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8.10 VHDL DESCRIPTION OF ARITHMETIC DEVICES 383

description of a four-bit adder composed of four full adders described in the previous
example. If Fig. 8.49 is used with reference to Figs. 8.4 and 8.5, the structure of the four-bit
adder is given as follows:

entity four_bit_adder is
port (aO, al, a2, a3, bO, bl, b2, b3, ci: in bit; sO, si, s2, s3, co: out bit;
end four_bit_adder;
architecture connectJour of four_bit_adder is
component fulLadder
port (a, b, ci: in bit; s, co: out bit);
end component;
for all: fulLadder use entity fulLadder_example;
signal cl,c2, c3: bit
begin
FAO: fulLadder port map (aO, bO, ci, sO, cl);
FA1: fulLadder port map (al, bl, cl, si, c2);
FA2: fulLadder port map (a2, b2, c2, s2, c3);
FA3: fulLadder port map (a3, b3, c3, s3, co);
end connect_four;
end four_bit_adder;

Just given is an architectural description for the full-adder primitive, followed by that for
a four-bit adder formed by cascading four full-adder primitives. However, within VHDL
compilers, encapsulations of such primitives are provided so that they can be easily re-
trieved and used in the architectural descriptions of larger systems. Thus, for well-known
primitives like those just considered, there is no need to construct the detailed architec-
tural descriptions — this has already been accomplished for the convenience of the user.
These primitives exist within standard logic packages. The IEEE 1164 standard library is
an example, and its contents are made available by making the statements

library ieee
use std_logic_1164.all

Once a standard library is made available in the design description, use can be made of data
types, functions, and operators provided by that standard. Standard data types include bit,
bit-vector, integer, time, and others, and the operators are of the type given in Fig. 6.44.
The standard package defined by the IEEE 1076 standard includes declarations for all the
standard data types. For detailed information on these and other subjects related to standard
libraries and packages the reader is referred to Further Reading.