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        Get disk serial number (INT 21h, 69h - Internal)

        Entry:
          AH    = 69h
          AL    = 0
          DS:DX = Buffer (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
            Buffer filled with appropriate values

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

          Buffer:
          Offset Size   Contens
          00-01  Word   ??? (zero)
          02-05  DWord  Disk serial number (binary)
          06-11  Bytes  Volume label or "NO NAME    " if none present
          12-1A  Bytes  8 BYTEs FAT type--string "FAT12   " or "FAT16  "