VHDL Programming

Predefined Attributes 145

The left bound of a type or subtype is the leftmost entry of the range

constraint. The right bound is the rightmost entry of the type or subtype.

In the following example, the left bound is -32,767, and the right bound

is 32,767:

TYPE smallint IS -32767 TO 32767;

The upper bound of a type or subtype is the bound with the largest

value, and the lower bound is the bound with the lowest value. In the pre-

ceding example, for the type smallint, the upper bound is 32,767, and the

lower bound is -32,767.

To use one of these value attributes, the type mark name is followed

by the attribute desired. For example, following is the syntax to return the

left bound of a type:

PROCESS(x)

SUBTYPE smallreal IS REAL RANGE -1.0E6 TO 1.0E6;

VARIABLE q : real;

BEGIN

q := smallreal’LEFT;

-- use of ’left returns

-- -1.0E6

END test;

In this example, variable qis assigned the left bound of type smallreal.

Variable qmust have the same type as the bounds of the type for the

assignment to occur. (The assignment could also occur if variable qwas cast

into the appropriate type.) After the assignment has occurred, variable q

contains -1.0E6, which is the left bound of type smallreal.

In the next example, all of the attributes are used to show what happens

when a DOWNTOrange is used for a type:

PROCESS(a)

TYPE bit_range IS ARRAY(31 DOWNTO 0) OF BIT;

VARIABLE left_range, right_range, uprange, lowrange :

integer;

BEGIN

left_range := bit_range’LEFT;

-- returns 31

right_range := bit_range’RIGHT;

-- returns 0

uprange := bit_range’HIGH;

-- returns 31

lowrange := bit_range’LOW;