source: trunk/binutils/gas/doc/as.info-2@ 3918

This is as.info, produced by makeinfo version 4.3 from as.texinfo.

START-INFO-DIR-ENTRY
* As: (as).                     The GNU assembler.
* Gas: (as).                    The GNU assembler.
END-INFO-DIR-ENTRY

   This file documents the GNU Assembler "as".

   Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002
Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License".


File: as.info,  Node: Comments,  Next: Symbol Intro,  Prev: Whitespace,  Up: Syntax

Comments
========

   There are two ways of rendering comments to `as'.  In both cases the
comment is equivalent to one space.

   Anything from `/*' through the next `*/' is a comment.  This means
you may not nest these comments.

     /*
       The only way to include a newline ('\n') in a comment
       is to use this sort of comment.
     */
     
     /* This sort of comment does not nest. */

   Anything from the "line comment" character to the next newline is
considered a comment and is ignored.  The line comment character is `;'
for the AMD 29K family; `;' on the ARC; `@' on the ARM; `;' for the
H8/300 family; `!' for the H8/500 family; `;' for the HPPA; `#' on the
i386 and x86-64; `#' on the i960; `;' for the PDP-11; `;' for picoJava;
`;' for Motorola PowerPC; `!' for the Renesas / SuperH SH; `!' on the
SPARC; `#' on the ip2k; `#' on the m32r; `|' on the 680x0; `#' on the
68HC11 and 68HC12; `;' on the M880x0; `#' on the Vax; `!' for the Z8000;
`#' on the V850; `#' for Xtensa systems; see *Note Machine
Dependencies::.

   On some machines there are two different line comment characters.
One character only begins a comment if it is the first non-whitespace
character on a line, while the other always begins a comment.

   The V850 assembler also supports a double dash as starting a comment
that extends to the end of the line.

   `--';

   To be compatible with past assemblers, lines that begin with `#'
have a special interpretation.  Following the `#' should be an absolute
expression (*note Expressions::): the logical line number of the _next_
line.  Then a string (*note Strings: Strings.) is allowed: if present
it is a new logical file name.  The rest of the line, if any, should be
whitespace.

   If the first non-whitespace characters on the line are not numeric,
the line is ignored.  (Just like a comment.)

                               # This is an ordinary comment.
     # 42-6 "new_file_name"    # New logical file name
                               # This is logical line # 36.
   This feature is deprecated, and may disappear from future versions
of `as'.


File: as.info,  Node: Symbol Intro,  Next: Statements,  Prev: Comments,  Up: Syntax

Symbols
=======

   A "symbol" is one or more characters chosen from the set of all
letters (both upper and lower case), digits and the three characters
`_.$'.  On most machines, you can also use `$' in symbol names;
exceptions are noted in *Note Machine Dependencies::.  No symbol may
begin with a digit.  Case is significant.  There is no length limit:
all characters are significant.  Symbols are delimited by characters
not in that set, or by the beginning of a file (since the source
program must end with a newline, the end of a file is not a possible
symbol delimiter).  *Note Symbols::.


File: as.info,  Node: Statements,  Next: Constants,  Prev: Symbol Intro,  Up: Syntax

Statements
==========

   A "statement" ends at a newline character (`\n') or line separator
character.  (The line separator is usually `;', unless this conflicts
with the comment character; *note Machine Dependencies::.)  The newline
or separator character is considered part of the preceding statement.
Newlines and separators within character constants are an exception:
they do not end statements.

   It is an error to end any statement with end-of-file:  the last
character of any input file should be a newline.

   An empty statement is allowed, and may include whitespace.  It is
ignored.

   A statement begins with zero or more labels, optionally followed by a
key symbol which determines what kind of statement it is.  The key
symbol determines the syntax of the rest of the statement.  If the
symbol begins with a dot `.' then the statement is an assembler
directive: typically valid for any computer.  If the symbol begins with
a letter the statement is an assembly language "instruction": it
assembles into a machine language instruction.  Different versions of
`as' for different computers recognize different instructions.  In
fact, the same symbol may represent a different instruction in a
different computer's assembly language.

   A label is a symbol immediately followed by a colon (`:').
Whitespace before a label or after a colon is permitted, but you may not
have whitespace between a label's symbol and its colon. *Note Labels::.

   For HPPA targets, labels need not be immediately followed by a
colon, but the definition of a label must begin in column zero.  This
also implies that only one label may be defined on each line.

     label:     .directive    followed by something
     another_label:           # This is an empty statement.
                instruction   operand_1, operand_2, ...


File: as.info,  Node: Constants,  Prev: Statements,  Up: Syntax

Constants
=========

   A constant is a number, written so that its value is known by
inspection, without knowing any context.  Like this:
     .byte  74, 0112, 092, 0x4A, 0X4a, 'J, '\J # All the same value.
     .ascii "Ring the bell\7"                  # A string constant.
     .octa  0x123456789abcdef0123456789ABCDEF0 # A bignum.
     .float 0f-314159265358979323846264338327\
     95028841971.693993751E-40                 # - pi, a flonum.

* Menu:

* Characters::                  Character Constants
* Numbers::                     Number Constants


File: as.info,  Node: Characters,  Next: Numbers,  Up: Constants

Character Constants
-------------------

   There are two kinds of character constants.  A "character" stands
for one character in one byte and its value may be used in numeric
expressions.  String constants (properly called string _literals_) are
potentially many bytes and their values may not be used in arithmetic
expressions.

* Menu:

* Strings::                     Strings
* Chars::                       Characters


File: as.info,  Node: Strings,  Next: Chars,  Up: Characters

Strings
.......

   A "string" is written between double-quotes.  It may contain
double-quotes or null characters.  The way to get special characters
into a string is to "escape" these characters: precede them with a
backslash `\' character.  For example `\\' represents one backslash:
the first `\' is an escape which tells `as' to interpret the second
character literally as a backslash (which prevents `as' from
recognizing the second `\' as an escape character).  The complete list
of escapes follows.

`\b'
     Mnemonic for backspace; for ASCII this is octal code 010.

`\f'
     Mnemonic for FormFeed; for ASCII this is octal code 014.

`\n'
     Mnemonic for newline; for ASCII this is octal code 012.

`\r'
     Mnemonic for carriage-Return; for ASCII this is octal code 015.

`\t'
     Mnemonic for horizontal Tab; for ASCII this is octal code 011.

`\ DIGIT DIGIT DIGIT'
     An octal character code.  The numeric code is 3 octal digits.  For
     compatibility with other Unix systems, 8 and 9 are accepted as
     digits: for example, `\008' has the value 010, and `\009' the
     value 011.

`\`x' HEX-DIGITS...'
     A hex character code.  All trailing hex digits are combined.
     Either upper or lower case `x' works.

`\\'
     Represents one `\' character.

`\"'
     Represents one `"' character.  Needed in strings to represent this
     character, because an unescaped `"' would end the string.

`\ ANYTHING-ELSE'
     Any other character when escaped by `\' gives a warning, but
     assembles as if the `\' was not present.  The idea is that if you
     used an escape sequence you clearly didn't want the literal
     interpretation of the following character.  However `as' has no
     other interpretation, so `as' knows it is giving you the wrong
     code and warns you of the fact.

   Which characters are escapable, and what those escapes represent,
varies widely among assemblers.  The current set is what we think the
BSD 4.2 assembler recognizes, and is a subset of what most C compilers
recognize.  If you are in doubt, do not use an escape sequence.


File: as.info,  Node: Chars,  Prev: Strings,  Up: Characters

Characters
..........

   A single character may be written as a single quote immediately
followed by that character.  The same escapes apply to characters as to
strings.  So if you want to write the character backslash, you must
write `'\\' where the first `\' escapes the second `\'.  As you can
see, the quote is an acute accent, not a grave accent.  A newline
immediately following an acute accent is taken as a literal character
and does not count as the end of a statement.  The value of a character
constant in a numeric expression is the machine's byte-wide code for
that character.  `as' assumes your character code is ASCII: `'A' means
65, `'B' means 66, and so on.


File: as.info,  Node: Numbers,  Prev: Characters,  Up: Constants

Number Constants
----------------

   `as' distinguishes three kinds of numbers according to how they are
stored in the target machine.  _Integers_ are numbers that would fit
into an `int' in the C language.  _Bignums_ are integers, but they are
stored in more than 32 bits.  _Flonums_ are floating point numbers,
described below.

* Menu:

* Integers::                    Integers
* Bignums::                     Bignums
* Flonums::                     Flonums


File: as.info,  Node: Integers,  Next: Bignums,  Up: Numbers

Integers
........

   A binary integer is `0b' or `0B' followed by zero or more of the
binary digits `01'.

   An octal integer is `0' followed by zero or more of the octal digits
(`01234567').

   A decimal integer starts with a non-zero digit followed by zero or
more digits (`0123456789').

   A hexadecimal integer is `0x' or `0X' followed by one or more
hexadecimal digits chosen from `0123456789abcdefABCDEF'.

   Integers have the usual values.  To denote a negative integer, use
the prefix operator `-' discussed under expressions (*note Prefix
Operators: Prefix Ops.).


File: as.info,  Node: Bignums,  Next: Flonums,  Prev: Integers,  Up: Numbers

Bignums
.......

   A "bignum" has the same syntax and semantics as an integer except
that the number (or its negative) takes more than 32 bits to represent
in binary.  The distinction is made because in some places integers are
permitted while bignums are not.


File: as.info,  Node: Flonums,  Prev: Bignums,  Up: Numbers

Flonums
.......

   A "flonum" represents a floating point number.  The translation is
indirect: a decimal floating point number from the text is converted by
`as' to a generic binary floating point number of more than sufficient
precision.  This generic floating point number is converted to a
particular computer's floating point format (or formats) by a portion
of `as' specialized to that computer.

   A flonum is written by writing (in order)
   * The digit `0'.  (`0' is optional on the HPPA.)

   * A letter, to tell `as' the rest of the number is a flonum.  `e' is
     recommended.  Case is not important.

     On the H8/300, H8/500, Renesas / SuperH SH, and AMD 29K
     architectures, the letter must be one of the letters `DFPRSX' (in
     upper or lower case).

     On the ARC, the letter must be one of the letters `DFRS' (in upper
     or lower case).

     On the Intel 960 architecture, the letter must be one of the
     letters `DFT' (in upper or lower case).

     On the HPPA architecture, the letter must be `E' (upper case only).

   * An optional sign: either `+' or `-'.

   * An optional "integer part": zero or more decimal digits.

   * An optional "fractional part": `.' followed by zero or more
     decimal digits.

   * An optional exponent, consisting of:

        * An `E' or `e'.

        * Optional sign: either `+' or `-'.

        * One or more decimal digits.


   At least one of the integer part or the fractional part must be
present.  The floating point number has the usual base-10 value.

   `as' does all processing using integers.  Flonums are computed
independently of any floating point hardware in the computer running
`as'.


File: as.info,  Node: Sections,  Next: Symbols,  Prev: Syntax,  Up: Top

Sections and Relocation
***********************

* Menu:

* Secs Background::             Background
* Ld Sections::                 Linker Sections
* As Sections::                 Assembler Internal Sections
* Sub-Sections::                Sub-Sections
* bss::                         bss Section


File: as.info,  Node: Secs Background,  Next: Ld Sections,  Up: Sections

Background
==========

   Roughly, a section is a range of addresses, with no gaps; all data
"in" those addresses is treated the same for some particular purpose.
For example there may be a "read only" section.

   The linker `ld' reads many object files (partial programs) and
combines their contents to form a runnable program.  When `as' emits an
object file, the partial program is assumed to start at address 0.
`ld' assigns the final addresses for the partial program, so that
different partial programs do not overlap.  This is actually an
oversimplification, but it suffices to explain how `as' uses sections.

   `ld' moves blocks of bytes of your program to their run-time
addresses.  These blocks slide to their run-time addresses as rigid
units; their length does not change and neither does the order of bytes
within them.  Such a rigid unit is called a _section_.  Assigning
run-time addresses to sections is called "relocation".  It includes the
task of adjusting mentions of object-file addresses so they refer to
the proper run-time addresses.  For the H8/300 and H8/500, and for the
Renesas / SuperH SH, `as' pads sections if needed to ensure they end on
a word (sixteen bit) boundary.

   An object file written by `as' has at least three sections, any of
which may be empty.  These are named "text", "data" and "bss" sections.

   When it generates COFF or ELF output, `as' can also generate
whatever other named sections you specify using the `.section'
directive (*note `.section': Section.).  If you do not use any
directives that place output in the `.text' or `.data' sections, these
sections still exist, but are empty.

   When `as' generates SOM or ELF output for the HPPA, `as' can also
generate whatever other named sections you specify using the `.space'
and `.subspace' directives.  See `HP9000 Series 800 Assembly Language
Reference Manual' (HP 92432-90001) for details on the `.space' and
`.subspace' assembler directives.

   Additionally, `as' uses different names for the standard text, data,
and bss sections when generating SOM output.  Program text is placed
into the `$CODE$' section, data into `$DATA$', and BSS into `$BSS$'.

   Within the object file, the text section starts at address `0', the
data section follows, and the bss section follows the data section.

   When generating either SOM or ELF output files on the HPPA, the text
section starts at address `0', the data section at address `0x4000000',
and the bss section follows the data section.

   To let `ld' know which data changes when the sections are relocated,
and how to change that data, `as' also writes to the object file
details of the relocation needed.  To perform relocation `ld' must
know, each time an address in the object file is mentioned:
   * Where in the object file is the beginning of this reference to an
     address?

   * How long (in bytes) is this reference?

   * Which section does the address refer to?  What is the numeric
     value of
          (ADDRESS) - (START-ADDRESS OF SECTION)?

   * Is the reference to an address "Program-Counter relative"?

   In fact, every address `as' ever uses is expressed as
     (SECTION) + (OFFSET INTO SECTION)

Further, most expressions `as' computes have this section-relative
nature.  (For some object formats, such as SOM for the HPPA, some
expressions are symbol-relative instead.)

   In this manual we use the notation {SECNAME N} to mean "offset N
into section SECNAME."

   Apart from text, data and bss sections you need to know about the
"absolute" section.  When `ld' mixes partial programs, addresses in the
absolute section remain unchanged.  For example, address `{absolute 0}'
is "relocated" to run-time address 0 by `ld'.  Although the linker
never arranges two partial programs' data sections with overlapping
addresses after linking, _by definition_ their absolute sections must
overlap.  Address `{absolute 239}' in one part of a program is always
the same address when the program is running as address `{absolute
239}' in any other part of the program.

   The idea of sections is extended to the "undefined" section.  Any
address whose section is unknown at assembly time is by definition
rendered {undefined U}--where U is filled in later.  Since numbers are
always defined, the only way to generate an undefined address is to
mention an undefined symbol.  A reference to a named common block would
be such a symbol: its value is unknown at assembly time so it has
section _undefined_.

   By analogy the word _section_ is used to describe groups of sections
in the linked program.  `ld' puts all partial programs' text sections
in contiguous addresses in the linked program.  It is customary to
refer to the _text section_ of a program, meaning all the addresses of
all partial programs' text sections.  Likewise for data and bss
sections.

   Some sections are manipulated by `ld'; others are invented for use
of `as' and have no meaning except during assembly.


File: as.info,  Node: Ld Sections,  Next: As Sections,  Prev: Secs Background,  Up: Sections

Linker Sections
===============

   `ld' deals with just four kinds of sections, summarized below.

*named sections*
*text section*
*data section*
     These sections hold your program.  `as' and `ld' treat them as
     separate but equal sections.  Anything you can say of one section
     is true of another.  When the program is running, however, it is
     customary for the text section to be unalterable.  The text
     section is often shared among processes: it contains instructions,
     constants and the like.  The data section of a running program is
     usually alterable: for example, C variables would be stored in the
     data section.

*bss section*
     This section contains zeroed bytes when your program begins
     running.  It is used to hold uninitialized variables or common
     storage.  The length of each partial program's bss section is
     important, but because it starts out containing zeroed bytes there
     is no need to store explicit zero bytes in the object file.  The
     bss section was invented to eliminate those explicit zeros from
     object files.

*absolute section*
     Address 0 of this section is always "relocated" to runtime address
     0.  This is useful if you want to refer to an address that `ld'
     must not change when relocating.  In this sense we speak of
     absolute addresses being "unrelocatable": they do not change
     during relocation.

*undefined section*
     This "section" is a catch-all for address references to objects
     not in the preceding sections.

   An idealized example of three relocatable sections follows.  The
example uses the traditional section names `.text' and `.data'.  Memory
addresses are on the horizontal axis.

                           +-----+----+--+
     partial program # 1:  |ttttt|dddd|00|
                           +-----+----+--+
     
                           text   data bss
                           seg.   seg. seg.
     
                           +---+---+---+
     partial program # 2:  |TTT|DDD|000|
                           +---+---+---+
     
                           +--+---+-----+--+----+---+-----+~~
     linked program:       |  |TTT|ttttt|  |dddd|DDD|00000|
                           +--+---+-----+--+----+---+-----+~~
     
         addresses:        0 ...


File: as.info,  Node: As Sections,  Next: Sub-Sections,  Prev: Ld Sections,  Up: Sections

Assembler Internal Sections
===========================

   These sections are meant only for the internal use of `as'.  They
have no meaning at run-time.  You do not really need to know about these
sections for most purposes; but they can be mentioned in `as' warning
messages, so it might be helpful to have an idea of their meanings to
`as'.  These sections are used to permit the value of every expression
in your assembly language program to be a section-relative address.

ASSEMBLER-INTERNAL-LOGIC-ERROR!
     An internal assembler logic error has been found.  This means
     there is a bug in the assembler.

expr section
     The assembler stores complex expression internally as combinations
     of symbols.  When it needs to represent an expression as a symbol,
     it puts it in the expr section.


File: as.info,  Node: Sub-Sections,  Next: bss,  Prev: As Sections,  Up: Sections

Sub-Sections
============

   Assembled bytes conventionally fall into two sections: text and data.
You may have separate groups of data in named sections that you want to
end up near to each other in the object file, even though they are not
contiguous in the assembler source.  `as' allows you to use
"subsections" for this purpose.  Within each section, there can be
numbered subsections with values from 0 to 8192.  Objects assembled
into the same subsection go into the object file together with other
objects in the same subsection.  For example, a compiler might want to
store constants in the text section, but might not want to have them
interspersed with the program being assembled.  In this case, the
compiler could issue a `.text 0' before each section of code being
output, and a `.text 1' before each group of constants being output.

   Subsections are optional.  If you do not use subsections, everything
goes in subsection number zero.

   Each subsection is zero-padded up to a multiple of four bytes.
(Subsections may be padded a different amount on different flavors of
`as'.)

   Subsections appear in your object file in numeric order, lowest
numbered to highest.  (All this to be compatible with other people's
assemblers.)  The object file contains no representation of
subsections; `ld' and other programs that manipulate object files see
no trace of them.  They just see all your text subsections as a text
section, and all your data subsections as a data section.

   To specify which subsection you want subsequent statements assembled
into, use a numeric argument to specify it, in a `.text EXPRESSION' or
a `.data EXPRESSION' statement.  When generating COFF or ELF output, you
can also use an extra subsection argument with arbitrary named
sections: `.section NAME, EXPRESSION'.  EXPRESSION should be an
absolute expression.  (*Note Expressions::.)  If you just say `.text'
then `.text 0' is assumed.  Likewise `.data' means `.data 0'.  Assembly
begins in `text 0'.  For instance:
     .text 0     # The default subsection is text 0 anyway.
     .ascii "This lives in the first text subsection. *"
     .text 1
     .ascii "But this lives in the second text subsection."
     .data 0
     .ascii "This lives in the data section,"
     .ascii "in the first data subsection."
     .text 0
     .ascii "This lives in the first text section,"
     .ascii "immediately following the asterisk (*)."

   Each section has a "location counter" incremented by one for every
byte assembled into that section.  Because subsections are merely a
convenience restricted to `as' there is no concept of a subsection
location counter.  There is no way to directly manipulate a location
counter--but the `.align' directive changes it, and any label
definition captures its current value.  The location counter of the
section where statements are being assembled is said to be the "active"
location counter.


File: as.info,  Node: bss,  Prev: Sub-Sections,  Up: Sections

bss Section
===========

   The bss section is used for local common variable storage.  You may
allocate address space in the bss section, but you may not dictate data
to load into it before your program executes.  When your program starts
running, all the contents of the bss section are zeroed bytes.

   The `.lcomm' pseudo-op defines a symbol in the bss section; see
*Note `.lcomm': Lcomm.

   The `.comm' pseudo-op may be used to declare a common symbol, which
is another form of uninitialized symbol; see *Note `.comm': Comm.

   When assembling for a target which supports multiple sections, such
as ELF or COFF, you may switch into the `.bss' section and define
symbols as usual; see *Note `.section': Section.  You may only assemble
zero values into the section.  Typically the section will only contain
symbol definitions and `.skip' directives (*note `.skip': Skip.).


File: as.info,  Node: Symbols,  Next: Expressions,  Prev: Sections,  Up: Top

Symbols
*******

   Symbols are a central concept: the programmer uses symbols to name
things, the linker uses symbols to link, and the debugger uses symbols
to debug.

     _Warning:_ `as' does not place symbols in the object file in the
     same order they were declared.  This may break some debuggers.

* Menu:

* Labels::                      Labels
* Setting Symbols::             Giving Symbols Other Values
* Symbol Names::                Symbol Names
* Dot::                         The Special Dot Symbol
* Symbol Attributes::           Symbol Attributes


File: as.info,  Node: Labels,  Next: Setting Symbols,  Up: Symbols

Labels
======

   A "label" is written as a symbol immediately followed by a colon
`:'.  The symbol then represents the current value of the active
location counter, and is, for example, a suitable instruction operand.
You are warned if you use the same symbol to represent two different
locations: the first definition overrides any other definitions.

   On the HPPA, the usual form for a label need not be immediately
followed by a colon, but instead must start in column zero.  Only one
label may be defined on a single line.  To work around this, the HPPA
version of `as' also provides a special directive `.label' for defining
labels more flexibly.


File: as.info,  Node: Setting Symbols,  Next: Symbol Names,  Prev: Labels,  Up: Symbols

Giving Symbols Other Values
===========================

   A symbol can be given an arbitrary value by writing a symbol,
followed by an equals sign `=', followed by an expression (*note
Expressions::).  This is equivalent to using the `.set' directive.
*Note `.set': Set.


File: as.info,  Node: Symbol Names,  Next: Dot,  Prev: Setting Symbols,  Up: Symbols

Symbol Names
============

   Symbol names begin with a letter or with one of `._'.  On most
machines, you can also use `$' in symbol names; exceptions are noted in
*Note Machine Dependencies::.  That character may be followed by any
string of digits, letters, dollar signs (unless otherwise noted in
*Note Machine Dependencies::), and underscores.  For the AMD 29K
family, `?' is also allowed in the body of a symbol name, though not at
its beginning.

   Case of letters is significant: `foo' is a different symbol name
than `Foo'.

   Each symbol has exactly one name.  Each name in an assembly language
program refers to exactly one symbol.  You may use that symbol name any
number of times in a program.

Local Symbol Names
------------------

   Local symbols help compilers and programmers use names temporarily.
They create symbols which are guaranteed to be unique over the entire
scope of the input source code and which can be referred to by a simple
notation.  To define a local symbol, write a label of the form `N:'
(where N represents any positive integer).  To refer to the most recent
previous definition of that symbol write `Nb', using the same number as
when you defined the label.  To refer to the next definition of a local
label, write `Nf'-- The `b' stands for"backwards" and the `f' stands
for "forwards".

   There is no restriction on how you can use these labels, and you can
reuse them too.  So that it is possible to repeatedly define the same
local label (using the same number `N'), although you can only refer to
the most recently defined local label of that number (for a backwards
reference) or the next definition of a specific local label for a
forward reference.  It is also worth noting that the first 10 local
labels (`0:'...`9:') are implemented in a slightly more efficient
manner than the others.

   Here is an example:

     1:        branch 1f
     2:        branch 1b
     1:        branch 2f
     2:        branch 1b

   Which is the equivalent of:

     label_1:  branch label_3
     label_2:  branch label_1
     label_3:  branch label_4
     label_4:  branch label_3

   Local symbol names are only a notational device.  They are
immediately transformed into more conventional symbol names before the
assembler uses them.  The symbol names stored in the symbol table,
appearing in error messages and optionally emitted to the object file.
The names are constructed using these parts:

`L'
     All local labels begin with `L'. Normally both `as' and `ld'
     forget symbols that start with `L'. These labels are used for
     symbols you are never intended to see.  If you use the `-L' option
     then `as' retains these symbols in the object file. If you also
     instruct `ld' to retain these symbols, you may use them in
     debugging.

`NUMBER'
     This is the number that was used in the local label definition.
     So if the label is written `55:' then the number is `55'.

`C-B'
     This unusual character is included so you do not accidentally
     invent a symbol of the same name.  The character has ASCII value
     of `\002' (control-B).

`_ordinal number_'
     This is a serial number to keep the labels distinct.  The first
     definition of `0:' gets the number `1'.  The 15th definition of
     `0:' gets the number `15', and so on.  Likewise the first
     definition of `1:' gets the number `1' and its 15th defintion gets
     `15' as well.

   So for example, the first `1:' is named `L1C-B1', the 44th `3:' is
named `L3C-B44'.

Dollar Local Labels
-------------------

   `as' also supports an even more local form of local labels called
dollar labels.  These labels go out of scope (ie they become undefined)
as soon as a non-local label is defined.  Thus they remain valid for
only a small region of the input source code.  Normal local labels, by
contrast, remain in scope for the entire file, or until they are
redefined by another occurrence of the same local label.

   Dollar labels are defined in exactly the same way as ordinary local
labels, except that instead of being terminated by a colon, they are
terminated by a dollar sign.  eg `55$'.

   They can also be distinguished from ordinary local labels by their
transformed name which uses ASCII character `\001' (control-A) as the
magic character to distinguish them from ordinary labels.  Thus the 5th
defintion of `6$' is named `L6C-A5'.


File: as.info,  Node: Dot,  Next: Symbol Attributes,  Prev: Symbol Names,  Up: Symbols

The Special Dot Symbol
======================

   The special symbol `.' refers to the current address that `as' is
assembling into.  Thus, the expression `melvin: .long .' defines
`melvin' to contain its own address.  Assigning a value to `.' is
treated the same as a `.org' directive.  Thus, the expression `.=.+4'
is the same as saying `.space 4'.


File: as.info,  Node: Symbol Attributes,  Prev: Dot,  Up: Symbols

Symbol Attributes
=================

   Every symbol has, as well as its name, the attributes "Value" and
"Type".  Depending on output format, symbols can also have auxiliary
attributes.

   If you use a symbol without defining it, `as' assumes zero for all
these attributes, and probably won't warn you.  This makes the symbol
an externally defined symbol, which is generally what you would want.

* Menu:

* Symbol Value::                Value
* Symbol Type::                 Type


* a.out Symbols::               Symbol Attributes: `a.out'

* COFF Symbols::                Symbol Attributes for COFF

* SOM Symbols::                Symbol Attributes for SOM


File: as.info,  Node: Symbol Value,  Next: Symbol Type,  Up: Symbol Attributes

Value
-----

   The value of a symbol is (usually) 32 bits.  For a symbol which
labels a location in the text, data, bss or absolute sections the value
is the number of addresses from the start of that section to the label.
Naturally for text, data and bss sections the value of a symbol changes
as `ld' changes section base addresses during linking.  Absolute
symbols' values do not change during linking: that is why they are
called absolute.

   The value of an undefined symbol is treated in a special way.  If it
is 0 then the symbol is not defined in this assembler source file, and
`ld' tries to determine its value from other files linked into the same
program.  You make this kind of symbol simply by mentioning a symbol
name without defining it.  A non-zero value represents a `.comm' common
declaration.  The value is how much common storage to reserve, in bytes
(addresses).  The symbol refers to the first address of the allocated
storage.


File: as.info,  Node: Symbol Type,  Next: a.out Symbols,  Prev: Symbol Value,  Up: Symbol Attributes

Type
----

   The type attribute of a symbol contains relocation (section)
information, any flag settings indicating that a symbol is external, and
(optionally), other information for linkers and debuggers.  The exact
format depends on the object-code output format in use.


File: as.info,  Node: a.out Symbols,  Next: COFF Symbols,  Prev: Symbol Type,  Up: Symbol Attributes

Symbol Attributes: `a.out'
--------------------------

* Menu:

* Symbol Desc::                 Descriptor
* Symbol Other::                Other


File: as.info,  Node: Symbol Desc,  Next: Symbol Other,  Up: a.out Symbols

Descriptor
..........

   This is an arbitrary 16-bit value.  You may establish a symbol's
descriptor value by using a `.desc' statement (*note `.desc': Desc.).
A descriptor value means nothing to `as'.


File: as.info,  Node: Symbol Other,  Prev: Symbol Desc,  Up: a.out Symbols

Other
.....

   This is an arbitrary 8-bit value.  It means nothing to `as'.


File: as.info,  Node: COFF Symbols,  Next: SOM Symbols,  Prev: a.out Symbols,  Up: Symbol Attributes

Symbol Attributes for COFF
--------------------------

   The COFF format supports a multitude of auxiliary symbol attributes;
like the primary symbol attributes, they are set between `.def' and
`.endef' directives.

Primary Attributes
..................

   The symbol name is set with `.def'; the value and type,
respectively, with `.val' and `.type'.

Auxiliary Attributes
....................

   The `as' directives `.dim', `.line', `.scl', `.size', and `.tag' can
generate auxiliary symbol table information for COFF.


File: as.info,  Node: SOM Symbols,  Prev: COFF Symbols,  Up: Symbol Attributes

Symbol Attributes for SOM
-------------------------

   The SOM format for the HPPA supports a multitude of symbol
attributes set with the `.EXPORT' and `.IMPORT' directives.

   The attributes are described in `HP9000 Series 800 Assembly Language
Reference Manual' (HP 92432-90001) under the `IMPORT' and `EXPORT'
assembler directive documentation.


File: as.info,  Node: Expressions,  Next: Pseudo Ops,  Prev: Symbols,  Up: Top

Expressions
***********

   An "expression" specifies an address or numeric value.  Whitespace
may precede and/or follow an expression.

   The result of an expression must be an absolute number, or else an
offset into a particular section.  If an expression is not absolute,
and there is not enough information when `as' sees the expression to
know its section, a second pass over the source program might be
necessary to interpret the expression--but the second pass is currently
not implemented.  `as' aborts with an error message in this situation.

* Menu:

* Empty Exprs::                 Empty Expressions
* Integer Exprs::               Integer Expressions


File: as.info,  Node: Empty Exprs,  Next: Integer Exprs,  Up: Expressions

Empty Expressions
=================

   An empty expression has no value: it is just whitespace or null.
Wherever an absolute expression is required, you may omit the
expression, and `as' assumes a value of (absolute) 0.  This is
compatible with other assemblers.


File: as.info,  Node: Integer Exprs,  Prev: Empty Exprs,  Up: Expressions

Integer Expressions
===================

   An "integer expression" is one or more _arguments_ delimited by
_operators_.

* Menu:

* Arguments::                   Arguments
* Operators::                   Operators
* Prefix Ops::                  Prefix Operators
* Infix Ops::                   Infix Operators


File: as.info,  Node: Arguments,  Next: Operators,  Up: Integer Exprs

Arguments
---------

   "Arguments" are symbols, numbers or subexpressions.  In other
contexts arguments are sometimes called "arithmetic operands".  In this
manual, to avoid confusing them with the "instruction operands" of the
machine language, we use the term "argument" to refer to parts of
expressions only, reserving the word "operand" to refer only to machine
instruction operands.

   Symbols are evaluated to yield {SECTION NNN} where SECTION is one of
text, data, bss, absolute, or undefined.  NNN is a signed, 2's
complement 32 bit integer.

   Numbers are usually integers.

   A number can be a flonum or bignum.  In this case, you are warned
that only the low order 32 bits are used, and `as' pretends these 32
bits are an integer.  You may write integer-manipulating instructions
that act on exotic constants, compatible with other assemblers.

   Subexpressions are a left parenthesis `(' followed by an integer
expression, followed by a right parenthesis `)'; or a prefix operator
followed by an argument.


File: as.info,  Node: Operators,  Next: Prefix Ops,  Prev: Arguments,  Up: Integer Exprs

Operators
---------

   "Operators" are arithmetic functions, like `+' or `%'.  Prefix
operators are followed by an argument.  Infix operators appear between
their arguments.  Operators may be preceded and/or followed by
whitespace.


File: as.info,  Node: Prefix Ops,  Next: Infix Ops,  Prev: Operators,  Up: Integer Exprs

Prefix Operator
---------------

   `as' has the following "prefix operators".  They each take one
argument, which must be absolute.

`-'
     "Negation".  Two's complement negation.

`~'
     "Complementation".  Bitwise not.


File: as.info,  Node: Infix Ops,  Prev: Prefix Ops,  Up: Integer Exprs

Infix Operators
---------------

   "Infix operators" take two arguments, one on either side.  Operators
have precedence, but operations with equal precedence are performed left
to right.  Apart from `+' or `-', both arguments must be absolute, and
the result is absolute.

  1. Highest Precedence

    `*'
          "Multiplication".

    `/'
          "Division".  Truncation is the same as the C operator `/'

    `%'
          "Remainder".

    `<'
    `<<'
          "Shift Left".  Same as the C operator `<<'.

    `>'
    `>>'
          "Shift Right".  Same as the C operator `>>'.

  2. Intermediate precedence

    `|'
          "Bitwise Inclusive Or".

    `&'
          "Bitwise And".

    `^'
          "Bitwise Exclusive Or".

    `!'
          "Bitwise Or Not".

  3. Low Precedence

    `+'
          "Addition".  If either argument is absolute, the result has
          the section of the other argument.  You may not add together
          arguments from different sections.

    `-'
          "Subtraction".  If the right argument is absolute, the result
          has the section of the left argument.  If both arguments are
          in the same section, the result is absolute.  You may not
          subtract arguments from different sections.

    `=='
          "Is Equal To"

    `<>'
          "Is Not Equal To"

    `<'
          "Is Less Than"

    `>'
          "Is Greater Than"

    `>='
          "Is Greater Than Or Equal To"

    `<='
          "Is Less Than Or Equal To"

          The comparison operators can be used as infix operators.  A
          true results has a value of -1 whereas a false result has a
          value of 0.   Note, these operators perform signed
          comparisons.

  4. Lowest Precedence

    `&&'
          "Logical And".

    `||'
          "Logical Or".

          These two logical operations can be used to combine the
          results of sub expressions.  Note, unlike the comparison
          operators a true result returns a value of 1 but a false
          results does still return 0.  Also note that the logical or
          operator has a slightly lower precedence than logical and.


   In short, it's only meaningful to add or subtract the _offsets_ in an
address; you can only have a defined section in one of the two
arguments.


File: as.info,  Node: Pseudo Ops,  Next: Machine Dependencies,  Prev: Expressions,  Up: Top

Assembler Directives
********************

   All assembler directives have names that begin with a period (`.').
The rest of the name is letters, usually in lower case.

   This chapter discusses directives that are available regardless of
the target machine configuration for the GNU assembler.  Some machine
configurations provide additional directives.  *Note Machine
Dependencies::.

* Menu:

* Abort::                       `.abort'

* ABORT::                       `.ABORT'

* Align::                       `.align ABS-EXPR , ABS-EXPR'
* Ascii::                       `.ascii "STRING"'...
* Asciz::                       `.asciz "STRING"'...
* Balign::                      `.balign ABS-EXPR , ABS-EXPR'
* Byte::                        `.byte EXPRESSIONS'
* Comm::                        `.comm SYMBOL , LENGTH '
* Data::                        `.data SUBSECTION'

* Def::                         `.def NAME'

* Desc::                        `.desc SYMBOL, ABS-EXPRESSION'

* Dim::                         `.dim'

* Double::                      `.double FLONUMS'
* Eject::                       `.eject'
* Else::                        `.else'
* Elseif::                      `.elseif'
* End::                         `.end'

* Endef::                       `.endef'

* Endfunc::                     `.endfunc'
* Endif::                       `.endif'
* Equ::                         `.equ SYMBOL, EXPRESSION'
* Equiv::                       `.equiv SYMBOL, EXPRESSION'
* Err::                         `.err'
* Exitm::                       `.exitm'
* Extern::                      `.extern'
* Fail::                        `.fail'

* File::                        `.file STRING'

* Fill::                        `.fill REPEAT , SIZE , VALUE'
* Float::                       `.float FLONUMS'
* Func::                        `.func'
* Global::                      `.global SYMBOL', `.globl SYMBOL'

* Hidden::                      `.hidden NAMES'

* hword::                       `.hword EXPRESSIONS'
* Ident::                       `.ident'
* If::                          `.if ABSOLUTE EXPRESSION'
* Incbin::                      `.incbin "FILE"[,SKIP[,COUNT]]'
* Include::                     `.include "FILE"'
* Int::                         `.int EXPRESSIONS'

* Internal::                    `.internal NAMES'

* Irp::                         `.irp SYMBOL,VALUES'...
* Irpc::                        `.irpc SYMBOL,VALUES'...
* Lcomm::                       `.lcomm SYMBOL , LENGTH'
* Lflags::                      `.lflags'

* Line::                        `.line LINE-NUMBER'

* Ln::                          `.ln LINE-NUMBER'
* Linkonce::                    `.linkonce [TYPE]'
* List::                        `.list'
* Long::                        `.long EXPRESSIONS'

* Macro::                       `.macro NAME ARGS'...
* MRI::                         `.mri VAL'
* Nolist::                      `.nolist'
* Octa::                        `.octa BIGNUMS'
* Org::                         `.org NEW-LC , FILL'
* P2align::                     `.p2align ABS-EXPR , ABS-EXPR'

* PopSection::                  `.popsection'
* Previous::                    `.previous'

* Print::                       `.print STRING'

* Protected::                   `.protected NAMES'

* Psize::                       `.psize LINES, COLUMNS'
* Purgem::                      `.purgem NAME'

* PushSection::                 `.pushsection NAME'

* Quad::                        `.quad BIGNUMS'
* Rept::                        `.rept COUNT'
* Sbttl::                       `.sbttl "SUBHEADING"'

* Scl::                         `.scl CLASS'

* Section::                     `.section NAME'

* Set::                         `.set SYMBOL, EXPRESSION'
* Short::                       `.short EXPRESSIONS'
* Single::                      `.single FLONUMS'

* Size::                        `.size [NAME , EXPRESSION]'

* Skip::                        `.skip SIZE , FILL'
* Sleb128::                     `.sleb128 EXPRESSIONS'
* Space::                       `.space SIZE , FILL'

* Stab::                        `.stabd, .stabn, .stabs'

* String::                      `.string "STR"'
* Struct::                      `.struct EXPRESSION'

* SubSection::                  `.subsection'
* Symver::                      `.symver NAME,NAME2@NODENAME'


* Tag::                         `.tag STRUCTNAME'

* Text::                        `.text SUBSECTION'
* Title::                       `.title "HEADING"'

* Type::                        `.type <INT | NAME , TYPE DESCRIPTION>'

* Uleb128::                     `.uleb128 EXPRESSIONS'

* Val::                         `.val ADDR'


* Version::                     `.version "STRING"'
* VTableEntry::                 `.vtable_entry TABLE, OFFSET'
* VTableInherit::               `.vtable_inherit CHILD, PARENT'
* Weak::                        `.weak NAMES'

* Word::                        `.word EXPRESSIONS'
* Deprecated::                  Deprecated Directives


File: as.info,  Node: Abort,  Next: ABORT,  Up: Pseudo Ops

`.abort'
========

   This directive stops the assembly immediately.  It is for
compatibility with other assemblers.  The original idea was that the
assembly language source would be piped into the assembler.  If the
sender of the source quit, it could use this directive tells `as' to
quit also.  One day `.abort' will not be supported.


File: as.info,  Node: ABORT,  Next: Align,  Prev: Abort,  Up: Pseudo Ops

`.ABORT'
========

   When producing COFF output, `as' accepts this directive as a synonym
for `.abort'.

   When producing `b.out' output, `as' accepts this directive, but
ignores it.


File: as.info,  Node: Align,  Next: Ascii,  Prev: ABORT,  Up: Pseudo Ops

`.align ABS-EXPR, ABS-EXPR, ABS-EXPR'
=====================================

   Pad the location counter (in the current subsection) to a particular
storage boundary.  The first expression (which must be absolute) is the
alignment required, as described below.

   The second expression (also absolute) gives the fill value to be
stored in the padding bytes.  It (and the comma) may be omitted.  If it
is omitted, the padding bytes are normally zero.  However, on some
systems, if the section is marked as containing code and the fill value
is omitted, the space is filled with no-op instructions.

   The third expression is also absolute, and is also optional.  If it
is present, it is the maximum number of bytes that should be skipped by
this alignment directive.  If doing the alignment would require
skipping more bytes than the specified maximum, then the alignment is
not done at all.  You can omit the fill value (the second argument)
entirely by simply using two commas after the required alignment; this
can be useful if you want the alignment to be filled with no-op
instructions when appropriate.

   The way the required alignment is specified varies from system to
system.  For the a29k, hppa, m68k, m88k, w65, sparc, Xtensa, and
Renesas / SuperH SH, and i386 using ELF format, the first expression is
the alignment request in bytes.  For example `.align 8' advances the
location counter until it is a multiple of 8.  If the location counter
is already a multiple of 8, no change is needed.

   For other systems, including the i386 using a.out format, and the
arm and strongarm, it is the number of low-order zero bits the location
counter must have after advancement.  For example `.align 3' advances
the location counter until it a multiple of 8.  If the location counter
is already a multiple of 8, no change is needed.

   This inconsistency is due to the different behaviors of the various
native assemblers for these systems which GAS must emulate.  GAS also
provides `.balign' and `.p2align' directives, described later, which
have a consistent behavior across all architectures (but are specific
to GAS).


File: as.info,  Node: Ascii,  Next: Asciz,  Prev: Align,  Up: Pseudo Ops

`.ascii "STRING"'...
====================

   `.ascii' expects zero or more string literals (*note Strings::)
separated by commas.  It assembles each string (with no automatic
trailing zero byte) into consecutive addresses.


File: as.info,  Node: Asciz,  Next: Balign,  Prev: Ascii,  Up: Pseudo Ops

`.asciz "STRING"'...
====================

   `.asciz' is just like `.ascii', but each string is followed by a
zero byte.  The "z" in `.asciz' stands for "zero".