PURPOSE: To expand flexibly a memory without requiring any complicated circuit by providing a means assigning an additional bit for memory mapping to a low-order bit of a memory address.

CONSTITUTION: A memory map controller 7 is provided to assign the additional bit for memory mapping to low-order bits MA0WMA2 of the memory address at an even number memory 8 and an odd number memory 9. Fifteen address lines A0WA14 from the system are required when three additional bits for mapping and a 256-Kbyte of memory capacity are used. In actual memory address lines MA0WMA17, 256-Kbyte is divided in this case into 32-Kbyte×8.