-- ALU Control Unit
-- Expected Behavior: The last 6 bits of each instruction along with
--   a 2 bit opcode that is produced by the Main Control Unit. Together,
--   they determine the type of operation and the appropriate op codes are
--   sent to the ALU. Possible instructions that use the ALU: XOR, NOR, ADD,
--   SUB, LW, SW, BEQ, and SLT.

library IEEE;
use IEEE.std_logic_1164.all;

entity alucontrol is                          -- New entity for ALU Control Unit
port(inv : in std_logic_vector(5 downto 0);   --  is defined
     ALUop : in std_logic_vector(1 downto 0);
     outv : out std_logic_vector(2 downto 0));
end alucontrol;

architecture behavior of alucontrol is
signal ALUop0not,ALUop1not,inv2not,s0,s1,s2,s3,s4: std_logic;
begin
  ALUop0not <= NOT ALUop(0) after 1 ns;
  ALUop1not <= NOT ALUop(1) after 1 ns;
  inv2not <= NOT inv(2) after 1 ns;

  s0 <= inv(1) NAND ALUop(1) after 2 ns;       -- Calculation for MSB
  s1 <= ALUop(1) NAND inv2not after 2 ns;
  s2 <= s1 NOR s0 after 2 ns;
  s3 <= s2 NOR ALUop(0) after 2 ns;
  outv(2) <= NOT s3 after 1 ns;

  outv(1) <= inv(2) NAND ALUop(1) after 2 ns;  -- Calculation for Middle Bit

  s4 <= inv(3) NOR inv(0) after 2 ns;          -- Calculation for LSB
  outv(0) <= s4 NOR ALUop1not after 2 ns;
end behavior;