Impact of Implementation Design Tradeoffs on Performance:
The PDP-11, A Case Study

As semiconductor technology has evolved, the digital systems designer has been presented with an ever increasing set of primitive components from which to construct systems: standard SSI, MSI, and LSI as well as custom LSI components. This expanding choice makes it more difficult to arrive at a near-optimal cost/performance ratio in a design. In the case of highly complex systems, the situation is even worse since different primitives may be cost-effective in different subareas of such systems.

Historically, digital system design has been more an art than a science. Good designs evolved from a mixture of experience, intuition, and trial and error. Only rarely have design methodologies been developed (e.g., two level combinational logic minimization, wire-wrap routing schemes, etc.). Effective design methodologies are essential for the cost-effective design of more complex systems. In addition, if the methodologies are sufficiently detailed, they can be applied in high level design automation systems [Siewiorek and Barbacci, 1976].

Design methodologies may be developed by studying the results of the human design process. There are at least two ways to study this process. The first involves a controlled design experiment where several designers perform the same task. By contrasting the results, the range of design variation and technique can be established [Thomas and Siewiorek, 1977]. However, this approach is limited to a fairly small number of design situations due to the redundant use of the human designers.

The second approach examines a series of existing designs that meet the same functional specification while spanning a wide range of de sign constraints in terms of cost, performance, etc. This paper considers the second approach and uses the DEC PDP-l1 minicomputer line as a basis of study. The PDP-l1 was selected due to the large number of implementations (eight are considered here) with designs spanning a wide range in performance (roughly 7:1) and component technology (bipolar SSI, MSI, MOS custom LSI). The designs are relatively complex and seem to embody good design tradeoffs as ultimately reflected by their price/performance and commercial success.

The design tradeoffs considered fall into three categories: circuit technology, control unit implementation, and data path topology. All