Flynn's Categorization

Flynn's Categorization is based on multiplicity of data streams and instruction streams observed by CPU at the time of program execution. Let I_s and D_s are least number of streams flowing at a random point in the execution, then computer organisation can be classified as follows:

Single Instruction and Single Data stream (SISD): In this organisation, serial execution of instructions is implemented by one CPU comprising a single processing element (PE), it implies that ALU under one CU like shown in figure below. Thus, SISD machines are conventional serial computers which process only single stream of instructions and one stream of data. This kind of computer organisation is depicted in the figure:

Figure:  SISD Organisation

Illustrations of SISD machines include:

• CDC 6600 it is unpipelined but has multiple functional units.
• CDC 7600 it has a pipelined arithmetic unit.
• Amdhal 470/6 it has pipelined instruction processing.
• Cray-1 it supports vector processing.