VHDL Programming

Subprograms and Packages 139

result := s(i);

ELSE

result := undriven;

ASSERT FALSE

REPORT “multiple drivers detected”

SEVERITY ERROR;

END IF;

END IF;

END LOOP;

RETURN result;

END resolve_cluster;

END cluspack;

The package body statement is very similar to the package declaration,

except for the keyword BODYafter package. The contents of the two design

units are very different, however. This package body example contains

only two items: the deferred constant value for deferred constant undriven

and the subprogram body for subprogram resolve_cluster. Notice how

the deferred constant value specification matches the deferred constant

declaration in the package declaration, and the subprogram body matches

the subprogram declaration in the package declaration. The subprogram

body must match the subprogram declaration exactly in the number of

parameters, the type of parameters, and the return type.

A package body can also contain local declarations that are used only

within the package body to create other subprogram bodies, or deferred

constant values. These declarations are not visible outside of the package

body but can be very useful within the package body. Following is an

example of a complete package making use of this feature:

USE LIBRARY IEEE;

USE IEEE.std_logic_1164.ALL;

PACKAGE math IS

TYPE st16 IS ARRAY(0 TO 15) OF std_logic;

FUNCTION add(a, b: IN st16) RETURN st16;

FUNCTION sub(a, b: IN st16) RETURN st16;

END math;

PACKAGE BODY math IS

FUNCTION vect_to_int(S : st16) RETURN INTEGER IS

VARIABLE result : INTEGER := 0;

BEGIN

FOR i IN 0 TO 7 LOOP

result := result * 2;

IF S(i) = ‘ 1 ’ THEN