1.1 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.

This manual also describes some of the machine-dependent features of various flavors of the assembler.

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. You may want to consult the manufacturer’s machine architecture manual for this information.

1.2 The GNU Assembler

GNU as is really a family of assemblers. 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. Any exceptions are documented explicitly (see Machine Dependent Features). This doesn’t mean as always uses the same syntax as another assembler for the same architecture; for example, we know of several incompatible versions of 680x0 assembly language syntax.

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).

1.3 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.

1.4 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

1.5 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 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 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. as understands the ‘#’ directives emitted by the gcc preprocessor. See also .file.

1.6 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. Its default name is a.out. You can give it another name by using the -o option. Conventionally, object file names end with .o. The default name is used for historical reasons: older assemblers were capable of assembling self-contained programs directly into a runnable program. (For some formats, this isn’t currently possible, but it can be done for the a.out format.)

The object file is meant for input to the linker ld. It contains assembled program code, information to help ld integrate the assembled program into a runnable file, and (optionally) symbolic information for the debugger.

1.7 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 both a logical file name (see .file) and a logical line number (see .line) have been given then they will be used, otherwise the file name and line number in the current assembler source file will be used. The message text is intended to be self explanatory (in the grand Unix tradition).

Note the file name must be set via the logical version of the .file directive, not the DWARF2 version of the .file directive. For example:

  .file 2 "bar.c"
     error_assembler_source
  .file "foo.c"
  .line 30
      error_c_source

produces this output:

  Assembler messages:
  asm.s:2: Error: no such instruction: `error_assembler_source'
  foo.c:31: Error: no such instruction: `error_c_source'

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.

2.1 Enable Listings: -a[cdghlns]

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 ‘-ag’ option to print a first section with general assembly information, like as version, switches passed, or time stamp.

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’.

Note if the assembler source is coming from the standard input (e.g., because it is being created by gcc and the ‘-pipe’ command-line switch is being used) then the listing will not contain any comments or preprocessor directives. This is because the listing code buffers input source lines from stdin only after they have been preprocessed by the assembler. This reduces memory usage and makes the code more efficient.

2.2 --alternate

Begin in alternate macro mode, see .altmacro.

2.3 -D

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

2.4 Work Faster: -f

‘-f’ should only be used when assembling programs written by a (trusted) compiler. ‘-f’ stops the assembler from doing whitespace and comment preprocessing on the input file(s) before assembling them. See Preprocessing.

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

2.5 .include Search Path: -I path

Use this option to add a path to the list of directories as searches 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.

2.6 Difference Tables: -K

as sometimes alters the code emitted for directives of the form ‘.word sym1-sym2’. See .word. You can use the ‘-K’ option if you want a warning issued when this is done.

2.7 Include Local Symbols: -L

Symbols beginning with system-specific local label prefixes, typically ‘.L’ for ELF systems or ‘L’ for traditional a.out systems, are called local symbols. See Symbol Names. Normally you do not see such symbols 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 symbols, so you do not normally debug with them.

This option tells as to retain those local symbols in the object file. Usually if you do this you also tell the linker ld to preserve those symbols.

2.8 Configuring listing output: --listing

The listing feature of the assembler can be enabled via the command-line switch ‘-a’ (see Enable Listings: -a[cdghlns]). This feature combines the input source file(s) with a hex dump of the corresponding locations in the output object file, and displays them as a listing file. The format of this listing can be controlled by directives inside the assembler source (i.e., .list (see .list), .title (see .title "heading"), .sbttl (see .sbttl "subheading"), .psize (see .psize lines , columns), and .eject (see .eject) and also by the following switches:

--listing-lhs-width=‘number’ ¶

Sets the maximum width, in words, of the first line of the hex byte dump. This dump appears on the left hand side of the listing output.

--listing-lhs-width2=‘number’ ¶

Sets the maximum width, in words, of any further lines of the hex byte dump for a given input source line. If this value is not specified, it defaults to being the same as the value specified for ‘--listing-lhs-width’. If neither switch is used the default is to one.

--listing-rhs-width=‘number’ ¶

Sets the maximum width, in characters, of the source line that is displayed alongside the hex dump. The default value for this parameter is 100. The source line is displayed on the right hand side of the listing output.

--listing-cont-lines=‘number’ ¶

Sets the maximum number of continuation lines of hex dump that will be displayed for a given single line of source input. The default value is 4.

2.9 Assemble in MRI Compatibility Mode: -M

The -M or --mri option selects MRI compatibility mode. This changes the syntax and pseudo-op handling of as to make it compatible with the ASM68K 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 as.

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:

• global symbols in common section

  The m68k MRI assembler supports common sections which are merged by the linker. Other object file formats do not support this. as handles common sections by treating them as a single common symbol. It permits local symbols to be defined within a common section, but it can not support global symbols, since it has no way to describe them.

• complex relocations

  The MRI assemblers support relocations against a negated section address, and relocations which combine the start addresses of two or more sections. These are not support by other object file formats.

• END pseudo-op specifying start address

  The MRI END pseudo-op permits the specification of a start address. This is not supported by other object file formats. The start address may instead be specified using the -e option to the linker, or in a linker script.

• IDNT, .ident and NAME pseudo-ops

  The MRI IDNT, .ident and NAME pseudo-ops assign a module name to the output file. This is not supported by other object file formats.

• ORG pseudo-op

  The m68k MRI ORG pseudo-op begins an absolute section at a given address. This differs from the usual as .org pseudo-op, which changes the location within the current section. Absolute sections are not supported by other object file formats. The address of a section may be assigned within a linker script.

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

• EBCDIC strings

  EBCDIC strings are not supported.

• packed binary coded decimal

  Packed binary coded decimal is not supported. This means that the DC.P and DCB.P pseudo-ops are not supported.

• FEQU pseudo-op

  The m68k FEQU pseudo-op is not supported.

• NOOBJ pseudo-op

  The m68k NOOBJ pseudo-op is not supported.

• OPT branch control options

  The m68k OPT branch control options—B, BRS, BRB, BRL, and BRW—are ignored. as automatically relaxes all branches, whether forward or backward, to an appropriate size, so these options serve no purpose.

• OPT list control options

  The following m68k OPT list control options are ignored: C, CEX, CL, CRE, E, G, I, M, MEX, MC, MD, X.

• other OPT options

  The following m68k OPT options are ignored: NEST, O, OLD, OP, P, PCO, PCR, PCS, R.

• OPT D option is default

  The m68k OPT D option is the default, unlike the MRI assembler. OPT NOD may be used to turn it off.

• XREF pseudo-op.

  The m68k XREF pseudo-op is ignored.

2.10 Dependency Tracking: --MD

as can generate a dependency file for the file it creates. This file consists of a single rule suitable for make describing the dependencies of the main source file.

The rule is written to the file named in its argument.

This feature is used in the automatic updating of makefiles.

2.11 Output Section Padding

Normally the assembler will pad the end of each output section up to its alignment boundary. But this can waste space, which can be significant on memory constrained targets. So the --no-pad-sections option will disable this behaviour.

2.12 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 overwrites any existing file of the same name.

2.13 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 do not 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.

When as is configured for COFF or ELF output, this option is only useful if you use sections named ‘.text’ and ‘.data’.

-R is not supported for any of the HPPA targets. Using -R generates a warning from as.

2.14 Display Assembly Statistics: --statistics

Use ‘--statistics’ to display two statistics about the resources used by as: the maximum amount of space allocated during the assembly (in bytes), and the total execution time taken for the assembly (in CPU seconds).

2.15 Compatible Output: --traditional-format

For some targets, the output of as is different in some ways from the output of some existing assembler. This switch requests as to use the traditional format instead.

For example, it disables the exception frame optimizations which as normally does by default on gcc output.

2.16 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.

2.17 Control Warnings: -W, --warn, --no-warn, --fatal-warnings

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 the -W and --no-warn options, no warnings are issued. This 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.

If you use the --fatal-warnings option, as considers files that generate warnings to be in error.

You can switch these options off again by specifying --warn, which causes warnings to be output as usual.

2.18 Generate Object File in Spite of Errors: -Z

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

3.1 Preprocessing

The as internal preprocessor:

• adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space.
• removes all comments, replacing them with a single space, or an appropriate number of newlines.
• converts character constants into the appropriate numeric values.

It does not do macro processing, include file handling, or anything else you may get from your C compiler’s preprocessor. You can do include file processing with the .include directive (see .include). You can use the GNU C compiler driver to get other “CPP” style preprocessing by giving the input file a ‘.S’ suffix. See the ’Options Controlling the Kind of Output’ section of the GCC manual for more details

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.

3.2 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.

3.3 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 a line comment character up to the next newline is considered a comment and is ignored. The line comment character is target specific, and some targets multiple comment characters. Some targets also have line comment characters that only work if they are the first character on a line. Some targets use a sequence of two characters to introduce a line comment. Some targets can also change their line comment characters depending upon command-line options that have been used. For more details see the Syntax section in the documentation for individual targets.

If the line comment character is the hash sign (‘#’) then it still has the special ability to enable and disable preprocessing (see Preprocessing) and to specify logical line numbers:

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

3.4 Symbols

A symbol is one or more characters chosen from the set of all letters (both upper and lower case), digits and the three characters ‘_.$’. On most machines, you can also use $ in symbol names; exceptions are noted in Machine Dependent Features. No symbol may begin with a digit. Case is significant. There is no length limit; all characters are significant. Multibyte characters are supported, but note that the setting of the --multibyte-handling option might prevent their use. 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.

Symbol names may also be enclosed in double quote " characters. In such cases any characters are allowed, except for the NUL character. If a double quote character is to be included in the symbol name it must be preceded by a backslash \ character.

3.5 Statements

A statement ends at a newline character (‘\n’) or a line separator character. The line separator character is target specific and described in the Syntax section of each target’s documentation. Not all targets support a line separator character. The newline or line separator character is considered to be part of the preceding statement. Newlines and separators within character constants are an exception: they do not end statements.

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

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

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

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

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

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

3.6 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
• Number Constants

4.1 Background

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

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

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

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

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

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

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

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

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

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

• Where in the object file is the beginning of this reference to an address?
• How long (in bytes) is this reference?
• Which section does the address refer to? What is the numeric value of

  (address) - (start-address of section)?
  

• Is the reference to an address “Program-Counter relative”?

In fact, every address as ever uses is expressed as

(section) + (offset into section)

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

In this manual we use the notation {secname N} to mean “offset N into section secname.”

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

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

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

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

4.2 Linker Sections

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

named sections

text section

data section

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

bss section

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

absolute section

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

undefined section

This “section” is a catch-all for address references to objects not in the preceding sections.

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

                      +-----+----+--+
partial program # 1:  |ttttt|dddd|00|
                      +-----+----+--+

                      text   data bss
                      seg.   seg. seg.

                      +---+---+---+
partial program # 2:  |TTT|DDD|000|
                      +---+---+---+

                      +--+---+-----+--+----+---+-----+~~
linked program:       |  |TTT|ttttt|  |dddd|DDD|00000|
                      +--+---+-----+--+----+---+-----+~~

    addresses:        0 …

4.3 Assembler Internal Sections

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

ASSEMBLER-INTERNAL-LOGIC-ERROR!

An internal assembler logic error has been found. This means there is a bug in the assembler.

expr section

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

4.4 Sub-Sections

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

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

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

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

To specify which subsection you want subsequent statements assembled into, use a numeric argument to specify it, in a ‘.text expression’ or a ‘.data expression’ statement. When generating COFF output, you can also use an extra subsection argument with arbitrary named sections: ‘.section name, expression’. When generating ELF output, you can also use the .subsection directive (see .subsection name) to specify a subsection: ‘.subsection expression’. Expression should be an absolute expression (see Expressions). If you just say ‘.text’ then ‘.text 0’ is assumed. Likewise ‘.data’ means ‘.data 0’. Assembly begins in text 0. For instance:

.text 0     # The default subsection is text 0 anyway.
.ascii "This lives in the first text subsection. *"
.text 1
.ascii "But this lives in the second text subsection."
.data 0
.ascii "This lives in the data section,"
.ascii "in the first data subsection."
.text 0
.ascii "This lives in the first text section,"
.ascii "immediately following the asterisk (*)."

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

4.5 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 .lcomm.

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

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

5.1 Labels

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

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

5.2 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. In the same way, using a double equals sign ‘=’‘=’ here represents an equivalent of the .eqv directive. See .eqv.

Blackfin does not support symbol assignment with ‘=’.

5.3 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 Dependent Features. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted for a particular target machine), and underscores. These restrictions do not apply when quoting symbol names by ‘"’, which is permitted for most targets. Escaping characters in quoted symbol names with ‘\’ generally extends only to ‘\’ itself and ‘"’, at the time of writing.

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

Symbol names do not start with a digit. An exception to this rule is made for Local Labels. See below.

Multibyte characters are supported, but note that the setting of the multibyte-handling option might prevent their use. To generate a symbol name containing multibyte characters enclose it within double quotes and use escape codes. cf See Strings. Generating a multibyte symbol name from a label is not currently supported.

Since multibyte symbol names are unusual, and could possibly be used maliciously, as provides a command line option (--multibyte-handling=warn-sym-only) which can be used to generate a warning message whenever a symbol name containing multibyte characters is defined.

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

A local symbol is any symbol beginning with certain local label prefixes. By default, the local label prefix is ‘.L’ for ELF systems or ‘L’ for traditional a.out systems, but each target may have its own set of local label prefixes. On the HPPA local symbols begin with ‘L$’.

Local symbols are defined and used within the assembler, but they are normally not saved in object files. Thus, they are not visible when debugging. You may use the ‘-L’ option (see Include Local Symbols) to retain the local symbols in the object files.

Local Labels

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

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

Here is an example:

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

Which is the equivalent of:

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

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

local label prefix

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

number

This is the number that was used in the local label definition. So if the label is written ‘55:’ then the number is ‘55’.

C-B

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

ordinal number

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

So for example, the first 1: may be named .L1C-B1, and the 44th 3: may be named .L3C-B44.

Dollar Local Labels

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

Dollar labels are defined in exactly the same way as ordinary local labels, except that they have a dollar sign suffix to their numeric value, e.g., ‘55$:’.

They can also be distinguished from ordinary local labels by their transformed names which use ASCII character ‘\001’ (control-A) as the magic character to distinguish them from ordinary labels. For example, the fifth definition of ‘6$’ may be named ‘.L6C-A5’.

5.4 The Special Dot Symbol

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

5.5 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
• Type
• Symbol Attributes: a.out
• Symbol Attributes for COFF
• Symbol Attributes for SOM

6.1 Empty Expressions

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

6.2 Integer Expressions

An integer expression is one or more arguments delimited by operators.

• Arguments
• Operators
• Prefix Operator
• Infix Operators

7.1 .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.

7.2 .ABORT (COFF)

When producing COFF output, as accepts this directive as a synonym for ‘.abort’.

7.3 .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. If this expression is omitted then a default value of 0 is used, effectively disabling alignment requirements.

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 most 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 arc, hppa, i386 using ELF, iq2000, m68k, or1k, s390, sparc, tic4x and xtensa, 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 the tic54x, the first expression is the alignment request in words.

For other systems, including ppc, i386 using a.out format, arm and strongarm, it is the number of low-order zero bits the location counter must have after advancement. For example ‘.align 3’ advances the location counter until it is 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).

7.4 .altmacro

Enable alternate macro mode, enabling:

LOCAL name [ , … ] ¶

One additional directive, LOCAL, is available. It is used to generate a string replacement for each of the name arguments, and replace any instances of name in each macro expansion. The replacement string is unique in the assembly, and different for each separate macro expansion. LOCAL allows you to write macros that define symbols, without fear of conflict between separate macro expansions.

String delimiters ¶

You can write strings delimited in these other ways besides "string":

'string'

You can delimit strings with single-quote characters.

<string>

You can delimit strings with matching angle brackets.

single-character string escape ¶

To include any single character literally in a string (even if the character would otherwise have some special meaning), you can prefix the character with ‘!’ (an exclamation mark). For example, you can write ‘<4.3 !> 5.4!!>’ to get the literal text ‘4.3 > 5.4!’.

Expression results as strings ¶

You can write ‘%expr’ to evaluate the expression expr and use the result as a string.

7.5 .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.

7.6 .asciz "string"…

.asciz is just like .ascii, but each string is followed by a zero byte. The “z” in ‘.asciz’ stands for “zero”. Note that multiple string arguments not separated by commas will be concatenated together and only one final zero byte will be stored.

7.7 .attach_to_group name

Attaches the current section to the named group. This is like declaring the section with the G attribute, but can be done after the section has been created. Note if the group section does not exist at the point that this directive is used then it will be created.

7.8 .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. If the expression is omitted then a default value of 0 is used, effectively disabling alignment requirements.

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 most 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.

7.9 .bss subsection

.bss tells as to assemble the following statements onto the end of the bss section. For ELF based targets an optional subsection expression (which must evaluate to a positive integer) can be provided. In this case the statements are appended to the end of the indicated bss subsection.

7.10 Bundle directives

• .bundle_align_mode abs-expr
• .bundle_lock and .bundle_unlock

7.11 .byte expressions

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

7.12 CFI directives

• .cfi_sections section_list
• .cfi_startproc [simple]
• .cfi_endproc
• .cfi_personality encoding [, exp]
• .cfi_personality_id id
• .cfi_fde_data [opcode1 [, …]]
• .cfi_lsda encoding [, exp]
• .cfi_inline_lsda [align]
• .cfi_def_cfa register, offset
• .cfi_def_cfa_register register
• .cfi_def_cfa_offset offset
• .cfi_adjust_cfa_offset offset
• .cfi_offset register, offset
• .cfi_val_offset register, offset
• .cfi_rel_offset register, offset
• .cfi_register register1, register2
• .cfi_restore register
• .cfi_undefined register
• .cfi_same_value register
• .cfi_remember_state and .cfi_restore_state
• .cfi_return_column register
• .cfi_signal_frame
• .cfi_window_save
• .cfi_escape expression[, …]
• .cfi_val_encoded_addr register, encoding, label

        je label
        popq %rbx
        .cfi_restore %rbx
        popq %r12
        .cfi_restore %r12
        popq %rbp
        .cfi_restore %rbp
        .cfi_def_cfa %rsp, 8
        ret
label:
        /* Do something else */

        je label
        popq %rbx
        .cfi_remember_state
        .cfi_restore %rbx
        popq %r12
        .cfi_restore %r12
        popq %rbp
        .cfi_restore %rbp
        .cfi_def_cfa %rsp, 8
        ret
label:
        .cfi_restore_state
        /* Do something else */

7.13 .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 ld 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 ld 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.

When using ELF or (as a GNU extension) PE, the .comm directive takes an optional third argument. This is the desired alignment of the symbol, specified for ELF as a byte boundary (for example, an alignment of 16 means that the least significant 4 bits of the address should be zero), and for PE as a power of two (for example, an alignment of 5 means aligned to a 32-byte boundary). The alignment must be an absolute expression, and it must be a power of two. If ld allocates uninitialized memory for the common symbol, it will use the alignment when placing the symbol. If no alignment is specified, as will set the alignment to the largest power of two less than or equal to the size of the symbol, up to a maximum of 16 on ELF, or the default section alignment of 4 on PE¹.

The syntax for .comm differs slightly on the HPPA. The syntax is ‘symbol .comm, length’; symbol is optional.

7.14 .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.

7.15 .dc[size] expressions

The .dc directive expects zero or more expressions separated by commas. These expressions are evaluated and their values inserted into the current section. The size of the emitted value depends upon the suffix to the .dc directive:

‘.a’

Emits N-bit values, where N is the size of an address on the target system.

‘.b’

Emits 8-bit values.

‘.d’

Emits double precision floating-point values.

‘.l’

Emits 32-bit values.

‘.s’

Emits single precision floating-point values.

‘.w’

Emits 16-bit values. Note - this is true even on targets where the .word directive would emit 32-bit values.

‘.x’

Emits long double precision floating-point values.

If no suffix is used then ‘.w’ is assumed.

The byte ordering is target dependent, as is the size and format of floating point values.

7.16 .dcb[size] number [,fill]

This directive emits number copies of fill, each of size bytes. Both number and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. The size suffix, if present, must be one of:

‘.b’

Emits single byte values.

‘.d’

Emits double-precision floating point values.

‘.l’

Emits 4-byte values.

‘.s’

Emits single-precision floating point values.

‘.w’

Emits 2-byte values.

‘.x’

Emits long double-precision floating point values.

If the size suffix is omitted then ‘.w’ is assumed.

The byte ordering is target dependent, as is the size and format of floating point values.

7.17 .ds[size] number [,fill]

This directive emits number copies of fill, each of size bytes. Both number and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. The size suffix, if present, must be one of:

‘.b’

Emits single byte values.

‘.d’

Emits 8-byte values.

‘.l’

Emits 4-byte values.

‘.p’

Emits values with size matching packed-decimal floating-point ones.

‘.s’

Emits 4-byte values.

‘.w’

Emits 2-byte values.

‘.x’

Emits values with size matching long double precision floating-point ones.

Note - unlike the .dcb directive the ‘.d’, ‘.s’ and ‘.x’ suffixes do not indicate that floating-point values are to be inserted.

If the size suffix is omitted then ‘.w’ is assumed.

The byte ordering is target dependent.

7.18 .def name

Begin defining debugging information for a symbol name; the definition extends until the .endef directive is encountered.

7.19 .desc symbol, abs-expression

This directive sets the descriptor of the symbol (see Symbol Attributes) to the low 16 bits of an absolute expression.

The ‘.desc’ directive is not available when as is configured for COFF output; it is only for a.out or b.out object format. For the sake of compatibility, as accepts it, but produces no output, when configured for COFF.

7.20 .dim

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs.

7.21 .double flonums

.double expects zero or more flonums, separated by commas. It assembles floating point numbers. The exact kind of floating point numbers emitted depends on how as is configured. See Machine Dependent Features.

7.22 .eject

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

7.23 .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.