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Single Instruction, Single Data stream(SISD)

A sequential computer which exploits no parallelism in either the instruction or data streams. Single

control unit (CU) fetches single Instruction Stream (IS) from memory. The CU then generates appropriate

control signals to direct single processing element (PE) to operate on single Data Stream (DS) i.e. one

operation at a time

Examples of SISD architecture are the traditionaluniprocessormachines like aPC(currently

manufactured PCs have multiple processors) or oldmainframes.

Single Instruction, Multiple Data streams(SIMD)

A computer which exploits multiple data streams against a single instruction stream to perform operations

which may be naturally parallelized. For example, anarray processororGPU.

Multiple Instruction, Single Data stream(MISD)

Multiple instructions operate on a single data stream. Uncommon architecture which is generally used for

fault tolerance. Heterogeneous systems operate on the same data stream and must agree on the result.

Examples include theSpace Shuttleflight control computer.[citation needed]

Multiple Instruction, Multiple Data streams(MIMD)

Multiple autonomous processors simultaneously executing different instructions on different

data.Distributed systemsare generally recognized to be MIMD architectures; either exploiting a single

shared memory space or a distributed memory space. Amulti-coresuperscalarprocessor is an MIMD

processor.

SISD MISD

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SIMD MIMD

Incomputing,shared memory ismemorythat may be simultaneously accessed by multiple programs

with an intent to provide communication among them or avoid redundant copies. Shared memory is an

efficient means of passing data between programs. Depending on context, programs may run on a single

processor or on multiple separate processors. Using memory for communication inside a single program,

for example among its multiplethreads, is generally not referred to as shared memory.

Incomputer science,distributed memory refers to amultiple-processor computer systemin which

eachprocessorhas its own privatememory. Computational tasks can only operate on local data, and if

remote data is required, the computational task must communicate with one or more remote processors.

In contrast, ashared memorymulti processor offers a single memory space used by all processors.

Processors do not have to be aware where data resides, except that there may be performance penalties,

and that race conditions are to be avoided.

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