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

-- TITLE: Program Counter Next

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

-- DATE CREATED: 2/8/01

-- FILENAME: pc_next.vhd

-- PROJECT: Plasma 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.mlite_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_future   : out std_logic_vector(31 downto 2);

        pc_current  : out std_logic_vector(31 downto 2);

        pc_plus4    : out std_logic_vector(31 downto 2));

end; --pc_next

architecture logic of pc_next is

   signal pc_reg : std_logic_vector(31 downto 2);

begin

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

                 opcode25_0, pc_source, pc_reg)

   variable pc_inc      : std_logic_vector(31 downto 2);

   variable pc_next : std_logic_vector(31 downto 2);

begin

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

   case pc_source is

   when FROM_INC4 =>

      pc_next := pc_inc;

   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 =>

      pc_next := pc_inc;

   end case;

   if pause_in = '1' then

      pc_next := pc_reg;

   end if;

   if reset_in = '1' then

      pc_reg <= ZERO(31 downto 2);

      pc_next := pc_reg;

   elsif rising_edge(clk) then

      pc_reg <= pc_next;

   end if;

   pc_future <= pc_next;

   pc_current <= pc_reg;

   pc_plus4 <= pc_inc;

end process;

end; --logic