Flynn's taxonomy

Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1966. The classification system has stuck, and has been used as a tool in design of modern processors and their functionalities. Since the rise of multiprocessing central processing units (CPUs), a multiprogramming context has evolved as an extension of the classification system.

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

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 older personal computers (PCs; by 2010, many PCs had multiple cores) and mainframe computers.

A computer which exploits multiple data streams against a single stream to perform operations which may be naturally parallelized. For example, an array processor or graphics processing unit (GPU)

Multiple instructions operate on one 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 autonomous processors simultaneously executing different instructions on different data. MIMD architectures include multi-core superscalar processors, and distributed systems, using either one shared memory space or a distributed memory space.

...
Wikipedia