bbs prng simulation done, but no result waveform

currently i am using Cyclone II family libraries to implement a bbs prng

i have insert a coding that i copy from internet, but i quite confuse on understanding the coding

the coding i'm using to make modification:

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity bbs is

Port ( p : in integer;

q : in integer;

s : in integer;

b : out STD_LOGIC_VECTOR(7 downto 0) );

end bbs;

architecture Behavioral of bbs is

begin

process(p,q,s)

variable x0,x1,s1,x_0:integer;

constant p:integer:=11;

constant q:integer:=19;

constant n:integer:=551;

variable b1:integer ;

variable b2:std_logic;

variable r:std_logic_vector(7 downto 0);

begin

s1:=s*s;

x0:=s1 mod 1024;

for i in 0 to 7 loop

x_0:=x0*x0;

x1:=x_0 mod 1024;

b1:=x1 mod 2;

if b1=1 then

b2:='1';

else b2:='0';

end if;

r(6):=r(7);

r(5):=r(6);

r(4):=r(5);

r(3):=r(4);

r(2):=r(3);

r(1):=r(2);

r(0):=r(1);

r(7):=b2;--so it will be serially transmitted but parallelly observed

x0:=x1;

b<=r;

end loop;

end process;

end Behavioral;

please if there is someone can help me on explaining the coding?