CISC ( English complex instruction set computing or complex instruction set computer ) is a type of processor architecture , which is characterized by the following set of properties:
- non-fixed value of command length;
- arithmetic operations are encoded in one command;
- a small number of registers , each of which performs a strictly defined function.
The methodology for building a CISC instruction system resists the methodology used in another common type of processor architectures - RISC , which uses a set of simplified instructions.
Typical representatives of the CISC architecture are x86- based processors , Motorola MC680x0 processors, and zSeries mainframe processors.
Due to the prevalence of x86 and x86-64 architecture processors , CISC systems are the most widespread in the world of computing - they dominate in the segments of workstations , personal computers , entry-level and mid-level servers , as well as mainframes (RISC systems prevail in the segments of mobile devices, microcontrollers and Unix servers of a higher level).
At the same time, the late x86 processors ( Intel Pentium 4 , Pentium D , Core , AMD Athlon , Phenom ), although CISC-compatible, are processors with a RISC core and are considered to be hybrid in the formal sense  . In such hybrid CISC processors, CISC instructions are converted to a set of internal RISC commands, while one x86 command can generate several RISC commands (in the case of processors like P6 , up to four RISC commands in most cases), the execution of commands occurs on a superscalar several times at a time.
The main drawback of CISC architecture in comparison with RISC is a more complex approach to parallelizing computations. .  
- Computer architecture
- CISC in the Open Directory Project Link Directory (dmoz)