DE2 SRAM read/write problem

I want make a program that can read/write data with sram on de2 board

But I has a trouble with read data! This only can read when I remove " this line" in the code! (I test this with de2 controlpanel) when I don't remove "this line" the program only can write!

library ieee;

use ieee.std_logic_1164.all;

entity sram is

port( clock_50:in std_logic;

sw: in std_logic_vector(17 downto 0);

ledr: out std_logic_vector(15 downto 0);

sram_dq: inout std_logic_vector(15 downto 0);

sram_addr: out std_logic_vector(17 downto 0);

sram_we_n,sram_oe_n,sram_ub_n,sram_lb_n,sram_ce_n: out std_logic

);

end sram;

architecture behavior of sram is

signal clk,w,r: std_logic;

signal datain,dataout: std_logic_vector(15 downto 0);

begin

datain<="00000000"&sw(15 downto 8) ;

sram_addr<="0000000000"&sw(7 downto 0);

ledr<=dataout;

sram_ce_n<='0';

sram_ub_n<='0';

sram_lb_n<='0';

process (sw(17 downto 16),clock_50) is

begin

case sw(17 downto 16) is

--read

when "10" =>

sram_oe_n<='0';

dataout<=sram_dq;

--write

when "01" =>

sram_we_n<='0';

sram_dq<=datain;--- this line

when others =>

sram_oe_n<='1';

sram_we_n<='1';

end case;

end process;

end behavior;