type
  T = record
    Str: string;
    Use: Boolean;
  end;

private
  A: Array[0..5] of T;
  CS: TRTLCriticalSection;

procedure TThread.Add(const S: string);
begin
  EnterCriticalSection(CS);
  try
    with A[GetFreeField] do
    begin
      Str := S;
      Use := True;
    end;
  finally
    LeaveCriticalSection(CS);
  end;
end;

procedure TThread.Execute;
begin
  while not Terminated do
  begin
    for I := Low(A) to High(A) do
    begin
      EnterCriticalSection(CS);
      if Use then
      begin
        LeaveCriticalSection(CS);
        //hier werden nur Felder von A[I] gelesen und geändert
      end
      else
        LeaveCriticalSection(CS);
    end;
    Sleep(1);
  end;
end;