By Jie Liu and

fundamentally from its desktop or Internet counterparts. Embedded computing is not simply computation on small devices. In most control applications, for example, embedded software engages the physical world. It reacts to physical and user-interaction events, performs computation on limited and competing resources, and produces results that further impact the environment. Of necessity, it acquires some properties of the physical world, most particularly, time. Despite the fact that both value and time affect the physical outputs of embedded systems, these two aspects are developed separately in typical embedded software design. The functionality is determined at design time with assumptions such as zero or a fixed nonzero run-time delay. The actual timing properties are determined at run time by a real-time operating system (RTOS). Typically, an RTOS offers as control of these timing properties one number for each task, a priority. Whether a piece of computation can be finished or brought to a quiescent state at a particular time is totally a dynamic phenomenon, and it depends largely on the hardware platform, when the inputs arrive, what other software is running at that time, and the relative priorities. These factors are usually out of the control of embedded system designers and may break the timing assumptions that the control algorithms may rely on. In most control applications, this run-time uncertainty is undesirable or even disastrous. We believe that two steps can be taken to improve the design process for embedded software and to bridge the gap between the functionality development and timing assurance: • rigorous software architectures that expose resource utilization and concurrent interactions among software components • specification, compilation, and execution mechanisms that preserve timing properties throughout the software life cycle. A component-based software architecture can help compilers to determine the logical dependencies and shared resources among components. By bringing the notion of time and concurrent interaction to the programming level, compilers and run-time systems can be developed to preserve both timing and functional properties at run time. Recent innovations in real-time programming models such as port-based objects (PBOs) [1] and Giotto [2] are examples that take a time-triggered approach to scheduling software components and to preserving their timing properties. These purely time-triggered approaches, although explicitly controlling the timing of each component, require tasks to be periodic and do not handle irregularly spaced new information (or events) well.