-- not in book

entity computer_system is
end entity computer_system;

-- end not in book


architecture top_level of computer_system is

  function resolve_bits ( bits : bit_vector ) return bit is
    variable result : bit := '0';
  begin
    for index in bits'range loop
      result := result or bits(index);
      exit when result = '1';
    end loop;
    return result;
  end function resolve_bits;

  signal write_en : resolve_bits bit bus;
  -- . . .

  -- not in book
  constant Tpd : delay_length := 2 ns;
  signal clock, hold_req : bit := '0';
  -- end not in book

begin

  CPU : process is
    -- . . .
  begin
    write_en <= '0' after Tpd;
    -- . . .
    loop
      wait until clock = '1';
      if hold_req = '1' then
        write_en <= null after Tpd;
        wait on clock until clock = '1' and hold_req = '0';
        write_en <= '0' after Tpd;
      end if;
      -- . . .
    end loop;
  end process CPU;

  -- . . .

  -- not in book

  clock_gen : clock <= '1' after 5 ns, '0' after 10 ns when clock = '0';

  stimulus : hold_req <= '1' after 40 ns, '0' after 80 ns;

  process is
  begin
    write_en <= null, '1' after 50 ns, '0' after 60 ns, null after 70 ns;
    wait;
  end process;

  -- end not in book

end architecture top_level;