Data processing apparatus comprises one or more interconnected processing elements; each processing element being configured to execute processing instructions of program tasks; each processing element being configured to save context data relating to a program task following execution of that program task by that processing element; and to load context data, previously saved by that processing element or another of the processing elements, at resumption of execution of a program task; each processing element having respective associated format definition data to define one or more sets of data items for inclusion in the context data; the apparatus comprising format selection circuitry to communicate the format definition data of each of the processing elements with others of the processing elements and to determine, in response to the format definition data for each of the processing elements, a common set of data items for inclusion in the context data.