justification used for this figure, save for historical confidence ("it worked last time"). As software and hardware have increased in complexity, they have driven the need for tool support. Complexity is increasingly becoming an issue in both static analysis and measurements. It is difficult to judge how wide the error margin should be and how well tested the software system is. System safety arguments based on a high-water mark achieved during testing are widely used, but become harder to justify as the software and hardware become less predictable. In the future, it is likely that a requirement for safety critical systems is that they are analyzed using both static and measurement-based approaches. Considerations The problem of finding WCET by analysis is equivalent to the halting problem and is therefore not solvable in the general. Fortunately, for the kind of systems that engineers typically want to find WCET for, the software is typically well structured, will always terminate and is analyzable. Most methods for finding a WCET involve approximations (usually a rounding upwards when there are uncertainties) and hence in practice the exact WCET itself is often regarded as unobtainable. Instead, different techniques for finding the WCET produce estimates for the WCET. Those estimates are typically pessimistic, meaning that the estimated WCET is known to be higher than the real WCET (which is usually what is desired). Much work on WCET analysis is on reducing the pessimism in analysis so that the estimated value is low enough to be valuable to the system designer. WCET analysis usually refers to the execution time of single thread, task or process. However, on modern hardware, especially multi-core, other tasks in the system will impact the WCET of a given task if they share cache, memory lines and other hardware features. Further, task scheduling events such as blocking or to be interruptions should be considered in WCET analysis if they can occur in a particular system. Therefore, it is important to consider the context in which WCET analysis is applied. Automated approaches There are many automated approaches to calculating WCET beyond the manual techniques above. These include: analytical techniques to improve test cases to increase confidence in end to end measurements static analysis of the software (“static” meaning without executing the software). combined approaches, often referred to as “hybrid” analysis, being a combination of measurements and structural analysis Static analysis techniques A static WCET tool