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<>VHDL编写二位数值比较器
二位数值比较器是由四个输入端和三个输出端组成的比较器，实现两个二位二进制数大小的比较（用高低电平来表示输入输出）真值表如下：
（xx表任意状态）
因此设计时，定义4个输入端和3个输出端的实体，分别为A1A0,B1B0和f1(a>b)f2(a<b)f3(a=b)
具体代码如下：
LIBRARY IEEE; USE IEEE.STD_LOGIC_1164.ALL; ENTITY homework2 IS PORT(A1:IN
STD_LOGIC; --定义4个输入端和3个输出端 A0:IN STD_LOGIC; B1:IN STD_LOGIC; B0:IN STD_LOGIC;
f1:OUT STD_LOGIC; f2:OUT STD_LOGIC; f3:OUT STD_LOGIC); END homework2;
ARCHITECTURE HA OF homework2 IS SIGNAL tmp:STD_LOGIC_VECTOR(3 DOWNTO
0);--定义一个信号数组 BEGIN tmp<=A1 & B1 & A0 & B0; --用&连接四个输入值，并赋值给tmp PROCESS(tmp)
BEGIN CASE tmp IS --CASE语句实现具体代码 WHEN "0000" =>f1<='0';f2<='0';f3<='1'; WHEN
"0001" =>f1<='0';f2<='1';f3<='0'; WHEN "0010" =>f1<='1';f2<='0';f3<='0'; WHEN
"0011" =>f1<='0';f2<='0';f3<='1'; WHEN "0100" =>f1<='0';f2<='1';f3<='0'; WHEN
"0101" =>f1<='0';f2<='1';f3<='0'; WHEN "0110" =>f1<='0';f2<='1';f3<='0'; WHEN
"0111" =>f1<='0';f2<='1';f3<='0'; WHEN "1000" =>f1<='1';f2<='0';f3<='0'; WHEN
"1001" =>f1<='1';f2<='0';f3<='0'; WHEN "1010" =>f1<='1';f2<='0';f3<='0'; WHEN
"1011" =>f1<='1';f2<='0';f3<='0'; WHEN "1100" =>f1<='0';f2<='0';f3<='1'; WHEN
"1101" =>f1<='0';f2<='1';f3<='0'; WHEN "1110" =>f1<='1';f2<='0';f3<='0'; WHEN
"1111" =>f1<='0';f2<='0';f3<='1'; END CASE; END PROCESS; END HA;
仿真结果如下：