Subversion Repositories special_functions_unit

----------------------------------------------------------------------------------
-- Right Shifter for FP addition-subtraction (right_shifter.vhd)
-- P: number of fractional (significant) bits
-- right shift up to E positions. Generates in the last bit the sticky bit.
-- PLOG is log (P); numbers of bits to represent P
-- Used in FP add of section 12.5.1
----------------------------------------------------------------------------------
 
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
entity right_shifter is
    generic (P: natural:= 27; E: natural := 8; PLOG: natural := 4);
    Port ( frac : in  std_logic_vector (P downto 0);
           diff_exp : in  std_logic_vector (E downto 0);
           frac_align : out  std_logic_vector (P downto 0));
end right_shifter;
 
architecture Behavioral of right_shifter is
   signal fracAlign_int : std_logic_vector(P downto 0);
   constant allShifted : std_logic_vector(P downto 0) := (0 =>'1', others =>'0'); 
   constant ZEROS : std_logic_vector(P downto PLOG) := (others =>'0'); 
 
begin
   --Right Shifter. Shifts up to p+3 positions
   process(diff_exp, 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);
      variable sticky : std_logic;
      constant zeros : std_logic_vector(P downto 0) := (others => '0');
   begin
      temp := frac;
      sticky := '0';
      for i in PLOG downto 0 loop --4 downto 0 for single (P=24+3=27)
         if (diff_exp(i) = '1') then
            dtemp := (others => '0');
            dtemp(P - 2**i downto 0) := temp(P downto 2**i);
            if temp(2**i -1 downto 0) /= zeros(2**i downto 0) then
               sticky := '1';
            end if;           
         else --if (diff_exp(i) = '0') 
            dtemp := temp;
         end if;
       temp := dtemp;
      end loop;
 
      fracAlign_int <= dtemp(P downto 1) & (dtemp(0) or sticky);
 
   end process;
 
   frac_align <= fracAlign_int when diff_exp(E downto PLOG+1) = ZEROS else allShifted; --if diffexp >> P -> allshifted
 
end Behavioral;