Worst-case Execution Time (WCET), refers to the upper bound on the execution time of a piece of program. In hard real-time systems, where the timing requirement is stringent, WCET is needed for the purpose of validation to show that the scheduler can safely schedule the designed system even under extreme conditions. Current research on WCET provide two approaches: (1) static analysis and (2) measurement-based analysis. Static method uses only the source code or the assembly code as its input and calculate the WCET based on an abstract model of the processor.
On your first lecture in Embedded Systems, you will definitely hear the term ‘hard’ real-time and ‘soft’ real-time. Systems such as avionic systems, automobile engine control systems and cardiac pacemakers could be included in the category of hard real-time systems, while web servers, human-machine interfaces and multimedia systems are soft real-time systems. By intuition, we can infer that these hard real-time systems are more critical and important than the soft ones.
Real-Time Scheduling Theory has been developed over the last 40 years, since the first published work of Liu and Layland in 1973. Before that, real-times systems were designed with cyclic executives, which is in a ad-hoc manner and very difficult to maintain. In general, the theory is consist of task modelling, scheduling policy and schedulability test. Some other issues, e.g., resource management, response time analysis, worst-case execution time analysis, are also involved in the design of real-time systems.