multiply two numbers

Hello.

I am a new to vhdl and as a part of my tranning i need to multiplay 2 numbers can u help me and tell my what i doing wrong?

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

use ieee.numeric_std.all;

ENTITY MUL IS PORT(

a,b: IN unsigned (3 downto 0);

y : OUT unsigned (7 downto 0));

END MUL ;

ARCHITECTURE Behavioral OF MUL IS

BEGIN

process (a,b)

variable temp1:unsigned(7 downto 0);

variable temp:unsigned(7 downto 0);

BEGIN

for j in 0 to 3 loop

if (b(j)='0') then

temp1:=(7 downto 0 =>'0');

else

temp1:=( j=>a(0),j+1=>a(1), j+2=>a(2),j+3=>a(3),others=>'0') ;

temp:=temp+temp1;

end if ;

end loop;

y<=temp;

end process;

END Behavioral;