Flynn’s Classifications in Computer System Architecture

Flynn’s Classifications – Michael J. Flynn is an American Professor Emeritus at Stanford University. He classifies computer architecture into four categories to enhance the computational speed of the computer.

The four classifications defined by Flynn are based upon the number of concurrent instruction (or control) and data streams available in the architecture.

Flynn’s Classifications

1. Single Instruction, Single Data Stream (SISD)
2. Single Instruction, Multiple Data Stream (SIMD)
3. Multiple Instruction, Single Data Stream (MISD)
4. Multiple Instruction, Multiple Data Stream (MIMD)

Single Instruction, Single Data (SISD) Stream

In computing, SISD is a term referring to a computer architecture in which a single processor executes a single instruction stream to operate on data stored in a single memory. This can be considered as the V_ON Neumann concept of serial computer design.

Every SISD computer has a special register, the program counter (PC), to ensure the serial execution of the program. Examples of SISD architecture are the traditional uniprocessor machine, like a personal computer (PC) or old mainframes, as shown in the diagram below.

Single Instruction Multiple Data (SIMD) Stream

The SIMD refers to a computer architecture in which a single instruction is applied to different data simultaneously. In such architecture, there are several processing units that are controlled by a control unit.

It exploits multiple data streams against a single instruction stream to perform operations that may be naturally parallelized. The examples of SIMD architectures are an array processor or GUP, as shown in the diagram below.

Multiple Instruction Single Data (MISD) Stream

The MISD architecture is a type of parallel computing architecture where many processing units perform different operations on the single data as shown in the image below. Pipeline architecture is an example of this classification.

It is a class of machines in which the data flows through a series of processing units. This uncommon architecture is generally used for fault tolerance. Heterogeneous systems operate on the same data stream and must agree on th result.

The space shuttle flight control computer is an example of this model.

Multiple Instruction Multiple Data (MIMD) Stream

MIMD architecture includes a machine with several processing units in which multiple instructions can be applied to different data simultaneously. Multiple autonomous processors simultaneously execute different instructions on different data.

Distributed systems are generally recognized to be MIMD architectures, either exploiting a single shared memory space or a distributed space. MIMD architecture may be used in a number of application areas, such as CAD/CAM, simulation, modeling, and as communication switches.