1. During the testing of the operation of processing unit, a system for identifying the occurrence of an interrupt service routine code flush condition in the pipeline flattener, the system comprising: timing trace apparatus responsive to signals from the processor unit, the timing trace apparatus generating a timing trace stream; program counter trace apparatus responsive to signals from the processing unit, the program counter trace apparatus generating a program counter trace stream; and synchronization apparatus applying periodic signals to the timing trace apparatus and to the program counter trace apparatus, the periodic signals resulting in periodic sync markers in the timing trace stream and in the program counter trace stream. wherein the program counter trace apparatus is responsive to an interrupt service routine code flush signal, the program counter trace apparatus generating a sync marker signal group identifying the occurrence of the interrupt service routine code flush signal and relating the interrupt service routine code flush signal to the timing trace stream and to the program code execution.