David RossDavid Ross 522 bronze badges 1 1 The SUB is really an artefact of CP/M, wherever it absolutely was applied being an EOF marker (CP/M "duration" was blocks, not bytes). MSDOS was designed to be able to examine CP/M files, & since it was likely at the time that CP/M could be used to read through the resuts from MSDOS, the ability to control