void ReservedSpace::protect_noaccess_prefix(const size_t size) {

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert( (_noaccess_prefix != 0) == (UseCompressedOops && _base != NULL &&
                                          (size_t(_base + _size) > OopEncodingHeapMax) &&
                                          Universe::narrow_oop_use_implicit_null_checks()),
             "noaccess_prefix should be used only with non zero based compressed oops");

  {- -------------------------------------------
  (1) もし access prefix が不要であれば, ここでリターン
      ---------------------------------------- -}

      // If there is no noaccess prefix, return.
      if (_noaccess_prefix == 0) return;

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert(_noaccess_prefix >= (size_t)os::vm_page_size(),
             "must be at least page size big");

  {- -------------------------------------------
  (1) os::protect_memory() で, ヒープの先頭を access prefix 分だけアクセス禁止にする.
      失敗したら, fatal() によりここで異常終了.
      ---------------------------------------- -}

      // Protect memory at the base of the allocated region.
      // If special, the page was committed (only matters on windows)
      if (!os::protect_memory(_base, _noaccess_prefix, os::MEM_PROT_NONE,
                              _special)) {
        fatal("cannot protect protection page");
      }

  {- -------------------------------------------
  (1) (トレース出力)
      ---------------------------------------- -}

      if (PrintCompressedOopsMode) {
        tty->cr();
        tty->print_cr("Protected page at the reserved heap base: " PTR_FORMAT " / " INTX_FORMAT " bytes", _base, _noaccess_prefix);
      }

  {- -------------------------------------------
  (1) アクセス禁止にした分だけ _base と _size を微調整しておく
      ---------------------------------------- -}

      _base += _noaccess_prefix;
      _size -= _noaccess_prefix;

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert((size == _size) && ((uintptr_t)_base % _alignment == 0),
             "must be exactly of required size and alignment");
    }