====== Attribute declarations ======
===== attribute_declaration =====
<code vhdl>
attribute identifier : type_mark ;
</code>

Attributes can be used to poll the characteristics of objects (e.g. signals). In many cases this allows a shorter and more elegant VHDL description to be created. The most important attributes are [[.:Predefined attributes|predefined ones]].

In order to be able to use one`s own attributes one has to write a corresponding function which then provides the

attribute`s value at any given time. For further information go to: [[.:Attribute specification]]. 

===== Parents =====
  * entity_declarative_part
  * architecture_declarative_part
  * package
  * block_declarative_part
  * function_declarative_part
  * procedure_declarative_part
  * process_declarative_part

===== Further definitions =====
==== identifier ====
  * [[.:bnf#basic_identifier]]
  * [[.:bnf#extended_identifier]]

==== type_mark ====
''type _[[.:bnf#name]]''

''subtype _[[.:bnf#name]]''

===== Examples =====
//capacitance_value// is declared as an attribute of the type //capacitance// .
<code vhdl>
ATTRIBUTE capacitance_value :
      capacitance ;
</code>

----

The attribute //pin_number// declared here is of the subtype //pin_natural// .
<code vhdl>
SUBTYPE pin_natural IS natural
      RANGE 1 TO 24 ;
ATTRIBUTE pin_number : pin_natural ;
</code>

----

Two different attributes are declared.

//location// is of the type //coordinate// which is a Record type.

//pin_no// is of the type //positive// which is a subtype of //integer// .
<code vhdl>
TYPE coordinate IS
   RECORD
      x, y : integer ;
   END RECORD ;
   TYPE positive IS integer
         RANGE 1 TO integer'high ;
   ATTRIBUTE location : coordinate ;
   ATTRIBUTE pin_no : positive ; 
</code>

----

By using [[.:predefined attributes]] the loop-instruction can be written in a different way as well:
<code vhdl>
FOR i IN a1'LOW TO a1'HIGH

FOR i IN a1'RIGHT TO a1'LEFT

FOR i IN a1'REVERSE_RANGE

FOR i IN a1'RANGE
</code>

<code vhdl>
SIGNAL a1: bit_vector(3 DOWNTO 0) ;
...
PROCESS (a)
   BEGIN
   z<= "0000";
   FOR i in 0 TO 3 LOOP
      IF (a = i) THEN
         z(i) <= '1' ;
      END IF ;
   END LOOP ;
END PROCESS ;
...
</code>