Subversion Repositories rv01_riscv_core

-----------------------------------------------------------------

--                                                             --

-----------------------------------------------------------------

--                                                             --

-- Copyright (C) 2015 Stefano Tonello                          --

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-----------------------------------------------------------------

---------------------------------------------------------------

-- 'Fast' adder (carry-select style)

---------------------------------------------------------------

library IEEE;

use IEEE.std_logic_1164.all;

use IEEE.numeric_std.all;

entity RV01_ADDER_F is

  generic(

    LEN1 : integer := 16;

    LEN2 : integer := 16

  );

  port(

    OPA_i : in signed(LEN1+LEN2-1 downto 0);

    OPB_i : in signed(LEN1+LEN2-1 downto 0);

    CI_i : in std_logic;

    SUM_o : out signed(LEN1+LEN2-1 downto 0)

  );

end RV01_ADDER_F;

architecture ARC of RV01_ADDER_F is

begin

  process(OPA_i,OPB_i,CI_i)

    variable A_LO,B_LO : signed(LEN1-1 downto 0);

    variable A_HI,B_HI : signed(LEN2-1 downto 0);

    variable SUM_LO : signed(LEN1+1 downto 0);

    variable SUM_HI0,SUM_HI1 : signed(LEN2 downto 0);

  begin

    A_LO := OPA_i(LEN1-1 downto 0);

    A_HI := OPA_i(LEN2+LEN1-1 downto LEN1);

    B_LO := OPB_i(LEN1-1 downto 0);

    B_HI := OPB_i(LEN2+LEN1-1 downto LEN1);

    -- low parts sum

    SUM_LO := ('0' & A_LO & '1') + ('0' & B_LO & CI_i);

    -- high parts sum (assuming carry-out is zero)

    SUM_HI0 := (A_HI & '0') + (B_HI & '0');

    -- high parts sum (assuming carry-out is one)

    SUM_HI1 := (A_HI & '1') + (B_HI & '1');

    -- SUM_o low part is low parts sum

    SUM_o(LEN1-1 downto 0) <= SUM_LO(LEN1 downto 1);

    -- select SUM_o high part according to low parts sum carry-out

    if(SUM_LO(LEN1+1) = '1') then

      SUM_o(LEN2+LEN1-1 downto LEN1) <= SUM_HI1(LEN2 downto 1);

    else

      SUM_o(LEN2+LEN1-1 downto LEN1) <= SUM_HI0(LEN2 downto 1);

    end if;

  end process;

end ARC;