----------------------------------------------------------------------------------
-- Company: Instituto Superior Técnico
-- Prof: Paulo Alexandre Crisóstomo Lopes
-- paulo.lopes@inesc-id.pt
--
-- Create Date:    14:01:57 12/27/2011
-- Design Name:
-- Module Name:    multiplier - Behavioral
-- Project Name:
-- Target Devices:
-- Tool versions:
-- Description: VHDL implementation of a 32 bit floating multiplier.
--			Float format: sign | 8 bits exponent + 127 | 23 bits normalized mantissa.
-- 		Uses IEEE 754-1985, with the following exceptions.
--			NaN is not implemented. Operations that would result in NaN
--			have a non definied result.
--			An exponent of all zeros will always mean zero, and an
--			exponent of all ones will always mean infinity.
--			Rounding is round nearest ties away from zero.
--			Non normalized numbers are not implemented.
--
-- Dependencies:
--
-- Revision:
-- Revision 1.02
-- - Removal of comparators to test undeflow and overflow.
-- - Adding of test for 0*x and inf*x.
-- - Added rounding.
--
-- Additional Comments:
--
----------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use ieee.std_logic_unsigned.all;

entity multiplier is
    Port ( x : in  STD_LOGIC_VECTOR (31 downto 0);
           y : in  STD_LOGIC_VECTOR (31 downto 0);
           z : out  STD_LOGIC_VECTOR (31 downto 0));
end multiplier;

architecture Behavioral of multiplier is

begin
	process(x,y)
		variable x_mantissa : STD_LOGIC_VECTOR (22 downto 0);
		variable x_exponent : STD_LOGIC_VECTOR (7 downto 0);
		variable x_sign : STD_LOGIC;
		variable y_mantissa : STD_LOGIC_VECTOR (22 downto 0);
		variable y_exponent : STD_LOGIC_VECTOR (7 downto 0);
		variable y_sign : STD_LOGIC;
		variable z_mantissa : STD_LOGIC_VECTOR (22 downto 0);
		variable z_exponent : STD_LOGIC_VECTOR (7 downto 0);
		variable z_sign : STD_LOGIC;
		variable aux : STD_LOGIC;
		variable aux2 : STD_LOGIC_VECTOR (47 downto 0);
		variable exponent_sum : STD_LOGIC_VECTOR (8 downto 0);
   begin
		x_mantissa := x(22 downto 0);
		x_exponent := x(30 downto 23);
		x_sign := x(31);
		y_mantissa := y(22 downto 0);
		y_exponent := y(30 downto 23);
		y_sign := y(31);

		-- inf*0 is not tested (result would be NaN)
		if (x_exponent=255 or y_exponent=255) then
		-- inf*x or x*inf
			z_exponent := "11111111";
			z_mantissa := (others => '0');
			z_sign := x_sign xor y_sign;

		elsif (x_exponent=0 or y_exponent=0) then
		-- 0*x or x*0
			z_exponent := (others => '0');
			z_mantissa := (others => '0');
			z_sign := '0';
		else

			aux2 := ('1' & x_mantissa) * ('1' & y_mantissa);
			-- args in Q23 result in Q46
			if (aux2(47)='1') then
				-- >=2, shift left and add one to exponent
				z_mantissa := aux2(46 downto 24) + aux2(23); -- with rounding
				aux := '1';
			else
				z_mantissa := aux2(45 downto 23) + aux2(22); -- with rounding
				aux := '0';
			end if;

			-- calculate exponent
			exponent_sum := ('0' & x_exponent) + ('0' & y_exponent) + aux - 127;

			if (exponent_sum(8)='1') then
				if (exponent_sum(7)='0') then           -- overflow
					z_exponent := "11111111";
					z_mantissa := (others => '0');
					z_sign := x_sign xor y_sign;
				else 									-- underflow
					z_exponent := (others => '0');
					z_mantissa := (others => '0');
					z_sign := '0';
				end if;
			else								  		 -- Ok
				z_exponent := exponent_sum(7 downto 0);
				z_sign := x_sign xor y_sign;
			end if;
		end if;


		z(22 downto 0) <= z_mantissa;
		z(30 downto 23) <= z_exponent;
		z(31) <= z_sign;

   end process;
end Behavioral;