We designed and implemented a new programming language called Hierarchical Timing Language (HTL) for hard realtime systems. Critical timing constraints are specified within the language, and ensured by the compiler. Programs in HTL are extensible in two dimensions without changing their timing behavior: new program modules can be added, and individual program tasks can be refined. The mechanism supporting time invariance under parallel composition is that different program modules communicate at specified instances of time. Time invariance under refinement is achieved by conservative scheduling of the top level. HTL is a coordination language, in that individual tasks can be implemented in "foreign" languages. As a case study, we present a distributed HTL implementation of an automotive steer-by-wire controller.

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Exotasks are a novel Java programming construct that achieve three important goals. They achieve low latency while allowing the fullest use of Java language features, compared to previous attempts to restrict the Java language for use in the submillisecond domain. They support pluggable schedulers, allowing easy implementation of new scheduling paradigms in a real-time Java system. They can achieve deterministic timing, even in the presence of other Java threads, and across changes of hardware and software platform. To achieve these goals, the program is divided into tasks with private heaps. Tasks may be strongly isolated, communicating only with each other and guaranteeing determinism, or weakly isolated, allowing some communication with the rest of the Java application. Scheduling of the tasks' execution, garbage collection, and value passing is accomplished by the pluggable scheduler. Schedulers that we have written employ logical execution time (LET) in association with strong isolation to achieve time portability. We have also built a quad-rotor model helicopter, the JAviator, which we use to evaluate our implementation of Exotasks in an experimental embedded version of IBM's J9 real-time virtual machine. Our experiments show that we are able to maintain very low scheduling jitter and deterministic behavior in the face of variations in both software load and hardware platform. We also show that Exotasks perform nearly as well as Eventrons on a benchmark audio application.

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