Question

2) Create the logical right shift circuit in VHDL. Using the & operator for concatenation might make things easier. For example, if A is a 4 bit input, the number “0”&A(3 downto 1) is A shifted one place right. The & operator sticks a zero onto the three most significant bits of A, A(3 downto 1). A(0) is lost, as expected. You must use std_logic_vectors with the & operator. “0” is a one bit std_logic_vector. A(3 downto 3) is also a one bit std_logic_vector.

Answer 1

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use work.DigEng.ALL;

-- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
use IEEE.NUMERIC_STD.ALL;

-- Uncomment the following library declaration if instantiating
-- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;

entity core is
    Generic (
           N : INTEGER);
    Port ( CLK : in  STD_LOGIC;
           A, B, X : in  STD_LOGIC_VECTOR(N downto 0);
              OPERATION : in STD_LOGIC_VECTOR(3 downto 0); --Allows us up to 16 operations
              FLAGS : in STD_LOGIC_VECTOR(7 downto 0);
           ALU_OUTPUT : out  STD_LOGIC_VECTOR(N downto 0));
end core;

architecture Behavioral of core is
    signal TEMP_OUTPUT : STD_LOGIC_VECTOR(N downto 0);
begin
    TEMP_OUTPUT <= (A) WHEN OPERATION = "0000" ELSE
                --Bit-wise Logic
                 (A AND B) WHEN OPERATION = "0001" ELSE
                 (A OR B) WHEN OPERATION = "0010" ELSE
                 (A XOR B) WHEN OPERATION = "0011" ELSE
                 (NOT A) WHEN OPERATION = "0100" ELSE
                --Arithmetic
                 STD_LOGIC_VECTOR(SIGNED(A) + 1) WHEN OPERATION = "0101" ELSE
                 STD_LOGIC_VECTOR(SIGNED(A) - 1) WHEN OPERATION = "0110" ELSE
                 STD_LOGIC_VECTOR(SIGNED(A) + SIGNED(B)) WHEN OPERATION = "0111" ELSE
                 STD_LOGIC_VECTOR(SIGNED(A) - SIGNED(B)) WHEN OPERATION = "1000" ELSE
                --Shift
                 --(A <= X(N) & X(N downto 1)) WHEN OPERATION = "1001" ELSE
                 "1111";

end Behavioral;