David RossDavid Ross 522 bronze badges 1 1 The SUB is really an artefact of CP/M, where by it absolutely was employed being an EOF marker (CP/M "length" was blocks, not bytes). MSDOS was made to be able to read CP/M documents, & as it was very likely at some time that CP/M could be accustomed to study the resuts from MSDOS, the ability to control t