A SYSTEM OF RT-LEVEL MODULES (RTM's)
This chapter presents a specific set of RT-level modules, called Register Transfer Modules \RTM's.(1) They will be used throughout the book as the given components out of which designs are to be constructed, except in Chapter 7, where we discuss the details of their construction, and in Chapter 8, where we discuss general RT-level design. The RTM components are the DEC PDP-16 modules, augmented by a few DEC modules that are not included formally in the DEC PDP-16 series. Throughout. we use the PMS notation for these modules, rather than the DEC names, believing the PMS names to be somewhat more descriptive functionally, hence making design easier. However, for some details of PDP-16's and their accessories, we shall refer the reader from time to time to the, "PDP-16 Computer Designer's Handbook" (DEC, 1971), or simply, to the PDP-16 handbook
This chapter differs from all those that follow in being a listing of information and specifications, rather than a series of design problems to be performed. It is a chapter that is to be used as a reference throughout the rest of the book and it should not be read through as if each of its pages should be understood in toto. We have organized it in several parts. The first introduces RTM's and describes a complete example system, carrying it all the way to photographs of the final constructed physical system. The second, gives the specifications of all modules, first a set of the four most important ones, and then description of the rest. To keep this part self contained as a reference, it is intentionally a little redundant with the preceding part that gives the example; The third part describes a major module, K(PCS), that provides a microprogrammed control. The final part describes two new modules that became available too late to be -used in the designs of the book.
A FIRST LOOK AT.RTM'S
A typical system implemented with RTM's, having external inputs and outputs, is shown in Figure 1 in PMS notation. This embodies a control portion (K) and a data-memory portion (DM). At a more detailed level an RTM system usually has the form shown in Figure 2. Figure 2 shows that the control portion of the system is in fact a network of K modules which is connected to the data-memory portion of the system by another network of three types of links (i.e., -- evoke-operation, ....operation-complete, and ---- Boolean data). The control network is isomorphic to the flowchart of the control algorithm for the system. Each K module carries out one step of the algorithm by sending control signals to the data-memory portion of the system to evoke the desired operation(s), -- for example, evoking the transfer of data from register X to Y, expressed as Y <__X. As each step is completed, control is passed on to the next K module in the network via control flow links (____)
Control flow enters a K module via an input port called "activate" which activates it. The K module requests that an operation be carried out in the data- memory part by using the evoke-operation link (--). As an operation is carried out in the DM part, it informs the control part of the completion so the next step can proceed. The operation-complete signal is used for this purpose (i.e., ....) The operation complete signal causes the currently active K module to
1. We use the backslash (\) to separate a name from its accepted alias.