Abstract: A tracing facility which is useful in a multiprocessing environment on a single processor is disclosed. Tracing statements are encoded within each process, each statement having arguments to indicate its location in the code and a message which is logged when the sybsystem and function corresponding to that tracing statement are selected for debugging. Each process includes a trace library that is called by each trace statement, and the trace library includes a local sync word and a local trace table having a list of the subsystems and functions currently selected for tracing. Each process has access to a global sync word and a global trace table having a list of the processes, subsystems and functions selected for tracing. A change in the trace operation is made by changing a parameter in the global trace table and changing the value of the global sync word to indicate that a change has been made in the global table.