-- *************************************************************
-- * FILENAME:  prob3_1b.vhd                                   *
-- * AUTHOR:    durant@msoe.edu and meier@msoe.edu             *
-- * DATE:      13 Dec 2006, rev. 12 Dec 2007                  *
-- * PROVIDES:                                                 *
-- * - 4-bit to 1-bit combinational function from 3.1b         *
-- *************************************************************

-- *************************************************************
-- * ENTITY DECLARATION                                        *
-- * - describes the input and output pins of the black box    *
-- * - each pin has direction and type bit declared            *
-- *************************************************************

entity PROB3_1B is
port (
       W : in bit;
       X : in bit;
       Y : in bit;
       Z : in bit;
       F : out bit
     );
end entity PROB3_1B;

-- *************************************************************
-- * ARCHITECTURE DESCRIPTION                                  *
-- * - describes the functional behavior of the black box      *
-- * - this example: with-select describes truth table rows    *
-- * -               commented-out is when-else alternative    *
-- * - note how W&X&Y&Z "concatenates" input pins into a number*
-- * - note that single signals have energy levels with ' '    *
-- * - note that multiple signals have energy levels with " "  *
-- * - note that binary sets are prefixed with B               *
-- *************************************************************

architecture DATAFLOW of PROB3_1B is
begin

  -- efficient since it generates a single MUX with 4 control inputs
  with W&X&Y&Z select
    F <= '1' when B"0010",
         '1' when B"0110",
         '1' when B"1010",
         '1' when B"1011",
         '1' when B"1100",
         '1' when B"1101",
         '1' when B"1110",
         '1' when B"1111",
         '0' when others;


  -- inefficient since it generates MUXes (several equality comparisons); with-select does better
  --  F <= '1' when W&X&Y&Z = B"0010" else
  --       '1' when W&X&Y&Z = B"0110" else
  --       '1' when W&X&Y&Z = B"1010" else
  --       '1' when W&X&Y&Z = B"1011" else
  --       '1' when W&X&Y&Z = B"1100" else
  --       '1' when W&X&Y&Z = B"1101" else
  --       '1' when W&X&Y&Z = B"1110" else
  --       '1' when W&X&Y&Z = B"1111" else
  --       '0';

end architecture DATAFLOW;