Function 3305h          Get Startup Drive

    Returns the drive that was used to load DOS.

        Entry   AX = 3305h

        Return  DL = Drive number (1 = A, 2 = B, 3 = C etc.)

    ──────────────────────────────────────────────────────────────────

    On a ROM-based version of DOS, there may be no startup drive; in
    this case, this function returns the number of the drive
    containing the CONFIG.SYS file.


    With DOS 5.0, function 3305h has been documented for DOS version
    2.0 and later.