David RossDavid Ross 522 bronze badges 1 one The SUB is surely an artefact of CP/M, in which it absolutely was employed as an EOF marker (CP/M "length" was blocks, not bytes). MSDOS was developed in order to examine CP/M data files, & since it was very likely at enough time that CP/M will be used to study the resuts from MSDOS, the chance to contro