LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;                		//定義實體comp4
ENTITY comp4 IS
    PORT(a, b:   IN STD_LOGIC_VECTOR(3 downto 0);	//a, b都是4位標準邏輯信號，作為輸入
            y:  OUT STD_LOGIC);                  		//y是1位標準邏輯信號，作為輸出
END comp4; 								//結構體 1：該結構體采用IF語句描述4位等值比										//較器設計
ARCHITECTURE  behavior  OF  comp4  IS            	//結構體名是behavior
BEGIN
Comp: PROCESS(a,b)
      BEGIN
         IF a=b THEN
             y<='1';
         ELSE
             y<='0';
         END  IF;
     END PROCESS comp;
END  behavior;									//結構體 2：該結構體采用條件賦值語句描述4位												//值比較器設計
ARCHITECTURE  dataflow  OF  comp4  IS         		//結構體名是dataflow
BEGIN
         y<='1'  WHEN(a=b)  ELSE  '0';
END  dataflow;								//結構體 3：該結構體采用結構化方法描述4位等												//值比較器設計
ARCHITECTURE  structural  OF  comp4  IS       		//結構體名是structural
        COMPONENT xnor2
              PORT(in1,in2:  IN STD_LOGIC;
                         Out: OUT STD_LOGIC);
         END COMPONENT;
         COMPONENT and4
               PORT(in1, in2, in3, in4: IN STD_LOGIC;
                           Out: OUT STD-LOGIC);
          END COMPONENT;
          SIGNAL s: STD_LOGIC_VECTOR(0 to 3);
BEGIN
      u0: xnor2 PORT MAP(a(0),b(0),s(0));
      u1: xnor2 PORT MAP(a(1),b(1),s(1));
      u2: xnor2 PORT MAP(a(2),b(2),s(2));
      u3: xnor2 PORT MAP(a(3),b(3),s(3));
      u4: and4  PORT MAP(s(0),s(1),s(2),s(3),y);
END  structural;
											//配置程序 
CONFIGURATION  comp4_con  OF  comp4  IS  		//comp4_con是配置名
       FOR  behavior                           		//指定為comp4_con配置的結構體是behavior
       END  FOR;
END  comp4_con;