Flynn's taxonomy

Flynn's taxonomy is a classification of computer architectures, proposed by Michael J. Flynn in 1966 and extended in 1972. The classification system has stuck, and it has been used as a tool in the design of modern processors and their functionalities. Since the rise of multiprocessing central processing units (CPUs), a multiprogramming context has evolved as an extension of the classification system. Vector processing, covered by Duncan's taxonomy, is missing from Flynn's work because the Cray-1 was released in 1977: Flynn's second paper was published in 1972. The four initial classifications defined by Flynn are based upon the number of concurrent instruction (or control) streams and data streams available in the architecture. Flynn defined three additional sub-categories of SIMD in 1972. Single instruction, single data A sequential computer which exploits no parallelism in either the instruction or data streams. Single control unit (CU) fetches a single instruction stream (IS) from memory. The CU then generates appropriate control signals to direct a single processing element (PE) to operate on a single data stream (DS) i.e., one operation at a time. Examples of SISD architectures are the traditional uniprocessor machines like older personal computers (PCs) (by 2010, many PCs had multiple cores) and mainframe computers. Single instruction, multiple data A single instruction is simultaneously applied to multiple different data streams. Instructions can be executed sequentially, such as by pipelining, or in parallel by multiple functional units. Flynn's 1972 paper subdivided SIMD down into three further categories: Array processor – These receive the one (same) instruction but each parallel processing unit has its own separate and distinct memory and register file. Pipelined processor – These receive the one (same) instruction but then read data from a central resource, each processes fragments of that data, then writes back the results to the same central resource.