VHDL problem?

QUESTION:

Develop a VHDL model for an accumulator that calculates the sum of a sequence of fixed-point numbers.

Each input number is signed with 4 pre-binary-point and 12 post-binary-point bits.

The accumulated sum has 8 pre-binary-point and 12 post-binary-point bits.

A new number arrives at the input during a clock cycle when the data_en control input is 1.

The accumulated sum is cleared to 0 when the reset control input is 1.

Both control inputs are synchronous.

DOUBT:

I want to ask how to insert the first input and second input as it using the pre and post numbers?

How to store the value for the first input for awhile before it add the second input?

The output will light up using LED at the board that we assign it by ourselves.

library ieee;

use ieee.std_logic_1164.all, ieee.numeric_std.all;

entity accumulator is

port (clk,reset, data_en : in std_logic;

data_in : in signed(15 downto 0);

data_out : out signed(19 downto 0) );

end entity accumulator;

architecture rtl of accumulator is

signal sum, new_sum :signed(19 downto 0);

begin

new_sum <= sum + resize(data_in, sum'length);

reg:process (clk) is

begin

if rising_edge(clk) then

if reset = '1' then

sum <= (others => '0');

elsif data_en = '1' then

sum <= new_sum;

end if;

end if;

end process reg;

data_out <= sum;

end architecture rtl;

-KYRA-:)