1. 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 is responsive to a memory access instruction: (i) to compare 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 (ii) if said base register value matches said predetermined value, then to branch to execution of a null value exception handler.