I am having trouble editing a signal in a sub program.

I declared a signal called reg_A. I then want to edit the signal in another subprogram. I basically want reg_A to be one of my inputs to a sub program (ALU_test) edit reg_A in the subprogram and then I want the output of my subprogram to change reg_A in the main program(test). When I tried to do this an error occurred ("Signal reg_A has multiple sources"). I am not sure what to do could some one please help me.

library ieee;
use ieee.std_logic_1164.all;

entity test is
port ( clk : in std_logic;
choze : in std_logic_vector(3 downto 0);
data_t : in std_logic_vector(7 downto 0);
Outw : out std_logic_vector(7 downto 0));
end entity test;

architecture no1 of test is
------------------------------------------------------------
component ALU_test is
port ( clk : in std_logic;
choose : in std_logic_vector(3 downto 0);
vlaggieIN : in std_logic_vector(7 downto 0);
vlaggie : out std_logic_vector(7 downto 0));
end component ALU_test;
------------------------------------------------------------
signal reg_A : std_logic_vector(7 downto 0);
------------------------------------------------------------
begin
process (clk)
begin
if rising_edge(clk) then

reg_A <= "11111111";
end if;--if rising_edge(clk) then
end process;

r1: ALU_toets4 port map (clk,choze,reg_A,reg_A);

Outw <= reg_A;

end architecture no1;
--------------------------------------------------------------------------
================================================== =======
--------------------------------------------------------------------------
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;

entity ALU_test is
port ( clk : in std_logic;
choose : in std_logic_vector(3 downto 0);
vlaggieIN : in std_logic_vector(7 downto 0);
vlaggie : out std_logic_vector(7 downto 0));
end entity ALU_test;

architecture alu1 of ALU_test is

begin
process (clk)
begin
if rising_edge(clk) then
case choose is
-----------------------------------
-- add
-----------------------------------
when "1000" =>
vlaggie <= "10101011" + vlaggieIN;
-----------------------------------
-- sub
-----------------------------------
when "0100" =>
vlaggie <= "10100100";
-----------------------------------
-- maal
-----------------------------------
when "0010" =>
vlaggie <= vlaggieIN;
-----------------------------------
when others =>
vlaggie <= "00000000";
-----------------------------------
end case;--case kies is

end if;--if rising_edge(clk) then
end process;
end architecture alu1;

Haai