Patent US 4,249,250

A computer store has a series of additional storage locations each associated with a main store address and each storing at least one digit. The additional locations have a "1" and a "0" written in when the associated address in the main store is written to and read from respectively. Before writing to the additional location its previous contents are read and fed to an AND gate together with the about to be written digit for that location. A "1" output from the AND gate, indicating an attempt to overwrite in the main store before its contents have been read, generates a warning or interrupt signal. At the end of a program block the corresponding block of additional storage locations can be read to determine whether any "1"s remain, thus indicating information in the main store written in during the program but unread.