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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Monday 31 December 2012


Formal Definition

A basic operation that combines one or more values into a composite value of a record or array type.


aggregate ::= ( element_association { , element association } ) element_association ::= [choices => ] expression choices ::= choice { | choice } choice ::= simple expression          | discrete_range          | element_simple_name          | others


The aggregate assigns one or more values to the elements of a record or array creating the composite value of this type. Aggregates are composed of element associations, which associate expressions to elements (one expression per one or more elements). Element associations are specified in parentheses and are separated by commas.

An expression assigned to an element or elements must be of the same type as the element(s).

Elements can be referred to either by textual order they have in the object declaration (so called positional associations - example 1) or by its name (named associations - example 2). Both methods can be used in the same aggregate, but in such a case all positional associations appear first (in textual order) and all named associations appearing next (in any order). In any case if the association others is used, it must be the last one in an aggregate.

The choice clause, denoting selection of element(s) can have any of the following forms: simple expression, discrete range, simple name or reserved word others.

A value of simple expression can be applied in arrays only and must belong to discrete range of an array type. A simple expression specifies the element at the corresponding index value. Example 2 illustrates this concept.

A discrete range must meet the same conditions as a simple expression. It is useful when several consecutive elements of an array are assigned the same value (example 3). A discrete range serves for defining the set of indexes only and the direction specified or implied has no significance.

The use of element simple name as a choice is restricted to records only. In this case, each element is identified by its name (Example 4).

When some elements are assigned different values and the remaining elements will receive some other value, reserved word others can be used to denote those elements (Example 5). Such a choice must be the last in an aggregate and can be used both in arrays and in records, provided that the remaining elements of the records are of the same type.

The choice others can serve as a very convenient way to assign the same value to all elements of some array, e.g. to reset a wide bus (Example 6).

If several elements are assigned the same value, a multiple choice can be used. In such a case a bar sign (|) separates references to elements (Example 7). If a multiple choice is used in an array aggregate, it may not be mixed with positional associations.


Example 1

variable Data_1 : BIT_VECTOR (0 to 3) := ('0','1','0','1');

Bits number 0 and 2 are assigned the value '0', while bits 1 and 3 are assigned '1'. All element associations here are positional.

Example 2

variable Data_2 : BIT_VECTOR (0 to 3) := (1=>'1',0=>'0',3=>'1',2=>'0');

Like in the previous example, bits number 0 and 2 are assigned the value '0', while bits 1 and 3 are assigned '1'. The element associations here, however, are named. Note that in this case the elements can be listed in arbitrary order.

Example 3

signal Data_Bus : Std_Logic_Vector (15 downto 0);
. . .
Data_Bus <= (15 downto 8 => '0', 7 downto 0 => '1');

Data_Bus will be assigned the value of "0000000011111111". The first element is associated a value in positional way (thus it is bit number 15), and the other two groups are assigned values using discrete ranges.

Example 4

type Status_Record is record
     Code : Integer;
     Name : String (1 to 4);
end record;
variable Status_Var : Status_Record := (Code => 57, Name => "MOVE");

Choice as an element simple name can be used in record aggregates - each element is associated a value (of the same type as the element itself).

Example 5

signal Data_Bus : Std_Logic_Vector (15 downto 0);
. . .
Data_Bus <= (14 downto 8 => '0', others => '1');

Data_Bus will be assigned the same value as in example 3 ("1000000011111111"), but this aggregate is written in more compact way. Apart from bits 14 through 8, which receive value '0' all the others (15 and 7 through 0) will be assigned '1'. Note that the choice others is the last in the aggregate.

Example 6

signal Data_Bus : Std_Logic_Vector (15 downto 0);
. . .
Data_Bus <= (others => 'Z');

Instead of assigning "ZZZZZZZZZZZZZZZZ" to Data_Bus in order to put it in high impedance state, an aggregate with the others choice representing all the elements can be used.

Example 7

signal Data_Bus : Std_Logic_Vector (15 downto 0);
. . .
Data_Bus <= (15 | 7 downto 0 => '1',
others => '0');

Note the multiple choice specification of the assignment to the bits 15 and 7 through 0. The result of the assignment to Data_Bus will be the same as in examples 3 and 5 ("1000000011111111").

Important Notes

· Associations with elements' simple names are allowed in record aggregates only.

· Associations with simple expressions or discrete ranges as choices are allowed only in array aggregates.

· Each element of the value defined by an aggregate must be represented once and only once in the aggregate.

· Aggregates containing the single element association must always be specified using named association in order to distinguish them from parenthesized expressions.

· The others choice can be only the last in an aggregate.

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