1. Apparatus for processing data, said apparatus comprising: (i) a valid word memory operable to store a plurality of valid words, each valid word having bits representing whether or not corresponding data storage locations in a further memory are storing valid data; and (ii) a plurality of flip-flop circuits operable to store values indicative of whether or not corresponding valid words within said valid memory are themselves valid; characterised in that (iii) a flip-flop circuit is operable to store a value indicative of validity of a number of valid words which varies in dependence upon how many valid words may be stored in said valid memory.