A method and system of identifying overlays used by a program. The overlays may be executable overlays (e.g., overlay programs and dynamically linked library programs), or the overlays may be data sets. Depending on the number of overlays and/or the type of information used to identify the overlays, an indication of the identity of the overlays may be written to a register (whose contents are inserted into the trace data stream), or the indication may comprise an entry in a log buffer and an index value written to the register (again whose contents are inserted into the trace data stream, and where the index value identifies the entry in the log buffer).