addsub_ovcy.vhd

Standard 8051 IP Core

--         Description: Adder/Subtractor with carry/borrow and arbitrary data
--                      width and overflow flag.
--
--
--
--
-------------------------------------------------------------------------------
library IEEE; 
use IEEE.std_logic_1164.all; 
use IEEE.std_logic_arith.all; 
  
-----------------------------ENTITY DECLARATION--------------------------------

entity addsub_ovcy is

  generic (DWIDTH : integer := 4);
  
  port (opa_i    : in  std_logic_vector(DWIDTH-1 downto 0);   -- Operand A
        opb_i    : in  std_logic_vector(DWIDTH-1 downto 0);   -- Operand B
        addsub_i : in  std_logic;       -- Add or subtract command
        cy_i     : in  std_logic;       -- Carry input
        cy_o     : out std_logic;       -- Carry/borrow bit
	ov_o     : out std_logic;  	-- Overflow flag
        rslt_o   : out std_logic_vector(DWIDTH-1 downto 0));  -- Result
  
end addsub_ovcy;
-------------------------------------------------------------------------------
architecture rtl of addsub_ovcy is

begin

  gen_equal_one: if (DWIDTH = 1) generate
    -- purpose: Simple adder/subtractor with carry/borrow and overflow
    -- type   : combinational
    -- inputs : opa_i, opb_i, addsub_i, cy_i
    -- outputs: cy_o, rslt_o
    p_addsub_ov: process (opa_i, opb_i, addsub_i, cy_i)
      variable v_la : unsigned(1 downto 0);
      variable v_lb : unsigned(1 downto 0);
      variable v_lresult : std_logic_vector(2 downto 0);
    begin  -- process p_addsub
      v_la(1) := opa_i(DWIDTH-1);
      v_lb(1) := opb_i(DWIDTH-1);
      if addsub_i = '1' then
        v_la(0) := '1';
        v_lb(0) := cy_i;
        v_lresult := conv_unsigned(v_la,3) + unsigned(v_lb);
      else
        v_la(0) := '0';
        v_lb(0) := cy_i;
        v_lresult := conv_unsigned(v_la,3) - unsigned(v_lb);
      end if;
      cy_o <= v_lresult(2);
      ov_o <= (cy_i and not(v_lresult(2))) or
  	    (v_lresult(2) and not(cy_i));
      rslt_o(DWIDTH-1) <= v_lresult(1);
    end process p_addsub_ov;
  end generate gen_equal_one;

  gen_greater_one: if (DWIDTH > 1) generate
    -- purpose: Simple adder/subtractor with carry/borrow and overflow
    -- type   : combinational
    -- inputs : opa_i, opb_i, addsub_i, cy_i
    -- outputs: cy_o, rslt_o
    p_addsub_ov: process (opa_i, opb_i, addsub_i, cy_i)
      variable v_a : unsigned(DWIDTH-1 downto 0);
      variable v_b : unsigned(DWIDTH-1 downto 0);
      variable v_result : std_logic_vector(DWIDTH downto 0);
      variable v_la : unsigned(1 downto 0);
      variable v_lb : unsigned(1 downto 0);
      variable v_lresult : std_logic_vector(2 downto 0);
    begin  -- process p_addsub
      v_a(DWIDTH-1 downto 1) := unsigned(opa_i(DWIDTH-2 downto 0));
      v_b(DWIDTH-1 downto 1) := unsigned(opb_i(DWIDTH-2 downto 0));
      v_la(1) := opa_i(DWIDTH-1);
      v_lb(1) := opb_i(DWIDTH-1);
      if addsub_i = '1' then
        v_a(0) := '1';
        v_b(0) := cy_i;
        v_result := conv_unsigned(v_a,DWIDTH+1) + unsigned(v_b);
        v_la(0) := '1';
        v_lb(0) := v_result(DWIDTH);
        v_lresult := conv_unsigned(v_la,3) + unsigned(v_lb);
      else
        v_a(0) := '0';
        v_b(0) := cy_i;
        v_result := conv_unsigned(v_a,DWIDTH+1) - unsigned(v_b);
        v_la(0) := '0';
        v_lb(0) := v_result(DWIDTH);      
        v_lresult := conv_unsigned(v_la,3) - unsigned(v_lb);
      end if;
      cy_o <= v_lresult(2);
      ov_o <= (v_result(DWIDTH) and not(v_lresult(2))) or
  	    (v_lresult(2) and not(v_result(DWIDTH)));
      rslt_o(DWIDTH-2 downto 0) <= v_result(DWIDTH-1 downto 1);
      rslt_o(DWIDTH-1) <= v_lresult(1);
    end process p_addsub_ov;
  end generate gen_greater_one;

end rtl;
-------------------------------------------------------------------------------