PURPOSE: To lay out a memory space of an external device (extention memory) which is added to a chip while securing a least necessary memory space for a storage such as a built-in ROM, etc., when a memory map is laid out for a single-chip microcomputer.

CONSTITUTION: In reference to a single-chip microcomputer, a memory space used for a built-in ROM is divided into a working memory space and a non- working memory space based on the variable address value set on an area register. Then the non-working memory space is laid out in a memory space set for an extension memory. Thus it is possible to set/change the memory space of the built-in ROM to a least necessary space in response to the acutaul working state just by making a user set/change the variable address value even if the memory space of the built-in ROM is decided according to the storage capacity of the ROM when the microcomputer 18 shipped plant. Therefore the non-working memory space can be best used.