Complex instruction set computing

Complex Instruction Set Computer ( CISC ) ​​(English for computer complex instruction set ) is a design philosophy for computer processors. The name is a retronym, which was marked with the introduction of RISC processors. The Intel x86 family of processors ( up to and including 80386 ) is one of the most famous representatives of this concept.


Initially, the instruction sets of processors were more extensive, in order to perform more complex calculation steps " at once " with only one machine instruction may thereby to be faster and more powerful. However, at the same time by the processor also is complex and difficult to develop. Many manufacturers initially focused on the micro- programming of the arithmetic units in order to correct problem cases more easily - yet the complexity increased more and more.

The term CISC was chosen in the 1970s by IBM to define classical instruction sets of a novel form, the Reduced Instruction Set Computer (RISC). A CISC instruction set is characterized by many relatively powerful individual commands, whereas RISC largely dispensed with in favor of a high execution speed and a lower decoding effort on complex commands.

CPUs with CISC instruction set were microprogrammed long time. Today you can find hardly microprogrammed CISC CPUs. From the Pentium Pro, Intel processors have an upstream functional unit that translates the complex instructions into RISC instructions. Depending on the manufacturer and this CPU units ROP, micro-op or μOp be called. Other examples of CPUs with CISC instruction set are the Intel 8086, Intel 80386, Motorola 68000, Zilog Z80 and the CPUs of the System z series of IBM.