private function _loadNorm($fieldNum)
    {
        if ($this->_hasSingleNormFile) {
            $normfFile = $this->openCompoundFile('.nrm');

            $header              = $normfFile->readBytes(3);
            $headerFormatVersion = $normfFile->readByte();

            if ($header != 'NRM'  ||  $headerFormatVersion != (int)0xFF) {
                require_once 'Zend/Search/Lucene/Exception.php';
                throw new  Zend_Search_Lucene_Exception('Wrong norms file format.');
            }

            foreach ($this->_fields as $fNum => $fieldInfo) {
                if ($fieldInfo->isIndexed) {
                    $this->_norms[$fNum] = $normfFile->readBytes($this->_docCount);
                }
            }
        } else {
            $fFile = $this->openCompoundFile('.f' . $fieldNum);
            $this->_norms[$fieldNum] = $fFile->readBytes($this->_docCount);
        }
    }