Computer Architecture: Fynn's Classification

Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1966. The four classifications defined by Flynn are based upon the number of concurrent instruction (or control) and data streams available in the architecture:

Single Instruction, Single Data stream (SISD)

sequential computer
exploits no parallelism in either the instruction or data streams.
Single control unit (CU)
fetches single Instruction Stream (IS) from memory
single Data Stream (DS) i.e. one operation at a time.

Examples of SISD architecture are the traditional uniprocessor machines like a PC (currently manufactured PCs have multiple processors) or old mainframes.

Single Instruction, Multiple Data streams (SIMD)

exploits multiple data streams
single instruction stream to perform operations which may be naturally parallelized.

For example, an array processor or GPU.

Multiple Instruction, Single Data stream (MISD)

Multiple instructions
single data stream.
Uncommon architecture
generally used for fault tolerance
Heterogeneous systems operate on the same data stream and must agree on the result.

Examples include the Space Shuttle flight control computer.

Multiple Instruction, Multiple Data streams (MIMD)

Multiple autonomous processors
simultaneously executing different instructions
on different data.

Example : Distributed systems are generally recognized to be MIMD architectures; either exploiting a single shared memory space or a distributed memory space. A multi-core superscalar processor is an MIMD processor.