It is safe to say that as of this writing there is no completely satisfactory characterization of the different types of parallel systems. The most popular taxonomy was defined by Flynn in 1966 . The classification is based on the notion of a stream of information. Two types of information flow into a processor: instructions and data. Conceptually these can be separated into two independent streams, whether or not the information actually arrives on a different set of wires. Flynn's taxonomy classifies machines according to whether they have one stream or more than one stream of each type (Figure 4). The four combinations are SISD (single instruction stream, single data stream), SIMD (single instruction stream, multiple data streams), MISD (multiple instruction streams, single data stream), and MIMD (multiple instruction streams, multiple data streams).