Impact of Implementation Design Tradeoffs on Performance:
The PDP-11, A Case Study
EDWARD A. SNOW and DANIEL P. SIEWIOREK
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