1. Apparatus for writing a plurality of times in a non-erasable memory comprising means for receiving incoming digital data and for forming sequential groups from said received data, means for reading successive groups of digital data from successive groups of positions in the non-erasable memory, means responsive to each said formed sequential group and to a respective read successive group from memory for generating successive new coded groups of digital data, and means for sequentially writing into said memory each said group of data at a said group of positions corresponding to said respective read group of data, whereby said coded groups of data can be later read and uniquely decoded to generate said formed sequential groups.