VHDL Programming

Predefined Attributes 147

of a value are equal to the value itself; but for an enumerated type, the

position numbers of a value are determined by the declaration of the type.

Values declared earlier have lower position numbers than values declared

later. Value bluefrom the preceding example has a position number of 0,

because it is the first value of the type. Value cyanhas a position number

1,greenhas 2, and so on. From these position numbers, the high and low

bounds of the type can be found.

Signals color5through color8are assigned attributes of the type

reverse_color. This type has a DOWNTOrange specification. Attributes

’HIGHand ’RIGHTdo not return the same value because the range is

reversed. Value redhas a higher position number than value green, and

therefore a DOWNTOis needed for the range specification.

Value Array Attributes

There is only one value array attribute:’LENGTH. Given an array type, this

attribute returns the total length of the array range specified. This

attribute works with array ranges of any scalar type and with multi-

dimensional arrays of scalar-type ranges. Following is a simple example:

PROCESS(a)

TYPE bit4 IS ARRAY(0 TO 3) of BIT;

TYPE bit_strange IS ARRAY(10 TO 20) OF BIT;

VARIABLE len1, len2 : INTEGER;

BEGIN

len1 := bit4’LENGTH; -- returns 4

len2 := bit_strange’LENGTH; -- returns 11

END PROCESS;

The assignment to len1assigns the value of the number of elements

in array type bit4. The assignment to len2assigns the value of the num-

ber of elements of type bit_strange.

This attribute also works with enumerated-type ranges, as shown by

the following example:

PACKAGE p_4val IS

TYPE t_4val IS (’x’, ’ 0 ’, ’ 1 ’, ’z’);

TYPE t_4valX1 IS ARRAY(t_4val’LOW TO t_4val’HIGH) OF

t_4val;

TYPE t_4valX2 IS ARRAY(t_4val’LOW TO t_4val’HIGH) OF

t_4valX1;

TYPE t_4valmd IS ARRAY(t_4val’LOW TO t_4val’HIGH,

t_4val’LOW TO t_4val’HIGH) OF t_4val;