David RossDavid Ross 522 bronze badges one one The SUB is an artefact of CP/M, in which it had been utilized as an EOF marker (CP/M "size" was blocks, not bytes). MSDOS was made to be able to go through CP/M documents, & since it was most likely at enough time that CP/M would be used to read through the resuts from MSDOS, the ability to control the