Complex Instruction Set Computer [CISC]

A complex instruction set computer (CISC /ˈsɪsk/) is a computer architecture in which single instructions can execute several low-level operations (such as a load from memory, an arithmetic 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 to reduced instruction set computer (RISC) and has therefore become something of an umbrella term for everything that is not RISC, where the typical differentiating characteristic^{[dubious – discuss]} is that most RISC designs use uniform instruction length for almost all instructions, and employ strictly separate load and store instructions.

Examples of CISC architectures include complex mainframe computers to simplistic microcontrollers where memory load and store operations are not separated from arithmetic instructions. Specific instruction set architectures that have been retroactively labeled CISC are System/360 through z/Architecture, the PDP-11 and VAX architectures, and many others. Well known microprocessors and microcontrollers that have also been labeled CISC in many academic publications^{[citation needed]} include the Motorola 6800, 6809 and 68000-families; the Intel 8080, iAPX432 and x86-family; the Zilog Z80, Z8 and Z8000-families; the National Semiconductor 32016 and NS320xx-line; the MOS Technology 6502-family; the Intel 8051-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. The 6502 and 6809 have both been described as RISC-like, although they have complex addressing modes as well as arithmetic instructions that operate on memory, contrary to the RISC principles.