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var
  F: File of Char;
  sFilename: String;
  sFileString: AnsiString;
  TmpStr: AnsiString;
  Ch: Char;
  iTmpLen: Integer;
  iAscii: Integer;

      AssignFile(F, sFilename);
      Reset(F);
        Seek(F, FileSize(F));
        for iTmpLen := 1 To Length(sFileString) Do
        begin
          //Ch := Copy(TmpStr, iTmpLen, 1);
          for iAscii := 0 To 255 Do
          begin
            if Chr(iAscii) = Copy(sFileString, iTmpLen, 1) Then
            begin
              Ch := Chr(iAscii);
              break;
            end;
          end;
          Write(F, Ch);
        end;
      CloseFile(F);					

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var
  FS: TFileStream;

begin
  FS := TFileStream.Create (bla bla)
  try
    fs.Write(PChar(MegaAnsiString)^, Length(MegaAnsiString));  
  finaly
    fs.Free;
  end;
end;