菜鸟基础问题，散分~~~~~~~~~~~~~~

Question

我要写一个替换二进制文件中字符串的程序，由于目标文件可能比较大，在20m左右，所以打算用内存映射，但是我不清楚如何读取指定地址的n个字符（n由EDIT读入），存为String，望高手指导procedure TForm1.Button1Click(Sender: TObject);
var
  h:Cardinal;
  p,p1:Pointer;
  ch: array[0..10] of char;
  fs:TFileStream;
begin
    if open.Execute then fs:=TFileStream.Create(open.FileName ,fmOpenReadWrite or fmShareExclusive);
    try
       h := CreateFileMapping(fs.Handle, nil, PAGE_READWRITE,0, 0,nil);
       if (h = INVALID_HANDLE_VALUE) or (h = 0) then
          raise Exception.Create('Error!')
       else
       begin
            try
               p := MapViewOfFile(h, FILE_MAP_ALL_ACCESS, 0, 0, 0);
     ***          try
                  p1:=(PChar(p)+$3A1C32);
                  copymemory(@ch,p1,20);                  showmessage(inttohex(ord(ch[strtoint(edit1.Text)]),8));
                               finally
                      UnMapViewOfFile(p);
     ***          end;
            finally
                   CloseHandle(h);
            end;
       end;
    finally
           fs.Free;
    end;其中加*的部分应该如何改进？？？