The `std_logic_vector` type

The std_logic_vector type is used for arrays of std_logic variables and signals.

The basic VHDL logic operations are defined on this type: and, nand, or, nor, xor, xnor. These must be given two arrays of the same size; they do the operation on ecah position and return another array. The not operation negates each position in the array.

Examples

signal s1, s2, s3 : std_logic_vector(3 downto 0);
...
s1(0) <= '0';
s1(1) <= '1';
s1(2) <= '1';
s1(3) <= '0';
s2 <= "1100";     -- sets s(3),s(2) to '1', s(1),s(0) to '0': same order as range in declaration
s3 <= s2          -- copies all of s2 into s3
s3 <= s1 and s2;  -- "0100"