The PDP-11 Family

The PDP-l1 has evolved quite differently from the other computers discussed in this book and, as a result, provides an independent and interesting story. Like the other computers, the factors that have created the various PDP- 11 machines have been market and technology based, but they have generated a large number of implementations (ten) over a relatively short (eight-year) lifetime. Because there are multiple implementations spanning a performance range at the same time, the PDP-1 1 provides problems and insight which did not occur in the evolutions of the traditional mini (PDP-8 Family), the optimal price/performance machines (18-bit), and the high performance timesharing machines (the DECsystem 10). The PDP-1 1 designs cover a range of 500:1 in system price ($500 to $250,000) and 500:1 in memory size (4 Kwords to 2 Mwords).

Rather than attempt to summarize the goals of designers, sentiments of users, or the thoughts of researchers, the discussion of the PDP- 11 is divided into chapters which, in most cases, consist of papers written contemporaneously with various important PDP-11 developments. The chapters are arranged in five categories: introduction to the PDP-11, conceptual basis for PDP-11 models, implementations of the PDP-l1, evaluation of the PDP-l1, and the virtual address extension of the PDP- 11.

Chapter 9, first published when the PDP-l1 was announced, introduces the PDP-11 architecture, gives its goals, and predicts how it might evolve. The concept of a family of machines is quite strong, but the actual development of that family has differed a good deal from the projections in this chapter. The major reasons (discussed in Chapter 16) for the disparity between the predicted and actual evolution are: