     [file asdoc-config.texi not included]

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

This file documents the GNU Assembler "AS".

Copyright (C) 1991, 1992, 1993 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled ``GNU General Public License'' is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU General Public License'' may be included in a translation approved by the Free Software Foundation instead of in the original English.

Using AS
********

This file is a user guide to the GNU assembler AS. This version of the file describes AS configured to generate code for TARGET architectures.

Overview
********

Here is a brief summary of how to invoke AS.  For details, see Comand-Line Options.

       AS [ -a[dhlns] ] [ -D ] [ -f ]
        [ -I PATH ] [ -K ] [ -L ]
        [ -o OBJFILE ] [ -R ] [ -v ] [ -w ]
        [ -- | FILES ... ]

-a[dhlns]
     Turn on listings; -ad, omit debugging pseudo-ops from listing, -ah, include high-level source, -al, assembly listing, -an, no forms processing, -as, symbols. These options may be combined; e.g., -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 AS.

-f
     ``fast''---skip preprocessing (assume source is compiler output)

-I PATH
     Add PATH to the search list for .include directives

-K
     This option is accepted but has no effect on the TARGET family.

-L
     Keep (in symbol table) local symbols, starting with L

-o OBJFILE
     Name the object-file output from AS

-R
     Fold data section into text section

-v
     Announce as version

-W
     Suppress warning messages

-- | FILES ...
     Standard input, or source files to assemble.

Structure of this Manual
========================

This manual is intended to describe what you need to know to use GNU AS.  We cover the syntax expected in source files, including notation for symbols, constants, and expressions; the directives that AS understands; and of course how to invoke AS.

We also cover special features in the TARGET configuration of AS, 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.

AS, the GNU Assembler
=====================

GNU as is really a family of assemblers. This manual describes AS, a member of that family which is configured for the TARGET 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.

AS 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 AS assemble correctly everything that other assemblers for the same machine would assemble.

Unlike older assemblers, AS 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).

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 TARGET, AS is configured to produce OBJ-NAME format object files.

Command Line
============

After the program name AS, 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 AS to assemble.

Except for -- any command line argument that begins with a hyphen (-) is an option.  Each option changes the behavior of AS.  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:

     AS -o my-object-file.o mumble.s
     AS -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 AS.  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 AS 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 AS 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 AS no file names it attempts to read one input file from the AS standard input, which is normally your terminal.  You may have to type ctl-D to tell AS 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, AS will produce 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 Error and Warning Messages.

Physical files are those files named in the command line given to AS.

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 AS source is itself synthesized from other files. See .app-file.

Output (Object) File
====================

Every time you run AS it produces an output file, which is your assembly language program translated into numbers.  This file is the object file, named a.out, unless you tell AS to give it another name by using the -o option.  Conventionally, object file names end with .o.  The default name of a.out 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 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
==========================

AS may write warnings and error messages to the standard error file (usually your terminal).  This should not happen when  a compiler runs AS automatically.  Warnings report an assumption made so that AS 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) it is used for the filename, otherwise the name of the current input file is used.  If a logical line number was given (see .line) 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 the TARGET.

If you are invoking AS 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

will cause a listing to be emitted to standard output with high-level and assembly source.

Many compiler command-line options, such as -R and many machine-specific options, will be automatically be passed to the assembler by the compiler, so usually you do not need to use this -Wa mechanism.

Enable Listings: -a[dhlns]
==========================

These options enable listing output from the assembler.  By itself, -a requests high-level, assembly, and symbols listing. Other letters may be used 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.

The -ad option may be used to omit debugging pseudo-ops 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 will also work with AS.

Work Faster: -f
===============

-f should only be used when assembling programs written by a (trusted) compiler.  -f stops the assembler from pre-processing the input file(s) before assembling them.  See Pre-processing.

     Warning: if the files actually need to be pre-processed (if they contain comments, for example), AS will not work correctly if -f is used.

.include search path: -I PATH
=============================

Use this option to add a PATH to the list of directories AS will search for files specified in .include directives (see .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, AS searches any -I directories in the same order as they were specified (left to right) on the command line.

Difference Tables: -K
=====================

On the TARGET family, this option is allowed, but has no effect.  It is permitted for compatibility with the GNU assembler on other platforms, where it can be used to warn when the assembler alters the machine code generated for .word directives in difference tables.  The TARGET family does not have the addressing limitations that sometimes lead to this alteration on other platforms.

Include Local Labels: -L
========================

Labels beginning with L (upper case only) are called local labels. See Symbol Names.  Normally you don't 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 AS and LD discard such labels, so you don't normally debug with them.

This option tells AS 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.

Name the Object File: -o
========================

There is always one object file output when you run AS.  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, AS will overwrite any existing file of the same name.

Join Data and Text Sections: -R
===============================

-R tells AS 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 and Relocation.)

When you specify -R it would be possible to generate shorter address displacements (because we don't have to cross between text and data section).  We refrain from doing this simply for compatibility with older versions of AS.  In future, -R may work this way.

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
=====================

AS should never give a warning or error message when assembling compiler output.  But programs written by people often cause AS 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 AS assembles your file.  Errors, which stop the assembly, are still reported.

Syntax
******

This chapter describes the machine-independent syntax allowed in a source file.  AS syntax is similar to what many other assemblers use; it is inspired by the BSD 4.2 assembler.

Pre-Processing
==============

The pre-processor:

  * 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.

Excess whitespace, comments, and character constants cannot be used in the portions of the input text that are not pre-processed.

If the first line of an input file is #NO_APP or the -f option is given, the input file will not be pre-processed.  Within such an input file, parts of the file can be pre-processed by putting a line that says #APP before the text that should be pre-processed, and putting a line that says #NO_APP after them.  This feature is mainly intend to support asm statements in compilers whose output normally does not need to be pre-processed.

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 Character Constants), any whitespace means the same as exactly one space.

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 see Machine Dependencies.

To be compatible with past assemblers, a special interpretation is given to lines that begin with #.  Following the # an absolute expression (see Expressions) is expected:  this will be the logical line number of the next line.  Then a string (see 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.

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 _.$. 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 don't 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 AS 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 will assemble 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 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.

\\
     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 \ will give a warning, but assemble 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, don't 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.  AS assumes your character code is ASCII: 'A means 65, 'B means 66, and so on.

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.

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 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 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.

  * A letter, to tell AS the rest of the number is a flonum.

  * 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.

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 AS emits an object file, the partial program is assumed to start at address 0.  LD will assign the final addresses the partial program occupies, so that different partial programs don't overlap.  This is actually an over-simplification, but it will suffice 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.

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.

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 will change 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, every expression AS computes is of this section-relative nature.  Absolute expression means an expression with section ``absolute'' (see Ld Sections).  A pass1 expression means an expression with section ``pass1'' (see AS Internal Sections).  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 two partial programs' data sections will not overlap addresses after linking, by definition their absolute sections will overlap. Address {absolute 239} in one partial program will always be the same address when the program is running as address {absolute 239} in any other partial 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 will be 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 program's 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.

LD Sections
===========

LD deals with just four kinds of sections, summarized below.

     These sections hold your program.  AS 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 don't 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. 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 ...

AS Internal Sections
====================

These sections are meant only for the internal use of AS.  They have no meaning at run-time.  You don't 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.

absent
     An expression was expected and none was found.

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

bignum/flonum
     If a number can't be written as a C int constant (a bignum or a flonum, but not an integer), it is recorded as belonging to this ``section''.  AS has to remember that a flonum or a bignum does not fit into 32 bits, and cannot be an argument (see Arguments) in an expression: this is done by making a flonum or bignum be in a separate internal section.  This is purely for internal AS convenience; bignum/flonum section behaves similarly to absolute section.

pass1 section
     The expression was impossible to evaluate in the first pass.  The assembler will attempt a second pass (second reading of the source) to evaluate the expression.  Your expression mentioned an undefined symbol in a way that defies the one-pass (section + offset in section) assembly process.  No compiler need emit such an expression.

          Warning: the second pass is currently not implemented.  AS will abort with an error message if one is required.

difference section
     As an assist to the C compiler, expressions of the forms

             (UNDEFINED SYMBOL) - (EXPRESSION)
             SOMETHING - (UNDEFINED SYMBOL)
             (UNDEFINED SYMBOL) - (UNDEFINED SYMBOL)

     are permitted, and belong to the difference section.  AS re-evaluates such expressions after the source file has been read and the symbol table built.  If by that time there are no undefined symbols in the expression then the expression assumes a new section.  The intention is to permit statements like .word label - base_of_table to be assembled in one pass where both label and base_of_table are undefined.  This is useful for compiling C and Algol switch statements, Pascal case statements, FORTRAN computed goto statements and the like.

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. 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 will be grouped with other objects in the same subsection when they are all put into the object file.  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 don't use subsections, everything will be stored 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 will 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. 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 AS there is no concept of a subsection location counter.  There is no way to directly manipulate a location counter---but the .align directive will change it, and any label definition will capture its current value.  The location counter of the section that statements are being assembled into 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, see .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: AS does not place symbols in the object file in the same order they were declared.  This may break some debuggers.

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.

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.

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 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 Nb, using the same digit as when you defined the label.  To refer to the next definition of a local label, write Nf---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 AS and LD forget symbols that start with L. These labels are used for symbols you are never intended to see.  If you give the -L option then AS will retain 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:.

A
     This unusual character is included so you don't 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:.

For instance, the first 1: is named L1A1, the 44th 3: is named L3A44.

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

The special symbol . refers to the current address that AS is assembling into.  Thus, the expression melvin:
.long . will cause 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, 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.

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 program, and LD will try to determine its value from other programs it is linked with.  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.

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

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

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 will assume 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 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.

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
---------------

AS 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.

 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. Lowest Precedence

     +
          Addition.  If either argument is absolute, the result has the section of the other argument. If either argument is pass1 or undefined, the result is pass1. Otherwise + is illegal.

     -
          Subtraction.  If the right argument is absolute, the result has the section of the left argument. If either argument is pass1 the result is pass1. If either argument is undefined the result is difference section. If both arguments are in the same section, the result is absolute---provided that section is one of text, data or bss. Otherwise subtraction is illegal.

The sense of the rule for addition is that it's only meaningful to add the offsets in an address; you can only have a defined section in one of the two arguments.

Similarly, you can't subtract quantities from two different sections.

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.

.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.

.align 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 number of low-order zero bits the location counter will have after advancement.  For example .align 3 will advance the location counter until it a multiple of 8.  If the location counter is already a multiple of 8, no change is needed.

The second expression (also absolute) gives the value to be stored in the padding bytes.  It (and the comma) may be omitted.  If it is omitted, the padding bytes are zero.

.app-file STRING
================

.app-file (which may also be spelled .file) tells AS that we are about to start a new logical file.  STRING is the new file name.  In general, the filename is recognized whether or not it is surrounded by quotes "; but if you wish to specify an empty file name is permitted, you must give the quotes--"".  This statement may go away in future: it is only recognized to be compatible with old AS programs.

.ascii "STRING"...
==================

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

.asciz "STRING"...
==================

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

.byte EXPRESSIONS
=================

.byte expects zero or more expressions, separated by commas. Each expression is assembled into the next byte.

.comm SYMBOL , LENGTH
=====================

.comm declares a named common area in the bss section.  Normally LD reserves memory addresses for it during linking, so no partial program defines the location of the symbol.  Use .comm to tell LD that it must be at least LENGTH bytes long.  LD will allocate space for each .comm symbol that is at least as long as the longest .comm request in any of the partial programs linked.  LENGTH is an absolute expression.

.data SUBSECTION
================

.data tells AS to assemble the following statements onto the end of the data subsection numbered SUBSECTION (which is an absolute expression).  If SUBSECTION is omitted, it defaults to zero.

.double FLONUMS
===============

.double expects zero or more flonums, separated by commas.  It assembles floating point numbers.

.eject
======

Force a page break at this point, when generating assembly listings.

.else
=====

.else is part of the AS support for conditional assembly; see .if.  It marks the beginning of a section of code to be assembled if the condition for the preceding .if was false.

.endif
======

.endif is part of the AS support for conditional assembly; it marks the end of a block of code that is only assembled conditionally.  See .if.

.equ SYMBOL, EXPRESSION
=======================

This directive sets the value of SYMBOL to EXPRESSION. It is synonymous with .set; see .set.

.extern
=======

.extern is accepted in the source program---for compatibility with other assemblers---but it is ignored.  AS treats all undefined symbols as external.

.file STRING
============

.file (which may also be spelled .app-file) tells AS that we are about to start a new logical file. STRING is the new file name.  In general, the filename is recognized whether or not it is surrounded by quotes "; but if you wish to specify an empty file name, you must give the quotes--"".  This statement may go away in future: it is only recognized to be compatible with old AS programs.

.fill REPEAT , SIZE , VALUE
===========================

RESULT, SIZE and VALUE are absolute expressions. This emits REPEAT copies of SIZE bytes.  REPEAT may be zero or more.  SIZE may be zero or more, but if it is more than 8, then it is deemed to have the value 8, compatible with other people's assemblers.  The contents of each REPEAT bytes is taken from an 8-byte number.  The highest order 4 bytes are zero.  The lowest order 4 bytes are VALUE rendered in the byte-order of an integer on the computer AS is assembling for. Each SIZE bytes in a repetition is taken from the lowest order SIZE bytes of this number.  Again, this bizarre behavior is compatible with other people's assemblers.

SIZE and VALUE are optional. If the second comma and VALUE are absent, VALUE is assumed zero.  If the first comma and following tokens are absent, SIZE is assumed to be 1.

.float FLONUMS
==============

This directive assembles zero or more flonums, separated by commas.  It has the same effect as .single.

.global SYMBOL, .globl SYMBOL
=============================

.global makes the symbol visible to LD.  If you define SYMBOL in your partial program, its value is made available to other partial programs that are linked with it.  Otherwise, SYMBOL will take its attributes from a symbol of the same name from another partial program it is linked with.

Both spellings (.globl and .global) are accepted, for compatibility with other assemblers.

.hword EXPRESSIONS
==================

This expects zero or more EXPRESSIONS, and emits a 16 bit number for each.

.ident
======

This directive is used by some assemblers to place tags in object files. AS simply accepts the directive for source-file compatibility with such assemblers, but does not actually emit anything for it.

.if ABSOLUTE EXPRESSION
=======================

.if marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an ABSOLUTE EXPRESSION) is non-zero.  The end of the conditional section of code must be marked by .endif (see .endif); optionally, you may include code for the alternative condition, flagged by .else (see .else.

The following variants of .if are also supported:

.ifdef SYMBOL
     Assembles the following section of code if the specified SYMBOL has been defined.

.ifndef SYMBOL
ifnotdef SYMBOL
     Assembles the following section of code if the specified SYMBOL has not been defined.  Both spelling variants are equivalent.

.include "FILE"
===============

This directive provides a way to include supporting files at specified points in your source program.  The code from FILE is assembled as if it followed the point of the .include; when the end of the included file is reached, assembly of the original file continues.  You can control the search paths used with the -I command-line option (see Command-Line Options).  Quotation marks are required around FILE.

.int EXPRESSIONS
================

Expect zero or more EXPRESSIONS, of any section, separated by commas.  For each expression, emit a 32-bit number that will, at run time, be the value of that expression.  The byte order of the expression depends on what kind of computer will run the program.

.lcomm SYMBOL , LENGTH
======================

Reserve LENGTH (an absolute expression) bytes for a local common denoted by SYMBOL.  The section and value of SYMBOL are those of the new local common.  The addresses are allocated in the bss section, so at run-time the bytes will start off zeroed.  SYMBOL is not declared global (see .global), so is normally not visible to LD.

.lflags
=======

AS accepts this directive, for compatibility with other assemblers, but ignores it.

.line LINE-NUMBER
=================

Even though this is a directive associated with the a.out or b.out object-code formats, AS will still recognize it when producing COFF output, and will treat .line as though it were the COFF .ln if it is found outside a .def/.endef pair.

Inside a .def, .line is, instead, one of the directives used by compilers to generate auxiliary symbol information for debugging.

.ln LINE-NUMBER
===============

.ln is a synonym for .line.

.list
=====

Control (in conjunction with the .nolist directive) whether or not assembly listings are generated.  These two directives maintain an internal counter (which is zero initially).   .list increments the counter, and .nolist decrements it.  Assembly listings are generated whenever the counter is greater than zero.

By default, listings are disabled.  When you enable them (with the -a command line option; see Command-Line Options), the initial value of the listing counter is one.

.long EXPRESSIONS
=================

.long is the same as .int, see .int.

.nolist
=======

Control (in conjunction with the .list directive) whether or not assembly listings are generated.  These two directives maintain an internal counter (which is zero initially).   .list increments the counter, and .nolist decrements it.  Assembly listings are generated whenever the counter is greater than zero.

.octa BIGNUMS
=============

This directive expects zero or more bignums, separated by commas.  For each bignum, it emits a 16-byte integer.

The term ``octa'' comes from contexts in which a ``word'' is two bytes; hence octa-word for 16 bytes.

.org NEW-LC , FILL
==================

.org will advance the location counter of the current section to NEW-LC.  NEW-LC is either an absolute expression or an expression with the same section as the current subsection.  That is, you can't use .org to cross sections: if NEW-LC has the wrong section, the .org directive is ignored.  To be compatible with former assemblers, if the section of NEW-LC is absolute, AS will issue a warning, then pretend the section of NEW-LC is the same as the current subsection.

.org may only increase the location counter, or leave it unchanged; you cannot use .org to move the location counter backwards.

Because AS tries to assemble programs in one pass NEW-LC may not be undefined.  If you really detest this restriction we eagerly await a chance to share your improved assembler.

Beware that the origin is relative to the start of the section, not to the start of the subsection.  This is compatible with other people's assemblers.

When the location counter (of the current subsection) is advanced, the intervening bytes are filled with FILL which should be an absolute expression.  If the comma and FILL are omitted, FILL defaults to zero.

.psize LINES , COLUMNS
======================

Use this directive to declare the number of lines---and, optionally, the number of columns---to use for each page, when generating listings.

If you don't use .psize, listings will use a default line-count of 60.  You may omit the comma and COLUMNS specification; the default width is 200 columns.

AS will generate formfeeds whenever the specified number of lines is exceeded (or whenever you explicitly request one, using .eject).

If you specify LINES as 0, no formfeeds are generated save those explicitly specified with .eject.

.quad BIGNUMS
=============

.quad expects zero or more bignums, separated by commas.  For each bignum, it emits an 8-byte integer.  If the bignum won't fit in 8 bytes, it prints a warning message; and just takes the lowest order 8 bytes of the bignum.

The term ``quad'' comes from contexts in which a ``word'' is two bytes; hence quad-word for 8 bytes.

.sbttl "SUBHEADING"
===================

Use SUBHEADING as the title (third line, immediately after the title line) when generating assembly listings.

This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.

.set SYMBOL, EXPRESSION
=======================

This directive sets the value of SYMBOL to EXPRESSION.  This will change SYMBOL's value and type to conform to EXPRESSION.  If SYMBOL was flagged as external, it remains flagged. (see Symbol Attributes.)

You may .set a symbol many times in the same assembly. If the expression's section is unknowable during pass 1, a second pass over the source program will be forced.  The second pass is currently not implemented.  AS will abort with an error message if one is required.

If you .set a global symbol, the value stored in the object file is the last value stored into it.

.short EXPRESSIONS
==================

.single FLONUMS
===============

This directive assembles zero or more flonums, separated by commas.  It has the same effect as .float.

.space SIZE , FILL
==================

This directive emits SIZE bytes, each of value FILL.  Both SIZE and FILL are absolute expressions.  If the comma and FILL are omitted, FILL is assumed to be zero.

.stabd, .stabn, .stabs
======================

There are three directives that begin .stab. All emit symbols (see Symbols), for use by symbolic debuggers. The symbols are not entered in the AS hash table: they cannot be referenced elsewhere in the source file. Up to five fields are required:

string
     This is the symbol's name.  It may contain any character except \000, so is more general than ordinary symbol names.  Some debuggers used to code arbitrarily complex structures into symbol names using this field.

type
     An absolute expression.  The symbol's type is set to the low 8 bits of this expression.  Any bit pattern is permitted, but LD and debuggers will choke on silly bit patterns.

other
     An absolute expression.  The symbol's ``other'' attribute is set to the low 8 bits of this expression.

desc
     An absolute expression.  The symbol's descriptor is set to the low 16 bits of this expression.

value
     An absolute expression which becomes the symbol's value.

If a warning is detected while reading a .stabd, .stabn, or .stabs statement, the symbol has probably already been created and you will get a half-formed symbol in your object file.  This is compatible with earlier assemblers!

.stabd TYPE , OTHER , DESC

     The ``name'' of the symbol generated is not even an empty string. It is a null pointer, for compatibility.  Older assemblers used a null pointer so they didn't waste space in object files with empty strings.

     The symbol's value is set to the location counter, relocatably.  When your program is linked, the value of this symbol will be where the location counter was when the .stabd was assembled.

.stabn TYPE , OTHER , DESC , VALUE
     The name of the symbol is set to the empty string "".

.stabs STRING ,  TYPE , OTHER , DESC , VALUE
     All five fields are specified.

.text SUBSECTION
================

Tells AS to assemble the following statements onto the end of the text subsection numbered SUBSECTION, which is an absolute expression.  If SUBSECTION is omitted, subsection number zero is used.

.title "HEADING"
================

Use HEADING as the title (second line, immediately after the source file name and pagenumber) when generating assembly listings.

This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.

.word EXPRESSIONS
=================

This directive expects zero or more EXPRESSIONS, of any section, separated by commas.

Deprecated Directives
=====================

One day these directives won't work. They are included for compatibility with older assemblers.

.abort

.app-file

.line