44个vhdl实例 注1： 含有不可综合语句

-- Multiplexer 16-to-4 using if-then-elsif-else Statement
-- download from www.pld.com.cn & www.fpga.com.cn

library ieee;
use ieee.std_logic_1164.all;

entity mux is port(
        a, b, c, d:     in std_logic_vector(3 downto 0);
        s:              in std_logic_vector(1 downto 0);
        x:              out std_logic_vector(3 downto 0));
end mux;

architecture archmux of mux is
begin
mux4_1: process (a, b, c, d)
        begin
                if s = "00" then
                        x <= a; 
                elsif s = "01" then
                        x <= b;
                elsif s = "10" then
                        x <= c;
                else
                        x <= d;
                end if;
        end process mux4_1;
end archmux;