David RossDavid Ross 522 bronze badges 1 1 The SUB is undoubtedly an artefact of CP/M, where it absolutely was used as an EOF marker (CP/M "duration" was blocks, not bytes). MSDOS was built in order to read CP/M documents, & because it was probably at the time that CP/M could well be accustomed to browse the resuts from MSDOS, the chance to control