David RossDavid Ross 522 bronze badges one one The SUB is really an artefact of CP/M, where it was utilized as an EOF marker (CP/M "size" was blocks, not bytes). MSDOS was made in order to read CP/M files, & since it was very likely at some time that CP/M could be used to go through the resuts from MSDOS, the opportunity to control the end-of-file