Chapter 20 ½ The Iliac IV System 307
2 Replication: One of the four major components (or subcomponents within a major component) could be duplicated many times. (Ten black boxes can produce the result of one black box in one-tenth of the time if the conditions are right.) The replication of I/O devices, for example, was a step taken very early in the evolution of digital computers-large installations had more than one tape drive, more than one card reader, more than one printer.
The overlapping philosophy was implemented largely through the buffer and pipeline mechanisms. The pipeline mechanism (see Fig. 2) breaks down an operation into suboperations, or stages, and decouples these stages from each other. After the stages are- decoupled they can be performed simultaneously or, equivalently, in parallel, The buffer mechanism allows an operation to be decoupled into parallel operation by providing a place to store information.
The replication philosophy is exemplified by the general multiprocessor which replicates three of the four major components (all but the I/O) many times. The cost of a general multiprocessor is, however, very high and further design options were considered which would decrease the cost without seriously degrading the power or efficiency of the system. The options consist merely of recentralizing one of the three major components which had been previously replicated in the general multiprocessor-the memory, the ALU, or the CU. Centralizing the CU gives rise to the basic organization of a vector or array processor such as Illiac IV. This particular option was chosen for two main reasons.
2 Structure: There is a large class of both scientific and business problems that can be solved by a computer with one CU (one instruction stream) and many ALUs. The same algorithm is performed repetitively on many sets of different data: the data are structured as a vector, and the vector processor of Illiac IV operates on the vector data. All of the components of data structured as a vector are processed simultaneously or in parallel.
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