The basic arithmetic operations, addition (+), subtraction (-) and multiplication (*) are defined on the unsigned and signed types.
The + and - operators can also be used to indicate a positive or negative value.
Each of the operators can take unsigned, signed and integer values as arguments. They can return unsigned, signed or std_logic_vector values as needed.
Note that this library doesn't allow you to do arithmetic on std_logic_vector values. That's because it's impossible for it to determine whether a particular std_logic_vector is representing a signed or unsigned value. To do this, you need to use either the std_logic_unsigned or std_logic_signed libraries.
signal u1, u2 : unsigned (3 downto 0); signal s1 : signed (3 downto 0); signal s2 : signed (4 downto 0); signal v1 : std_logic_vector (3 downto 0); signal v2 : std_logic_vector (4 downto 0); signal i1, i2 : integer; ... u1 <= "1001"; -- = 9 s1 <= "1001"; -- = -7 i1 <= 16; wait for 10 ns; s2 <= u1 + s1; -- = 2 v2 <= u1 + s1; -- = "0010" u2 <= i1 - u1; -- = 7 s1 <= -s1; -- = 7