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---------------------------------------------------------------------

-- TITLE: Program Counter Next

-- AUTHOR: Steve Rhoads (rhoadss@yahoo.com)

-- DATE CREATED: 2/8/01

-- FILENAME: pc_next.vhd

-- PROJECT: MIPS CPU core

-- COPYRIGHT: Software placed into the public domain by the author.

--    Software 'as is' without warranty.  Author liable for nothing.

-- DESCRIPTION:

--    Implements the Program Counter logic.

---------------------------------------------------------------------

library ieee;

use ieee.std_logic_1164.all;

use work.mips_pack.all;

entity pc_next is

   port(clk          : in std_logic;

        reset_in     : in std_logic;

        pc_new       : in std_logic_vector(31 downto 2);

        take_branch  : in std_logic;

        pause_in     : in std_logic;

        opcode25_0   : in std_logic_vector(25 downto 0);

        pc_source    : in pc_source_type;

        pc_out       : out std_logic_vector(31 downto 0);

        pc_out_plus4 : out std_logic_vector(31 downto 0));

end; --pc_next

architecture logic of pc_next is

--   type pc_source_type is (from_inc4, from_opcode25_0, from_branch, 

--      from_lbranch);

   signal pc_reg : std_logic_vector(31 downto 2); --:= ZERO(31 downto 2);

begin

pc_next: process(clk, reset_in, pc_new, take_branch, pause_in,

                 opcode25_0, pc_source, pc_reg)

   variable pc_inc, pc_next : std_logic_vector(31 downto 2);

begin

   pc_inc := bv_increment(pc_reg);  --pc_reg+1

   pc_next := pc_reg;

   case pc_source is

   when from_inc4 =>

      if pause_in = '0' then

         pc_next := pc_inc;

      end if;

   when from_opcode25_0 =>

      pc_next := pc_reg(31 downto 28) & opcode25_0;

   when from_branch | from_lbranch =>

      if take_branch = '1' then

         pc_next := pc_new;

      else

         pc_next := pc_inc;

      end if;

   when others =>

   end case;

   if reset_in = '1' then

      pc_next := ZERO(31 downto 2);

   end if;

   if rising_edge(clk) then

      pc_reg <= pc_next;

   end if;

   pc_out <= pc_reg & "00";

   pc_out_plus4 <= pc_inc & "00";

end process;

end; --logic