Base converter binary to octal

I'm writing a base converter because I have a test soon and I need to convert a binary number in 3 different bases: octal, decimal and hexadecimal. I've already written the code that convert a binary string into decimal and hexadecimal.

function bintodec(Value:string;dec:TEdit;hexadec:TEdit): Integer;
 var             //dec and hexadec are the TEdits where I will put the result
  i, iValueSize: Integer;
  Edit2,f:TEdit;
begin
  Result := 0;
  iValueSize := Length(Value);
  for i := iValueSize downto 1 do
    begin
      if Value[i] = '1' then Result := Result + (1 shl (iValueSize - i));
    end;
  dec.Text:=(IntToStr(Result));        //dec. number
  hexadec.Text:=(IntToHex(Result,8));  //hexadec. number
end;

As you can see here, the function takes a string (for example 10101001) and puts into 2 different edit the result.

I've made a function that convert a decimal number into an octal number but when I press the SpeedButton Calc. I have an error. It says that project1 raised a class exception 'External: SIGSEGV' and then near to Unit1 I see the page control.inc. I've searched on google a solution but I didn't find useful answers.

function dec2oct(mystring:Integer): String;
  var
  a: String;
  getal_met_rest : Double;
  Edit2:TEdit;
  begin
    while mystring> 0 do
         begin
            getal_met_rest := getal / 8;
            a:= a + IntToStr(mystring - (trunc(getal_met_rest)*8));
            getal := trunc(getal_met_rest);
         end;
    dec2oct:=ReverseString(a);
    Edit2.text:=dec2oct
  end; 

I didn't find a way for binary-octal conversion, so once I've converted from binary to decimal, I call the function dec2oct . I call the functions in this way:

var a:smallint;
begin
 bintodec(Edit1.Text,Edit3,Edit4);
 dec2oct(Edit3.Text); //Edit3 contains the number on base 10
end;

Could you help me?

I would usually use string or character arrays and bit arithmetic for such conversions. For instance:

function Int2Oct(invalue: integer): ShortString;
  const
    tt: array[0..7] of char = ('0', '1', '2', '3', '4', '5', '6', '7');
  var
    tempval: integer;
  begin
    Result := '';
    tempval := invalue;
    if tempval = 0 then
      Result := '0'
    else
      while (tempval <> 0) do
        begin
          Result := tt[(tempval and $7)] + Result;
          tempval := (tempval shr 3);
        end;
  end;

Seems to work in the short little bit of time that I tested it as long as you don't expect it to handle negative numbers. Edit: It handles zero now.