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Sunday 16 December 2012

Variable Assignment

Formal Definition

A variable assignment statement replaces the current value of a variable with a new value specified by an expression.

Simplified Syntax

variable_name := expression ;


The variable assignment statement modifies the value of the variable. The new value of the variable is obtained by assigning an expression to this variable. In order to distinguish variable assignment from signal assignment, the variable assignment symbol is different (:=).

The expression assigned to a variable must give results of the same type as the variable. The target at the left-hand side of the assignment can be either a name of a variable or anaggregate.

In the first case, the target can be in the form of simple name, selected name, indexed name or slice name (Example 1).

In case of aggregate as the target of the assignment, the type of the aggregate must be determinable from the context, including the fact that the type must be of the composite type. Each element of the aggregate must be in the form of the locally static name, which represents variable (Example 2).

The element association of aggregate, similarly to names, may have forms that are more complex: selected name, indexed name or slice name (Example 3).


Example 1

variable X, Y : REAL;
variable A, B : BIT_VECTOR (0 to 7);
type BIT_RECORD is record
  bitfield : BIT;
  intfield : Integer;
end record;
variable C, D : BIT_RECORD;
X := 1000.0;
A := B;
A := "11111111";
A (3 to 6) := ('1','1','1','1');
A (0 to 5) := B (2 to 7);
A (7) := '0';
B (0) := A (6);
C.bitfield := '1';
D.intfield := C.intfield;

The above examples of variable assignments are grouped in the following way: after the declarations the first group is a group of assignments with simple names as targets, then slice names, indexed names and finally selected names.

Example 2

variable E : BIT;
variable I : INTEGER;
(E, I) := C;

The aggregate used above as a target for a variable assignment could be used for the variable C declared in such a way as in the Example 1. E will be assigned the value of C.bitfield and I - C.intfield.

Example 3

type BIT_VECTOR_RECORD is record
  a: BIT_VECTOR(0 to 7);
  b: Integer;
end record;
variable G, H : BIT_VECTOR_RECORD;
(C.bitfield, C.intfield) := D; -- aggregate with selected name
(G.a(0 to 7), K) := H; -- aggregate with sliced name
(G.a(0), K) := D; -- aggregate with indexed name

Aggregates can use different forms of names.

Important Notes

· Variable assignment can be labeled.

· Variable assignment takes effect immediately.

· Variable assignment can not be specified with a delay.

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