A data processing device 2 has a processor 4 which executes software directly from non-volatile memory 6, 8. The processor 4 has a runtime component 20 which dynamically maps software element identifiers specified by the software to corresponding software elements in memory 6, 8. Mapping information 22 is used to determine which software elements identifiers correspond to which software elements. This provides a level of indirection which can be used to make software updates more efficient, by updating only parts of the software while leaving old parts of the software as they are. Updated software elements can be stored to memory and the mapping information updated to point to the new elements, while existing mappings may be retained.