array of byte : how to compute its length

[virtual]

function getPtrLength(_ptr : PBYTE) : integer;
begin
Result := 0;
  while ( _ptr <> NIL ) do
  begin
     inc(Result);
     inc(_ptr);
  end;
end;

why this function cause a loop for ever when passed an array of byte

when use it like this

var 
  ptr , _start : PByte;
begin
  getmem(ptr,100);
  _start := ptr;
  for i := 1 to 100 do
  begin
     ptr^ := Random(100); // fill our array with random numbers
     inc(ptr);
  end;
//use our function here
ShowMessage(inttostr(getPtrLength(_start))); // expected result is 100 !! but its not
end.

whats wrong here , thanks