Flynn's taxonomy

Different types of computer architecture are classified using a system which was first proposed by Michael J. Flynn, back in 1966. This classification system knows four categories, defining the capabilities of the processing hardware in terms of the number of input and output streams:

• Single Instruction, Single Data (SISD): A single instruction is fetched to operate on a single data stream. This is the traditional model for CPUs.
• Single Instruction, Multiple Data (SIMD): With this model, a single instruction operates on multiple data streams in parallel. This is what vector processors such as graphics processing units (GPUs) use.
• Multiple Instruction, Single Data (MISD): This model is most commonly used for redundant systems, whereby the same operation is performed on the same data by different processing units, validating the results at the end to detect hardware failure. This is commonly used by avionics systems and similar.
• Multiple Instruction, Multiple Data (MIMD): For this model, a multiprocessing system lends itself very well. Multiple threads across multiple processors process multiple streams of data. These threads are not identical, as is the case with SIMD.

An important thing to note with these categories is that they are all defined in terms of multiprocessing, meaning that they refer to the intrinsic capabilities of the hardware. Using software techniques, virtually any method can be approximated on even a regular SISD-style architecture. This is, however, part of multithreading.