Designing an asynchronous bus interface

By presenting the design of an asynchronous bus interface for the 80C51 microcontroller we show that nonchannel communications are needed to come to a modular and efficient solution. We derive the bus design by applying five transformations to an initial design that is completely based on channel communications. In each of the steps we first discuss the problem to be solved. The final design uses both channel and nonchannel communications, the latter kind of communications being based on shared variables. In principle, communicating through variables is less safe than communicating through channels. We propose so-called communication sessions to obtain safe communications through variables. Communication sessions are well-defined periods of time during which the access rights with respect to a set of variables are transferred from one activity to another We also show that such sessions can be implemented using conventional channels.