CHAPTER 7 THE DESIGN OF RTM's
The design of the RTM modules is presented with two goals: to communicate detailed information on the structure of the modules in order that the operation may be better understood; and to stimulate the reader to think about the process of choosing and designing a set of register-transfer level components for implementing digital systems. This latter goal is important both because it is likely that other sets of such components will be developed in the future (particularly some useful integrated circuits for control), and because all logical design should involve a conceptual structuring into modules. Thus, the chapter is organized into two major sections: the first presents the general engineering design considerations that entered into the choice of the RTM set of modules; and the second discusses the important aspects of the switching circuit realization of, the modules.
CHOOSING A MODULE SET
The design of a set of register-transfer level components presents unique problems that are not encountered in other digital components. For example, in choosing a set of gates and flip flops at the switching circuit level, the Junctions of the components are very basic; and many small sets exist that are universal, i.e., can be used, to realize any function (e.g., NAND|NOR|AND+NOT|OR+NOT). At the other extreme large scale computers are rarely designed for the primary purpose of being components, so they are usually universal in themselves.
However, register-transfer level components do not have functions as basic as those at the switching circuit level, yet they are used as components in larger systems. Thus, the choice of the functions that the modules perform becomes nontrivial. Furthermore, the type of systems that they will be employed in influences their design. Therefore potential applications became a big factor in determining the RTM set of modules. As for universality, there are many more possible universal sets at the RT level than at the switching circuit level. Thus, in a sense, the choice becomes more difficult.
Three main areas of application were considered in the design of the RTM set and the interconnection scheme: special purpose digital systems, computer-related systems, and educational systems. In Figure 1 we present a rough categorization of examples within each of these areas, showing those cases in which RTM's would be an appropriate solution.
Within each application area there is a wide range of problems. In the special purpose digital systems area, problems range from A-D and D-A conversion to instrument control and analysis. In the computer-related area, problems range from controllers for plotters and card handling equipment to special processors and the emulation of older computers which are no longer produced, but which have adequately seasoned software. In the educational area, problems range from simple computer arithmetic to the construction of small computers. The reader should be already quite familiar with this full range of problems, as they are covered in previous chapters of this book.
Depending on how the various application areas and their requirements are weighted, a different set of modules emerge. For example, weighting the educational requirement heavily might cause highly rugged components to be built. In the actual design of the modules all three applications areas were weighted about equally.