protected function getOSPrefix($os, $flags = 0x000000) {

    $letter = NULL;
    if ($this->isWindowsDrivePrefix($letter)) {
      $osPrefix = '';

      if (($flags & self::WINDOWS_WITH_CYGWIN) && $os === self::WINDOWS) {
        $osPrefix .= '/cygdrive/'.mb_strtolower($letter).'/';
      } else {

        if (!($flags & self::WINDOWS_DRIVE_WINDOWS_STYLE) && $os === self::UNIX) {
          $osPrefix  .= '/';
        }

        $osPrefix .= $letter.':'.$this->getOSDS($os);
      }

    } else {
      $osPrefix = $this->prefix;
    }

    return $osPrefix;
  }