gasdocs.txt

From: "A.Appleyard" <A.APPLEYARD@fs2.mt.umist.ac.uk>
To: dj
Date: Thu, 24 Aug 1995 10:04:07 BST
Subject: GAS manual in ascii
Priority: normal
X-Mailer: Pegasus Mail v3.22
DJ-Mail-Sort: djgpp, djgpp

  Herewith djgpp\docs\gas\as.tex converted to ascii. I have removed the matter
that does not seem to refer to the IBM PC. Please check it through first in
case I went astray anywhere among a tangle of Tex @if --- @end and suchlike.
It is now 1510 lines long.
  ............................................

[The GNU Assembler for the IBM PC family] [January 1994]
  This file is a user guide to the Gnu assembler `GAS'. This version of the
file describes `GAS' configured to generate code for IBM PC architectures.

  =================================================
[Overview]

Here is a brief summary of how to invoke `GAS'. For details, see Invoking
Command Line Options.
  GAS [ -a[dhlns] ] [ -D ] [ -f ] [ -I <path> ] [ -K ] [ -L ] [ -o <objfile> ]
  [ -R ] [ --statistics] [ -v ] [ -W ] [ -Z ] [ -- | <files> ... ]
-a[dhlns]       Turn on listings, in any of a variety of ways:-
-ad             omit debugging directives from listing
-ah             include high-level source
-al             assembly listing
-an             no forms processing
-as             symbols
  You may combine these options; for example, use `-aln' for assembly listing
without forms processing. By itself, `-a' defaults to `-ahls' - that is, all
listings turned on.

-D      This option is accepted only for script compatibility with calls to
        other assemblers; it has no effect on `GAS'.
-f      ``fast''---skip whitespace and comment preprocessing (assume source is
        compiler output)
-I <path>       Add <path> to the search list for `.include' directives
-K      According to version, this option may or may not issue warnings when difference tables altered for long displacements.
-L      Keep (in symbol table) local symbols, starting with `L'
-o <objfile>    Name the object-file output from `GAS'
-R      Fold data section into text section
--statistics    Display maximum space (in bytes), and total time (in seconds), taken by
        assembly.
-v      Announce `as' version
-W      Suppress warning messages
-Z      Generate object file even after errors
--      Standard input
<files> ...     Source files to assemble.

  --------------------------------------------------------
[Structure of this Manual]
  This manual is intended to describe what you need to know to use Gnu `GAS'.
We cover the syntax expected in source files, including notation for symbols,
constants, and expressions; the directives that `GAS' understands; and of
course how to invoke `GAS'. We also cover special features in the IBM PC
configuration of `GAS', including assembler directives. On the other hand,
this manual is NOT intended as an introduction to programming in assembly
language---let alone programming in general! In a similar vein, we make no
attempt to introduce the machine architecture; we do NOT describe the
instruction set, standard mnemonics, registers or addressing modes that are
standard to a particular architecture.

  --------------------------------------------------------
[GAS, the GNU Assembler]
  Gnu `as' is really a family of assemblers. This manual describes `GAS', a
member of that family which is configured for the IBM PC architectures. If you
use (or have used) the Gnu assembler on one architecture, you should find a
fairly similar environment when you use it on another architecture. Each
version has much in common with the others, including object file formats,
most assembler directives (often called `pseudo-ops') and assembler syntax.
`GAS' is primarily intended to assemble the output of the Gnu C compiler `GCC'
for use by the linker `LD'. Nevertheless, we've tried to make `GAS' assemble
correctly everything that other assemblers for the same machine would
assemble. Unlike older assemblers, `GAS' is designed to assemble a source
program in one pass of the source file. This has a subtle impact on the `.org'
directive (see Org,,`.org').

  --------------------------------------------------------
[Object File Formats]
  The Gnu assembler can be configured to produce several alternative object
file formats. For the most part, this does not affect how you write assembly
language programs; but directives for debugging symbols are typically
different in different file formats. See Symbol Attributes. On the IBM PC,
`GAS' is configured to produce `a.out' format object files.

  --------------------------------------------------------
[Command Line]
  After the program name `GAS', the command line may contain options and file
names. Options may appear in any order, and may be before, after, or between
file names. The order of file names is significant.
  `--' (two hyphens) by itself names the standard input file explicitly, as
one of the files for `GAS' to assemble.
  Except for `--' any command line argument that begins with a hyphen (`-') is
an option. Each option changes the behavior of `GAS'. No option changes the
way another option works. An option is a `-' followed by one or more letters;
the case of the letter is important. All options are optional.
  Some options expect exactly one file name to follow them. The file name may
either immediately follow the option's letter (compatible with older
assemblers) or it may be the next command argument (Gnu standard). These two
command lines are equivalent:
    GAS -o my-object-file.o mumble.s
    GAS -omy-object-file.o mumble.s

  --------------------------------------------------------
[Input Files]
  We use the phrase `source program', abbreviated `source', to describe the
program input to one run of `GAS'. The program may be in one or more files;
how the source is partitioned into files doesn't change the meaning of the
source. The source program is a concatenation of the text in all the files, in
the order specified. Each time you run `GAS' it assembles exactly one source
program. The source program is made up of one or more files. (The standard
input is also a file.)
  You give `GAS' a command line that has zero or more input file names. The
input files are read (from left file name to right). A command line argument
(in any position) that has no special meaning is taken to be an input file
name. If you give `GAS' no file names it attempts to read one input file from
the `GAS' standard input, which is normally your terminal. You may have to
type `ctrl-D' to tell `GAS' there is no more program to assemble. Use `--' if
you need to explicitly name the standard input file in your command line. If
the source is empty, `GAS' produces a small, empty object file.

  --------------------------------------------------------
[Filenames and Line-numbers]
  There are two ways of locating a line in the input file (or files) and
either may be used in reporting error messages. One way refers to a line
number in a physical file; the other refers to a line number in a ``logical''
file. See Errors, ,Error and Warning Messages.
  `Physical files' are those files named in the command line given to `GAS'.
  `Logical files' are simply names declared explicitly by assembler
directives; they bear no relation to physical files. Logical file names help
error messages reflect the original source file, when `GAS' source is itself
synthesized from other files. See App-File,,`.app-file'.

  --------------------------------------------------------
[Output (Object) File]
  Every time you run `GAS' it produces an output file, which is your assembly
language program translated into numbers. This file is the object file. Its
default name is `a.out'. You can give it another name by using the `-o'
option. Conventionally, object file names end with `.o'. The default name is
used for historical reasons: older assemblers were capable of assembling
self-contained programs directly into a runnable program. (For some formats,
this isn't currently possible, but it can be done for the `a.out' format.)
  The object file is meant for input to the linker `LD'. It contains assembled
program code, information to help `LD' integrate the assembled program into a
runnable file, and (optionally) symbolic information for the debugger.

  --------------------------------------------------------
[Error and Warning Messages]
  `GAS' may write warnings and error messages to the standard error file
(usually your terminal). This should not happen when  a compiler runs `GAS'
automatically. Warnings report an assumption made so that `GAS' could keep
assembling a flawed program; errors report a grave problem that stops the
assembly. Warning messages have the format
    file_name:<NNN>:Warning Message Text
  (where <NNN> is a line number). If a logical file name has been given (see
App-File,,`.app-file') it is used for the filename, otherwise the name of the
current input file is used. If a logical line number was given, then it is
used to calculate the number printed, otherwise the actual line in the current
source file is printed. The message text is intended to be self explanatory
(in the grand Unix tradition). Error messages have the format
    file_name:<NNN>:FATAL:Error Message Text
  The file name and line number are derived as for warning messages. The
actual message text may be rather less explanatory because many of them aren't
supposed to happen.

  =================================================
[Command-Line Options]
  This chapter describes command-line options available in ALL versions of the
Gnu assembler; see Machine Dependencies, for options specific to particular
machine architectures. If you are invoking `GAS' via the Gnu C compiler
(version 2), you can use the `-Wa' option to pass arguments through to the
assembler. The assembler arguments must be separated from each other (and the
`-Wa') by commas. For example:
    gcc -c -g -O -Wa,-alh,-L file.c
 emits a listing to standard output with high-level and assembly source.
Usually you do not need to use this `-Wa' mechanism, since many compiler
command-line options are automatically passed to the assembler by the
compiler. (You can call the Gnu compiler driver with the `-v' option to see
precisely what options it passes to each compilation pass, including the
assembler.)

  --------------------------------------------------------
[Enable Listings: `-a[dhlns]']
  These options enable listing output from the assembler. By itself, `-a'
requests high-level, assembly, and symbols listing. You can use other letters
to select specific options for the list: `-ah' requests a high-level language
listing, `-al' requests an output-program assembly listing, and `-as' requests
a symbol table listing. High-level listings require that a compiler debugging
option like `-g' be used, and that assembly listings (`-al') be requested
also. Use the `-ad' option to omit debugging directives from the listing.
  Once you have specified one of these options, you can further control
listing output and its appearance using the directives `.list', `.nolist',
`.psize', `.eject', `.title', and `.sbttl'. The `-an' option turns off all
forms processing. If you do not request listing output with one of the `-a'
options, the listing-control directives have no effect.
  The letters after `-a' may be combined into one option, E.G., `-aln'.

  --------------------------------------------------------
[`-D']
  This option has no effect whatsoever, but it is accepted to make it more
likely that scripts written for other assemblers also work with `GAS'.

  --------------------------------------------------------
[Work Faster: `-f']
  `-f' should only be used when assembling programs written by a (trusted)
compiler. `-f' stops the assembler from doing whitespace and comment
preprocessing on the input file(s) before assembling them. See Preprocessing:
if you use `-f' when the files actually need to be preprocessed (if they
contain comments, for example), `GAS' does not work correctly.

  --------------------------------------------------------
[`.include' search path: `-I' <path>]
  Use this option to add a <path> to the list of directories `GAS' searches
for files specified in `.include' directives (see Include,,`.include'). You
may use `-I' as many times as necessary to include a variety of paths. The
current working directory is always searched first; after that, `GAS' searches
any `-I' directories in the same order as they were specified (left to right)
on the command line.

  --------------------------------------------------------
[Difference Tables: `-K']
  According to version (the `DIFF-TBL-KLUGE'), `GAS' may or may not sometimes
alter the code emitted for directives of the form `.word <sym1>-<sym2>'; see
Word,,`.word'. You can use the `-K' option if you want a warning issued when
this is done.

  --------------------------------------------------------
[Include Local Labels: `-L']
  Labels beginning with `L' (upper case only) are called `local labels'. See
Symbol Names. Normally you do not see such labels when debugging, because they
are intended for the use of programs (like compilers) that compose assembler
programs, not for your notice. Normally both `GAS' and `LD' discard such
labels, so you do not normally debug with them. This option tells `GAS' to
retain those `L...' symbols in the object file. Usually if you do this you
also tell the linker `LD' to preserve symbols whose names begin with `L'. y
default, a local label is any label beginning with `L', but each target is
allowed to redefine the local label prefix.

  --------------------------------------------------------
[Name the Object File: `-o']
  There is always one object file output when you run `GAS'. By default it has
the name `a.out'. You use this option (which takes exactly one filename) to
give the object file a different name. Whatever the object file is called,
`GAS' overwrites any existing file of the same name.

  --------------------------------------------------------
[Join Data and Text Sections: `-R']
  `-R' tells `GAS' to write the object file as if all data-section data lives
in the text section. This is only done at the very last moment:  your binary
data are the same, but data section parts are relocated differently. The data
section part of your object file is zero bytes long because all its bytes are
appended to the text section. (See Sections,,Sections and Relocation.) When
you specify `-R' it would be possible to generate shorter address
displacements (because we do not have to cross between text and data section).
We refrain from doing this simply for compatibility with older versions of
`GAS'. In future, `-R' may work this way.

  --------------------------------------------------------
[Display Assembly Statistics: `--statistics']
  Use `--statistics' to display two statistics about the resources used by
`GAS': the maximum amount of space allocated during the assembly (in bytes),
and the total execution time taken for the assembly (in `cpu' seconds).

  --------------------------------------------------------
[Announce Version: `-v']
  You can find out what version of as is running by including the option `-v'
(which you can also spell as `-version') on the command line.

  --------------------------------------------------------
[Suppress Warnings: `-W']
  `GAS' should never give a warning or error message when assembling compiler
output. But programs written by people often cause `GAS' to give a warning
that a particular assumption was made. All such warnings are directed to the
standard error file. If you use this option, no warnings are issued. This
option only affects the warning messages: it does not change any particular of
how `GAS' assembles your file. Errors, which stop the assembly, are still
reported.

  --------------------------------------------------------
[Generate Object File in Spite of Errors: `-Z']
  After an error message, `GAS' normally produces no output. If for some
reason you are interested in object file output even after `GAS' gives an
error message on your program, use the `-Z' option. If there are any errors,
`GAS' continues anyways, and writes an object file after a final warning
message of the form `<n> errors, <m> warnings, generating bad object file.'

  =================================================
[Syntax]
  This chapter describes the machine-independent syntax allowed in a source
file. `GAS' syntax is similar to what many other assemblers use; it is
inspired by the BSD 4.2 assembler.

  --------------------------------------------------------
[Preprocessing]
  The `GAS' internal preprocessor:-
  - Adjusts and removes extra whitespace. It leaves one space or tab before
the keywords on a line, and turns any other whitespace on the line into a
single space.
  - Removes all comments, replacing them with a single space, or an
appropriate number of newlines.
  - converts character constants into the appropriate numeric values.
  It does not do macro processing, include file handling, or anything else you
may get from your C compiler's preprocessor. You can do include file
processing with the `.include' directive (see Include,,`.include'). You can
use the Gnu C compiler driver to get other ``CPP'' style preprocessing, by
giving the input file a `.S' suffix. See Overall Options,, Options Controlling
the Kind of Output, gcc.info, Using GNU CC.
  Excess whitespace, comments, and character constants cannot be used in the
portions of the input text that are not preprocessed.
  If the first line of an input file is `#NO_APP' or if you use the `-f'
option, whitespace and comments are not removed from the input file. Within an
input file, you can ask for whitespace and comment removal in specific
portions of the by putting a line that says `#APP' before the text that may
contain whitespace or comments, and putting a line that says `#NO_APP' after
this text. This feature is mainly intend to support `asm' statements in
compilers whose output is otherwise free of comments and whitespace.

  --------------------------------------------------------
[Whitespace]
  `Whitespace' is one or more blanks or tabs, in any order. Whitespace is used
to separate symbols, and to make programs neater for people to read. Unless
within character constants (see Characters,,Character Constants), any
whitespace means the same as exactly one space.

  --------------------------------------------------------
[Comments]
  There are two ways of rendering comments to `GAS'. In both cases the comment
is equivalent to one space. Anything from `/*' to the next `*/' is a comment.
This means you may not nest these comments. The only way to include a newline
in a comment is to use a /* */ comment.
  Anything from the `line comment' character to the next newline is considered
a comment and is ignored. The line comment character is
  To be compatible with past assemblers, lines that begin with `#' have a
special interpretation. Following the `#' should be an absolute expression
(see Expressions): the logical line number of the NEXT line. Then a string
(see 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 `GAS'.

  --------------------------------------------------------
[Symbols]
  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). See
Symbols.

  --------------------------------------------------------
[Statements]
  A `statement' ends at a newline character (`\n') or at a semicolon (`;').
The newline or semicolon is considered part of the preceding statement.
Newlines and semicolons 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.
  You may write a statement on more than one line if you put a backslash (`\')
immediately in front of any newlines within the statement. When `GAS' reads a
backslashed newline both characters are ignored. You can even put backslashed
newlines in the middle of symbol names without changing the meaning of your
source program.
  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.
  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. See Labels.
    label:     .directive    followed by something
    another_label:           # This is an empty statement.
               instruction   operand_1, operand_2, ...

  --------------------------------------------------------
[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.

  ..............................................
[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.

  . . . . . . . . . . . . . . . . . . . . . . .
[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
`GAS' to interpret the second character literally as a backslash (which
prevents `GAS' 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.
\\      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 `GAS' has no other interpretation, so `GAS' 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.

  . . . . . . . . . . . . . . . . . . . . . . .
[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 (or semicolon `;') 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. `GAS' assumes your character code
is ASCII: `'A' means 65, `'B' means 66, and so on.

  ..............................................
[Number Constants]
  `GAS' 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.

  . . . . . . . . . . . . . . . . . . . . . . .
[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 (see Prefix Ops,,Prefix Operators).

  . . . . . . . . . . . . . . . . . . . . . . .
[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.

  . . . . . . . . . . . . . . . . . . . . . . .
[Flonums]
  A `flonum' represents a floating point number. The translation is indirect:
a decimal floating point number from the text is converted by `GAS' 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 `GAS' specialized to that computer.
A flonum is written by writing (in order)
    The digit `0'.
    A letter, to tell `GAS' the rest of the number is a flonum.
        `e' is recommended. Case is not important.
    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. `GAS' does all processing
using integers. Flonums are computed independently of any floating point
hardware in the computer running `GAS'.

  =================================================
[Sections and Relocation]

  --------------------------------------------------------
[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 `GAS' 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 `GAS' 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.
  An object file written by `GAS' has at least three sections, any of which
may be empty. These are named `text', `data' and `bss' sections.
  Within the object file, the text section starts at address `0', the data
section follows, 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, `GAS' 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 `GAS' ever uses is expressed as
      (<section>) + (<offset into section>)
  Further, most expressions `GAS' computes have this section-relative nature.
  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 `GAS'
and have no meaning except during assembly.

  --------------------------------------------------------
[LD Sections]
  `LD' deals with just four kinds of sections, summarized below. These
sections hold your program. `GAS' and `LD' treat them as separate but equal
sections. Anything you can say of one section is true another.

  bss section: This section contains zeroed bytes when your program begins
running. It is used to hold unitialized 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. @end table
  An idealized example of three relocatable sections follows. 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 ...
  --------------------------------------------------------
[GAS Internal Sections]
  These sections are meant only for the internal use of `GAS'. 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 `GAS' warning messages, so it
might be helpful to have an idea of their meanings to `GAS'. 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.

  --------------------------------------------------------
[Sub-Sections]
  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. `GAS' 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.
  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. You can also use an extra subsection argument with
arbitrary named sections: `.section <name>, <expression>'. <Expression> should
be an absolute expression. (See 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 `GAS' 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.

  --------------------------------------------------------
[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.
  Addresses in the bss section are allocated with special directives; you may
not assemble anything directly into the bss section. Hence there are no bss
subsections. See Comm,,`.comm', see Lcomm,,`.lcomm'.

  =================================================
[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: `GAS' does not place symbols in the object file in the same order
they were declared. This may break some debuggers.

  --------------------------------------------------------
[Labels]
 `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.

  --------------------------------------------------------
[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 (see Expressions). This is
equivalent to using the `.set' directive. See Set,,`.set'.

  --------------------------------------------------------
[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 @ref{Machine
Dependencies}. That character may be followed by any string of digits,
letters, dollar signs (unless otherwise noted in `Machine Dependencies'), and
underscores. 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. There
are ten local symbol names, which are re-used throughout the program. You may
refer to them using the names `0' `1' ... `9'. To define a local symbol, write
a label of the form `<N>:' (where <N> represents any digit). To refer to the
most recent previous definition of that symbol write `<N>'b, using the same
digit as when you defined the label. To refer to the next definition of a
local label, write `<N>'f---where <N> gives you a choice of 10 forward
references. The `b' stands for ``backwards'' and the `f' stands for
``forwards''. Local symbols are not emitted by the current Gnu C compiler.
  There is no restriction on how you can use these labels, but remember that
at any point in the assembly you can refer to at most 10 prior local labels
and to at most 10 forward local labels.
  Local symbol names are only a notation 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 have these parts:

  L : All local labels begin with `L'. Normally both `GAS' 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 `GAS' retains these symbols
in the object file. If you also instruct `LD' to retain these symbols, you may
use them in debugging.
  <digit> : If the label is written `0:' then the digit is `0'. If the label
is written `1:' then the digit is `1'. And so on up through `9:'.
  <ctrl-A> : This unusual character is included so you do not accidentally
invent a symbol of the same name. The character has ASCII value `\001'.
  ORDINAL NUMBER : This is a serial number to keep the labels distinct. The
first `0:' gets the number `1'; The 15th `0:' gets the number `15'; ETC..
Likewise for the other labels `1:' through `9:'. @end table For instance, the
first `1:' is named `L1<ctrl-A>1', the 44th `3:' is named `L3<ctrl-A>44'.

  --------------------------------------------------------
[The Special Dot Symbol]
  The special symbol `.' refers to the current address that `GAS' 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'.

  --------------------------------------------------------
[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, `GAS' 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.

  ..............................................
[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.

  ..............................................
[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.

  . . . . . . . . . . . . . . . . . . . . . . .
[Descriptor]
  This is an arbitrary 16-bit value. You may establish a symbol's descriptor
value by using a `.desc' statement (see Desc,,`.desc'). A descriptor value
means nothing to `GAS'.

  . . . . . . . . . . . . . . . . . . . . . . .
[Other]
  This is an arbitrary 8-bit value. It means nothing to `GAS'.

  =================================================
[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 `GAS' 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. `GAS' aborts with an error message in this situation.

  --------------------------------------------------------
[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 `GAS'
assumes a value of (absolute) 0. This is compatible with other assemblers.

  --------------------------------------------------------
[Integer Expressions]
  An `integer expression' is one or more ARGUMENTS delimited by OPERATORS.

  ..............................................
[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 `GAS' 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.

  ..............................................
[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.

  ..............................................
[Prefix Operator]
  `GAS' has the following `prefix operators'. They each take one argument,
which must be absolute.
 -      `Negation'. Two's complement negation.
 ~      `Complementation'. Bitwise not.

  ..............................................
[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.
    Highest Precedence
 *      `Multiplication'.
 /      `Division'. Truncation is the same as the C operator `/'
 %      `Remainder'.
 <      `Shift Left'. Same as the C operator `<<'.
 <<     `Shift Left'. Same as the C operator `<<'.
 >      `Shift Right'. Same as the C operator `>>'.
 >>     `Shift Right'. Same as the C operator `>>'.

    Intermediate precedence
 |      `Bitwise Inclusive Or'.
 &      `Bitwise And'.
 ^      `Bitwise Exclusive Or'.
 !      `Bitwise Or Not'.

    Lowest 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.
  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.

  =================================================
[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 t