Ошибка "Access violations at address . in module Project1.exe ." Как обнаружить/устранить?

procedure TForm3.btn4Click(Sender: TObject);
 var d10 : Real; kk,h,j,j1,z : Integer;
  begin
  edt4.Text := edt1.Text;
  j1 := k - b;   
  j := j1;
   for h := b+1 to k do
   begin
    edt6.Text := edt6.Text + edt1.Text[h];
     end;   
    for z := 1 to j1 do
   begin
    edt4.Text := edt4.Text + edt1.Text[z];
     end;
   while k >= b do
   begin
    begin
    i := 1;
    l := 0;
    while i < k do
    if strtoint ( edt4.Text[i] ) <> strtoint (edt2.Text[i] )
    then begin l := i; i := k+1; end
     else begin i := i+1;j := j+1; end;
         l2 := l + b - 1;
         if l2>k then l2 := k;
      for l1 := l to l2 do
    if l1 > b then  else
      if strtoint( edt4.Text[l1] ) = strtoint( edt2.Text[l1] ) then edt5.Text := edt5.Text+'0' else edt5.Text := edt5.Text+'1';
     end;
    edt4.Text := edt5.Text;
    edt5.Clear;
    d10 := StrToFloat(edt4.Text);
    begin
      kk := 0;
       while d10 >= 1 do
      begin
       d10 := d10 / 10;
       kk := kk + 1;
       end;
       end;
    k := kk;
     while ( k < b ) and ( j > 0 ) do
     begin
     edt4.Text := edt4.Text + edt6.Text[1];
     k := k + 1;
     j := j - 1;
      if j < 2 then j := 0 else
     edt6.Text := Copy(edt6.Text,2,length(edt6.Text)-0);
     end;
   end;
   end;

  d12, d11 : Real;
  km, d121, l1, l2, l, j, i, k, k1, p0, p11, b : Integer;

edt4.Text := edt4.Text + edt6.Text[1];

edt4.Text := edt4.Text + edt1.Text[z];

[Fatal Error] Unit3.pas(137): Internal error: C1091