Abstract: An expanded memory field is obtained by forming a physical address for accessing a main memory unit according to the present invention. The physical address is developed following an address-mode provided in an instruction including an address field. When the address-mode is a "1", physical address is provided by (i) obtaining a base offset address from a base register which is given by a base register bank as determined by a first field of the address field, and (ii) adding the base address to a displacement number determined by a second field of the address to obtain an offset address, (iii) obtaining a segment base address from a segment base address register corresponding to a segment number register determined by the first field of the address, and (iv) adding together the segment base address and the offset address. A 24 bit physical address is obtained. When the address-mode is a "0", the address field constitutes the offset address and the above additions are carried out.