~bs~~i~tio~ as a el for-the Ar~h~t~~~re of Embedded Systems
M. Boasson · 1996
Embedded systems have one characten'stic that directly infiuences their design more than any other: they need to keep up with events in their environment. Whereas ordinary systems generally require their environment to adapt to the behaviour of the system, embedded systems need to have the opposite artitude and must themselves adapt to their environment. This requirement implies that there must be progress at all times, regardless of what the intemal status of the system may be: hardware mljbctions or sopware errors may neve< to the extent possible, result in halting such a system. One direct consequence of this observation is that traditional architectures are less than ideally suited for designing this qpe of system. In many architectures there is synchronous communication between cooperating sofnvare functions, and even if asynchrony is introduced, there is generally an asymmetric relation between such functions: at least one of them has knowledge of the othel: nese properties reduce the resilience of systems to problems of VQ~~OUS kinds, and lead to vely complex design problems. It will be argued that subscription is a much better model for organizing the communication inside embedded systems. In this model, components have no knowledge about each othel; communication channels are established only when the need to receive certain data elements is explicitly expressed by afinction, and transfer of data is autonomously handled by the architecture upon production of a new element.