A data processor assigns a unique identifier to each instruction. As there are a finite number of unique identifiers, the identifiers are reused during execution of a program within the data processing system. To maintain an age relationship between instructions executing in the pipeline...http://www.google.de/patents/US5805849?utm_source=gb-gplus-sharePatent US5805849 - Data processing system and method for using an unique identifier to maintain an age relationship between executing instructions 
