use ieee.numeric_std."*";

signal a, b : std_logic_vector (7 downto 0);
signal sc : std_logic_vector (15 downto 0)
signal uc : std_logic_vector (15 downto 0)

-- signed multiplication
sc = std_logic_vector (signed (a) * signed (b));
-- unsigned multiplication
uc = std_logic_vector (unsigned (a) * unsigned (b));