As discussed in Chapter 1, a computer system can be viewed as a hierarchy of structural levels, each level consisting of a set of elements that are aggregates of those at the next lower level. From that point of view, the PDP-l was constructed from elements or building blocks that were DEC Systems Modules, each containing elements from the switching circuit level of the structural hierarchy (AND gates, OR gates, etc.). When the integrated circuit was introduced, the number of components in one indivisible package became an order of magnitude larger than it had been with discrete components. The functionality contained in a single DEC module increased accordingly, and it was not long before computers were constructed using building blocks from the next higher level in the structural hierarchy. At that level, the register transfer (RT) level, modules each contained register files, multiplexers, arithmetic logic units, and so on. The functions available in a single integrated circuit, and the functionality available in a single module, have been dictated by the search for universal functions discussed in the section "LSI dilemma," in Chapter 2.

While Chapters 4 and 5 are devoted to the history of DEC modules and the circuit and logic level characteristics that developed in the various module families as a result of the advances in semiconductor technology, the chapters in Part IV emphasize the role of modules as digital systems and computer building blocks. Thus, the emphasis is on the use of modules at the register transfer and processor-memory-switch (PMS) levels of the structural hierarchy.

Two types of building block are discussed:

The most complete examples of the module set building blocks are the Register Transfer Modules (RTMs) produced by DEC in the late 1960s and the Macromodules proposed by Wes Clark in 1967 [Clark, 1967; Ornstein et al., 1967]. The Register Transfer Modules are of interest because they were building blocks of a