take many cycles to perform due to the required additional memory accesses. It was argued that such functions would be better performed by sequences of simpler instructions if this could yield implementations small enough to leave room for many registers,[2] reducing the number of slow memory accesses. In these simple designs, most instructions are of uniform length and similar structure, arithmetic operations are restricted to CPU registers and only separate load and store instructions access memory. These properties enable a better balancing of pipeline stages than before, making RISC pipelines significantly more efficient and allowing higher clock frequencies.


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