code: public final long getRootDirOffset() {
          long sectSize = this.getBytesPerSector();
          long sectsPerFat = this.getSectorsPerFat();
          int fats = this.getNrFats();
  
          long offset = getFatOffset(0);
          
          offset += fats * sectsPerFat * sectSize;
  
          return offset;
      }
docs: Gets the offset (in bytes) of the root directory with the given index
  
  @param bs
  @return long
  @throws IOException