VHDL Programming

146 Chapter Six

-- returns 0

END PROCESS;

This example shows how the different attributes can be used to return

information about a type. When ranges of a type are defined using (a TO

b)where b > a, the ’LEFTattribute will always equal the ’LOWattribute;

but when a range specification using (b DOWNTO a)where b > a is used,

the ’HIGHand ’LOWcan be used to determine the upper and lower bounds

of the type.

Value type attributes are not restricted to numeric types. These attributes

can also be used with any scalar type. Following is an example using

enumerated types:

ARCHITECTURE b OF a IS

TYPE color IS (blue, cyan, green, yellow, red, magenta);

SUBTYPE reverse_color IS color RANGE red DOWNTO green;

SIGNAL color1, color2, color3,

color4, color5, color6,

color7, color8 : color;

BEGIN

color1 <= color’LEFT; -- returns blue

color2 <= color’RIGHT; -- returns magenta

color3 <= color’HIGH; -- returns magenta

color4 <= color’LOW; -- returns blue

color5 <= reverse_color’LEFT;

-- returns red

color6 <= reverse_color’RIGHT;

-- returns green

color7 <= reverse_color’HIGH;

-- returns red

color8 <= reverse_color’LOW;

-- returns green

END b;

This example illustrates how value type attributes can be used with

enumerated types to return information about the type. Signals color1

and color2are assigned blueand magenta, respectively, the left and right

bounds of the type. It is easy to see how these values are obtained by

examining the declaration of the type. The left bound of the type is blue

and the right bound is magenta. What is returned for the ’HIGHand ’LOW

attributes of an enumerated type? The answer relates to the position

numbers of the type. For an integer and real type, the position numbers