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 [9]. 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).