NOTES

Allowable replacements for the special characters vertical line(|), number sign (#), and quotation mark (") are defined in the last clause of this section.

1--The font design of graphical symbols (for example, whether they are in italic or bold typeface) is not part of ISO 8859-1:1987.

2--The meanings of the acronyms used in this section are as follows: ASCII stands for American Standard Code for Information Interchange, ISO stands for International Organization for Standardization.

3--There are no uppercase equivalents for the characters ß and ÿ.

4--The following names are used when referring to special characters:

Character	Name		Character	Name
"	quotation mark		¢	cent sign
#	number sign		£	pound sign
&	ampersand		¤	currency sign
'	apostrophe, tick		¥	yen sign
(	left parenthesis		¦	broken bar
)	right parenthesis		§	paragraph sign, section sign
*	asterisk, multiply		¨	diaeresis
+	plus sign		©	copyright sign
,	comma		ª	feminine ordinal indicator
-	hyphen, minus sign		«	left angle quotation mark
.	dot, point, period, full stop		­	not sign
/	slash, divide, solidus		-	soft hyphen*
:	colon		®	registered trade mark sign
;	semicolon		¯	macron
<	less-than sign		°	ring above, degree sign
=	equals sign		±	plus-minus sign
>	greater-than sign		²	superscript two
_	underline, low line		³	superscript three
|	vertical line, vertical bar		'	acute accent
!	exclamation mark		µ	micro sign
$	dollar sign		¶	pilcrow sign
%	percent sign		·	middle dot
?	question mark		¸	cedilla
@	commercial at		¹	superscript one
[	left square bracket		º	masculine ordinal indicator
\	backslash, reverse solidus		»	right angle quotation mark
]	right square bracket		¼	vulgar fraction one quarter
^	circumflex accent		½	vulgar fraction one half
`	grave accent		¾	vulgar fraction three quarters
{	left curly bracket		¿	inverted question mark
}	right curly bracket		×	multiplication sign
~	tilde		÷	division sign
¡	inverted exclamation mark

*The soft hyphen is a graphic character that is imaged by a graphic symbol identical with, or similar to, that representing HYPHEN, for use when a line break has been established within a work.

13.2 Lexical elements, separators, and delimiters

The text of each design unit is a sequence of separate lexical elements. Each lexical element is either a delimiter, an identifier (which may be a reserved word), an abstract literal, a character literal, a string literal, a bit string literal, or a comment.

In some cases an explicit separator is required to separate adjacent lexical elements (namely when, without separation, interpretation as a single lexical element is possible). A separator is either a space character (SPACE or NBSP),a format effector, or the end of a line. A space character (SPACE or NBSP) is a separator except within a comment, a string literal, or a space character literal.

The end of a line is always a separator. The language does not define what causes the end of a line. However if, for a given implementation, the end of aline is signified by one or more characters, then these characters must be format effectors other than horizontal tabulation. In any case, a sequence of one or more format effectors other than horizontal tabulation must cause atleast one end-of-line.

One or more separators are allowed between any two adjacent lexical elements,before the first of each design unit or after the last. At least one separator is required between an identifier or an abstract literal and an adjacent identifier or abstract literal.

A delimiter is either one of the following special characters (in the basic character set):

& ' ( ) * + , - . / : ; < = > | [ ]

or one of the following compound delimiters, each composed of two adjacent special characters:

=> ** := /= >= <= <>

Each of the special characters listed for single character delimiters is a single delimiter except if this character is used as a character of a compound delimiter or as a character of a comment, string literal, character literal, or abstract literal.

The remaining forms of lexical elements are described in other clauses of this section.

NOTES

1--Each lexical element must fit on one line, since the end of a line is a separator. The quotation mark, number sign, and underline characters, likewise two adjacent hyphens, are not delimiters, but may form part of other lexical elements.

2--The following names are used when referring to compound delimiters:

Delimiter		Name
=>		arrow
**		double star, exponentiate
:=		variable assignment
/=		inequality (pronounced "not equal")
>=		greater than or equal
<=		less than or equal; signal assignment
<>		box

13.3 Identifiers

Identifiers are used as names and also as reserved words.

     identifier ::=  basic_identifier | extended_identifier

13.3.1 Basic identifiers

A basic identifier consists only of letters, digits, and underlines.

     basic_identifier ::=
         letter  { [ underline ] letter_or_digit }

     letter_or_digit ::=  letter | digit

     letter ::=  upper_case_letter | lower_case_letter

All characters of a basic identifier are significant, including any underline character inserted between a letter or digit and an adjacent letter or digit. Basic identifiers differing only in the use of corresponding upper and lowercase letters are considered the same.

Examples:

     COUNT    X     c_out      FFT     Decoder

     VHSIC    X1    PageCount  STORE_NEXT_ITEM

NOTE--No space (SPACE or NBSP) is allowed within a basic identifier since a space is a separator.

13.3.2 Extended identifiers

Extended identifiers may contain any graphic character.

     extended_identifier ::=
        \ graphic_character { graphic_character } \

If a backslash is to be used as one of the graphic characters of an extended literal, it must be doubled. All characters of an extended identifier are significant (a doubled backslash counting as one character). Extended identifiers differing only in the use of corresponding upper and lowercase letters are distinct. Moreover, every extended identifier is distinct from any basic identifier.

Examples:

     \BUS\     \bus\                         --  Two different identifiers, neither of which is
                                             --  the reserved word bus.

     \a\\b\                                  --  An identifier containing three characters.

     VHDL     \VHDL\     \vhdl\              --  Three distinct identifiers.

13.4 Abstract literals

There are two classes of abstract literals: real literals and integer literals. A real literal is an abstract literal that includes a point; an integer literal is an abstract literal without a point. Real literals are the literals of the type universal_real. Integer literals are the literals of the type universal_integer.

     abstract_literal ::=  decimal_literal | based_literal

13.4.1 Decimal literals

A decimal literal is an abstract literal expressed in the conventional decimal notation (that is, the base is implicitly ten).

     decimal_literal ::=  integer [ . integer ] [ exponent ]

     integer ::=  digit  { [ underline ] digit }

     exponent ::=  E [ + ] integer | E - integer

An underline character inserted between adjacent digits of a decimal literal does not affect the value of this abstract literal. The letter E of the exponent,if any, can be written either in lowercase or in uppercase, with the same meaning.

An exponent indicates the power of ten by which the value of the decimal literal without the exponent is to be multiplied to obtain the value of the decimal literal with the exponent. An exponent for an integer literal must not have a minus sign.

Examples:

     12         0         1E6         123_456              --  Integer literals

     12.0       0.0       0.456       3.14159_26           --  Real literals

     1.34E-12   1.0E+6    6.023E+24                        --  Real literals with exponents

13.4.2 Based literals

A based literal is an abstract literal expressed in a form that specifies the base explicitly. The base must be at least two and at most sixteen.

     based_literal ::=
         base # based_integer [ . based_integer ] # [ exponent ]

     base ::=  integer

     based_integer ::=
         extended_digit { [ underline ] extended_digit }
   
     extended_digit ::=  digit | letter

An underline character inserted between adjacent digits of a based literal does not affect the value of this abstract literal. The base and the exponent, if any, are in decimal notation. The only letters allowed as extended digits are the letters A through F for the digits ten through fifteen. A letter in a based literal (either an extended digit or the letter E of an exponent) can be written either in lowercase or in uppercase, with the same meaning.

The conventional meaning of based notation is assumed; in particular the value of each extended digit of a based literal must be less than the base. An exponent indicates the power of the base by which the value of the based literal without the exponent is to be multiplied to obtain the value of the based literal with the exponent. An exponent for a based integer literal must not have a minus sign.

Examples:

     --  Integer literals of value 255:
          2#1111_1111#               16#FF#             016#0FF#
     --  Integer literals of value 224:
          16#E#E1                    2#1110_0000#
     --  Real literals of value   4095.0 :
          16#F.FF#E+2                2#1.1111_1111_111#E11

13.5 Character literals

A character literal is formed by enclosing one of the 191 graphic characters(including the space and nonbreaking space characters) between two apostrophe characters. A character literal has a value that belongs to a character type.

     character_literal ::= ' graphic_character '

Examples:

     'A'   '*'   '''   ' '

13.6 String literals

A string literal is formed by a sequence of graphic characters (possibly none)enclosed between two quotation marks used as string brackets.

     string_literal ::=  " { graphic_character } "

A string literal has a value that is a sequence of character values corresponding to the graphic characters of the string literal apart from the quotation mark itself. If a quotation-mark value is to be represented in the sequence of character values, then a pair of adjacent quotation marks must be written at the corresponding place within the string literal. (This means that a string literal that includes two adjacent quotation marks is never interpreted as two adjacent string literals.)

The length of a string literal is the number of character values in the sequence represented. (Each doubled quotation mark is counted as a single character.)

Examples:

     "Setup time is too short"                --  An error message.

     ""                                       --  An empty string literal.

     " "       "A"      """"                  --  Three string literals of length 1.

     "Characters such as $, %, and } are allowed in string literals."

NOTE--A string literal must fit on one line, since it is a lexical element (see 13.2 ).Longer sequences of graphic character values can be obtained by concatenation of string literals. The concatenation operation may also be used to obtain string literals containing nongraphic character values. The predefined type CHARACTER in package STANDARD specifies the enumeration literals denoting both graphic and nongraphic characters. Examples of such uses of concatenation are

     "FIRST PART OF A SEQUENCE OF CHARACTERS " &
     "THAT CONTINUES ON THE NEXT LINE"

     "Sequence that includes the" & ACK & "control character"

13.7 Bit string literals

A bit string literal is formed by a sequence of extended digits (possibly none)enclosed between two quotations used as bit string brackets, preceded by a base specifier.

     bit_string_literal ::=  base_specifier " [ bit_value ] "

     bit_value ::=  extended_digit { [ underline ] extended_digit }

     base_specifier ::=  B | O | X

An underline character inserted between adjacent digits of a bit string literal does not affect the value of this literal. The only letters allowed as extended digits are the letters A through F for the digits ten through fifteen. A letter in a bit string literal (either an extended digit or the base specifier) can be written either in lowercase or in uppercase, with the same meaning.

If the base specifier is 'B', the extended digits in the bit value are restricted to 0 and 1. If the base specifier is 'O', the extended digits in the bit value are restricted to legal digits in the octal number system, i.e.,the digits 0 through 7. If the base specifier is 'X', the extended digits are all digits together with the letters A through F.

A bit string literal has a value that is a string literal consisting of the character literals '0' and '1'. If the base specifier is 'B', the value of the bit string literal is the sequence given explicitly by the bit value itself after any underlines have been removed.

If the base specifier is 'O' (respectively 'X'), the value of the bit string literal is the sequence obtained by replacing each extended digit in the bit_value by a sequence consisting of the three (respectively four) values representing that extended digit taken from the character literals '0' and '1';as in the case of the base specifier 'B', underlines are first removed. Each extended digit is replaced according to the table on the following page:

Extended digit	Replacement when the base specifier is	Replacement when the base specifier is
	'O'	'X'
0	000	0000
1	001	0001
2	010	0010
3	011	0011
4	100	0100
5	101	0101
6	110	0110
7	111	0111
8	(illegal)	1000
9	(illegal)	1001
A	(illegal)	1010
B	(illegal)	1011
C	(illegal)	1100
D	(illegal)	1101
E	(illegal)	1110
F	(illegal)	1111

The length of a bit string literal is the length of its string literal value.

Example:

     B"1111_1111_1111"         --  Equivalent to the string literal "111111111111"
     X"FFF"                    --  Equivalent to B"1111_1111_1111"
     O"777"                    --  Equivalent to B"111_111_111"
     X"777"                    --  Equivalent to B"0111_0111_0111"

     constant c1: STRING := B"1111_1111_1111";

     constant c2: BIT_VECTOR := X"FFF";

     type MVL is ('X', '0', '1', 'Z');
     type MVL_VECTOR is array (NATURAL range <>) of MVL;
     constant c3: MVL_VECTOR := O"777";

     assert    c1'LENGTH = 12 and
               c2'LENGTH = 12 and
               c3 = "111111111";

13.8 Comments

A comment starts with two adjacent hyphens and extends up to the end of the line. A comment can appear on any line of a VHDL description. The presence or absence of comments has no influence on whether a description is legal or illegal. Furthermore, comments do not influence the execution of a simulation module; their sole purpose is to enlighten the human reader.

Examples:

     --  The last sentence above echoes the Algol 68 report.

     end;                                     -- Processing of LINE is complete

     --  A long comment may be split onto
     --  two or more consecutive lines.

     -----------  The first two hyphens start the comment.

NOTE--Horizontal tabulation can be used in comments, after the double hyphen, and is equivalent to one or more spaces (SPACE characters) (see 13.2 ).

13.9 Reserved words

The identifiers listed below are called reserved words and are reserved for significance in the language. For readability of this manual, the reserved words appear in lowercase boldface.

abs		file		nand		select
access		for		new		severity
after		function		next		signal
alias				nor		shared
all		generate		not		sla
and		generic		null		sll
architecture		group				sra
array		guarded		of		srl
assert				on		subtype
attribute		if		open
		impure		or		then
begin		in		others		to
block		inertial		out		transport
body		inout				type
buffer		is		package
bus				port		unaffected
		label		postponed		units
case		library		procedure		until
component		linkage		process		use
configuration		literal		pure
constant		loop				variable
				range
disconnect		map		record		wait
downto		mod		register		when
				reject		while
else				rem		with
elsif				report
end				return		xnor
entity				rol		xor
exit				ror

A reserved word must not be used as an explicitly declared identifier.

NOTES

1--Reserved words differing only in the use of corresponding upper and lowercase letters are considered as the same (see 13.3.1 ). The reserved word range is also used as the name of a predefined attribute.

2--An extended identifier whose sequence of characters inside the leading and trailing backslashes is identical to a reserved word is not a reserved word. For example, \next\ is a legal (extended) identifier and is not the reserved word next.

13.10 Allowable replacements of characters

The following replacements are allowed for the vertical line, number sign, and quotation mark basic characters:

-- A vertical line (|) can be replaced by an exclamation mark (!) where used as a delimiter.

-- The number sign (#) of a based literal can be replaced by colons (:),provided that the replacement is done for both occurrences.

-- The quotation marks (") used as string brackets at both ends of a string literal can be replaced by percent signs (%), provided that the enclosed sequence of characters contains no quotation marks, and provided that both string brackets are replaced. Any percent sign within the sequence of characters must then be doubled, and each such doubled percent sign is interpreted as a single percent sign value. The same replacement is allowed for a bit string literal, provided that both bit string brackets are replaced.

These replacements do not change the meaning of the description.

NOTES

1--It is recommended that use of the replacements for the vertical line,number sign, and quotation marks be restricted to cases where the corresponding graphical symbols are not available. Note that the vertical line appears as a broken line on some equipment; replacement is not recommended in this case.

2--The rules given for identifiers and abstract literals are such that lowercase and uppercase letters can be used indifferently; these lexical elements can thus be written using only characters of the basic character set.