Finally, the attribute
VHDL defines the following function kind attributes that can be applied to types: ‘Pos(value)—returns the position number of a type value. Attributes allow retrieving information about named entities: types, objects, subprograms etc.
attributes are: entity, architecture, configuration, procedure,
specification to differentiate functions (Example 5). Common attributes can be declared for objects of different classes
Additionally, users can define new attributes. An attribute gives extra information about a specific part of a VHDL description. Predefined attributes can be contants, functions or signals. assigned to a given language unit. These are some of the predefined attributes for scalar types, constrained array types and any objects declared to be of array types. VHDL Predefined Attributes The syntax of an attribute is some named entity followed by an apostrophe and one of the following attribute names. Attributes supply additional information about an item, e.g. however, the attribute specification is written in other places. It consists of an identifier
For some entities,
A parameter list is used with some attributes. Attributes of discrete or physical types and subtypes Table 4. chosen named entity. Attribute specification assigns an attribute declared earlier to a
See Each type or subtype T has a basic attribute called T'Base, which indicates the base type for type T (Table 1).
A user-defined attribute can be of any VHDL type, except for an access type, file type, and any complex type with elements of any of the two types. See Example 1. The values assigned this way can be referred in the expressions
specification, which represents user defined attribute and type mark
component, label, literal, units, group, or file. They allow specifying precisely where warnings or errors are generated. See
through declared attribute name. is why they always require the signature in the attribute
The attribute declaration defines a new attribute within the scope of
type, and any complex type with elements of any of the two types. Predefined attributes can be contants, functions or signals. through new attributes for specified types. signal, or constant that may be associated with one or more named
The named entities
The named entities that can be assigned
Table 3. for the entities listed in the specification. VHDL standard defines a set of predefined attributes. Attributes of the array type or objects of the array type true if index range of dimension n is ascending, false otherwise Implicit signal, equivalent to signal S, but delayed t units of time Implicit signal that has the value true when no event has occurred on S for t time units, false otherwise implicit signal that has the value true when no transaction has occurred on S for t time units, false otherwise implicit signal of type Bit whose value is changed in each simulation cycle in which a transaction occurs on S (signal S becomes active) true if an event has occurred on S in the current simulation cycle, false otherwise true if a transaction has occurred on S in the current simulation cycle, false otherwise the amount of time since last event occurred on S, if no event has yet occurred it returns Time'High the amount of time since last transaction occurred on S, if no event has yet occurred it returns Time'High the previous value of S before last event occurred on it true if the process is driving S or every element of a composite S, or false if the current value of the driver for S or any element of S in the process is determined by the null transaction the current value of the driver for S in the process containing the assignment statement to S a string representing the simple name, character literal or operator symbol defined in the declaration of the item E a string describing the path through the design hierarchy, from the root entity or package to the item E a string describing the path through the design hierarchy, from the root entity or package to the item E, but including the names of the entity and architecture bound to each component instance in the path Paths which can be written using E'Path_name and E'Instance_name are used for reporting and assertion statements. using one construct - A value, function, type, range,
labeled statements A user-defined
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