怎样实现加减乘除的混合运算?

我编写了一个计算器程序,但不知怎样实现加减乘除的混合运算,请各位高手指点.下面是我的源程序,我想请问一下,如果我想把m:=1;这句加在procedure外面,不知要放在哪里?
unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
Button2: TButton;
Button3: TButton;
Button4: TButton;
Button5: TButton;
Button6: TButton;
Button7: TButton;
Button8: TButton;
Button9: TButton;
Button10: TButton;
Button11: TButton;
Button12: TButton;
Button13: TButton;
Button14: TButton;
Button15: TButton;
Button16: TButton;
Button17: TButton;
Button18: TButton;
Edit1: TEdit;
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure Button3Click(Sender: TObject);
procedure Button4Click(Sender: TObject);
procedure Button5Click(Sender: TObject);
procedure Button6Click(Sender: TObject);
procedure Button7Click(Sender: TObject);
procedure Button8Click(Sender: TObject);
procedure Button9Click(Sender: TObject);
procedure Button10Click(Sender: TObject);
procedure Button11Click(Sender: TObject);
procedure Button12Click(Sender: TObject);
procedure Button13Click(Sender: TObject);
procedure Button14Click(Sender: TObject);
procedure Button15Click(Sender: TObject);
procedure Button16Click(Sender: TObject);
procedure Button17Click(Sender: TObject);
procedure Button18Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
x,y,z,m: real;
flag: char;

implementation

{$R *.dfm}



procedure TForm1.Button1Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='1'
else
edit1.Text:=edit1.Text+'1';
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='2' else
edit1.Text:=edit1.Text+'2';
end;

procedure TForm1.Button3Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='3' else
edit1.Text:=edit1.Text+'3';
end;

procedure TForm1.Button4Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='4' else
edit1.Text:=edit1.Text+'4';
end;

procedure TForm1.Button5Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='5' else
edit1.Text:=edit1.Text+'5';
end;

procedure TForm1.Button6Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='6' else
edit1.Text:=edit1.Text+'6';
end;

procedure TForm1.Button7Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='7' else
edit1.Text:=edit1.Text+'7';
end;

procedure TForm1.Button8Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='8' else
edit1.Text:=edit1.Text+'8';
end;

procedure TForm1.Button9Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='9' else
edit1.Text:=edit1.Text+'9';
end;

procedure TForm1.Button10Click(Sender: TObject);
begin
if edit1.Text='0' then edit1.Text:='0' else
edit1.Text:=edit1.Text+'0';
end;

procedure TForm1.Button11Click(Sender: TObject);
begin
if edit1.Text='' then edit1.Text:=''
else
begin
if pos('.',edit1.Text)<>0
then edit1.Text:=edit1.Text
else edit1.Text:=edit1.Text+'.';
end;
end;

procedure TForm1.Button12Click(Sender: TObject);
begin
edit1.Clear;
end;

procedure TForm1.Button13Click(Sender: TObject);
begin
if edit1.Text='' then edit1.Text:=''
else begin

flag:='+';
x:=strtofloat(edit1.Text);
z:=x+z;

edit1.Clear;
end;
end;

procedure TForm1.Button14Click(Sender: TObject);
begin
if edit1.Text='' then edit1.Text:=''
else begin
flag:='-';
x:=strtofloat(edit1.Text);
z:=x-z;
edit1.Clear;
end;
end;

procedure TForm1.Button15Click(Sender: TObject);
begin
if edit1.Text='' then edit1.Text:=''
else begin
flag:='*';
x:=strtofloat(edit1.Text);
z:=x*m;
edit1.Clear;
end;
end;

procedure TForm1.Button16Click(Sender: TObject);
begin
if edit1.Text='' then edit1.Text:=''
else begin
flag:='/';
x:=strtofloat(edit1.Text);
z:=x/m;
edit1.Clear;
end;
end;

procedure TForm1.Button17Click(Sender: TObject);
begin
if edit1.Text='' then edit1.Text:=''
else
case flag of
'+': begin
edit1.text:=floattostr(z+strtofloat(edit1.Text));
z:=0;
end;
'-':begin
edit1.text:=floattostr(z-strtofloat(edit1.Text));
z:=0;
end;
'*': begin
edit1.text:=floattostr(m*strtofloat(edit1.Text));
m:=1;
end;
'/': begin
y:=strtofloat(edit1.Text);
if y=0 then edit1.Text:='99999999999999999999999999999999'
else begin
edit1.text:=floattostr(m/strtofloat(edit1.Text));
m:=1;
end;
end;
end;

end;
procedure TForm1.Button18Click(Sender: TObject);
begin
close;
end;

end.