- •Contents
- •Foreword
- •IEEE Introduction
- •0. Overview of this standard
- •0.1 Intent and scope of this standard
- •0.2 Structure and terminology of this standard
- •1.1 Entity declarations
- •1.2 Architecture bodies
- •2. Subprograms and packages
- •2.1 Subprogram declarations
- •2.2 Subprogram bodies
- •2.3 Subprogram overloading
- •2.4 Resolution functions
- •2.5 Package declarations
- •2.6 Package bodies
- •2.7 Conformance rules
- •3. Types
- •3.1 Scalar types
- •3.2 Composite types
- •3.3 Access types
- •3.4 File types
- •3.5 Protected types
- •4. Declarations
- •4.1 Type declarations
- •4.2 Subtype declarations
- •4.3 Objects
- •4.4 Attribute declarations
- •4.5 Component declarations
- •4.6 Group template declarations
- •4.7 Group declarations
- •6. Names
- •6.1 Names
- •6.2 Simple names
- •6.3 Selected names
- •6.4 Indexed names
- •6.5 Slice names
- •6.6 Attribute names
- •7. Expressions
- •7.1 Expressions
- •7.2 Operators
- •7.3 Operands
- •7.4 Static expressions
- •7.5 Universal expressions
- •8. Sequential statements
- •8.1 Wait statement
- •8.2 Assertion statement
- •8.3 Report statement
- •8.4 Signal assignment statement
- •8.5 Variable assignment statement
- •8.6 Procedure call statement
- •8.7 If statement
- •8.8 Case statement
- •8.9 Loop statement
- •8.10 Next statement
- •8.11 Exit statement
- •8.12 Return statement
- •8.13 Null statement
- •9. Concurrent statements
- •9.1 Block statement
- •9.2 Process statement
- •9.3 Concurrent procedure call statements
- •9.4 Concurrent assertion statements
- •9.5 Concurrent signal assignment statements
- •9.6 Component instantiation statements
- •9.7 Generate statements
- •10. Scope and visibility
- •10.1 Declarative region
- •10.2 Scope of declarations
- •10.3 Visibility
- •10.4 Use clauses
- •10.5 The context of overload resolution
- •11. Design units and their analysis
- •11.1 Design units
- •11.2 Design libraries
- •11.3 Context clauses
- •11.4 Order of analysis
- •12. Elaboration and execution
- •12.1 Elaboration of a design hierarchy
- •12.2 Elaboration of a block header
- •12.3 Elaboration of a declarative part
- •12.4 Elaboration of a statement part
- •12.5 Dynamic elaboration
- •12.6 Execution of a model
- •13. Lexical elements
- •13.1 Character set
- •13.2 Lexical elements, separators, and delimiters
- •13.4 Abstract literals
- •13.5 Character literals
- •13.6 String literals
- •13.7 Bit string literals
- •13.8 Comments
- •13.9 Reserved words
- •13.10 Allowable replacements of characters
- •14.2 Package STANDARD
- •14.3 Package TEXTIO
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IEC 61691-1-1:2004(E) |
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IEEE 1076-2002(E)IEEE |
LANGUAGE REFERENCE MANUAL |
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Std 1076-2002 |
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iteration_scheme ::= |
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while |
condition |
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|for loop _parameter_specification |
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parameter_specification ::= |
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identifier |
in |
discrete_range |
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If a label appears at |
the end |
of a loop statement, it must repeat the label at the beginning of |
the loop |
statement. |
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Execution of a loop statement is complete when the loop is left as a consequence of the completion of the
iteration scheme (see below), if any, or the execution of a next statement, an exit statement, or a return statement.
A loop statement without an iteration scheme specifies repeated execution of the sequence of statements.
For a loop statement with a |
while |
iteration scheme, the condition is evaluated before each execution of the |
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sequence of statements; if the value of the condition is TRUE, the sequence of statements is executed; if |
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FALSE, the iteration scheme is said to be |
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complete |
and the execution of the loop statement is complete. |
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For a loop statement with a |
for iteration scheme, the loop parameter specification is the declaration of the |
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loop parameter |
with the given identifier. The loop parameter is an object whose type is the base type of the |
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discrete range. Within the sequence of statements, the loop parameter is a constant. Hence, a loop parameter |
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is not allowed as the target of an assignment statement. Similarly, the loop parameter must not be given as an |
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actual corresponding to a formal of mode |
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out |
or |
inout |
in an association list. |
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For the execution of a loop with a |
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for iteration scheme, the discrete range is first evaluated. If the discrete |
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range is a null range, the iteration scheme |
is said |
to be |
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complete |
and the execution of the loop statement is |
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therefore complete; otherwise, the sequence of statements is executed once for |
each value of the discrete |
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range (subject to the loop not being left as a consequence of the execution of a next statement, an exit |
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statement, or a return statement), after which the iteration scheme is said to be |
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complete |
. Prior to each such |
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iteration, the corresponding value of the discrete range is assigned to the loop parameter. These values are |
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assigned in left-to-right order. |
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NOTE—A loop may be left as the result of the execution of a next statement if the loop is nested inside of an outer loop and the next statement has a loop label that denotes the outer loop.
8.10 Next statement
A next statement is used to complete the execution of one of the iterations of an enclosing loop statement (called “loop” in the following text). The completion is conditional if the statement includes a condition.
next_statement ::= |
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[ label : ]next [ loop _label ] [ when |
condition ] ; |
A next statement with a loop label is only allowed within the labeled loop and applies to that loop; a next statement without a loop label is only allowed within a loop and applies only to the innermost enclosing loop (whether labeled or not).
For the execution of a next statement, the condition, if present, is first evaluated. The current iteration of the loop is terminated if the value of the condition is TRUE or if there is no condition.
Copyright © 2002 |
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Published byIEEEEC under licence from IEEE. © 2004 IEEE. All rights reserved. |
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IEC 61691-1-1:2004(E) |
– 133 – |
IEIEEE 1076-2002(E) |
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Std 1076-2002 |
IEEE STANDARD VHDL |
8.11 Exit statement
An exit statement is used to complete the execution of an enclosing loop statement (called “loop” in the following text). The completion is conditional if the statement includes a condition.
exit_statement ::= |
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[ label : ]exit [ loop _label ] [ when |
condition ] ; |
An exit statement with a loop label is only allowed within the labeled loop and applies to that loop; an exit statement without a loop label is only allowed within a loop and applies only to the innermost enclosing loop (whether labeled or not).
For the execution of an exit statement, the condition, if present, is first evaluated. Exit from the loop then takes place if the value of the condition is TRUE or if there is no condition.
8.12 Return statement
A return statement is used to complete the execution of the innermost enclosing function or procedure body.
return_statement ::= |
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[ label : ]return |
[ expression ] ; |
A return statement is only allowed within the body of a function or procedure, and it applies to the innermost enclosing function or procedure.
A return statement appearing in a procedure body must not have an expression. A return statement appearing in a function body must have an expression.
The value of the expression defines the result returned by the function. The type of this expression must be
the base |
type of the type mark |
given after the reserved word |
return |
in the specification of the function. It is |
an error |
if execution of a function |
completes by any means other than the execution of a return statement. |
For the execution of a return statement, the expression (if any) is first evaluated and a check is made that the value belongs to the result subtype. The execution of the return statement is thereby completed if the check succeeds; so also is the execution of the enclosing subprogram. An error occurs at the place of the return statement if the check fails.
NOTES |
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1—If the expression |
is either a numeric literal, or an attribute that |
yields a |
result |
of type |
universal_integer |
or |
universal_real |
, then an implicit conversion of the result is performed. |
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2—If the return type mark of a function denotes a constrained array subtype, then no implicit subtype conversions are |
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performed on the values of the expressions of the return statements within the subprogram body of that function. Thus, |
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for each index position of each value, the bounds of the discrete range must |
be the same |
as the |
discrete range of |
the |
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return subtype, and the directions must be the same. |
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8.13 Null statement
A null statement performs no action.
null_statement ::=
[ label : ]null ;
130 |
Copyright © 2002 IEEE. All rights reserved. |
Published by IEC under licence from IEEE. © 2004 IEEE. All rights reserved. |
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– 134 – |
IEC 61691-1-1:2004(E) |
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IEEE 1076-2002(E)IEEE |
LANGUAGE REFERENCE MANUAL |
Std 1076-2002 |
The execution of the null statement has no effect other than to pass on to the next statement.
NOTE—The null statement can be used to specify explicitly that no action is to be performed when certain conditions
are true, although it is never mandatory for this (or any other) purpose. This is particularly useful in conjunction with the case statement, in which all possible values of the case expression must be covered by choices: for certain choices, it may be that no action is required.
Copyright © 2002 |
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Published byIEEEEC under licence from IEEE. © 2004 IEEE. All rights reserved. |
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