--變量的賦值是立即生效的，所以他的新值可以在下一行代碼中使用
--如下的是計算一個二進制矢量中１的個數(shù)的代碼：
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library ieee;
use ieee.std_logic_1164.all;
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entity jishuqi is
  port (din: in std_logic_vector(7 downto 0);
         ones: out integer range 0 to 8);
end jishuqi;
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architecture ok of jishuqi is
   begin
process(din)
  variable  temp: integer range 0 to 8;--變量切記在局部中聲明使用
   begin
    temp := 0;
    for i in 0 to 7 loop
   if (din(i) = '1') then 
    temp := temp+1;--注意變量用:=賦值,信號<=用賦值
    end if;
  end loop;
  ones <= temp;
end process;
end ok;
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