add4.vhd

出租车计费器,VHDL实现

LIBRARY  IEEE;
USE  IEEE.STD_LOGIC_1164.ALL;
USE  IEEE.STD_LOGIC_UNSIGNED.ALL; 
ENTITY add4 IS
    PORT(
           cin:IN STD_LOGIC;
          a,b :IN STD_LOGIC_VECTOR(3 DOWNTO 0);
            s :OUT STD_LOGIC_VECTOR(3 DOWNTO 0);
          cout:OUT STD_LOGIC);
END  add4;

ARCHITECTURE  execute  OF add4  IS
  SIGNAL  sint,aa,bb:STD_LOGIC_VECTOR(4 DOWNTO 0);
BEGIN
  aa<='0'&a;     
  bb<='0'&b;
  sint<=aa+bb+cin;
  s(3 DOWNTO 0)<=sint(3 DOWNTO 0);
  cout<=sint(4);
END execute;