为啥这程序波形实现不了辨向，求解释啊

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
entity time4 is
port(a:in std_logic;
     b:in std_logic;
          clk:in std_logic;
          pp: out std_logic;
          np: out std_logic);
end time4;

architecture Behavioral of time4 is
signal prestate,state: std_logic_vector(1 downto 0);
signal snp,spp:std_logic;
begin
process(a,b,clk)
begin
if(clk'event and clk='1')then
state<=a&b;
if state="00" then
case prestate is
when "01"=>prestate<="00";spp<='1';
when "10"=>prestate<="00";snp<='1';
when others=>prestate<="00";spp<='0';snp<='0';
end case;
elsif state="10" then
case prestate is
when "00"=>prestate<="10";spp<='1';
when "11"=>prestate<="10";snp<='1';
when others=>prestate<="10";spp<='0';snp<='0';
end case;
elsif state="11" then
case prestate is
when "10"=>prestate<="11";spp<='1';
when "01"=>prestate<="11";snp<='1';
when others=>prestate<="11";spp<='0';snp<='0';
end case;
else
case prestate is
when "11"=>prestate<="01";spp<='1';
when "00"=>prestate<="01";snp<='1';
when others=>prestate<="01";spp<='0';snp<='0';
end case;
end if;
end if;
end process;
pp<=spp;
np<=snp;
end Behavioral;