接上篇：http://m.zg4o1577.cn/bbs/dpj-28500-1.html
五、八位二進制數減法器
一）、不帶符號
該減法器只能大數減小數，不帶正負號，且運算在時鐘脈沖上升沿到來時進行計算。
Library ieee;
Use ieee.std_logic_1164.all;
Entity jianfaqi8 is
Port(a,b:in std_logic_vector(7 downto 0);
CLK:in std_logic;
S:out std_logic_vector(7 downto 0);
co:out std_logic);
end jianfaqi8;
Architecture add of jianfaqi8 is
signal a1,b1:std_logic_vector(7 downto 0);
component adder8 is
Port(a,b:in std_logic_vector(7 downto 0);
cin:in std_logic;
S:out std_logic_vector(7 downto 0);
co:out std_logic);
end component;
begin
process(a,b,CLK)
begin
if CLK'event and CLK='1' then
a1<=a;
b1<=b;
end if;
end process;
U:adder8 port map(a1,(not b1),'1',S,co);
end;
二）帶符號
可以大減小，也可小減大，輸出正負的計算結果
Library ieee;
Use ieee.std_logic_1164.all;
Use ieee.std_logic_unsigned.all;
Entity jianfaqi8 is
Port(a,b:in std_logic_vector(7 downto 0);
CLK:in std_logic;
S:out std_logic_vector(7 downto 0);
co:out std_logic);
end jianfaqi8;
Architecture add of jianfaqi8 is
signal a1,b1,S1,S2,S3:std_logic_vector(7 downto 0);
signal c1,c2:std_logic;
component adder8 is
Port(a,b:in std_logic_vector(7 downto 0);
cin:in std_logic;
S:out std_logic_vector(7 downto 0);
co:out std_logic);
end component;
begin
process(a,b,CLK)
begin
if CLK'event and CLK='1' then
a1<=a;b1<=b;
end if;
end process;
U:adder8 port map(a1,(not b1),'1',S1,c1);
c2<=c1 xor '1';
co<=c2;
Process(S1,c1)
begin
if c2='0' then
S<=S1;
else
S2<= S1-1;
S<=not S2;
end if;
end process;
end;
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