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Function 1Bh (27)        Get FAT Information for Current Drive

    Gets disk characteristics and the media descriptor byte for the
    current drive. (The media descriptor byte specifies the type of
    drive.)

       On entry:      AH         1Bh

       Returns:       AL         Number of sectors per cluster
                      CX         Sector size in bytes
                      DX         Total number of disk clusters
                      DS:BX      Pointer to the media descriptor byte
                                 (FAT ID byte)

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  Media Descriptor (FAT ID) Byte

    The media descriptor byte is limited to the values F0 through FF. This
    range is no longer sufficient to provide unique identifiers for each
    media type. The following values are defined:

          FFh            Double-sided 5 1/4" floppy, 8 sectors
          FEh            Single-sided 5 1/4" floppy, 8 sectors or
                         Single-sided 8" floppy, 26 sectors or
                         Single-sided 8" floppy, 8 sectors
          FDh            Double-sided 5 1/4" floppy, 9 sectors or
                         Single-sided 8" floppy, 26 sectors
          FCh            Single-sided floppy, 9 sectors per track
          F9h            Double-sided 5 1/4" floppy, 15 sectors or
                         Double-sided 3 1/2" floppy, 9 sectors
          F8h            Hard disk
          F0h            Double-sided 3 1/2" floppy, 18 sectors or
                         Any other undefined media type

    For more reliable information about media type, use Interrupt 25h to
    read the BIOS Parameter Block (BPB) in the boot record.

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       Notes:         This function changes the current value of DS. To
                      ensure the stability of your program, it's best to
                      PUSH DS before calling this service.

                      To perform the same function for a drive other than
                      the default, use Function 1Ch.