Using as

The GNU Assembler

January 1994

Table of Contents

• 1 Overview
• 1.1 Structure of this Manual
• 1.2 The GNU Assembler
• 1.3 Object File Formats
• 1.4 Command Line
• 1.5 Input Files
• 1.6 Output (Object) File
• 1.7 Error and Warning Messages
• 2 Command-Line Options
• 2.1 Enable Listings: -a[cdhlns]
• 2.2 -D
• 2.3 Work Faster: -f
• 2.4 .include search path: -I path
• 2.5 Difference Tables: -K
• 2.6 Include Local Labels: -L
• 2.7 Assemble in MRI Compatibility Mode: -M
• 2.8 Dependency tracking: --MD
• 2.9 Name the Object File: -o
• 2.10 Join Data and Text Sections: -R
• 2.11 Display Assembly Statistics: --statistics
• 2.12 Compatible output: --traditional-format
• 2.13 Announce Version: -v
• 2.14 Suppress Warnings: -W
• 2.15 Generate Object File in Spite of Errors: -Z
• 3 Syntax
• 3.1 Preprocessing
• 3.2 Whitespace
• 3.3 Comments
• 3.4 Symbols
• 3.5 Statements
• 3.6 Constants
• 3.6.1 Character Constants
• 3.6.1.1 Strings
• 3.6.1.2 Characters
• 3.6.2 Number Constants
• 3.6.2.1 Integers
• 3.6.2.2 Bignums
• 3.6.2.3 Flonums
• 4 Sections and Relocation
• 4.1 Background
• 4.2 Linker Sections
• 4.3 Assembler Internal Sections
• 4.4 Sub-Sections
• 4.5 bss Section
• 5 Symbols
• 5.1 Labels
• 5.2 Giving Symbols Other Values
• 5.3 Symbol Names
• 5.4 The Special Dot Symbol
• 5.5 Symbol Attributes
• 5.5.1 Value
• 5.5.2 Type
• 5.5.3 Symbol Attributes: a.out
• 5.5.3.1 Descriptor
• 5.5.3.2 Other
• 5.5.4 Symbol Attributes for COFF
• 5.5.4.1 Primary Attributes
• 5.5.4.2 Auxiliary Attributes
• 5.5.5 Symbol Attributes for SOM
• 6 Expressions
• 6.1 Empty Expressions
• 6.2 Integer Expressions
• 6.2.1 Arguments
• 6.2.2 Operators
• 6.2.3 Prefix Operator
• 6.2.4 Infix Operators
• 7 Assembler Directives
• 7.1 .abort
• 7.2 .ABORT
• 7.3 .align abs-expr, abs-expr, abs-expr
• 7.4 .app-file string
• 7.5 .ascii "string"...
• 7.6 .asciz "string"...
• 7.7 .balign[wl] abs-expr, abs-expr, abs-expr
• 7.8 .byte expressions
• 7.9 .comm symbol , length
• 7.10 .data subsection
• 7.11 .def name
• 7.12 .desc symbol, abs-expression
• 7.13 .dim
• 7.14 .double flonums
• 7.15 .eject
• 7.16 .else
• 7.17 .endef
• 7.18 .endif
• 7.19 .equ symbol, expression
• 7.20 .equiv symbol, expression
• 7.21 .err
• 7.22 .extern
• 7.23 .file string
• 7.24 .fill repeat , size , value
• 7.25 .float flonums
• 7.26 .global symbol, .globl symbol
• 7.27 .hword expressions
• 7.28 .ident
• 7.29 .if absolute expression
• 7.30 .include "file"
• 7.31 .int expressions
• 7.32 .irp symbol,values...
• 7.33 .irpc symbol,values...
• 7.34 .lcomm symbol , length
• 7.35 .lflags
• 7.36 .line line-number
• 7.37 .linkonce [type]
• 7.38 .ln line-number
• 7.39 .mri val
• 7.40 .list
• 7.41 .long expressions
• 7.42 .macro
• 7.43 .nolist
• 7.44 .octa bignums
• 7.45 .org new-lc , fill
• 7.46 .p2align[wl] abs-expr, abs-expr, abs-expr
• 7.47 .psize lines , columns
• 7.48 .quad bignums
• 7.49 .rept count
• 7.50 .sbttl "subheading"
• 7.51 .scl class
• 7.52 .section name
• 7.53 .set symbol, expression
• 7.54 .short expressions
• 7.55 .single flonums
• 7.56 .size
• 7.57 .sleb128 expressions
• 7.58 .skip size , fill
• 7.59 .space size , fill
• 7.60 .stabd, .stabn, .stabs
• 7.61 .string "str"
• 7.62 .symver
• 7.63 .tag structname
• 7.64 .text subsection
• 7.65 .title "heading"
• 7.66 .type int
• 7.67 .val addr
• 7.68 .uleb128 expressions
• 7.69 .word expressions
• 7.70 Deprecated Directives
• 8 Machine Dependent Features
• 9 ARC Dependent Features
• 9.1 Options
• 9.2 Floating Point
• 9.3 ARC Machine Directives
• 10 AMD 29K Dependent Features
• 10.1 Options
• 10.2 Syntax
• 10.2.1 Macros
• 10.2.2 Special Characters
• 10.2.3 Register Names
• 10.3 Floating Point
• 10.4 AMD 29K Machine Directives
• 10.5 Opcodes
• 11 ARM Dependent Features
• 11.1 Options
• 11.2 Syntax
• 11.2.1 Special Characters
• 11.2.2 Register Names
• 11.3 Floating Point
• 11.4 ARM Machine Directives
• 11.5 Opcodes
• 12 D10V Dependent Features
• 12.1 D10V Options
• 12.2 Syntax
• 12.2.1 Size Modifiers
• 12.2.2 Sub-Instructions
• 12.2.3 Special Characters
• 12.2.4 Register Names
• 12.2.5 Addressing Modes
• 12.2.6 @WORD Modifier
• 12.3 Floating Point
• 12.4 Opcodes
• 13 H8/300 Dependent Features
• 13.1 Options
• 13.2 Syntax
• 13.2.1 Special Characters
• 13.2.2 Register Names
• 13.2.3 Addressing Modes
• 13.3 Floating Point
• 13.4 H8/300 Machine Directives
• 13.5 Opcodes
• 14 H8/500 Dependent Features
• 14.1 Options
• 14.2 Syntax
• 14.2.1 Special Characters
• 14.2.2 Register Names
• 14.2.3 Addressing Modes
• 14.3 Floating Point
• 14.4 H8/500 Machine Directives
• 14.5 Opcodes
• 15 HPPA Dependent Features
• 15.1 Notes
• 15.2 Options
• 15.3 Syntax
• 15.4 Floating Point
• 15.5 HPPA Assembler Directives
• 15.6 Opcodes
• 16 80386 Dependent Features
• 16.1 Options
• 16.2 AT&T Syntax versus Intel Syntax
• 16.3 Opcode Naming
• 16.4 Register Naming
• 16.5 Opcode Prefixes
• 16.6 Memory References
• 16.7 Handling of Jump Instructions
• 16.8 Floating Point
• 16.9 Writing 16-bit Code
• 16.10 Notes
• 17 Intel 80960 Dependent Features
• 17.1 i960 Command-line Options
• 17.2 Floating Point
• 17.3 i960 Machine Directives
• 17.4 i960 Opcodes
• 17.4.1 callj
• 17.4.2 Compare-and-Branch
• 18 M680x0 Dependent Features
• 18.1 M680x0 Options
• 18.2 Syntax
• 18.3 Motorola Syntax
• 18.4 Floating Point
• 18.5 680x0 Machine Directives
• 18.6 Opcodes
• 18.6.1 Branch Improvement
• 18.6.2 Special Characters
• 19 MIPS Dependent Features
• 19.1 Assembler options
• 19.2 MIPS ECOFF object code
• 19.3 Directives for debugging information
• 19.4 Directives to override the ISA level
• 19.5 Directives for extending MIPS 16 bit instructions
• 19.6 Directive to mark data as an instruction
• 19.7 Directives to save and restore options
• 20 Hitachi SH Dependent Features
• 20.1 Options
• 20.2 Syntax
• 20.2.1 Special Characters
• 20.2.2 Register Names
• 20.2.3 Addressing Modes
• 20.3 Floating Point
• 20.4 SH Machine Directives
• 20.5 Opcodes
• 21 SPARC Dependent Features
• 21.1 Options
• 21.2 Enforcing aligned data
• 21.3 Floating Point
• 21.4 Sparc Machine Directives
• 22 Z8000 Dependent Features
• 22.1 Options
• 22.2 Syntax
• 22.2.1 Special Characters
• 22.2.2 Register Names
• 22.2.3 Addressing Modes
• 22.3 Assembler Directives for the Z8000
• 22.4 Opcodes
• 23 VAX Dependent Features
• 23.1 VAX Command-Line Options
• 23.2 VAX Floating Point
• 23.3 Vax Machine Directives
• 23.4 VAX Opcodes
• 23.5 VAX Branch Improvement
• 23.6 VAX Operands
• 23.7 Not Supported on VAX
• 24 v850 Dependent Features
• 24.1 Options
• 24.2 Syntax
• 24.2.1 Special Characters
• 24.2.2 Register Names
• 24.3 Floating Point
• 24.4 V850 Machine Directives
• 24.5 Opcodes
• 25 Reporting Bugs
• 25.1 Have you found a bug?
• 25.2 How to report bugs
• 26 Acknowledgements
• Index

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, 97, 1998 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.

1 Overview

Here is a brief summary of how to invoke as. For details, see section 2 Command-Line Options.

as [ -a[cdhlns][=file] ] [ -D ]  [ --defsym sym=val ]
 [ -f ] [ --gstabs ] [ --help ] [ -I dir ] [ -J ] [ -K ] [ -L ]
 [ --keep-locals ] [ -o objfile ] [ -R ] [ --statistics ] [ -v ]
 [ -version ] [ --version ] [ -W ] [ -w ] [ -x ] [ -Z ]
 [ -mbig-endian | -mlittle-endian ]
 [ -m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]7[t][[d]m[i]] ]
 [ -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t ]
 [ -mthumb | -mall ]
 [ -mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu ]
 [ -EB | -EL ]
 [ -mapcs-32 | -mapcs-26 ]
 [ -O ]
 [ -Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
   -Av8plus | -Av8plusa | -Av9 | -Av9a ]
 [ -xarch=v8plus | -xarch=v8plusa ] [ -bump ] [ -32 | -64 ]
 [ -ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC ]
 [ -b ] [ -no-relax ]
 [ -l ] [ -m68000 | -m68010 | -m68020 | ... ]
 [ -nocpp ] [ -EL ] [ -EB ] [ -G num ] [ -mcpu=CPU ]
 [ -mips1 ] [ -mips2 ] [ -mips3 ] [ -m4650 ] [ -no-m4650 ]
 [ --trap ] [ --break ]
 [ --emulation=name ]
 [ -- | files ... ]

-a[cdhlmns]

Turn on listings, in any of a variety of ways:

-ac

omit false conditionals

-ad

omit debugging directives

-ah

include high-level source

-al

include assembly

-am

include macro expansions

-an

omit forms processing

-as

include symbols

=file

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'.

-D

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.

-f

"fast"---skip whitespace and comment preprocessing (assume source is compiler output).

--gstabs

Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.

--help

Print a summary of the command line options and exit.

-I dir

Add directory dir to the search list for .include directives.

-J

Don't warn about signed overflow.

-K

Issue warnings when difference tables altered for long displacements.

-L

--keep-locals

Keep (in the symbol table) local symbols. On traditional a.out systems these start with `L', but different systems have different local label prefixes.

-o objfile

Name the object-file output from as objfile.

-R

Fold the data section into the text section.

--statistics

Print the maximum space (in bytes) and total time (in seconds) used by assembly.

--strip-local-absolute

Remove local absolute symbols from the outgoing symbol table.

-v

-version

Print the as version.

--version

Print the as version and exit.

-W

Suppress warning messages.

-w

Ignored.

-x

Ignored.

-Z

Generate an object file even after errors.

-- | files ...

Standard input, or source files to assemble.

-mbig-endian

Generate "big endian" format output. 

-mlittle-endian

Generate "little endian" format output.

-m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]7[t][[d]m] | -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t

Specify which variant of the ARM architecture is the target.

-mthumb | -mall

Enable or disable Thumb only instruction decoding.

-mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu

Select which Floating Point architcture is the target.

-mapcs-32 | -mapcs-26

Select which procedure calling convention is in use.

-EB | -EL

Select either big-endian (-EB) or little-endian (-EL) output.

-O

Optimize output by parallelizing instructions.

-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC

Specify which variant of the 960 architecture is the target.

-b

Add code to collect statistics about branches taken.

-no-relax

Do not alter compare-and-branch instructions for long displacements; error if necessary.

-l

Shorten references to undefined symbols, to one word instead of two.

-m68000 | -m68008 | -m68010 | -m68020 | -m68030 | -m68040 | -m68060

| -m68302 | -m68331 | -m68332 | -m68333 | -m68340 | -mcpu32 | -m5200

Specify what processor in the 68000 family is the target. The default is normally the 68020, but this can be changed at configuration time.

-m68881 | -m68882 | -mno-68881 | -mno-68882

The target machine does (or does not) have a floating-point coprocessor. The default is to assume a coprocessor for 68020, 68030, and cpu32. Although the basic 68000 is not compatible with the 68881, a combination of the two can be specified, since it's possible to do emulation of the coprocessor instructions with the main processor.

-m68851 | -mno-68851

The target machine does (or does not) have a memory-management unit coprocessor. The default is to assume an MMU for 68020 and up.

-Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite

-Av8plus | -Av8plusa | -Av9 | -Av9a

Explicitly select a variant of the SPARC architecture. `-Av8plus' and `-Av8plusa' select a 32 bit environment. `-Av9' and `-Av9a' select a 64 bit environment. `-Av8plusa' and `-Av9a' enable the SPARC V9 instruction set with UltraSPARC extensions.

-xarch=v8plus | -xarch=v8plusa

For compatibility with the Solaris v9 assembler. These options are equivalent to -Av8plus and -Av8plusa, respectively.

-bump

Warn when the assembler switches to another architecture.

-G num

This option sets the largest size of an object that can be referenced implicitly with the gp register. It is only accepted for targets that use ECOFF format, such as a DECstation running Ultrix. The default value is 8. 

-EB

Generate "big endian" format output. 

-EL

Generate "little endian" format output. 

-mips1

-mips2

-mips3

Generate code for a particular MIPS Instruction Set Architecture level. `-mips1' corresponds to the R2000 and R3000 processors, `-mips2' to the R6000 processor, and `-mips3' to the R4000 processor.

-m4650

-no-m4650

Generate code for the MIPS R4650 chip. This tells the assembler to accept the `mad' and `madu' instruction, and to not schedule `nop' instructions around accesses to the `HI' and `LO' registers. `-no-m4650' turns off this option.

-mcpu=CPU

Generate code for a particular MIPS cpu. This has little effect on the assembler, but it is passed by gcc. 

--emulation=name

This option causes as to emulate as configured for some other target, in all respects, including output format (choosing between ELF and ECOFF only), handling of pseudo-opcodes which may generate debugging information or store symbol table information, and default endianness. The available configuration names are: `mipsecoff', `mipself', `mipslecoff', `mipsbecoff', `mipslelf', `mipsbelf'. The first two do not alter the default endianness from that of the primary target for which the assembler was configured; the others change the default to little- or big-endian as indicated by the `b' or `l' in the name. Using `-EB' or `-EL' will override the endianness selection in any case. This option is currently supported only when the primary target as is configured for is a MIPS ELF or ECOFF target. Furthermore, the primary target or others specified with `--enable-targets=...' at configuration time must include support for the other format, if both are to be available. For example, the Irix 5 configuration includes support for both. Eventually, this option will support more configurations, with more fine-grained control over the assembler's behavior, and will be supported for more processors.

-nocpp

as ignores this option. It is accepted for compatibility with the native tools.

--trap

--no-trap

--break

--no-break

Control how to deal with multiplication overflow and division by zero. `--trap' or `--no-break' (which are synonyms) take a trap exception (and only work for Instruction Set Architecture level 2 and higher); `--break' or `--no-trap' (also synonyms, and the default) take a break exception.

1.1 Structure of this Manual

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

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 section 8 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 section 7.45 .org new-lc , fill).

1.3 Object File Formats

1.4 Command Line

`--' (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

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.

1.5.1 Filenames and Line-numbers

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

Logical files are simply names declared explicitly by assembler directives; they bear no relation to physical files. Logical file names help error messages reflect the original source file, when as source is itself synthesized from other files. See section 7.4 .app-file string.

1.6 Output (Object) File

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

Warning messages have the format

file_name:NNN:Warning Message Text

Error messages have the format

file_name:NNN:FATAL:Error Message Text

2 Command-Line Options

If you are invoking as via the GNU C compiler (version 2), you can use the `-Wa' option to pass arguments through to the assembler. The assembler arguments must be separated from each other (and the `-Wa') by commas. For example:

gcc -c -g -O -Wa,-alh,-L file.c

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.)

2.1 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'.

2.2 -D

2.3 Work Faster: -f

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

2.4 .include search path: -I path

2.5 Difference Tables: -K

as sometimes alters the code emitted for directives of the form `.word sym1-sym2'; see section 7.69 .word expressions. You can use the `-K' option if you want a warning issued when this is done.

2.6 Include Local Labels: -L

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

By default, a local label is any label beginning with `L', but each target is allowed to redefine the local label prefix. On the HPPA local labels begin with `L$'. `;' for the ARM family;

2.7 Assemble in MRI Compatibility Mode: -M

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.

• 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.
• .debug pseudo-op The i960 .debug pseudo-op is not supported.
• .extended pseudo-op The i960 .extended pseudo-op is not supported.
• .list pseudo-op. The various options of the i960 .list pseudo-op are not supported.
• .optimize pseudo-op The i960 .optimize pseudo-op is not supported.
• .output pseudo-op The i960 .output pseudo-op is not supported.
• .setreal pseudo-op The i960 .setreal pseudo-op is not supported.

2.8 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.

2.9 Name the Object File: -o

Whatever the object file is called, as overwrites any existing file of the same name.

2.10 Join Data and Text Sections: -R

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 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.

2.11 Display Assembly Statistics: --statistics

2.12 Compatible output: --traditional-format

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

2.13 Announce Version: -v

2.14 Suppress Warnings: -W

2.15 Generate Object File in Spite of Errors: -Z

3 Syntax

3.1 Preprocessing

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

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

3.3 Comments

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. */

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

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

`--';

To be compatible with past assemblers, lines that begin with `#' have a special interpretation. Following the `#' should be an absolute expression (see section 6 Expressions): the logical line number of the next line. Then a string (see section 3.6.1.1 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.

3.4 Symbols

3.5 Statements

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

You may write a statement on more than one line if you put a backslash (\) immediately in front of any newlines within the statement. When as reads a backslashed newline both characters are ignored. You can even put backslashed newlines in the middle of symbol names without changing the meaning of your source program.

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

A statement begins with zero or more labels, optionally followed by a key symbol which determines what kind of statement it is. The key symbol determines the syntax of the rest of the statement. If the symbol begins with a dot `.' then the statement is an assembler directive: typically valid for any computer. If the symbol begins with a letter the statement is an assembly language instruction: it 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 section 5.1 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

.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.

3.6.1 Character Constants

3.6.1.1 Strings

\b

Mnemonic for backspace; for ASCII this is octal code 010. 

\f

Mnemonic for FormFeed; for ASCII this is octal code 014. 

\n

Mnemonic for newline; for ASCII this is octal code 012. 

\r

Mnemonic for carriage-Return; for ASCII this is octal code 015. 

\t

Mnemonic for horizontal Tab; for ASCII this is octal code 011. 

\ digit digit digit

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

\x hex-digits...

A hex character code. All trailing hex digits are combined. Either upper or lower case x works. 

\\

Represents one `\' character. 

\"

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

\ anything-else

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

3.6.1.2 Characters

3.6.2 Number Constants

3.6.2.1 Integers

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

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

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

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

Integers have the usual values. To denote a negative integer, use the prefix operator `-' discussed under expressions (see section 6.2.3 Prefix Operator).

3.6.2.2 Bignums

3.6.2.3 Flonums

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

A flonum is written by writing (in order)

• The digit `0'. (`0' is optional on the HPPA.)
• A letter, to tell as the rest of the number is a flonum. e is recommended. Case is not important. On the H8/300, H8/500, Hitachi SH, and AMD 29K architectures, the letter must be one of the letters `DFPRSX' (in upper or lower case). On the ARC, the letter must be one of the letters `DFRS' (in upper or lower case). On the Intel 960 architecture, the letter must be one of the letters `DFT' (in upper or lower case). On the HPPA architecture, the letter must be `E' (upper case only).
• An optional sign: either `+' or `-'.
• An optional integer part: zero or more decimal digits.
• An optional fractional part: `.' followed by zero or more decimal digits.
• An optional exponent, consisting of:
• An `E' or `e'.
• Optional sign: either `+' or `-'.
• One or more decimal digits.

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

4 Sections and Relocation

4.1 Background

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

ld moves blocks of bytes of your program to their run-time addresses. These blocks slide to their run-time addresses as rigid units; their length does not change and neither does the order of bytes within them. Such a rigid unit is called a section. Assigning run-time addresses to sections is called relocation. It includes the task of adjusting mentions of object-file addresses so they refer to the proper run-time addresses. For the H8/300 and H8/500, and for the Hitachi 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 output, as can also generate whatever other named sections you specify using the `.section' directive (see section 7.52 .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.

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"?

(section) + (offset into section)

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

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 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 unitialized variables or common storage. The length of each partial program's bss section is important, but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in the object file. The bss section was invented to eliminate those explicit zeros from object files. 

absolute section

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

undefined section

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

4.3 Assembler Internal Sections

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

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'. Expression should be an absolute expression. (See section 6 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 (*)."

4.5 bss Section

The .lcomm pseudo-op defines a symbol in the bss section; see section 7.34 .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 7.9 .comm symbol , length .

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 7.52 .section name. You may only assemble zero values into the section. Typically the section will only contain symbol definitions and .skip directives (see section 7.58 .skip size , fill).

5 Symbols

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

5.1 Labels

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

5.3 Symbol Names

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.

5.3.1 Local Symbol Names

Local symbols are not emitted by the current GNU C compiler.

There is no restriction on how you can use these labels, but remember that at any point in the assembly you can refer to at most 10 prior local labels and to at most 10 forward local labels.

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

L

All local labels begin with `L'. Normally both as and ld forget symbols that start with `L'. These labels are used for symbols you are never intended to see. If you 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.

digit

If the label is written `0:' then the digit is `0'. If the label is written `1:' then the digit is `1'. And so on up through `9:'.

C-A

This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value `\001'.

ordinal number

This is a serial number to keep the labels distinct. The first `0:' gets the number `1'; The 15th `0:' gets the number `15'; etc.. Likewise for the other labels `1:' through `9:'.

5.4 The Special Dot Symbol

5.5 Symbol 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.

5.5.1 Value

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

5.5.2 Type

5.5.3 Symbol Attributes: a.out

5.5.3.1 Descriptor

5.5.3.2 Other

5.5.4 Symbol Attributes for COFF

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

5.5.4.1 Primary Attributes

5.5.4.2 Auxiliary Attributes

5.5.5 Symbol Attributes for SOM

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

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

6 Expressions

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

6.1 Empty Expressions

6.2 Integer Expressions

6.2.1 Arguments

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

Numbers are usually integers.

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

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

6.2.2 Operators

6.2.3 Prefix Operator

-

Negation. Two's complement negation.

~

Complementation. Bitwise not.

6.2.4 Infix Operators

1. Highest Precedence

*

Multiplication.

/

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

%

Remainder.

<

<<

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

>

>>

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

2. Intermediate precedence

|

Bitwise Inclusive Or.

&

Bitwise And.

^

Bitwise Exclusive Or.

!

Bitwise Or Not.

3. Lowest Precedence

+

Addition. If either argument is absolute, the result has the section of the other argument. 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.

7 Assembler Directives

This chapter discusses directives that are available regardless of the target machine configuration for the GNU assembler. Some machine configurations provide additional directives. See section 8 Machine Dependent Features.

7.1 .abort

7.2 .ABORT

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

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

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).

7.4 .app-file string

7.5 .ascii "string"...

7.6 .asciz "string"...

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

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.

7.8 .byte expressions

7.9 .comm symbol , length