An electrically-modifiable, non-volatile, semiconductor memory comprising a plurality of user memory locations which can be addressed individually from outside the memory in order to read and to modify the data held therein is characterized in that, for each user memory location, there is a corresponding pair of physical memory locations in the memory, which assume, alternatively, the functions of an active memory location and of a non-active memory location, the active memory location containing a previously-written datum and the non-active memory location being available for the writing of a new datum to replace the previously-written datum, so that, upon a request to replace the previous datum with the new datum, the previous datum is kept in the memory until the new datum has been written.