给以下vhdl代码添加注释：library ieee; use ieee.std_logic_1164.all; use ieee.std_logic_unsigned.all; use ieee.std_logic_arith.all; entity adder_4bit is port ( ain1,bin1,cin1,ain2,bin2,ain3,bin3,ain4,bin4:in std_logic; so1,so2,so3,so4,co5:out std_logic ); end adder_4bit; architecture hav of adder_4bit is component adder_1bit port (bin: IN STD_LOGIC; cin: IN STD_LOGIC; ain: IN STD_LOGIC; so :OUT STD_LOGIC; co :OUT STD_LOGIC); end component; signal cin2,cin3,cin4,cin5:std_logic; begin u1:adder_1bit port map(ain=>ain1,bin=>bin1,cin=>cin1,co=>cin2,so=>so1); u2:adder_1bit port map(ain=>ain2,bin=>bin2,cin=>cin2,co=>cin3,so=>so2); u3:adder_1bit port map(ain=>ain3,bin=>bin3,cin=>cin3,co=>cin4,so=>so3); u4:adder_1bit port map(ain=>ain4,bin=>bin4,cin=>cin4,co=>cin5,so=>so4); end architecture;

-- This is a VHDL code for a 4-bit adder
-- The code defines an entity named "adder_4bit" with 9 input ports and 5 output ports
-- The entity uses 4 instances of a component named "adder_1bit" to perform the addition of 4-bit numbers
-- The component "adder_1bit" has 3 input ports and 2 output ports
-- The architecture "hav" instantiates the 4 components and maps their input and output ports
-- The signals cin2, cin3, cin4, and cin5 are used to carry the carry bits between the components