a data processing system having an address conversion system for translating a virtual address into a real address , including a main memory for storing instructions and data and a second memory for storing an address conversion table including a plurality of entries , each of which consists of a plurality of control bits and a portion of a real address . a central processing unit executes the instruction stored in the main memory according to a microprogram . an addressing means addresses the second memory by a continuous portion , which includes the effective upper most bit of the virtual address for the address conversion and for updating of the conversion table . the main memory need not store the conversion table .