David RossDavid Ross 522 bronze badges 1 one The SUB is definitely an artefact of CP/M, where it absolutely was made use of as an EOF marker (CP/M "size" was blocks, not bytes). MSDOS was intended to be able to go through CP/M information, & because it was probably at time that CP/M can be used to study the resuts from MSDOS, a chance to control th