CISC (complex instruction set computers) are a broad type of instruction set architecture where single instructions can execute several low-level operations, and can span more than one word of memory.
This is contrasted with RISC, which restricts instructions to one word. CISC was prevalent prior to the introduction of RISC on the market, and very few commercial architectures use CISC. A key example is x86.
Some key properties of CISC architecture:
- Memory access is directly available to most types of instructions.
- There are substantial numbers of addressing modes.
- Instruction formats are of different lengths.
- Instructions can perform both elementary and complex operations.
The idea with CISC is to closer match the operations used in programming languages.