INT 21 U - DOS 4+ internal - GET/SET DISK SERIAL NUMBER
        AH = 69h
        AL = subfunction
            00h get serial number
            01h set serial number
        BL = drive (0=default, 1=A, 2=B, etc)
        DS:DX -> disk info (see below)
Return: CF set on error
            AX = error code (see AH=59h)
        CF clear if successful
            AX destroyed
            (AL = 00h) buffer filled with appropriate values from extended BPB
            (AL = 01h) extended BPB on disk set to values from buffer
Notes:  does not generate a critical error; all errors are returned in AX
        error 0005h given if no extended BPB on disk
        does not work on network drives (error 0001h)
        buffer after first two bytes is exact copy of bytes 27h thru 3Dh of
          extended BPB on disk
        this function is supported under Novell NetWare versions 2.0A through
          3.11; the returned serial number is the one a DIR would display,
          the volume label is the NetWare volume label, and the file system
          is set to "FAT16".
        the serial number is computed from the current date and time when the
          disk is created; the first part is the sum of the seconds/hundredths
          and month/day, the second part is the sum of the hours/minutes and
          year
        the volume label which is read or set is the one stored in the extended
          BPB on disks formatted with DOS 4.0+, rather than the special root
          directory entry used by the DIR command in COMMAND.COM (use AH=11h
          to find that volume label)
SeeAlso: AX=440Dh

Format of disk info:
Offset  Size    Description
 00h    WORD    info level (zero)
 02h    DWORD   disk serial number (binary)
 06h 11 BYTEs   volume label or "NO NAME    " if none present
 11h  8 BYTEs   (AL=00h only) filesystem type--string "FAT12   " or "FAT16   "

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