在vhdl设计中常常将常用、典型的功能实体放在同一个目录下面，在设计复杂的电路的时候使用这些已经设计好的实例，这样就使设计变得简单了。
关键字：component
8位加法器，使用两个四位加法器实现。
四位加法器实现为adder4.vhd：
----------------------------------------------------------
-- 4 bits adder
-- xiaoyang@2011.4.14
----------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
entity adder4 is
port(
cin:in std_logic;
a:in std_logic_vector(3 downto 0);
b:in std_logic_vector(3 downto 0);
sum:out std_logic_vector(3 downto 0);
cout:out std_logic
);
end adder4;
architecture behav of adder4 is
signal sint:std_logic_vector(4 downto 0);
signal aa,bb:std_logic_vector(4 downto 0);
begin
--位连接--
aa<='0' & a(3 downto 0);
bb<='0' & b(3 downto 0);
sint<=aa+bb+cin;
sum(3 downto 0)<=sint(3 downto 0);
cout<=sint(4);
end behav;
模拟仿真波形如下图所示，仿真了5个加法计算：
1+1
0+0
3+6
1110（B） +1111（B）= （1）1101（B）产生进位count=1
4+1
八位加法器实现为adder8.vhd
测试数据加法有两个：
----------------------------------------------------------
-- 4 bits adder
-- xiaoyang@2011.4.14
----------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
entity adder8 is
port(
cin:in std_logic;
a:in std_logic_vector(7 downto 0);
b:in std_logic_vector(7 downto 0);
sum:out std_logic_vector(7 downto 0);
cout:out std_logic
);
end adder8;
architecture behav of adder8 is
--添加组件和声明--
component adder4
port(
cin:in std_logic;
a:in std_logic_vector(3 downto 0);
b:in std_logic_vector(3 downto 0);
sum:out std_logic_vector(3 downto 0);
cout:out std_logic
);
end component;
signal carry_out:std_logic;
begin
u1:adder4 port map(cin=>cin,a=>a(3 downto 0),b=>b(3 downto 0),sum=>sum(3 downto 0),cout=>carry_out);
u2:adder4 port map(cin=>carry_out,a=>a(7 downto 4),b=>b(7 downto 4),sum=>sum(7 downto 4),cout=>cout);
end behav;
其一是0xe2 + 0xc7 = oxa9进一位cout=1
其二是0x80+0x08 +进位= 0x88有进位故为0x89
测试结果：
