Instruction Set - Classification of Instruction Sets

Classification of Instruction Sets

A complex instruction set computer (CISC) has many specialized instructions, which may only be rarely used in practical programs. A reduced instruction set computer (RISC) simplifies the processor by only implementing instructions that are frequently used in programs; unusual operations are implemented as subroutines, where the extra processor execution time is offset by their rare use. Theoretically important types are the minimal instruction set computer and the one instruction set computer but these are not implemented in commercial processors. Another variation is the very long instruction word (VLIW) where the processor receives many instructions encoded and retrieved in one instruction word.