In the previous version of VHDL, operators such as and, nand, or, took two different values and returned a bit or boolean value. For VHDL-2008, unary support has been added for these operators. They return the logical function of the input. For example, the code:
out1 <= and("0101");would AND the 4 bits together and return 0 . The
logical functions have unary support are: and, nand,
or, nor, xor, and
xnor.