Abstract: Data processing apparatus, particularly for use with a computer, is described which includes a primary store and an address translator. The primary store which has a fast access time compared with the main computer store is addressed by virtual addresses applied to the translator, each virtual address being associated with a particular real address for the primary store. The associations between real and virtual addresses can be changed by program instructions so leading to shorter programs and faster computer operation. Similar advantages are also gained by logic circuits for changing one or more of the more significant bits of the virtual addresses, under program instructions, and of addresses for a secondary store also of fast access time.