算数运算时FPGA编程设计中常会用到的功能，其规则直接影响变成效果，调用use  ieee.std_logic_unsigned.all;此程序包对不同的数据类型可以进行适当的算数运算：
   1.对std_logic_vector()可进行相同位数的加减运算（被加数必须和输出位数相同）；
   2.对std_logic_vector()可进行相乘法运算（积的位数等于俩乘数位数之和）；
   
Library ieee;
Use ieee.std_logic_1164.all;
Use ieee.std_logic_unsigned.all;
entity adder_n is
port(a,b:in std_logic_vector(4 downto 0);     
     c:in std_logic_vector(2 downto 0);
     Sum1,sum2:out std_logic_vector(9 downto 0);
     sum4,sum3 :out std_logic_vector(4 downto 0) );
end;
Architecture add of adder_n is
begin
Sum1<="00000"&a + b;
Sum2<=a * b; 
sum3<=a - b;
sum4<=a + b;
end;
3.对Integer（整数）可以进行加、减、乘、除和取余运算；
Library ieee;
Useieee.std_logic_1164.all;
Useieee.std_logic_unsigned.all;
entity adder_n is
generic(n:integer:=4);----改变w的值可以改变运算宽度
port(a:in integerrange 0 to 4095;
     dd,b:in integer range 0 to 255;
     cin:in integer range 0 to 64;
     Sum1,sum2:out integer range 0 to 4095;
     ddd:out integer range 0 to 1023;
     co1,co2:out integer range 0 to 1023);
end;
Architecture add of adder_n is  
begin
Sum1<=a * b;---a,b一个可以是常数，如 Sum1<=2 * b
sum2<=a + b;
ddd<=a / b;---- 虽然是除，但实际为商 取整运算
co1<=a - dd;  
co2<= a REM cin;  ---  取余
  end;