Conventional single processor computers are classified as SISD systems. Each arithmetic instruction initiates an operation on a data item taken from a single stream of data elements. Historical supercomputers such as the Control Data Corporation 6600 and 7600 fit this category as do most contemporary microprocessors.
Vector processors such as the Cray-1 and its descendants are often classified as SIMD machines, although they are more properly regarded as SISD machines. Vector processors achieve their high performance by passing successive elements of vectors through separate pieces of hardware dedicated to independent phases of a complex operation. For example, in order to add two numbers such as and , the numbers must have the same exponent. The processor must shift the mantissa (and decrement the exponent) of one number until its exponent matches the exponent of the other number. In this example is adjusted to so it can be added to , and the sum is . A vector processor is specially constructed to feed a data stream into the processor at a high rate, so that as one part of the processor is adding the mantissas in the pair another part of the processor is adjusting the exponents in .