< VHDL for FPGA Design
4-Bit ALU VHDL Code
A combinatorial ALU with the following operations:
| Operation | Result | Flag | Description |
|---|---|---|---|
| 000 | Nibble1 + Nibble2 | Carry = Overflow | Addition |
| 001 | | Nibble1 - Nibble2 | | 1 if Nibble2 > Nibble1, 0 otherwise |
Test / diff |
| 010 | Nibble1 AND Nibble2 | 0 | Bitwise AND |
| 011 | Nibble1 OR Nibble2 | 0 | Bitwise OR |
| 100 | Nibble1 XOR Nibble2 | 0 | Bitwise XOR |
| 101 | 15 - Nibble1 | 0 | Bitwise inverse of Nibble1 |
| 110 | 15 - Nibble2 | 0 | Bitwise inverse of Nibble2 |
| 111 | Nibble1 + Nibble2 + 1 | Carry = Overflow | Addition |
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
entity ALU_VHDL is
port
(
Nibble1, Nibble2 : in std_logic_vector(3 downto 0);
Operation : in std_logic_vector(2 downto 0);
Carry_Out : out std_logic;
Flag : out std_logic;
Result : out std_logic_vector(3 downto 0)
);
end entity ALU_VHDL;
architecture Behavioral of ALU_VHDL is
signal Temp: std_logic_vector(4 downto 0);
begin
process(Nibble1, Nibble2, Operation, temp) is
begin
Flag <= '0';
case Operation is
when "000" => -- res = nib1 + nib2, flag = carry = overflow
Temp <= std_logic_vector((unsigned("0" & Nibble1) + unsigned(Nibble2)));
Result <= temp(3 downto 0);
Carry_Out <= temp(4);
when "001" => -- res = |nib1 - nib2|, flag = 1 iff nib2 > nib1
if (Nibble1 >= Nibble2) then
Result <= std_logic_vector(unsigned(Nibble1) - unsigned(Nibble2));
Flag <= '0';
else
Result <= std_logic_vector(unsigned(Nibble2) - unsigned(Nibble1));
Flag <= '1';
end if;
when "010" =>
Result <= Nibble1 and Nibble2;
when "011" =>
Result <= Nibble1 or Nibble2;
when "100" =>
Result <= Nibble1 xor Nibble2;
when "101" =>
Result <= not Nibble1;
when "110" =>
Result <= not Nibble2;
when others => -- res = nib1 + nib2 + 1, flag = 0
Temp <= std_logic_vector((unsigned("0" & Nibble1) + unsigned(not Nibble2)) + 1);
Result <= temp(3 downto 0);
Flag <= temp(4);
end case;
end process;
end architecture Behavioral;
Simulation Waveform
Generated Symbol
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