8-bit binary counter cant count down

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;

entity counter_code is
	port (
	output  :out std_logic_vector (7 downto 0);    
	up_down :in  std_logic;              
	clk     :in  std_logic;             
	reset   :in  std_logic         
			);
end counter_code;

architecture start of counter_code is
	signal count :std_logic_vector (7 downto 0);
	
	begin
		process (clk, reset, up_down)
		begin
		
			if (reset = '1' and rising_edge(clk)) then
			
				count <= (others=>'0');
				
			elsif (rising_edge(clk)) then -- rising edge
			
				if (up_down = '1') then
						count <= count + 1;
				elsif (up_down = '0' and count /= "00000000") then

						count <= count - 1;
				
				end if;
				

			end if;
			
		end process;
	output <= count;
end start;