Methods and apparatus for the encoding of an input sequence of digit data into a sequence of storage cells of a ROM device are disclosed. The input sequence is divided into a first kind of groups and a second kind of groups. A first kind of group comprises a plurality of consecutive first digits, two first kind of groups are separated by a second kind of group, the second kind of group comprises consecutive digits without any consecutive first digits, and the second kind of group has a starting digit which is the second digit. A starting storage cell is programmed to the active state to store the starting digit of the second kind of group. The rest digits of the second kind of group are programmed one digit at a time, based on a shared terminal which has been programmed for the proceeding storage cell.