VHDL Syntax :        Libraries and Types


library ieee;
use ieee.STD_LOGIC_1164.all;
use ieee.numeric_std.all;


Types ( just a few of them )

std_ulogic, std_logic -- nine possible values  UX01ZWLH-
std_ulogic_vector, std_logic_vector,
bit, bit_vector -- two possible values  01
                  -- can be redefined to hold UZ10
unsigned, signed -- used in arithmetic