8-Bit ALU in VHDL

This arithmetic logic unit accepts 8-bit inputs, but it can easily be modded to higher bits. It supports the addition, subtraction, set if less than, AND, and OR operations. The operation to perform is determined by the 3-bit address bus.

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
use ieee.std_logic_arith.all;

ENTITY alu8bit IS
	port(a, b : in std_logic_vector(7 downto 0);	-- a and b are busses
		op : in std_logic_vector(2 downto 0);
		zero : out std_logic;
	    f : out std_logic_vector(7 downto 0));
END alu8bit;

architecture behavioral of alu8bit is
begin
	process(op)
	variable temp: std_logic_vector(7 downto 0);
	begin
	case op is
		when "000" =>
			temp := a and b;
		when "100" =>
			temp := a and b;
		when "001" =>
			temp := a or b;
		when "101" =>
			temp := a or b;
		when "010" =>
			temp := a + b;
		when "110" =>
			temp := a - b;
		when "111" =>
			if a < b then
			temp := "11111111";
			else
			temp := "00000000";
			end if;
		when others =>
			temp := a - b;
	end case;
	if temp="00000000" then
	zero <= '1';
	else
	zero <= '0';
	end if;
	f <= temp;
	end process;
end behavioral;


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