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3.1.1 SISD Computers     continued...

The ambiguity over the classification of vector machines depends on how one views the flow of data. A static ``snapshot'' of the processor during the processing of a vector would show several pieces of data being operated on at one time, and under this view one could say one instruction (a vector add) initiates several data operations (adjust exponents, add mantissas, etc.) and the machine might be classified SIMD. A more dynamic view shows that there is just one stream of data, and elements of this stream are passed sequentially through a single pipeline (which implements addition in this example). Another argument for not including vector machines in the SIMD category will be presented when we see how SIMD machines implement vector addition.