仿真代码，但实际上无论什么代码仿真都是这样，
LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;
ENtiTY bit8X8 IS
PORT(cout: OUT STD_LOGIC_VECTOR(7 DOWNTO 0);
     addr: IN STD_LOGIC_VECTOR(2 DOWNTO 0));
END bit8X8;
ARCHITECTURE ROM_a OF bit8X8 IS
BEGIN
cout<="00001111" WHEN addr="000" ELSE
   "11110000" WHEN addr="001" ELSE
   "00110011" WHEN addr="010" ELSE
   "01000111" WHEN addr="011" ELSE
   "01100011" WHEN addr="100" ELSE
   "00111001" WHEN addr="101" ELSE
   "01110001" WHEN addr="110" ELSE
   "00011101" WHEN addr="111" ;
END ROM_a;

仿真时停的地方，一直不动