help understanding code

Altera_Forum · ‎04-03-2009

What is "zero" doing in this program.


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;

Also see the fist case "000" what does it mean by a and b. When I test this condition I always get the result of b?

Altera_Forum · ‎04-06-2009

It looks like a simple ALU.

op is the opcode. SO when op is 000 the ALU performs a logical AND of a and b, When Op is 001 it performs a logical OR etc

The result of the operation is stored in the variable temp. The signal zero is one when the result in temp is 0. i.e. it indicates that the result of the requested operation (AND, OR, +, -) etc is zero.

When you say you always get the result of b, what value is a?