Embedded Systems October 2000 Vol13_11

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sage desc ribing the triggering incide nt is passed to the DVUI and stored in a log fil e. Trace poin ts are a great way to fo llow system execution at a high level. They can even be post-processed to generate a sequence diagram describ- ing the scenario executed, or they can be used in regression testing. Break and trace point controls can be implemented using the publish- subscribe pattern . The agent supports registration of each type of application execution incident. The DVUl can then subscribe to incidents, such as the creation of a particular instance or ini tiation of a state machine u-ansi- tion. Instrumentation within the appli- cation code notifies thc agent when an incident occurs, and the agent then notifies the subsc riber wi th the appro- pl-iate action, break, or trace. One implementation of a system modeled with communicating state machi nes includes an event queue. Events sent by obj ects and from out- side the domain are placed on the queue. An event loop executes conti n- uously, pulling the next event off the queue and passing it to its destination instance, which executes the specified action. With this implementation, the event loop serves as a central place to monitor system execution. Within the event loop, an insn-umen- tation interface is added ulat watches ule events going by and looks up ule next state to be executed by ule receiv- ing instance. These conditions can be compared against ule set of break and n

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