---------------------------------------------------------------
--             Four Bit Register Example                    --
--                  weiwang 4.12.98                         --
---------------------------------------------------------------

PACKAGE resources IS
                        -- user defined enumerated type
   TYPE level IS ('X', '0', '1', 'Z');
--    ALIAS level IS X01Z ;                   
                        -- type for vectors (buses)
   TYPE level_vector IS ARRAY (NATURAL RANGE <>) OF level;
                        -- subtype used for delays
   SUBTYPE delay IS time range 0 fs to 1 sec;
  
  FUNCTION level_it(char : character) return level; 
END resources;

PACKAGE BODY resources IS   

FUNCTION level_it(char   : character) RETURN level IS
    begin
                if char = '0' then
                        return '0';
                elsif char = '1' then
                        return '1';
                elsif char = 'X' then
                        return 'X';
                elsif char = 'Z' then
                        return 'Z';
                else  
                        return 'X';
                end if;
  end level_it;

 END resources;


USE work.resources.all;

ENTITY reg4 IS
  GENERIC(tprop : delay := 8 ns;
          tsu   : delay := 2 ns);

  PORT(d   : IN bit_vector(3 downto 0);
       clk : IN bit;
       q   : OUT bit_vector(3 downto 0));
       --qn  : OUT bit_vector(3 downto 0));
END reg4;

ARCHITECTURE behavioral OF reg4 IS

  BEGIN

    one : PROCESS (clk)

      BEGIN
        IF (clk = '1' AND clk'LAST_VALUE = '0') THEN    -- rising clock edge
          IF (d'STABLE(tsu)) THEN                       -- check setup
            FOR i IN 3 DOWNTO 0 LOOP
              IF (d(i) = '0') THEN                      -- check valid input data
                q(i) <= '0' AFTER tprop;
                --qn(i) <= '1' AFTER tprop;
              ELSIF (d(i) = '1') THEN
                q(i) <= '1' AFTER tprop;
                --qn(i) <= '0' AFTER tprop;
              END IF;
            END LOOP;
          END IF;
        END IF;
    END PROCESS one;

END behavioral;