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Attributes
Attribute ---- used in ----> Entity
Architecture
Package
Package Body

Syntax

object'attribute_name

See LRM sections 14.1, 4.4, 5.1 and 6.6

Rules and Examples

Attributes supply additional information about an item, e.g. a signal, variable, type or component. Certain attributes are predefined for types, array objects and signals. These are some of the predefined attributes for scalar types, constrained array types and any objects declared to be of array types. They are the same type as the object (scalar), or the index (array):
Name Definition
X'highThe upper bound of X
X'lowThe lower bound of X
X'leftThe leftmost bound of X
X'rightThe rightmost bound of X
These are predefined only constrained array types and any objects declared to be of array types:
Name Definition
X'rangeThe range of X
X'reverse_rangeThe range of X "back to front"
X'lengthX'high - X'low + 1 (integer)
These attributes are predefined for any signal X:
Name Definition
X'eventTrue when signal X changes (boolean)
X'activeTrue when signal X assigned to (boolean)
X'last_eventWhen signal X last changed (time)
X'last_activeWhen signal X was last assigned to (time)
X'last_valuePrevious value of X (same type as X)
These attributes create a new signal, based on signal X:
Name Definition
X'delayed(T)Signal X delayed by T (same type as X)
X'stable(T)True if X unaltered for time T (boolean)
X'quiet(T)True if X is unassigned for time T (boolean)
X'transaction"Toggles" when X is assigned (bit)
User defined attributes may be declared. These do not affect simulation, but may be used to supply information to other tools, e.g. for layout or synthesis: 
type IC_PACKAGE is (DIL, PLCC, PGA);
attribute PTYPE: ICPACKAGE;
attribute PTYPE of U1 : component is PLCC;
attribute PTYPE of U2 : component is DIL;

Synthesis Issues

Logic synthesis tools usually support the predefined attributes 'high, 'low, 'left, 'right, 'range, reverse_range, 'length and 'event. Some tools support 'last_value and 'stable

Several synthesis vendors define a set of attributes to supply synthesis directives such as area or timing constraints, enumeration encoding etc.

Whats New in '93

VHDL-93 has several new predefined attributes:

Name Definition
X'driving True if a process is driving signal X
X'driving_value Value a process is driving signal X with
X'ascending True if index range of X is ascending
X'image(literal) String representation of enumeration literal
X'simple_name String equivalent to the name of X
X'instance_name Path downto and including X, excluding entity and architecture names
X'path_name Path downto and including X, excluding entity and architecture names

The group construct allows collections of VHDL objects of different classes to be grouped together to allow common attributes to be set for the elements of these groups.