David dos or pronto dos ( one of them ) supported 40 tracks.. then there was the trick to format a data disk and recoup the first 3 tracks by not putting a dos image on it.. 40 Tracks * 16 Sectors per track = 640 sectors - boot sector and catalog track ( 17 sectors ) = 623 Sectors = 155.75k.. that sure beat the 128k of usable space per dos formattted disk.. K.