Abstract: A single chip microcomputer comprises a central processing unit (CPU) 2, a on-chip RAM 3, a on-chip ROM 5, a first bus DBUS for connecting the CPU, RAM, and ROM with one another and transferring data between them, a second bus ABUS for passing address data corresponding to the data passed through the first bus, a third bus SDBUS for connecting the CPU 2 with the RAM 3 and transferring data between them, the number of bits of the third bus SDBUS being larger than that of the first bus DBUS, and a fourth bus BABUS for connecting the CPU 2 with the RAM 3 and passing address data corresponding to the data passed through the third bus SDBUS. The CPU 2 has a data memory RF serving as general purpose registers for providing internal data to the third bus SDBUS, and a bank specifying register BP for holding positional data of a mapping region in the RAM 3 where the contents of the data memory RF are mapped and providing the positional data to the fourth bus BABUS.