ABSTRACT
The fundamental distinction between the two classes is that one class is MIMD, while the other is SIMD. In MIMD architectures several processors operate in parallel in an asynchronous manner and generally share access to a common memory. In tightly coupled MIMD multiprocessors, the number of processing units is fixed, and they operate under the supervision of a strict control scheme. In SIMD architectures a single control unit (CU) fetches and decodes instructions. Then the instruction is executed either in the CU itself or it is broadcast to a collection of processing elements. In shared memory designs all processors have direct access to all of the memory; in distributed memory computers each processor has direct access only to its own local memory. MIMD computers use shared memory, while SIMD computers use distributed memory.
