To debug applications that make use of z/OS subtasking, IDF must be invoked with the SVC97 option (see Breakpoint method selection (TSO)).

Debugging z/OS applications that make use of subtasking can be unreliable. IDF cannot handle more than one event at a time. For example, a program check in one subtask while statement stepping in another subtask causes an ABEND within IDF.

Breakpoints can be set in different subtasks at the same time, but this is only reliable when only one subtask is "active" at a time (that is, the other subtask remains non-dispatchable). This requires judicious use of breakpoints at the appropriate points in the application. It is strongly recommended that you do not place breakpoints in code which can be executed by multiple tasks if it is possible for the breakpoints to be encountered simultaneously. Failure to do this can cause unpredictable results.