a single chip type microcomputer includes at least a central processing unit , a random - access memory , a mask read - only memory and an electrically writable rom such as an electrically erasable and programmable read - only memory . the electrically writable rom stores both the user program and data to be preserved . the microcomputer includes further memory for storing a write control program for controlling the write operation to the writable rom , and the electrically writable rom and the memory are disposed to mutually different address positions on the address space of cpu . the proportion of size of the user program region and the data region in the writable rom can be selected in a free proportion . a timer governs writes to the writable rom .