####
**Arithmetic Logic Unit (ALU) is one of the most important digital logic components in CPUs. It normally executes logic and arithmetic operations such as addition, subtraction, multiplication, division, etc.**

## In this VHDL project, an ALU is designed and implemented in VHDL. VHDL code for the ALU is fully presented.

####
**The logic and arithmetic operations being implemented in the ALU are as follows:**

**1. Arithmetic Addition**

**ALU_Out = A + B;**

**2. Arithmetic Subtraction**

**ALU_Out = A - B;**

**3. Arithmetic Multiplication**

**ALU_Out = A * B;**

**4. Arithmetic Division**

**ALU_Out = A / B;**

**5. Logical Shift Left**

**ALU_Out = A logical shifted left by 1;**

**6. Logical Shift Right**

**ALU_Out = A logical shifted right by 1;**

**7. Rotate Left**

**ALU_Out = A rotated left by 1;**

**8. Rotate Right**

**ALU_Out = A rotated right by 1;**

**9. Logical AND**

**ALU_Out = A AND B;**

**10. Logical OR**

**ALU_Out = A OR B;**

**11. Logical XOR**

**ALU_Out = A XOR B;**

**12. Logical NOR**

**ALU_Out = A NOR B;**

**13. Logical NAND**

**ALU_Out = A NAND B;**

**14. Logical XNOR**

**ALU_Out = A XNOR B;**

**15. Greater comparison**

**ALU_Out = 1 if A > B else 0;**

**16. Equal comparison**

**ALU_Out = 1 A = B else 0;**

### VHDL code for ALU:

-- fpga4student.com: FPGA Projects, Verilog projects, VHDL projects -- VHDL code for ALU -- @fpga4student library IEEE; use IEEE.STD_LOGIC_1164.ALL; use IEEE.STD_LOGIC_UNSIGNED.ALL; use ieee.NUMERIC_STD.all; ----------------------------------------------- ---------- ALU 8-bit VHDL --------------------- ----------------------------------------------- entity ALU is generic ( constant N: natural := 1 -- number of shited or rotated bits ); Port ( A, B : in STD_LOGIC_VECTOR(7 downto 0); -- 2 inputs 8-bit ALU_Sel : in STD_LOGIC_VECTOR(3 downto 0); -- 1 input 4-bit for selecting function ALU_Out : out STD_LOGIC_VECTOR(7 downto 0); -- 1 output 8-bit Carryout : out std_logic -- Carryout flag ); end ALU; architecture Behavioral of ALU is signal ALU_Result : std_logic_vector (7 downto 0); signal tmp: std_logic_vector (8 downto 0); begin process(A,B,ALU_Sel) begin case(ALU_Sel) is when "0000" => -- Addition ALU_Result <= A + B ; when "0001" => -- Subtraction ALU_Result <= A - B ; when "0010" => -- Multiplication ALU_Result <= std_logic_vector(to_unsigned((to_integer(unsigned(A)) * to_integer(unsigned(B))),8)) ; when "0011" => -- Division ALU_Result <= std_logic_vector(to_unsigned(to_integer(unsigned(A)) / to_integer(unsigned(B)),8)) ; when "0100" => -- Logical shift left ALU_Result <= std_logic_vector(unsigned(A) sll N); when "0101" => -- Logical shift right ALU_Result <= std_logic_vector(unsigned(A) srl N); when "0110" => -- Rotate left ALU_Result <= std_logic_vector(unsigned(A) rol N); when "0111" => -- Rotate right ALU_Result <= std_logic_vector(unsigned(A) ror N); when "1000" => -- Logical and ALU_Result <= A and B; when "1001" => -- Logical or ALU_Result <= A or B; when "1010" => -- Logical xor ALU_Result <= A xor B; when "1011" => -- Logical nor ALU_Result <= A nor B; when "1100" => -- Logical nand ALU_Result <= A nand B; when "1101" => -- Logical xnor ALU_Result <= A xnor B; when "1110" => -- Greater comparison if(A>B) then ALU_Result <= x"01" ; else ALU_Result <= x"00" ; end if; when "1111" => -- Equal comparison if(A=B) then ALU_Result <= x"01" ; else ALU_Result <= x"00" ; end if; when others => ALU_Result <= A + B ; end case; end process; ALU_Out <= ALU_Result; -- ALU out tmp <= ('0' & A) + ('0' & B); Carryout <= tmp(8); -- Carryout flag end Behavioral;

### Testbench VHDL code for ALU:

-- fpga4student.com: FPGA Projects, Verilog projects, VHDL projects -- Testbench VHDL code for ALU -- @fpga4student LIBRARY ieee; USE ieee.std_logic_1164.ALL; use IEEE.std_logic_unsigned.all; -- Uncomment the following library declaration if using -- arithmetic functions with Signed or Unsigned values --USE ieee.numeric_std.ALL; ENTITY tb_ALU IS END tb_ALU; ARCHITECTURE behavior OF tb_ALU IS -- Component Declaration for the Unit Under Test (UUT) COMPONENT ALU PORT( A : IN std_logic_vector(7 downto 0); B : IN std_logic_vector(7 downto 0); ALU_Sel : IN std_logic_vector(3 downto 0); ALU_Out : OUT std_logic_vector(7 downto 0); Carryout : OUT std_logic ); END COMPONENT; --Inputs signal A : std_logic_vector(7 downto 0) := (others => '0'); signal B : std_logic_vector(7 downto 0) := (others => '0'); signal ALU_Sel : std_logic_vector(3 downto 0) := (others => '0'); --Outputs signal ALU_Out : std_logic_vector(7 downto 0); signal Carryout : std_logic; signal i:integer; BEGIN -- Instantiate the Unit Under Test (UUT) uut: ALU PORT MAP ( A => A, B => B, ALU_Sel => ALU_Sel, ALU_Out => ALU_Out, Carryout => Carryout ); -- Stimulus process stim_proc: process begin -- hold reset state for 100 ns. A <= x"0A"; B <= x"02"; ALU_Sel <= x"0"; for i in 0 to 15 loop ALU_Sel <= ALU_Sel + x"1"; wait for 100 ns; end loop; A <= x"F6"; B <= x"0A"; wait; end process; END;

####
**Simulation waveform for the ALU:**

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In this line " ALU_Result <= std_logic_vector(to_unsigned((to_integer(unsigned(A)) * to_integer(unsigned(B))),8)) ;"

ReplyDeletewhy cannot we just do A*B ?

Because the * multiplication operator is not supported for std_logic_vector type in VHDL. You need to convert to integer which multiplication is supported. Then, you need to convert the result back to std_logic_vector type.

DeleteIs could one of the functions be NOT A

ReplyDeleteIt can be done.

DeleteMy lecture asked me to write ALU code in 2 different ways, can I use dataflow or structural model to write? or I just need to split into 2 process? thank you

ReplyDelete