4选1多路选择器的VHDL描述
要求：THEN语句和CASE语句实现4选1多路选择器，其中选择控制信号s1和s0的数据类型为STD_LOGIC_VECTOR;当s1=‟0‟,s0=‟0‟；s1=‟0‟,s0=‟1‟；s1=‟1‟,s0=‟0‟和s1=‟1‟,s0=‟1‟时,分别执行y<=a、y<=b、y<=c、y<=d。

一、解法1：用IF_THEN语句实现4选1多路选择器
（1）程序：
LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY mux41 IS
PORT (a,b,c,d: IN STD_LOGIC;
s0: IN STD_LOGIC;
s1: IN STD_LOGIC;
y: OUT STD_LOGIC);
END ENTITY mux41;
ARCHITECTURE if_mux41 OF mux41 IS
SIGNAL s0s1 : STD_LOGIC_VECTOR(1 DOWNTO 0);--定义标准逻辑位矢量数据BEGIN
s0s1<=s1&s0; --s1相并s0,即s1与s0并置操作
PROCESS(s0s1,a,b,c,d)
BEGIN
IF s0s1 = "00" THEN y <= a;
ELSIF s0s1 = "01" THEN y <= b;
ELSIF s0s1 = "10" THEN y <= c;
ELSE y <= d;
END IF;
END PROCESS;
END ARCHITECTURE if_mux41;
(2)编译的结果如下：
编译报告：
二、解法2：用CASE语句实现4选1多路选择器（1）程序:
LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY mux41 IS
PORT (a,b,c,d: IN STD_LOGIC;
s0: IN STD_LOGIC;
s1: IN STD_LOGIC;
y: OUT STD_LOGIC);
END ENTITY mux41;
ARCHITECTURE case_mux41 OF mux41 IS
SIGNAL s0s1 : STD_LOGIC_VECTOR(1 DOWNTO 0);--定义标准逻辑位矢量数据类型
BEGIN
s0s1<=s1&s0; --s1相并s0,即s1与s0并置操作
PROCESS(s0s1,a,b,c,d)
BEGIN
CASE s0s1 IS --类似于真值表的case语句
WHEN "00" => y <= a;
WHEN "01" => y <= b;
WHEN "10" => y <= c;
WHEN "11" => y <= d;
WHEN OTHERS =>NULL ;
END CASE;
END PROCESS;
END case_mux41;
（2）编译结果：
编译报告：。