1. Apparatus for use in carrying out data processing operations in which data items are called from storage by an address, the apparatus including a primary store for storing items of data at a plurality of storage locations, the primary store having an address input and each location having a unique real address which when applied to the said address input in the form of a real address allows transfer of data items to and from that location of the primary store, a main store having a greater number of storage locations than the primary store, means for allowing transfer of stored items between the primary and main stores, an address translator, having its output coupled to the address input of the primary store, for translating into real addresses for data items from virtual addresses for said items to allow programs of data processing operations which use virtual addresses to be used, and a program storage means for storing said programs which use virtual addresses, each real address being changeably associated with a virtual address particular thereto, changes in the associations between real and virtual addresses effecting changes in the said programs, and the translator including translator storage means, separate from the primary and main stores, for storing addresses, and conversion means for changing the associations between real and virtual addresses independently of transfer of data items between the main and primary stores, said conversion means including logic means for changing said associations while real addresses of data items remain the same by operating upon addresses already held by the translator storage means, the said changes in associations being carried out by the conversion means in response to instruction signals applied to the conversion means.