Package that makes normally distributed random numbers from uniform numbers

In order to be able to have normal distribution noise, I made a package that
takes the uniform distribution random numbers and uses the central limit theorem
to give an (approximately) normal distribution. Here's the package:

library ieee;
use ieee.std_logic_1164.all;
use ieee.math_real.all;
use ieee.numeric_std.all;
use work.random_int.all;

--by MEP 22 February 2011
--usage:
--this is a function, which means it can be on the right-hand side
--of an assignment. It returns a mean-zero random number from a
--normal distribution. The argument is a real number that indicates
--the standard deviation desired.
--
--random_noise(sigma);
--

package normal_distribution_random_noise is

function random_noise (
sigma : real)
return real;

end package normal_distribution_random_noise;

package body normal_distribution_random_noise is

function random_noise (
sigma : real
)
return real is

--variables
variable u_noise: real; --uniform distribution noise
variable n_noise: real := 0.0; --normal distribution noise
variable seed1 : positive;
variable seed2 : positive;

begin

--obtain a uniformly distributed random number
uniform(seed1, seed2, u_noise);
--report "Random uniform noise is " & real'image(u_noise) & ".";

for normal_count in 0 to 12 loop
--Turn the uniform distributed number
--into a normally distributed number
--by using the central limit theorem.
--Make it mean zero and make it have
--the range of the uniform numbers
--that it is composed from.
n_noise := n_noise + u_noise;
end loop;

n_noise := n_noise - (0.5)*(real(12)); --normal distribution with a mean
of zero
--report "Random normal noise is " & real'image(n_noise) & ".";
n_noise := n_noise/(real(12));
--report "Random normal noise using range of uniform is " &
real'image(n_noise) & ".";
n_noise := sigma*n_noise;

return n_noise;
end function random_noise;
end package body normal_distribution_random_noise;