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---------------------------------------------------------------------
-- TITLE: Arithmetic Logic Unit
-- AUTHOR: Steve Rhoads (rhoadss@yahoo.com)
-- DATE CREATED: 2/8/01
-- FILENAME: alu.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 ALU.
---------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;
use work.mlite_pack.all;
 
entity alu is
   generic(alu_type  : string := "DEFAULT");
   port(a_in         : in  std_logic_vector(31 downto 0);
        b_in         : in  std_logic_vector(31 downto 0);
        alu_function : in  alu_function_type;
        c_alu        : out std_logic_vector(31 downto 0));
end; --alu
 
architecture logic of alu is
   signal do_add    : std_logic;
   signal sum       : std_logic_vector(32 downto 0);
   signal less_than : std_logic;
begin
 
   do_add <= '1' when alu_function = ALU_ADD else '0';
   sum <= bv_adder(a_in, b_in, do_add);
   less_than <= sum(32) when a_in(31) = b_in(31) or alu_function = ALU_LESS_THAN 
                else a_in(31);
 
   GENERIC_ALU: if alu_type = "DEFAULT" generate
      c_alu <= sum(31 downto 0) when alu_function=ALU_ADD or
		                               alu_function=ALU_SUBTRACT else
               ZERO(31 downto 1) & less_than when alu_function=ALU_LESS_THAN or 
                                alu_function=ALU_LESS_THAN_SIGNED else
               a_in or  b_in    when alu_function=ALU_OR else
               a_in and b_in    when alu_function=ALU_AND else
               a_in xor b_in    when alu_function=ALU_XOR else
               a_in nor b_in    when alu_function=ALU_NOR else
               ZERO;
   end generate;
 
   AREA_OPTIMIZED_ALU: if alu_type/="DEFAULT" generate
      c_alu <= sum(31 downto 0) when alu_function=ALU_ADD or 
		                          alu_function=ALU_SUBTRACT else (others => 'Z');
      c_alu <= ZERO(31 downto 1) & less_than when alu_function=ALU_LESS_THAN or 
		                          alu_function=ALU_LESS_THAN_SIGNED else 
										  (others => 'Z');
      c_alu <= a_in or  b_in    when alu_function=ALU_OR else (others => 'Z');
      c_alu <= a_in and b_in    when alu_function=ALU_AND else (others => 'Z');
      c_alu <= a_in xor b_in    when alu_function=ALU_XOR else (others => 'Z');
      c_alu <= a_in nor b_in    when alu_function=ALU_NOR else (others => 'Z');
      c_alu <= ZERO             when alu_function=ALU_NOTHING else (others => 'Z');
   end generate;
 
end; --architecture logic