Package
Formal Definition
A package declaration defines the
interface to a package.
Simplified Syntax
package package_name is
package_declarations
end package package_name;
Description
The package is a unit that groups various declarations, which can be
shared among several designs. Packages are stored in libraries for
greater convenience. A package consists of package declaration
(mandatory) and may contain a single optional package
body.
The purpose of a package is to declare shareable types, subtypes,
constants, signals, files, aliases, component, attributes and groups.
Once a package is defined, it can be used in multiple independent designs.
Items declared in a package declaration are visible in other design
units if the use clause is
applied (Example 1).
The two-part specification of a package (declaration and body) allows
to declare the so-called deferred constants
which have no value assigned in the package declaration (Example 2).
The value for a deferred constant, however, must be declared in the
package body accompanying the package declaration.
The VHDL Language Standard defines two standard packages, which must
be available in any VHDL environment - package STANDARD
and package TEXTIO. The
former contains basic declarations of types, constants and operators,
while the latter defines operations for manipulating text files. Both
are located in the library STD. See respective topics for details.
Apart from the VHDL Language Standard there is another standard,
which extends the language and supports the extensions in the form of
a package: Std_Logic_1164.
Examples
Example 1
library Packages;
use Packages.AUXILIARY.all;
architecture STRUCT of
Adder is
................
end architecture STRUCT;
All declarations, which are inside the AUXILIARY package, may be used
in the architecture body STRUCT of the design entity Adder. The
package itself is stored in the library Packages.
Example 2
library IEEE;
use IEEE.STD_LOGIC_1164.all;
package AUXILIARY is
type MUX_input is array
(INTEGER range<>) of
STD_LOGIC_VECTOR (0 to 7);
type operation_set is
(SHIFT_LEFT, ADD);
subtype MUX_address is POSITIVE;
function Compute_Adress (IN1
: MUX_input) return MUX_address;
constant Deferred_Con : Integer;
end AUXILIARY;
Package AUXILIARY contains a function declaration and a deferred
constant, thus a package body had to be declared for this package.
Important Notes
-
Package declaration may
contain a subprogram (function or procedure) declaration; subprogram
body is not allowed here and must appear in the package body.
-
Package body must accompany
a package declaration if the declaration contains subprogram
declarations or deferred constants.
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