function string_address (var str : [readonly] packed array [l..h:integer] of char) : integer;
  begin
  string_address := iaddress(str);
  end;

  { Calculate Button }
  Procedure Calculate_Button_Pressed;
  var s           : varying [10] of char;
      i           : integer;
      work_proc   : Dwt$Work_Proc;
      client_data : Dwt$Opaque;
      args        : array [1..2] of dwt$arg;
  begin
  writeln('in Calculate_Button_Pressed');

    { Clear out the old entries in the list box }
    if interval_number > 0 then
      begin
      i := string_address('items'(0));          { Dwt$C_Nitems }
      args[1].dwt$a_arg_name := i::Dwt$Strng;
      args[1].dwt$l_arg_value := 0;
      i := string_address('itemsCount'(0));     { Dwt$C_NitemsCount }
      args[2].dwt$a_arg_name := i::Dwt$Strng;
      args[2].dwt$l_arg_value := 0;
      Xt$Set_Values (list_box, args, 2);
      interval_number := 0;
      end;

    { Update the input fields to reflect the values in use }
    writev(s,initial_loan:1);        Dwt$S_Text_Set_String (input_fields[principal].widget, s);
    writev(s,interest_rate:1:2);     Dwt$S_Text_Set_String (input_fields[rate].widget, s);
    writev(s,repayment_amount:1:2);  Dwt$S_Text_Set_String (input_fields[payment].widget, s);

    i := iaddress(Calculate_One_Interval);
    work_proc := i::Dwt$Work_Proc;
    client_data := i::Dwt$Opaque;
    Xt$Add_Work_Proc (work_proc, client_data);

    end;
  end;