1. Data storage apparatus comprising: (a) a main store having an address input and a data output, (b) first address generating means for generating a first address signal, (c) means for applying the first address signal to the address input of the main store thereby reading out a data item from the data output of the main store, (d) an additional store having an address input and a data output, (e) second address generating means responsive to a previously generated first address signal for generating a second address signal, predicting said first address signal, (f) means for applying said second address signal to the address input of the additional store thereby reading out a data item from the data output of the additional store, (g) selection means connected to the data outputs of the main store and the additional store and responsive to the first address signal, for selecting the data item read out from the data output of the main store when the first address is in a first predetermined range and selecting the data item read out from the data output of the additional store when the first address is in a second predetermined range, (h) comparison means for comparing said first and second address signals and, if they are unequal, producing an inequality signal, and (i) means connected to the second address generating means and responsive to said inequality signal, for setting the second address signal equal to the first address signal.