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---------------------------------------------------------------------
-- TITLE: Pipeline
-- AUTHOR: Steve Rhoads (rhoadss@yahoo.com)
-- DATE CREATED: 6/24/02
-- FILENAME: pipeline.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:
--    Controls the three stage pipeline by delaying the signals:
--      a_bus, b_bus, alu/shift/mult_func, c_source, and rs_index.
---------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;
use work.mlite_pack.all;
 
--Note: sigD <= sig after rising_edge(clk)
entity pipeline is
   port(clk            : in  std_logic;
        reset          : in  std_logic;
        a_bus          : in  std_logic_vector(31 downto 0);
        a_busD         : out std_logic_vector(31 downto 0);
        b_bus          : in  std_logic_vector(31 downto 0);
        b_busD         : out std_logic_vector(31 downto 0);
        alu_func       : in  alu_function_type;
        alu_funcD      : out alu_function_type;
        shift_func     : in  shift_function_type;
        shift_funcD    : out shift_function_type;
        mult_func      : in  mult_function_type;
        mult_funcD     : out mult_function_type;
        reg_dest       : in  std_logic_vector(31 downto 0);
        reg_destD      : out std_logic_vector(31 downto 0);
        rd_index       : in  std_logic_vector(5 downto 0);
        rd_indexD      : out std_logic_vector(5 downto 0);
 
        rs_index       : in  std_logic_vector(5 downto 0);
        rt_index       : in  std_logic_vector(5 downto 0);
        pc_source      : in  pc_source_type;
        mem_source     : in  mem_source_type;
        a_source       : in  a_source_type;
        b_source       : in  b_source_type;
        c_source       : in  c_source_type;
        c_bus          : in  std_logic_vector(31 downto 0);
        take_branch    : in  std_logic;
        take_branchD   : out std_logic;
        pause_any      : in  std_logic;
        pause_pipeline : out std_logic);
end; --entity pipeline
 
architecture logic of pipeline is
   signal rd_index_reg   : std_logic_vector(5 downto 0);
   signal reg_dest_reg   : std_logic_vector(31 downto 0);
   signal c_source_reg   : c_source_type;
   signal pause_reg      : std_logic;
begin
 
--When operating in three stage pipeline mode, the following signals
--are delayed by one clock cycle:  a_bus, b_bus, alu/shift/mult_func,
--c_source, and rd_index.
pipeline3: process(clk, reset, a_bus, b_bus, alu_func, shift_func, mult_func,
      rd_index, rd_index_reg, pause_any, pause_reg, 
      take_branch, rs_index, rt_index,
      pc_source, mem_source, c_source, c_source_reg, 
      reg_dest, reg_dest_reg)
   variable pause_mult_clock : std_logic;
begin
   if (pc_source /= from_inc4 and pc_source /= from_opcode25_0) or
      mem_source /= mem_fetch or
      (mult_func = mult_read_lo or mult_func = mult_read_hi) then
      pause_mult_clock := '1';
   else
      pause_mult_clock := '0';
   end if;
 
   pause_pipeline <= pause_mult_clock and pause_reg;
   take_branchD <= take_branch and not pause_any;
   rd_indexD <= rd_index_reg;
 
   if c_source_reg = c_from_alu then
      reg_destD <= c_bus;
   else
      reg_destD <= reg_dest_reg;
   end if;
 
   if rising_edge(clk) then
      if (rs_index = "000000" or rs_index /= rd_index_reg) or 
            (a_source /= a_from_reg_source or pause_reg = '0') then
         a_busD <= a_bus;
      elsif c_source_reg = c_from_alu then
         a_busD <= c_bus;  --rs from previous operation (bypass stage)
      else
         a_busD <= reg_dest_reg;
      end if;
 
      if (rt_index = "000000" or rt_index /= rd_index_reg) or
            (b_source /= b_from_reg_target or pause_reg = '0') then
         b_busD <= b_bus;
      elsif c_source_reg = c_from_alu then
         b_busD <= c_bus;  --rt from previous operation
      else
         b_busD <= reg_dest_reg;
      end if;
 
      alu_funcD <= alu_func;
      shift_funcD <= shift_func;
      mult_funcD <= mult_func;
      reg_dest_reg <= reg_dest;
      c_source_reg <= c_source;
   end if;
 
   if reset = '1' then
      pause_reg <= '1';
      rd_index_reg <= "000000";
   elsif rising_edge(clk) then
      if pause_reg = '0' and pause_any = '0' then
         pause_reg <= '1';   --enable pause_pipeline
      elsif pause_mult_clock = '1' then
         pause_reg <= '0';   --disable pause_pipeline
      end if;
      rd_index_reg <= rd_index;
   end if;
 
end process; --pipeline3
 
end; --logic