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  Interrupt:   21h     Functions:  1Bh and 1Ch
 
  Title:       Get Drive Data
 
  See also:    Select Disk, Get Drive Parameters, @ChkDrv
 
  Description:
 
     Obtains allocation information about the default disk drive
     (Function 1Bh) or about a specified disk drive (Function 1Ch).
     Gets a pointer to the media identification byte from its file
     allocation table.
 
     Input                    Output
 
     AH = 1Bh for default     If function successful
          drive                 AL = Sectors per cluster
        = 1Ch for any           DS:BX = segment:offset of media ID byte
          drive                 CX = Size of physical sector (bytes)
     DL = Drive code for        DX = Number of clusters for default or
          1Ch only                   specified drive
          (0 = default,
           1 = A,...)         If function unsuccessful (invalid drive or
                              critical error)
                                AL = FFh
                                    -♦-