(1) LIBRARY statement and PACHAGE part
Action: library (Library) is used to store pre-compiled programs (Package), defines a data package assembly, logical operations and components. Mainly in the constant declarations, data types, elements and subsystems in the physical design or the like is used.
Use Format: LIBRARY library name;
. USE library name package name .ALL;
For example: LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
(2) ENTITY defined
Action: the definition of the design input / output ports, i.e. the appearance of the circuit, i.e., I / O type and number of interfaces using the format:
format:
ENTITY entity name of the IS
Port (Port: port mode data type;
........
Port Name: Port mode data types;
)
END entity name;
Example:
ENTITY MUX41A IS
PORT (A,B,C,D,s0,s1,s2,s3 :IN STD_LOGIC;
Y : OUT STD_LOGIC);
END ENTITY MUX41A;
(3) ARCHITECTURE defined
Role: implementation-defined entity. That is specifically described circuit, a circuit or what action to realize the function.
Using the format:
ARCHITECTURE structure name of the entity name IS
begin
Description Statement;
end structure name;
E.g:
ARCHITECTURE BHV OF MUX41A IS
SIGNAL S:STD_LOGIC_VECTOR(3 DOWNTO 0);
BEGIN
S <= s3 & s2 & s1 & s0 ;
Y<=A WHEN S="1110" ELSE
B WHEN S="1101" ELSE
C WHEN S="1011" ELSE
D WHEN S="0111" ELSE
‘1’;
END BHV;