how to do zero padding al the lsb

Question

I would like to add zero at the lsb (zero padding). my input is

 m  :   IN STD_LOGIC_VECTOR (31 DOWNTO 0);

and another vector (lets say a) that his length is changing all the time.
I didn't manage doing that by using bitwise "OR" because the length is always not the same.

Answer 1

Assuming you don't know the length of a, but want to zero pad the LSB of the resulting vector b it to length of b.

b(b'left downto b'left - a'left) <= a;
b(b'left - a'left - 1 downto 0) <= (others => '0');

I hope I didn't mix up the - 1.