David RossDavid Ross 522 bronze badges 1 1 The SUB is an artefact of CP/M, exactly where it had been applied as an EOF marker (CP/M "size" was blocks, not bytes). MSDOS was made in order to go through CP/M files, & as it was very likely at enough time that CP/M could well be accustomed to read the resuts from MSDOS, the chance to control the end-of