a microcode controlled processor having an overwritable memory device for microcode storage includes a setting device for setting a virtual memory space and a conversion table device showing the relationship between the address information of the virtual memory space designated by the setting device and the address in a real memory space inclusive of the memory device . a predetermined address in the virtual memory space is designated to read out data and the contents of the conversion table device may be modified . in this manner , a large size program or a plurality of programs can be executed immediately without the size of the executable program being limited by the size of the real memory space .