PURPOSE: To execute a program without reading it on a main memory unit and to unitarily manage the program and data on a randomly accessible and rewritable memory without distinguishing them. CONSTITUTION: A computer system is provided with a CPU for converting addresses by referring to a real/virtual address conversion table provided with a page table 25 and an external storage device for using the randomly accessible and rewritable memory connected to the CPU. The page table 25 is constituted so as to make the codes of the program be continuous on the virtual address space of the CPU corresponding to an execution order, and by referring to the real/virtual address conversion table provided with the page table, the program stored in the external storage device is read from the external storage device and executed by using a virtual address. Also, the real address of the CPU is allocated only to the data area of the randomly accessible memory. COPYRIGHT: (C)1994,JPO