1 indicator the next store that occurs is that of register r 5 this is out of order and thus the explicit index value relating to this data transfer needs to be output in the trace stream and thus an explicit 4 is output similarly with the storing of register r 3 an explicit 2 needs to be output trace is then disabled and thus no trace is output the final instruction in this example instruction stream is a single load to register r 1 in this embodiment single loads are treated in the same way as multiple loads and thus one always outputs an index indicting that the first data transfer for this instruction has been performed fig5 shows a flow diagram illustrating steps of a method according to an embodiment of the present invention in this embodiment the execution of a stream of instructions is monitored for each instruction monitored it is determined whether the instruction is a data transfer instruction or not if it is a datatransfer instruction then a predetermined index value for identifying that individual data transfer is associated with that data transfer it is then determined if trace is enabled if trace is enabled then a value indicative of the index value that has been associated with that data transfer is output a trace element indicating perhaps the address of the transfer may also.