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398 Part 5 The PMS level

Section 2 Computers with one central processor and multiple input/output processors

Fig. 1. National Bureau of Standards' PILOT computer PMS diagram.

unlike present multiprocessors with several identical processors, each PILOT processor is different.

PILOT is a good example of an early attempt to use multiprocessors; successors look little like it. It has one of the best analytical discussions of any computer [Leiner et al., 1957]. With this machine there was an attempt to resolve the controversy between the short-word EDSAC (17 bits) and the long-word Institute for Advanced Studies computers (40 bits) by providing a processor and memory (i.e., computers) for each problem. Only the first computer had substantial Mp, and the other computers, or processors, could be concerned only with the first computer. The third computer was introduced to process devices such as Ms(magnetic tape) and used a plugboard program memory. The idea of an independent processor (IBM 7094) or computer (ODO 6600) for input/output processing is used now, though it is doubtful that PILOT inspired these designs.

The capacitor-diode store is novel and daring for the technology. Two- and three-address computers are used in the primary and secondary computers. The secondary computer, with 16-bit words, is not very useful; its memory is very limited, and it is essentially used only for address calculations. The book-keeping operation for a three-address computer could easily keep a small processor busy.

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