code: public final long getFatOffset(int fatNr) {
          long sectSize = this.getBytesPerSector();
          long sectsPerFat = this.getSectorsPerFat();
          long resSects = this.getNrReservedSectors();
  
          long offset = resSects * sectSize;
          long fatSize = sectsPerFat * sectSize;
  
          offset += fatNr * fatSize;
  
          return offset;
      }
docs: Gets the offset (in bytes) of the fat with the given index
  
  @param bs
  @param fatNr (0..)
  @return long
  @throws IOException