PARALLEL PROCESSING

PARALLEL processing is a term used to denote a large class of techniques that are used to provide simultaneous data processing tasks for the purpose of increasing the computational speed of a computer system.Instead of processing each instruction sequentially as in a conventional computer,a parallel processing system is able to perform 

Concurrent data processing to achieve faster execution time .

   For example:- While an instruction is being executed in the ALU, the next instruction can be read from memory.The system may have two or more ALU'S and be able to execute two or more instructions at the same time.

   Furthermore,the system may have two or more processors operating concurrently.The purpose of PARALLEL PROCESSING IS to speed up the computer processing capability & increase its throughput,i.e the amount of processing that can be accomplished during a 

Given interval of time.The amount of hardware increases with parallel processing,&with it, the cost of the system increases.However, technological developments have reduced hardware costs to the point where parallel processing techniques are economically feasible.

.      Parallel processing can be viewed from various levels of complexity.At the lowest level,we distinguish between parallel &serial operations by the type of registers used.Shift register operate in serial fashion one bit at a time ,while registers with parallel load operate with all the bits of the word simultaneously.parallel processing at a higher level of complexity can be achieved by having a multiplicity of functional units that perform identical or different operations simultaneously.Parallel processing is established by distributing the data among the multiple functional units.

Ex:-The arthimetic logic,&shift operations can be separated into three units &the operand diverted to each unit under the supervision of a control unit.

Multiple functional units:-    

The above fig 1 shows one possible way of separating the execution unit into 8 functional units operating parallel.The operands in the registers are applied to one of the units depending on the operation specified by the instruction associated with the operands.

     The operation performed in each functional unit is indicated in each block of the diagram.

The adder and integer multiplier perform the operations with integer numbers.The floating-point operations are separated into three circuits operating in parallel.The logic,shift&increment operations can be performed concurrently on different data.All units are independent of each other ,so one number can be shifted while another number is being incremented.

    A multifunctional organisation is usually associated with a complex control unit to coordinate all the activities among the various components.

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    There are 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 information through the system.One classification introduced by M.J.Flynn considers the items that are manipulated simultaneously.The normal operation of a computer is to fetch instructions from memory constitutes an instruction stream.

The operations performed on the data in the processor constitutes the data stream.

Parallel processing may occur in both or any one.

Flynn's classification divides computer into four major group as follows:-

Single instruction stream,single data stream(SISD)

Single instruction stream,multiple data stream(SIMD)

Multiple instruction stream,single data stream(MISD)

Multiple instruction stream,multiple data stream(MIMD)

     SiSD represents the organization of a single 

Control unit, aprocessor unit,& a memory unit. Instruction are executed sequentially &the system may or may not have internal parallel processing capabilities. parallel processing in this case may be achieved by means of multiple functional units or by pipeline processing.

    SIMD represents an organization that includes many processing units under the supervision of a common control unit.All processor recieve 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 processor simultaneously.

    MISD structure is only of the theoretical interest since no practical system has been constructed using the organization.

    MIMD organization refers to the computer system capable of processing several programs at the same time.Most multiprocessors &multi computer systems can be classified in this category.

Flynn's classification depends on the distinction between the performance of the control unit &data processing unit.It emphasizes the havioral characteristics of the computer system rather than its operational &structural interconnections.One type of parallel processing that doesn't fit this classification are SIMD array processors &MIMD multiprocessors .