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Sunday 23 December 2012

Package Body

Formal Definition

A package body defines the bodies of subprograms and the values of deferred constants declared in the interface to the package.

Simplified Syntax

package body package_name is


subprogram bodies declarations

deferred constants declarations

end package body package_name;


The package body includes complete definitions of subprogram body declarations as well as values of deferred constants declared in corresponding package declarations. Other declarations (similar to those of package declaration) are also allowed here, but are visible only inside the package body.

The deferred constant, which has been declared in a package declaration, may be used before its full declaration only in a default expression for a local generic parameter, local port or formal parameter of subprogram.


Example 1

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_Address (IN1 : MUX_input) return MUX_address;
  constant Deferred_Con : Integer;
package body AUXILIARY is
  function Compute_Address (IN1 : MUX_input) return MUX_address is
  constant Deferred_Con : Integer := 177;
end package body AUXILIARY;

First, the package is specified here and then the accompanying package body. Note that both have the same name.

Important Notes

· Declarations other than values of deferred constants and subprogram bodies are not visible outside the package body and can be used only locally, inside it.

· Each package can have only one body.

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