1. An exception handling system for a packet processing system comprising: a plurality of exception handlers; a register for holding a data element having a data field with a value selecting one of the plurality of exception handlers, the selected exception handler configured to check for the presence of one or more potential exception conditions associated with the selected exception handler; logic for allowing the selected exception handler, and, therefore, the one or more potential exception conditions that are checked, to dynamically vary as packets are processed by the packet processing system; a memory for holding packet processing state data relating to a packet undergoing processing by the packet processing system; and a packet processor for selectively modifying the packet processing state data relating to the packet if one or more of the associated potential exception conditions is determined to be present by the selected exception handler; wherein the one or more of the potential exception conditions that are checked are determined using a mask associated with the selected exception handler; and wherein the mask is selected from a plurality of masks associated with the selected exception handler.