Subversion Repositories v6502

library IEEE;

use IEEE.std_logic_1164.all;  -- defines std_logic types

use IEEE.STD_LOGIC_unsigned.all;

use IEEE.STD_LOGIC_arith.all;

-- 16 bit two-way multiplexer

entity addrmux is

  port(  sel:  in STD_LOGIC_VECTOR(1 downto 0);

           a:  in STD_LOGIC_VECTOR(15 downto 0);

           b:  in STD_LOGIC_VECTOR(15 downto 0);

           s:  in STD_LOGIC_VECTOR(15 downto 0);

           y: out STD_LOGIC_VECTOR(15 downto 0)

      );

end addrmux;

architecture comb of addrmux is

constant ADPC: STD_LOGIC_VECTOR(1 downto 0) := "00";  -- select PC

constant ADMP: STD_LOGIC_VECTOR(1 downto 0) := "01";  -- select MP

constant ADSP: STD_LOGIC_VECTOR(1 downto 0) := "10";  -- select SP

constant ADNP: STD_LOGIC_VECTOR(1 downto 0) := "00";  -- no operation

begin

  process(sel,a,b,s)

  begin

    case sel is

      when ADPC   => y <= a;

      when ADMP   => y <= b;

      when ADSP   => y <= s;

      when others => y <= a;

    end case;

  end process;

end comb;