create two elements connecting to one mux 41 and 21

Question

I have big problem because i dont uderstand properly how make my homework. Well i have to make something like this:
http://tomaszewicz.zpt.tele.pw.edu.pl/files/u1/zad4.gif
I have code which create b1 but i dont knwo how to create the second and make them connect to b3.

My code is:

library ieee;
use ieee.std_logic_1164.all;

entity test is
generic(
n : integer := 4
);
port(
a, b, c, d : in std_logic_vector(n-1 downto 0);
s : in std_logic_vector(1 downto 0);
y : out std_logic_vector(n-1 downto 0)
);
end test;



-- przypisanie sekwencyjne - case
architecture arch_mux5 of test is
begin
pr_case: process(a,b,c,d,s)
begin
case s is
when "00" => y <= a;
when "01" => y <= b;
when "10" => y <= c;
when others => y <= d;
end case;
end process;
end arch_mux5;

architecture arch_mux6 of test is
begin
pr_if: process(a,b,c,d,s)
begin
y <= (others => '0'); -- latch jesli zakomentujemy, dlaczego?
if s = "00" then
y <= a;
end if;
if s = "01" then
y <= b;
end if;
if s = "10" then
y <= c;
end if;
if s = "11" then
y <= d;
end if;
end process;
end arch_mux6;

configuration cfg of test is
for arch_mux5
end for;
end cfg;

mux5 and mux6 seems to be the same but in different write method.

Answer 1

You have to instantiate those multiplexers, e.g.:

entity top is
  generic (
    n: integer:=4
  );
  port (
    a, b, c, d, e, f, g, h: in std_logic_vector(n-1 downto 0);
    s: in std_logic_vector(2 downto 0);
    y: out std_logic_vector(n-1 downto 0)
  );
end entity top;

architecture struct of top is
  signal t1, t2: std_logic_vector(n-1 downto 0);
  component test is
    generic(
      n : integer := 4
    );
    port (
      a, b, c, d : in std_logic_vector(n-1 downto 0);
      s : in std_logic_vector(1 downto 0);
      y : out std_logic_vector(n-1 downto 0)
    );
  end component test;
  component mux2 is
    generic(
      n : integer := 4
    );
    port (
      a, b : in std_logic_vector(n-1 downto 0);
      s : in std_logic;
      y : out std_logic_vector(n-1 downto 0)
    );
  end component test;
begin
  b1: test
    generic_map (
      n => n
    );
    port map (
      a => a,
      b => b,
      c => c,
      d => d,
      s => s(1 downto 0),
      y => t1
    );
  b2: test
    generic_map (
      n => n
    );
    port map (
      e => a,
      f => b,
      g => c,
      h => d,
      s => s(1 downto 0),
      y => t2
    );
  b3: mux2
    generic_map (
      n => n
    );
    port map (
      a => t1,
      b => t2,
      s => s(2),
      y => y
    );
end architecture struct;

Of course you still have to write the entity+architecture for mux2. I didn't test this code (don't have a VHDL compiler here) but that should at least lead you into the correct direction.