Flynn's Taxonomy - Classifications

Classifications

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)

A sequential computer which exploits no parallelism in either the instruction or data streams. Single control unit (CU) fetches single Instruction Stream (IS) from memory. The CU then generates appropriate control signals to direct single processing element (PE) to operate on 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)

A computer which exploits multiple data streams against a 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 operate on a single data stream. Uncommon architecture which is 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. 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.