This is nasm.info, produced by makeinfo version 4.11 from nasmdoc.texi.

INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* NASM: (nasm).                The Netwide Assembler for x86.
END-INFO-DIR-ENTRY

   This file documents NASM, the Netwide Assembler: an assembler
targetting the Intel x86 series of processors, with portable source.

   Copyright 2008 The NASM Development Team

   All rights reserved. This document is redistributable under the
license given in the file "COPYING" distributed in the NASM archive.


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The Netwide Assembler for x86
*****************************

This file documents NASM, the Netwide Assembler: an assembler
targetting the Intel x86 series of processors, with portable source.

* Menu:

* Chapter 1:: Introduction
* Chapter 2:: Running NASM
* Chapter 3:: The NASM Language
* Chapter 4:: The NASM Preprocessor
* Chapter 5:: Standard Macro Packages
* Chapter 6:: Assembler Directives
* Chapter 7:: Output Formats
* Chapter 8:: Writing 16-bit Code (DOS, Windows 3/3.1)
* Chapter 9:: Writing 32-bit Code (Unix, Win32, DJGPP)
* Chapter 10:: Mixing 16 and 32 Bit Code
* Chapter 11:: Writing 64-bit Code (Unix, Win64)
* Chapter 12:: Troubleshooting
* Appendix A:: Ndisasm
* Appendix B:: Instruction List
* Appendix C:: NASM Version History
* Index::


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Chapter 1: Introduction
***********************

* Menu:

* Section 1.1:: What Is NASM?
* Section 1.2:: Contact Information
* Section 1.3:: Installation


File: nasm.info,  Node: Section 1.1,  Next: Section 1.1.1,  Prev: Chapter 1,  Up: Chapter 1

1.1. What Is NASM?
==================

The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed
for portability and modularity. It supports a range of object file
formats, including Linux and `*BSD' `a.out', `ELF', `COFF', `Mach-O',
Microsoft 16-bit `OBJ', `Win32' and `Win64'.  It will also output plain
binary files. Its syntax is designed to be simple and easy to
understand, similar to Intel's but less complex. It supports all
currently known x86 architectural extensions, and has strong support
for macros.

* Menu:

* Section 1.1.1:: Why Yet Another Assembler?
* Section 1.1.2:: License Conditions


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1.1.1. Why Yet Another Assembler?
---------------------------------

The Netwide Assembler grew out of an idea on `comp.lang.asm.x86' (or
possibly `alt.lang.asm' - I forget which), which was essentially that
there didn't seem to be a good _free_ x86-series assembler around, and
that maybe someone ought to write one.

   * `a86' is good, but not free, and in particular you don't get any
     32- bit capability until you pay. It's DOS only, too.

   * `gas' is free, and ports over to DOS and Unix, but it's not very
     good, since it's designed to be a back end to `gcc', which always
     feeds it correct code. So its error checking is minimal. Also, its
     syntax is horrible, from the point of view of anyone trying to
     actually _write_ anything in it. Plus you can't write 16-bit code
     in it (properly.)

   * `as86' is specific to Minix and Linux, and (my version at least)
     doesn't seem to have much (or any) documentation.

   * `MASM' isn't very good, and it's (was) expensive, and it runs only
     under DOS.

   * `TASM' is better, but still strives for MASM compatibility, which
     means millions of directives and tons of red tape. And its syntax
     is essentially MASM's, with the contradictions and quirks that
     entails (although it sorts out some of those by means of Ideal
     mode.) It's expensive too. And it's DOS-only.

   So here, for your coding pleasure, is NASM. At present it's still in
prototype stage - we don't promise that it can outperform any of these
assemblers. But please, _please_ send us bug reports, fixes, helpful
information, and anything else you can get your hands on (and thanks to
the many people who've done this already! You all know who you are),
and we'll improve it out of all recognition. Again.


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1.1.2. License Conditions
-------------------------

Please see the file `COPYING', supplied as part of any NASM
distribution archive, for the license conditions under which you may use
NASM. NASM is now under the so-called GNU Lesser General Public License,
LGPL.


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1.2. Contact Information
========================

The current version of NASM (since about 0.98.08) is maintained by a
team of developers, accessible through the `nasm-devel' mailing list
(see below for the link). If you want to report a bug, please read
*note Section 12.2:: first.

   NASM has a WWW page at `http://nasm.sourceforge.net'. If it's not
there, google for us!

   The original authors are e-mailable as `jules@dsf.org.uk' and
`anakin@pobox.com'. The latter is no longer involved in the development
team.

   New releases of NASM are uploaded to the official sites
`http://nasm.sourceforge.net' and to `ftp.kernel.org' and `ibiblio.org'.

   Announcements are posted to `comp.lang.asm.x86', `alt.lang.asm' and
`comp.os.linux.announce'

   If you want information about NASM beta releases, and the current
development status, please subscribe to the `nasm-devel' email list by
registering at `http://sourceforge.net/projects/nasm'.


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1.3. Installation
=================

* Menu:

* Section 1.3.1:: Installing NASM under MS-DOS or Windows
* Section 1.3.2:: Installing NASM under Unix


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1.3.1. Installing NASM under MS-DOS or Windows
----------------------------------------------

Once you've obtained the appropriate archive for NASM,
`nasm-XXX-dos.zip' or `nasm-XXX-win32.zip' (where `XXX' denotes the
version number of NASM contained in the archive), unpack it into its
own directory (for example `c:\nasm').

   The archive will contain a set of executable files: the NASM
executable file `nasm.exe', the NDISASM executable file `ndisasm.exe',
and possibly additional utilities to handle the RDOFF file format.

   The only file NASM needs to run is its own executable, so copy
`nasm.exe' to a directory on your PATH, or alternatively edit
`autoexec.bat' to add the `nasm' directory to your `PATH' (to do that
under Windows XP, go to Start > Control Panel > System > Advanced >
Environment Variables; these instructions may work under other versions
of Windows as well.)

   That's it - NASM is installed. You don't need the nasm directory to
be present to run NASM (unless you've added it to your `PATH'), so you
can delete it if you need to save space; however, you may want to keep
the documentation or test programs.

   If you've downloaded the DOS source archive, `nasm-XXX.zip', the
`nasm' directory will also contain the full NASM source code, and a
selection of Makefiles you can (hopefully) use to rebuild your copy of
NASM from scratch. See the file `INSTALL' in the source archive.

   Note that a number of files are generated from other files by Perl
scripts.  Although the NASM source distribution includes these
generated files, you will need to rebuild them (and hence, will need a
Perl interpreter) if you change insns.dat, standard.mac or the
documentation. It is possible future source distributions may not
include these files at all. Ports of Perl for a variety of platforms,
including DOS and Windows, are available from www.cpan.org.


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1.3.2. Installing NASM under Unix
---------------------------------

Once you've obtained the Unix source archive for NASM,
`nasm-XXX.tar.gz' (where `XXX' denotes the version number of NASM
contained in the archive), unpack it into a directory such as
`/usr/local/src'. The archive, when unpacked, will create its own
subdirectory `nasm-XXX'.

   NASM is an auto-configuring package: once you've unpacked it, `cd' to
the directory it's been unpacked into and type `./configure'. This
shell script will find the best C compiler to use for building NASM and
set up Makefiles accordingly.

   Once NASM has auto-configured, you can type `make' to build the
`nasm' and `ndisasm' binaries, and then `make install' to install them
in `/usr/local/bin' and install the man pages `nasm.1' and `ndisasm.1'
in `/usr/local/man/man1'.  Alternatively, you can give options such as
`--prefix' to the configure script (see the file `INSTALL' for more
details), or install the programs yourself.

   NASM also comes with a set of utilities for handling the `RDOFF'
custom object-file format, which are in the `rdoff' subdirectory of the
NASM archive. You can build these with `make rdf' and install them with
`make rdf_install', if you want them.


File: nasm.info,  Node: Chapter 2,  Next: Section 2.1,  Prev: Section 1.3.2,  Up: Top

Chapter 2: Running NASM
***********************

* Menu:

* Section 2.1:: NASM Command-Line Syntax
* Section 2.2:: Quick Start for MASM Users


File: nasm.info,  Node: Section 2.1,  Next: Section 2.1.1,  Prev: Chapter 2,  Up: Chapter 2

2.1. NASM Command-Line Syntax
=============================

To assemble a file, you issue a command of the form

     nasm -f <format> <filename> [-o <output>]

   For example,

     nasm -f elf myfile.asm

   will assemble `myfile.asm' into an `ELF' object file `myfile.o'. And

     nasm -f bin myfile.asm -o myfile.com

   will assemble `myfile.asm' into a raw binary file `myfile.com'.

   To produce a listing file, with the hex codes output from NASM
displayed on the left of the original sources, use the `-l' option to
give a listing file name, for example:

     nasm -f coff myfile.asm -l myfile.lst

   To get further usage instructions from NASM, try typing

     nasm -h

   As `-hf', this will also list the available output file formats, and
what they are.

   If you use Linux but aren't sure whether your system is `a.out' or
`ELF', type

     file nasm

   (in the directory in which you put the NASM binary when you
installed it).  If it says something like

     nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1

   then your system is `ELF', and you should use the option `-f elf'
when you want NASM to produce Linux object files. If it says

     nasm: Linux/i386 demand-paged executable (QMAGIC)

   or something similar, your system is `a.out', and you should use `-f
aout' instead (Linux `a.out' systems have long been obsolete, and are
rare these days.)

   Like Unix compilers and assemblers, NASM is silent unless it goes
wrong: you won't see any output at all, unless it gives error messages.

* Menu:

* Section 2.1.1:: The `-o' Option: Specifying the Output File Name
* Section 2.1.2:: The `-f' Option: Specifying the Output File Format
* Section 2.1.3:: The `-l' Option: Generating a Listing File
* Section 2.1.4:: The `-M' Option: Generate Makefile Dependencies
* Section 2.1.5:: The `-MG' Option: Generate Makefile Dependencies
* Section 2.1.6:: The `-MF' Option: Set Makefile Dependency File
* Section 2.1.7:: The `-MD' Option: Assemble and Generate Dependencies
* Section 2.1.8:: The `-MT' Option: Dependency Target Name
* Section 2.1.9:: The `-MQ' Option: Dependency Target Name (Quoted)
* Section 2.1.10:: The `-MP' Option: Emit phony targets
* Section 2.1.11:: The `-F' Option: Selecting a Debug Information Format
* Section 2.1.12:: The `-g' Option: Enabling Debug Information.
* Section 2.1.13:: The `-X' Option: Selecting an Error Reporting Format
* Section 2.1.14:: The `-Z' Option: Send Errors to a File
* Section 2.1.15:: The `-s' Option: Send Errors to `stdout'
* Section 2.1.16:: The `-i' Option: Include File Search Directories
* Section 2.1.17:: The `-p' Option: Pre-Include a File
* Section 2.1.18:: The `-d' Option: Pre-Define a Macro
* Section 2.1.19:: The `-u' Option: Undefine a Macro
* Section 2.1.20:: The `-E' Option: Preprocess Only
* Section 2.1.21:: The `-a' Option: Don't Preprocess At All
* Section 2.1.22:: The `-O' Option: Specifying Multipass Optimization
* Section 2.1.23:: The `-t' option: Enable TASM Compatibility Mode
* Section 2.1.24:: The `-w' Option: Enable or Disable Assembly Warnings
* Section 2.1.25:: The `-v' Option: Display Version Info
* Section 2.1.26:: The `-y' Option: Display Available Debug Info Formats
* Section 2.1.27:: The `--prefix' and `--postfix' Options.
* Section 2.1.28:: The `NASMENV' Environment Variable


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2.1.1. The `-o' Option: Specifying the Output File Name
-------------------------------------------------------

NASM will normally choose the name of your output file for you;
precisely how it does this is dependent on the object file format. For
Microsoft object file formats (`obj' and `win32'), it will remove the
`.asm' extension (or whatever extension you like to use - NASM doesn't
care) from your source file name and substitute `.obj'. For Unix object
file formats (`aout', `coff', `elf', `macho' and `as86') it will
substitute `.o'. For `rdf', it will use `.rdf', and for the `bin'
format it will simply remove the extension, so that `myfile.asm'
produces the output file `myfile'.

   If the output file already exists, NASM will overwrite it, unless it
has the same name as the input file, in which case it will give a
warning and use `nasm.out' as the output file name instead.

   For situations in which this behaviour is unacceptable, NASM
provides the `-o' command-line option, which allows you to specify your
desired output file name. You invoke `-o' by following it with the name
you wish for the output file, either with or without an intervening
space. For example:

     nasm -f bin program.asm -o program.com
     nasm -f bin driver.asm -odriver.sys

   Note that this is a small o, and is different from a capital O ,
which is used to specify the number of optimisation passes required. See
*note Section 2.1.22::.


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2.1.2. The `-f' Option: Specifying the Output File Format
---------------------------------------------------------

If you do not supply the `-f' option to NASM, it will choose an output
file format for you itself. In the distribution versions of NASM, the
default is always `bin'; if you've compiled your own copy of NASM, you
can redefine `OF_DEFAULT' at compile time and choose what you want the
default to be.

   Like `-o', the intervening space between `-f' and the output file
format is optional; so `-f elf' and `-felf' are both valid.

   A complete list of the available output file formats can be given by
issuing the command `nasm -hf'.


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2.1.3. The `-l' Option: Generating a Listing File
-------------------------------------------------

If you supply the `-l' option to NASM, followed (with the usual
optional space) by a file name, NASM will generate a source-listing file
for you, in which addresses and generated code are listed on the left,
and the actual source code, with expansions of multi-line macros
(except those which specifically request no expansion in source
listings: see *note Section 4.3.9::) on the right. For example:

     nasm -f elf myfile.asm -l myfile.lst

   If a list file is selected, you may turn off listing for a section
of your source with `[list -]', and turn it back on with `[list +]',
(the default, obviously). There is no "user form" (without the
brackets). This can be used to list only sections of interest, avoiding
excessively long listings.


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2.1.4. The `-M' Option: Generate Makefile Dependencies
------------------------------------------------------

This option can be used to generate makefile dependencies on stdout.
This can be redirected to a file for further processing. For example:

     nasm -M myfile.asm > myfile.dep


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2.1.5. The `-MG' Option: Generate Makefile Dependencies
-------------------------------------------------------

This option can be used to generate makefile dependencies on stdout.
This differs from the `-M' option in that if a nonexisting file is
encountered, it is assumed to be a generated file and is added to the
dependency list without a prefix.


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2.1.6. The `-MF' Option: Set Makefile Dependency File
-----------------------------------------------------

This option can be used with the `-M' or `-MG' options to send the
output to a file, rather than to stdout. For example:

     nasm -M -MF myfile.dep myfile.asm


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2.1.7. The `-MD' Option: Assemble and Generate Dependencies
-----------------------------------------------------------

The `-MD' option acts as the combination of the `-M' and `-MF' options
(i.e. a filename has to be specified.) However, unlike the `-M' or
`-MG' options, `-MD' does _not_ inhibit the normal operation of the
assembler. Use this to automatically generate updated dependencies with
every assembly session. For example:

     nasm -f elf -o myfile.o -MD myfile.dep myfile.asm


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2.1.8. The `-MT' Option: Dependency Target Name
-----------------------------------------------

The `-MT' option can be used to override the default name of the
dependency target. This is normally the same as the output filename,
specified by the `-o' option.


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2.1.9. The `-MQ' Option: Dependency Target Name (Quoted)
--------------------------------------------------------

The `-MQ' option acts as the `-MT' option, except it tries to quote
characters that have special meaning in Makefile syntax. This is not
foolproof, as not all characters with special meaning are quotable in
Make.


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2.1.10. The `-MP' Option: Emit phony targets
--------------------------------------------

When used with any of the dependency generation options, the `-MP'
option causes NASM to emit a phony target without dependencies for each
header file. This prevents Make from complaining if a header file has
been removed.


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2.1.11. The `-F' Option: Selecting a Debug Information Format
-------------------------------------------------------------

This option is used to select the format of the debug information
emitted into the output file, to be used by a debugger (or _will_ be).
Prior to version 2.03.01, the use of this switch did _not_ enable
output of the selected debug info format. Use `-g', see *note Section
2.1.12::, to enable output. Versions 2.03.01 and later automatically
enable `-g' if `-F' is specified.

   A complete list of the available debug file formats for an output
format can be seen by issuing the command `nasm -f <format> -y'. Not all
output formats currently support debugging output. See *note Section
2.1.26::.

   This should not be confused with the `-f dbg' output format option
which is not built into NASM by default. For information on how to
enable it when building from the sources, see *note Section 7.12::.


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2.1.12. The `-g' Option: Enabling Debug Information.
----------------------------------------------------

This option can be used to generate debugging information in the
specified format. See *note Section 2.1.11::. Using `-g' without `-F'
results in emitting debug info in the default format, if any, for the
selected output format. If no debug information is currently
implemented in the selected output format, `-g' is _silently ignored_.


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2.1.13. The `-X' Option: Selecting an Error Reporting Format
------------------------------------------------------------

This option can be used to select an error reporting format for any
error messages that might be produced by NASM.

   Currently, two error reporting formats may be selected. They are the
`-Xvc' option and the `-Xgnu' option. The GNU format is the default and
looks like this:

     filename.asm:65: error: specific error message

   where `filename.asm' is the name of the source file in which the
error was detected, `65' is the source file line number on which the
error was detected, `error' is the severity of the error (this could be
`warning'), and `specific error message' is a more detailed text
message which should help pinpoint the exact problem.

   The other format, specified by `-Xvc' is the style used by Microsoft
Visual C++ and some other programs. It looks like this:

     filename.asm(65) : error: specific error message

   where the only difference is that the line number is in parentheses
instead of being delimited by colons.

   See also the `Visual C++' output format, *note Section 7.3::.


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2.1.14. The `-Z' Option: Send Errors to a File
----------------------------------------------

Under `MS-DOS' it can be difficult (though there are ways) to redirect
the standard-error output of a program to a file. Since NASM usually
produces its warning and error messages on `stderr', this can make it
hard to capture the errors if (for example) you want to load them into
an editor.

   NASM therefore provides the `-Z' option, taking a filename argument
which causes errors to be sent to the specified files rather than
standard error. Therefore you can redirect the errors into a file by
typing

     nasm -Z myfile.err -f obj myfile.asm

   In earlier versions of NASM, this option was called `-E', but it was
changed since `-E' is an option conventionally used for preprocessing
only, with disastrous results. See *note Section 2.1.20::.


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2.1.15. The `-s' Option: Send Errors to `stdout'
------------------------------------------------

The `-s' option redirects error messages to `stdout' rather than
`stderr', so it can be redirected under `MS-DOS'. To assemble the file
`myfile.asm' and pipe its output to the `more' program, you can type:

     nasm -s -f obj myfile.asm | more

   See also the `-Z' option, *note Section 2.1.14::.


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2.1.16. The `-i' Option: Include File Search Directories
--------------------------------------------------------

When NASM sees the `%include' or `%pathsearch' directive in a source
file (see *note Section 4.6.1::, *note Section 4.6.2:: or *note Section
3.2.3::), it will search for the given file not only in the current
directory, but also in any directories specified on the command line by
the use of the `-i' option. Therefore you can include files from a macro
library, for example, by typing

     nasm -ic:\macrolib\ -f obj myfile.asm

   (As usual, a space between `-i' and the path name is allowed, and
optional).

   NASM, in the interests of complete source-code portability, does not
understand the file naming conventions of the OS it is running on; the
string you provide as an argument to the `-i' option will be prepended
exactly as written to the name of the include file. Therefore the
trailing backslash in the above example is necessary. Under Unix, a
trailing forward slash is similarly necessary.

   (You can use this to your advantage, if you're really perverse, by
noting that the option `-ifoo' will cause `%include "bar.i"' to search
for the file `foobar.i'...)

   If you want to define a _standard_ include search path, similar to
`/usr/include' on Unix systems, you should place one or more `-i'
directives in the `NASMENV' environment variable (see *note Section
2.1.28::).

   For Makefile compatibility with many C compilers, this option can
also be specified as `-I'.


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2.1.17. The `-p' Option: Pre-Include a File
-------------------------------------------

NASM allows you to specify files to be _pre-included_ into your source
file, by the use of the `-p' option. So running

     nasm myfile.asm -p myinc.inc

   is equivalent to running `nasm myfile.asm' and placing the directive
`%include "myinc.inc"' at the start of the file.

   For consistency with the `-I', `-D' and `-U' options, this option
can also be specified as `-P'.


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2.1.18. The `-d' Option: Pre-Define a Macro
-------------------------------------------

Just as the `-p' option gives an alternative to placing `%include'
directives at the start of a source file, the `-d' option gives an
alternative to placing a `%define' directive. You could code

     nasm myfile.asm -dFOO=100

   as an alternative to placing the directive

     %define FOO 100

   at the start of the file. You can miss off the macro value, as well:
the option `-dFOO' is equivalent to coding `%define FOO'. This form of
the directive may be useful for selecting assembly-time options which
are then tested using `%ifdef', for example `-dDEBUG'.

   For Makefile compatibility with many C compilers, this option can
also be specified as `-D'.


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2.1.19. The `-u' Option: Undefine a Macro
-----------------------------------------

The `-u' option undefines a macro that would otherwise have been pre-
defined, either automatically or by a `-p' or `-d' option specified
earlier on the command lines.

   For example, the following command line:

     nasm myfile.asm -dFOO=100 -uFOO

   would result in `FOO' _not_ being a predefined macro in the program.
This is useful to override options specified at a different point in a
Makefile.

   For Makefile compatibility with many C compilers, this option can
also be specified as `-U'.


File: nasm.info,  Node: Section 2.1.20,  Next: Section 2.1.21,  Prev: Section 2.1.19,  Up: Section 2.1

2.1.20. The `-E' Option: Preprocess Only
----------------------------------------

NASM allows the preprocessor to be run on its own, up to a point. Using
the `-E' option (which requires no arguments) will cause NASM to
preprocess its input file, expand all the macro references, remove all
the comments and preprocessor directives, and print the resulting file
on standard output (or save it to a file, if the `-o' option is also
used).

   This option cannot be applied to programs which require the
preprocessor to evaluate expressions which depend on the values of
symbols: so code such as

     %assign tablesize ($-tablestart)

   will cause an error in preprocess-only mode.

   For compatiblity with older version of NASM, this option can also be
written `-e'. `-E' in older versions of NASM was the equivalent of the
current `-Z' option, *note Section 2.1.14::.


File: nasm.info,  Node: Section 2.1.21,  Next: Section 2.1.22,  Prev: Section 2.1.20,  Up: Section 2.1

2.1.21. The `-a' Option: Don't Preprocess At All
------------------------------------------------

If NASM is being used as the back end to a compiler, it might be
desirable to suppress preprocessing completely and assume the compiler
has already done it, to save time and increase compilation speeds. The
`-a' option, requiring no argument, instructs NASM to replace its
powerful preprocessor with a stub preprocessor which does nothing.


File: nasm.info,  Node: Section 2.1.22,  Next: Section 2.1.23,  Prev: Section 2.1.21,  Up: Section 2.1

2.1.22. The `-O' Option: Specifying Multipass Optimization
----------------------------------------------------------

NASM defaults to not optimizing operands which can fit into a signed
byte.  This means that if you want the shortest possible object code,
you have to enable optimization.

   Using the `-O' option, you can tell NASM to carry out different
levels of optimization. The syntax is:

   * `-O0': No optimization. All operands take their long forms, if a
     short form is not specified, except conditional jumps. This is
     intended to match NASM 0.98 behavior.

   * `-O1': Minimal optimization. As above, but immediate operands which
     will fit in a signed byte are optimized, unless the long form is
     specified.  Conditional jumps default to the long form unless
     otherwise specified.

   * `-Ox' (where `x' is the actual letter `x'): Multipass
     optimization. Minimize branch offsets and signed immediate bytes,
     overriding size specification unless the `strict' keyword has been
     used (see *note Section 3.7::). For compatability with earlier
     releases, the letter `x' may also be any number greater than one.
     This number has no effect on the actual number of passes.

   The `-Ox' mode is recommended for most uses.

   Note that this is a capital `O', and is different from a small `o',
which is used to specify the output file name. See *note Section
2.1.1::.


File: nasm.info,  Node: Section 2.1.23,  Next: Section 2.1.24,  Prev: Section 2.1.22,  Up: Section 2.1

2.1.23. The `-t' option: Enable TASM Compatibility Mode
-------------------------------------------------------

NASM includes a limited form of compatibility with Borland's `TASM'.
When NASM's `-t' option is used, the following changes are made:

   * local labels may be prefixed with `@@' instead of `.'

   * size override is supported within brackets. In TASM compatible
     mode, a size override inside square brackets changes the size of
     the operand, and not the address type of the operand as it does in
     NASM syntax. E.g.  `mov eax,[DWORD val]' is valid syntax in TASM
     compatibility mode. Note that you lose the ability to override the
     default address type for the instruction.

   * unprefixed forms of some directives supported (`arg', `elif',
     `else', `endif', `if', `ifdef', `ifdifi', `ifndef', `include',
     `local')


File: nasm.info,  Node: Section 2.1.24,  Next: Section 2.1.25,  Prev: Section 2.1.23,  Up: Section 2.1

2.1.24. The `-w' Option: Enable or Disable Assembly Warnings
------------------------------------------------------------

NASM can observe many conditions during the course of assembly which are
worth mentioning to the user, but not a sufficiently severe error to
justify NASM refusing to generate an output file. These conditions are
reported like errors, but come up with the word `warning' before the
message. Warnings do not prevent NASM from generating an output file and
returning a success status to the operating system.

   Some conditions are even less severe than that: they are only
sometimes worth mentioning to the user. Therefore NASM supports the `-w'
command-line option, which enables or disables certain classes of
assembly warning. Such warning classes are described by a name, for
example `orphan-labels'; you can enable warnings of this class by the
command- line option `-w+orphan-labels' and disable it by
`-w-orphan-labels'.

   The suppressible warning classes are:

   * `error' decides if warnings should be treated as errors. It is
     disabled by default.

   * `macro-params' covers warnings about multi-line macros being
     invoked with the wrong number of parameters. This warning class is
     enabled by default; see *note Section 4.3.1:: for an example of
     why you might want to disable it.

   * `macro-selfref' warns if a macro references itself. This warning
     class is disabled by default.

   * `macro-defaults' warns when a macro has more default parameters
     than optional parameters. This warning class is enabled by
     default; see *note Section 4.3.4:: for why you might want to
     disable it.

   * `orphan-labels' covers warnings about source lines which contain no
     instruction but define a label without a trailing colon. NASM
     warns about this somewhat obscure condition by default; see *note
     Section 3.1:: for more information.

   * `number-overflow' covers warnings about numeric constants which
     don't fit in 64 bits. This warning class is enabled by default.

   * `gnu-elf-extensions' warns if 8-bit or 16-bit relocations are used
     in `-f elf' format. The GNU extensions allow this. This warning
     class is disabled by default.

   * `float-overflow' warns about floating point overflow. Enabled by
     default.

   * `float-denorm' warns about floating point denormals. Disabled by
     default.

   * `float-underflow' warns about floating point underflow. Disabled by
     default.

   * `float-toolong' warns about too many digits in floating-point
     numbers.  Enabled by default.

   * `user' controls `%warning' directives (see *note Section 4.9::).
     Enabled by default.

   In addition, you can set warning classes across sections. Warning
classes may be enabled with `[warning +warning-name]', disabled with
`[warning -warning-name]' or reset to their original value with
`[warning *warning-name]'. No "user form" (without the brackets) exists.


File: nasm.info,  Node: Section 2.1.25,  Next: Section 2.1.26,  Prev: Section 2.1.24,  Up: Section 2.1

2.1.25. The `-v' Option: Display Version Info
---------------------------------------------

Typing `NASM -v' will display the version of NASM which you are using,
and the date on which it was compiled.

   You will need the version number if you report a bug.


File: nasm.info,  Node: Section 2.1.26,  Next: Section 2.1.27,  Prev: Section 2.1.25,  Up: Section 2.1

2.1.26. The `-y' Option: Display Available Debug Info Formats
-------------------------------------------------------------

Typing `nasm -f <option> -y' will display a list of the available debug
info formats for the given output format. The default format is
indicated by an asterisk. For example:

     nasm -f elf -y

     valid debug formats for 'elf32' output format are
       ('*' denotes default):
       * stabs     ELF32 (i386) stabs debug format for Linux
         dwarf     elf32 (i386) dwarf debug format for Linux


File: nasm.info,  Node: Section 2.1.27,  Next: Section 2.1.28,  Prev: Section 2.1.26,  Up: Section 2.1

2.1.27. The `--prefix' and `--postfix' Options.
-----------------------------------------------

The `--prefix' and `--postfix' options prepend or append (respectively)
the given argument to all `global' or `extern' variables. E.g.
`--prefix _' will prepend the underscore to all global and external
variables, as C sometimes (but not always) likes it.


File: nasm.info,  Node: Section 2.1.28,  Next: Section 2.2,  Prev: Section 2.1.27,  Up: Section 2.1

2.1.28. The `NASMENV' Environment Variable
------------------------------------------

If you define an environment variable called `NASMENV', the program
will interpret it as a list of extra command-line options, which are
processed before the real command line. You can use this to define
standard search directories for include files, by putting `-i' options
in the `NASMENV' variable.

   The value of the variable is split up at white space, so that the
value `-s -ic:\nasmlib' will be treated as two separate options.
However, that means that the value `-dNAME="my name"' won't do what you
might want, because it will be split at the space and the NASM
command-line processing will get confused by the two nonsensical words
`-dNAME="my' and `name"'.

   To get round this, NASM provides a feature whereby, if you begin the
`NASMENV' environment variable with some character that isn't a minus
sign, then NASM will treat this character as the separator character for
options. So setting the `NASMENV' variable to the value
`!-s!-ic:\nasmlib' is equivalent to setting it to `-s -ic:\nasmlib',
but `!-dNAME="my name"' will work.

   This environment variable was previously called `NASM'. This was
changed with version 0.98.31.


File: nasm.info,  Node: Section 2.2,  Next: Section 2.2.1,  Prev: Section 2.1.28,  Up: Chapter 2

2.2. Quick Start for MASM Users
===============================

If you're used to writing programs with MASM, or with TASM in MASM-
compatible (non-Ideal) mode, or with `a86', this section attempts to
outline the major differences between MASM's syntax and NASM's. If
you're not already used to MASM, it's probably worth skipping this
section.

* Menu:

* Section 2.2.1:: NASM Is Case-Sensitive
* Section 2.2.2:: NASM Requires Square Brackets For Memory References
* Section 2.2.3:: NASM Doesn't Store Variable Types
* Section 2.2.4:: NASM Doesn't `ASSUME'
* Section 2.2.5:: NASM Doesn't Support Memory Models
* Section 2.2.6:: Floating-Point Differences
* Section 2.2.7:: Other Differences


File: nasm.info,  Node: Section 2.2.1,  Next: Section 2.2.2,  Prev: Section 2.2,  Up: Section 2.2

2.2.1. NASM Is Case-Sensitive
-----------------------------

One simple difference is that NASM is case-sensitive. It makes a
difference whether you call your label `foo', `Foo' or `FOO'. If you're
assembling to `DOS' or `OS/2' `.OBJ' files, you can invoke the
`UPPERCASE' directive (documented in *note Section 7.2::) to ensure
that all symbols exported to other code modules are forced to be upper
case; but even then, _within_ a single module, NASM will distinguish
between labels differing only in case.


File: nasm.info,  Node: Section 2.2.2,  Next: Section 2.2.3,  Prev: Section 2.2.1,  Up: Section 2.2

2.2.2. NASM Requires Square Brackets For Memory References
----------------------------------------------------------

NASM was designed with simplicity of syntax in mind. One of the design
goals of NASM is that it should be possible, as far as is practical, for
the user to look at a single line of NASM code and tell what opcode is
generated by it. You can't do this in MASM: if you declare, for example,

     foo     equ     1
     bar     dw      2

   then the two lines of code

             mov     ax,foo
             mov     ax,bar

   generate completely different opcodes, despite having
identical-looking syntaxes.

   NASM avoids this undesirable situation by having a much simpler
syntax for memory references. The rule is simply that any access to the
_contents_ of a memory location requires square brackets around the
address, and any access to the _address_ of a variable doesn't. So an
instruction of the form `mov ax,foo' will _always_ refer to a
compile-time constant, whether it's an `EQU' or the address of a
variable; and to access the _contents_ of the variable `bar', you must
code `mov ax,[bar]'.

   This also means that NASM has no need for MASM's `OFFSET' keyword,
since the MASM code `mov ax,offset bar' means exactly the same thing as
NASM's `mov ax,bar'. If you're trying to get large amounts of MASM code
to assemble sensibly under NASM, you can always code `%idefine offset'
to make the preprocessor treat the `OFFSET' keyword as a no-op.

   This issue is even more confusing in `a86', where declaring a label
with a trailing colon defines it to be a `label' as opposed to a
`variable' and causes `a86' to adopt NASM-style semantics; so in `a86',
`mov ax,var' has different behaviour depending on whether `var' was
declared as `var: dw 0' (a label) or `var dw 0' (a word-size variable).
NASM is very simple by comparison: _everything_ is a label.

   NASM, in the interests of simplicity, also does not support the
hybrid syntaxes supported by MASM and its clones, such as `mov
ax,table[bx]', where a memory reference is denoted by one portion
outside square brackets and another portion inside. The correct syntax
for the above is `mov ax,[table+bx]'. Likewise, `mov ax,es:[di]' is
wrong and `mov ax,[es:di]' is right.


File: nasm.info,  Node: Section 2.2.3,  Next: Section 2.2.4,  Prev: Section 2.2.2,  Up: Section 2.2

2.2.3. NASM Doesn't Store Variable Types
----------------------------------------

NASM, by design, chooses not to remember the types of variables you
declare. Whereas MASM will remember, on seeing `var dw 0', that you
declared `var' as a word-size variable, and will then be able to fill
in the ambiguity in the size of the instruction `mov var,2', NASM will
deliberately remember nothing about the symbol `var' except where it
begins, and so you must explicitly code `mov word [var],2'.

   For this reason, NASM doesn't support the `LODS', `MOVS', `STOS',
`SCAS', `CMPS', `INS', or `OUTS' instructions, but only supports the
forms such as `LODSB', `MOVSW', and `SCASD', which explicitly specify
the size of the components of the strings being manipulated.


File: nasm.info,  Node: Section 2.2.4,  Next: Section 2.2.5,  Prev: Section 2.2.3,  Up: Section 2.2

2.2.4. NASM Doesn't `ASSUME'
----------------------------

As part of NASM's drive for simplicity, it also does not support the
`ASSUME' directive. NASM will not keep track of what values you choose
to put in your segment registers, and will never _automatically_
generate a segment override prefix.


File: nasm.info,  Node: Section 2.2.5,  Next: Section 2.2.6,  Prev: Section 2.2.4,  Up: Section 2.2

2.2.5. NASM Doesn't Support Memory Models
-----------------------------------------

NASM also does not have any directives to support different 16-bit
memory models. The programmer has to keep track of which functions are
supposed to be called with a far call and which with a near call, and
is responsible for putting the correct form of `RET' instruction
(`RETN' or `RETF'; NASM accepts `RET' itself as an alternate form for
`RETN'); in addition, the programmer is responsible for coding CALL FAR
instructions where necessary when calling _external_ functions, and
must also keep track of which external variable definitions are far and
which are near.


File: nasm.info,  Node: Section 2.2.6,  Next: Section 2.2.7,  Prev: Section 2.2.5,  Up: Section 2.2

2.2.6. Floating-Point Differences
---------------------------------

NASM uses different names to refer to floating-point registers from
MASM: where MASM would call them `ST(0)', `ST(1)' and so on, and `a86'
would call them simply `0', `1' and so on, NASM chooses to call them
`st0', `st1' etc.

   As of version 0.96, NASM now treats the instructions with `nowait'
forms in the same way as MASM-compatible assemblers. The idiosyncratic
treatment employed by 0.95 and earlier was based on a misunderstanding
by the authors.


File: nasm.info,  Node: Section 2.2.7,  Next: Chapter 3,  Prev: Section 2.2.6,  Up: Section 2.2

2.2.7. Other Differences
------------------------

For historical reasons, NASM uses the keyword `TWORD' where MASM and
compatible assemblers use `TBYTE'.

   NASM does not declare uninitialized storage in the same way as MASM:
where a MASM programmer might use `stack db 64 dup (?)', NASM requires
`stack resb 64', intended to be read as `reserve 64 bytes'. For a
limited amount of compatibility, since NASM treats `?' as a valid
character in symbol names, you can code `? equ 0' and then writing `dw
?' will at least do something vaguely useful. `DUP' is still not a
supported syntax, however.

   In addition to all of this, macros and directives work completely
differently to MASM. See *note Chapter 4:: and *note Chapter 6:: for
further details.


File: nasm.info,  Node: Chapter 3,  Next: Section 3.1,  Prev: Section 2.2.7,  Up: Top

Chapter 3: The NASM Language
****************************

* Menu:

* Section 3.1:: Layout of a NASM Source Line
* Section 3.2:: Pseudo-Instructions
* Section 3.3:: Effective Addresses
* Section 3.4:: Constants
* Section 3.5:: Expressions
* Section 3.6:: `SEG' and `WRT'
* Section 3.7:: `STRICT': Inhibiting Optimization
* Section 3.8:: Critical Expressions
* Section 3.9:: Local Labels


File: nasm.info,  Node: Section 3.1,  Next: Section 3.2,  Prev: Chapter 3,  Up: Chapter 3

3.1. Layout of a NASM Source Line
=================================

Like most assemblers, each NASM source line contains (unless it is a
macro, a preprocessor directive or an assembler directive: see *note
Chapter 4:: and *note Chapter 6::) some combination of the four fields

     label:    instruction operands        ; comment

   As usual, most of these fields are optional; the presence or absence
of any combination of a label, an instruction and a comment is allowed.
Of course, the operand field is either required or forbidden by the
presence and nature of the instruction field.

   NASM uses backslash (\) as the line continuation character; if a
line ends with backslash, the next line is considered to be a part of
the backslash- ended line.

   NASM places no restrictions on white space within a line: labels may
have white space before them, or instructions may have no space before
them, or anything. The colon after a label is also optional. (Note that
this means that if you intend to code `lodsb' alone on a line, and type
`lodab' by accident, then that's still a valid source line which does
nothing but define a label. Running NASM with the command-line option
`-w+orphan-labels' will cause it to warn you if you define a label
alone on a line without a trailing colon.)

   Valid characters in labels are letters, numbers, `_', `$', `#', `@',
`~', `.', and `?'. The only characters which may be used as the _first_
character of an identifier are letters, `.' (with special meaning: see
*note Section 3.9::), `_' and `?'. An identifier may also be prefixed
with a `$' to indicate that it is intended to be read as an identifier
and not a reserved word; thus, if some other module you are linking
with defines a symbol called `eax', you can refer to `$eax' in NASM
code to distinguish the symbol from the register. Maximum length of an
identifier is 4095 characters.

   The instruction field may contain any machine instruction: Pentium
and P6 instructions, FPU instructions, MMX instructions and even
undocumented instructions are all supported. The instruction may be
prefixed by `LOCK', `REP', `REPE'/`REPZ' or `REPNE'/`REPNZ', in the
usual way. Explicit address-size and operand-size prefixes `A16',
`A32', `A64', `O16' and `O32', `O64' are provided - one example of
their use is given in *note Chapter 10::. You can also use the name of
a segment register as an instruction prefix: coding `es mov [bx],ax' is
equivalent to coding `mov [es:bx],ax'. We recommend the latter syntax,
since it is consistent with other syntactic features of the language,
but for instructions such as `LODSB', which has no operands and yet can
require a segment override, there is no clean syntactic way to proceed
apart from `es lodsb'.

   An instruction is not required to use a prefix: prefixes such as
`CS', `A32', `LOCK' or `REPE' can appear on a line by themselves, and
NASM will just generate the prefix bytes.

   In addition to actual machine instructions, NASM also supports a
number of pseudo-instructions, described in *note Section 3.2::.

   Instruction operands may take a number of forms: they can be
registers, described simply by the register name (e.g. `ax', `bp',
`ebx', `cr0': NASM does not use the `gas'-style syntax in which
register names must be prefixed by a `%' sign), or they can be
effective addresses (see *note Section 3.3::), constants (*note Section
3.4::) or expressions (*note Section 3.5::).

   For x87 floating-point instructions, NASM accepts a wide range of
syntaxes: you can use two-operand forms like MASM supports, or you can
use NASM's native single-operand forms in most cases. For example, you
can code:

             fadd    st1             ; this sets st0 := st0 + st1
             fadd    st0,st1         ; so does this

             fadd    st1,st0         ; this sets st1 := st1 + st0
             fadd    to st1          ; so does this

   Almost any x87 floating-point instruction that references memory
must use one of the prefixes `DWORD', `QWORD' or `TWORD' to indicate
what size of memory operand it refers to.


File: nasm.info,  Node: Section 3.2,  Next: Section 3.2.1,  Prev: Section 3.1,  Up: Chapter 3

3.2. Pseudo-Instructions
========================

Pseudo-instructions are things which, though not real x86 machine
instructions, are used in the instruction field anyway because that's
the most convenient place to put them. The current pseudo-instructions
are `DB', `DW', `DD', `DQ', `DT', `DO' and `DY'; their uninitialized
counterparts `RESB', `RESW', `RESD', `RESQ', `REST', `RESO' and `RESY';
the `INCBIN' command, the `EQU' command, and the `TIMES' prefix.

* Menu:

* Section 3.2.1:: `DB' and Friends: Declaring Initialized Data
* Section 3.2.2:: `RESB' and Friends: Declaring Uninitialized Data
* Section 3.2.3:: `INCBIN': Including External Binary Files
* Section 3.2.4:: `EQU': Defining Constants
* Section 3.2.5:: `TIMES': Repeating Instructions or Data


File: nasm.info,  Node: Section 3.2.1,  Next: Section 3.2.2,  Prev: Section 3.2,  Up: Section 3.2

3.2.1. `DB' and Friends: Declaring Initialized Data
---------------------------------------------------

`DB', `DW', `DD', `DQ', `DT', `DO' and `DY' are used, much as in MASM,
to declare initialized data in the output file. They can be invoked in
a wide range of ways:

           db    0x55                ; just the byte 0x55
           db    0x55,0x56,0x57      ; three bytes in succession
           db    'a',0x55            ; character constants are OK
           db    'hello',13,10,'$'   ; so are string constants
           dw    0x1234              ; 0x34 0x12
           dw    'a'                 ; 0x61 0x00 (it's just a number)
           dw    'ab'                ; 0x61 0x62 (character constant)
           dw    'abc'               ; 0x61 0x62 0x63 0x00 (string)
           dd    0x12345678          ; 0x78 0x56 0x34 0x12
           dd    1.234567e20         ; floating-point constant
           dq    0x123456789abcdef0  ; eight byte constant
           dq    1.234567e20         ; double-precision float
           dt    1.234567e20         ; extended-precision float

   `DT', `DO' and `DY' do not accept numeric constants as operands.


File: nasm.info,  Node: Section 3.2.2,  Next: Section 3.2.3,  Prev: Section 3.2.1,  Up: Section 3.2

3.2.2. `RESB' and Friends: Declaring Uninitialized Data
-------------------------------------------------------

`RESB', `RESW', `RESD', `RESQ', `REST', `RESO' and `RESY' are designed
to be used in the BSS section of a module: they declare _uninitialized_
storage space. Each takes a single operand, which is the number of
bytes, words, doublewords or whatever to reserve. As stated in *note
Section 2.2.7::, NASM does not support the MASM/TASM syntax of
reserving uninitialized space by writing `DW ?' or similar things: this
is what it does instead. The operand to a `RESB'-type
pseudo-instruction is a _critical expression_: see *note Section 3.8::.

   For example:

     buffer:         resb    64              ; reserve 64 bytes
     wordvar:        resw    1               ; reserve a word
     realarray       resq    10              ; array of ten reals
     ymmval:         resy    1               ; one YMM register


File: nasm.info,  Node: Section 3.2.3,  Next: Section 3.2.4,  Prev: Section 3.2.2,  Up: Section 3.2

3.2.3. `INCBIN': Including External Binary Files
------------------------------------------------

`INCBIN' is borrowed from the old Amiga assembler DevPac: it includes a
binary file verbatim into the output file. This can be handy for (for
example) including graphics and sound data directly into a game
executable file. It can be called in one of these three ways:

         incbin  "file.dat"             ; include the whole file
         incbin  "file.dat",1024        ; skip the first 1024 bytes
         incbin  "file.dat",1024,512    ; skip the first 1024, and
                                        ; actually include at most 512

   `INCBIN' is both a directive and a standard macro; the standard macro
version searches for the file in the include file search path and adds
the file to the dependency lists. This macro can be overridden if
desired.


File: nasm.info,  Node: Section 3.2.4,  Next: Section 3.2.5,  Prev: Section 3.2.3,  Up: Section 3.2

3.2.4. `EQU': Defining Constants
--------------------------------

`EQU' defines a symbol to a given constant value: when `EQU' is used,
the source line must contain a label. The action of `EQU' is to define
the given label name to the value of its (only) operand. This
definition is absolute, and cannot change later. So, for example,

     message         db      'hello, world'
     msglen          equ     $-message

   defines `msglen' to be the constant 12. `msglen' may not then be
redefined later. This is not a preprocessor definition either: the
value of `msglen' is evaluated _once_, using the value of `$' (see
*note Section 3.5:: for an explanation of `$') at the point of
definition, rather than being evaluated wherever it is referenced and
using the value of `$' at the point of reference.


File: nasm.info,  Node: Section 3.2.5,  Next: Section 3.3,  Prev: Section 3.2.4,  Up: Section 3.2

3.2.5. `TIMES': Repeating Instructions or Data
----------------------------------------------

The `TIMES' prefix causes the instruction to be assembled multiple
times. This is partly present as NASM's equivalent of the `DUP' syntax
supported by MASM-compatible assemblers, in that you can code

     zerobuf:        times 64 db 0

   or similar things; but `TIMES' is more versatile than that. The
argument to `TIMES' is not just a numeric constant, but a numeric
_expression_, so you can do things like

     buffer: db      'hello, world'
             times 64-$+buffer db ' '

   which will store exactly enough spaces to make the total length of
`buffer' up to 64. Finally, `TIMES' can be applied to ordinary
instructions, so you can code trivial unrolled loops in it:

             times 100 movsb

   Note that there is no effective difference between `times 100 resb 1'
and `resb 100', except that the latter will be assembled about 100
times faster due to the internal structure of the assembler.

   The operand to `TIMES' is a critical expression (*note Section
3.8::).

   Note also that `TIMES' can't be applied to macros: the reason for
this is that `TIMES' is processed after the macro phase, which allows
the argument to `TIMES' to contain expressions such as `64-$+buffer' as
above. To repeat more than one line of code, or a complex macro, use the
preprocessor `%rep' directive.


File: nasm.info,  Node: Section 3.3,  Next: Section 3.4,  Prev: Section 3.2.5,  Up: Chapter 3

3.3. Effective Addresses
========================

An effective address is any operand to an instruction which references
memory. Effective addresses, in NASM, have a very simple syntax: they
consist of an expression evaluating to the desired address, enclosed in
square brackets. For example:

     wordvar dw      123
             mov     ax,[wordvar]
             mov     ax,[wordvar+1]
             mov     ax,[es:wordvar+bx]

   Anything not conforming to this simple system is not a valid memory
reference in NASM, for example `es:wordvar[bx]'.

   More complicated effective addresses, such as those involving more
than one register, work in exactly the same way:

             mov     eax,[ebx*2+ecx+offset]
             mov     ax,[bp+di+8]

   NASM is capable of doing algebra on these effective addresses, so
that things which don't necessarily _look_ legal are perfectly all
right:

         mov     eax,[ebx*5]             ; assembles as [ebx*4+ebx]
         mov     eax,[label1*2-label2]   ; ie [label1+(label1-label2)]

   Some forms of effective address have more than one assembled form;
in most such cases NASM will generate the smallest form it can. For
example, there are distinct assembled forms for the 32-bit effective
addresses `[eax*2+0]' and `[eax+eax]', and NASM will generally generate
the latter on the grounds that the former requires four bytes to store
a zero offset.

   NASM has a hinting mechanism which will cause `[eax+ebx]' and
`[ebx+eax]' to generate different opcodes; this is occasionally useful
because `[esi+ebp]' and `[ebp+esi]' have different default segment
registers.

   However, you can force NASM to generate an effective address in a
particular form by the use of the keywords `BYTE', `WORD', `DWORD' and
`NOSPLIT'. If you need `[eax+3]' to be assembled using a double-word
offset field instead of the one byte NASM will normally generate, you
can code `[dword eax+3]'. Similarly, you can force NASM to use a byte
offset for a small value which it hasn't seen on the first pass (see
*note Section 3.8:: for an example of such a code fragment) by using
`[byte eax+offset]'. As special cases, `[byte eax]' will code `[eax+0]'
with a byte offset of zero, and `[dword eax]' will code it with a
double-word offset of zero. The normal form, `[eax]', will be coded
with no offset field.

   The form described in the previous paragraph is also useful if you
are trying to access data in a 32-bit segment from within 16 bit code.
For more information on this see the section on mixed-size addressing
(*note Section 10.2::). In particular, if you need to access data with
a known offset that is larger than will fit in a 16-bit value, if you
don't specify that it is a dword offset, nasm will cause the high word
of the offset to be lost.

   Similarly, NASM will split `[eax*2]' into `[eax+eax]' because that
allows the offset field to be absent and space to be saved; in fact, it
will also split `[eax*2+offset]' into `[eax+eax+offset]'. You can
combat this behaviour by the use of the `NOSPLIT' keyword: `[nosplit
eax*2]' will force `[eax*2+0]' to be generated literally.

   In 64-bit mode, NASM will by default generate absolute addresses. The
`REL' keyword makes it produce `RIP'-relative addresses. Since this is
frequently the normally desired behaviour, see the `DEFAULT' directive
(*note Section 6.2::). The keyword `ABS' overrides `REL'.


File: nasm.info,  Node: Section 3.4,  Next: Section 3.4.1,  Prev: Section 3.3,  Up: Chapter 3

3.4. Constants
==============

NASM understands four different types of constant: numeric, character,
string and floating-point.

* Menu:

* Section 3.4.1:: Numeric Constants
* Section 3.4.2:: Character Strings
* Section 3.4.3:: Character Constants
* Section 3.4.4:: String Constants
* Section 3.4.5:: Unicode Strings
* Section 3.4.6:: Floating-Point Constants
* Section 3.4.7:: Packed BCD Constants


File: nasm.info,  Node: Section 3.4.1,  Next: Section 3.4.2,  Prev: Section 3.4,  Up: Section 3.4

3.4.1. Numeric Constants
------------------------

A numeric constant is simply a number. NASM allows you to specify
numbers in a variety of number bases, in a variety of ways: you can
suffix `H', `Q' or `O', and `B' for hex, octal and binary, or you can
prefix `0x' for hex in the style of C, or you can prefix `$' for hex in
the style of Borland Pascal. Note, though, that the `$' prefix does
double duty as a prefix on identifiers (see *note Section 3.1::), so a
hex number prefixed with a `$' sign must have a digit after the `$'
rather than a letter.

   Numeric constants can have underscores (`_') interspersed to break up
long strings.

   Some examples:

             mov     ax,100          ; decimal
             mov     ax,0a2h         ; hex
             mov     ax,$0a2         ; hex again: the 0 is required
             mov     ax,0xa2         ; hex yet again
             mov     ax,777q         ; octal
             mov     ax,777o         ; octal again
             mov     ax,10010011b    ; binary
             mov     ax,1001_0011b   ; same binary constant


File: nasm.info,  Node: Section 3.4.2,  Next: Section 3.4.3,  Prev: Section 3.4.1,  Up: Section 3.4

3.4.2. Character Strings
------------------------

A character string consists of up to eight characters enclosed in either
single quotes (`'...''), double quotes (`"..."') or backquotes
(``...`'). Single or double quotes are equivalent to NASM (except of
course that surrounding the constant with single quotes allows double
quotes to appear within it and vice versa); the contents of those are
represented verbatim. Strings enclosed in backquotes support C-style
`\'-escapes for special characters.

   The following escape sequences are recognized by backquoted strings:

           \'          single quote (')
           \"          double quote (")
           \`          backquote (`)
           \\          backslash (\)
           \?          question mark (?)
           \a          BEL (ASCII 7)
           \b          BS  (ASCII 8)
           \t          TAB (ASCII 9)
           \n          LF  (ASCII 10)
           \v          VT  (ASCII 11)
           \f          FF  (ASCII 12)
           \r          CR  (ASCII 13)
           \e          ESC (ASCII 27)
           \377        Up to 3 octal digits - literal byte
           \xFF        Up to 2 hexadecimal digits - literal byte
           \u1234      4 hexadecimal digits - Unicode character
           \U12345678  8 hexadecimal digits - Unicode character

   All other escape sequences are reserved. Note that `\0', meaning a
`NUL' character (ASCII 0), is a special case of the octal escape
sequence.

   Unicode characters specified with `\u' or `\U' are converted to
UTF-8. For example, the following lines are all equivalent:

           db `\u263a`            ; UTF-8 smiley face
           db `\xe2\x98\xba`      ; UTF-8 smiley face
           db 0E2h, 098h, 0BAh    ; UTF-8 smiley face


File: nasm.info,  Node: Section 3.4.3,  Next: Section 3.4.4,  Prev: Section 3.4.2,  Up: Section 3.4

3.4.3. Character Constants
--------------------------

A character constant consists of a string up to eight bytes long, used
in an expression context. It is treated as if it was an integer.

   A character constant with more than one byte will be arranged with
little-endian order in mind: if you code

               mov eax,'abcd'

   then the constant generated is not `0x61626364', but `0x64636261',
so that if you were then to store the value into memory, it would read
`abcd' rather than `dcba'. This is also the sense of character
constants understood by the Pentium's `CPUID' instruction.


File: nasm.info,  Node: Section 3.4.4,  Next: Section 3.4.5,  Prev: Section 3.4.3,  Up: Section 3.4

3.4.4. String Constants
-----------------------

String constants are character strings used in the context of some
pseudo- instructions, namely the `DB' family and `INCBIN' (where it
represents a filename.) They are also used in certain preprocessor
directives.

   A string constant looks like a character constant, only longer. It is
treated as a concatenation of maximum-size character constants for the
conditions. So the following are equivalent:

           db    'hello'               ; string constant
           db    'h','e','l','l','o'   ; equivalent character constants

   And the following are also equivalent:

           dd    'ninechars'           ; doubleword string constant
           dd    'nine','char','s'     ; becomes three doublewords
           db    'ninechars',0,0,0     ; and really looks like this

   Note that when used in a string-supporting context, quoted strings
are treated as a string constants even if they are short enough to be a
character constant, because otherwise `db 'ab'' would have the same
effect as `db 'a'', which would be silly. Similarly, three-character or
four-character constants are treated as strings when they are operands
to `DW', and so forth.


File: nasm.info,  Node: Section 3.4.5,  Next: Section 3.4.6,  Prev: Section 3.4.4,  Up: Section 3.4

3.4.5. Unicode Strings
----------------------

The special operators `__utf16__' and `__utf32__' allows definition of
Unicode strings. They take a string in UTF-8 format and converts it to
(littleendian) UTF-16 or UTF-32, respectively.

   For example:

     %define u(x) __utf16__(x)
     %define w(x) __utf32__(x)

           dw u('C:\WINDOWS'), 0       ; Pathname in UTF-16
           dd w(`A + B = \u206a`), 0   ; String in UTF-32

   `__utf16__' and `__utf32__' can be applied either to strings passed
to the `DB' family instructions, or to character constants in an
expression context.


File: nasm.info,  Node: Section 3.4.6,  Next: Section 3.4.7,  Prev: Section 3.4.5,  Up: Section 3.4

3.4.6. Floating-Point Constants
-------------------------------

Floating-point constants are acceptable only as arguments to `DB',
`DW', `DD', `DQ', `DT', and `DO', or as arguments to the special
operators `__float8__', `__float16__', `__float32__', `__float64__',
`__float80m__', `__float80e__', `__float128l__', and `__float128h__'.

   Floating-point constants are expressed in the traditional form:
digits, then a period, then optionally more digits, then optionally an
`E' followed by an exponent. The period is mandatory, so that NASM can
distinguish between `dd 1', which declares an integer constant, and `dd
1.0' which declares a floating-point constant. NASM also support
C99-style hexadecimal floating-point: `0x', hexadecimal digits, period,
optionally more hexadeximal digits, then optionally a `P' followed by a
_binary_ (not hexadecimal) exponent in decimal notation.

   Underscores to break up groups of digits are permitted in
floating-point constants as well.

   Some examples:

           db    -0.2                    ; "Quarter precision"
           dw    -0.5                    ; IEEE 754r/SSE5 half precision
           dd    1.2                     ; an easy one
           dd    1.222_222_222           ; underscores are permitted
           dd    0x1p+2                  ; 1.0x2^2 = 4.0
           dq    0x1p+32                 ; 1.0x2^32 = 4 294 967 296.0
           dq    1.e10                   ; 10 000 000 000.0
           dq    1.e+10                  ; synonymous with 1.e10
           dq    1.e-10                  ; 0.000 000 000 1
           dt    3.141592653589793238462 ; pi
           do    1.e+4000                ; IEEE 754r quad precision

   The 8-bit "quarter-precision" floating-point format is
sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears
to be the most frequently used 8-bit floating-point format, although it
is not covered by any formal standard. This is sometimes called a
"minifloat."

   The special operators are used to produce floating-point numbers in
other contexts. They produce the binary representation of a specific
floating- point number as an integer, and can use anywhere integer
constants are used in an expression. `__float80m__' and `__float80e__'
produce the 64-bit mantissa and 16-bit exponent of an 80-bit
floating-point number, and `__float128l__' and `__float128h__' produce
the lower and upper 64-bit halves of a 128-bit floating-point number,
respectively.

   For example:

           mov    rax,__float64__(3.141592653589793238462)

   ... would assign the binary representation of pi as a 64-bit
floating point number into `RAX'. This is exactly equivalent to:

           mov    rax,0x400921fb54442d18

   NASM cannot do compile-time arithmetic on floating-point constants.
This is because NASM is designed to be portable - although it always
generates code to run on x86 processors, the assembler itself can run
on any system with an ANSI C compiler. Therefore, the assembler cannot
guarantee the presence of a floating-point unit capable of handling the
Intel number formats, and so for NASM to be able to do floating
arithmetic it would have to include its own complete set of
floating-point routines, which would significantly increase the size of
the assembler for very little benefit.

   The special tokens `__Infinity__', `__QNaN__' (or `__NaN__') and
`__SNaN__' can be used to generate infinities, quiet NaNs, and
signalling NaNs, respectively. These are normally used as macros:

     %define Inf __Infinity__
     %define NaN __QNaN__

           dq    +1.5, -Inf, NaN         ; Double-precision constants


File: nasm.info,  Node: Section 3.4.7,  Next: Section 3.5,  Prev: Section 3.4.6,  Up: Section 3.4

3.4.7. Packed BCD Constants
---------------------------

x87-style packed BCD constants can be used in the same contexts as
80-bit floating-point numbers. They are suffixed with `p' or prefixed
with `0p', and can include up to 18 decimal digits.

   As with other numeric constants, underscores can be used to separate
digits.

   For example:

           dt 12_345_678_901_245_678p
           dt -12_345_678_901_245_678p
           dt +0p33
           dt 33p


File: nasm.info,  Node: Section 3.5,  Next: Section 3.5.1,  Prev: Section 3.4.7,  Up: Chapter 3

3.5. Expressions
================

Expressions in NASM are similar in syntax to those in C. Expressions are
evaluated as 64-bit integers which are then adjusted to the appropriate
size.

   NASM supports two special tokens in expressions, allowing
calculations to involve the current assembly position: the `$' and `$$'
tokens.  `$' evaluates to the assembly position at the beginning of the
line containing the expression; so you can code an infinite loop using
`JMP $'. `$$' evaluates to the beginning of the current section; so you
can tell how far into the section you are by using `($-$$)'.

   The arithmetic operators provided by NASM are listed here, in
increasing order of precedence.

* Menu:

* Section 3.5.1:: `|': Bitwise OR Operator
* Section 3.5.2:: `^': Bitwise XOR Operator
* Section 3.5.3:: `&': Bitwise AND Operator
* Section 3.5.4:: `<<' and `>>': Bit Shift Operators
* Section 3.5.5:: `+' and `-': Addition and Subtraction Operators
* Section 3.5.6:: `*', `/', `//', `%' and `%%': Multiplication and Division
* Section 3.5.7:: Unary Operators: `+', `-', `~', `!' and `SEG'


File: nasm.info,  Node: Section 3.5.1,  Next: Section 3.5.2,  Prev: Section 3.5,  Up: Section 3.5

3.5.1. `|': Bitwise OR Operator
-------------------------------

The `|' operator gives a bitwise OR, exactly as performed by the `OR'
machine instruction. Bitwise OR is the lowest-priority arithmetic
operator supported by NASM.


File: nasm.info,  Node: Section 3.5.2,  Next: Section 3.5.3,  Prev: Section 3.5.1,  Up: Section 3.5

3.5.2. `^': Bitwise XOR Operator
--------------------------------

`^' provides the bitwise XOR operation.


File: nasm.info,  Node: Section 3.5.3,  Next: Section 3.5.4,  Prev: Section 3.5.2,  Up: Section 3.5

3.5.3. `&': Bitwise AND Operator
--------------------------------

`&' provides the bitwise AND operation.


File: nasm.info,  Node: Section 3.5.4,  Next: Section 3.5.5,  Prev: Section 3.5.3,  Up: Section 3.5

3.5.4. `<<' and `>>': Bit Shift Operators
-----------------------------------------

`<<' gives a bit-shift to the left, just as it does in C. So `5<<3'
evaluates to 5 times 8, or 40. `>>' gives a bit-shift to the right; in
NASM, such a shift is _always_ unsigned, so that the bits shifted in
from the left-hand end are filled with zero rather than a
sign-extension of the previous highest bit.


File: nasm.info,  Node: Section 3.5.5,  Next: Section 3.5.6,  Prev: Section 3.5.4,  Up: Section 3.5

3.5.5. `+' and `-': Addition and Subtraction Operators
------------------------------------------------------

The `+' and `-' operators do perfectly ordinary addition and
subtraction.


File: nasm.info,  Node: Section 3.5.6,  Next: Section 3.5.7,  Prev: Section 3.5.5,  Up: Section 3.5

3.5.6. `*', `/', `//', `%' and `%%': Multiplication and Division
----------------------------------------------------------------

`*' is the multiplication operator. `/' and `//' are both division
operators: `/' is unsigned division and `//' is signed division.
Similarly, `%' and `%%' provide unsigned and signed modulo operators
respectively.

   NASM, like ANSI C, provides no guarantees about the sensible
operation of the signed modulo operator.

   Since the `%' character is used extensively by the macro
preprocessor, you should ensure that both the signed and unsigned
modulo operators are followed by white space wherever they appear.


File: nasm.info,  Node: Section 3.5.7,  Next: Section 3.6,  Prev: Section 3.5.6,  Up: Section 3.5

3.5.7. Unary Operators: `+', `-', `~', `!' and `SEG'
----------------------------------------------------

The highest-priority operators in NASM's expression grammar are those
which only apply to one argument. `-' negates its operand, `+' does
nothing (it's provided for symmetry with `-'), `~' computes the one's
complement of its operand, `!' is the logical negation operator, and
`SEG' provides the segment address of its operand (explained in more
detail in *note Section 3.6::).


File: nasm.info,  Node: Section 3.6,  Next: Section 3.7,  Prev: Section 3.5.7,  Up: Chapter 3

3.6. `SEG' and `WRT'
====================

When writing large 16-bit programs, which must be split into multiple
segments, it is often necessary to be able to refer to the segment part
of the address of a symbol. NASM supports the `SEG' operator to perform
this function.

   The `SEG' operator returns the _preferred_ segment base of a symbol,
defined as the segment base relative to which the offset of the symbol
makes sense. So the code

             mov     ax,seg symbol
             mov     es,ax
             mov     bx,symbol

   will load `ES:BX' with a valid pointer to the symbol `symbol'.

   Things can be more complex than this: since 16-bit segments and
groups may overlap, you might occasionally want to refer to some symbol
using a different segment base from the preferred one. NASM lets you do
this, by the use of the `WRT' (With Reference To) keyword. So you can
do things like

             mov     ax,weird_seg        ; weird_seg is a segment base
             mov     es,ax
             mov     bx,symbol wrt weird_seg

   to load `ES:BX' with a different, but functionally equivalent,
pointer to the symbol `symbol'.

   NASM supports far (inter-segment) calls and jumps by means of the
syntax `call segment:offset', where `segment' and `offset' both
represent immediate values. So to call a far procedure, you could code
either of

             call    (seg procedure):procedure
             call    weird_seg:(procedure wrt weird_seg)

   (The parentheses are included for clarity, to show the intended
parsing of the above instructions. They are not necessary in practice.)

   NASM supports the syntax `call far procedure' as a synonym for the
first of the above usages. `JMP' works identically to `CALL' in these
examples.

   To declare a far pointer to a data item in a data segment, you must
code

             dw      symbol, seg symbol

   NASM supports no convenient synonym for this, though you can always
invent one using the macro processor.


File: nasm.info,  Node: Section 3.7,  Next: Section 3.8,  Prev: Section 3.6,  Up: Chapter 3

3.7. `STRICT': Inhibiting Optimization
======================================

When assembling with the optimizer set to level 2 or higher (see *note
Section 2.1.22::), NASM will use size specifiers (`BYTE', `WORD',
`DWORD', `QWORD', `TWORD', `OWORD' or `YWORD'), but will give them the
smallest possible size. The keyword `STRICT' can be used to inhibit
optimization and force a particular operand to be emitted in the
specified size. For example, with the optimizer on, and in `BITS 16'
mode,

             push dword 33

   is encoded in three bytes `66 6A 21', whereas

             push strict dword 33

   is encoded in six bytes, with a full dword immediate operand `66 68
21 00 00 00'.

   With the optimizer off, the same code (six bytes) is generated
whether the `STRICT' keyword was used or not.


File: nasm.info,  Node: Section 3.8,  Next: Section 3.9,  Prev: Section 3.7,  Up: Chapter 3

3.8. Critical Expressions
=========================

Although NASM has an optional multi-pass optimizer, there are some
expressions which must be resolvable on the first pass. These are called
_Critical Expressions_.

   The first pass is used to determine the size of all the assembled
code and data, so that the second pass, when generating all the code,
knows all the symbol addresses the code refers to. So one thing NASM
can't handle is code whose size depends on the value of a symbol
declared after the code in question. For example,

             times (label-$) db 0
     label:  db      'Where am I?'

   The argument to `TIMES' in this case could equally legally evaluate
to anything at all; NASM will reject this example because it cannot
tell the size of the `TIMES' line when it first sees it. It will just
as firmly reject the slightly paradoxical code

             times (label-$+1) db 0
     label:  db      'NOW where am I?'

   in which _any_ value for the `TIMES' argument is by definition wrong!

   NASM rejects these examples by means of a concept called a _critical
expression_, which is defined to be an expression whose value is
required to be computable in the first pass, and which must therefore
depend only on symbols defined before it. The argument to the `TIMES'
prefix is a critical expression.


File: nasm.info,  Node: Section 3.9,  Next: Chapter 4,  Prev: Section 3.8,  Up: Chapter 3

3.9. Local Labels
=================

NASM gives special treatment to symbols beginning with a period. A label
beginning with a single period is treated as a _local_ label, which
means that it is associated with the previous non-local label. So, for
example:

     label1  ; some code

     .loop
             ; some more code

             jne     .loop
             ret

     label2  ; some code

     .loop
             ; some more code

             jne     .loop
             ret

   In the above code fragment, each `JNE' instruction jumps to the line
immediately before it, because the two definitions of `.loop' are kept
separate by virtue of each being associated with the previous non-local
label.

   This form of local label handling is borrowed from the old Amiga
assembler DevPac; however, NASM goes one step further, in allowing
access to local labels from other parts of the code. This is achieved
by means of _defining_ a local label in terms of the previous non-local
label: the first definition of `.loop' above is really defining a
symbol called `label1.loop', and the second defines a symbol called
`label2.loop'. So, if you really needed to, you could write

     label3  ; some more code
             ; and some more

             jmp label1.loop

   Sometimes it is useful - in a macro, for instance - to be able to
define a label which can be referenced from anywhere but which doesn't
interfere with the normal local-label mechanism. Such a label can't be
non-local because it would interfere with subsequent definitions of,
and references to, local labels; and it can't be local because the
macro that defined it wouldn't know the label's full name. NASM
therefore introduces a third type of label, which is probably only
useful in macro definitions: if a label begins with the special prefix
`..@', then it does nothing to the local label mechanism. So you could
code

     label1:                         ; a non-local label
     .local:                         ; this is really label1.local
     ..@foo:                         ; this is a special symbol
     label2:                         ; another non-local label
     .local:                         ; this is really label2.local

             jmp     ..@foo          ; this will jump three lines up

   NASM has the capacity to define other special symbols beginning with
a double period: for example, `..start' is used to specify the entry
point in the `obj' output format (see *note Section 7.2.6::).


File: nasm.info,  Node: Chapter 4,  Next: Section 4.1,  Prev: Section 3.9,  Up: Top

Chapter 4: The NASM Preprocessor
********************************

NASM contains a powerful macro processor, which supports conditional
assembly, multi-level file inclusion, two forms of macro (single-line
and multi-line), and a `context stack' mechanism for extra macro power.
Preprocessor directives all begin with a `%' sign.

   The preprocessor collapses all lines which end with a backslash (\)
character into a single line. Thus:

     %define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \
             THIS_VALUE

   will work like a single-line macro without the backslash-newline
sequence.

* Menu:

* Section 4.1:: Single-Line Macros
* Section 4.2:: String Manipulation in Macros
* Section 4.3:: Multi-Line Macros: `%macro'
* Section 4.4:: Conditional Assembly
* Section 4.5:: Preprocessor Loops: `%rep'
* Section 4.6:: Source Files and Dependencies
* Section 4.7:: The Context Stack
* Section 4.8:: Stack Relative Preprocessor Directives
* Section 4.9:: Reporting User-Defined Errors: `%error', `%warning', `%fatal'
* Section 4.10:: Other Preprocessor Directives
* Section 4.11:: Standard Macros


File: nasm.info,  Node: Section 4.1,  Next: Section 4.1.1,  Prev: Chapter 4,  Up: Chapter 4

4.1. Single-Line Macros
=======================

* Menu:

* Section 4.1.1:: The Normal Way: `%define'
* Section 4.1.2:: Resolving `%define': `%xdefine'
* Section 4.1.3:: Concatenating Single Line Macro Tokens: `%+'
* Section 4.1.4:: The Macro Name Itself: `%?' and `%??'
* Section 4.1.5:: Undefining Single-Line Macros: `%undef'
* Section 4.1.6:: Preprocessor Variables: `%assign'
* Section 4.1.7:: Defining Strings: `%defstr'


File: nasm.info,  Node: Section 4.1.1,  Next: Section 4.1.2,  Prev: Section 4.1,  Up: Section 4.1

4.1.1. The Normal Way: `%define'
--------------------------------

Single-line macros are defined using the `%define' preprocessor
directive. The definitions work in a similar way to C; so you can do
things like

     %define ctrl    0x1F &
     %define param(a,b) ((a)+(a)*(b))

             mov     byte [param(2,ebx)], ctrl 'D'

   which will expand to

             mov     byte [(2)+(2)*(ebx)], 0x1F & 'D'

   When the expansion of a single-line macro contains tokens which
invoke another macro, the expansion is performed at invocation time,
not at definition time. Thus the code

     %define a(x)    1+b(x)
     %define b(x)    2*x

             mov     ax,a(8)

   will evaluate in the expected way to `mov ax,1+2*8', even though the
macro `b' wasn't defined at the time of definition of `a'.

   Macros defined with `%define' are case sensitive: after `%define foo
bar', only `foo' will expand to `bar': `Foo' or `FOO' will not. By
using `%idefine' instead of `%define' (the `i' stands for
`insensitive') you can define all the case variants of a macro at once,
so that `%idefine foo bar' would cause `foo', `Foo', `FOO', `fOO' and
so on all to expand to `bar'.

   There is a mechanism which detects when a macro call has occurred as
a result of a previous expansion of the same macro, to guard against
circular references and infinite loops. If this happens, the
preprocessor will only expand the first occurrence of the macro. Hence,
if you code

     %define a(x)    1+a(x)

             mov     ax,a(3)

   the macro `a(3)' will expand once, becoming `1+a(3)', and will then
expand no further. This behaviour can be useful: see *note Section 9.1::
for an example of its use.

   You can overload single-line macros: if you write

     %define foo(x)   1+x
     %define foo(x,y) 1+x*y

   the preprocessor will be able to handle both types of macro call, by
counting the parameters you pass; so `foo(3)' will become `1+3' whereas
`foo(ebx,2)' will become `1+ebx*2'. However, if you define

     %define foo bar

   then no other definition of `foo' will be accepted: a macro with no
parameters prohibits the definition of the same name as a macro _with_
parameters, and vice versa.

   This doesn't prevent single-line macros being _redefined_: you can
perfectly well define a macro with

     %define foo bar

   and then re-define it later in the same source file with

     %define foo baz

   Then everywhere the macro `foo' is invoked, it will be expanded
according to the most recent definition. This is particularly useful
when defining single-line macros with `%assign' (see *note Section
4.1.6::).

   You can pre-define single-line macros using the `-d' option on the
NASM command line: see *note Section 2.1.18::.


File: nasm.info,  Node: Section 4.1.2,  Next: Section 4.1.3,  Prev: Section 4.1.1,  Up: Section 4.1

4.1.2. Resolving `%define': `%xdefine'
--------------------------------------

To have a reference to an embedded single-line macro resolved at the
time that the embedding macro is _defined_, as opposed to when the
embedding macro is _expanded_, you need a different mechanism to the
one offered by `%define'. The solution is to use `%xdefine', or it's
case-insensitive counterpart `%ixdefine'.

   Suppose you have the following code:

     %define  isTrue  1
     %define  isFalse isTrue
     %define  isTrue  0

     val1:    db      isFalse

     %define  isTrue  1

     val2:    db      isFalse

   In this case, `val1' is equal to 0, and `val2' is equal to 1.  This
is because, when a single-line macro is defined using `%define', it is
expanded only when it is called. As `isFalse' expands to `isTrue', the
expansion will be the current value of `isTrue'.  The first time it is
called that is 0, and the second time it is 1.

   If you wanted `isFalse' to expand to the value assigned to the
embedded macro `isTrue' at the time that `isFalse' was defined, you
need to change the above code to use `%xdefine'.

     %xdefine isTrue  1
     %xdefine isFalse isTrue
     %xdefine isTrue  0

     val1:    db      isFalse

     %xdefine isTrue  1

     val2:    db      isFalse

   Now, each time that `isFalse' is called, it expands to 1, as that is
what the embedded macro `isTrue' expanded to at the time that `isFalse'
was defined.


File: nasm.info,  Node: Section 4.1.3,  Next: Section 4.1.4,  Prev: Section 4.1.2,  Up: Section 4.1

4.1.3. Concatenating Single Line Macro Tokens: `%+'
---------------------------------------------------

Individual tokens in single line macros can be concatenated, to produce
longer tokens for later processing. This can be useful if there are
several similar macros that perform similar functions.

   Please note that a space is required after `%+', in order to
disambiguate it from the syntax `%+1' used in multiline macros.

   As an example, consider the following:

     %define BDASTART 400h                ; Start of BIOS data area

     struc   tBIOSDA                      ; its structure
             .COM1addr       RESW    1
             .COM2addr       RESW    1
             ; ..and so on
     endstruc

   Now, if we need to access the elements of tBIOSDA in different
places, we can end up with:

             mov     ax,BDASTART + tBIOSDA.COM1addr
             mov     bx,BDASTART + tBIOSDA.COM2addr

   This will become pretty ugly (and tedious) if used in many places,
and can be reduced in size significantly by using the following macro:

     ; Macro to access BIOS variables by their names (from tBDA):

     %define BDA(x)  BDASTART + tBIOSDA. %+ x

   Now the above code can be written as:

             mov     ax,BDA(COM1addr)
             mov     bx,BDA(COM2addr)

   Using this feature, we can simplify references to a lot of macros
(and, in turn, reduce typing errors).


File: nasm.info,  Node: Section 4.1.4,  Next: Section 4.1.5,  Prev: Section 4.1.3,  Up: Section 4.1

4.1.4. The Macro Name Itself: `%?' and `%??'
--------------------------------------------

The special symbols `%?' and `%??' can be used to reference the macro
name itself inside a macro expansion, this is supported for both
single-and multi-line macros. `%?' refers to the macro name as
_invoked_, whereas `%??' refers to the macro name as _declared_. The
two are always the same for case-sensitive macros, but for
case-insensitive macros, they can differ.

   For example:

     %idefine Foo mov %?,%??

             foo
             FOO

   will expand to:

             mov foo,Foo
             mov FOO,Foo

   The sequence:

     %idefine keyword $%?

   can be used to make a keyword "disappear", for example in case a new
instruction has been used as a label in older code. For example:

     %idefine pause $%?                  ; Hide the PAUSE instruction


File: nasm.info,  Node: Section 4.1.5,  Next: Section 4.1.6,  Prev: Section 4.1.4,  Up: Section 4.1

4.1.5. Undefining Single-Line Macros: `%undef'
----------------------------------------------

Single-line macros can be removed with the `%undef' directive. For
example, the following sequence:

     %define foo bar
     %undef  foo

             mov     eax, foo

   will expand to the instruction `mov eax, foo', since after `%undef'
the macro `foo' is no longer defined.

   Macros that would otherwise be pre-defined can be undefined on the
command- line using the `-u' option on the NASM command line: see *note
Section 2.1.19::.


File: nasm.info,  Node: Section 4.1.6,  Next: Section 4.1.7,  Prev: Section 4.1.5,  Up: Section 4.1

4.1.6. Preprocessor Variables: `%assign'
----------------------------------------

An alternative way to define single-line macros is by means of the
`%assign' command (and its case-insensitive counterpart `%iassign',
which differs from `%assign' in exactly the same way that `%idefine'
differs from `%define').

   `%assign' is used to define single-line macros which take no
parameters and have a numeric value. This value can be specified in the
form of an expression, and it will be evaluated once, when the
`%assign' directive is processed.

   Like `%define', macros defined using `%assign' can be re-defined
later, so you can do things like

     %assign i i+1

   to increment the numeric value of a macro.

   `%assign' is useful for controlling the termination of `%rep'
preprocessor loops: see *note Section 4.5:: for an example of this.
Another use for `%assign' is given in *note Section 8.4:: and *note
Section 9.1::.

   The expression passed to `%assign' is a critical expression (see
*note Section 3.8::), and must also evaluate to a pure number (rather
than a relocatable reference such as a code or data address, or
anything involving a register).


File: nasm.info,  Node: Section 4.1.7,  Next: Section 4.2,  Prev: Section 4.1.6,  Up: Section 4.1

4.1.7. Defining Strings: `%defstr'
----------------------------------

`%defstr', and its case-insensitive counterpart `%idefstr', define or
redefine a single-line macro without parameters but converts the entire
right-hand side, after macro expansion, to a quoted string before
definition.

   For example:

     %defstr test TEST

   is equivalent to

     %define test 'TEST'

   This can be used, for example, with the `%!' construct (see *note
Section 4.10.2::):

     %defstr PATH %!PATH          ; The operating system PATH variable


File: nasm.info,  Node: Section 4.2,  Next: Section 4.2.1,  Prev: Section 4.1.7,  Up: Chapter 4

4.2. String Manipulation in Macros
==================================

It's often useful to be able to handle strings in macros. NASM supports
two simple string handling macro operators from which more complex
operations can be constructed.

   All the string operators define or redefine a value (either a string
or a numeric value) to a single-line macro.

* Menu:

* Section 4.2.1:: Concatenating Strings: `%strcat'
* Section 4.2.2:: String Length: `%strlen'
* Section 4.2.3:: Extracting Substrings: `%substr'


File: nasm.info,  Node: Section 4.2.1,  Next: Section 4.2.2,  Prev: Section 4.2,  Up: Section 4.2

4.2.1. Concatenating Strings: `%strcat'
---------------------------------------

The `%strcat' operator concatenates quoted strings and assign them to a
single-line macro. In doing so, it may change the type of quotes and
possibly use `\'-escapes inside ``'-quoted strings in order to make
sure the string is still a valid quoted string.

   For example:

     %strcat alpha "Alpha: ", '12" screen'

   ... would assign the value `'Alpha: 12" screen'' to `alpha'.
Similarly:

     %strcat beta '"', "'"

   ... would assign the value ``"'`' to `beta'.

   The use of commas to separate strings is permitted but optional.


File: nasm.info,  Node: Section 4.2.2,  Next: Section 4.2.3,  Prev: Section 4.2.1,  Up: Section 4.2

4.2.2. String Length: `%strlen'
-------------------------------

The `%strlen' operator assigns the length of a string to a macro. For
example:

     %strlen charcnt 'my string'

   In this example, `charcnt' would receive the value 9, just as if an
`%assign' had been used. In this example, `'my string'' was a literal
string but it could also have been a single-line macro that expands to
a string, as in the following example:

     %define sometext 'my string'
     %strlen charcnt sometext

   As in the first case, this would result in `charcnt' being assigned
the value of 9.


File: nasm.info,  Node: Section 4.2.3,  Next: Section 4.3,  Prev: Section 4.2.2,  Up: Section 4.2

4.2.3. Extracting Substrings: `%substr'
---------------------------------------

Individual letters or substrings in strings can be extracted using the
`%substr' operator. An example of its use is probably more useful than
the description:

     %substr mychar 'xyzw' 1       ; equivalent to %define mychar 'x'
     %substr mychar 'xyzw' 2       ; equivalent to %define mychar 'y'
     %substr mychar 'xyzw' 3       ; equivalent to %define mychar 'z'
     %substr mychar 'xyzw' 2,2     ; equivalent to %define mychar 'yz'
     %substr mychar 'xyzw' 2,-1    ; equivalent to %define mychar 'yzw'
     %substr mychar 'xyzw' 2,-2    ; equivalent to %define mychar 'yz'

   As with `%strlen' (see *note Section 4.2.2::), the first parameter is
the single-line macro to be created and the second is the string. The
third parameter specifies the first character to be selected, and the
optional fourth parameter preceeded by comma) is the length. Note that
the first index is 1, not 0 and the last index is equal to the value
that `%strlen' would assign given the same string. Index values out of
range result in an empty string. A negative length means "until N-1
characters before the end of string", i.e. `-1' means until end of
string, `-2' until one character before, etc.


File: nasm.info,  Node: Section 4.3,  Next: Section 4.3.1,  Prev: Section 4.2.3,  Up: Chapter 4

4.3. Multi-Line Macros: `%macro'
================================

Multi-line macros are much more like the type of macro seen in MASM and
TASM: a multi-line macro definition in NASM looks something like this.

     %macro  prologue 1

             push    ebp
             mov     ebp,esp
             sub     esp,%1

     %endmacro

   This defines a C-like function prologue as a macro: so you would
invoke the macro with a call such as

     myfunc:   prologue 12

   which would expand to the three lines of code

     myfunc: push    ebp
             mov     ebp,esp
             sub     esp,12

   The number `1' after the macro name in the `%macro' line defines the
number of parameters the macro `prologue' expects to receive. The use
of `%1' inside the macro definition refers to the first parameter to
the macro call. With a macro taking more than one parameter, subsequent
parameters would be referred to as `%2', `%3' and so on.

   Multi-line macros, like single-line macros, are case-sensitive,
unless you define them using the alternative directive `%imacro'.

   If you need to pass a comma as _part_ of a parameter to a multi-line
macro, you can do that by enclosing the entire parameter in braces. So
you could code things like

     %macro  silly 2

         %2: db      %1

     %endmacro

             silly 'a', letter_a             ; letter_a:  db 'a'
             silly 'ab', string_ab           ; string_ab: db 'ab'
             silly {13,10}, crlf             ; crlf:      db 13,10

* Menu:

* Section 4.3.1:: Overloading Multi-Line Macros
* Section 4.3.2:: Macro-Local Labels
* Section 4.3.3:: Greedy Macro Parameters
* Section 4.3.4:: Default Macro Parameters
* Section 4.3.5:: `%0': Macro Parameter Counter
* Section 4.3.6:: `%rotate': Rotating Macro Parameters
* Section 4.3.7:: Concatenating Macro Parameters
* Section 4.3.8:: Condition Codes as Macro Parameters
* Section 4.3.9:: Disabling Listing Expansion
* Section 4.3.10:: Undefining Multi-Line Macros: `%unmacro'


File: nasm.info,  Node: Section 4.3.1,  Next: Section 4.3.2,  Prev: Section 4.3,  Up: Section 4.3

4.3.1. Overloading Multi-Line Macros
------------------------------------

As with single-line macros, multi-line macros can be overloaded by
defining the same macro name several times with different numbers of
parameters.  This time, no exception is made for macros with no
parameters at all. So you could define

     %macro  prologue 0

             push    ebp
             mov     ebp,esp

     %endmacro

   to define an alternative form of the function prologue which
allocates no local stack space.

   Sometimes, however, you might want to `overload' a machine
instruction; for example, you might want to define

     %macro  push 2

             push    %1
             push    %2

     %endmacro

   so that you could code

             push    ebx             ; this line is not a macro call
             push    eax,ecx         ; but this one is

   Ordinarily, NASM will give a warning for the first of the above two
lines, since `push' is now defined to be a macro, and is being invoked
with a number of parameters for which no definition has been given. The
correct code will still be generated, but the assembler will give a
warning. This warning can be disabled by the use of the
`-w-macro-params' command- line option (see *note Section 2.1.24::).


File: nasm.info,  Node: Section 4.3.2,  Next: Section 4.3.3,  Prev: Section 4.3.1,  Up: Section 4.3

4.3.2. Macro-Local Labels
-------------------------

NASM allows you to define labels within a multi-line macro definition in
such a way as to make them local to the macro call: so calling the same
macro multiple times will use a different label each time. You do this
by prefixing `%%' to the label name. So you can invent an instruction
which executes a `RET' if the `Z' flag is set by doing this:

     %macro  retz 0

             jnz     %%skip
             ret
         %%skip:

     %endmacro

   You can call this macro as many times as you want, and every time
you call it NASM will make up a different `real' name to substitute for
the label `%%skip'. The names NASM invents are of the form
`..@2345.skip', where the number 2345 changes with every macro call.
The `..@' prefix prevents macro-local labels from interfering with the
local label mechanism, as described in *note Section 3.9::. You should
avoid defining your own labels in this form (the `..@' prefix, then a
number, then another period) in case they interfere with macro-local
labels.


File: nasm.info,  Node: Section 4.3.3,  Next: Section 4.3.4,  Prev: Section 4.3.2,  Up: Section 4.3

4.3.3. Greedy Macro Parameters
------------------------------

Occasionally it is useful to define a macro which lumps its entire
command line into one parameter definition, possibly after extracting
one or two smaller parameters from the front. An example might be a
macro to write a text string to a file in MS-DOS, where you might want
to be able to write

             writefile [filehandle],"hello, world",13,10

   NASM allows you to define the last parameter of a macro to be
_greedy_, meaning that if you invoke the macro with more parameters
than it expects, all the spare parameters get lumped into the last
defined one along with the separating commas. So if you code:

     %macro  writefile 2+

             jmp     %%endstr
       %%str:        db      %2
       %%endstr:
             mov     dx,%%str
             mov     cx,%%endstr-%%str
             mov     bx,%1
             mov     ah,0x40
             int     0x21

     %endmacro

   then the example call to `writefile' above will work as expected: the
text before the first comma, `[filehandle]', is used as the first macro
parameter and expanded when `%1' is referred to, and all the subsequent
text is lumped into `%2' and placed after the `db'.

   The greedy nature of the macro is indicated to NASM by the use of the
`+' sign after the parameter count on the `%macro' line.

   If you define a greedy macro, you are effectively telling NASM how it
should expand the macro given _any_ number of parameters from the
actual number specified up to infinity; in this case, for example, NASM
now knows what to do when it sees a call to `writefile' with 2, 3, 4 or
more parameters. NASM will take this into account when overloading
macros, and will not allow you to define another form of `writefile'
taking 4 parameters (for example).

   Of course, the above macro could have been implemented as a
non-greedy macro, in which case the call to it would have had to look
like

               writefile [filehandle], {"hello, world",13,10}

   NASM provides both mechanisms for putting commas in macro
parameters, and you choose which one you prefer for each macro
definition.

   See *note Section 6.3.1:: for a better way to write the above macro.


File: nasm.info,  Node: Section 4.3.4,  Next: Section 4.3.5,  Prev: Section 4.3.3,  Up: Section 4.3

4.3.4. Default Macro Parameters
-------------------------------

NASM also allows you to define a multi-line macro with a _range_ of
allowable parameter counts. If you do this, you can specify defaults for
omitted parameters. So, for example:

     %macro  die 0-1 "Painful program death has occurred."

             writefile 2,%1
             mov     ax,0x4c01
             int     0x21

     %endmacro

   This macro (which makes use of the `writefile' macro defined in
*note Section 4.3.3::) can be called with an explicit error message,
which it will display on the error output stream before exiting, or it
can be called with no parameters, in which case it will use the default
error message supplied in the macro definition.

   In general, you supply a minimum and maximum number of parameters
for a macro of this type; the minimum number of parameters are then
required in the macro call, and then you provide defaults for the
optional ones. So if a macro definition began with the line

     %macro foobar 1-3 eax,[ebx+2]

   then it could be called with between one and three parameters, and
`%1' would always be taken from the macro call. `%2', if not specified
by the macro call, would default to `eax', and `%3' if not specified
would default to `[ebx+2]'.

   You can provide extra information to a macro by providing too many
default parameters:

     %macro quux 1 something

   This will trigger a warning by default; see *note Section 2.1.24::
for more information. When `quux' is invoked, it receives not one but
two parameters. `something' can be referred to as `%2'. The difference
between passing `something' this way and writing `something' in the
macro body is that with this way `something' is evaluated when the
macro is defined, not when it is expanded.

   You may omit parameter defaults from the macro definition, in which
case the parameter default is taken to be blank. This can be useful for
macros which can take a variable number of parameters, since the `%0'
token (see *note Section 4.3.5::) allows you to determine how many
parameters were really passed to the macro call.

   This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the `die' macro above could be made more powerful, and
more useful, by changing the first line of the definition to

     %macro die 0-1+ "Painful program death has occurred.",13,10

   The maximum parameter count can be infinite, denoted by `*'. In this
case, of course, it is impossible to provide a _full_ set of default
parameters. Examples of this usage are shown in *note Section 4.3.6::.


File: nasm.info,  Node: Section 4.3.5,  Next: Section 4.3.6,  Prev: Section 4.3.4,  Up: Section 4.3

4.3.5. `%0': Macro Parameter Counter
------------------------------------

The parameter reference `%0' will return a numeric constant giving the
number of parameters received, that is, if `%0' is n then `%'n is the
last parameter. `%0' is mostly useful for macros that can take a
variable number of parameters. It can be used as an argument to `%rep'
(see *note Section 4.5::) in order to iterate through all the
parameters of a macro. Examples are given in *note Section 4.3.6::.


File: nasm.info,  Node: Section 4.3.6,  Next: Section 4.3.7,  Prev: Section 4.3.5,  Up: Section 4.3

4.3.6. `%rotate': Rotating Macro Parameters
-------------------------------------------

Unix shell programmers will be familiar with the `shift' shell command,
which allows the arguments passed to a shell script (referenced as
`$1', `$2' and so on) to be moved left by one place, so that the
argument previously referenced as `$2' becomes available as `$1', and
the argument previously referenced as `$1' is no longer available at
all.

   NASM provides a similar mechanism, in the form of `%rotate'. As its
name suggests, it differs from the Unix `shift' in that no parameters
are lost: parameters rotated off the left end of the argument list
reappear on the right, and vice versa.

   `%rotate' is invoked with a single numeric argument (which may be an
expression). The macro parameters are rotated to the left by that many
places. If the argument to `%rotate' is negative, the macro parameters
are rotated to the right.

   So a pair of macros to save and restore a set of registers might
work as follows:

     %macro  multipush 1-*

       %rep  %0
             push    %1
       %rotate 1
       %endrep

     %endmacro

   This macro invokes the `PUSH' instruction on each of its arguments in
turn, from left to right. It begins by pushing its first argument,
`%1', then invokes `%rotate' to move all the arguments one place to the
left, so that the original second argument is now available as `%1'.
Repeating this procedure as many times as there were arguments
(achieved by supplying `%0' as the argument to `%rep') causes each
argument in turn to be pushed.

   Note also the use of `*' as the maximum parameter count, indicating
that there is no upper limit on the number of parameters you may supply
to the `multipush' macro.

   It would be convenient, when using this macro, to have a `POP'
equivalent, which _didn't_ require the arguments to be given in reverse
order. Ideally, you would write the `multipush' macro call, then
cut-and-paste the line to where the pop needed to be done, and change
the name of the called macro to `multipop', and the macro would take
care of popping the registers in the opposite order from the one in
which they were pushed.

   This can be done by the following definition:

     %macro  multipop 1-*

       %rep %0
       %rotate -1
             pop     %1
       %endrep

     %endmacro

   This macro begins by rotating its arguments one place to the _right_,
so that the original _last_ argument appears as `%1'. This is then
popped, and the arguments are rotated right again, so the second-to-
last argument becomes `%1'. Thus the arguments are iterated through in
reverse order.


File: nasm.info,  Node: Section 4.3.7,  Next: Section 4.3.8,  Prev: Section 4.3.6,  Up: Section 4.3

4.3.7. Concatenating Macro Parameters
-------------------------------------

NASM can concatenate macro parameters on to other text surrounding them.
This allows you to declare a family of symbols, for example, in a macro
definition. If, for example, you wanted to generate a table of key codes
along with offsets into the table, you could code something like

     %macro keytab_entry 2

         keypos%1    equ     $-keytab
                     db      %2

     %endmacro

     keytab:
               keytab_entry F1,128+1
               keytab_entry F2,128+2
               keytab_entry Return,13

   which would expand to

     keytab:
     keyposF1        equ     $-keytab
                     db     128+1
     keyposF2        equ     $-keytab
                     db      128+2
     keyposReturn    equ     $-keytab
                     db      13

   You can just as easily concatenate text on to the other end of a
macro parameter, by writing `%1foo'.

   If you need to append a _digit_ to a macro parameter, for example
defining labels `foo1' and `foo2' when passed the parameter `foo', you
can't code `%11' because that would be taken as the eleventh macro
parameter. Instead, you must code `%{1}1', which will separate the
first `1' (giving the number of the macro parameter) from the second
(literal text to be concatenated to the parameter).

   This concatenation can also be applied to other preprocessor in-line
objects, such as macro-local labels (*note Section 4.3.2::) and
context-local labels (*note Section 4.7.2::). In all cases, ambiguities
in syntax can be resolved by enclosing everything after the `%' sign
and before the literal text in braces: so `%{%foo}bar' concatenates the
text `bar' to the end of the real name of the macro-local label
`%%foo'. (This is unnecessary, since the form NASM uses for the real
names of macro-local labels means that the two usages `%{%foo}bar' and
`%%foobar' would both expand to the same thing anyway; nevertheless,
the capability is there.)

   See also the `%+' operator, *note Section 4.1.3::.


File: nasm.info,  Node: Section 4.3.8,  Next: Section 4.3.9,  Prev: Section 4.3.7,  Up: Section 4.3

4.3.8. Condition Codes as Macro Parameters
------------------------------------------

NASM can give special treatment to a macro parameter which contains a
condition code. For a start, you can refer to the macro parameter `%1'
by means of the alternative syntax `%+1', which informs NASM that this
macro parameter is supposed to contain a condition code, and will cause
the preprocessor to report an error message if the macro is called with
a parameter which is _not_ a valid condition code.

   Far more usefully, though, you can refer to the macro parameter by
means of `%-1', which NASM will expand as the _inverse_ condition code.
So the `retz' macro defined in *note Section 4.3.2:: can be replaced by
a general conditional-return macro like this:

     %macro  retc 1

             j%-1    %%skip
             ret
       %%skip:

     %endmacro

   This macro can now be invoked using calls like `retc ne', which will
cause the conditional-jump instruction in the macro expansion to come
out as `JE', or `retc po' which will make the jump a `JPE'.

   The `%+1' macro-parameter reference is quite happy to interpret the
arguments `CXZ' and `ECXZ' as valid condition codes; however, `%-1'
will report an error if passed either of these, because no inverse
condition code exists.


File: nasm.info,  Node: Section 4.3.9,  Next: Section 4.3.10,  Prev: Section 4.3.8,  Up: Section 4.3

4.3.9. Disabling Listing Expansion
----------------------------------

When NASM is generating a listing file from your program, it will
generally expand multi-line macros by means of writing the macro call
and then listing each line of the expansion. This allows you to see
which instructions in the macro expansion are generating what code;
however, for some macros this clutters the listing up unnecessarily.

   NASM therefore provides the `.nolist' qualifier, which you can
include in a macro definition to inhibit the expansion of the macro in
the listing file. The `.nolist' qualifier comes directly after the
number of parameters, like this:

     %macro foo 1.nolist

   Or like this:

     %macro bar 1-5+.nolist a,b,c,d,e,f,g,h


File: nasm.info,  Node: Section 4.3.10,  Next: Section 4.4,  Prev: Section 4.3.9,  Up: Section 4.3

4.3.10. Undefining Multi-Line Macros: `%unmacro'
------------------------------------------------

Multi-line macros can be removed with the `%unmacro' directive. Unlike
the `%undef' directive, however, `%unmacro' takes an argument
specification, and will only remove exact matches with that argument
specification.

   For example:

     %macro foo 1-3
             ; Do something
     %endmacro
     %unmacro foo 1-3

   removes the previously defined macro `foo', but

     %unmacro bar 1-3
             ; Do something
     %endmacro
     %unmacro bar 1

   does _not_ remove the macro `bar', since the argument specification
does not match exactly.


File: nasm.info,  Node: Section 4.4,  Next: Section 4.4.1,  Prev: Section 4.3.10,  Up: Chapter 4

4.4. Conditional Assembly
=========================

Similarly to the C preprocessor, NASM allows sections of a source file
to be assembled only if certain conditions are met. The general syntax
of this feature looks like this:

     %if<condition>
         ; some code which only appears if <condition> is met
     %elif<condition2>
         ; only appears if <condition> is not met but <condition2> is
     %else
         ; this appears if neither <condition> nor <condition2> was met
     %endif

   The inverse forms `%ifn' and `%elifn' are also supported.

   The `%else' clause is optional, as is the `%elif' clause. You can
have more than one `%elif' clause as well.

   There are a number of variants of the `%if' directive. Each has its
corresponding `%elif', `%ifn', and `%elifn' directives; for example,
the equivalents to the `%ifdef' directive are `%elifdef', `%ifndef',
and `%elifndef'.

* Menu:

* Section 4.4.1:: `%ifdef': Testing Single-Line Macro Existence
* Section 4.4.2:: `%ifmacro': Testing Multi-Line Macro Existence
* Section 4.4.3:: `%ifctx': Testing the Context Stack
* Section 4.4.4:: `%if': Testing Arbitrary Numeric Expressions
* Section 4.4.5:: `%ifidn' and `%ifidni': Testing Exact Text Identity
* Section 4.4.6:: `%ifid', `%ifnum', `%ifstr': Testing Token Types
* Section 4.4.7:: `%iftoken': Test for a Single Token
* Section 4.4.8:: `%ifempty': Test for Empty Expansion


File: nasm.info,  Node: Section 4.4.1,  Next: Section 4.4.2,  Prev: Section 4.4,  Up: Section 4.4

4.4.1. `%ifdef': Testing Single-Line Macro Existence
----------------------------------------------------

Beginning a conditional-assembly block with the line `%ifdef MACRO'
will assemble the subsequent code if, and only if, a single-line macro
called `MACRO' is defined. If not, then the `%elif' and `%else' blocks
(if any) will be processed instead.

   For example, when debugging a program, you might want to write code
such as

               ; perform some function
     %ifdef DEBUG
               writefile 2,"Function performed successfully",13,10
     %endif
               ; go and do something else

   Then you could use the command-line option `-dDEBUG' to create a
version of the program which produced debugging messages, and remove the
option to generate the final release version of the program.

   You can test for a macro _not_ being defined by using `%ifndef'
instead of `%ifdef'. You can also test for macro definitions in `%elif'
blocks by using `%elifdef' and `%elifndef'.


File: nasm.info,  Node: Section 4.4.2,  Next: Section 4.4.3,  Prev: Section 4.4.1,  Up: Section 4.4

4.4.2. `%ifmacro': Testing Multi-Line Macro Existence
-----------------------------------------------------

The `%ifmacro' directive operates in the same way as the `%ifdef'
directive, except that it checks for the existence of a multi-line
macro.

   For example, you may be working with a large project and not have
control over the macros in a library. You may want to create a macro
with one name if it doesn't already exist, and another name if one with
that name does exist.

   The `%ifmacro' is considered true if defining a macro with the given
name and number of arguments would cause a definitions conflict. For
example:

     %ifmacro MyMacro 1-3

          %error "MyMacro 1-3" causes a conflict with an existing macro.

     %else

          %macro MyMacro 1-3

                  ; insert code to define the macro

          %endmacro

     %endif

   This will create the macro "MyMacro 1-3" if no macro already exists
which would conflict with it, and emits a warning if there would be a
definition conflict.

   You can test for the macro not existing by using the `%ifnmacro'
instead of `%ifmacro'. Additional tests can be performed in `%elif'
blocks by using `%elifmacro' and `%elifnmacro'.


File: nasm.info,  Node: Section 4.4.3,  Next: Section 4.4.4,  Prev: Section 4.4.2,  Up: Section 4.4

4.4.3. `%ifctx': Testing the Context Stack
------------------------------------------

The conditional-assembly construct `%ifctx' will cause the subsequent
code to be assembled if and only if the top context on the
preprocessor's context stack has the same name as one of the arguments.
As with `%ifdef', the inverse and `%elif' forms `%ifnctx', `%elifctx'
and `%elifnctx' are also supported.

   For more details of the context stack, see *note Section 4.7::. For
a sample use of `%ifctx', see *note Section 4.7.5::.


File: nasm.info,  Node: Section 4.4.4,  Next: Section 4.4.5,  Prev: Section 4.4.3,  Up: Section 4.4

4.4.4. `%if': Testing Arbitrary Numeric Expressions
---------------------------------------------------

The conditional-assembly construct `%if expr' will cause the subsequent
code to be assembled if and only if the value of the numeric expression
`expr' is non-zero. An example of the use of this feature is in
deciding when to break out of a `%rep' preprocessor loop: see *note
Section 4.5:: for a detailed example.

   The expression given to `%if', and its counterpart `%elif', is a
critical expression (see *note Section 3.8::).

   `%if' extends the normal NASM expression syntax, by providing a set
of relational operators which are not normally available in
expressions. The operators `=', `<', `>', `<=', `>=' and `<>' test
equality, less-than, greater-than, less-or-equal, greater-or-equal and
not-equal respectively. The C-like forms `==' and `!=' are supported as
alternative forms of `=' and `<>'. In addition, low- priority logical
operators `&&', `^^' and `||' are provided, supplying logical AND,
logical XOR and logical OR. These work like the C logical operators
(although C has no logical XOR), in that they always return either 0 or
1, and treat any non-zero input as 1 (so that `^^', for example,
returns 1 if exactly one of its inputs is zero, and 0 otherwise). The
relational operators also return 1 for true and 0 for false.

   Like other `%if' constructs, `%if' has a counterpart `%elif', and
negative forms `%ifn' and `%elifn'.


File: nasm.info,  Node: Section 4.4.5,  Next: Section 4.4.6,  Prev: Section 4.4.4,  Up: Section 4.4

4.4.5. `%ifidn' and `%ifidni': Testing Exact Text Identity
----------------------------------------------------------

The construct `%ifidn text1,text2' will cause the subsequent code to be
assembled if and only if `text1' and `text2', after expanding
single-line macros, are identical pieces of text. Differences in white
space are not counted.

   `%ifidni' is similar to `%ifidn', but is case-insensitive.

   For example, the following macro pushes a register or number on the
stack, and allows you to treat `IP' as a real register:

     %macro  pushparam 1

       %ifidni %1,ip
             call    %%label
       %%label:
       %else
             push    %1
       %endif

     %endmacro

   Like other `%if' constructs, `%ifidn' has a counterpart `%elifidn',
and negative forms `%ifnidn' and `%elifnidn'.  Similarly, `%ifidni' has
counterparts `%elifidni', `%ifnidni' and `%elifnidni'.


File: nasm.info,  Node: Section 4.4.6,  Next: Section 4.4.7,  Prev: Section 4.4.5,  Up: Section 4.4

4.4.6. `%ifid', `%ifnum', `%ifstr': Testing Token Types
-------------------------------------------------------

Some macros will want to perform different tasks depending on whether
they are passed a number, a string, or an identifier. For example, a
string output macro might want to be able to cope with being passed
either a string constant or a pointer to an existing string.

   The conditional assembly construct `%ifid', taking one parameter
(which may be blank), assembles the subsequent code if and only if the
first token in the parameter exists and is an identifier. `%ifnum'
works similarly, but tests for the token being a numeric constant;
`%ifstr' tests for it being a string.

   For example, the `writefile' macro defined in *note Section 4.3.3::
can be extended to take advantage of `%ifstr' in the following fashion:

     %macro writefile 2-3+

       %ifstr %2
             jmp     %%endstr
         %if %0 = 3
           %%str:    db      %2,%3
         %else
           %%str:    db      %2
         %endif
           %%endstr: mov     dx,%%str
                     mov     cx,%%endstr-%%str
       %else
                     mov     dx,%2
                     mov     cx,%3
       %endif
                     mov     bx,%1
                     mov     ah,0x40
                     int     0x21

     %endmacro

   Then the `writefile' macro can cope with being called in either of
the following two ways:

             writefile [file], strpointer, length
             writefile [file], "hello", 13, 10

   In the first, `strpointer' is used as the address of an already-
declared string, and `length' is used as its length; in the second, a
string is given to the macro, which therefore declares it itself and
works out the address and length for itself.

   Note the use of `%if' inside the `%ifstr': this is to detect whether
the macro was passed two arguments (so the string would be a single
string constant, and `db %2' would be adequate) or more (in which case,
all but the first two would be lumped together into `%3', and `db
%2,%3' would be required).

   The usual `%elif'..., `%ifn'..., and `%elifn'... versions exist for
each of `%ifid', `%ifnum' and `%ifstr'.


File: nasm.info,  Node: Section 4.4.7,  Next: Section 4.4.8,  Prev: Section 4.4.6,  Up: Section 4.4

4.4.7. `%iftoken': Test for a Single Token
------------------------------------------

Some macros will want to do different things depending on if it is
passed a single token (e.g. paste it to something else using `%+')
versus a multi-token sequence.

   The conditional assembly construct `%iftoken' assembles the
subsequent code if and only if the expanded parameters consist of
exactly one token, possibly surrounded by whitespace.

   For example:

     %iftoken 1

   will assemble the subsequent code, but

     %iftoken -1

   will not, since `-1' contains two tokens: the unary minus operator
`-', and the number `1'.

   The usual `%eliftoken', `%ifntoken', and `%elifntoken' variants are
also provided.


File: nasm.info,  Node: Section 4.4.8,  Next: Section 4.5,  Prev: Section 4.4.7,  Up: Section 4.4

4.4.8. `%ifempty': Test for Empty Expansion
-------------------------------------------

The conditional assembly construct `%ifempty' assembles the subsequent
code if and only if the expanded parameters do not contain any tokens at
all, whitespace excepted.

   The usual `%elifempty', `%ifnempty', and `%elifnempty' variants are
also provided.


File: nasm.info,  Node: Section 4.5,  Next: Section 4.6,  Prev: Section 4.4.8,  Up: Chapter 4

4.5. Preprocessor Loops: `%rep'
===============================

NASM's `TIMES' prefix, though useful, cannot be used to invoke a
multi-line macro multiple times, because it is processed by NASM after
macros have already been expanded. Therefore NASM provides another form
of loop, this time at the preprocessor level: `%rep'.

   The directives `%rep' and `%endrep' (`%rep' takes a numeric
argument, which can be an expression; `%endrep' takes no arguments) can
be used to enclose a chunk of code, which is then replicated as many
times as specified by the preprocessor:

     %assign i 0
     %rep    64
             inc     word [table+2*i]
     %assign i i+1
     %endrep

   This will generate a sequence of 64 `INC' instructions, incrementing
every word of memory from `[table]' to `[table+126]'.

   For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the `%exitrep' directive to terminate
the loop, like this:

     fibonacci:
     %assign i 0
     %assign j 1
     %rep 100
     %if j > 65535
         %exitrep
     %endif
             dw j
     %assign k j+i
     %assign i j
     %assign j k
     %endrep

     fib_number equ ($-fibonacci)/2

   This produces a list of all the Fibonacci numbers that will fit in
16 bits.  Note that a maximum repeat count must still be given to
`%rep'. This is to prevent the possibility of NASM getting into an
infinite loop in the preprocessor, which (on multitasking or multi-user
systems) would typically cause all the system memory to be gradually
used up and other applications to start crashing.


File: nasm.info,  Node: Section 4.6,  Next: Section 4.6.1,  Prev: Section 4.5,  Up: Chapter 4

4.6. Source Files and Dependencies
==================================

These commands allow you to split your sources into multiple files.

* Menu:

* Section 4.6.1:: `%include': Including Other Files
* Section 4.6.2:: `%pathsearch': Search the Include Path
* Section 4.6.3:: `%depend': Add Dependent Files
* Section 4.6.4:: `%use': Include Standard Macro Package


File: nasm.info,  Node: Section 4.6.1,  Next: Section 4.6.2,  Prev: Section 4.6,  Up: Section 4.6

4.6.1. `%include': Including Other Files
----------------------------------------

Using, once again, a very similar syntax to the C preprocessor, NASM's
preprocessor lets you include other source files into your code. This is
done by the use of the `%include' directive:

     %include "macros.mac"

   will include the contents of the file `macros.mac' into the source
file containing the `%include' directive.

   Include files are searched for in the current directory (the
directory you're in when you run NASM, as opposed to the location of
the NASM executable or the location of the source file), plus any
directories specified on the NASM command line using the `-i' option.

   The standard C idiom for preventing a file being included more than
once is just as applicable in NASM: if the file `macros.mac' has the
form

     %ifndef MACROS_MAC
         %define MACROS_MAC
         ; now define some macros
     %endif

   then including the file more than once will not cause errors,
because the second time the file is included nothing will happen
because the macro `MACROS_MAC' will already be defined.

   You can force a file to be included even if there is no `%include'
directive that explicitly includes it, by using the `-p' option on the
NASM command line (see *note Section 2.1.17::).


File: nasm.info,  Node: Section 4.6.2,  Next: Section 4.6.3,  Prev: Section 4.6.1,  Up: Section 4.6

4.6.2. `%pathsearch': Search the Include Path
---------------------------------------------

The `%pathsearch' directive takes a single-line macro name and a
filename, and declare or redefines the specified single-line macro to be
the include-path-resolved verson of the filename, if the file exists
(otherwise, it is passed unchanged.)

   For example,

     %pathsearch MyFoo "foo.bin"

   ... with `-Ibins/' in the include path may end up defining the macro
`MyFoo' to be `"bins/foo.bin"'.


File: nasm.info,  Node: Section 4.6.3,  Next: Section 4.6.4,  Prev: Section 4.6.2,  Up: Section 4.6

4.6.3. `%depend': Add Dependent Files
-------------------------------------

The `%depend' directive takes a filename and adds it to the list of
files to be emitted as dependency generation when the `-M' options and
its relatives (see *note Section 2.1.4::) are used. It produces no
output.

   This is generally used in conjunction with `%pathsearch'. For
example, a simplified version of the standard macro wrapper for the
`INCBIN' directive looks like:

     %imacro incbin 1-2+ 0
     %pathsearch dep %1
     %depend dep
             incbin dep,%2
     %endmacro

   This first resolves the location of the file into the macro `dep',
then adds it to the dependency lists, and finally issues the assembler-
level `INCBIN' directive.


File: nasm.info,  Node: Section 4.6.4,  Next: Section 4.7,  Prev: Section 4.6.3,  Up: Section 4.6

4.6.4. `%use': Include Standard Macro Package
---------------------------------------------

The `%use' directive is similar to `%include', but rather than
including the contents of a file, it includes a named standard macro
package. The standard macro packages are part of NASM, and are
described in *note Chapter 5::.

   Unlike the `%include' directive, package names for the `%use'
directive do not require quotes, but quotes are permitted; using quotes
will prevent unwanted macro expansion. Thus, the following lines are
equivalent, unless `altreg' is defined as a macro:

     %use altreg
     %use 'altreg'

   Standard macro packages are protected from multiple inclusion. When a
standard macro package is used, a testable single-line macro of the form
`__USE_'_package_`__' is also defined, see *note Section 4.11.8::.


File: nasm.info,  Node: Section 4.7,  Next: Section 4.7.1,  Prev: Section 4.6.4,  Up: Chapter 4

4.7. The Context Stack
======================

Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a `REPEAT' ... `UNTIL'
loop, in which the expansion of the `REPEAT' macro would need to be
able to refer to a label which the `UNTIL' macro had defined. However,
for such a macro you would also want to be able to nest these loops.

   NASM provides this level of power by means of a _context stack_. The
preprocessor maintains a stack of _contexts_, each of which is
characterized by a name. You add a new context to the stack using the
`%push' directive, and remove one using `%pop'. You can define labels
that are local to a particular context on the stack.

* Menu:

* Section 4.7.1:: `%push' and `%pop': Creating and Removing Contexts
* Section 4.7.2:: Context-Local Labels
* Section 4.7.3:: Context-Local Single-Line Macros
* Section 4.7.4:: `%repl': Renaming a Context
* Section 4.7.5:: Example Use of the Context Stack: Block IFs


File: nasm.info,  Node: Section 4.7.1,  Next: Section 4.7.2,  Prev: Section 4.7,  Up: Section 4.7

4.7.1. `%push' and `%pop': Creating and Removing Contexts
---------------------------------------------------------

The `%push' directive is used to create a new context and place it on
the top of the context stack. `%push' requires one argument, which is
the name of the context. For example:

     %push    foobar

   This pushes a new context called `foobar' on the stack. You can have
several contexts on the stack with the same name: they can still be
distinguished.

   The directive `%pop', requiring no arguments, removes the top context
from the context stack and destroys it, along with any labels associated
with it.


File: nasm.info,  Node: Section 4.7.2,  Next: Section 4.7.3,  Prev: Section 4.7.1,  Up: Section 4.7

4.7.2. Context-Local Labels
---------------------------

Just as the usage `%%foo' defines a label which is local to the
particular macro call in which it is used, the usage `%$foo' is used to
define a label which is local to the context on the top of the context
stack. So the `REPEAT' and `UNTIL' example given above could be
implemented by means of:

     %macro repeat 0

         %push   repeat
         %$begin:

     %endmacro

     %macro until 1

             j%-1    %$begin
         %pop

     %endmacro

   and invoked by means of, for example,

             mov     cx,string
             repeat
             add     cx,3
             scasb
             until   e

   which would scan every fourth byte of a string in search of the byte
in `AL'.

   If you need to define, or access, labels local to the context _below_
the top one on the stack, you can use `%$$foo', or `%$$$foo' for the
context below that, and so on.


File: nasm.info,  Node: Section 4.7.3,  Next: Section 4.7.4,  Prev: Section 4.7.2,  Up: Section 4.7

4.7.3. Context-Local Single-Line Macros
---------------------------------------

NASM also allows you to define single-line macros which are local to a
particular context, in just the same way:

     %define %$localmac 3

   will define the single-line macro `%$localmac' to be local to the top
context on the stack. Of course, after a subsequent `%push', it can
then still be accessed by the name `%$$localmac'.


File: nasm.info,  Node: Section 4.7.4,  Next: Section 4.7.5,  Prev: Section 4.7.3,  Up: Section 4.7

4.7.4. `%repl': Renaming a Context
----------------------------------

If you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to `%ifctx'), you can
execute a `%pop' followed by a `%push'; but this will have the side
effect of destroying all context-local labels and macros associated
with the context that was just popped.

   NASM provides the directive `%repl', which _replaces_ a context with
a different name, without touching the associated macros and labels.
So you could replace the destructive code

     %pop
     %push   newname

   with the non-destructive version `%repl newname'.


File: nasm.info,  Node: Section 4.7.5,  Next: Section 4.8,  Prev: Section 4.7.4,  Up: Section 4.7

4.7.5. Example Use of the Context Stack: Block IFs
--------------------------------------------------

This example makes use of almost all the context-stack features,
including the conditional-assembly construct `%ifctx', to implement a
block IF statement as a set of macros.

     %macro if 1

         %push if
         j%-1  %$ifnot

     %endmacro

     %macro else 0

       %ifctx if
             %repl   else
             jmp     %$ifend
             %$ifnot:
       %else
             %error  "expected `if' before `else'"
       %endif

     %endmacro

     %macro endif 0

       %ifctx if
             %$ifnot:
             %pop
       %elifctx      else
             %$ifend:
             %pop
       %else
             %error  "expected `if' or `else' before `endif'"
       %endif

     %endmacro

   This code is more robust than the `REPEAT' and `UNTIL' macros given
in *note Section 4.7.2::, because it uses conditional assembly to check
that the macros are issued in the right order (for example, not calling
`endif' before `if') and issues a `%error' if they're not.

   In addition, the `endif' macro has to be able to cope with the two
distinct cases of either directly following an `if', or following an
`else'. It achieves this, again, by using conditional assembly to do
different things depending on whether the context on top of the stack is
`if' or `else'.

   The `else' macro has to preserve the context on the stack, in order
to have the `%$ifnot' referred to by the `if' macro be the same as the
one defined by the `endif' macro, but has to change the context's name
so that `endif' will know there was an intervening `else'.  It does
this by the use of `%repl'.

   A sample usage of these macros might look like:

             cmp     ax,bx

             if ae
                    cmp     bx,cx

                    if ae
                            mov     ax,cx
                    else
                            mov     ax,bx
                    endif

             else
                    cmp     ax,cx

                    if ae
                            mov     ax,cx
                    endif

             endif

   The block-`IF' macros handle nesting quite happily, by means of
pushing another context, describing the inner `if', on top of the one
describing the outer `if'; thus `else' and `endif' always refer to the
last unmatched `if' or `else'.


File: nasm.info,  Node: Section 4.8,  Next: Section 4.8.1,  Prev: Section 4.7.5,  Up: Chapter 4

4.8. Stack Relative Preprocessor Directives
===========================================

The following preprocessor directives provide a way to use labels to
refer to local variables allocated on the stack.

   * `%arg' (see *note Section 4.8.1::)

   * `%stacksize' (see *note Section 4.8.2::)

   * `%local' (see *note Section 4.8.3::)

* Menu:

* Section 4.8.1:: `%arg' Directive
* Section 4.8.2:: `%stacksize' Directive
* Section 4.8.3:: `%local' Directive


File: nasm.info,  Node: Section 4.8.1,  Next: Section 4.8.2,  Prev: Section 4.8,  Up: Section 4.8

4.8.1. `%arg' Directive
-----------------------

The `%arg' directive is used to simplify the handling of parameters
passed on the stack. Stack based parameter passing is used by many high
level languages, including C, C++ and Pascal.

   While NASM has macros which attempt to duplicate this functionality
(see *note Section 8.4.5::), the syntax is not particularly convenient
to use. and is not TASM compatible. Here is an example which shows the
use of `%arg' without any external macros:

     some_function:

         %push     mycontext        ; save the current context
         %stacksize large           ; tell NASM to use bp
         %arg      i:word, j_ptr:word

             mov     ax,[i]
             mov     bx,[j_ptr]
             add     ax,[bx]
             ret

         %pop                       ; restore original context

   This is similar to the procedure defined in *note Section 8.4.5::
and adds the value in i to the value pointed to by j_ptr and returns
the sum in the ax register. See *note Section 4.7.1:: for an
explanation of `push' and `pop' and the use of context stacks.


File: nasm.info,  Node: Section 4.8.2,  Next: Section 4.8.3,  Prev: Section 4.8.1,  Up: Section 4.8

4.8.2. `%stacksize' Directive
-----------------------------

The `%stacksize' directive is used in conjunction with the `%arg' (see
*note Section 4.8.1::) and the `%local' (see *note Section 4.8.3::)
directives. It tells NASM the default size to use for subsequent `%arg'
and `%local' directives. The `%stacksize' directive takes one required
argument which is one of `flat', `flat64', `large' or `small'.

     %stacksize flat

   This form causes NASM to use stack-based parameter addressing
relative to `ebp' and it assumes that a near form of call was used to
get to this label (i.e. that `eip' is on the stack).

     %stacksize flat64

   This form causes NASM to use stack-based parameter addressing
relative to `rbp' and it assumes that a near form of call was used to
get to this label (i.e. that `rip' is on the stack).

     %stacksize large

   This form uses `bp' to do stack-based parameter addressing and
assumes that a far form of call was used to get to this address (i.e.
that `ip' and `cs' are on the stack).

     %stacksize small

   This form also uses `bp' to address stack parameters, but it is
different from `large' because it also assumes that the old value of bp
is pushed onto the stack (i.e. it expects an `ENTER' instruction).  In
other words, it expects that `bp', `ip' and `cs' are on the top of the
stack, underneath any local space which may have been allocated by
`ENTER'. This form is probably most useful when used in combination
with the `%local' directive (see *note Section 4.8.3::).


File: nasm.info,  Node: Section 4.8.3,  Next: Section 4.9,  Prev: Section 4.8.2,  Up: Section 4.8

4.8.3. `%local' Directive
-------------------------

The `%local' directive is used to simplify the use of local temporary
stack variables allocated in a stack frame. Automatic local variables
in C are an example of this kind of variable. The `%local' directive is
most useful when used with the `%stacksize' (see *note Section 4.8.2::
and is also compatible with the `%arg' directive (see *note Section
4.8.1::). It allows simplified reference to variables on the stack which
have been allocated typically by using the `ENTER' instruction. An
example of its use is the following:

     silly_swap:

         %push mycontext             ; save the current context
         %stacksize small            ; tell NASM to use bp
         %assign %$localsize 0       ; see text for explanation
         %local old_ax:word, old_dx:word

             enter   %$localsize,0   ; see text for explanation
             mov     [old_ax],ax     ; swap ax & bx
             mov     [old_dx],dx     ; and swap dx & cx
             mov     ax,bx
             mov     dx,cx
             mov     bx,[old_ax]
             mov     cx,[old_dx]
             leave                   ; restore old bp
             ret                     ;

         %pop                        ; restore original context

   The `%$localsize' variable is used internally by the `%local'
directive and _must_ be defined within the current context before the
`%local' directive may be used. Failure to do so will result in one
expression syntax error for each `%local' variable declared. It then
may be used in the construction of an appropriately sized ENTER
instruction as shown in the example.


File: nasm.info,  Node: Section 4.9,  Next: Section 4.10,  Prev: Section 4.8.3,  Up: Chapter 4

4.9. Reporting User-Defined Errors: `%error', `%warning', `%fatal'
==================================================================

The preprocessor directive `%error' will cause NASM to report an error
if it occurs in assembled code. So if other users are going to try to
assemble your source files, you can ensure that they define the right
macros by means of code like this:

     %ifdef F1
         ; do some setup
     %elifdef F2
         ; do some different setup
     %else
         %error "Neither F1 nor F2 was defined."
     %endif

   Then any user who fails to understand the way your code is supposed
to be assembled will be quickly warned of their mistake, rather than
having to wait until the program crashes on being run and then not
knowing what went wrong.

   Similarly, `%warning' issues a warning, but allows assembly to
continue:

     %ifdef F1
         ; do some setup
     %elifdef F2
         ; do some different setup
     %else
         %warning "Neither F1 nor F2 was defined, assuming F1."
         %define F1
     %endif

   `%error' and `%warning' are issued only on the final assembly pass.
This makes them safe to use in conjunction with tests that depend on
symbol values.

   `%fatal' terminates assembly immediately, regardless of pass. This is
useful when there is no point in continuing the assembly further, and
doing so is likely just going to cause a spew of confusing error
messages.

   It is optional for the message string after `%error', `%warning' or
`%fatal' to be quoted. If it is _not_, then single-line macros are
expanded in it, which can be used to display more information to the
user. For example:

     %if foo > 64
         %assign foo_over foo-64
         %error foo is foo_over bytes too large
     %endif


File: nasm.info,  Node: Section 4.10,  Next: Section 4.10.1,  Prev: Section 4.9,  Up: Chapter 4

4.10. Other Preprocessor Directives
===================================

NASM also has preprocessor directives which allow access to information
from external sources. Currently they include:

   The following preprocessor directive is supported to allow NASM to
correctly handle output of the cpp C language preprocessor.

   * `%line' enables NASM to correctly handle the output of the cpp C
     language preprocessor (see *note Section 4.10.1::).

   * `%!' enables NASM to read in the value of an environment variable,
     which can then be used in your program (see *note Section
     4.10.2::).

* Menu:

* Section 4.10.1:: `%line' Directive
* Section 4.10.2:: `%!'`<env>': Read an environment variable.


File: nasm.info,  Node: Section 4.10.1,  Next: Section 4.10.2,  Prev: Section 4.10,  Up: Section 4.10

4.10.1. `%line' Directive
-------------------------

The `%line' directive is used to notify NASM that the input line
corresponds to a specific line number in another file. Typically this
other file would be an original source file, with the current NASM
input being the output of a pre-processor. The `%line' directive allows
NASM to output messages which indicate the line number of the original
source file, instead of the file that is being read by NASM.

   This preprocessor directive is not generally of use to programmers,
by may be of interest to preprocessor authors. The usage of the `%line'
preprocessor directive is as follows:

     %line nnn[+mmm] [filename]

   In this directive, `nnn' identifies the line of the original source
file which this line corresponds to. `mmm' is an optional parameter
which specifies a line increment value; each line of the input file
read in is considered to correspond to `mmm' lines of the original
source file. Finally, `filename' is an optional parameter which
specifies the file name of the original source file.

   After reading a `%line' preprocessor directive, NASM will report all
file name and line numbers relative to the values specified therein.


File: nasm.info,  Node: Section 4.10.2,  Next: Section 4.11,  Prev: Section 4.10.1,  Up: Section 4.10

4.10.2. `%!'`<env>': Read an environment variable.
--------------------------------------------------

The `%!<env>' directive makes it possible to read the value of an
environment variable at assembly time. This could, for example, be used
to store the contents of an environment variable into a string, which
could be used at some other point in your code.

   For example, suppose that you have an environment variable `FOO', and
you want the contents of `FOO' to be embedded in your program. You
could do that as follows:

     %defstr FOO    %!FOO

   See *note Section 4.1.7:: for notes on the `%defstr' directive.


File: nasm.info,  Node: Section 4.11,  Next: Section 4.11.1,  Prev: Section 4.10.2,  Up: Chapter 4

4.11. Standard Macros
=====================

NASM defines a set of standard macros, which are already defined when it
starts to process any source file. If you really need a program to be
assembled with no pre-defined macros, you can use the `%clear'
directive to empty the preprocessor of everything but context-local
preprocessor variables and single-line macros.

   Most user-level assembler directives (see *note Chapter 6::) are
implemented as macros which invoke primitive directives; these are
described in *note Chapter 6::. The rest of the standard macro set is
described here.

* Menu:

* Section 4.11.1:: NASM Version Macros
* Section 4.11.2:: `__NASM_VERSION_ID__': NASM Version ID
* Section 4.11.3:: `__NASM_VER__': NASM Version string
* Section 4.11.4:: `__FILE__' and `__LINE__': File Name and Line Number
* Section 4.11.5:: `__BITS__': Current BITS Mode
* Section 4.11.6:: `__OUTPUT_FORMAT__': Current Output Format
* Section 4.11.7:: Assembly Date and Time Macros
* Section 4.11.8:: `__USE_'_package_`__': Package Include Test
* Section 4.11.9:: `__PASS__': Assembly Pass
* Section 4.11.10:: `STRUC' and `ENDSTRUC': Declaring Structure Data Types
* Section 4.11.11:: `ISTRUC', `AT' and `IEND': Declaring Instances of Structures
* Section 4.11.12:: `ALIGN' and `ALIGNB': Data Alignment


File: nasm.info,  Node: Section 4.11.1,  Next: Section 4.11.2,  Prev: Section 4.11,  Up: Section 4.11

4.11.1. NASM Version Macros
---------------------------

The single-line macros `__NASM_MAJOR__', `__NASM_MINOR__',
`__NASM_SUBMINOR__' and `___NASM_PATCHLEVEL__' expand to the major,
minor, subminor and patch level parts of the version number of NASM
being used. So, under NASM 0.98.32p1 for example, `__NASM_MAJOR__'
would be defined to be 0, `__NASM_MINOR__' would be defined as 98,
`__NASM_SUBMINOR__' would be defined to 32, and `___NASM_PATCHLEVEL__'
would be defined as 1.

   Additionally, the macro `__NASM_SNAPSHOT__' is defined for
automatically generated snapshot releases _only_.


File: nasm.info,  Node: Section 4.11.2,  Next: Section 4.11.3,  Prev: Section 4.11.1,  Up: Section 4.11

4.11.2. `__NASM_VERSION_ID__': NASM Version ID
----------------------------------------------

The single-line macro `__NASM_VERSION_ID__' expands to a dword integer
representing the full version number of the version of nasm being used.
The value is the equivalent to `__NASM_MAJOR__', `__NASM_MINOR__',
`__NASM_SUBMINOR__' and `___NASM_PATCHLEVEL__' concatenated to produce
a single doubleword. Hence, for 0.98.32p1, the returned number would be
equivalent to:

             dd      0x00622001

   or

             db      1,32,98,0

   Note that the above lines are generate exactly the same code, the
second line is used just to give an indication of the order that the
separate values will be present in memory.


File: nasm.info,  Node: Section 4.11.3,  Next: Section 4.11.4,  Prev: Section 4.11.2,  Up: Section 4.11

4.11.3. `__NASM_VER__': NASM Version string
-------------------------------------------

The single-line macro `__NASM_VER__' expands to a string which defines
the version number of nasm being used. So, under NASM 0.98.32 for
example,

             db      __NASM_VER__

   would expand to

             db      "0.98.32"


File: nasm.info,  Node: Section 4.11.4,  Next: Section 4.11.5,  Prev: Section 4.11.3,  Up: Section 4.11

4.11.4. `__FILE__' and `__LINE__': File Name and Line Number
------------------------------------------------------------

Like the C preprocessor, NASM allows the user to find out the file name
and line number containing the current instruction. The macro `__FILE__'
expands to a string constant giving the name of the current input file
(which may change through the course of assembly if `%include'
directives are used), and `__LINE__' expands to a numeric constant
giving the current line number in the input file.

   These macros could be used, for example, to communicate debugging
information to a macro, since invoking `__LINE__' inside a macro
definition (either single-line or multi-line) will return the line
number of the macro _call_, rather than _definition_. So to determine
where in a piece of code a crash is occurring, for example, one could
write a routine `stillhere', which is passed a line number in `EAX' and
outputs something like `line 155: still here'. You could then write a
macro

     %macro  notdeadyet 0

             push    eax
             mov     eax,__LINE__
             call    stillhere
             pop     eax

     %endmacro

   and then pepper your code with calls to `notdeadyet' until you find
the crash point.


File: nasm.info,  Node: Section 4.11.5,  Next: Section 4.11.6,  Prev: Section 4.11.4,  Up: Section 4.11

4.11.5. `__BITS__': Current BITS Mode
-------------------------------------

The `__BITS__' standard macro is updated every time that the BITS mode
is set using the `BITS XX' or `[BITS XX]' directive, where XX is a
valid mode number of 16, 32 or 64. `__BITS__' receives the specified
mode number and makes it globally available. This can be very useful for
those who utilize mode-dependent macros.


File: nasm.info,  Node: Section 4.11.6,  Next: Section 4.11.7,  Prev: Section 4.11.5,  Up: Section 4.11

4.11.6. `__OUTPUT_FORMAT__': Current Output Format
--------------------------------------------------

The `__OUTPUT_FORMAT__' standard macro holds the current Output Format,
as given by the `-f' option or NASM's default. Type `nasm -hf' for a
list.

     %ifidn __OUTPUT_FORMAT__, win32
      %define NEWLINE 13, 10
     %elifidn __OUTPUT_FORMAT__, elf32
      %define NEWLINE 10
     %endif


File: nasm.info,  Node: Section 4.11.7,  Next: Section 4.11.8,  Prev: Section 4.11.6,  Up: Section 4.11

4.11.7. Assembly Date and Time Macros
-------------------------------------

NASM provides a variety of macros that represent the timestamp of the
assembly session.

   * The `__DATE__' and `__TIME__' macros give the assembly date and
     time as strings, in ISO 8601 format (`"YYYY-MM-DD"' and
     `"HH:MM:SS"', respectively.)

   * The `__DATE_NUM__' and `__TIME_NUM__' macros give the assembly
     date and time in numeric form; in the format `YYYYMMDD' and
     `HHMMSS' respectively.

   * The `__UTC_DATE__' and `__UTC_TIME__' macros give the assembly
     date and time in universal time (UTC) as strings, in ISO 8601
     format (`"YYYY-MM-DD"' and `"HH:MM:SS"', respectively.) If the host
     platform doesn't provide UTC time, these macros are undefined.

   * The `__UTC_DATE_NUM__' and `__UTC_TIME_NUM__' macros give the
     assembly date and time universal time (UTC) in numeric form; in
     the format `YYYYMMDD' and `HHMMSS' respectively. If the host
     platform doesn't provide UTC time, these macros are undefined.

   * The `__POSIX_TIME__' macro is defined as a number containing the
     number of seconds since the POSIX epoch, 1 January 1970 00:00:00
     UTC; excluding any leap seconds. This is computed using UTC time
     if available on the host platform, otherwise it is computed using
     the local time as if it was UTC.

   All instances of time and date macros in the same assembly session
produce consistent output. For example, in an assembly session started
at 42 seconds after midnight on January 1, 2010 in Moscow (timezone
UTC+3) these macros would have the following values, assuming, of
course, a properly configured environment with a correct clock:

     __DATE__             "2010-01-01"
     __TIME__             "00:00:42"
     __DATE_NUM__         20100101
     __TIME_NUM__         000042
     __UTC_DATE__         "2009-12-31"
     __UTC_TIME__         "21:00:42"
     __UTC_DATE_NUM__     20091231
     __UTC_TIME_NUM__     210042
     __POSIX_TIME__       1262293242


File: nasm.info,  Node: Section 4.11.8,  Next: Section 4.11.9,  Prev: Section 4.11.7,  Up: Section 4.11

4.11.8. `__USE_'_package_`__': Package Include Test
---------------------------------------------------

When a standard macro package (see *note Chapter 5::) is included with
the `%use' directive (see *note Section 4.6.4::), a single-line macro
of the form `__USE_'_package_`__' is automatically defined. This allows
testing if a particular package is invoked or not.

   For example, if the `altreg' package is included (see *note Section
5.1::), then the macro `__USE_ALTREG__' is defined.


File: nasm.info,  Node: Section 4.11.9,  Next: Section 4.11.10,  Prev: Section 4.11.8,  Up: Section 4.11

4.11.9. `__PASS__': Assembly Pass
---------------------------------

The macro `__PASS__' is defined to be `1' on preparatory passes, and
`2' on the final pass. In preprocess-only mode, it is set to `3', and
when running only to generate dependencies (due to the `-M' or `-MG'
option, see *note Section 2.1.4::) it is set to `0'.

   _Avoid using this macro if at all possible. It is tremendously easy
to generate very strange errors by misusing it, and the semantics may
change in future versions of NASM._


File: nasm.info,  Node: Section 4.11.10,  Next: Section 4.11.11,  Prev: Section 4.11.9,  Up: Section 4.11

4.11.10. `STRUC' and `ENDSTRUC': Declaring Structure Data Types
---------------------------------------------------------------

The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that
data structures can be implemented as a set of macros. The macros
`STRUC' and `ENDSTRUC' are used to define a structure data type.

   `STRUC' takes one parameter, which is the name of the data type. This
name is defined as a symbol with the value zero, and also has the suffix
`_size' appended to it and is then defined as an `EQU' giving the size
of the structure. Once `STRUC' has been issued, you are defining the
structure, and should define fields using the `RESB' family of
pseudo-instructions, and then invoke `ENDSTRUC' to finish the
definition.

   For example, to define a structure called `mytype' containing a
longword, a word, a byte and a string of bytes, you might code

     struc   mytype

       mt_long:      resd    1
       mt_word:      resw    1
       mt_byte:      resb    1
       mt_str:       resb    32

     endstruc

   The above code defines six symbols: `mt_long' as 0 (the offset from
the beginning of a `mytype' structure to the longword field), `mt_word'
as 4, `mt_byte' as 6, `mt_str' as 7, `mytype_size' as 39, and `mytype'
itself as zero.

   The reason why the structure type name is defined at zero is a side
effect of allowing structures to work with the local label mechanism:
if your structure members tend to have the same names in more than one
structure, you can define the above structure like this:

     struc mytype

       .long:        resd    1
       .word:        resw    1
       .byte:        resb    1
       .str:         resb    32

     endstruc

   This defines the offsets to the structure fields as `mytype.long',
`mytype.word', `mytype.byte' and `mytype.str'.

   NASM, since it has no _intrinsic_ structure support, does not support
any form of period notation to refer to the elements of a structure once
you have one (except the above local-label notation), so code such as
`mov ax,[mystruc.mt_word]' is not valid. `mt_word' is a constant just
like any other constant, so the correct syntax is `mov
ax,[mystruc+mt_word]' or `mov ax,[mystruc+mytype.word]'.


File: nasm.info,  Node: Section 4.11.11,  Next: Section 4.11.12,  Prev: Section 4.11.10,  Up: Section 4.11

4.11.11. `ISTRUC', `AT' and `IEND': Declaring Instances of Structures
---------------------------------------------------------------------

Having defined a structure type, the next thing you typically want to
do is to declare instances of that structure in your data segment. NASM
provides an easy way to do this in the `ISTRUC' mechanism. To declare a
structure of type `mytype' in a program, you code something like this:

     mystruc:
         istruc mytype

             at mt_long, dd      123456
             at mt_word, dw      1024
             at mt_byte, db      'x'
             at mt_str,  db      'hello, world', 13, 10, 0

         iend

   The function of the `AT' macro is to make use of the `TIMES' prefix
to advance the assembly position to the correct point for the specified
structure field, and then to declare the specified data.  Therefore the
structure fields must be declared in the same order as they were
specified in the structure definition.

   If the data to go in a structure field requires more than one source
line to specify, the remaining source lines can easily come after the
`AT' line. For example:

             at mt_str,  db      123,134,145,156,167,178,189
                         db      190,100,0

   Depending on personal taste, you can also omit the code part of the
`AT' line completely, and start the structure field on the next line:

             at mt_str
                     db      'hello, world'
                     db      13,10,0


File: nasm.info,  Node: Section 4.11.12,  Next: Chapter 5,  Prev: Section 4.11.11,  Up: Section 4.11

4.11.12. `ALIGN' and `ALIGNB': Data Alignment
---------------------------------------------

The `ALIGN' and `ALIGNB' macros provides a convenient way to align code
or data on a word, longword, paragraph or other boundary. (Some
assemblers call this directive `EVEN'.) The syntax of the `ALIGN' and
`ALIGNB' macros is

             align   4               ; align on 4-byte boundary
             align   16              ; align on 16-byte boundary
             align   8,db 0          ; pad with 0s rather than NOPs
             align   4,resb 1        ; align to 4 in the BSS
             alignb  4               ; equivalent to previous line

   Both macros require their first argument to be a power of two; they
both compute the number of additional bytes required to bring the
length of the current section up to a multiple of that power of two,
and then apply the `TIMES' prefix to their second argument to perform
the alignment.

   If the second argument is not specified, the default for `ALIGN' is
`NOP', and the default for `ALIGNB' is `RESB 1'. So if the second
argument is specified, the two macros are equivalent. Normally, you can
just use `ALIGN' in code and data sections and `ALIGNB' in BSS
sections, and never need the second argument except for special
purposes.

   `ALIGN' and `ALIGNB', being simple macros, perform no error
checking: they cannot warn you if their first argument fails to be a
power of two, or if their second argument generates more than one byte
of code.  In each of these cases they will silently do the wrong thing.

   `ALIGNB' (or `ALIGN' with a second argument of `RESB 1') can be used
within structure definitions:

     struc mytype2

       mt_byte:
             resb 1
             alignb 2
       mt_word:
             resw 1
             alignb 4
       mt_long:
             resd 1
       mt_str:
             resb 32

     endstruc

   This will ensure that the structure members are sensibly aligned
relative to the base of the structure.

   A final caveat: `ALIGN' and `ALIGNB' work relative to the beginning
of the _section_, not the beginning of the address space in the final
executable. Aligning to a 16-byte boundary when the section you're in
is only guaranteed to be aligned to a 4-byte boundary, for example, is
a waste of effort. Again, NASM does not check that the section's
alignment characteristics are sensible for the use of `ALIGN' or
`ALIGNB'.

   See also the `smartalign' standard macro package, *note Section
5.2::.


File: nasm.info,  Node: Chapter 5,  Next: Section 5.1,  Prev: Section 4.11.12,  Up: Top

Chapter 5: Standard Macro Packages
**********************************

The `%use' directive (see *note Section 4.6.4::) includes one of the
standard macro packages included with the NASM distribution and compiled
into the NASM binary. It operates like the `%include' directive (see
*note Section 4.6.1::), but the included contents is provided by NASM
itself.

   The names of standard macro packages are case insensitive, and can be
quoted or not.

* Menu:

* Section 5.1:: `altreg': Alternate Register Names
* Section 5.2:: `smartalign': Smart `ALIGN' Macro


File: nasm.info,  Node: Section 5.1,  Next: Section 5.2,  Prev: Chapter 5,  Up: Chapter 5

5.1. `altreg': Alternate Register Names
=======================================

The `altreg' standard macro package provides alternate register names.
It provides numeric register names for all registers (not just `R8'-
`R15'), the Intel-defined aliases `R8L'-`R15L' for the low bytes of
register (as opposed to the NASM/AMD standard names `R8B'- `R15B'), and
the names `R0H'-`R3H' (by analogy with `R0L'-`R3L') for `AH', `CH',
`DH', and `BH'.

   Example use:

     %use altreg

     proc:
           mov r0l,r3h                    ; mov al,bh
           ret

   See also *note Section 11.1::.


File: nasm.info,  Node: Section 5.2,  Next: Chapter 6,  Prev: Section 5.1,  Up: Chapter 5

5.2. `smartalign': Smart `ALIGN' Macro
======================================

The `smartalign' standard macro package provides for an `ALIGN' macro
which is more powerful than the default (and backwards-compatible) one
(see *note Section 4.11.12::). When the `smartalign' package is
enabled, when `ALIGN' is used without a second argument, NASM will
generate a sequence of instructions more efficient than a series of
`NOP'. Furthermore, if the padding exceeds a specific threshold, then
NASM will generate a jump over the entire padding sequence.

   The specific instructions generated can be controlled with the new
`ALIGNMODE' macro. This macro takes two parameters: one mode, and an
optional jump threshold override. The modes are as follows:

   * `generic': Works on all x86 CPUs and should have reasonable
     performance. The default jump threshold is 8. This is the default.

   * `nop': Pad out with `NOP' instructions. The only difference
     compared to the standard `ALIGN' macro is that NASM can still jump
     over a large padding area. The default jump threshold is 16.

   * `k7': Optimize for the AMD K7 (Athlon/Althon XP). These
     instructions should still work on all x86 CPUs. The default jump
     threshold is 16.

   * `k8': Optimize for the AMD K8 (Opteron/Althon 64). These
     instructions should still work on all x86 CPUs. The default jump
     threshold is 16.

   * `p6': Optimize for Intel CPUs. This uses the long `NOP'
     instructions first introduced in Pentium Pro. This is incompatible
     with all CPUs of family 5 or lower, as well as some VIA CPUs and
     several virtualization solutions. The default jump threshold is 16.

   The macro `__ALIGNMODE__' is defined to contain the current alignment
mode. A number of other macros beginning with `__ALIGN_' are used
internally by this macro package.


File: nasm.info,  Node: Chapter 6,  Next: Section 6.1,  Prev: Section 5.2,  Up: Top

Chapter 6: Assembler Directives
*******************************

NASM, though it attempts to avoid the bureaucracy of assemblers like
MASM and TASM, is nevertheless forced to support a _few_ directives.
These are described in this chapter.

   NASM's directives come in two types: _user-level_ directives and
_primitive_ directives. Typically, each directive has a user-level form
and a primitive form. In almost all cases, we recommend that users use
the user-level forms of the directives, which are implemented as macros
which call the primitive forms.

   Primitive directives are enclosed in square brackets; user-level
directives are not.

   In addition to the universal directives described in this chapter,
each object file format can optionally supply extra directives in order
to control particular features of that file format. These
_format-specific_ directives are documented along with the formats that
implement them, in *note Chapter 7::.

* Menu:

* Section 6.1:: `BITS': Specifying Target Processor Mode
* Section 6.2:: `DEFAULT': Change the assembler defaults
* Section 6.3:: `SECTION' or `SEGMENT': Changing and Defining Sections
* Section 6.4:: `ABSOLUTE': Defining Absolute Labels
* Section 6.5:: `EXTERN': Importing Symbols from Other Modules
* Section 6.6:: `GLOBAL': Exporting Symbols to Other Modules
* Section 6.7:: `COMMON': Defining Common Data Areas
* Section 6.8:: `CPU': Defining CPU Dependencies
* Section 6.9:: `FLOAT': Handling of floating-point constants


File: nasm.info,  Node: Section 6.1,  Next: Section 6.1.1,  Prev: Chapter 6,  Up: Chapter 6

6.1. `BITS': Specifying Target Processor Mode
=============================================

The `BITS' directive specifies whether NASM should generate code
designed to run on a processor operating in 16-bit mode, 32-bit mode or
64- bit mode. The syntax is `BITS XX', where XX is 16, 32 or 64.

   In most cases, you should not need to use `BITS' explicitly. The
`aout', `coff', `elf', `macho', `win32' and `win64' object formats,
which are designed for use in 32-bit or 64-bit operating systems, all
cause NASM to select 32-bit or 64-bit mode, respectively, by default.
The `obj' object format allows you to specify each segment you define
as either `USE16' or `USE32', and NASM will set its operating mode
accordingly, so the use of the `BITS' directive is once again
unnecessary.

   The most likely reason for using the `BITS' directive is to write 32-
bit or 64-bit code in a flat binary file; this is because the `bin'
output format defaults to 16-bit mode in anticipation of it being used
most frequently to write DOS `.COM' programs, DOS `.SYS' device drivers
and boot loader software.

   You do _not_ need to specify `BITS 32' merely in order to use 32-
bit instructions in a 16-bit DOS program; if you do, the assembler will
generate incorrect code because it will be writing code targeted at a
32- bit platform, to be run on a 16-bit one.

   When NASM is in `BITS 16' mode, instructions which use 32-bit data
are prefixed with an 0x66 byte, and those referring to 32-bit addresses
have an 0x67 prefix. In `BITS 32' mode, the reverse is true: 32-bit
instructions require no prefixes, whereas instructions using 16-bit data
need an 0x66 and those working on 16-bit addresses need an 0x67.

   When NASM is in `BITS 64' mode, most instructions operate the same as
they do for `BITS 32' mode. However, there are 8 more general and SSE
registers, and 16-bit addressing is no longer supported.

   The default address size is 64 bits; 32-bit addressing can be
selected with the 0x67 prefix. The default operand size is still 32
bits, however, and the 0x66 prefix selects 16-bit operand size. The
`REX' prefix is used both to select 64-bit operand size, and to access
the new registers. NASM automatically inserts REX prefixes when
necessary.

   When the `REX' prefix is used, the processor does not know how to
address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead, it
is possible to access the the low 8-bits of the SP, BP SI and DI
registers as SPL, BPL, SIL and DIL, respectively; but only when the REX
prefix is used.

   The `BITS' directive has an exactly equivalent primitive form,
`[BITS 16]', `[BITS 32]' and `[BITS 64]'. The user-level form is a
macro which has no function other than to call the primitive form.

   Note that the space is neccessary, e.g. `BITS32' will _not_ work!

* Menu:

* Section 6.1.1:: `USE16' & `USE32': Aliases for BITS


File: nasm.info,  Node: Section 6.1.1,  Next: Section 6.2,  Prev: Section 6.1,  Up: Section 6.1

6.1.1. `USE16' & `USE32': Aliases for BITS
------------------------------------------

The ``USE16'' and ``USE32'' directives can be used in place of ``BITS
16'' and ``BITS 32'', for compatibility with other assemblers.


File: nasm.info,  Node: Section 6.2,  Next: Section 6.3,  Prev: Section 6.1.1,  Up: Chapter 6

6.2. `DEFAULT': Change the assembler defaults
=============================================

The `DEFAULT' directive changes the assembler defaults. Normally, NASM
defaults to a mode where the programmer is expected to explicitly
specify most features directly. However, this is occationally
obnoxious, as the explicit form is pretty much the only one one wishes
to use.

   Currently, the only `DEFAULT' that is settable is whether or not
registerless instructions in 64-bit mode are `RIP'-relative or not.  By
default, they are absolute unless overridden with the `REL' specifier
(see *note Section 3.3::). However, if `DEFAULT REL' is specified,
`REL' is default, unless overridden with the `ABS' specifier, _except
when used with an FS or GS segment override_.

   The special handling of `FS' and `GS' overrides are due to the fact
that these registers are generally used as thread pointers or other
special functions in 64-bit mode, and generating `RIP'-relative
addresses would be extremely confusing.

   `DEFAULT REL' is disabled with `DEFAULT ABS'.


File: nasm.info,  Node: Section 6.3,  Next: Section 6.3.1,  Prev: Section 6.2,  Up: Chapter 6

6.3. `SECTION' or `SEGMENT': Changing and Defining Sections
===========================================================

The `SECTION' directive (`SEGMENT' is an exactly equivalent synonym)
changes which section of the output file the code you write will be
assembled into. In some object file formats, the number and names of
sections are fixed; in others, the user may make up as many as they
wish.  Hence `SECTION' may sometimes give an error message, or may
define a new section, if you try to switch to a section that does not
(yet) exist.

   The Unix object formats, and the `bin' object format (but see *note
Section 7.1.3::, all support the standardized section names `.text',
`.data' and `.bss' for the code, data and uninitialized-data sections.
The `obj' format, by contrast, does not recognize these section names
as being special, and indeed will strip off the leading period of any
section name that has one.

* Menu:

* Section 6.3.1:: The `__SECT__' Macro


File: nasm.info,  Node: Section 6.3.1,  Next: Section 6.4,  Prev: Section 6.3,  Up: Section 6.3

6.3.1. The `__SECT__' Macro
---------------------------

The `SECTION' directive is unusual in that its user-level form
functions differently from its primitive form. The primitive form,
`[SECTION xyz]', simply switches the current target section to the one
given. The user-level form, `SECTION xyz', however, first defines the
single-line macro `__SECT__' to be the primitive `[SECTION]' directive
which it is about to issue, and then issues it. So the user-level
directive

             SECTION .text

   expands to the two lines

     %define __SECT__        [SECTION .text]
             [SECTION .text]

   Users may find it useful to make use of this in their own macros. For
example, the `writefile' macro defined in *note Section 4.3.3:: can be
usefully rewritten in the following more sophisticated form:

     %macro  writefile 2+

             [section .data]

       %%str:        db      %2
       %%endstr:

             __SECT__

             mov     dx,%%str
             mov     cx,%%endstr-%%str
             mov     bx,%1
             mov     ah,0x40
             int     0x21

     %endmacro

   This form of the macro, once passed a string to output, first
switches temporarily to the data section of the file, using the
primitive form of the `SECTION' directive so as not to modify
`__SECT__'. It then declares its string in the data section, and then
invokes `__SECT__' to switch back to _whichever_ section the user was
previously working in. It thus avoids the need, in the previous version
of the macro, to include a `JMP' instruction to jump over the data, and
also does not fail if, in a complicated `OBJ' format module, the user
could potentially be assembling the code in any of several separate code
sections.


File: nasm.info,  Node: Section 6.4,  Next: Section 6.5,  Prev: Section 6.3.1,  Up: Chapter 6

6.4. `ABSOLUTE': Defining Absolute Labels
=========================================

The `ABSOLUTE' directive can be thought of as an alternative form of
`SECTION': it causes the subsequent code to be directed at no physical
section, but at the hypothetical section starting at the given absolute
address. The only instructions you can use in this mode are the `RESB'
family.

   `ABSOLUTE' is used as follows:

     absolute 0x1A

         kbuf_chr    resw    1
         kbuf_free   resw    1
         kbuf        resw    16

   This example describes a section of the PC BIOS data area, at segment
address 0x40: the above code defines `kbuf_chr' to be 0x1A, `kbuf_free'
to be 0x1C, and `kbuf' to be 0x1E.

   The user-level form of `ABSOLUTE', like that of `SECTION', redefines
the `__SECT__' macro when it is invoked.

   `STRUC' and `ENDSTRUC' are defined as macros which use `ABSOLUTE'
(and also `__SECT__').

   `ABSOLUTE' doesn't have to take an absolute constant as an argument:
it can take an expression (actually, a critical expression: see *note
Section 3.8::) and it can be a value in a segment. For example, a TSR
can re-use its setup code as run-time BSS like this:

             org     100h               ; it's a .COM program

             jmp     setup              ; setup code comes last

             ; the resident part of the TSR goes here
     setup:
             ; now write the code that installs the TSR here

     absolute setup

     runtimevar1     resw    1
     runtimevar2     resd    20

     tsr_end:

   This defines some variables `on top of' the setup code, so that
after the setup has finished running, the space it took up can be
re-used as data storage for the running TSR. The symbol `tsr_end' can
be used to calculate the total size of the part of the TSR that needs
to be made resident.


File: nasm.info,  Node: Section 6.5,  Next: Section 6.6,  Prev: Section 6.4,  Up: Chapter 6

6.5. `EXTERN': Importing Symbols from Other Modules
===================================================

`EXTERN' is similar to the MASM directive `EXTRN' and the C keyword
`extern': it is used to declare a symbol which is not defined anywhere
in the module being assembled, but is assumed to be defined in some
other module and needs to be referred to by this one. Not every
object-file format can support external variables: the `bin' format
cannot.

   The `EXTERN' directive takes as many arguments as you like. Each
argument is the name of a symbol:

     extern  _printf
     extern  _sscanf,_fscanf

   Some object-file formats provide extra features to the `EXTERN'
directive. In all cases, the extra features are used by suffixing a
colon to the symbol name followed by object-format specific text. For
example, the `obj' format allows you to declare that the default
segment base of an external should be the group `dgroup' by means of
the directive

     extern  _variable:wrt dgroup

   The primitive form of `EXTERN' differs from the user-level form only
in that it can take only one argument at a time: the support for
multiple arguments is implemented at the preprocessor level.

   You can declare the same variable as `EXTERN' more than once: NASM
will quietly ignore the second and later redeclarations. You can't
declare a variable as `EXTERN' as well as something else, though.


File: nasm.info,  Node: Section 6.6,  Next: Section 6.7,  Prev: Section 6.5,  Up: Chapter 6

6.6. `GLOBAL': Exporting Symbols to Other Modules
=================================================

`GLOBAL' is the other end of `EXTERN': if one module declares a symbol
as `EXTERN' and refers to it, then in order to prevent linker errors,
some other module must actually _define_ the symbol and declare it as
`GLOBAL'. Some assemblers use the name `PUBLIC' for this purpose.

   The `GLOBAL' directive applying to a symbol must appear _before_ the
definition of the symbol.

   `GLOBAL' uses the same syntax as `EXTERN', except that it must refer
to symbols which _are_ defined in the same module as the `GLOBAL'
directive. For example:

     global _main
     _main:
             ; some code

   `GLOBAL', like `EXTERN', allows object formats to define private
extensions by means of a colon. The `elf' object format, for example,
lets you specify whether global data items are functions or data:

     global  hashlookup:function, hashtable:data

   Like `EXTERN', the primitive form of `GLOBAL' differs from the
user-level form only in that it can take only one argument at a time.


File: nasm.info,  Node: Section 6.7,  Next: Section 6.8,  Prev: Section 6.6,  Up: Chapter 6

6.7. `COMMON': Defining Common Data Areas
=========================================

The `COMMON' directive is used to declare _common variables_. A common
variable is much like a global variable declared in the uninitialized
data section, so that

     common  intvar  4

   is similar in function to

     global  intvar
     section .bss

     intvar  resd    1

   The difference is that if more than one module defines the same
common variable, then at link time those variables will be _merged_, and
references to `intvar' in all modules will point at the same piece of
memory.

   Like `GLOBAL' and `EXTERN', `COMMON' supports object-format specific
extensions. For example, the `obj' format allows common variables to be
NEAR or FAR, and the `elf' format allows you to specify the alignment
requirements of a common variable:

     common  commvar  4:near  ; works in OBJ
     common  intarray 100:4   ; works in ELF: 4 byte aligned

   Once again, like `EXTERN' and `GLOBAL', the primitive form of
`COMMON' differs from the user-level form only in that it can take only
one argument at a time.


File: nasm.info,  Node: Section 6.8,  Next: Section 6.9,  Prev: Section 6.7,  Up: Chapter 6

6.8. `CPU': Defining CPU Dependencies
=====================================

The `CPU' directive restricts assembly to those instructions which are
available on the specified CPU.

   Options are:

   * `CPU 8086' Assemble only 8086 instruction set

   * `CPU 186' Assemble instructions up to the 80186 instruction set

   * `CPU 286' Assemble instructions up to the 286 instruction set

   * `CPU 386' Assemble instructions up to the 386 instruction set

   * `CPU 486' 486 instruction set

   * `CPU 586' Pentium instruction set

   * `CPU PENTIUM' Same as 586

   * `CPU 686' P6 instruction set

   * `CPU PPRO' Same as 686

   * `CPU P2' Same as 686

   * `CPU P3' Pentium III (Katmai) instruction sets

   * `CPU KATMAI' Same as P3

   * `CPU P4' Pentium 4 (Willamette) instruction set

   * `CPU WILLAMETTE' Same as P4

   * `CPU PRESCOTT' Prescott instruction set

   * `CPU X64' x86-64 (x64/AMD64/Intel 64) instruction set

   * `CPU IA64' IA64 CPU (in x86 mode) instruction set

   All options are case insensitive. All instructions will be selected
only if they apply to the selected CPU or lower. By default, all
instructions are available.


File: nasm.info,  Node: Section 6.9,  Next: Chapter 7,  Prev: Section 6.8,  Up: Chapter 6

6.9. `FLOAT': Handling of floating-point constants
==================================================

By default, floating-point constants are rounded to nearest, and IEEE
denormals are supported. The following options can be set to alter this
behaviour:

   * `FLOAT DAZ' Flush denormals to zero

   * `FLOAT NODAZ' Do not flush denormals to zero (default)

   * `FLOAT NEAR' Round to nearest (default)

   * `FLOAT UP' Round up (toward +Infinity)

   * `FLOAT DOWN' Round down (toward -Infinity)

   * `FLOAT ZERO' Round toward zero

   * `FLOAT DEFAULT' Restore default settings

   The standard macros `__FLOAT_DAZ__', `__FLOAT_ROUND__', and
`__FLOAT__' contain the current state, as long as the programmer has
avoided the use of the brackeded primitive form, (`[FLOAT]').

   `__FLOAT__' contains the full set of floating-point settings; this
value can be saved away and invoked later to restore the setting.


File: nasm.info,  Node: Chapter 7,  Next: Section 7.1,  Prev: Section 6.9,  Up: Top

Chapter 7: Output Formats
*************************

NASM is a portable assembler, designed to be able to compile on any
ANSI C- supporting platform and produce output to run on a variety of
Intel x86 operating systems. For this reason, it has a large number of
available output formats, selected using the `-f' option on the NASM
command line. Each of these formats, along with its extensions to the
base NASM syntax, is detailed in this chapter.

   As stated in *note Section 2.1.1::, NASM chooses a default name for
your output file based on the input file name and the chosen output
format. This will be generated by removing the extension (`.asm', `.s',
or whatever you like to use) from the input file name, and substituting
an extension defined by the output format. The extensions are given
with each format below.

* Menu:

* Section 7.1:: `bin': Flat-Form Binary Output
* Section 7.2:: `obj': Microsoft OMF Object Files
* Section 7.3:: `win32': Microsoft Win32 Object Files
* Section 7.4:: `win64': Microsoft Win64 Object Files
* Section 7.5:: `coff': Common Object File Format
* Section 7.6:: `macho': Mach Object File Format
* Section 7.7:: `elf, elf32, and elf64': Executable and Linkable Format Object Files
* Section 7.8:: `aout': Linux `a.out' Object Files
* Section 7.9:: `aoutb': NetBSD/FreeBSD/OpenBSD `a.out' Object Files
* Section 7.10:: `as86': Minix/Linux `as86' Object Files
* Section 7.11:: `rdf': Relocatable Dynamic Object File Format
* Section 7.12:: `dbg': Debugging Format


File: nasm.info,  Node: Section 7.1,  Next: Section 7.1.1,  Prev: Chapter 7,  Up: Chapter 7

7.1. `bin': Flat-Form Binary Output
===================================

The `bin' format does not produce object files: it generates nothing in
the output file except the code you wrote. Such `pure binary' files are
used by MS-DOS: `.COM' executables and `.SYS' device drivers are pure
binary files. Pure binary output is also useful for operating system
and boot loader development.

   The `bin' format supports multiple section names. For details of how
nasm handles sections in the `bin' format, see *note Section 7.1.3::.

   Using the `bin' format puts NASM by default into 16-bit mode (see
*note Section 6.1::). In order to use `bin' to write 32-bit or 64-bit
code, such as an OS kernel, you need to explicitly issue the `BITS 32'
or `BITS 64' directive.

   `bin' has no default output file name extension: instead, it leaves
your file name as it is once the original extension has been removed.
Thus, the default is for NASM to assemble `binprog.asm' into a binary
file called `binprog'.

* Menu:

* Section 7.1.1:: `ORG': Binary File Program Origin
* Section 7.1.2:: `bin' Extensions to the `SECTION' Directive
* Section 7.1.3:: `Multisection' support for the BIN format.
* Section 7.1.4:: Map files


File: nasm.info,  Node: Section 7.1.1,  Next: Section 7.1.2,  Prev: Section 7.1,  Up: Section 7.1

7.1.1. `ORG': Binary File Program Origin
----------------------------------------

The `bin' format provides an additional directive to the list given in
*note Chapter 6::: `ORG'. The function of the `ORG' directive is to
specify the origin address which NASM will assume the program begins at
when it is loaded into memory.

   For example, the following code will generate the longword
`0x00000104':

             org     0x100
             dd      label
     label:

   Unlike the `ORG' directive provided by MASM-compatible assemblers,
which allows you to jump around in the object file and overwrite code
you have already generated, NASM's `ORG' does exactly what the directive
says: _origin_. Its sole function is to specify one offset which is
added to all internal address references within the section; it does not
permit any of the trickery that MASM's version does. See *note Section
12.1.3:: for further comments.


File: nasm.info,  Node: Section 7.1.2,  Next: Section 7.1.3,  Prev: Section 7.1.1,  Up: Section 7.1

7.1.2. `bin' Extensions to the `SECTION' Directive
--------------------------------------------------

The `bin' output format extends the `SECTION' (or `SEGMENT') directive
to allow you to specify the alignment requirements of segments.  This
is done by appending the `ALIGN' qualifier to the end of the
section-definition line. For example,

     section .data   align=16

   switches to the section `.data' and also specifies that it must be
aligned on a 16-byte boundary.

   The parameter to `ALIGN' specifies how many low bits of the section
start address must be forced to zero. The alignment value given may be
any power of two.


File: nasm.info,  Node: Section 7.1.3,  Next: Section 7.1.4,  Prev: Section 7.1.2,  Up: Section 7.1

7.1.3. `Multisection' support for the BIN format.
-------------------------------------------------

The `bin' format allows the use of multiple sections, of arbitrary
names, besides the "known" `.text', `.data', and `.bss' names.

   * Sections may be designated `progbits' or `nobits'. Default is
     `progbits' (except `.bss', which defaults to `nobits', of course).

   * Sections can be aligned at a specified boundary following the
     previous section with `align=', or at an arbitrary byte-granular
     position with `start='.

   * Sections can be given a virtual start address, which will be used
     for the calculation of all memory references within that section
     with `vstart='.

   * Sections can be ordered using `follows='`<section>' or
     `vfollows='`<section>' as an alternative to specifying an explicit
     start address.

   * Arguments to `org', `start', `vstart', and `align=' are critical
     expressions. See *note Section 3.8::. E.g.  `align=(1 <<
     ALIGN_SHIFT)' - `ALIGN_SHIFT' must be defined before it is used
     here.

   * Any code which comes before an explicit `SECTION' directive is
     directed by default into the `.text' section.

   * If an `ORG' statement is not given, `ORG 0' is used by default.

   * The `.bss' section will be placed after the last `progbits'
     section, unless `start=', `vstart=', `follows=', or `vfollows='
     has been specified.

   * All sections are aligned on dword boundaries, unless a different
     alignment has been specified.

   * Sections may not overlap.

   * NASM creates the `section.<secname>.start' for each section, which
     may be used in your code.


File: nasm.info,  Node: Section 7.1.4,  Next: Section 7.2,  Prev: Section 7.1.3,  Up: Section 7.1

7.1.4. Map files
----------------

Map files can be generated in `-f bin' format by means of the `[map]'
option. Map types of `all' (default), `brief', `sections', `segments',
or `symbols' may be specified.  Output may be directed to `stdout'
(default), `stderr', or a specified file. E.g. `[map symbols
myfile.map]'. No "user form" exists, the square brackets must be used.


File: nasm.info,  Node: Section 7.2,  Next: Section 7.2.1,  Prev: Section 7.1.4,  Up: Chapter 7

7.2. `obj': Microsoft OMF Object Files
======================================

The `obj' file format (NASM calls it `obj' rather than `omf' for
historical reasons) is the one produced by MASM and TASM, which is
typically fed to 16-bit DOS linkers to produce `.EXE' files. It is also
the format used by OS/2.

   `obj' provides a default output file-name extension of `.obj'.

   `obj' is not exclusively a 16-bit format, though: NASM has full
support for the 32-bit extensions to the format. In particular, 32-bit
`obj' format files are used by Borland's Win32 compilers, instead of
using Microsoft's newer `win32' object file format.

   The `obj' format does not define any special segment names: you can
call your segments anything you like. Typical names for segments in
`obj' format files are `CODE', `DATA' and `BSS'.

   If your source file contains code before specifying an explicit
`SEGMENT' directive, then NASM will invent its own segment called
`__NASMDEFSEG' for you.

   When you define a segment in an `obj' file, NASM defines the segment
name as a symbol as well, so that you can access the segment address of
the segment. So, for example:

     segment data

     dvar:   dw      1234

     segment code

     function:
             mov     ax,data         ; get segment address of data
             mov     ds,ax           ; and move it into DS
             inc     word [dvar]     ; now this reference will work
             ret

   The `obj' format also enables the use of the `SEG' and `WRT'
operators, so that you can write code which does things like

     extern  foo

           mov   ax,seg foo            ; get preferred segment of foo
           mov   ds,ax
           mov   ax,data               ; a different segment
           mov   es,ax
           mov   ax,[ds:foo]           ; this accesses `foo'
           mov   [es:foo wrt data],bx  ; so does this

* Menu:

* Section 7.2.1:: `obj' Extensions to the `SEGMENT' Directive
* Section 7.2.2:: `GROUP': Defining Groups of Segments
* Section 7.2.3:: `UPPERCASE': Disabling Case Sensitivity in Output
* Section 7.2.4:: `IMPORT': Importing DLL Symbols
* Section 7.2.5:: `EXPORT': Exporting DLL Symbols
* Section 7.2.6:: `..start': Defining the Program Entry Point
* Section 7.2.7:: `obj' Extensions to the `EXTERN' Directive
* Section 7.2.8:: `obj' Extensions to the `COMMON' Directive


File: nasm.info,  Node: Section 7.2.1,  Next: Section 7.2.2,  Prev: Section 7.2,  Up: Section 7.2

7.2.1. `obj' Extensions to the `SEGMENT' Directive
--------------------------------------------------

The `obj' output format extends the `SEGMENT' (or `SECTION') directive
to allow you to specify various properties of the segment you are
defining. This is done by appending extra qualifiers to the end of the
segment-definition line. For example,

     segment code private align=16

   defines the segment `code', but also declares it to be a private
segment, and requires that the portion of it described in this code
module must be aligned on a 16-byte boundary.

   The available qualifiers are:

   * `PRIVATE', `PUBLIC', `COMMON' and `STACK' specify the combination
     characteristics of the segment. `PRIVATE' segments do not get
     combined with any others by the linker; `PUBLIC' and `STACK'
     segments get concatenated together at link time; and `COMMON'
     segments all get overlaid on top of each other rather than stuck
     end-to-end.

   * `ALIGN' is used, as shown above, to specify how many low bits of
     the segment start address must be forced to zero. The alignment
     value given may be any power of two from 1 to 4096; in reality,
     the only values supported are 1, 2, 4, 16, 256 and 4096, so if 8
     is specified it will be rounded up to 16, and 32, 64 and 128 will
     all be rounded up to 256, and so on. Note that alignment to
     4096-byte boundaries is a PharLap extension to the format and may
     not be supported by all linkers.

   * `CLASS' can be used to specify the segment class; this feature
     indicates to the linker that segments of the same class should be
     placed near each other in the output file. The class name can be
     any word, e.g.  `CLASS=CODE'.

   * `OVERLAY', like `CLASS', is specified with an arbitrary word as an
     argument, and provides overlay information to an overlay-capable
     linker.

   * Segments can be declared as `USE16' or `USE32', which has the
     effect of recording the choice in the object file and also
     ensuring that NASM's default assembly mode when assembling in that
     segment is 16-bit or 32-bit respectively.

   * When writing OS/2 object files, you should declare 32-bit segments
     as `FLAT', which causes the default segment base for anything in
     the segment to be the special group `FLAT', and also defines the
     group if it is not already defined.

   * The `obj' file format also allows segments to be declared as
     having a pre-defined absolute segment address, although no linkers
     are currently known to make sensible use of this feature;
     nevertheless, NASM allows you to declare a segment such as
     `SEGMENT SCREEN ABSOLUTE=0xB800' if you need to. The `ABSOLUTE'
     and `ALIGN' keywords are mutually exclusive.

   NASM's default segment attributes are `PUBLIC', `ALIGN=1', no class,
no overlay, and `USE16'.


File: nasm.info,  Node: Section 7.2.2,  Next: Section 7.2.3,  Prev: Section 7.2.1,  Up: Section 7.2

7.2.2. `GROUP': Defining Groups of Segments
-------------------------------------------

The `obj' format also allows segments to be grouped, so that a single
segment register can be used to refer to all the segments in a group.
NASM therefore supplies the `GROUP' directive, whereby you can code

     segment data

             ; some data

     segment bss

             ; some uninitialized data

     group dgroup data bss

   which will define a group called `dgroup' to contain the segments
`data' and `bss'. Like `SEGMENT', `GROUP' causes the group name to be
defined as a symbol, so that you can refer to a variable `var' in the
`data' segment as `var wrt data' or as `var wrt dgroup', depending on
which segment value is currently in your segment register.

   If you just refer to `var', however, and `var' is declared in a
segment which is part of a group, then NASM will default to giving you
the offset of `var' from the beginning of the _group_, not the
_segment_. Therefore `SEG var', also, will return the group base rather
than the segment base.

   NASM will allow a segment to be part of more than one group, but will
generate a warning if you do this. Variables declared in a segment
which is part of more than one group will default to being relative to
the first group that was defined to contain the segment.

   A group does not have to contain any segments; you can still make
`WRT' references to a group which does not contain the variable you are
referring to. OS/2, for example, defines the special group `FLAT' with
no segments in it.


File: nasm.info,  Node: Section 7.2.3,  Next: Section 7.2.4,  Prev: Section 7.2.2,  Up: Section 7.2

7.2.3. `UPPERCASE': Disabling Case Sensitivity in Output
--------------------------------------------------------

Although NASM itself is case sensitive, some OMF linkers are not;
therefore it can be useful for NASM to output single-case object files.
The `UPPERCASE' format-specific directive causes all segment, group and
symbol names that are written to the object file to be forced to upper
case just before being written. Within a source file, NASM is still
case- sensitive; but the object file can be written entirely in upper
case if desired.

   `UPPERCASE' is used alone on a line; it requires no parameters.


File: nasm.info,  Node: Section 7.2.4,  Next: Section 7.2.5,  Prev: Section 7.2.3,  Up: Section 7.2

7.2.4. `IMPORT': Importing DLL Symbols
--------------------------------------

The `IMPORT' format-specific directive defines a symbol to be imported
from a DLL, for use if you are writing a DLL's import library in NASM.
You still need to declare the symbol as `EXTERN' as well as using the
`IMPORT' directive.

   The `IMPORT' directive takes two required parameters, separated by
white space, which are (respectively) the name of the symbol you wish to
import and the name of the library you wish to import it from. For
example:

         import  WSAStartup wsock32.dll

   A third optional parameter gives the name by which the symbol is
known in the library you are importing it from, in case this is not the
same as the name you wish the symbol to be known by to your code once
you have imported it. For example:

         import  asyncsel wsock32.dll WSAAsyncSelect


File: nasm.info,  Node: Section 7.2.5,  Next: Section 7.2.6,  Prev: Section 7.2.4,  Up: Section 7.2

7.2.5. `EXPORT': Exporting DLL Symbols
--------------------------------------

The `EXPORT' format-specific directive defines a global symbol to be
exported as a DLL symbol, for use if you are writing a DLL in NASM. You
still need to declare the symbol as `GLOBAL' as well as using the
`EXPORT' directive.

   `EXPORT' takes one required parameter, which is the name of the
symbol you wish to export, as it was defined in your source file. An
optional second parameter (separated by white space from the first)
gives the _external_ name of the symbol: the name by which you wish the
symbol to be known to programs using the DLL. If this name is the same
as the internal name, you may leave the second parameter off.

   Further parameters can be given to define attributes of the exported
symbol. These parameters, like the second, are separated by white
space. If further parameters are given, the external name must also be
specified, even if it is the same as the internal name. The available
attributes are:

   * `resident' indicates that the exported name is to be kept resident
     by the system loader. This is an optimisation for frequently used
     symbols imported by name.

   * `nodata' indicates that the exported symbol is a function which
     does not make use of any initialized data.

   * `parm=NNN', where `NNN' is an integer, sets the number of
     parameter words for the case in which the symbol is a call gate
     between 32- bit and 16-bit segments.

   * An attribute which is just a number indicates that the symbol
     should be exported with an identifying number (ordinal), and gives
     the desired number.

   For example:

         export  myfunc
         export  myfunc TheRealMoreFormalLookingFunctionName
         export  myfunc myfunc 1234  ; export by ordinal
         export  myfunc myfunc resident parm=23 nodata


File: nasm.info,  Node: Section 7.2.6,  Next: Section 7.2.7,  Prev: Section 7.2.5,  Up: Section 7.2

7.2.6. `..start': Defining the Program Entry Point
--------------------------------------------------

`OMF' linkers require exactly one of the object files being linked to
define the program entry point, where execution will begin when the
program is run. If the object file that defines the entry point is
assembled using NASM, you specify the entry point by declaring the
special symbol `..start' at the point where you wish execution to begin.


File: nasm.info,  Node: Section 7.2.7,  Next: Section 7.2.8,  Prev: Section 7.2.6,  Up: Section 7.2

7.2.7. `obj' Extensions to the `EXTERN' Directive
-------------------------------------------------

If you declare an external symbol with the directive

         extern  foo

   then references such as `mov ax,foo' will give you the offset of
`foo' from its preferred segment base (as specified in whichever module
`foo' is actually defined in). So to access the contents of `foo' you
will usually need to do something like

             mov     ax,seg foo      ; get preferred segment base
             mov     es,ax           ; move it into ES
             mov     ax,[es:foo]     ; and use offset `foo' from it

   This is a little unwieldy, particularly if you know that an external
is going to be accessible from a given segment or group, say `dgroup'.
So if `DS' already contained `dgroup', you could simply code

             mov     ax,[foo wrt dgroup]

   However, having to type this every time you want to access `foo' can
be a pain; so NASM allows you to declare `foo' in the alternative form

         extern  foo:wrt dgroup

   This form causes NASM to pretend that the preferred segment base of
`foo' is in fact `dgroup'; so the expression `seg foo' will now return
`dgroup', and the expression `foo' is equivalent to `foo wrt dgroup'.

   This default-`WRT' mechanism can be used to make externals appear to
be relative to any group or segment in your program. It can also be
applied to common variables: see *note Section 7.2.8::.


File: nasm.info,  Node: Section 7.2.8,  Next: Section 7.3,  Prev: Section 7.2.7,  Up: Section 7.2

7.2.8. `obj' Extensions to the `COMMON' Directive
-------------------------------------------------

The `obj' format allows common variables to be either near or far; NASM
allows you to specify which your variables should be by the use of the
syntax

     common  nearvar 2:near   ; `nearvar' is a near common
     common  farvar  10:far   ; and `farvar' is far

   Far common variables may be greater in size than 64Kb, and so the OMF
specification says that they are declared as a number of _elements_ of
a given size. So a 10-byte far common variable could be declared as ten
one-byte elements, five two-byte elements, two five-byte elements or one
ten-byte element.

   Some `OMF' linkers require the element size, as well as the variable
size, to match when resolving common variables declared in more than one
module. Therefore NASM must allow you to specify the element size on
your far common variables. This is done by the following syntax:

     common  c_5by2  10:far 5        ; two five-byte elements
     common  c_2by5  10:far 2        ; five two-byte elements

   If no element size is specified, the default is 1. Also, the `FAR'
keyword is not required when an element size is specified, since only
far commons may have element sizes at all. So the above declarations
could equivalently be

     common  c_5by2  10:5            ; two five-byte elements
     common  c_2by5  10:2            ; five two-byte elements

   In addition to these extensions, the `COMMON' directive in `obj'
also supports default-`WRT' specification like `EXTERN' does (explained
in *note Section 7.2.7::). So you can also declare things like

     common  foo     10:wrt dgroup
     common  bar     16:far 2:wrt data
     common  baz     24:wrt data:6


File: nasm.info,  Node: Section 7.3,  Next: Section 7.3.1,  Prev: Section 7.2.8,  Up: Chapter 7

7.3. `win32': Microsoft Win32 Object Files
==========================================

The `win32' output format generates Microsoft Win32 object files,
suitable for passing to Microsoft linkers such as Visual C++. Note that
Borland Win32 compilers do not use this format, but use `obj' instead
(see *note Section 7.2::).

   `win32' provides a default output file-name extension of `.obj'.

   Note that although Microsoft say that Win32 object files follow the
`COFF' (Common Object File Format) standard, the object files produced
by Microsoft Win32 compilers are not compatible with COFF linkers such
as DJGPP's, and vice versa. This is due to a difference of opinion over
the precise semantics of PC-relative relocations. To produce COFF files
suitable for DJGPP, use NASM's `coff' output format; conversely, the
`coff' format does not produce object files that Win32 linkers can
generate correct output from.

* Menu:

* Section 7.3.1:: `win32' Extensions to the `SECTION' Directive
* Section 7.3.2:: `win32': Safe Structured Exception Handling


File: nasm.info,  Node: Section 7.3.1,  Next: Section 7.3.2,  Prev: Section 7.3,  Up: Section 7.3

7.3.1. `win32' Extensions to the `SECTION' Directive
----------------------------------------------------

Like the `obj' format, `win32' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names `.text',
`.data' and `.bss', but may still be overridden by these qualifiers.

   The available qualifiers are:

   * `code', or equivalently `text', defines the section to be a code
     section. This marks the section as readable and executable, but not
     writable, and also indicates to the linker that the type of the
     section is code.

   * `data' and `bss' define the section to be a data section,
     analogously to `code'. Data sections are marked as readable and
     writable, but not executable. `data' declares an initialized data
     section, whereas `bss' declares an uninitialized data section.

   * `rdata' declares an initialized data section that is readable but
     not writable. Microsoft compilers use this section to place
     constants in it.

   * `info' defines the section to be an informational section, which is
     not included in the executable file by the linker, but may (for
     example) pass information _to_ the linker. For example, declaring
     an `info'-type section called `.drectve' causes the linker to
     interpret the contents of the section as command-line options.

   * `align=', used with a trailing number as in `obj', gives the
     alignment requirements of the section. The maximum you may specify
     is 64: the Win32 object file format contains no means to request a
     greater section alignment than this. If alignment is not
     explicitly specified, the defaults are 16-byte alignment for code
     sections, 8-byte alignment for rdata sections and 4-byte alignment
     for data (and BSS) sections. Informational sections get a default
     alignment of 1 byte (no alignment), though the value does not
     matter.

   The defaults assumed by NASM if you do not specify the above
qualifiers are:

     section .text    code  align=16
     section .data    data  align=4
     section .rdata   rdata align=8
     section .bss     bss   align=4

   Any other section name is treated by default like `.text'.


File: nasm.info,  Node: Section 7.3.2,  Next: Section 7.4,  Prev: Section 7.3.1,  Up: Section 7.3

7.3.2. `win32': Safe Structured Exception Handling
--------------------------------------------------

Among other improvements in Windows XP SP2 and Windows Server 2003
Microsoft has introduced concept of "safe structured exception
handling."  General idea is to collect handlers' entry points in
designated read-only table and have alleged entry point verified
against this table prior exception control is passed to the handler. In
order for an executable module to be equipped with such "safe exception
handler table," all object modules on linker command line has to comply
with certain criteria. If one single module among them does not, then
the table in question is omitted and above mentioned run-time checks
will not be performed for application in question. Table omission is by
default silent and therefore can be easily overlooked. One can instruct
linker to refuse to produce binary without such table by passing
`/safeseh' command line option.

   Without regard to this run-time check merits it's natural to expect
NASM to be capable of generating modules suitable for `/safeseh'
linking. From developer's viewpoint the problem is two-fold:

   * how to adapt modules not deploying exception handlers of their own;

   * how to adapt/develop modules utilizing custom exception handling;

   Former can be easily achieved with any NASM version by adding
following line to source code:

     $@feat.00 equ 1

   As of version 2.03 NASM adds this absolute symbol automatically. If
it's not already present to be precise. I.e. if for whatever reason
developer would choose to assign another value in source file, it would
still be perfectly possible.

   Registering custom exception handler on the other hand requires
certain "magic." As of version 2.03 additional directive is implemented,
`safeseh', which instructs the assembler to produce appropriately
formatted input data for above mentioned "safe exception handler table."
Its typical use would be:

     section .text
     extern  _MessageBoxA@16
     %if     __NASM_VERSION_ID__ >= 0x02030000
     safeseh handler         ; register handler as "safe handler"
     %endif
     handler:
             push    DWORD 1 ; MB_OKCANCEL
             push    DWORD caption
             push    DWORD text
             push    DWORD 0
             call    _MessageBoxA@16
             sub     eax,1   ; incidentally suits as return value
                             ; for exception handler
             ret
     global  _main
     _main:
             push    DWORD handler
             push    DWORD [fs:0]
             mov     DWORD [fs:0],esp ; engage exception handler
             xor     eax,eax
             mov     eax,DWORD[eax]   ; cause exception
             pop     DWORD [fs:0]     ; disengage exception handler
             add     esp,4
             ret
     text:   db      'OK to rethrow, CANCEL to generate core dump',0
     caption:db      'SEGV',0

     section .drectve info
             db      '/defaultlib:user32.lib /defaultlib:msvcrt.lib '

   As you might imagine, it's perfectly possible to produce .exe binary
with "safe exception handler table" and yet engage unregistered
exception handler. Indeed, handler is engaged by simply manipulating
`[fs:0]' location at run-time, something linker has no power over,
run-time that is.  It should be explicitly mentioned that such failure
to register handler's entry point with `safeseh' directive has
undesired side effect at run- time. If exception is raised and
unregistered handler is to be executed, the application is abruptly
terminated without any notification whatsoever.  One can argue that
system could at least have logged some kind "non-safe exception handler
in x.exe at address n" message in event log, but no, literally no
notification is provided and user is left with no clue on what caused
application failure.

   Finally, all mentions of linker in this paragraph refer to Microsoft
linker version 7.x and later. Presence of `@feat.00' symbol and input
data for "safe exception handler table" causes no backward
incompatibilities and "safeseh" modules generated by NASM 2.03 and
later can still be linked by earlier versions or non-Microsoft linkers.


File: nasm.info,  Node: Section 7.4,  Next: Section 7.4.1,  Prev: Section 7.3.2,  Up: Chapter 7

7.4. `win64': Microsoft Win64 Object Files
==========================================

The `win64' output format generates Microsoft Win64 object files, which
is nearly 100% identical to the `win32' object format (*note Section
7.3::) with the exception that it is meant to target 64-bit code and
the x86-64 platform altogether. This object file is used exactly the
same as the `win32' object format (*note Section 7.3::), in NASM, with
regard to this exception.

* Menu:

* Section 7.4.1:: `win64': Writing Position-Independent Code
* Section 7.4.2:: `win64': Structured Exception Handling


File: nasm.info,  Node: Section 7.4.1,  Next: Section 7.4.2,  Prev: Section 7.4,  Up: Section 7.4

7.4.1. `win64': Writing Position-Independent Code
-------------------------------------------------

While `REL' takes good care of RIP-relative addressing, there is one
aspect that is easy to overlook for a Win64 programmer: indirect
references. Consider a switch dispatch table:

             jmp     QWORD[dsptch+rax*8]
             ...
     dsptch: dq      case0
             dq      case1
             ...

   Even novice Win64 assembler programmer will soon realize that the
code is not 64-bit savvy. Most notably linker will refuse to link it
with "`'ADDR32' relocation to '.text' invalid without
/LARGEADDRESSAWARE:NO'".  So [s]he will have to split jmp instruction
as following:

             lea     rbx,[rel dsptch]
             jmp     QWORD[rbx+rax*8]

   What happens behind the scene is that effective address in `lea' is
encoded relative to instruction pointer, or in perfectly position-
independent manner. But this is only part of the problem! Trouble is
that in .dll context `caseN' relocations will make their way to the
final module and might have to be adjusted at .dll load time. To be
specific when it can't be loaded at preferred address. And when this
occurs, pages with such relocations will be rendered private to current
process, which kind of undermines the idea of sharing .dll. But no
worry, it's trivial to fix:

             lea     rbx,[rel dsptch]
             add     rbx,QWORD[rbx+rax*8]
             jmp     rbx
             ...
     dsptch: dq      case0-dsptch
             dq      case1-dsptch
             ...

   NASM version 2.03 and later provides another alternative, `wrt
..imagebase' operator, which returns offset from base address of the
current image, be it .exe or .dll module, therefore the name. For those
acquainted with PE-COFF format base address denotes start of
`IMAGE_DOS_HEADER' structure. Here is how to implement switch with
these image-relative references:

             lea     rbx,[rel dsptch]
             mov     eax,DWORD[rbx+rax*4]
             sub     rbx,dsptch wrt ..imagebase
             add     rbx,rax
             jmp     rbx
             ...
     dsptch: dd      case0 wrt ..imagebase
             dd      case1 wrt ..imagebase

   One can argue that the operator is redundant. Indeed, snippet before
last works just fine with any NASM version and is not even Windows
specific...  The real reason for implementing `wrt ..imagebase' will
become apparent in next paragraph.

   It should be noted that `wrt ..imagebase' is defined as 32-bit
operand only:

             dd      label wrt ..imagebase           ; ok
             dq      label wrt ..imagebase           ; bad
             mov     eax,label wrt ..imagebase       ; ok
             mov     rax,label wrt ..imagebase       ; bad


File: nasm.info,  Node: Section 7.4.2,  Next: Section 7.5,  Prev: Section 7.4.1,  Up: Section 7.4

7.4.2. `win64': Structured Exception Handling
---------------------------------------------

Structured exception handing in Win64 is completely different matter
from Win32. Upon exception program counter value is noted, and
linker-generated table comprising start and end addresses of all the
functions [in given executable module] is traversed and compared to the
saved program counter.  Thus so called `UNWIND_INFO' structure is
identified. If it's not found, then offending subroutine is assumed to
be "leaf" and just mentioned lookup procedure is attempted for its
caller. In Win64 leaf function is such function that does not call any
other function _nor_ modifies any Win64 non-volatile registers,
including stack pointer. The latter ensures that it's possible to
identify leaf function's caller by simply pulling the value from the
top of the stack.

   While majority of subroutines written in assembler are not calling
any other function, requirement for non-volatile registers'
immutability leaves developer with not more than 7 registers and no
stack frame, which is not necessarily what [s]he counted with.
Customarily one would meet the requirement by saving non-volatile
registers on stack and restoring them upon return, so what can go
wrong? If [and only if] an exception is raised at run-time and no
`UNWIND_INFO' structure is associated with such "leaf" function, the
stack unwind procedure will expect to find caller's return address on
the top of stack immediately followed by its frame. Given that
developer pushed caller's non-volatile registers on stack, would the
value on top point at some code segment or even addressable space? Well,
developer can attempt copying caller's return address to the top of
stack and this would actually work in some very specific circumstances.
But unless developer can guarantee that these circumstances are always
met, it's more appropriate to assume worst case scenario, i.e. stack
unwind procedure going berserk. Relevant question is what happens then?
Application is abruptly terminated without any notification whatsoever.
Just like in Win32 case, one can argue that system could at least have
logged "unwind procedure went berserk in x.exe at address n" in event
log, but no, no trace of failure is left.

   Now, when we understand significance of the `UNWIND_INFO' structure,
let's discuss what's in it and/or how it's processed. First of all it is
checked for presence of reference to custom language-specific exception
handler. If there is one, then it's invoked. Depending on the return
value, execution flow is resumed (exception is said to be "handled"),
_or_ rest of `UNWIND_INFO' structure is processed as following. Beside
optional reference to custom handler, it carries information about
current callee's stack frame and where non-volatile registers are saved.
Information is detailed enough to be able to reconstruct contents of
caller's non-volatile registers upon call to current callee. And so
caller's context is reconstructed, and then unwind procedure is
repeated, i.e. another `UNWIND_INFO' structure is associated, this
time, with caller's instruction pointer, which is then checked for
presence of reference to language-specific handler, etc. The procedure
is recursively repeated till exception is handled. As last resort
system "handles" it by generating memory core dump and terminating the
application.

   As for the moment of this writing NASM unfortunately does not
facilitate generation of above mentioned detailed information about
stack frame layout. But as of version 2.03 it implements building
blocks for generating structures involved in stack unwinding. As
simplest example, here is how to deploy custom exception handler for
leaf function:

     default rel
     section .text
     extern  MessageBoxA
     handler:
             sub     rsp,40
             mov     rcx,0
             lea     rdx,[text]
             lea     r8,[caption]
             mov     r9,1    ; MB_OKCANCEL
             call    MessageBoxA
             sub     eax,1   ; incidentally suits as return value
                             ; for exception handler
             add     rsp,40
             ret
     global  main
     main:
             xor     rax,rax
             mov     rax,QWORD[rax]  ; cause exception
             ret
     main_end:
     text:   db      'OK to rethrow, CANCEL to generate core dump',0
     caption:db      'SEGV',0

     section .pdata  rdata align=4
             dd      main wrt ..imagebase
             dd      main_end wrt ..imagebase
             dd      xmain wrt ..imagebase
     section .xdata  rdata align=8
     xmain:  db      9,0,0,0
             dd      handler wrt ..imagebase
     section .drectve info
             db      '/defaultlib:user32.lib /defaultlib:msvcrt.lib '

   What you see in `.pdata' section is element of the "table comprising
start and end addresses of function" along with reference to associated
`UNWIND_INFO' structure. And what you see in `.xdata' section is
`UNWIND_INFO' structure describing function with no frame, but with
designated exception handler. References are _required_ to be image-
relative (which is the real reason for implementing `wrt ..imagebase'
operator). It should be noted that `rdata align=n', as well as `wrt
..imagebase', are optional in these two segments' contexts, i.e.  can
be omitted. Latter means that _all_ 32-bit references, not only above
listed required ones, placed into these two segments turn out image-
relative. Why is it important to understand? Developer is allowed to
append handler-specific data to `UNWIND_INFO' structure, and if [s]he
adds a 32-bit reference, then [s]he will have to remember to adjust its
value to obtain the real pointer.

   As already mentioned, in Win64 terms leaf function is one that does
not call any other function _nor_ modifies any non-volatile register,
including stack pointer. But it's not uncommon that assembler programmer
plans to utilize every single register and sometimes even have variable
stack frame. Is there anything one can do with bare building blocks?
I.e.  besides manually composing fully-fledged `UNWIND_INFO' structure,
which would surely be considered error-prone? Yes, there is. Recall that
exception handler is called first, before stack layout is analyzed. As
it turned out, it's perfectly possible to manipulate current callee's
context in custom handler in manner that permits further stack
unwinding. General idea is that handler would not actually "handle" the
exception, but instead restore callee's context, as it was at its entry
point and thus mimic leaf function. In other words, handler would
simply undertake part of unwinding procedure. Consider following
example:

     function:
             mov     rax,rsp         ; copy rsp to volatile register
             push    r15             ; save non-volatile registers
             push    rbx
             push    rbp
             mov     r11,rsp         ; prepare variable stack frame
             sub     r11,rcx
             and     r11,-64
             mov     QWORD[r11],rax  ; check for exceptions
             mov     rsp,r11         ; allocate stack frame
             mov     QWORD[rsp],rax  ; save original rsp value
     magic_point:
             ...
             mov     r11,QWORD[rsp]  ; pull original rsp value
             mov     rbp,QWORD[r11-24]
             mov     rbx,QWORD[r11-16]
             mov     r15,QWORD[r11-8]
             mov     rsp,r11         ; destroy frame
             ret

   The keyword is that up to `magic_point' original `rsp' value remains
in chosen volatile register and no non-volatile register, except for
`rsp', is modified. While past `magic_point' `rsp' remains constant
till the very end of the `function'. In this case custom
language-specific exception handler would look like this:

     EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame,
             CONTEXT *context,DISPATCHER_CONTEXT *disp)
     {   ULONG64 *rsp;
         if (context->Rip<(ULONG64)magic_point)
             rsp = (ULONG64 *)context->Rax;
         else
         {   rsp = ((ULONG64 **)context->Rsp)[0];
             context->Rbp = rsp[-3];
             context->Rbx = rsp[-2];
             context->R15 = rsp[-1];
         }
         context->Rsp = (ULONG64)rsp;

         memcpy (disp->ContextRecord,context,sizeof(CONTEXT));
         RtlVirtualUnwind(UNW_FLAG_NHANDLER,disp->ImageBase,
             dips->ControlPc,disp->FunctionEntry,disp->ContextRecord,
             &disp->HandlerData,&disp->EstablisherFrame,NULL);
         return ExceptionContinueSearch;
     }

   As custom handler mimics leaf function, corresponding `UNWIND_INFO'
structure does not have to contain any information about stack frame and
its layout.


File: nasm.info,  Node: Section 7.5,  Next: Section 7.6,  Prev: Section 7.4.2,  Up: Chapter 7

7.5. `coff': Common Object File Format
======================================

The `coff' output type produces `COFF' object files suitable for
linking with the DJGPP linker.

   `coff' provides a default output file-name extension of `.o'.

   The `coff' format supports the same extensions to the `SECTION'
directive as `win32' does, except that the `align' qualifier and the
`info' section type are not supported.


File: nasm.info,  Node: Section 7.6,  Next: Section 7.7,  Prev: Section 7.5,  Up: Chapter 7

7.6. `macho': Mach Object File Format
=====================================

The `macho' output type produces `Mach-O' object files suitable for
linking with the Mac OSX linker.

   `macho' provides a default output file-name extension of `.o'.


File: nasm.info,  Node: Section 7.7,  Next: Section 7.7.1,  Prev: Section 7.6,  Up: Chapter 7

7.7. `elf, elf32, and elf64': Executable and Linkable Format Object Files
=========================================================================

The `elf32' and `elf64' output formats generate `ELF32 and ELF64'
(Executable and Linkable Format) object files, as used by Linux as well
as Unix System V, including Solaris x86, UnixWare and SCO Unix. `elf'
provides a default output file-name extension of `.o'. `elf' is a
synonym for `elf32'.

* Menu:

* Section 7.7.1:: ELF specific directive `osabi'
* Section 7.7.2:: `elf' Extensions to the `SECTION' Directive
* Section 7.7.3:: Position-Independent Code: `elf' Special Symbols and `WRT'
* Section 7.7.4:: `elf' Extensions to the `GLOBAL' Directive
* Section 7.7.5:: `elf' Extensions to the `COMMON' Directive
* Section 7.7.6:: 16-bit code and ELF
* Section 7.7.7:: Debug formats and ELF


File: nasm.info,  Node: Section 7.7.1,  Next: Section 7.7.2,  Prev: Section 7.7,  Up: Section 7.7

7.7.1. ELF specific directive `osabi'
-------------------------------------

The ELF header specifies the application binary interface for the target
operating system (OSABI). This field can be set by using the `osabi'
directive with the numeric value (0-255) of the target system. If this
directive is not used, the default value will be "UNIX System V ABI" (0)
which will work on most systems which support ELF.


File: nasm.info,  Node: Section 7.7.2,  Next: Section 7.7.3,  Prev: Section 7.7.1,  Up: Section 7.7

7.7.2. `elf' Extensions to the `SECTION' Directive
--------------------------------------------------

Like the `obj' format, `elf' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names `.text',
`.data' and `.bss', but may still be overridden by these qualifiers.

   The available qualifiers are:

   * `alloc' defines the section to be one which is loaded into memory
     when the program is run. `noalloc' defines it to be one which is
     not, such as an informational or comment section.

   * `exec' defines the section to be one which should have execute
     permission when the program is run. `noexec' defines it as one
     which should not.

   * `write' defines the section to be one which should be writable when
     the program is run. `nowrite' defines it as one which should not.

   * `progbits' defines the section to be one with explicit contents
     stored in the object file: an ordinary code or data section, for
     example, `nobits' defines the section to be one with no explicit
     contents given, such as a BSS section.

   * `align=', used with a trailing number as in `obj', gives the
     alignment requirements of the section.

   The defaults assumed by NASM if you do not specify the above
qualifiers are:

     section .text    progbits  alloc  exec    nowrite  align=16
     section .rodata  progbits  alloc  noexec  nowrite  align=4
     section .data    progbits  alloc  noexec  write    align=4
     section .bss     nobits    alloc  noexec  write    align=4
     section other    progbits  alloc  noexec  nowrite  align=1

   (Any section name other than `.text', `.rodata', `.data' and `.bss'
is treated by default like `other' in the above code.)


File: nasm.info,  Node: Section 7.7.3,  Next: Section 7.7.4,  Prev: Section 7.7.2,  Up: Section 7.7

7.7.3. Position-Independent Code: `elf' Special Symbols and `WRT'
-----------------------------------------------------------------

The `ELF' specification contains enough features to allow position-
independent code (PIC) to be written, which makes ELF shared libraries
very flexible. However, it also means NASM has to be able to generate a
variety of strange relocation types in ELF object files, if it is to be
an assembler which can write PIC.

   Since `ELF' does not support segment-base references, the `WRT'
operator is not used for its normal purpose; therefore NASM's `elf'
output format makes use of `WRT' for a different purpose, namely the
PIC-specific relocation types.

   `elf' defines five special symbols which you can use as the
right-hand side of the `WRT' operator to obtain PIC relocation types.
They are `..gotpc', `..gotoff', `..got', `..plt' and `..sym'. Their
functions are summarized here:

   * Referring to the symbol marking the global offset table base using
     `wrt ..gotpc' will end up giving the distance from the beginning of
     the current section to the global offset table.
     (`_GLOBAL_OFFSET_TABLE_' is the standard symbol name used to refer
     to the GOT.) So you would then need to add `$$' to the result to
     get the real address of the GOT.

   * Referring to a location in one of your own sections using `wrt
     ..gotoff' will give the distance from the beginning of the GOT to
     the specified location, so that adding on the address of the GOT
     would give the real address of the location you wanted.

   * Referring to an external or global symbol using `wrt ..got' causes
     the linker to build an entry _in_ the GOT containing the address
     of the symbol, and the reference gives the distance from the
     beginning of the GOT to the entry; so you can add on the address
     of the GOT, load from the resulting address, and end up with the
     address of the symbol.

   * Referring to a procedure name using `wrt ..plt' causes the linker
     to build a procedure linkage table entry for the symbol, and the
     reference gives the address of the PLT entry. You can only use
     this in contexts which would generate a PC-relative relocation
     normally (i.e. as the destination for `CALL' or `JMP'), since ELF
     contains no relocation type to refer to PLT entries absolutely.

   * Referring to a symbol name using `wrt ..sym' causes NASM to write
     an ordinary relocation, but instead of making the relocation
     relative to the start of the section and then adding on the offset
     to the symbol, it will write a relocation record aimed directly at
     the symbol in question. The distinction is a necessary one due to
     a peculiarity of the dynamic linker.

   A fuller explanation of how to use these relocation types to write
shared libraries entirely in NASM is given in *note Section 9.2::.


File: nasm.info,  Node: Section 7.7.4,  Next: Section 7.7.5,  Prev: Section 7.7.3,  Up: Section 7.7

7.7.4. `elf' Extensions to the `GLOBAL' Directive
-------------------------------------------------

`ELF' object files can contain more information about a global symbol
than just its address: they can contain the size of the symbol and its
type as well. These are not merely debugger conveniences, but are
actually necessary when the program being written is a shared library.
NASM therefore supports some extensions to the `GLOBAL' directive,
allowing you to specify these features.

   You can specify whether a global variable is a function or a data
object by suffixing the name with a colon and the word `function' or
`data'. (`object' is a synonym for `data'.) For example:

     global   hashlookup:function, hashtable:data

   exports the global symbol `hashlookup' as a function and `hashtable'
as a data object.

   Optionally, you can control the ELF visibility of the symbol. Just
add one of the visibility keywords: `default', `internal', `hidden', or
`protected'. The default is `default' of course. For example, to make
`hashlookup' hidden:

     global   hashlookup:function hidden

   You can also specify the size of the data associated with the
symbol, as a numeric expression (which may involve labels, and even
forward references) after the type specifier. Like this:

     global  hashtable:data (hashtable.end - hashtable)

     hashtable:
             db this,that,theother  ; some data here
     .end:

   This makes NASM automatically calculate the length of the table and
place that information into the `ELF' symbol table.

   Declaring the type and size of global symbols is necessary when
writing shared library code. For more information, see *note Section
9.2.4::.


File: nasm.info,  Node: Section 7.7.5,  Next: Section 7.7.6,  Prev: Section 7.7.4,  Up: Section 7.7

7.7.5. `elf' Extensions to the `COMMON' Directive
-------------------------------------------------

`ELF' also allows you to specify alignment requirements on common
variables. This is done by putting a number (which must be a power of
two) after the name and size of the common variable, separated (as
usual) by a colon. For example, an array of doublewords would benefit
from 4-byte alignment:

     common  dwordarray 128:4

   This declares the total size of the array to be 128 bytes, and
requires that it be aligned on a 4-byte boundary.


File: nasm.info,  Node: Section 7.7.6,  Next: Section 7.7.7,  Prev: Section 7.7.5,  Up: Section 7.7

7.7.6. 16-bit code and ELF
--------------------------

The `ELF32' specification doesn't provide relocations for 8- and 16-
bit values, but the GNU `ld' linker adds these as an extension. NASM
can generate GNU-compatible relocations, to allow 16-bit code to be
linked as ELF using GNU `ld'. If NASM is used with the
`-w+gnu-elf-extensions' option, a warning is issued when one of these
relocations is generated.


File: nasm.info,  Node: Section 7.7.7,  Next: Section 7.8,  Prev: Section 7.7.6,  Up: Section 7.7

7.7.7. Debug formats and ELF
----------------------------

`ELF32' and `ELF64' provide debug information in `STABS' and `DWARF'
formats. Line number information is generated for all executable
sections, but please note that only the ".text" section is executable
by default.


File: nasm.info,  Node: Section 7.8,  Next: Section 7.9,  Prev: Section 7.7.7,  Up: Chapter 7

7.8. `aout': Linux `a.out' Object Files
=======================================

The `aout' format generates `a.out' object files, in the form used by
early Linux systems (current Linux systems use ELF, see *note Section
7.7::.) These differ from other `a.out' object files in that the magic
number in the first four bytes of the file is different; also, some
implementations of `a.out', for example NetBSD's, support
position-independent code, which Linux's implementation does not.

   `a.out' provides a default output file-name extension of `.o'.

   `a.out' is a very simple object format. It supports no special
directives, no special symbols, no use of `SEG' or `WRT', and no
extensions to any standard directives. It supports only the three
standard section names `.text', `.data' and `.bss'.


File: nasm.info,  Node: Section 7.9,  Next: Section 7.10,  Prev: Section 7.8,  Up: Chapter 7

7.9. `aoutb': NetBSD/FreeBSD/OpenBSD `a.out' Object Files
=========================================================

The `aoutb' format generates `a.out' object files, in the form used by
the various free `BSD Unix' clones, `NetBSD', `FreeBSD' and `OpenBSD'.
For simple object files, this object format is exactly the same as
`aout' except for the magic number in the first four bytes of the file.
However, the `aoutb' format supports position-independent code in the
same way as the `elf' format, so you can use it to write `BSD' shared
libraries.

   `aoutb' provides a default output file-name extension of `.o'.

   `aoutb' supports no special directives, no special symbols, and only
the three standard section names `.text', `.data' and `.bss'. However,
it also supports the same use of `WRT' as `elf' does, to provide
position-independent code relocation types. See *note Section 7.7.3::
for full documentation of this feature.

   `aoutb' also supports the same extensions to the `GLOBAL' directive
as `elf' does: see *note Section 7.7.4:: for documentation of this.


File: nasm.info,  Node: Section 7.10,  Next: Section 7.11,  Prev: Section 7.9,  Up: Chapter 7

7.10. `as86': Minix/Linux `as86' Object Files
=============================================

The Minix/Linux 16-bit assembler `as86' has its own non-standard object
file format. Although its companion linker `ld86' produces something
close to ordinary `a.out' binaries as output, the object file format
used to communicate between `as86' and `ld86' is not itself `a.out'.

   NASM supports this format, just in case it is useful, as `as86'.
`as86' provides a default output file-name extension of `.o'.

   `as86' is a very simple object format (from the NASM user's point of
view). It supports no special directives, no special symbols, no use of
`SEG' or `WRT', and no extensions to any standard directives. It
supports only the three standard section names `.text', `.data' and
`.bss'.


File: nasm.info,  Node: Section 7.11,  Next: Section 7.11.1,  Prev: Section 7.10,  Up: Chapter 7

7.11. `rdf': Relocatable Dynamic Object File Format
===================================================

The `rdf' output format produces `RDOFF' object files.  `RDOFF'
(Relocatable Dynamic Object File Format) is a home-grown object-file
format, designed alongside NASM itself and reflecting in its file
format the internal structure of the assembler.

   `RDOFF' is not used by any well-known operating systems. Those
writing their own systems, however, may well wish to use `RDOFF' as
their object format, on the grounds that it is designed primarily for
simplicity and contains very little file-header bureaucracy.

   The Unix NASM archive, and the DOS archive which includes sources,
both contain an `rdoff' subdirectory holding a set of RDOFF utilities:
an RDF linker, an `RDF' static-library manager, an RDF file dump
utility, and a program which will load and execute an RDF executable
under Linux.

   `rdf' supports only the standard section names `.text', `.data' and
`.bss'.

* Menu:

* Section 7.11.1:: Requiring a Library: The `LIBRARY' Directive
* Section 7.11.2:: Specifying a Module Name: The `MODULE' Directive
* Section 7.11.3:: `rdf' Extensions to the `GLOBAL' directive
* Section 7.11.4:: `rdf' Extensions to the `EXTERN' directive


File: nasm.info,  Node: Section 7.11.1,  Next: Section 7.11.2,  Prev: Section 7.11,  Up: Section 7.11

7.11.1. Requiring a Library: The `LIBRARY' Directive
----------------------------------------------------

`RDOFF' contains a mechanism for an object file to demand a given
library to be linked to the module, either at load time or run time.
This is done by the `LIBRARY' directive, which takes one argument which
is the name of the module:

         library  mylib.rdl


File: nasm.info,  Node: Section 7.11.2,  Next: Section 7.11.3,  Prev: Section 7.11.1,  Up: Section 7.11

7.11.2. Specifying a Module Name: The `MODULE' Directive
--------------------------------------------------------

Special `RDOFF' header record is used to store the name of the module.
It can be used, for example, by run-time loader to perform dynamic
linking.  `MODULE' directive takes one argument which is the name of
current module:

         module  mymodname

   Note that when you statically link modules and tell linker to strip
the symbols from output file, all module names will be stripped too. To
avoid it, you should start module names with `$', like:

         module  $kernel.core


File: nasm.info,  Node: Section 7.11.3,  Next: Section 7.11.4,  Prev: Section 7.11.2,  Up: Section 7.11

7.11.3. `rdf' Extensions to the `GLOBAL' directive
--------------------------------------------------

`RDOFF' global symbols can contain additional information needed by the
static linker. You can mark a global symbol as exported, thus telling
the linker do not strip it from target executable or library file. Like
in `ELF', you can also specify whether an exported symbol is a procedure
(function) or data object.

   Suffixing the name with a colon and the word `export' you make the
symbol exported:

         global  sys_open:export

   To specify that exported symbol is a procedure (function), you add
the word `proc' or `function' after declaration:

         global  sys_open:export proc

   Similarly, to specify exported data object, add the word `data' or
`object' to the directive:

         global  kernel_ticks:export data


File: nasm.info,  Node: Section 7.11.4,  Next: Section 7.12,  Prev: Section 7.11.3,  Up: Section 7.11

7.11.4. `rdf' Extensions to the `EXTERN' directive
--------------------------------------------------

By default the `EXTERN' directive in `RDOFF' declares a "pure external"
symbol (i.e. the static linker will complain if such a symbol is not
resolved). To declare an "imported" symbol, which must be resolved
later during a dynamic linking phase, `RDOFF' offers an additional
`import' modifier. As in `GLOBAL', you can also specify whether an
imported symbol is a procedure (function) or data object. For example:

         library $libc
         extern  _open:import
         extern  _printf:import proc
         extern  _errno:import data

   Here the directive `LIBRARY' is also included, which gives the
dynamic linker a hint as to where to find requested symbols.


File: nasm.info,  Node: Section 7.12,  Next: Chapter 8,  Prev: Section 7.11.4,  Up: Chapter 7

7.12. `dbg': Debugging Format
=============================

The `dbg' output format is not built into NASM in the default
configuration. If you are building your own NASM executable from the
sources, you can define `OF_DBG' in `outform.h' or on the compiler
command line, and obtain the `dbg' output format.

   The `dbg' format does not output an object file as such; instead, it
outputs a text file which contains a complete list of all the
transactions between the main body of NASM and the output-format back
end module. It is primarily intended to aid people who want to write
their own output drivers, so that they can get a clearer idea of the
various requests the main program makes of the output driver, and in
what order they happen.

   For simple files, one can easily use the `dbg' format like this:

     nasm -f dbg filename.asm

   which will generate a diagnostic file called `filename.dbg'. However,
this will not work well on files which were designed for a different
object format, because each object format defines its own macros
(usually user- level forms of directives), and those macros will not be
defined in the `dbg' format. Therefore it can be useful to run NASM
twice, in order to do the preprocessing with the native object format
selected:

     nasm -e -f rdf -o rdfprog.i rdfprog.asm
     nasm -a -f dbg rdfprog.i

   This preprocesses `rdfprog.asm' into `rdfprog.i', keeping the `rdf'
object format selected in order to make sure RDF special directives are
converted into primitive form correctly. Then the preprocessed source
is fed through the `dbg' format to generate the final diagnostic output.

   This workaround will still typically not work for programs intended
for `obj' format, because the `obj' `SEGMENT' and `GROUP' directives
have side effects of defining the segment and group names as symbols;
`dbg' will not do this, so the program will not assemble. You will have
to work around that by defining the symbols yourself (using `EXTERN',
for example) if you really need to get a `dbg' trace of an
`obj'-specific source file.

   `dbg' accepts any section name and any directives at all, and logs
them all to its output file.


File: nasm.info,  Node: Chapter 8,  Next: Section 8.1,  Prev: Section 7.12,  Up: Top

Chapter 8: Writing 16-bit Code (DOS, Windows 3/3.1)
***************************************************

This chapter attempts to cover some of the common issues encountered
when writing 16-bit code to run under `MS-DOS' or `Windows 3.x'. It
covers how to link programs to produce `.EXE' or `.COM' files, how to
write `.SYS' device drivers, and how to interface assembly language
code with 16-bit C compilers and with Borland Pascal.

* Menu:

* Section 8.1:: Producing `.EXE' Files
* Section 8.2:: Producing `.COM' Files
* Section 8.3:: Producing `.SYS' Files
* Section 8.4:: Interfacing to 16-bit C Programs
* Section 8.5:: Interfacing to Borland Pascal Programs


File: nasm.info,  Node: Section 8.1,  Next: Section 8.1.1,  Prev: Chapter 8,  Up: Chapter 8

8.1. Producing `.EXE' Files
===========================

Any large program written under DOS needs to be built as a `.EXE' file:
only `.EXE' files have the necessary internal structure required to
span more than one 64K segment. Windows programs, also, have to be built
as `.EXE' files, since Windows does not support the `.COM' format.

   In general, you generate `.EXE' files by using the `obj' output
format to produce one or more `.OBJ' files, and then linking them
together using a linker. However, NASM also supports the direct
generation of simple DOS `.EXE' files using the `bin' output format (by
using `DB' and `DW' to construct the `.EXE' file header), and a macro
package is supplied to do this. Thanks to Yann Guidon for contributing
the code for this.

   NASM may also support `.EXE' natively as another output format in
future releases.

* Menu:

* Section 8.1.1:: Using the `obj' Format To Generate `.EXE' Files
* Section 8.1.2:: Using the `bin' Format To Generate `.EXE' Files


File: nasm.info,  Node: Section 8.1.1,  Next: Section 8.1.2,  Prev: Section 8.1,  Up: Section 8.1

8.1.1. Using the `obj' Format To Generate `.EXE' Files
------------------------------------------------------

This section describes the usual method of generating `.EXE' files by
linking `.OBJ' files together.

   Most 16-bit programming language packages come with a suitable
linker; if you have none of these, there is a free linker called VAL,
available in `LZH' archive format from `x2ftp.oulu.fi'. An LZH archiver
can be found at `ftp.simtel.net'. There is another `free' linker
(though this one doesn't come with sources) called FREELINK, available
from `www.pcorner.com'. A third, `djlink', written by DJ Delorie, is
available at `www.delorie.com'. A fourth linker, `ALINK', written by
Anthony A.J. Williams, is available at `alink.sourceforge.net'.

   When linking several `.OBJ' files into a `.EXE' file, you should
ensure that exactly one of them has a start point defined (using the
`..start' special symbol defined by the `obj' format: see *note Section
7.2.6::). If no module defines a start point, the linker will not know
what value to give the entry-point field in the output file header; if
more than one defines a start point, the linker will not know _which_
value to use.

   An example of a NASM source file which can be assembled to a `.OBJ'
file and linked on its own to a `.EXE' is given here. It demonstrates
the basic principles of defining a stack, initialising the segment
registers, and declaring a start point. This file is also provided in
the `test' subdirectory of the NASM archives, under the name
`objexe.asm'.

     segment code

     ..start:
             mov     ax,data
             mov     ds,ax
             mov     ax,stack
             mov     ss,ax
             mov     sp,stacktop

   This initial piece of code sets up `DS' to point to the data segment,
and initializes `SS' and `SP' to point to the top of the provided
stack. Notice that interrupts are implicitly disabled for one
instruction after a move into `SS', precisely for this situation, so
that there's no chance of an interrupt occurring between the loads of
`SS' and `SP' and not having a stack to execute on.

   Note also that the special symbol `..start' is defined at the
beginning of this code, which means that will be the entry point into
the resulting executable file.

             mov     dx,hello
             mov     ah,9
             int     0x21

   The above is the main program: load `DS:DX' with a pointer to the
greeting message (`hello' is implicitly relative to the segment `data',
which was loaded into `DS' in the setup code, so the full pointer is
valid), and call the DOS print-string function.

             mov     ax,0x4c00
             int     0x21

   This terminates the program using another DOS system call.

     segment data

     hello:  db      'hello, world', 13, 10, '$'

   The data segment contains the string we want to display.

     segment stack stack
             resb 64
     stacktop:

   The above code declares a stack segment containing 64 bytes of
uninitialized stack space, and points `stacktop' at the top of it. The
directive `segment stack stack' defines a segment _called_ `stack', and
also of _type_ `STACK'. The latter is not necessary to the correct
running of the program, but linkers are likely to issue warnings or
errors if your program has no segment of type `STACK'.

   The above file, when assembled into a `.OBJ' file, will link on its
own to a valid `.EXE' file, which when run will print `hello, world'
and then exit.


File: nasm.info,  Node: Section 8.1.2,  Next: Section 8.2,  Prev: Section 8.1.1,  Up: Section 8.1

8.1.2. Using the `bin' Format To Generate `.EXE' Files
------------------------------------------------------

The `.EXE' file format is simple enough that it's possible to build a
`.EXE' file by writing a pure-binary program and sticking a 32-byte
header on the front. This header is simple enough that it can be
generated using `DB' and `DW' commands by NASM itself, so that you can
use the `bin' output format to directly generate `.EXE' files.

   Included in the NASM archives, in the `misc' subdirectory, is a file
`exebin.mac' of macros. It defines three macros: `EXE_begin',
`EXE_stack' and `EXE_end'.

   To produce a `.EXE' file using this method, you should start by using
`%include' to load the `exebin.mac' macro package into your source
file. You should then issue the `EXE_begin' macro call (which takes no
arguments) to generate the file header data. Then write code as normal
for the `bin' format - you can use all three standard sections `.text',
`.data' and `.bss'. At the end of the file you should call the
`EXE_end' macro (again, no arguments), which defines some symbols to
mark section sizes, and these symbols are referred to in the header
code generated by `EXE_begin'.

   In this model, the code you end up writing starts at `0x100', just
like a `.COM' file - in fact, if you strip off the 32-byte header from
the resulting `.EXE' file, you will have a valid `.COM' program. All
the segment bases are the same, so you are limited to a 64K program,
again just like a `.COM' file. Note that an `ORG' directive is issued
by the `EXE_begin' macro, so you should not explicitly issue one of
your own.

   You can't directly refer to your segment base value, unfortunately,
since this would require a relocation in the header, and things would
get a lot more complicated. So you should get your segment base by
copying it out of `CS' instead.

   On entry to your `.EXE' file, `SS:SP' are already set up to point to
the top of a 2Kb stack. You can adjust the default stack size of 2Kb by
calling the `EXE_stack' macro. For example, to change the stack size of
your program to 64 bytes, you would call `EXE_stack 64'.

   A sample program which generates a `.EXE' file in this way is given
in the `test' subdirectory of the NASM archive, as `binexe.asm'.


File: nasm.info,  Node: Section 8.2,  Next: Section 8.2.1,  Prev: Section 8.1.2,  Up: Chapter 8

8.2. Producing `.COM' Files
===========================

While large DOS programs must be written as `.EXE' files, small ones
are often better written as `.COM' files. `.COM' files are pure binary,
and therefore most easily produced using the `bin' output format.

* Menu:

* Section 8.2.1:: Using the `bin' Format To Generate `.COM' Files
* Section 8.2.2:: Using the `obj' Format To Generate `.COM' Files


File: nasm.info,  Node: Section 8.2.1,  Next: Section 8.2.2,  Prev: Section 8.2,  Up: Section 8.2

8.2.1. Using the `bin' Format To Generate `.COM' Files
------------------------------------------------------

`.COM' files expect to be loaded at offset `100h' into their segment
(though the segment may change). Execution then begins at `100h', i.e.
right at the start of the program. So to write a `.COM' program, you
would create a source file looking like

             org 100h

     section .text

     start:
             ; put your code here

     section .data

             ; put data items here

     section .bss

             ; put uninitialized data here

   The `bin' format puts the `.text' section first in the file, so you
can declare data or BSS items before beginning to write code if you
want to and the code will still end up at the front of the file where it
belongs.

   The BSS (uninitialized data) section does not take up space in the
`.COM' file itself: instead, addresses of BSS items are resolved to
point at space beyond the end of the file, on the grounds that this
will be free memory when the program is run. Therefore you should not
rely on your BSS being initialized to all zeros when you run.

   To assemble the above program, you should use a command line like

     nasm myprog.asm -fbin -o myprog.com

   The `bin' format would produce a file called `myprog' if no explicit
output file name were specified, so you have to override it and give
the desired file name.


File: nasm.info,  Node: Section 8.2.2,  Next: Section 8.3,  Prev: Section 8.2.1,  Up: Section 8.2

8.2.2. Using the `obj' Format To Generate `.COM' Files
------------------------------------------------------

If you are writing a `.COM' program as more than one module, you may
wish to assemble several `.OBJ' files and link them together into a
`.COM' program. You can do this, provided you have a linker capable of
outputting `.COM' files directly (TLINK does this), or alternatively a
converter program such as `EXE2BIN' to transform the `.EXE' file output
from the linker into a `.COM' file.

   If you do this, you need to take care of several things:

   * The first object file containing code should start its code
     segment with a line like `RESB 100h'. This is to ensure that the
     code begins at offset `100h' relative to the beginning of the code
     segment, so that the linker or converter program does not have to
     adjust address references within the file when generating the
     `.COM' file. Other assemblers use an `ORG' directive for this
     purpose, but `ORG' in NASM is a format-specific directive to the
     `bin' output format, and does not mean the same thing as it does
     in MASM-compatible assemblers.

   * You don't need to define a stack segment.

   * All your segments should be in the same group, so that every time
     your code or data references a symbol offset, all offsets are
     relative to the same segment base. This is because, when a `.COM'
     file is loaded, all the segment registers contain the same value.


File: nasm.info,  Node: Section 8.3,  Next: Section 8.4,  Prev: Section 8.2.2,  Up: Chapter 8

8.3. Producing `.SYS' Files
===========================

MS-DOS device drivers - `.SYS' files - are pure binary files, similar
to `.COM' files, except that they start at origin zero rather than
`100h'. Therefore, if you are writing a device driver using the `bin'
format, you do not need the `ORG' directive, since the default origin
for `bin' is zero. Similarly, if you are using `obj', you do not need
the `RESB 100h' at the start of your code segment.

   `.SYS' files start with a header structure, containing pointers to
the various routines inside the driver which do the work. This
structure should be defined at the start of the code segment, even
though it is not actually code.

   For more information on the format of `.SYS' files, and the data
which has to go in the header structure, a list of books is given in the
Frequently Asked Questions list for the newsgroup
`comp.os.msdos.programmer'.


File: nasm.info,  Node: Section 8.4,  Next: Section 8.4.1,  Prev: Section 8.3,  Up: Chapter 8

8.4. Interfacing to 16-bit C Programs
=====================================

This section covers the basics of writing assembly routines that call,
or are called from, C programs. To do this, you would typically write an
assembly module as a `.OBJ' file, and link it with your C modules to
produce a mixed-language program.

* Menu:

* Section 8.4.1:: External Symbol Names
* Section 8.4.2:: Memory Models
* Section 8.4.3:: Function Definitions and Function Calls
* Section 8.4.4:: Accessing Data Items
* Section 8.4.5:: `c16.mac': Helper Macros for the 16-bit C Interface


File: nasm.info,  Node: Section 8.4.1,  Next: Section 8.4.2,  Prev: Section 8.4,  Up: Section 8.4

8.4.1. External Symbol Names
----------------------------

C compilers have the convention that the names of all global symbols
(functions or data) they define are formed by prefixing an underscore to
the name as it appears in the C program. So, for example, the function
a C programmer thinks of as `printf' appears to an assembly language
programmer as `_printf'. This means that in your assembly programs, you
can define symbols without a leading underscore, and not have to worry
about name clashes with C symbols.

   If you find the underscores inconvenient, you can define macros to
replace the `GLOBAL' and `EXTERN' directives as follows:

     %macro  cglobal 1

       global  _%1
       %define %1 _%1

     %endmacro

     %macro  cextern 1

       extern  _%1
       %define %1 _%1

     %endmacro

   (These forms of the macros only take one argument at a time; a `%rep'
construct could solve this.)

   If you then declare an external like this:

     cextern printf

   then the macro will expand it as

     extern  _printf
     %define printf _printf

   Thereafter, you can reference `printf' as if it was a symbol, and the
preprocessor will put the leading underscore on where necessary.

   The `cglobal' macro works similarly. You must use `cglobal' before
defining the symbol in question, but you would have had to do that
anyway if you used `GLOBAL'.

   Also see *note Section 2.1.27::.


File: nasm.info,  Node: Section 8.4.2,  Next: Section 8.4.3,  Prev: Section 8.4.1,  Up: Section 8.4

8.4.2. Memory Models
--------------------

NASM contains no mechanism to support the various C memory models
directly; you have to keep track yourself of which one you are writing
for. This means you have to keep track of the following things:

   * In models using a single code segment (tiny, small and compact),
     functions are near. This means that function pointers, when stored
     in data segments or pushed on the stack as function arguments, are
     16 bits long and contain only an offset field (the `CS' register
     never changes its value, and always gives the segment part of the
     full function address), and that functions are called using
     ordinary near `CALL' instructions and return using `RETN' (which,
     in NASM, is synonymous with `RET' anyway). This means both that
     you should write your own routines to return with `RETN', and that
     you should call external C routines with near `CALL' instructions.

   * In models using more than one code segment (medium, large and
     huge), functions are far. This means that function pointers are 32
     bits long (consisting of a 16-bit offset followed by a 16-bit
     segment), and that functions are called using `CALL FAR' (or `CALL
     seg:offset') and return using `RETF'. Again, you should therefore
     write your own routines to return with `RETF' and use `CALL FAR'
     to call external routines.

   * In models using a single data segment (tiny, small and medium),
     data pointers are 16 bits long, containing only an offset field
     (the `DS' register doesn't change its value, and always gives the
     segment part of the full data item address).

   * In models using more than one data segment (compact, large and
     huge), data pointers are 32 bits long, consisting of a 16-bit
     offset followed by a 16- bit segment. You should still be careful
     not to modify `DS' in your routines without restoring it
     afterwards, but `ES' is free for you to use to access the contents
     of 32-bit data pointers you are passed.

   * The huge memory model allows single data items to exceed 64K in
     size. In all other memory models, you can access the whole of a
     data item just by doing arithmetic on the offset field of the
     pointer you are given, whether a segment field is present or not;
     in huge model, you have to be more careful of your pointer
     arithmetic.

   * In most memory models, there is a _default_ data segment, whose
     segment address is kept in `DS' throughout the program. This data
     segment is typically the same segment as the stack, kept in `SS',
     so that functions' local variables (which are stored on the stack)
     and global data items can both be accessed easily without changing
     `DS'.  Particularly large data items are typically stored in other
     segments.  However, some memory models (though not the standard
     ones, usually) allow the assumption that `SS' and `DS' hold the
     same value to be removed. Be careful about functions' local
     variables in this latter case.

   In models with a single code segment, the segment is called `_TEXT',
so your code segment must also go by this name in order to be linked
into the same place as the main code segment. In models with a single
data segment, or with a default data segment, it is called `_DATA'.


File: nasm.info,  Node: Section 8.4.3,  Next: Section 8.4.4,  Prev: Section 8.4.2,  Up: Section 8.4

8.4.3. Function Definitions and Function Calls
----------------------------------------------

The C calling convention in 16-bit programs is as follows. In the
following description, the words _caller_ and _callee_ are used to
denote the function doing the calling and the function which gets
called.

   * The caller pushes the function's parameters on the stack, one after
     another, in reverse order (right to left, so that the first
     argument specified to the function is pushed last).

   * The caller then executes a `CALL' instruction to pass control to
     the callee. This `CALL' is either near or far depending on the
     memory model.

   * The callee receives control, and typically (although this is not
     actually necessary, in functions which do not need to access their
     parameters) starts by saving the value of `SP' in `BP' so as to be
     able to use `BP' as a base pointer to find its parameters on the
     stack.  However, the caller was probably doing this too, so part
     of the calling convention states that `BP' must be preserved by
     any C function. Hence the callee, if it is going to set up `BP' as
     a _frame pointer_, must push the previous value first.

   * The callee may then access its parameters relative to `BP'. The
     word at `[BP]' holds the previous value of `BP' as it was pushed;
     the next word, at `[BP+2]', holds the offset part of the return
     address, pushed implicitly by `CALL'. In a small-model (near)
     function, the parameters start after that, at `[BP+4]'; in a
     large-model (far) function, the segment part of the return address
     lives at `[BP+4]', and the parameters begin at `[BP+6]'. The
     leftmost parameter of the function, since it was pushed last, is
     accessible at this offset from `BP'; the others follow, at
     successively greater offsets. Thus, in a function such as `printf'
     which takes a variable number of parameters, the pushing of the
     parameters in reverse order means that the function knows where to
     find its first parameter, which tells it the number and type of
     the remaining ones.

   * The callee may also wish to decrease `SP' further, so as to
     allocate space on the stack for local variables, which will then
     be accessible at negative offsets from `BP'.

   * The callee, if it wishes to return a value to the caller, should
     leave the value in `AL', `AX' or `DX:AX' depending on the size of
     the value. Floating-point results are sometimes (depending on the
     compiler) returned in `ST0'.

   * Once the callee has finished processing, it restores `SP' from
     `BP' if it had allocated local stack space, then pops the previous
     value of `BP', and returns via `RETN' or `RETF' depending on
     memory model.

   * When the caller regains control from the callee, the function
     parameters are still on the stack, so it typically adds an
     immediate constant to `SP' to remove them (instead of executing a
     number of slow `POP' instructions). Thus, if a function is
     accidentally called with the wrong number of parameters due to a
     prototype mismatch, the stack will still be returned to a sensible
     state since the caller, which _knows_ how many parameters it
     pushed, does the removing.

   It is instructive to compare this calling convention with that for
Pascal programs (described in *note Section 8.5.1::). Pascal has a
simpler convention, since no functions have variable numbers of
parameters.  Therefore the callee knows how many parameters it should
have been passed, and is able to deallocate them from the stack itself
by passing an immediate argument to the `RET' or `RETF' instruction, so
the caller does not have to do it. Also, the parameters are pushed in
left-to- right order, not right-to-left, which means that a compiler
can give better guarantees about sequence points without performance
suffering.

   Thus, you would define a function in C style in the following way.
The following example is for small model:

     global  _myfunc

     _myfunc:
             push    bp
             mov     bp,sp
             sub     sp,0x40         ; 64 bytes of local stack space
             mov     bx,[bp+4]       ; first parameter to function

             ; some more code

             mov     sp,bp           ; undo "sub sp,0x40" above
             pop     bp
             ret

   For a large-model function, you would replace `RET' by `RETF', and
look for the first parameter at `[BP+6]' instead of `[BP+4]'.  Of
course, if one of the parameters is a pointer, then the offsets of
_subsequent_ parameters will change depending on the memory model as
well: far pointers take up four bytes on the stack when passed as a
parameter, whereas near pointers take up two.

   At the other end of the process, to call a C function from your
assembly code, you would do something like this:

     extern  _printf

           ; and then, further down...

           push    word [myint]        ; one of my integer variables
           push    word mystring       ; pointer into my data segment
           call    _printf
           add     sp,byte 4           ; `byte' saves space

           ; then those data items...

     segment _DATA

     myint         dw    1234
     mystring      db    'This number -> %d <- should be 1234',10,0

   This piece of code is the small-model assembly equivalent of the C
code

         int myint = 1234;
         printf("This number -> %d <- should be 1234\n", myint);

   In large model, the function-call code might look more like this. In
this example, it is assumed that `DS' already holds the segment base of
the segment `_DATA'. If not, you would have to initialize it first.

           push    word [myint]
           push    word seg mystring   ; Now push the segment, and...
           push    word mystring       ; ... offset of "mystring"
           call    far _printf
           add    sp,byte 6

   The integer value still takes up one word on the stack, since large
model does not affect the size of the `int' data type. The first
argument (pushed last) to `printf', however, is a data pointer, and
therefore has to contain a segment and offset part. The segment should
be stored second in memory, and therefore must be pushed first. (Of
course, `PUSH DS' would have been a shorter instruction than `PUSH WORD
SEG mystring', if `DS' was set up as the above example assumed.) Then
the actual call becomes a far call, since functions expect far calls in
large model; and `SP' has to be increased by 6 rather than 4 afterwards
to make up for the extra word of parameters.


File: nasm.info,  Node: Section 8.4.4,  Next: Section 8.4.5,  Prev: Section 8.4.3,  Up: Section 8.4

8.4.4. Accessing Data Items
---------------------------

To get at the contents of C variables, or to declare variables which C
can access, you need only declare the names as `GLOBAL' or `EXTERN'.
(Again, the names require leading underscores, as stated in *note
Section 8.4.1::.) Thus, a C variable declared as `int i' can be
accessed from assembler as

     extern _i

             mov ax,[_i]

   And to declare your own integer variable which C programs can access
as `extern int j', you do this (making sure you are assembling in the
`_DATA' segment, if necessary):

     global  _j

     _j      dw      0

   To access a C array, you need to know the size of the components of
the array. For example, `int' variables are two bytes long, so if a C
program declares an array as `int a[10]', you can access `a[3]' by
coding `mov ax,[_a+6]'. (The byte offset 6 is obtained by multiplying
the desired array index, 3, by the size of the array element, 2.) The
sizes of the C base types in 16-bit compilers are: 1 for `char', 2 for
`short' and `int', 4 for `long' and `float', and 8 for `double'.

   To access a C data structure, you need to know the offset from the
base of the structure to the field you are interested in. You can
either do this by converting the C structure definition into a NASM
structure definition (using `STRUC'), or by calculating the one offset
and using just that.

   To do either of these, you should read your C compiler's manual to
find out how it organizes data structures. NASM gives no special
alignment to structure members in its own `STRUC' macro, so you have to
specify alignment yourself if the C compiler generates it. Typically,
you might find that a structure like

     struct {
         char c;
         int i;
     } foo;

   might be four bytes long rather than three, since the `int' field
would be aligned to a two-byte boundary. However, this sort of feature
tends to be a configurable option in the C compiler, either using
command- line options or `#pragma' lines, so you have to find out how
your own compiler does it.


File: nasm.info,  Node: Section 8.4.5,  Next: Section 8.5,  Prev: Section 8.4.4,  Up: Section 8.4

8.4.5. `c16.mac': Helper Macros for the 16-bit C Interface
----------------------------------------------------------

Included in the NASM archives, in the `misc' directory, is a file
`c16.mac' of macros. It defines three macros: `proc', `arg' and
`endproc'. These are intended to be used for C-style procedure
definitions, and they automate a lot of the work involved in keeping
track of the calling convention.

   (An alternative, TASM compatible form of `arg' is also now built into
NASM's preprocessor. See *note Section 4.8:: for details.)

   An example of an assembly function using the macro set is given here:

     proc    _nearproc

     %$i     arg
     %$j     arg
             mov     ax,[bp + %$i]
             mov     bx,[bp + %$j]
             add     ax,[bx]

     endproc

   This defines `_nearproc' to be a procedure taking two arguments, the
first (`i') an integer and the second (`j') a pointer to an integer. It
returns `i + *j'.

   Note that the `arg' macro has an `EQU' as the first line of its
expansion, and since the label before the macro call gets prepended to
the first line of the expanded macro, the `EQU' works, defining `%$i'
to be an offset from `BP'. A context-local variable is used, local to
the context pushed by the `proc' macro and popped by the `endproc'
macro, so that the same argument name can be used in later procedures.
Of course, you don't _have_ to do that.

   The macro set produces code for near functions (tiny, small and
compact- model code) by default. You can have it generate far functions
(medium, large and huge-model code) by means of coding `%define
FARCODE'. This changes the kind of return instruction generated by
`endproc', and also changes the starting point for the argument
offsets. The macro set contains no intrinsic dependency on whether data
pointers are far or not.

   `arg' can take an optional parameter, giving the size of the
argument.  If no size is given, 2 is assumed, since it is likely that
many function parameters will be of type `int'.

   The large-model equivalent of the above function would look like
this:

     %define FARCODE

     proc    _farproc

     %$i     arg
     %$j     arg     4
             mov     ax,[bp + %$i]
             mov     bx,[bp + %$j]
             mov     es,[bp + %$j + 2]
             add     ax,[bx]

     endproc

   This makes use of the argument to the `arg' macro to define a
parameter of size 4, because `j' is now a far pointer. When we load
from `j', we must load a segment and an offset.


File: nasm.info,  Node: Section 8.5,  Next: Section 8.5.1,  Prev: Section 8.4.5,  Up: Chapter 8

8.5. Interfacing to Borland Pascal Programs
===========================================

Interfacing to Borland Pascal programs is similar in concept to
interfacing to 16-bit C programs. The differences are:

   * The leading underscore required for interfacing to C programs is
     not required for Pascal.

   * The memory model is always large: functions are far, data pointers
     are far, and no data item can be more than 64K long. (Actually,
     some functions are near, but only those functions that are local
     to a Pascal unit and never called from outside it. All assembly
     functions that Pascal calls, and all Pascal functions that
     assembly routines are able to call, are far.)  However, all static
     data declared in a Pascal program goes into the default data
     segment, which is the one whose segment address will be in `DS'
     when control is passed to your assembly code. The only things that
     do not live in the default data segment are local variables (they
     live in the stack segment) and dynamically allocated variables.
     All data _pointers_, however, are far.

   * The function calling convention is different - described below.

   * Some data types, such as strings, are stored differently.

   * There are restrictions on the segment names you are allowed to use
     - Borland Pascal will ignore code or data declared in a segment it
     doesn't like the name of. The restrictions are described below.

* Menu:

* Section 8.5.1:: The Pascal Calling Convention
* Section 8.5.2:: Borland Pascal Segment Name Restrictions
* Section 8.5.3:: Using `c16.mac' With Pascal Programs


File: nasm.info,  Node: Section 8.5.1,  Next: Section 8.5.2,  Prev: Section 8.5,  Up: Section 8.5

8.5.1. The Pascal Calling Convention
------------------------------------

The 16-bit Pascal calling convention is as follows. In the following
description, the words _caller_ and _callee_ are used to denote the
function doing the calling and the function which gets called.

   * The caller pushes the function's parameters on the stack, one after
     another, in normal order (left to right, so that the first argument
     specified to the function is pushed first).

   * The caller then executes a far `CALL' instruction to pass control
     to the callee.

   * The callee receives control, and typically (although this is not
     actually necessary, in functions which do not need to access their
     parameters) starts by saving the value of `SP' in `BP' so as to be
     able to use `BP' as a base pointer to find its parameters on the
     stack.  However, the caller was probably doing this too, so part
     of the calling convention states that `BP' must be preserved by
     any function. Hence the callee, if it is going to set up `BP' as a
     frame pointer, must push the previous value first.

   * The callee may then access its parameters relative to `BP'. The
     word at `[BP]' holds the previous value of `BP' as it was pushed.
     The next word, at `[BP+2]', holds the offset part of the return
     address, and the next one at `[BP+4]' the segment part. The
     parameters begin at `[BP+6]'. The rightmost parameter of the
     function, since it was pushed last, is accessible at this offset
     from `BP'; the others follow, at successively greater offsets.

   * The callee may also wish to decrease `SP' further, so as to
     allocate space on the stack for local variables, which will then
     be accessible at negative offsets from `BP'.

   * The callee, if it wishes to return a value to the caller, should
     leave the value in `AL', `AX' or `DX:AX' depending on the size of
     the value. Floating-point results are returned in `ST0'. Results
     of type `Real' (Borland's own custom floating-point data type, not
     handled directly by the FPU) are returned in `DX:BX:AX'. To return
     a result of type `String', the caller pushes a pointer to a
     temporary string before pushing the parameters, and the callee
     places the returned string value at that location. The pointer is
     not a parameter, and should not be removed from the stack by the
     `RETF' instruction.

   * Once the callee has finished processing, it restores `SP' from
     `BP' if it had allocated local stack space, then pops the previous
     value of `BP', and returns via `RETF'. It uses the form of `RETF'
     with an immediate parameter, giving the number of bytes taken up
     by the parameters on the stack. This causes the parameters to be
     removed from the stack as a side effect of the return instruction.

   * When the caller regains control from the callee, the function
     parameters have already been removed from the stack, so it needs
     to do nothing further.

   Thus, you would define a function in Pascal style, taking two
`Integer'-type parameters, in the following way:

     global  myfunc

     myfunc: push    bp
             mov     bp,sp
             sub     sp,0x40         ; 64 bytes of local stack space
             mov     bx,[bp+8]       ; first parameter to function
             mov     bx,[bp+6]       ; second parameter to function

             ; some more code

             mov     sp,bp           ; undo "sub sp,0x40" above
             pop     bp
             retf    4               ; total size of params is 4

   At the other end of the process, to call a Pascal function from your
assembly code, you would do something like this:

     extern  SomeFunc

            ; and then, further down...

            push   word seg mystring   ; Now push the segment, and...
            push   word mystring       ; ... offset of "mystring"
            push   word [myint]        ; one of my variables
            call   far SomeFunc

   This is equivalent to the Pascal code

     procedure SomeFunc(String: PChar; Int: Integer);
         SomeFunc(@mystring, myint);


File: nasm.info,  Node: Section 8.5.2,  Next: Section 8.5.3,  Prev: Section 8.5.1,  Up: Section 8.5

8.5.2. Borland Pascal Segment Name Restrictions
-----------------------------------------------

Since Borland Pascal's internal unit file format is completely different
from `OBJ', it only makes a very sketchy job of actually reading and
understanding the various information contained in a real `OBJ' file
when it links that in. Therefore an object file intended to be linked
to a Pascal program must obey a number of restrictions:

   * Procedures and functions must be in a segment whose name is either
     `CODE', `CSEG', or something ending in `_TEXT'.

   * initialized data must be in a segment whose name is either `CONST'
     or something ending in `_DATA'.

   * Uninitialized data must be in a segment whose name is either
     `DATA', `DSEG', or something ending in `_BSS'.

   * Any other segments in the object file are completely ignored.
     `GROUP' directives and segment attributes are also ignored.


File: nasm.info,  Node: Section 8.5.3,  Next: Chapter 9,  Prev: Section 8.5.2,  Up: Section 8.5

8.5.3. Using `c16.mac' With Pascal Programs
-------------------------------------------

The `c16.mac' macro package, described in *note Section 8.4.5::, can
also be used to simplify writing functions to be called from Pascal
programs, if you code `%define PASCAL'. This definition ensures that
functions are far (it implies `FARCODE'), and also causes procedure
return instructions to be generated with an operand.

   Defining `PASCAL' does not change the code which calculates the
argument offsets; you must declare your function's arguments in reverse
order. For example:

     %define PASCAL

     proc    _pascalproc

     %$j     arg 4
     %$i     arg
             mov     ax,[bp + %$i]
             mov     bx,[bp + %$j]
             mov     es,[bp + %$j + 2]
             add     ax,[bx]

     endproc

   This defines the same routine, conceptually, as the example in *note
Section 8.4.5::: it defines a function taking two arguments, an integer
and a pointer to an integer, which returns the sum of the integer and
the contents of the pointer. The only difference between this code and
the large-model C version is that `PASCAL' is defined instead of
`FARCODE', and that the arguments are declared in reverse order.


File: nasm.info,  Node: Chapter 9,  Next: Section 9.1,  Prev: Section 8.5.3,  Up: Top

Chapter 9: Writing 32-bit Code (Unix, Win32, DJGPP)
***************************************************

This chapter attempts to cover some of the common issues involved when
writing 32-bit code, to run under Win32 or Unix, or to be linked with C
code generated by a Unix-style C compiler such as DJGPP. It covers how
to write assembly code to interface with 32-bit C routines, and how to
write position-independent code for shared libraries.

   Almost all 32-bit code, and in particular all code running under
`Win32', `DJGPP' or any of the PC Unix variants, runs in _flat_ memory
model. This means that the segment registers and paging have already
been set up to give you the same 32-bit 4Gb address space no matter
what segment you work relative to, and that you should ignore all
segment registers completely. When writing flat-model application code,
you never need to use a segment override or modify any segment
register, and the code-section addresses you pass to `CALL' and `JMP'
live in the same address space as the data-section addresses you access
your variables by and the stack-section addresses you access local
variables and procedure parameters by. Every address is 32 bits long
and contains only an offset part.

* Menu:

* Section 9.1:: Interfacing to 32-bit C Programs
* Section 9.2:: Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries


File: nasm.info,  Node: Section 9.1,  Next: Section 9.1.1,  Prev: Chapter 9,  Up: Chapter 9

9.1. Interfacing to 32-bit C Programs
=====================================

A lot of the discussion in *note Section 8.4::, about interfacing to
16-bit C programs, still applies when working in 32 bits. The absence
of memory models or segmentation worries simplifies things a lot.

* Menu:

* Section 9.1.1:: External Symbol Names
* Section 9.1.2:: Function Definitions and Function Calls
* Section 9.1.3:: Accessing Data Items
* Section 9.1.4:: `c32.mac': Helper Macros for the 32-bit C Interface


File: nasm.info,  Node: Section 9.1.1,  Next: Section 9.1.2,  Prev: Section 9.1,  Up: Section 9.1

9.1.1. External Symbol Names
----------------------------

Most 32-bit C compilers share the convention used by 16-bit compilers,
that the names of all global symbols (functions or data) they define
are formed by prefixing an underscore to the name as it appears in the
C program.  However, not all of them do: the `ELF' specification states
that C symbols do _not_ have a leading underscore on their
assembly-language names.

   The older Linux `a.out' C compiler, all `Win32' compilers, `DJGPP',
and `NetBSD' and `FreeBSD', all use the leading underscore; for these
compilers, the macros `cextern' and `cglobal', as given in *note
Section 8.4.1::, will still work. For `ELF', though, the leading
underscore should not be used.

   See also *note Section 2.1.27::.


File: nasm.info,  Node: Section 9.1.2,  Next: Section 9.1.3,  Prev: Section 9.1.1,  Up: Section 9.1

9.1.2. Function Definitions and Function Calls
----------------------------------------------

The C calling convention in 32-bit programs is as follows. In the
following description, the words _caller_ and _callee_ are used to
denote the function doing the calling and the function which gets
called.

   * The caller pushes the function's parameters on the stack, one after
     another, in reverse order (right to left, so that the first
     argument specified to the function is pushed last).

   * The caller then executes a near `CALL' instruction to pass control
     to the callee.

   * The callee receives control, and typically (although this is not
     actually necessary, in functions which do not need to access their
     parameters) starts by saving the value of `ESP' in `EBP' so as to
     be able to use `EBP' as a base pointer to find its parameters on
     the stack.  However, the caller was probably doing this too, so
     part of the calling convention states that `EBP' must be preserved
     by any C function.  Hence the callee, if it is going to set up
     `EBP' as a frame pointer, must push the previous value first.

   * The callee may then access its parameters relative to `EBP'. The
     doubleword at `[EBP]' holds the previous value of `EBP' as it was
     pushed; the next doubleword, at `[EBP+4]', holds the return
     address, pushed implicitly by `CALL'. The parameters start after
     that, at `[EBP+8]'. The leftmost parameter of the function, since
     it was pushed last, is accessible at this offset from `EBP'; the
     others follow, at successively greater offsets. Thus, in a
     function such as `printf' which takes a variable number of
     parameters, the pushing of the parameters in reverse order means
     that the function knows where to find its first parameter, which
     tells it the number and type of the remaining ones.

   * The callee may also wish to decrease `ESP' further, so as to
     allocate space on the stack for local variables, which will then
     be accessible at negative offsets from `EBP'.

   * The callee, if it wishes to return a value to the caller, should
     leave the value in `AL', `AX' or `EAX' depending on the size of the
     value. Floating-point results are typically returned in `ST0'.

   * Once the callee has finished processing, it restores `ESP' from
     `EBP' if it had allocated local stack space, then pops the previous
     value of `EBP', and returns via `RET' (equivalently, `RETN').

   * When the caller regains control from the callee, the function
     parameters are still on the stack, so it typically adds an
     immediate constant to `ESP' to remove them (instead of executing a
     number of slow `POP' instructions). Thus, if a function is
     accidentally called with the wrong number of parameters due to a
     prototype mismatch, the stack will still be returned to a sensible
     state since the caller, which _knows_ how many parameters it
     pushed, does the removing.

   There is an alternative calling convention used by Win32 programs for
Windows API calls, and also for functions called _by_ the Windows API
such as window procedures: they follow what Microsoft calls the
`__stdcall' convention. This is slightly closer to the Pascal
convention, in that the callee clears the stack by passing a parameter
to the `RET' instruction. However, the parameters are still pushed in
right-to-left order.

   Thus, you would define a function in C style in the following way:

     global  _myfunc

     _myfunc:
             push    ebp
             mov     ebp,esp
             sub     esp,0x40        ; 64 bytes of local stack space
             mov     ebx,[ebp+8]     ; first parameter to function

             ; some more code

             leave                   ; mov esp,ebp / pop ebp
             ret

   At the other end of the process, to call a C function from your
assembly code, you would do something like this:

     extern  _printf

             ; and then, further down...

             push    dword [myint]   ; one of my integer variables
             push    dword mystring  ; pointer into my data segment
             call    _printf
             add     esp,byte 8      ; `byte' saves space

             ; then those data items...

     segment _DATA

     myint       dd   1234
     mystring    db   'This number -> %d <- should be 1234',10,0

   This piece of code is the assembly equivalent of the C code

         int myint = 1234;
         printf("This number -> %d <- should be 1234\n", myint);


File: nasm.info,  Node: Section 9.1.3,  Next: Section 9.1.4,  Prev: Section 9.1.2,  Up: Section 9.1

9.1.3. Accessing Data Items
---------------------------

To get at the contents of C variables, or to declare variables which C
can access, you need only declare the names as `GLOBAL' or `EXTERN'.
(Again, the names require leading underscores, as stated in *note
Section 9.1.1::.) Thus, a C variable declared as `int i' can be
accessed from assembler as

               extern _i
               mov eax,[_i]

   And to declare your own integer variable which C programs can access
as `extern int j', you do this (making sure you are assembling in the
`_DATA' segment, if necessary):

               global _j
     _j        dd 0

   To access a C array, you need to know the size of the components of
the array. For example, `int' variables are four bytes long, so if a C
program declares an array as `int a[10]', you can access `a[3]' by
coding `mov ax,[_a+12]'. (The byte offset 12 is obtained by multiplying
the desired array index, 3, by the size of the array element, 4.) The
sizes of the C base types in 32-bit compilers are: 1 for `char', 2 for
`short', 4 for `int', `long' and `float', and 8 for `double'. Pointers,
being 32-bit addresses, are also 4 bytes long.

   To access a C data structure, you need to know the offset from the
base of the structure to the field you are interested in. You can
either do this by converting the C structure definition into a NASM
structure definition (using `STRUC'), or by calculating the one offset
and using just that.

   To do either of these, you should read your C compiler's manual to
find out how it organizes data structures. NASM gives no special
alignment to structure members in its own `STRUC' macro, so you have to
specify alignment yourself if the C compiler generates it. Typically,
you might find that a structure like

     struct {
         char c;
         int i;
     } foo;

   might be eight bytes long rather than five, since the `int' field
would be aligned to a four-byte boundary. However, this sort of feature
is sometimes a configurable option in the C compiler, either using
command- line options or `#pragma' lines, so you have to find out how
your own compiler does it.


File: nasm.info,  Node: Section 9.1.4,  Next: Section 9.2,  Prev: Section 9.1.3,  Up: Section 9.1

9.1.4. `c32.mac': Helper Macros for the 32-bit C Interface
----------------------------------------------------------

Included in the NASM archives, in the `misc' directory, is a file
`c32.mac' of macros. It defines three macros: `proc', `arg' and
`endproc'. These are intended to be used for C-style procedure
definitions, and they automate a lot of the work involved in keeping
track of the calling convention.

   An example of an assembly function using the macro set is given here:

     proc    _proc32

     %$i     arg
     %$j     arg
             mov     eax,[ebp + %$i]
             mov     ebx,[ebp + %$j]
             add     eax,[ebx]

     endproc

   This defines `_proc32' to be a procedure taking two arguments, the
first (`i') an integer and the second (`j') a pointer to an integer. It
returns `i + *j'.

   Note that the `arg' macro has an `EQU' as the first line of its
expansion, and since the label before the macro call gets prepended to
the first line of the expanded macro, the `EQU' works, defining `%$i'
to be an offset from `BP'. A context-local variable is used, local to
the context pushed by the `proc' macro and popped by the `endproc'
macro, so that the same argument name can be used in later procedures.
Of course, you don't _have_ to do that.

   `arg' can take an optional parameter, giving the size of the
argument.  If no size is given, 4 is assumed, since it is likely that
many function parameters will be of type `int' or pointers.


File: nasm.info,  Node: Section 9.2,  Next: Section 9.2.1,  Prev: Section 9.1.4,  Up: Chapter 9

9.2. Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries
==================================================================

`ELF' replaced the older `a.out' object file format under Linux because
it contains support for position-independent code (PIC), which makes
writing shared libraries much easier. NASM supports the `ELF'
position-independent code features, so you can write Linux `ELF' shared
libraries in NASM.

   NetBSD, and its close cousins FreeBSD and OpenBSD, take a different
approach by hacking PIC support into the `a.out' format. NASM supports
this as the `aoutb' output format, so you can write BSD shared
libraries in NASM too.

   The operating system loads a PIC shared library by memory-mapping the
library file at an arbitrarily chosen point in the address space of the
running process. The contents of the library's code section must
therefore not depend on where it is loaded in memory.

   Therefore, you cannot get at your variables by writing code like
this:

             mov     eax,[myvar]             ; WRONG

   Instead, the linker provides an area of memory called the _global
offset table_, or GOT; the GOT is situated at a constant distance from
your library's code, so if you can find out where your library is
loaded (which is typically done using a `CALL' and `POP' combination),
you can obtain the address of the GOT, and you can then load the
addresses of your variables out of linker-generated entries in the GOT.

   The _data_ section of a PIC shared library does not have these
restrictions: since the data section is writable, it has to be copied
into memory anyway rather than just paged in from the library file, so
as long as it's being copied it can be relocated too. So you can put
ordinary types of relocation in the data section without too much worry
(but see *note Section 9.2.4:: for a caveat).

* Menu:

* Section 9.2.1:: Obtaining the Address of the GOT
* Section 9.2.2:: Finding Your Local Data Items
* Section 9.2.3:: Finding External and Common Data Items
* Section 9.2.4:: Exporting Symbols to the Library User
* Section 9.2.5:: Calling Procedures Outside the Library
* Section 9.2.6:: Generating the Library File


File: nasm.info,  Node: Section 9.2.1,  Next: Section 9.2.2,  Prev: Section 9.2,  Up: Section 9.2

9.2.1. Obtaining the Address of the GOT
---------------------------------------

Each code module in your shared library should define the GOT as an
external symbol:

     extern  _GLOBAL_OFFSET_TABLE_   ; in ELF
     extern  __GLOBAL_OFFSET_TABLE_  ; in BSD a.out

   At the beginning of any function in your shared library which plans
to access your data or BSS sections, you must first calculate the
address of the GOT. This is typically done by writing the function in
this form:

     func:   push    ebp
             mov     ebp,esp
             push    ebx
             call    .get_GOT
     .get_GOT:
             pop     ebx
             add     ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc

             ; the function body comes here

             mov     ebx,[ebp-4]
             mov     esp,ebp
             pop     ebp
             ret

   (For BSD, again, the symbol `_GLOBAL_OFFSET_TABLE' requires a second
leading underscore.)

   The first two lines of this function are simply the standard C
prologue to set up a stack frame, and the last three lines are standard
C function epilogue. The third line, and the fourth to last line, save
and restore the `EBX' register, because PIC shared libraries use this
register to store the address of the GOT.

   The interesting bit is the `CALL' instruction and the following two
lines. The `CALL' and `POP' combination obtains the address of the
label `.get_GOT', without having to know in advance where the program
was loaded (since the `CALL' instruction is encoded relative to the
current position). The `ADD' instruction makes use of one of the
special PIC relocation types: GOTPC relocation. With the `WRT ..gotpc'
qualifier specified, the symbol referenced (here
`_GLOBAL_OFFSET_TABLE_', the special symbol assigned to the GOT) is
given as an offset from the beginning of the section. (Actually, `ELF'
encodes it as the offset from the operand field of the `ADD'
instruction, but NASM simplifies this deliberately, so you do things the
same way for both `ELF' and `BSD'.) So the instruction then _adds_ the
beginning of the section, to get the real address of the GOT, and
subtracts the value of `.get_GOT' which it knows is in `EBX'.
Therefore, by the time that instruction has finished, `EBX' contains
the address of the GOT.

   If you didn't follow that, don't worry: it's never necessary to
obtain the address of the GOT by any other means, so you can put those
three instructions into a macro and safely ignore them:

     %macro  get_GOT 0

             call    %%getgot
       %%getgot:
             pop     ebx
             add     ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc

     %endmacro


File: nasm.info,  Node: Section 9.2.2,  Next: Section 9.2.3,  Prev: Section 9.2.1,  Up: Section 9.2

9.2.2. Finding Your Local Data Items
------------------------------------

Having got the GOT, you can then use it to obtain the addresses of your
data items. Most variables will reside in the sections you have
declared; they can be accessed using the `..gotoff' special `WRT' type.
The way this works is like this:

             lea     eax,[ebx+myvar wrt ..gotoff]

   The expression `myvar wrt ..gotoff' is calculated, when the shared
library is linked, to be the offset to the local variable `myvar' from
the beginning of the GOT. Therefore, adding it to `EBX' as above will
place the real address of `myvar' in `EAX'.

   If you declare variables as `GLOBAL' without specifying a size for
them, they are shared between code modules in the library, but do not
get exported from the library to the program that loaded it. They will
still be in your ordinary data and BSS sections, so you can access them
in the same way as local variables, using the above `..gotoff'
mechanism.

   Note that due to a peculiarity of the way BSD `a.out' format handles
this relocation type, there must be at least one non-local symbol in the
same section as the address you're trying to access.

