Michael K. Johnson
Erik W. Troan


The GNU Assembler

for the family

January 1994

Dean Elsner, Jay Fenlason & friends

Table of Contents

The Free Software Foundation Inc. thanks The Nice Computer Company of Australia for loaning Dean Elsner to write the first (Vax) version of as for Project GNU. The proprietors, management and staff of TNCCA thank FSF for distracting the boss while they got some work done.

Copyright (C) 1991, 92, 93, 94, 95, 96, 1997 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 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.


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

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

 [ -a[cdhlns][=file] ] [ -D ]  [ --defsym sym=val ]
 [ -f ] [ --help ] [ -I dir ] [ -J ] [ -K ] [ -L ]
 [ -o objfile ] [ -R ] [ --statistics ] [ -v ] [ -version ]
 [ --version ] [ -W ] [ -w ] [ -x ] [ -Z ]

 [ -- | files ... ]
Turn on listings, in any of a variety of ways:
omit debugging directives
include high-level source
include assembly
omit forms processing
include symbols
set the name of the listing file
You may combine these options; for example, use `-aln' for assembly listing without forms processing. The `=file' option, if used, must be the last one. By itself, `-a' defaults to `-ahls'---that is, all listings turned on.
Ignored. This option is accepted for script compatibility with calls to other assemblers.
--defsym sym=value
Define the symbol sym to be value before assembling the input file. value must be an integer constant. As in C, a leading `0x' indicates a hexadecimal value, and a leading `0' indicates an octal value.
"fast"---skip whitespace and comment preprocessing (assume source is compiler output).
Print a summary of the command line options and exit.
-I dir
Add directory dir to the search list for .include directives.
Don't warn about signed overflow.
This option is accepted but has no effect on the family.
Keep (in the symbol table) local symbols, starting with `L'.
-o objfile
Name the object-file output from objfile.
Fold the data section into the text section.
Print the maximum space (in bytes) and total time (in seconds) used by assembly.
Print the as version.
Print the as version and exit.
Suppress warning messages.
Generate an object file even after errors.
-- | 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 . We cover the syntax expected in source files, including notation for symbols, constants, and expressions; the directives that understands; and of course how to invoke .

We also cover special features in the configuration of , 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.

The GNU Assembler

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

is primarily intended to assemble the output of the GNU C compiler for use by the linker . Nevertheless, we've tried to make assemble correctly everything that other assemblers for the same machine would assemble.

Unlike older assemblers, is designed to assemble a source program in one pass of the source file. This has a subtle impact on the .org directive (see section .org new-lc , fill).

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 section Symbol Attributes. On the , is configured to produce format object files.

Command Line

After the program name , 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 to assemble.

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

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

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

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

Output (Object) File

Every time you run 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. b.out when is configured for the Intel 80960. 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 . It contains assembled program code, information to help integrate the assembled program into a runnable file, and (optionally) symbolic information for the debugger.

Error and Warning Messages

may write warnings and error messages to the standard error file (usually your terminal). This should not happen when a compiler runs automatically. Warnings report an assumption made so that 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 section .app-file string) it is used for the filename, otherwise the name of the current input file is used. If a logical line number was given (see section .line line-number) 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; @xref{Machine Dependencies}, for options specific to the .

If you are invoking 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[cdhlns]

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 `-ac' option to omit false conditionals from a listing. Any lines which are not assembled because of a false .if (or .ifdef, or any other conditional), or a true .if followed by an .else, will be omitted from the listing.

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'.


This option has no effect whatsoever, but it is accepted to make it more likely that scripts written for other assemblers also work with .

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 section Preprocessing.

Warning: if you use `-f' when the files actually need to be preprocessed (if they contain comments, for example), does not work correctly.

.include search path: -I path

Use this option to add a path to the list of directories searches for files specified in .include directives (see section .include "file"). 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, searches any `-I' directories in the same order as they were specified (left to right) on the command line.

Difference Tables: -K

On the 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 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 section 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 and discard such labels, so you do not normally debug with them.

This option tells to retain those `L...' symbols in the object file. Usually if you do this you also tell the linker to preserve symbols whose names begin with `L'.

By default, a local label is any label beginning with `L', but each target is allowed to redefine the local label prefix.

Assemble in MRI Compatibility Mode: -M

The -M or --mri option selects MRI compatibility mode. This changes the syntax and pseudo-op handling of to make it compatible with the ASM68K or the ASM960 (depending upon the configured target) assembler from Microtec Research. The exact nature of the MRI syntax will not be documented here; see the MRI manuals for more information. Note in particular that the handling of macros and macro arguments is somewhat different. The purpose of this option is to permit assembling existing MRI assembler code using .

The MRI compatibility is not complete. Certain operations of the MRI assembler depend upon its object file format, and can not be supported using other object file formats. Supporting these would require enhancing each object file format individually. These are:

There are some other features of the MRI assembler which are not supported by , typically either because they are difficult or because they seem of little consequence. Some of these may be supported in future releases.

Name the Object File: -o

There is always one object file output when you run . By default it has the name `a.out'. `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, overwrites any existing file of the same name.

Join Data and Text Sections: -R

-R tells 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 section 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 . In future, -R may work this way.

Display Assembly Statistics: --statistics

Use `--statistics' to display two statistics about the resources used by : 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

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

Generate Object File in Spite of Errors: -Z

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


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


The internal preprocessor:

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 section .include "file"). You can use the GNU C compiler driver to get other "CPP" style preprocessing, by giving the input file a `.S' suffix. See section `Options Controlling the Kind of Output' in 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 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 section Character Constants), any whitespace means the same as exactly one space.


There are two ways of rendering comments to . 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 @xref{Machine Dependencies}.

To be compatible with past assemblers, lines that begin with `#' have a special interpretation. Following the `#' should be an absolute expression (see section Expressions): the logical line number of the next line. Then a string (see section 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 .


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 section Symbols.


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 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 section Labels.

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


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.


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 to interpret the second character literally as a backslash (which prevents from recognizing the second \ as an escape character). The complete list of escapes follows.

Mnemonic for backspace; for ASCII this is octal code 010.
Mnemonic for FormFeed; for ASCII this is octal code 014.
Mnemonic for newline; for ASCII this is octal code 012.
Mnemonic for carriage-Return; for ASCII this is octal code 015.
Mnemonic for horizontal Tab; for ASCII this is octal code 011.
\ digit digit digit
An octal character code. The numeric code is 3 octal digits. For compatibility with other Unix systems, 8 and 9 are accepted as digits: for example, \008 has the value 010, and \009 the value 011.
\x hex-digits...
A hex character code. All trailing hex digits are combined. Either upper or lower case x works.
Represents one `\' character.
Represents one `"' character. Needed in strings to represent this character, because an unescaped `"' would end the string.
\ anything-else
Any other character when escaped by \ gives a warning, but assembles as if the `\' was not present. The idea is that if you used an escape sequence you clearly didn't want the literal interpretation of the following character. However has no other interpretation, so 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.


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. assumes your character code is ASCII: 'A means 65, 'B means 66, and so on.

Number Constants

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.


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 section Prefix Operator).


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.


A flonum represents a floating point number. The translation is indirect: a decimal floating point number from the text is converted by 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 specialized to that computer.

A flonum is written by writing (in order)

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

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

into a field whose width depends on which assembler directive has the bit-field as its argument. Overflow (a result from the bitwise and requiring more binary digits to represent) is not an error; instead, more constants are generated, of the specified width, beginning with the least significant digits.

The directives .byte, .hword, .int, .long, .short, and .word accept bit-field arguments.

Sections and Relocation


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 reads many object files (partial programs) and combines their contents to form a runnable program. When emits an object file, the partial program is assumed to start at address 0. 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 uses sections.

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 has at least three sections, any of which may be empty. These are named text, data and bss sections.

can also generate whatever other named sections you specify using the `.section' directive (see section .section name). If you do not use any directives that place output in the `.text' or `.data' sections, these sections still exist, but are empty.

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

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 know which data changes when the sections are relocated, and how to change that data, also writes to the object file details of the relocation needed. To perform relocation must know, each time an address in the object file is mentioned:

In fact, every address ever uses is expressed as

(section) + (offset into section)

Further, most expressions 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 mixes partial programs, addresses in the absolute section remain unchanged. For example, address {absolute 0} is "relocated" to run-time address 0 by . 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. 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 ; others are invented for use of and have no meaning except during assembly.

Linker Sections

deals with just four kinds of sections, summarized below.

These sections hold your program. and 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 must not change when relocating. In this sense we speak of absolute addresses being "unrelocatable": they do not change during relocation.
undefined section
This "section" is a catch-all for address references to objects not in the preceding sections.

An idealized example of three relocatable sections follows. Memory addresses are on the horizontal axis.

Assembler Internal Sections

These sections are meant only for the internal use of . 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 warning messages, so it might be helpful to have an idea of their meanings to . These sections are used to permit the value of every expression in your assembly language program to be a section-relative address.

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.


fall into two sections: text and data. You may have separate groups of data in named sections that you want to end up near to each other in the object file, even though they are not contiguous in the assembler source. 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; 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 section 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 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.

The .lcomm pseudo-op defines a symbol in the bss section; see section .lcomm symbol , length.

The .comm pseudo-op may be used to declare a common symbol, which is another form of uninitialized symbol; see See section .comm symbol , length .


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


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 section Expressions). This is equivalent to using the .set directive. See section .set symbol, expression.

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 @xref{Machine Dependencies}. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted in @xref{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:

All local labels begin with `L'. Normally both and 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 retains these symbols in the object file. If you also instruct to retain these symbols, you may use them in debugging.
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:'.
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:'.

For instance, the first 1: is named L1C-A1, the 44th 3: is named L3C-A44.

The Special Dot Symbol

The special symbol `.' refers to the current address that 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, 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.


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 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 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.


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.

Symbol Attributes: a.out


This is an arbitrary 16-bit value. You may establish a symbol's descriptor value by using a .desc statement (@xref{Desc,,.desc}). A descriptor value means nothing to .


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


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 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. 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 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 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 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 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

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
    Division. Truncation is the same as the C operator `/'
    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. 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 that are available regardless of the target machine configuration for the GNU assembler.


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 to quit also. One day .abort will not be supported.

.align abs-expr, abs-expr, abs-expr

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

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

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

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

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

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

.app-file string

.app-file (which may also be spelled `.file') tells 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 programs.

.ascii "string"...

.ascii expects zero or more string literals (see section 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".

.balign[wl] abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment request in bytes. For example `.balign 8' advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed.

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

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

The .balignw and .balignl directives are variants of the .balign directive. The .balignw directive treats the fill pattern as a two byte word value. The .balignl directives treats the fill pattern as a four byte longword value. For example, .balignw 4,0x368d will align to a multiple of 4. If it skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.

.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 common symbol named symbol. When linking, a common symbol in one object file may be merged with a defined or common symbol of the same name in another object file. If does not see a definition for the symbol--just one or more common symbols--then it will allocate length bytes of uninitialized memory. length must be an absolute expression. If sees multiple common symbols with the same name, and they do not all have the same size, it will allocate space using the largest size.

.data subsection

.data tells 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.


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


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


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

.equ symbol, expression

This directive sets the value of symbol to expression. It is synonymous with `.set'; see section .set symbol, expression.

.equiv symbol, expression

The .equiv directive is like .equ and .set, except that the assembler will signal an error if symbol is already defined.

Except for the contents of the error message, this is roughly equivalent to

.ifdef SYM
.equ SYM,VAL


If assembles a .err directive, it will print an error message and, unless the -Z option was used, it will not generate an object file. This can be used to signal error an conditionally compiled code.


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

.file string

.file (which may also be spelled `.app-file') tells 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 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 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 . If you define symbol in your partial program, its value is made available to other partial programs that are linked with it. Otherwise, symbol takes its attributes from a symbol of the same name from another file linked into the same program.

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.


This directive is used by some assemblers to place tags in object files. 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 section .endif); optionally, you may include code for the alternative condition, flagged by .else (see section .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 section 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 number that, at run time, is the value of that expression. The byte order and bit size of the number depends on what kind of target the assembly is for.

.irp symbol,values...

Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irp directive, and is terminated by an .endr directive. For each value, symbol is set to value, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.

For example, assembling

        .irp    param,1,2,3
        move    d\param,sp@-

is equivalent to assembling

        move    d1,sp@-
        move    d2,sp@-
        move    d3,sp@-

.irpc symbol,values...

Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irpc directive, and is terminated by an .endr directive. For each character in value, symbol is set to the character, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.

For example, assembling

        .irpc    param,123
        move    d\param,sp@-

is equivalent to assembling

        move    d1,sp@-
        move    d2,sp@-
        move    d3,sp@-

.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 that at run-time the bytes start off zeroed. Symbol is not declared global (see section .global symbol, .globl symbol), so is normally not visible to .


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, still recognizes it when producing COFF output, and treats `.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.

.linkonce [type]

Mark the current section so that the linker only includes a single copy of it. This may be used to include the same section in several different object files, but ensure that the linker will only include it once in the final output file. The .linkonce pse