@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/syntax.info
@node Syntax Tables, Abbrevs, Searching and Matching, Top
@chapter Syntax Tables
@cindex parsing
@cindex syntax table
@cindex text parsing

  A @dfn{syntax table} specifies the syntactic textual function of each
character.  This information is used by the parsing commands, the
complex movement commands, and others to determine where words, symbols,
and other syntactic constructs begin and end.  The current syntax table
controls the meaning of the word motion functions (@pxref{Word Motion})
and the list motion functions (@pxref{List Motion}) as well as the
functions in this chapter.

@menu
* Basics: Syntax Basics.     Basic concepts of syntax tables.
* Desc: Syntax Descriptors.  How characters are classified.
* Syntax Table Functions::   How to create, examine and alter syntax tables.
* Motion and Syntax::	     Moving over characters with certain syntaxes.
* Parsing Expressions::      Parsing balanced expressions
                                using the syntax table.
* Standard Syntax Tables::   Syntax tables used by various major modes.
* Syntax Table Internals::   How syntax table information is stored.
@end menu

@node Syntax Basics
@section Syntax Table Concepts

@ifinfo
  A @dfn{syntax table} provides Emacs with the information that
determines the syntactic use of each character in a buffer.  This
information is used by the parsing commands, the complex movement
commands, and others to determine where words, symbols, and other
syntactic constructs begin and end.  The current syntax table controls
the meaning of the word motion functions (@pxref{Word Motion}) and the
list motion functions (@pxref{List Motion}) as well as the functions in
this chapter.
@end ifinfo

  Under XEmacs 20, a syntax table is a particular subtype of the
primitive char table type (@pxref{Char Tables}), and each element of the
char table is an integer that encodes the syntax of the character in
question, or a cons of such an integer and a matching character (for
characters with parenthesis syntax).

  Under XEmacs 19, a syntax table is a vector of 256 elements; it
contains one entry for each of the 256 possible characters in an 8-bit
byte.  Each element is an integer that encodes the syntax of the
character in question. (The matching character, if any, is embedded
in the bits of this integer.)

  Syntax tables are used only for moving across text, not for the Emacs
Lisp reader.  XEmacs Lisp uses built-in syntactic rules when reading Lisp
expressions, and these rules cannot be changed.

  Each buffer has its own major mode, and each major mode has its own
idea of the syntactic class of various characters.  For example, in Lisp
mode, the character @samp{;} begins a comment, but in C mode, it
terminates a statement.  To support these variations, XEmacs makes the
choice of syntax table local to each buffer.  Typically, each major
mode has its own syntax table and installs that table in each buffer
that uses that mode.  Changing this table alters the syntax in all
those buffers as well as in any buffers subsequently put in that mode.
Occasionally several similar modes share one syntax table.
@xref{Example Major Modes}, for an example of how to set up a syntax
table.

A syntax table can inherit the data for some characters from the
standard syntax table, while specifying other characters itself.  The
``inherit'' syntax class means ``inherit this character's syntax from
the standard syntax table.''  Most major modes' syntax tables inherit
the syntax of character codes 0 through 31 and 128 through 255.  This is
useful with character sets such as ISO Latin-1 that have additional
alphabetic characters in the range 128 to 255.  Just changing the
standard syntax for these characters affects all major modes.

@defun syntax-table-p object
This function returns @code{t} if @var{object} is a vector of length 256
elements.  This means that the vector may be a syntax table.  However,
according to this test, any vector of length 256 is considered to be a
syntax table, no matter what its contents.
@end defun

@node Syntax Descriptors
@section Syntax Descriptors
@cindex syntax classes

  This section describes the syntax classes and flags that denote the
syntax of a character, and how they are represented as a @dfn{syntax
descriptor}, which is a Lisp string that you pass to
@code{modify-syntax-entry} to specify the desired syntax.

  XEmacs defines a number of @dfn{syntax classes}.  Each syntax table
puts each character into one class.  There is no necessary relationship
between the class of a character in one syntax table and its class in
any other table.

  Each class is designated by a mnemonic character, which serves as the
name of the class when you need to specify a class.  Usually the
designator character is one that is frequently in that class; however,
its meaning as a designator is unvarying and independent of what syntax
that character currently has.

@cindex syntax descriptor
  A syntax descriptor is a Lisp string that specifies a syntax class, a
matching character (used only for the parenthesis classes) and flags.
The first character is the designator for a syntax class.  The second
character is the character to match; if it is unused, put a space there.
Then come the characters for any desired flags.  If no matching
character or flags are needed, one character is sufficient.

  For example, the descriptor for the character @samp{*} in C mode is
@samp{@w{. 23}} (i.e., punctuation, matching character slot unused,
second character of a comment-starter, first character of an
comment-ender), and the entry for @samp{/} is @samp{@w{. 14}} (i.e.,
punctuation, matching character slot unused, first character of a
comment-starter, second character of a comment-ender).

@menu
* Syntax Class Table::      Table of syntax classes.
* Syntax Flags::            Additional flags each character can have.
@end menu

@node Syntax Class Table
@subsection Table of Syntax Classes

  Here is a table of syntax classes, the characters that stand for them,
their meanings, and examples of their use.

@deffn {Syntax class} @w{whitespace character}
@dfn{Whitespace characters} (designated with @w{@samp{@ }} or @samp{-})
separate symbols and words from each other.  Typically, whitespace
characters have no other syntactic significance, and multiple whitespace
characters are syntactically equivalent to a single one.  Space, tab,
newline and formfeed are almost always classified as whitespace.
@end deffn

@deffn {Syntax class} @w{word constituent}
@dfn{Word constituents} (designated with @samp{w}) are parts of normal
English words and are typically used in variable and command names in
programs.  All upper- and lower-case letters, and the digits, are typically
word constituents.
@end deffn

@deffn {Syntax class} @w{symbol constituent}
@dfn{Symbol constituents} (designated with @samp{_}) are the extra
characters that are used in variable and command names along with word
constituents.  For example, the symbol constituents class is used in
Lisp mode to indicate that certain characters may be part of symbol
names even though they are not part of English words.  These characters
are @samp{$&*+-_<>}.  In standard C, the only non-word-constituent
character that is valid in symbols is underscore (@samp{_}).
@end deffn

@deffn {Syntax class} @w{punctuation character}
@dfn{Punctuation characters} (@samp{.}) are those characters that are
used as punctuation in English, or are used in some way in a programming
language to separate symbols from one another.  Most programming
language modes, including Emacs Lisp mode, have no characters in this
class since the few characters that are not symbol or word constituents
all have other uses.
@end deffn

@deffn {Syntax class} @w{open parenthesis character}
@deffnx {Syntax class} @w{close parenthesis character}
@cindex parenthesis syntax
Open and close @dfn{parenthesis characters} are characters used in
dissimilar pairs to surround sentences or expressions.  Such a grouping
is begun with an open parenthesis character and terminated with a close.
Each open parenthesis character matches a particular close parenthesis
character, and vice versa.  Normally, XEmacs indicates momentarily the
matching open parenthesis when you insert a close parenthesis.
@xref{Blinking}.

The class of open parentheses is designated with @samp{(}, and that of
close parentheses with @samp{)}.

In English text, and in C code, the parenthesis pairs are @samp{()},
@samp{[]}, and @samp{@{@}}.  In XEmacs Lisp, the delimiters for lists and
vectors (@samp{()} and @samp{[]}) are classified as parenthesis
characters.
@end deffn

@deffn {Syntax class} @w{string quote}
@dfn{String quote characters} (designated with @samp{"}) are used in
many languages, including Lisp and C, to delimit string constants.  The
same string quote character appears at the beginning and the end of a
string.  Such quoted strings do not nest.

The parsing facilities of XEmacs consider a string as a single token.
The usual syntactic meanings of the characters in the string are
suppressed.

The Lisp modes have two string quote characters: double-quote (@samp{"})
and vertical bar (@samp{|}).  @samp{|} is not used in XEmacs Lisp, but it
is used in Common Lisp.  C also has two string quote characters:
double-quote for strings, and single-quote (@samp{'}) for character
constants.

English text has no string quote characters because English is not a
programming language.  Although quotation marks are used in English,
we do not want them to turn off the usual syntactic properties of
other characters in the quotation.
@end deffn

@deffn {Syntax class} @w{escape}
An @dfn{escape character} (designated with @samp{\}) starts an escape
sequence such as is used in C string and character constants.  The
character @samp{\} belongs to this class in both C and Lisp.  (In C, it
is used thus only inside strings, but it turns out to cause no trouble
to treat it this way throughout C code.)

Characters in this class count as part of words if
@code{words-include-escapes} is non-@code{nil}.  @xref{Word Motion}.
@end deffn

@deffn {Syntax class} @w{character quote}
A @dfn{character quote character} (designated with @samp{/}) quotes the
following character so that it loses its normal syntactic meaning.  This
differs from an escape character in that only the character immediately
following is ever affected.

Characters in this class count as part of words if
@code{words-include-escapes} is non-@code{nil}.  @xref{Word Motion}.

This class is used for backslash in @TeX{} mode.
@end deffn

@deffn {Syntax class} @w{paired delimiter}
@dfn{Paired delimiter characters} (designated with @samp{$}) are like
string quote characters except that the syntactic properties of the
characters between the delimiters are not suppressed.  Only @TeX{} mode
uses a paired delimiter presently---the @samp{$} that both enters and
leaves math mode.
@end deffn

@deffn {Syntax class} @w{expression prefix}
An @dfn{expression prefix operator} (designated with @samp{'}) is used
for syntactic operators that are part of an expression if they appear
next to one.  These characters in Lisp include the apostrophe, @samp{'}
(used for quoting), the comma, @samp{,} (used in macros), and @samp{#}
(used in the read syntax for certain data types).
@end deffn

@deffn {Syntax class} @w{comment starter}
@deffnx {Syntax class} @w{comment ender}
@cindex comment syntax
The @dfn{comment starter} and @dfn{comment ender} characters are used in
various languages to delimit comments.  These classes are designated
with @samp{<} and @samp{>}, respectively.

English text has no comment characters.  In Lisp, the semicolon
(@samp{;}) starts a comment and a newline or formfeed ends one.
@end deffn

@deffn {Syntax class} @w{inherit}
This syntax class does not specify a syntax.  It says to look in the
standard syntax table to find the syntax of this character.  The
designator for this syntax code is @samp{@@}.
@end deffn

@node Syntax Flags
@subsection Syntax Flags
@cindex syntax flags

  In addition to the classes, entries for characters in a syntax table
can include flags.  There are six possible flags, represented by the
characters @samp{1}, @samp{2}, @samp{3}, @samp{4}, @samp{b} and
@samp{p}.

  All the flags except @samp{p} are used to describe multi-character
comment delimiters.  The digit flags indicate that a character can
@emph{also} be part of a comment sequence, in addition to the syntactic
properties associated with its character class.  The flags are
independent of the class and each other for the sake of characters such
as @samp{*} in C mode, which is a punctuation character, @emph{and} the
second character of a start-of-comment sequence (@samp{/*}), @emph{and}
the first character of an end-of-comment sequence (@samp{*/}).

The flags for a character @var{c} are:

@itemize @bullet
@item
@samp{1} means @var{c} is the start of a two-character comment-start
sequence.

@item
@samp{2} means @var{c} is the second character of such a sequence.

@item
@samp{3} means @var{c} is the start of a two-character comment-end
sequence.

@item
@samp{4} means @var{c} is the second character of such a sequence.

@item
@c Emacs 19 feature
@samp{b} means that @var{c} as a comment delimiter belongs to the
alternative ``b'' comment style.

Emacs supports two comment styles simultaneously in any one syntax
table.  This is for the sake of C++.  Each style of comment syntax has
its own comment-start sequence and its own comment-end sequence.  Each
comment must stick to one style or the other; thus, if it starts with
the comment-start sequence of style ``b'', it must also end with the
comment-end sequence of style ``b''.

The two comment-start sequences must begin with the same character; only
the second character may differ.  Mark the second character of the
``b''-style comment-start sequence with the @samp{b} flag.

A comment-end sequence (one or two characters) applies to the ``b''
style if its first character has the @samp{b} flag set; otherwise, it
applies to the ``a'' style.

The appropriate comment syntax settings for C++ are as follows:

@table @asis
@item @samp{/}
@samp{124b}
@item @samp{*}
@samp{23}
@item newline
@samp{>b}
@end table

This defines four comment-delimiting sequences:

@table @asis
@item @samp{/*}
This is a comment-start sequence for ``a'' style because the
second character, @samp{*}, does not have the @samp{b} flag.

@item @samp{//}
This is a comment-start sequence for ``b'' style because the second
character, @samp{/}, does have the @samp{b} flag.

@item @samp{*/}
This is a comment-end sequence for ``a'' style because the first
character, @samp{*}, does not have the @samp{b} flag

@item newline
This is a comment-end sequence for ``b'' style, because the newline
character has the @samp{b} flag.
@end table

@item
@c Emacs 19 feature
@samp{p} identifies an additional ``prefix character'' for Lisp syntax.
These characters are treated as whitespace when they appear between
expressions.  When they appear within an expression, they are handled
according to their usual syntax codes.

The function @code{backward-prefix-chars} moves back over these
characters, as well as over characters whose primary syntax class is
prefix (@samp{'}).  @xref{Motion and Syntax}.
@end itemize

@node Syntax Table Functions
@section Syntax Table Functions

  In this section we describe functions for creating, accessing and
altering syntax tables.

@defun make-syntax-table &optional table
This function creates a new syntax table.  Character codes 0 through
31 and 128 through 255 are set up to inherit from the standard syntax
table.  The other character codes are set up by copying what the
standard syntax table says about them.

Most major mode syntax tables are created in this way.
@end defun

@defun copy-syntax-table &optional table
This function constructs a copy of @var{table} and returns it.  If
@var{table} is not supplied (or is @code{nil}), it returns a copy of the
current syntax table.  Otherwise, an error is signaled if @var{table} is
not a syntax table.
@end defun

@deffn Command modify-syntax-entry char syntax-descriptor  &optional table
This function sets the syntax entry for @var{char} according to
@var{syntax-descriptor}.  The syntax is changed only for @var{table},
which defaults to the current buffer's syntax table, and not in any
other syntax table.  The argument @var{syntax-descriptor} specifies the
desired syntax; this is a string beginning with a class designator
character, and optionally containing a matching character and flags as
well.  @xref{Syntax Descriptors}.

This function always returns @code{nil}.  The old syntax information in
the table for this character is discarded.

An error is signaled if the first character of the syntax descriptor is not
one of the twelve syntax class designator characters.  An error is also
signaled if @var{char} is not a character.

@example
@group
@exdent @r{Examples:}

;; @r{Put the space character in class whitespace.}
(modify-syntax-entry ?\  " ")
     @result{} nil
@end group

@group
;; @r{Make @samp{$} an open parenthesis character,}
;;   @r{with @samp{^} as its matching close.}
(modify-syntax-entry ?$ "(^")
     @result{} nil
@end group

@group
;; @r{Make @samp{^} a close parenthesis character,}
;;   @r{with @samp{$} as its matching open.}
(modify-syntax-entry ?^ ")$")
     @result{} nil
@end group

@group
;; @r{Make @samp{/} a punctuation character,}
;;   @r{the first character of a start-comment sequence,}
;;   @r{and the second character of an end-comment sequence.}
;;   @r{This is used in C mode.}
(modify-syntax-entry ?/ ". 14")
     @result{} nil
@end group
@end example
@end deffn

@defun char-syntax character
This function returns the syntax class of @var{character}, represented
by its mnemonic designator character.  This @emph{only} returns the
class, not any matching parenthesis or flags.

An error is signaled if @var{char} is not a character.

The following examples apply to C mode.  The first example shows that
the syntax class of space is whitespace (represented by a space).  The
second example shows that the syntax of @samp{/} is punctuation.  This
does not show the fact that it is also part of comment-start and -end
sequences.  The third example shows that open parenthesis is in the class
of open parentheses.  This does not show the fact that it has a matching
character, @samp{)}.

@example
@group
(char-to-string (char-syntax ?\ ))
     @result{} " "
@end group

@group
(char-to-string (char-syntax ?/))
     @result{} "."
@end group

@group
(char-to-string (char-syntax ?\())
     @result{} "("
@end group
@end example
@end defun

@defun set-syntax-table table &optional buffer
This function makes @var{table} the syntax table for @var{buffer}, which
defaults to the current buffer if omitted.  It returns @var{table}.
@end defun

@defun syntax-table &optional buffer
This function returns the syntax table for @var{buffer}, which defaults
to the current buffer if omitted.
@end defun

@node Motion and Syntax
@section Motion and Syntax

  This section describes functions for moving across characters in
certain syntax classes.  None of these functions exists in Emacs
version 18 or earlier.

@defun skip-syntax-forward syntaxes &optional limit buffer
This function moves point forward across characters having syntax classes
mentioned in @var{syntaxes}.  It stops when it encounters the end of
the buffer, or position @var{limit} (if specified), or a character it is
not supposed to skip.  Optional argument @var{buffer} defaults to the
current buffer if omitted.
@ignore @c may want to change this.
The return value is the distance traveled, which is a nonnegative
integer.
@end ignore
@end defun

@defun skip-syntax-backward syntaxes &optional limit buffer
This function moves point backward across characters whose syntax
classes are mentioned in @var{syntaxes}.  It stops when it encounters
the beginning of the buffer, or position @var{limit} (if specified), or a
character it is not supposed to skip.  Optional argument @var{buffer}
defaults to the current buffer if omitted.

@ignore @c may want to change this.
The return value indicates the distance traveled.  It is an integer that
is zero or less.
@end ignore
@end defun

@defun backward-prefix-chars &optional buffer
This function moves point backward over any number of characters with
expression prefix syntax.  This includes both characters in the
expression prefix syntax class, and characters with the @samp{p} flag.
Optional argument @var{buffer} defaults to the current buffer if
omitted.
@end defun

@node Parsing Expressions
@section Parsing Balanced Expressions

  Here are several functions for parsing and scanning balanced
expressions, also known as @dfn{sexps}, in which parentheses match in
pairs.  The syntax table controls the interpretation of characters, so
these functions can be used for Lisp expressions when in Lisp mode and
for C expressions when in C mode.  @xref{List Motion}, for convenient
higher-level functions for moving over balanced expressions.

@defun parse-partial-sexp start limit &optional target-depth stop-before state stop-comment buffer
This function parses a sexp in the current buffer starting at
@var{start}, not scanning past @var{limit}.  It stops at position
@var{limit} or when certain criteria described below are met, and sets
point to the location where parsing stops.  It returns a value
describing the status of the parse at the point where it stops.

If @var{state} is @code{nil}, @var{start} is assumed to be at the top
level of parenthesis structure, such as the beginning of a function
definition.  Alternatively, you might wish to resume parsing in the
middle of the structure.  To do this, you must provide a @var{state}
argument that describes the initial status of parsing.

@cindex parenthesis depth
If the third argument @var{target-depth} is non-@code{nil}, parsing
stops if the depth in parentheses becomes equal to @var{target-depth}.
The depth starts at 0, or at whatever is given in @var{state}.

If the fourth argument @var{stop-before} is non-@code{nil}, parsing
stops when it comes to any character that starts a sexp.  If
@var{stop-comment} is non-@code{nil}, parsing stops when it comes to the
start of a comment.

@cindex parse state
The fifth argument @var{state} is an eight-element list of the same
form as the value of this function, described below.  The return value
of one call may be used to initialize the state of the parse on another
call to @code{parse-partial-sexp}.

The result is a list of eight elements describing the final state of
the parse:

@enumerate 0
@item 
The depth in parentheses, counting from 0.

@item 
@cindex innermost containing parentheses
The character position of the start of the innermost parenthetical
grouping containing the stopping point; @code{nil} if none.

@item 
@cindex previous complete subexpression
The character position of the start of the last complete subexpression
terminated; @code{nil} if none.

@item 
@cindex inside string
Non-@code{nil} if inside a string.  More precisely, this is the
character that will terminate the string.

@item 
@cindex inside comment
@code{t} if inside a comment (of either style).

@item 
@cindex quote character
@code{t} if point is just after a quote character.

@item 
The minimum parenthesis depth encountered during this scan.

@item
@code{t} if inside a comment of style ``b''.
@end enumerate

Elements 0, 3, 4, 5 and 7 are significant in the argument @var{state}.

@cindex indenting with parentheses
This function is most often used to compute indentation for languages
that have nested parentheses.
@end defun

@defun scan-lists from count depth &optional buffer noerror
This function scans forward @var{count} balanced parenthetical groupings
from character number @var{from}.  It returns the character position
where the scan stops.

If @var{depth} is nonzero, parenthesis depth counting begins from that
value.  The only candidates for stopping are places where the depth in
parentheses becomes zero; @code{scan-lists} counts @var{count} such
places and then stops.  Thus, a positive value for @var{depth} means go
out @var{depth} levels of parenthesis.

Scanning ignores comments if @code{parse-sexp-ignore-comments} is
non-@code{nil}.

If the scan reaches the beginning or end of the buffer (or its
accessible portion), and the depth is not zero, an error is signaled.
If the depth is zero but the count is not used up, @code{nil} is
returned.

If optional arg @var{buffer} is non-@code{nil}, scanning occurs in that
buffer instead of in the current buffer.

If optional arg @var{noerror} is non-@code{nil}, @code{scan-lists}
will return @code{nil} instead of signalling an error.
@end defun

@defun scan-sexps from count &optional buffer noerror
This function scans forward @var{count} sexps from character position
@var{from}.  It returns the character position where the scan stops.

Scanning ignores comments if @code{parse-sexp-ignore-comments} is
non-@code{nil}.

If the scan reaches the beginning or end of (the accessible part of) the
buffer in the middle of a parenthetical grouping, an error is signaled.
If it reaches the beginning or end between groupings but before count is
used up, @code{nil} is returned.

If optional arg @var{buffer} is non-@code{nil}, scanning occurs in
that buffer instead of in the current buffer.

If optional arg @var{noerror} is non-@code{nil}, @code{scan-sexps}
will return nil instead of signalling an error.
@end defun

@defvar parse-sexp-ignore-comments
@cindex skipping comments
If the value is non-@code{nil}, then comments are treated as
whitespace by the functions in this section and by @code{forward-sexp}.

In older Emacs versions, this feature worked only when the comment
terminator is something like @samp{*/}, and appears only to end a
comment.  In languages where newlines terminate comments, it was
necessary make this variable @code{nil}, since not every newline is the
end of a comment.  This limitation no longer exists.
@end defvar

You can use @code{forward-comment} to move forward or backward over
one comment or several comments.

@defun forward-comment count &optional buffer
This function moves point forward across @var{count} comments (backward,
if @var{count} is negative).  If it finds anything other than a comment
or whitespace, it stops, leaving point at the place where it stopped.
It also stops after satisfying @var{count}.

  Optional argument @var{buffer} defaults to the current buffer.
@end defun

To move forward over all comments and whitespace following point, use
@code{(forward-comment (buffer-size))}.  @code{(buffer-size)} is a good
argument to use, because the number of comments in the buffer cannot
exceed that many.

@node Standard Syntax Tables
@section Some Standard Syntax Tables

  Most of the major modes in XEmacs have their own syntax tables.  Here
are several of them:

@defun standard-syntax-table
This function returns the standard syntax table, which is the syntax
table used in Fundamental mode.
@end defun

@defvar text-mode-syntax-table
The value of this variable is the syntax table used in Text mode.
@end defvar

@defvar c-mode-syntax-table
The value of this variable is the syntax table for C-mode buffers.
@end defvar

@defvar emacs-lisp-mode-syntax-table
The value of this variable is the syntax table used in Emacs Lisp mode
by editing commands.  (It has no effect on the Lisp @code{read}
function.)
@end defvar

@node Syntax Table Internals
@section Syntax Table Internals
@cindex syntax table internals

  Each element of a syntax table is an integer that encodes the syntax
of one character: the syntax class, possible matching character, and
flags.  Lisp programs don't usually work with the elements directly; the
Lisp-level syntax table functions usually work with syntax descriptors
(@pxref{Syntax Descriptors}).

  The low 8 bits of each element of a syntax table indicate the
syntax class.

@table @asis
@item @i{Integer}
@i{Class}
@item 0
whitespace
@item 1
punctuation
@item 2
word
@item 3
symbol
@item 4
open parenthesis
@item 5
close parenthesis
@item 6
expression prefix
@item 7
string quote
@item 8
paired delimiter
@item 9
escape
@item 10
character quote
@item 11
comment-start
@item 12
comment-end
@item 13
inherit
@end table

  The next 8 bits are the matching opposite parenthesis (if the
character has parenthesis syntax); otherwise, they are not meaningful.
The next 6 bits are the flags.
