Как создать свою библиотеку?

library CalcLib;
 
{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }
 
uses
  SysUtils;
 
{$R *.res}
 
//Площадь прямоугольника.
function RectArea(const aLen, aWidth : Extended) : Extended; stdcall;
begin
  Result := aLen * aWidth;
end;
 
//Площадь круга.
function CircleArea(const aR : Extended) : Extended; stdcall;
begin
  Result := Pi * aR * aR;
end;
 
exports RectArea, CircleArea;
 
begin
end.

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
 
type
  TForm1 = class(TForm)
    Button1: TButton;
    Memo1: TMemo;
    Edit1: TEdit;
    Edit2: TEdit;
    Edit3: TEdit;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
 
//Площадь прямоугольника.
function RectArea(const aLen, aWidth : Extended) : Extended; stdcall;
  external 'CalcLib.dll' name 'RectArea';
 
//Площадь круга.
function CircleArea(const aR : Extended) : Extended; stdcall;
  external 'CalcLib.dll' name 'CircleArea';
 
implementation
 
{$R *.dfm}
 
procedure TForm1.Button1Click(Sender: TObject);
var
  L, W, R, A : Extended;
begin
  Memo1.Clear;
  Memo1.Lines.Add('Площади фигур:');
 
  //Прямоугольник.
  //Длина.
  L := StrToFloat(Edit1.Text);
  //Ширина.
  W := StrToFloat(Edit2.Text);
  //Площадь.
  A := RectArea(L, W);
  Memo1.Lines.Add('Площадь прямоугольника = ' + FloatToStr(A));
 
  //Круг.
  //Радиус.
  R := StrToFloat(Edit3.Text);
  //Площадь.
  A := CircleArea(R);
  Memo1.Lines.Add('Площадь круга = ' + FloatToStr(A));
end;
 
end.