Compositional Petri net models of advanced tasking in Ada-95

The Ada language has been designed to support development of concurrent and distributed software. While the Ada-83 standard defined the basic mechanisms of rendezvous-based tasking, the Ada-95 standard significantly extended this capability with the introduction of several advanced tasking constructs. We present and discuss formal models of these key tasking constructs using the Petri net model. We also provide some formal evaluation of the models using one particular net-based method, invariant analysis. The constructs considered are the asynchronous transfer of control, the protected object, and the requeue statement. By modeling these advanced Ada tasking constructs with Petri nets, we obtain compositional models of the constructs that are complementary to earlier work in net-based modeling of Ada tasking, both in terms of defining precise behavior for tasking semantics, and also in terms of providing support for automated analysis of concurrent software.