OS-Classification of Sequential and Parallel Architectures

The purpose of parallel processing is to speed up the computer processing capability and increase its throughput, that is, the amount of processing that can be accomplished during a given interval of time. There are a variety of ways that parallel processing can be classified. It can be considered from the internal organization of the processors, from the interconnection structure between processors, or from the flow of information through the system. One classification introduced by M.J.FLYNN considers the organization of a computer system by the number of instructions and data items that are manipulated simultaneously. The normal operation of a computer is to fetch instructions from memory and execute them in the processor. The sequence of instructions read from memory constitutes an instruction stream. The operations performed on the data in the processor constitutes a data stream.

Flynn’s classification divides computers into four major groups as follows.
  1. Single Instruction Stream, Single Data Stream (SISD)
  2. Single Instruction Stream, Multiple Data Stream (SIMD)
  3. Multiple Instruction Stream, Single Data Stream (MISD)
  4. Multiple Instruction Stream, Multiple Data Stream (MIMD) 



SISD represents the organization of a single computer containing a control unit, a processor unit a memory unit. Instructions are executed sequentially and the system may or may not have internal parallel processing capabilities. These are uniprocessor computer that process one instruction at a time. SISD machines process data from a single data stream. 

SIMD represents an organization that includes many processing units under the supervision of a common control unit. All processors receive the same instruction from the control unit but operate on different items of data. The shared memory unit must contain multiple modules so that it can communicate with all the processors simultaneously. It is commonly referred as an array processor, a device that essentially performs the same operation simultaneously on every element of an array. Vector processors and pipeline processors are sometimes included in this category.

The MISD(Multiple Instruction Stream, Single Data Stream) machine has not found application in industry.

The MIMD(Multiple Instruction Stream, Multiple Data Stream) machine is a true parallel processor machines in this class are commonly called multiprocessors. MIMD organization refers to a computer system capable of processing several programs at the same time. 
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