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Chapter 39

Implementation and Performance Evaluation of the PDP-11 Family

Edward A. Snow / Daniel P. Siewiorek

In order that methodologies useful in the design of complex systems may be developed, existing designs must be studied. The DEC PDP-11 was selected for a case study because there are a number of designs (eight are considered here), because the designs span a wide range in basic performance (7 to 1) and component technology (bipolar SSI to MOS LSI), and because the designs represent relatively complex systems.

The goals of the chapter are twofold: (1) to provide actual data about design tradeoffs and (2) to suggest design methodologies based on these data. An archetypical PDP-11 implementation is described.

Two methodologies are presented. A top-down approach uses micro-cycle and memory-read-pause times to account for 90 percent of the variation in processor performance. This approach can be used in initial system planning. A bottom-up approach uses relative frequency of functions to determine the impact of design tradeoffs on performance. This approach can be used in design-space exploration of a single design. Finally, the general cost/performance design tradeoffs used in the PDP-11 are summarized.

1. Introduction

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 of an art than a science. Good designs have evolved from a mixture of experience, intuition, and trial and error. Only rarely have design methodologies been developed (among those that have are two-level combinational logic minimization and wire-wrap routing schemes, for example). 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 fairly small design situations because of 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 design constraints in terms of cost, performance, etc. This paper considers the second approach and uses the DEC PDP-111 minicomputer line as a basis of study. The PDP-11 was selected on account of the large number of implementations (eight are considered here) with designs spanning a wide range in performance (roughly 7 to 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.

Attention here is focused mainly upon the CPU. Memory performance enhancements such as caching are considered only insofar as they impinge upon CPU performance.

This paper is divided into three major parts. The first part (Sec. 2) provides an overview of the PDP- 11 functional specification (its architecture) and serves as background for subsequent discussion of design tradeoffs. The second part (Sec. 3) presents an archetypical implementation. The last part (Secs. 4 and 5) presents methodologies for determining the impact of various design parameters on system performance. The magnitude of the impact is quantified for several parameters, and the use of the results in design situations is discussed.

2. Architectural Overview

The PDP-11 family is a set of small- to medium-scale stored-program central processors with compatible instruction sets [Bell et al., 1970]. The family evolution in terms of increased performance, constant cost, and constant performance successors is traced in Fig. 1.2 Since the 11/45, 11/55, and 11/70 use the same processor, only the 11/45 is treated in this study.

A PDP-11 system consists of three parts: a PDP-11 processor, a collection of memories and peripherals, and a link called the Unibus over which they all communicate (Fig. 2).

A number of features, not otherwise considered here, are available as options on certain processors. These include memory management and floating-point arithmetic. The next three sub-

1DEC, PDP, LSI-11, Unibus, and Fastbus are registered trademarks of Digital Equipment Corporation.

2The original equipment manufacturer (OEM) versions of the 11/10, 11/20, and 11/40 are the 11/05, 11/15, and 11/35 respectively. The OEM machines are electrically identical (or nearly so) to their end-user counterparts, the distinction being made for marketing purposes only.


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