1. Apparatus operable in a first mode to perform processing operations specified by Java bytecodes and operable in a second mode to perform processing operations specified by other instructions, said apparatus comprising: a register bank operable to store data values to be manipulated; processing logic operable to perform processing operation upon data values stored in said register bank; and a Java bytecode decoder operable in said first mode to store a copy of a Java Local Variable held in a memory location within a register of said register bank; wherein said Java bytecode decoder is operable when decoding a Java bytecode which reads said Java Local Variable to read said Java Local Variable from said register and is operable when decoding a Java bytecode which writes said Java Local Variable to write said Java Local Variable to said register and to said memory location.