--  A,B are inputs 
--  C   is output ( A x B )
--  based on IEEE standard
--  s   <- exponent  ->    <-       mantissa         ->
--  64  62   11bits  52    51       52 bits          0
-- written by N.M.Duc 16th June 1997
-- completely tested

library IEEE;
use 	IEEE.std_logic_1164.all;
use		IEEE.std_logic_unsigned.all;
--      this statement is needed when complile with Peak tools
--use	    IEEE.std_logic_arith.all;
--      this statement is needed when complile with Cadence tools

entity MULT is
  port(	A,B	:in  std_logic_vector(63 downto 0);
    		C	:out std_logic_vector(63 downto 0) );
end MULT;

architecture RTL of MULT is
constant OFFSET : std_logic_vector(11 downto 0):="001111111111";
begin

  process(A,B) 
	variable Sa,Sb,Sc	: std_logic;
	variable Ea,Eb 		: std_logic_vector(10  downto 0);
	variable Ecbuf		: std_logic_vector(12  downto 0);
	variable Ma,Mb,Mc	: std_logic_vector(51 downto 0);
	variable Mmultp		: std_logic_vector(105 downto 0);
	variable Mround		: std_logic_vector(54 downto 0);
  begin
	Sa := A(63);	Sb := B(63);   Sc := Sa xor Sb;
	Ea := A(62 downto 52); 	Eb := B(62 downto 52);

	Ecbuf := (("00" & Ea) + ("00" & Eb)) - OFFSET;

	if( Ecbuf(12)='1' ) then
	-- Ecbuff's underflow
		C <= Sc & "00000000000" & "000000000000000000000000000000000000000000";
	else
		Ma := A(51 downto 0);	Mb := B(51 downto 0);
		Mmultp := ( '1' & Ma ) * ( '1' & Mb );

		if( Mmultp(105)='1' ) then
		-- right shift Mmultp 1 bit and increte Ecbuff
			Mmultp := '0' &  Mmultp(105  downto 1); 
			Ecbuf   := Ecbuf +1;
		end if;

		if( Ecbuf(11)='1' ) then
		-- Ecbuf's overflow 
			Ecbuf := "0011111111111";
		end if;

		if(  Mmultp(51)='1' ) then
		-- round off
			Mround := Mmultp(105 downto 51) + 1;   
		else
			Mround := Mmultp(105 downto 51);
		end if;

		if( Mround(54)='1' ) then
		-- Mround's overflow
			Mc := Mround(53 downto 2);
			Ecbuf  := Ecbuf +1; 
		else
			Mc := Mround(52 downto 1);
		end if;

		if( Ecbuf(11)='1' ) then
		-- Ecbuf's overflow
			Ecbuf := "0011111111111";
		end if;

		C <= Sc & Ecbuf(10 downto 0) & Mc;

	end if;  -- of if(E_c(12)='1')

  end process;
end RTL;