Composite semiconductor storage device and operating method therefor

摘要

A semiconductor storage device formed on a single chip includes a ROM, a RAM and an input/output port. When a mode setting signal designates a normal mode, access is made to one of the ROM, RAM and input/output port in response to an address signal. The ROM is accessed when the mode setting signal designates a ROM write mode and the address signal designates an address assigned to the ROM. A dummy data is output from a data input/output terminal when the mode setting signal designates the ROM write mode, the address signal designates an address outside an address region assigned to the ROM, and a read signal is applied to the device.