Buses, The Skeleton of Computer Structures
JOHN V. LEVY
A bus is a communication pathway connecting two or more electrical devices. In the context of minicomputer design, buses are the physical and electrical structures that determine how the building blocks are interconnected.
In every computer system, there are many buses: internal pathways connect the registers and arithmetic logic of a central processor; input/output pathways connect processors, memories, and peripheral devices; and external communication buses attach computer systems to the telephone and other data communication pathways. In this chapter, the discussion is restricted to buses that interconnect computer system components that are designed by different engineering groups.
This particular approach may sound out of place, but one of the most important functions of a bus is to provide a well specified interface between complex subsystems. We exclude from discussion internal processor register transfer buses, as well as external buses whose specifications are determined by engineers not involved in the minicomputer design process. Although none of the examples in this chapter is drawn from multiprocessor systems, most of the design experience presented is relevant to such systems.
What Does a Bus Do?
A bus is a communication medium. Each one exists in order to transfer information from place to place within a computer system. In this chapter, we attempt to illustrate the complexities of bus design by drawing on the real history of some PDP-l 1 Family designs.* In computer systems being manufactured and sold, the success of bus designs is measured by the following criteria:
1. Does the bus successfully establish the communication pathway required?
*All of the real buses presented as examples are proprietary products of Digital Equipment Corporation, protected by United States and foreign patents.