1. Apparatus for processing data, said apparatus comprising: data processing logic operable to perform data processing operations; and an instruction decoder operable to decode program instructions specifying data processing operations to be performed by said data processing logic and to control said data processing logic to perform said data processing operations; wherein said instruction decoder is operable in a first mode in which program instructions of a first instruction set are decoded and in a second mode in which program instructions of a second instruction set are decoded, a subset of program instructions of said first instruction set having a common storage order compensated encoding with a subset of program instructions of said second instruction set and forming a common subset of instructions representing at least one class of instructions, said common subset of instructions controlling said data processing logic to perform the same data processing operations independent of whether said instruction decoder is operating in said first mode or said second mode.