Subversion Repositories special_functions_unit

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-- FP leading zeros and normalization (fp_leading_zeros_and_shift.vhd)

--

-- The fractional P includes the guard digits and the sticky bit.

-- Returns the normalized number and the adjusted exponent

-- Used in FP add of section 12.5.1

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library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

entity fp_leading_zeros_and_shift is

    generic (P: natural:= 27; E: natural := 8; PLOG: natural := 4);

    Port ( frac : in  std_logic_vector (P downto 0);

           exp  : in  std_logic_vector (E-1 downto 0);

           frac_Norm : out  std_logic_vector (P downto 0);

           exp_Norm : out  std_logic_vector (E-1 downto 0);

           underFlow : out std_logic);

end fp_leading_zeros_and_shift;

architecture Behavioral of fp_leading_zeros_and_shift is

   constant ZEROS : std_logic_vector(P downto 0):= (others => '0');

   signal leadZerosBin : std_logic_vector(PLOG downto 0);

   signal exp_Norm_int : std_logic_vector(E-1 downto 0);

   signal isZ : std_logic;

begin

    --count zeros: elegant version.

    detectZeros: process(frac, exp)

       variable leadZerosBinVar : std_logic_vector(PLOG downto 0);

    begin

      leadZerosBinVar := ZEROS(PLOG downto 0);

      for i in P downto 1 loop

       if (frac(P-1 downto P-i)= ZEROS(P-1 downto P-i) ) then

          leadZerosBinVar := conv_std_logic_vector(i,PLOG+1);

          exit;

       end if;

      end loop;

      leadZerosBin <= leadZerosBinVar;

    end process;

   adjustExponent: process(leadZerosBin, exp, isZ)

   begin

      if isZ = '1' then

         exp_Norm_int <= (others => '0');

      else

         exp_Norm_int <= exp - ("000" & leadZerosBin);

      end if;

   end process;

   isZ <= '1' when frac(P-1 downto 0) = ZEROS (P-1 downto 0) else '0';

   underFlow <= '0' when isZ='0' and exp > leadZerosBin else '1';

   exp_Norm <= exp_Norm_int;

   shift: process(leadZerosBin, frac)

      variable temp : std_logic_vector(P downto 0);

      variable dtemp : std_logic_vector(P downto 0);

      variable fracAgnVar : std_logic_vector(P downto 0);

   begin

      temp := frac;

      for i in PLOG downto 0 loop

         if (leadZerosBin(i) = '1') then

            dtemp := (others => '0');

            dtemp(P downto 2**i) := temp(P - 2**i downto 0);

         else

            dtemp := temp;

         end if;

         temp := dtemp;

      end loop;

      frac_Norm <= dtemp;

   end process;

end Behavioral;