Abstract: A data item input processor operates on the smallest subdivision of data as it is entered into a data processing system to provide control and/or error functions nearly simultaneously with the entry of each such data item. As each item is entered a tree logic search is conducted within a stored data base to determine if the entered item is permissible, that is, whether it completes an allowable sequence of data items. If the item cannot be found a signal is provided which may be used to initiate any function desired by the user.