Subversion Repositories v65c816

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(2 downto 0);

           a:  in STD_LOGIC_VECTOR(23 downto 0);

           b:  in STD_LOGIC_VECTOR(23 downto 0);

                        dbr:  in STD_LOGIC_VECTOR(7 downto 0);

           s:  in STD_LOGIC_VECTOR(15 downto 0);

                         xr:  in STD_LOGIC_VECTOR(15 downto 0);

                         yr:  in STD_LOGIC_VECTOR(15 downto 0);

           y: out STD_LOGIC_VECTOR(23 downto 0)

      );

end addrmux;

architecture comb of addrmux is

constant ADPC: STD_LOGIC_VECTOR(2 downto 0) := "000";  -- select PC

constant ADMP: STD_LOGIC_VECTOR(2 downto 0) := "001";  -- select MP

constant ADSP: STD_LOGIC_VECTOR(2 downto 0) := "010";  -- select SP

constant ADDI: STD_LOGIC_VECTOR(2 downto 0) := "011";  -- select Direct

constant ADXR: STD_LOGIC_VECTOR(2 downto 0) := "100";  -- select X register

constant ADYR: STD_LOGIC_VECTOR(2 downto 0) := "101";  -- select Y register

constant ADNP: STD_LOGIC_VECTOR(2 downto 0) := "000";  -- no operation (PC)

begin

  process(sel,a,b,s,xr,yr,dbr)

  begin

    case sel is

      when ADPC   => y <= a;                             -- program counter

      when ADMP   => y <= b;                             -- memory data pointer

      when ADSP   => y <= "00000000" & s;                -- stack address space

      when ADDI   => y <= "00000000" & b(15 downto 0);   -- direct address space

                when ADXR   => y <= dbr & xr;                      -- DBR\X register

                when ADYR   => y <= dbr & yr;                      -- DBR\Y register

      when others => y <= a;

    end case;

  end process;

end comb;