• Action notation includes a communicative action notation for specifying information processing by distributed systems of agents.

• Communicative actions are concerned with permanent information.

• Chapter 17 illustrates the use of communicative action notation in the semantic description of tasks and entry calls.

So fax, we have dealt with sequential performance of actions by a single agent in isolation. Let us now consider concurrent performance by a distributed system of agents, where each agent can communicate with the other agents, sending and receiving messages and offering contracts. Even when only one agent is active, this generalizes action performance sufficiently to allow the representation of interactive input-output behaviour, where nonterminating information processing is especially significant.

An agent represents the identity of a single process, embedded in a universal communication medium, or ‘ether’. Implementations of processes may run them on physically separate processors, linked by buses or networks, or on a single processor using time-sharing. Of course each processor itself may consist of a number of connected parts, such as CPU, memory modules, video cards, etc. Agents may correspond to the behaviours of such special-purpose subprocessors, as well as to processes specified directly in high-level programming languages.

Communication between agents is asynchronous: to send a message, an agent emits the message into the medium, with another agent specified as the receiver, and then carries on performing, not waiting until the message has been delivered to the other agent.