Flynn's taxonomy

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Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1972. 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. Examples of SISD architecture are the traditional uniprocessor machines like a PC or old mainframes.
Multiple instruction, single data stream (MISD) - unusual due to the fact that multiple instruction streams generally require multiple data streams to be effective. However, this type is used when it comes to redundant parallelism, as for example on airplanes that need to have several backup systems in case one fails. Some theoretical computer architectures have also been proposed which make use of MISD, but none have entered mass production.
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 parallelised. For example, an array processor.
Multiple instruction, multiple data streams (MIMD) - multiple autonomous processors simultaneously executing different instructions on different data. Distributed systems are generally recognised to be MIMD architectures; either exploiting a single shared memory space or a distributed memory space.

There is also a common extension to this taxonomy which is:

Single Program, multiple data streams (SPMD) - multiple autonomous processors simultaneously executing the same instruction set (but at independent points) on different data. Also referred to as 'Single Process, multiple data'.

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References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

A Survey of Parallel Computer Architectures", Duncan, Ralph, IEEE Computer. February 1990, pp. 5-16.
Flynn, M., Some Computer Organizations and Their Effectiveness, IEEE Trans. Comput., Vol. C-21, pp. 948, 1972.

This is with reference to SPMD discussion. In no way SPMD should mean Single Process Multiple Instruction. This is because if we use the term Process in place of Program, this would take us to somewhere which is totally different context.

Program is the blueprint of the process or we can say that Program in execution is called process. If we say SPMD to Single Process Multiple Instruction then we mean that all processors are executing the same Process then the question arises how?

The true case is the program, which is common to all the processors under consideration.

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