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Complex instruction set computing (SISC)

Complex instruction set computing (CISC /ˈsɪsk/) is a processor design where single instructions can execute several low-level operations (such as a load from memory, anarithmetic operation, and a memory store) or are capable of multi-step operations or addressing modes within single instructions. The term was retroactively coined in contrast toreduced instruction set computer (RISC)[1][2] and has therefore become something of an umbrella term for everything that is not RISC, from large and complex mainframe computersto simplistic microcontrollers where memory load and store operations are not separated from arithmetic instructions.

A modern RISC processor can therefore be much more complex than, say, a modern microcontroller using a CISC-labeled instruction set, especially in terms of electronic circuit complexity, but also in terms of the number of instructions or the complexity of their encoding patterns. The only typical differentiating characteristic is that most RISC designs use uniform instruction length for almost all instructions, and employ strictly separate load/store-instructions.

Examples of instruction set architectures that have been retroactively labeled CISC are System/360 through z/Architecture, the PDP-11 and VAX architectures, Data General Nova and many others. Well known microprocessors and microcontrollers that have also been labeled CISC in many academic publications include the Motorola 68006809 and 68000-families; the Intel 8080iAPX432 and x86-family; the Zilog Z80Z8 and Z8000-families; the National Semiconductor 32016 and NS320xx-line; the MOS Technology 6502-family; the Intel8051-family; and others.

Some designs have been regarded as borderline cases by some writers. For instance, the Microchip Technology PIC has been labeled RISC in some circles and CISC in others and the6502 and 6809 have both been described as “RISC-like”, although they have complex addressing modes as well as arithmetic instructions that access memory, contrary to the RISC-principles.