When you call external code from VisualAge, all of the VisualAge processing and the external function's processing are executed on a single thread. This means that while the function is being run, processing for the rest of VisualAge is halted, including processing for the normal operation of the VisualAge user interface.
To overcome this problem, VisualAge enables you to reroute the execution of any external function onto a separate thread so that normal processing can continue while the function is running.