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VHDL Program Format :

The various sections used in VHDL program are shown in figure below. These sections are,

1) LIBRARY declaration which contains the list of libraries used in the program.
e.g. : ieee, std, work, etc.

2) ENTITY which declares the I/O pins of the circuit and

3) ARCHITECTURE which describes the detailed logical design aspects of the IC.

LIBRARY Declarations :

LIBRARY is a set of functions which are used in the program. The structure of the library is shown in figure. The LIBRARY code is normally written in the form of FUNCTIONS, PROCEDURES, or COMPONENTS and kept in the form of PACKAGES. To declare the LIBRARY two lines are required in the program, one is the name of the library and second is the USE clause, as shown in the syntax.

        LIBRARY library_name;

The packages required in the program are,

1) ieee.std_logic_1164 (from the ieee library),

2) standard (from the std library) and

3) work (work library).

The declarations of the libraries are as follows,

        LIBRARY ieee; 
        USE ieee.std_logic_1164.all; 

        LIBRARY std;  
        USE std.standard.all;  

        LIBRARY work;
        USE work.all;

Since the libraries std and work are by default libraries, these libraries are need not to be declared. In the program the ieee library must be declared along with the STD_LOGIC (or STD_ULOGIC) data type used in the design.

The purpose of packages is :

1) std_logic_1164 package of ieee library declares a multi-level logic system and indicates the data types, text i/o, etc. for the VHDL design.

2) std_logic_1164 : Shows the STD_LOGIC (8 levels) and STD_ULOGIC (9 levels) multi-valued logic systems.

3) std_logic_arith shows the SIGNED and UNSIGNED data types and related arithmetic and comparison operations.

4) std_logic_signed has functions that allow operations with STD_LOGIC_VECTOR data to be performed of type SIGNED.

5) std_logic_unsigned has functions that allow operations with STD_LOGIC_VECTOR data to be performed as if the data were of type UNSIGNED.