Flynn's taxonomy is a system for classifying computer architectures. It is based on two main concepts:

• Instruction flow: A system with n CPU has n program counters and, therefore, n instructions flows. This corresponds to a program counter.
• Data flow: A program that calculates a function on a list of data has a data flow. The program that calculates the same function on several different lists of data has more data flows. This is made up of a set of operands.

As the instruction and data flows are independent, there are four categories of parallel machines: Single Instruction Single Data (SISD), Single Instruction Multiple Data (SIMD), Multiple Instruction Single Data (MISD), and Multiple Instruction Multiple Data (MIMD):