Example: --allow-const-ram-index

subtype t_dim1 is std_logic_vector(7 downto 0);
type t_dim1_vector is array(natural range <>) of t_dim1;
subtype t_dim2 is t_dim1_vector(0 to 511);
type t_dim3_vector is array(natural range <>) of t_dim2;
subtype t_dim3 is t_dim3_vector(0 to 3);

signal RAM : t_dim3 := (others => (others => (others => '0')));
...
RAM3Proc_t : process(Clk)
begin
if(rising_edge(Clk)) then
  if(WriteEn = '1') then
    RAM(3)(to_integer(unsigned(Addr))) <= WriteData3_t;
  end if;
  ReadData3_t <= RAM(3)(to_integer(unsigned(Addr)));
end if;
end process RAM3Proc_t;

RAM2Proc_t : process(Clk)
begin
if(rising_edge(Clk)) then
  if(WriteEn = '1') then
    RAM(2)(to_integer(unsigned(Addr))) <= WriteData2_t;
  end if;
  ReadData2_t <= RAM(2)(to_integer(unsigned(Addr)));
end if;
end process RAM2Proc_t;

...