1. A method for address mapping, comprising the steps of: setting a first physical address region that is accessible by a processor when a system is booted and a second physical address region that is expanded by a virtual address; and mapping the first and second physical address regions so that a part of a region of a main storage is doubly mapped to both the first physical address region and the second physical address region, and a remaining portion of the region of the main storage is mapped to the second physical address region only.