不带使能端的3线8线译码器：

1、用CASE语句

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

ENTITY         LS138        IS

  PORT ( S:in STD_LOGIC_vector(2 downto 0);

         Y: out STD_LOGIC_vector(7 downto 0));

end LS138;

ARCHITECTURE mux_behave OF LS138 IS

begin

  PROCESS(S)       IS

    BEGIN

    CASE        S        IS

             when "000"=>Y<=(0=>'0',OTHERS=>'1');

             when "001"=>Y<=(1=>'0',OTHERS=>'1');

             when "010"=>Y<=(2=>'0',OTHERS=>'1');

             when "011"=>Y<=(3=>'0',OTHERS=>'1');

             when "100"=>Y<=(4=>'0',OTHERS=>'1');

             when "101"=>Y<=(5=>'0',OTHERS=>'1');

             when "110"=>Y<=(6=>'0',OTHERS=>'1');

             when "111"=>Y<=(7=>'0',OTHERS=>'1');

      END CASE;

   END PROCESS;

  end  mux_behave;

2、用公式

LIBRARY    IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY       yimaqi138      IS
  PORT (A,B,C:IN STD_LOGIC;
          S1,notS2,notS3:IN STD_LOGIC;
        Y0,Y1,Y2,Y3: out STD_LOGIC;
        Y4,Y5,Y6,Y7: OUT STD_LOGIC);
End    yimaqi138;
ARCHITECTURE    mux_behave    OF     yimaqi138      IS
  SIGNAL S:STD_LOGIC;
  BEGIN
     S<=S1 AND (NOT notS2) AND (NOT notS3);
     Y0<=NOT(((NOT A)AND(NOT B)AND (NOT C)) AND S);
     Y1<=NOT(((NOT A)AND (NOT B) AND C)AND S);
     Y2<=NOT(((NOT A) AND  B AND (not C))AND S);
     Y3<=NOT(((NOT A) AND B AND  C )AND S);
     Y4<=NOT((A AND(NOT B)AND (NOT C))AND S);
     Y5<=NOT((A AND (NOT B) AND C)AND S);
     Y6<=NOT((A AND  B AND (not C))AND S);
     Y7<=NOT((A AND B AND  C )AND S);       
end  mux_behave;