VHDL Programming

138 Chapter Five

actual subprogram body specified yet. The subprogram body must exist

before the simulator is built, during elaboration.

Package Body

The main purpose of the package body is to define the values for deferred

constants and specify the subprogram bodies for any subprogram decla-

rations from the package declaration. However, the package body can also

contain the following declarations:

Subprogram declaration

Subprogram body

Type, subtype declaration

Constant declaration, which fills in the value for the deferred con-

stant

File declaration

Alias declaration

Use clause

All of the declarations in the package body, except for the constant

declaration that is specifying the value of a deferred constant and the sub-

program body declaration, are local to the package body.

Let’s examine a package body for the package declaration that was

discussed in the last section:

PACKAGE BODY cluspack IS

CONSTANT undriven : t_wclus :=

(ZX, ZX, ZX, ZX,

ZX, ZX, ZX, ZX,

ZX, ZX, ZX, ZX,

ZX, ZX, ZX, ZX);

FUNCTION resolve_cluster ( s: t_clus_vec )

return t_cluster IS

VARIABLE result : t_cluster;

VARIABLE drive_count : INTEGER;

BEGIN

IF s’LENGTH = 0 THEN RETURN undriven;

END IF;

FOR i in s’RANGE LOOP

IF s(i) /= undriven THEN

drive_count := drive_count + 1;

IF drive_count = 1 THEN