Function 19h (25)        Get Current Disk

    Gets the drive number of the current default drive.

       On entry:      AH         19h

       Returns:       AL         Number of default drive (A = 0, B = 1, etc.)

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       Note:          The drive numbers returned by this function are one
                      less than the drive numbers used by many functions;
                      many functions use 0 for the default drive and start
                      with A = 1 (rather than A = 0).