it contains half-adder vhdl code and simulate form also, there is test bench coding for half-adder, which means writers can gave a clock by himself. like a <= '0', '1' after 5ns, '0' after 10ns, '1' after 15ns, '0' after 20ns, '1'after 25ns, '0' after 30ns, '1' after 35ns; b <= '0', '1' after 10ns, '0' after 20ns, '1' after 30ns, '0' after 40ns;