Some observations on modular design technology and the use of microprogramming

摘要

As modules become more complex the advantages and disadvantages of modularity have become more pronounced. The cost of modularity is measured not only in added hardware but also in a loss of flexibility. Functions that are easy to implement at a submodule level may be very difficult, or even impossible, to duplicate at the modular level. We term this a loss of transparency.

PMS (Processor-Memory-Switch) level modules could be available in the next four-six years. Their existence will open many significiant areas of research. It appears that the overhead for PMS modular systems will be on the order of 30%-50% but with decreasing hardware costs this will be tolerable. The expendable components will be processors and there will be no effort to obtain a high utilization factor for the individual processors in a system. An 80%-90% idle time may be acceptable. The high sales volume required by the semiconductor industry suggests that, in the foreseeable future, PMS level components will be oriented towards mass market applications like personal calculators and intelligent terminals. It is interesting to note that as the cost per digital function has decreased the design time and cost per system has remained relatively constant. So instead of obtaining a cheaper system with the same functions a user gets a more complex system at the same cost. This is best exemplified by observing the evolution of minicomputers and noting that the cost per system of a 1965 vintage minicomputer (e.g., PDP-8) is about as costly as a ]974 minicomputer (e.g., the PDP-]I)*. Finally, microprogrammed modules are an attractive control element for PMS level modules from both an economic and a transparency point of view.