Set STD_LOGIC_VECTOR with constant integer
You can do it, but not directly.
Something like this should work:

s1 &lt;= std_logic_vector(to_unsigned(12,7));


or 

s1 &lt;= std_logic_vector(to_unsigned(12,s1'length)); 


Of course at the begining of your file you should declare:

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;