1. An apparatus for processing data comprising: processing logic operable to perform data processing operations; and an instruction decoder operable to decode program instructions to control said processing logic to perform data processing operations specified by said program instructions; wherein said instruction decoder, in response to a memory access instruction, compares a base register value, stored within a base register specified by a base register field of said memory access instruction, with a predetermined null value; and, if said base register value matches said predetermined null value, then said decoder triggers branching to execution of a null value exception handler.