Reduced Instruction Set Computing

Reduced instruction set computing, or RISC ( /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 called RISC. The opposing architecture is known as complex instruction set computing, i.e. CISC.

Various suggestions have been made regarding a precise definition of RISC, but the general concept is that of a system that uses a small, highly-optimized set of instructions, rather than a more specialized set of instructions often found in other types of architectures. Another common trait is that RISC systems use the load/store architecture, where memory is normally accessed only through specific instructions, rather than accessed as part of other instructions like add.

Although a number of systems from the 1960s and 70s have been identified as being forerunners of RISC, the modern version of the design dates to the 1980s. In particular, two projects at Stanford University and Berkeley University are most associated with the popularization of the concept. Stanford's design would go on to be commercialized as the successful MIPS architecture, while Berkeley's RISC gave its name to the entire concept, commercialized as the SPARC. Another success from this era were IBM's efforts that eventually lead to the Power Architecture. As these projects matured, a wide variety of similar designs flourished in the late 1980s and especially the early 1990s, representing a major force in the Unix workstation market as well as embedded processors in laser printers, routers and similar products.

Well known RISC families include DEC Alpha, AMD 29k, ARC, ARM, Atmel AVR, Blackfin, Intel i860 and i960, MIPS, Motorola 88000, PA-RISC, Power (including PowerPC), SuperH, and SPARC. In the 21st century, the use of ARM architecture processors in smart phones and tablet computers such as the iPad and Android tablets provided a wide user base for RISC-based systems. RISC processors are also used in supercomputers such as the K computer, the fastest on the TOP500 list in 2011, and the second at the 2012 list.