Select scan line

        Entry:
          AX = CD03h
          BX = scan line
          CX = requested density in dots per inch (300, 600, or 1200)
        Return:
          AL = 00h succesful
                CX = density at which scan line was mapped
                ES:DI -> start of scan line
          AL = 80h unsuccessful
             = 81h scan line out of range
             = 82h unsupported scan line density
             = 83h out of memory