The category of MIMD machines is the most diverse of the four classifications in Flynn's taxonomy. It includes machines with processors and memory units specifically designed to be components of a parallel architecture, large scale parallel machines built from ``off the shelf'' microprocessors, small scale multiprocessors made by connecting four vector processors together, and a wide variety of other designs. With the continued improvement in network communication and the development of software packages that allow programs running on one machine to communicate with programs on other machines, users are even starting to use local networks of workstations as MIMD systems.
Computer systems with two or more independent processors have been available commercially for a long time. For example, the Burroughs Corporation sold dual processor versions of its B6700 systems in the 1970s. These were rarely, if ever, used to work on the same job, however. Multiprocessors of this era were intended to be used for job level parallelism, i.e. each would run a separate program. Parallel processing, in the sense of using more than one processor in the execution of a single program, has been an active area in corporate and academic research labs since the early 1970s. The c.mmp and cm* projects at Carnegie Mellon University used DEC PDP-11 microcomputers as processing elements and pioneered several important developments in parallel hardware and software. Commercial parallel processors started to become widely used in the mid 1980s. By the early 1990s these systems began to approach top of the line vector processors in computing power, and the trend for future high performance computing is clearly with parallel processing.