----------------------------------------------------------------
-- Rebuild the 4-bit latch using structural description using 
-- positional signal association. Use latch as the building 
-- component. Write a configuration declaration for the latch.
----------------------------------------------------------------

ENTITY latch4 IS
  PORT( d: IN bit_vector(3 DOWNTO 0); 
        en,clk: IN bit;
        q: OUT bit_vector(3 DOWNTO 0));
END ENTITY latch4;

ARCHITECTURE struct OF latch4 IS
  COMPONENT latch
    PORT( d, clk: IN bit; 
          q, nq: OUT bit);
  END COMPONENT latch;

  COMPONENT and2
    PORT( a, b: IN bit; c: OUT bit);
  END COMPONENT and2;

  SIGNAL int_clk: bit;
BEGIN
  bit3: latch
    PORT MAP (d(3), int_clk, q(3));
  bit2: latch
    PORT MAP (d(2), int_clk, q(2));
  bit1: latch
    PORT MAP (d(1), int_clk, q(1));
  bit0: latch
    PORT MAP (d(0), int_clk, q(0));
  gate: and2
    PORT MAP (en,clk, int_clk);
END ARCHITECTURE struct;

CONFIGURATION struct_conf OF latch4 IS
  FOR struct
    FOR ALL: latch
      USE ENTITY WORK.latch(behav);
    END FOR;
    FOR gate: and2
      USE ENTITY WORK.and2(and2);
    END FOR;
  END FOR;
END CONFIGURATION struct_conf;