What are SISD, MIMD, SPMD, and Vector?

The categorization of parallel hardware is based on the number of instruction streams and the number of data streams.

SISD: Single Instruction Stream, Single Data stream. A uniprocessor.

MIMD: Multiple Instruction streams, Multiple Data streams. A multiprocessor.

SPMD: Single Program, Multiple Data Streams. The conventional MIMD programming model, where a single program runs across all processors.

SIMD: Single Instruction stream, Multiple Data streams. The same instruction is applied to many data streams, as in vector processor.

More details about the difference between them can be found in: