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\#
\# Source code to NASM documentation
\#
\M{category}{Programming}
\M{title}{NASM - The Netwide Assembler}
\M{year}{2007}
\M{author}{The NASM Development Team}
\M{license}{All rights reserved. This document is redistributable under the license given in the file "COPYING" distributed in the NASM archive.}
\M{summary}{This file documents NASM, the Netwide Assembler: an assembler targetting the Intel x86 series of processors, with portable source.}
\M{infoname}{NASM}
\M{infofile}{nasm}
\M{infotitle}{The Netwide Assembler for x86}
\M{epslogo}{nasmlogo.eps}
\IR{-D} \c{-D} option
\IR{-E} \c{-E} option
\IR{-F} \c{-F} option
\IR{-I} \c{-I} option
\IR{-M} \c{-M} option
\IR{-On} \c{-On} option
\IR{-P} \c{-P} option
\IR{-U} \c{-U} option
\IR{-X} \c{-X} option
\IR{-a} \c{-a} option
\IR{-d} \c{-d} option
\IR{-e} \c{-e} option
\IR{-f} \c{-f} option
\IR{-g} \c{-g} option
\IR{-i} \c{-i} option
\IR{-l} \c{-l} option
\IR{-o} \c{-o} option
\IR{-p} \c{-p} option
\IR{-s} \c{-s} option
\IR{-u} \c{-u} option
\IR{-v} \c{-v} option
\IR{-w} \c{-w} option
\IR{-y} \c{-y} option
\IR{!=} \c{!=} operator
\IR{$, here} \c{$}, Here token
\IR{$, prefix} \c{$}, prefix
\IR{$$} \c{$$} token
\IR{%} \c{%} operator
\IR{%%} \c{%%} operator
\IR{%+1} \c{%+1} and \c{%-1} syntax
\IA{%-1}{%+1}
\IR{%0} \c{%0} parameter count
\IR{&} \c{&} operator
\IR{&&} \c{&&} operator
\IR{*} \c{*} operator
\IR{..@} \c{..@} symbol prefix
\IR{/} \c{/} operator
\IR{//} \c{//} operator
\IR{<} \c{<} operator
\IR{<<} \c{<<} operator
\IR{<=} \c{<=} operator
\IR{<>} \c{<>} operator
\IR{=} \c{=} operator
\IR{==} \c{==} operator
\IR{>} \c{>} operator
\IR{>=} \c{>=} operator
\IR{>>} \c{>>} operator
\IR{?} \c{?} MASM syntax
\IR{^} \c{^} operator
\IR{^^} \c{^^} operator
\IR{|} \c{|} operator
\IR{||} \c{||} operator
\IR{~} \c{~} operator
\IR{%$} \c{%$} and \c{%$$} prefixes
\IA{%$$}{%$}
\IR{+ opaddition} \c{+} operator, binary
\IR{+ opunary} \c{+} operator, unary
\IR{+ modifier} \c{+} modifier
\IR{- opsubtraction} \c{-} operator, binary
\IR{- opunary} \c{-} operator, unary
\IR{! opunary} \c{!} operator, unary
\IR{alignment, in bin sections} alignment, in \c{bin} sections
\IR{alignment, in elf sections} alignment, in \c{elf} sections
\IR{alignment, in win32 sections} alignment, in \c{win32} sections
\IR{alignment, of elf common variables} alignment, of \c{elf} common
variables
\IR{alignment, in obj sections} alignment, in \c{obj} sections
\IR{a.out, bsd version} \c{a.out}, BSD version
\IR{a.out, linux version} \c{a.out}, Linux version
\IR{autoconf} Autoconf
\IR{bin} bin
\IR{bitwise and} bitwise AND
\IR{bitwise or} bitwise OR
\IR{bitwise xor} bitwise XOR
\IR{block ifs} block IFs
\IR{borland pascal} Borland, Pascal
\IR{borland's win32 compilers} Borland, Win32 compilers
\IR{braces, after % sign} braces, after \c{%} sign
\IR{bsd} BSD
\IR{c calling convention} C calling convention
\IR{c symbol names} C symbol names
\IA{critical expressions}{critical expression}
\IA{command line}{command-line}
\IA{case sensitivity}{case sensitive}
\IA{case-sensitive}{case sensitive}
\IA{case-insensitive}{case sensitive}
\IA{character constants}{character constant}
\IR{common object file format} Common Object File Format
\IR{common variables, alignment in elf} common variables, alignment
in \c{elf}
\IR{common, elf extensions to} \c{COMMON}, \c{elf} extensions to
\IR{common, obj extensions to} \c{COMMON}, \c{obj} extensions to
\IR{declaring structure} declaring structures
\IR{default-wrt mechanism} default-\c{WRT} mechanism
\IR{devpac} DevPac
\IR{djgpp} DJGPP
\IR{dll symbols, exporting} DLL symbols, exporting
\IR{dll symbols, importing} DLL symbols, importing
\IR{dos} DOS
\IR{dos archive} DOS archive
\IR{dos source archive} DOS source archive
\IA{effective address}{effective addresses}
\IA{effective-address}{effective addresses}
\IR{elf} ELF
\IR{elf, 16-bit code and} ELF, 16-bit code and
\IR{elf shared libraries} ELF, shared libraries
\IR{executable and linkable format} Executable and Linkable Format
\IR{extern, obj extensions to} \c{EXTERN}, \c{obj} extensions to
\IR{extern, rdf extensions to} \c{EXTERN}, \c{rdf} extensions to
\IR{freebsd} FreeBSD
\IR{freelink} FreeLink
\IR{functions, c calling convention} functions, C calling convention
\IR{functions, pascal calling convention} functions, Pascal calling
convention
\IR{global, aoutb extensions to} \c{GLOBAL}, \c{aoutb} extensions to
\IR{global, elf extensions to} \c{GLOBAL}, \c{elf} extensions to
\IR{global, rdf extensions to} \c{GLOBAL}, \c{rdf} extensions to
\IR{got} GOT
\IR{got relocations} \c{GOT} relocations
\IR{gotoff relocation} \c{GOTOFF} relocations
\IR{gotpc relocation} \c{GOTPC} relocations
\IR{intel number formats} Intel number formats
\IR{linux, elf} Linux, ELF
\IR{linux, a.out} Linux, \c{a.out}
\IR{linux, as86} Linux, \c{as86}
\IR{logical and} logical AND
\IR{logical or} logical OR
\IR{logical xor} logical XOR
\IR{masm} MASM
\IA{memory reference}{memory references}
\IR{minix} Minix
\IA{misc directory}{misc subdirectory}
\IR{misc subdirectory} \c{misc} subdirectory
\IR{microsoft omf} Microsoft OMF
\IR{mmx registers} MMX registers
\IA{modr/m}{modr/m byte}
\IR{modr/m byte} ModR/M byte
\IR{ms-dos} MS-DOS
\IR{ms-dos device drivers} MS-DOS device drivers
\IR{multipush} \c{multipush} macro
\IR{nasm version} NASM version
\IR{netbsd} NetBSD
\IR{omf} OMF
\IR{openbsd} OpenBSD
\IR{operating system} operating system
\IR{os/2} OS/2
\IR{pascal calling convention}Pascal calling convention
\IR{passes} passes, assembly
\IR{perl} Perl
\IR{pic} PIC
\IR{pharlap} PharLap
\IR{plt} PLT
\IR{plt} \c{PLT} relocations
\IA{pre-defining macros}{pre-define}
\IA{preprocessor expressions}{preprocessor, expressions}
\IA{preprocessor loops}{preprocessor, loops}
\IA{preprocessor variables}{preprocessor, variables}
\IA{rdoff subdirectory}{rdoff}
\IR{rdoff} \c{rdoff} subdirectory
\IR{relocatable dynamic object file format} Relocatable Dynamic
Object File Format
\IR{relocations, pic-specific} relocations, PIC-specific
\IA{repeating}{repeating code}
\IR{section alignment, in elf} section alignment, in \c{elf}
\IR{section alignment, in bin} section alignment, in \c{bin}
\IR{section alignment, in obj} section alignment, in \c{obj}
\IR{section alignment, in win32} section alignment, in \c{win32}
\IR{section, elf extensions to} \c{SECTION}, \c{elf} extensions to
\IR{section, win32 extensions to} \c{SECTION}, \c{win32} extensions to
\IR{segment alignment, in bin} segment alignment, in \c{bin}
\IR{segment alignment, in obj} segment alignment, in \c{obj}
\IR{segment, obj extensions to} \c{SEGMENT}, \c{elf} extensions to
\IR{segment names, borland pascal} segment names, Borland Pascal
\IR{shift command} \c{shift} command
\IA{sib}{sib byte}
\IR{sib byte} SIB byte
\IR{solaris x86} Solaris x86
\IA{standard section names}{standardized section names}
\IR{symbols, exporting from dlls} symbols, exporting from DLLs
\IR{symbols, importing from dlls} symbols, importing from DLLs
\IR{test subdirectory} \c{test} subdirectory
\IR{tlink} \c{TLINK}
\IR{underscore, in c symbols} underscore, in C symbols
\IR{unix} Unix
\IA{sco unix}{unix, sco}
\IR{unix, sco} Unix, SCO
\IA{unix source archive}{unix, source archive}
\IR{unix, source archive} Unix, source archive
\IA{unix system v}{unix, system v}
\IR{unix, system v} Unix, System V
\IR{unixware} UnixWare
\IR{val} VAL
\IR{version number of nasm} version number of NASM
\IR{visual c++} Visual C++
\IR{www page} WWW page
\IR{win32} Win32
\IR{win32} Win64
\IR{windows} Windows
\IR{windows 95} Windows 95
\IR{windows nt} Windows NT
\# \IC{program entry point}{entry point, program}
\# \IC{program entry point}{start point, program}
\# \IC{MS-DOS device drivers}{device drivers, MS-DOS}
\# \IC{16-bit mode, versus 32-bit mode}{32-bit mode, versus 16-bit mode}
\# \IC{c symbol names}{symbol names, in C}


\C{intro} Introduction

\H{whatsnasm} 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 \c{*BSD} \c{a.out}, \c{ELF}, \c{COFF}, \c{Mach-O},
Microsoft 16-bit \c{OBJ}, \c{Win32} and \c{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 from the upto and including \c{Pentium},
\c{P6}, \c{MMX}, \c{3DNow!}, \c{SSE}, \c{SSE2}, \c{SSE3} and \c{x64} opcodes. NASM has
a strong support for macro conventions.


\S{yaasm} Why Yet Another Assembler?

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

\b \i\c{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.

\b \i\c{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 \i\c{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 \e{write} anything in it. Plus you can't write 16-bit code in
it (properly.)

\b \i\c{as86} is specific to Minix and Linux, and (my version at least)
doesn't seem to have much (or any) documentation.

\b \i\c{MASM} isn't very good, and it's (was) expensive, and it runs only under
DOS.

\b \i\c{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, \e{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.


\S{legal} License Conditions

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


\H{contact} Contact Information

The current version of NASM (since about 0.98.08) is maintained by a
team of developers, accessible through the \c{nasm-devel} mailing list
(see below for the link).
If you want to report a bug, please read \k{bugs} first.

NASM has a \i{WWW page} at
\W{http://nasm.sourceforge.net}\c{http://nasm.sourceforge.net}. If it's
not there, google for us!


The original authors are \i{e\-mail}able as
\W{mailto:jules@dsf.org.uk}\c{jules@dsf.org.uk} and
\W{mailto:anakin@pobox.com}\c{anakin@pobox.com}.
The latter is no longer involved in the development team.

\i{New releases} of NASM are uploaded to the official sites
\W{http://nasm.sourceforge.net}\c{http://nasm.sourceforge.net}
and to
\W{ftp://ftp.kernel.org/pub/software/devel/nasm/}\i\c{ftp.kernel.org}
and
\W{ftp://ibiblio.org/pub/Linux/devel/lang/assemblers/}\i\c{ibiblio.org}.

Announcements are posted to
\W{news:comp.lang.asm.x86}\i\c{comp.lang.asm.x86},
\W{news:alt.lang.asm}\i\c{alt.lang.asm} and
\W{news:comp.os.linux.announce}\i\c{comp.os.linux.announce}

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


\H{install} Installation

\S{instdos} \i{Installing} NASM under MS-\i{DOS} or Windows

Once you've obtained the \i{DOS archive} for NASM, \i\c{nasmXXX.zip}
(where \c{XXX} denotes the version number of NASM contained in the
archive), unpack it into its own directory (for example \c{c:\\nasm}).

The archive will contain four executable files: the NASM executable
files \i\c{nasm.exe} and \i\c{nasmw.exe}, and the NDISASM executable
files \i\c{ndisasm.exe} and \i\c{ndisasmw.exe}. In each case, the
file whose name ends in \c{w} is a \I{Win32}\c{Win32} executable,
designed to run under \I{Windows 95}\c{Windows 95} or \I{Windows NT}
\c{Windows NT} Intel, and the other one is a 16-bit \I{DOS}\c{DOS}
executable.

The only file NASM needs to run is its own executable, so copy
(at least) one of \c{nasm.exe} and \c{nasmw.exe} to a directory on
your PATH, or alternatively edit \i\c{autoexec.bat} to add the
\c{nasm} directory to your \i\c{PATH}. (If you're only installing the
\c{Win32} version, you may wish to rename it to \c{nasm.exe}.)

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 \c{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 \i{DOS source archive}, \i\c{nasmXXXs.zip},
the \c{nasm} directory will also contain the full NASM \i{source
code}, and a selection of \i{Makefiles} you can (hopefully) use to
rebuild your copy of NASM from scratch.

Note that the source files \c{insnsa.c}, \c{insnsd.c}, \c{insnsi.h}
and \c{insnsn.c} are automatically generated from the master
instruction table \c{insns.dat} by a Perl script; the file
\c{macros.c} is generated from \c{standard.mac} by another Perl
script. 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 \i{Perl} for a variety of
platforms, including DOS and Windows, are available from
\W{http://www.cpan.org/ports/}\i{www.cpan.org}.


\S{instdos} Installing NASM under \i{Unix}

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

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

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

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

If NASM fails to auto-configure, you may still be able to make it
compile by using the fall-back Unix makefile \i\c{Makefile.unx}.
Copy or rename that file to \c{Makefile} and try typing \c{make}.
There is also a Makefile.unx file in the \c{rdoff} subdirectory.


\C{running} Running NASM

\H{syntax} NASM \i{Command-Line} Syntax

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

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

For example,

\c nasm -f elf myfile.asm

will assemble \c{myfile.asm} into an \c{ELF} object file \c{myfile.o}. And

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

will assemble \c{myfile.asm} into a raw binary file \c{myfile.com}.

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

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

To get further usage instructions from NASM, try typing

\c nasm -h

As \c{-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 \c{a.out}
or \c{ELF}, type

\c file nasm

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

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

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

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

or something similar, your system is \c{a.out}, and you should use
\c{-f aout} instead (Linux \c{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.


\S{opt-o} The \i\c{-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 (\i\c{obj} and \i\c{win32}), it
will remove the \c{.asm} \i{extension} (or whatever extension you
like to use - NASM doesn't care) from your source file name and
substitute \c{.obj}. For Unix object file formats (\i\c{aout},
\i\c{coff}, \i\c{elf}, \i\c{macho} and \i\c{as86}) it will substitute \c{.o}. For
\i\c{rdf}, it will use \c{.rdf}, and for the \i\c{bin} format it
will simply remove the extension, so that \c{myfile.asm} produces
the output file \c{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 \i\c{nasm.out} as the output file name instead.

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

\c nasm -f bin program.asm -o program.com
\c 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 \k{opt-On}.


\S{opt-f} The \i\c{-f} Option: Specifying the \i{Output File Format}

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

Like \c{-o}, the intervening space between \c{-f} and the output
file format is optional; so \c{-f elf} and \c{-felf} are both valid.

A complete list of the available output file formats can be given by
issuing the command \i\c{nasm -hf}.


\S{opt-l} The \i\c{-l} Option: Generating a \i{Listing File}

If you supply the \c{-l} option to NASM, followed (with the usual
optional space) by a file name, NASM will generate a
\i{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 \k{nolist}) on the
right. For example:

\c 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 \c{[list -]}, and turn it back on
with \c{[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.


\S{opt-M} The \i\c{-M} Option: Generate \i{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:

\c NASM -M myfile.asm > myfile.dep


\S{opt-F} The \i\c{-F} Option: Selecting a \i{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 \e{will} be). Use
of this switch does \e{not} enable output of the selected debug info format.
Use \c{-g}, see \k{opt-g}, to enable output.

A complete list of the available debug file formats for an output format
can be seen by issuing the command \i\c{nasm -f <format> -y}. (only 
"borland" in "-f obj", as of 0.98.35, but "watch this space") 
See: \k{opt-y}.

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 \k{dbgfmt}


\S{opt-g} The \i\c{-g} Option: Enabling \i{Debug Information}.

This option can be used to generate debugging information in the specified
format. See: \k{opt-F}. Using \c{-g} without \c{-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, \c{-g} is \e{silently ignored}.


\S{opt-X} The \i\c{-X} Option: Selecting an \i{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 \c{-Xvc} option and the \c{-Xgnu} option.  The GNU format is 
the default and looks like this:

\c filename.asm:65: error: specific error message 

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

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

\c 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 \c{Visual C++} output format, \k{win32fmt}.

\S{opt-E} The \i\c{-E} Option: Send Errors to a File

Under \I{DOS}\c{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 \i{error messages} on
\i\c{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 \c{-E} option, taking a filename argument
which causes errors to be sent to the specified files rather than
standard error. Therefore you can \I{redirecting errors}redirect
the errors into a file by typing

\c nasm -E myfile.err -f obj myfile.asm


\S{opt-s} The \i\c{-s} Option: Send Errors to \i\c{stdout}

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

\c nasm -s -f obj myfile.asm | more

See also the \c{-E} option, \k{opt-E}.


\S{opt-i} The \i\c{-i}\I\c{-I} Option: Include File Search Directories

When NASM sees the \i\c{%include} or \i\c{incbin} directive in 
a source file (see \k{include} or \k{incbin}), 
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 \c{-i} option. Therefore you can
include files from a \i{macro library}, for example, by typing

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

(As usual, a space between \c{-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 \c{-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 \i{perverse},
by noting that the option \c{-ifoo} will cause \c{%include "bar.i"}
to search for the file \c{foobar.i}...)

If you want to define a \e{standard} \i{include search path},
similar to \c{/usr/include} on Unix systems, you should place one or
more \c{-i} directives in the \c{NASMENV} environment variable (see
\k{nasmenv}).

For Makefile compatibility with many C compilers, this option can also
be specified as \c{-I}.


\S{opt-p} The \i\c{-p}\I\c{-P} Option: \I{pre-including files}Pre-Include a File

\I\c{%include}NASM allows you to specify files to be
\e{pre-included} into your source file, by the use of the \c{-p}
option. So running

\c nasm myfile.asm -p myinc.inc

is equivalent to running \c{nasm myfile.asm} and placing the
directive \c{%include "myinc.inc"} at the start of the file.

For consistency with the \c{-I}, \c{-D} and \c{-U} options, this
option can also be specified as \c{-P}.


\S{opt-d} The \i\c{-d}\I\c{-D} Option: \I{pre-defining macros}Pre-Define a Macro

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

\c nasm myfile.asm -dFOO=100

as an alternative to placing the directive

\c %define FOO 100

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

For Makefile compatibility with many C compilers, this option can also
be specified as \c{-D}.


\S{opt-u} The \i\c{-u}\I\c{-U} Option: \I{Undefining macros}Undefine a Macro

\I\c{%undef}The \c{-u} option undefines a macro that would otherwise
have been pre-defined, either automatically or by a \c{-p} or \c{-d}
option specified earlier on the command lines.

For example, the following command line:

\c nasm myfile.asm -dFOO=100 -uFOO

would result in \c{FOO} \e{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 \c{-U}.


\S{opt-e} The \i\c{-e} Option: Preprocess Only

NASM allows the \i{preprocessor} to be run on its own, up to a
point. Using the \c{-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 \c{-o} option is also used).

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

\c %assign tablesize ($-tablestart)

will cause an error in \i{preprocess-only mode}.


\S{opt-a} The \i\c{-a} Option: Don't Preprocess At All

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


\S{opt-On} The \i\c{-On} Option: Specifying \i{Multipass Optimization}.

NASM defaults to being a two pass assembler. This means that if you
have a complex source file which needs more than 2 passes to assemble
optimally, you have to enable extra passes.

Using the \c{-O} option, you can tell NASM to carry out multiple passes.
The syntax is:

\b \c{-O0} strict two-pass assembly, JMP and Jcc are handled more
        like v0.98, except that backward JMPs are short, if possible.
        Immediate operands take their long forms if a short form is
        not specified.

\b \c{-O1} strict two-pass assembly, but forward branches are assembled
        with code guaranteed to reach; may produce larger code than
        -O0, but will produce successful assembly more often if
        branch offset sizes are not specified.
        Additionally, immediate operands which will fit in a signed byte
        are optimized, unless the long form is specified.

\b \c{-On} multi-pass optimization, minimize branch offsets; also will
        minimize signed immediate bytes, overriding size specification
        unless the \c{strict} keyword has been used (see \k{strict}).
        The number specifies the maximum number of passes.  The more
        passes, the better the code, but the slower is the assembly.

Note that this is a capital O, and is different from a small o, which
is used to specify the output format. See \k{opt-o}.


\S{opt-t} The \i\c{-t} option: Enable TASM Compatibility Mode

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

\b local labels may be prefixed with \c{@@} instead of \c{.}

\b TASM-style response files beginning with \c{@} may be specified on
the command line. This is different from the \c{-@resp} style that NASM
natively supports.

\b 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.
\c{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.

\b \c{%arg} preprocessor directive is supported which is similar to
TASM's \c{ARG} directive.

\b \c{%local} preprocessor directive

\b \c{%stacksize} preprocessor directive

\b unprefixed forms of some directives supported (\c{arg}, \c{elif},
\c{else}, \c{endif}, \c{if}, \c{ifdef}, \c{ifdifi}, \c{ifndef},
\c{include}, \c{local})

\b more...

For more information on the directives, see the section on TASM
Compatiblity preprocessor directives in \k{tasmcompat}.


\S{opt-w} The \i\c{-w} Option: Enable or Disable Assembly \i{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
\c{-w} command-line option, which enables or disables certain
classes of assembly warning. Such warning classes are described by a
name, for example \c{orphan-labels}; you can enable warnings of
this class by the command-line option \c{-w+orphan-labels} and
disable it by \c{-w-orphan-labels}.

The \i{suppressible warning} classes are:

\b \i\c{macro-params} covers warnings about \i{multi-line macros}
being invoked with the wrong number of parameters. This warning
class is enabled by default; see \k{mlmacover} for an example of why
you might want to disable it.

\b \i\c{macro-selfref} warns if a macro references itself. This 
warning class is enabled by default.

\b \i\c{orphan-labels} covers warnings about source lines which
contain no instruction but define a label without a trailing colon.
NASM does not warn about this somewhat obscure condition by default;
see \k{syntax} for an example of why you might want it to.

\b \i\c{number-overflow} covers warnings about numeric constants which
don't fit in 32 bits (for example, it's easy to type one too many Fs
and produce \c{0x7ffffffff} by mistake). This warning class is
enabled by default.

\b \i\c{gnu-elf-extensions} warns if 8-bit or 16-bit relocations 
are used in \c{-f elf} format. The GNU extensions allow this. 
This warning class is enabled by default.

\b In addition, warning classes may be enabled or disabled across 
sections of source code with \i\c{[warning +warning-name]} or 
\i\c{[warning -warning-name]}. No "user form" (without the 
brackets) exists. 


\S{opt-v} The \i\c{-v} Option: Display \i{Version} Info

Typing \c{NASM -v} will display the version of NASM which you are using,
and the date on which it was compiled. This replaces the deprecated 
\c{-r}.

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

\S{opt-y} The \i\c{-y} Option: Display Available Debug Info Formats

Typing \c{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. E.g. \c{nasm -f obj -y} yields \c{* borland}.
(as of 0.98.35, the \e{only} debug info format implemented).


\S{opt-pfix} The \i\c{--prefix} and \i\c{--postfix} Options.

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


\S{nasmenv} The \c{NASMENV} \i{Environment} Variable

If you define an environment variable called \c{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 \c{-i}
options in the \c{NASMENV} variable.

The value of the variable is split up at white space, so that the
value \c{-s -ic:\\nasmlib} will be treated as two separate options.
However, that means that the value \c{-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 \c{-dNAME="my} and \c{name"}.

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

This environment variable was previously called \c{NASM}. This was
changed with version 0.98.31.


\H{qstart} \i{Quick Start} for \i{MASM} Users

If you're used to writing programs with MASM, or with \i{TASM} in
MASM-compatible (non-Ideal) mode, or with \i\c{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.


\S{qscs} NASM Is \I{case sensitivity}Case-Sensitive

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


\S{qsbrackets} NASM Requires \i{Square Brackets} For \i{Memory References}

NASM was designed with simplicity of syntax in mind. One of the
\i{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,

\c foo     equ     1
\c bar     dw      2

then the two lines of code

\c         mov     ax,foo
\c         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 \e{contents} of a memory location requires square brackets
around the address, and any access to the \e{address} of a variable
doesn't. So an instruction of the form \c{mov ax,foo} will
\e{always} refer to a compile-time constant, whether it's an \c{EQU}
or the address of a variable; and to access the \e{contents} of the
variable \c{bar}, you must code \c{mov ax,[bar]}.

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

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

NASM, in the interests of simplicity, also does not support the
\i{hybrid syntaxes} supported by MASM and its clones, such as
\c{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 \c{mov ax,[table+bx]}. Likewise,
\c{mov ax,es:[di]} is wrong and \c{mov ax,[es:di]} is right.


\S{qstypes} NASM Doesn't Store \i{Variable Types}

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

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


\S{qsassume} NASM Doesn't \i\c{ASSUME}

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


\S{qsmodel} NASM Doesn't Support \i{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 \i{far call} and which with a
\i{near call}, and is responsible for putting the correct form of
\c{RET} instruction (\c{RETN} or \c{RETF}; NASM accepts \c{RET}
itself as an alternate form for \c{RETN}); in addition, the
programmer is responsible for coding CALL FAR instructions where
necessary when calling \e{external} functions, and must also keep
track of which external variable definitions are far and which are
near.


\S{qsfpu} \i{Floating-Point} Differences

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

As of version 0.96, NASM now treats the instructions with
\i{`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.


\S{qsother} Other Differences

For historical reasons, NASM uses the keyword \i\c{TWORD} where MASM
and compatible assemblers use \i\c{TBYTE}.

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

In addition to all of this, macros and directives work completely
differently to MASM. See \k{preproc} and \k{directive} for further
details.


\C{lang} The NASM Language

\H{syntax} 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
\k{preproc} and \k{directive}) some combination of the four fields

\c 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 \i{colon} after a label is also
optional. (Note that this means that if you intend to code \c{lodsb}
alone on a line, and type \c{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
\I{orphan-labels}\c{-w+orphan-labels} will cause it to warn you if
you define a label alone on a line without a \i{trailing colon}.)

\i{Valid characters} in labels are letters, numbers, \c{_}, \c{$},
\c{#}, \c{@}, \c{~}, \c{.}, and \c{?}. The only characters which may
be used as the \e{first} character of an identifier are letters,
\c{.} (with special meaning: see \k{locallab}), \c{_} and \c{?}.
An identifier may also be prefixed with a \I{$, prefix}\c{$} to
indicate that it is intended to be read as an identifier and not a
reserved word; thus, if some other module you are linking with
defines a symbol called \c{eax}, you can refer to \c{$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 \c{LOCK}, \c{REP}, \c{REPE}/\c{REPZ} or
\c{REPNE}/\c{REPNZ}, in the usual way. Explicit \I{address-size
prefixes}address-size and \i{operand-size prefixes} \c{A16},
\c{A32}, \c{O16} and \c{O32} are provided - one example of their use
is given in \k{mixsize}. You can also use the name of a \I{segment
override}segment register as an instruction prefix: coding
\c{es mov [bx],ax} is equivalent to coding \c{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
\c{LODSB}, which has no operands and yet can require a segment
override, there is no clean syntactic way to proceed apart from
\c{es lodsb}.

An instruction is not required to use a prefix: prefixes such as
\c{CS}, \c{A32}, \c{LOCK} or \c{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 \k{pseudop}.

Instruction \i{operands} may take a number of forms: they can be
registers, described simply by the register name (e.g. \c{ax},
\c{bp}, \c{ebx}, \c{cr0}: NASM does not use the \c{gas}-style
syntax in which register names must be prefixed by a \c{%} sign), or
they can be \i{effective addresses} (see \k{effaddr}), constants
(\k{const}) or expressions (\k{expr}).

For x87 \i{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.
\# Details of
\# all forms of each supported instruction are given in
\# \k{iref}.
For example, you can code:

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

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


\H{pseudop} \i{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 \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT} and \i\c{DO};
their \i{uninitialized} counterparts \i\c{RESB}, \i\c{RESW},
\i\c{RESD}, \i\c{RESQ}, \i\c{REST} and \i\c{RESO}; the \i\c{INCBIN}
command, the \i\c{EQU} command, and the \i\c{TIMES} prefix.


\S{db} \c{DB} and friends: Declaring initialized Data

\i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT} and \i\c{DO} are
used, much as in MASM, to declare initialized data in the output
file. They can be invoked in a wide range of ways:
\I{floating-point}\I{character constant}\I{string constant}

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

\c{DT} and \c{DO} do not accept \i{numeric constants} as operands.
\c{DB} does not accept \i{floating-point} numbers as operands.


\S{resb} \c{RESB} and friends: Declaring \i{Uninitialized} Data

\i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST} and
\i\c{RESO} are designed to be used in the BSS section of a module:
they declare \e{uninitialized} storage space. Each takes a single
operand, which is the number of bytes, words, doublewords or whatever
to reserve.  As stated in \k{qsother}, NASM does not support the
MASM/TASM syntax of reserving uninitialized space by writing
\I\c{?}\c{DW ?} or similar things: this is what it does instead. The
operand to a \c{RESB}-type pseudo-instruction is a \i\e{critical
expression}: see \k{crit}.

For example:

\c buffer:         resb    64              ; reserve 64 bytes
\c wordvar:        resw    1               ; reserve a word
\c realarray       resq    10              ; array of ten reals


\S{incbin} \i\c{INCBIN}: Including External \i{Binary Files}

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

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


\S{equ} \i\c{EQU}: Defining Constants

\c{EQU} defines a symbol to a given constant value: when \c{EQU} is
used, the source line must contain a label. The action of \c{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,

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

defines \c{msglen} to be the constant 12. \c{msglen} may not then be
redefined later. This is not a \i{preprocessor} definition either:
the value of \c{msglen} is evaluated \e{once}, using the value of
\c{$} (see \k{expr} for an explanation of \c{$}) at the point of
definition, rather than being evaluated wherever it is referenced
and using the value of \c{$} at the point of reference. Note that
the operand to an \c{EQU} is also a \i{critical expression}
(\k{crit}).


\S{times} \i\c{TIMES}: \i{Repeating} Instructions or Data

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

\c zerobuf:        times 64 db 0

or similar things; but \c{TIMES} is more versatile than that. The
argument to \c{TIMES} is not just a numeric constant, but a numeric
\e{expression}, so you can do things like

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

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

\c         times 100 movsb

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

The operand to \c{TIMES}, like that of \c{EQU} and those of \c{RESB}
and friends, is a critical expression (\k{crit}).

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


\H{effaddr} Effective Addresses

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

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

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

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

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

NASM is capable of doing \i{algebra} on these effective addresses,
so that things which don't necessarily \e{look} legal are perfectly
all right:

\c     mov     eax,[ebx*5]             ; assembles as [ebx*4+ebx]
\c     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 \c{[eax*2+0]} and \c{[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 \c{[eax+ebx]} and
\c{[ebx+eax]} to generate different opcodes; this is occasionally
useful because \c{[esi+ebp]} and \c{[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 \c{BYTE}, \c{WORD},
\c{DWORD} and \c{NOSPLIT}. If you need \c{[eax+3]} to be assembled
using a double-word offset field instead of the one byte NASM will
normally generate, you can code \c{[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 \k{crit} for an example of such a
code fragment) by using \c{[byte eax+offset]}. As special cases,
\c{[byte eax]} will code \c{[eax+0]} with a byte offset of zero, and
\c{[dword eax]} will code it with a double-word offset of zero. The
normal form, \c{[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
(\k{mixaddr}). 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 \c{[eax*2]} into \c{[eax+eax]} because
that allows the offset field to be absent and space to be saved; in
fact, it will also split \c{[eax*2+offset]} into
\c{[eax+eax+offset]}. You can combat this behaviour by the use of
the \c{NOSPLIT} keyword: \c{[nosplit eax*2]} will force
\c{[eax*2+0]} to be generated literally.

In 64-bit mode, NASM will by default generate absolute addresses.  The
\i\c{REL} keyword makes it produce \c{RIP}-relative addresses. Since
this is frequently the normally desired behaviour, see the \c{DEFAULT}
directive (\k{default}). The keyword \i\c{ABS} overrides \i\c{REL}.


\H{const} \i{Constants}

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


\S{numconst} \i{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 \c{H}, \c{Q} or \c{O}, and \c{B} for \i{hex}, \i{octal} and \i{binary},
or you can prefix \c{0x} for hex in the style of C, or you can
prefix \c{$} for hex in the style of Borland Pascal. Note, though,
that the \I{$, prefix}\c{$} prefix does double duty as a prefix on
identifiers (see \k{syntax}), so a hex number prefixed with a \c{$}
sign must have a digit after the \c{$} rather than a letter.

Some examples:

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


\S{chrconst} \i{Character Constants}

A character constant consists of up to four characters enclosed in
either single or double quotes. The type of quote makes no
difference to NASM, except of course that surrounding the constant
with single quotes allows double quotes to appear within it and vice
versa.

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

\c           mov eax,'abcd'

then the constant generated is not \c{0x61626364}, but
\c{0x64636261}, so that if you were then to store the value into
memory, it would read \c{abcd} rather than \c{dcba}. This is also
the sense of character constants understood by the Pentium's
\i\c{CPUID} instruction.
\# (see \k{insCPUID})


\S{strconst} String Constants

String constants are only acceptable to some pseudo-instructions,
namely the \I\c{DW}\I\c{DD}\I\c{DQ}\I\c{DT}\i\c{DB} family and
\i\c{INCBIN}.

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:

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

And the following are also equivalent:

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

Note that when used as an operand to \c{db}, a constant like
\c{'ab'} is treated as a string constant despite being short enough
to be a character constant, because otherwise \c{db 'ab'} would have
the same effect as \c{db 'a'}, which would be silly. Similarly,
three-character or four-character constants are treated as strings
when they are operands to \c{dw}.


\S{fltconst} \I{floating-point, constants}Floating-Point Constants

\i{Floating-point} constants are acceptable only as arguments to
\i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, and \i\c{DO}. They are
expressed in the traditional form: digits, then a period, then
optionally more digits, then optionally an \c{E} followed by an
exponent. The period is mandatory, so that NASM can distinguish
between \c{dd 1}, which declares an integer constant, and \c{dd 1.0}
which declares a floating-point constant.

NASM also support C99-style hexadecimal floating-point: \c{0x},
hexadecimal digits, period, optionally more hexadeximal digits, then
optionally a \c{P} followed by a \e{binary} (not hexadecimal) exponent
in decimal notation.

Some examples:

\c       dw    -0.5                    ; IEEE half precision
\c       dd    1.2                     ; an easy one
\c       dd    0x1p+2                  ; 1.0x2^2 = 4.0
\c       dq    1.e10                   ; 10,000,000,000
\c       dq    1.e+10                  ; synonymous with 1.e10
\c       dq    1.e-10                  ; 0.000 000 000 1
\c       dt    3.141592653589793238462 ; pi
\c       do    1.e+4000                ; IEEE quad precision

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 \i{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.


\H{expr} \i{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
\I{$, here}\c{$} and \i\c{$$} tokens. \c{$} evaluates to the assembly
position at the beginning of the line containing the expression; so
you can code an \i{infinite loop} using \c{JMP $}. \c{$$} evaluates
to the beginning of the current section; so you can tell how far
into the section you are by using \c{($-$$)}.

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


\S{expor} \i\c{|}: \i{Bitwise OR} Operator

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


\S{expxor} \i\c{^}: \i{Bitwise XOR} Operator

\c{^} provides the bitwise XOR operation.


\S{expand} \i\c{&}: \i{Bitwise AND} Operator

\c{&} provides the bitwise AND operation.


\S{expshift} \i\c{<<} and \i\c{>>}: \i{Bit Shift} Operators

\c{<<} gives a bit-shift to the left, just as it does in C. So \c{5<<3}
evaluates to 5 times 8, or 40. \c{>>} gives a bit-shift to the
right; in NASM, such a shift is \e{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.


\S{expplmi} \I{+ opaddition}\c{+} and \I{- opsubtraction}\c{-}:
\i{Addition} and \i{Subtraction} Operators

The \c{+} and \c{-} operators do perfectly ordinary addition and
subtraction.


\S{expmul} \i\c{*}, \i\c{/}, \i\c{//}, \i\c{%} and \i\c{%%}:
\i{Multiplication} and \i{Division}

\c{*} is the multiplication operator. \c{/} and \c{//} are both
division operators: \c{/} is \i{unsigned division} and \c{//} is
\i{signed division}. Similarly, \c{%} and \c{%%} provide \I{unsigned
modulo}\I{modulo operators}unsigned and
\i{signed modulo} operators respectively.

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

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


\S{expmul} \i{Unary Operators}: \I{+ opunary}\c{+}, \I{- opunary}\c{-},
\i\c{~}, \I{! opunary}\c{!} and \i\c{SEG}

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


\H{segwrt} \i\c{SEG} and \i\c{WRT}

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

The \c{SEG} operator returns the \i\e{preferred} segment base of a
symbol, defined as the segment base relative to which the offset of
the symbol makes sense. So the code

\c         mov     ax,seg symbol
\c         mov     es,ax
\c         mov     bx,symbol

will load \c{ES:BX} with a valid pointer to the symbol \c{symbol}.

Things can be more complex than this: since 16-bit segments and
\i{groups} may \I{overlapping segments}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 \c{WRT}
(With Reference To) keyword. So you can do things like

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

to load \c{ES:BX} with a different, but functionally equivalent,
pointer to the symbol \c{symbol}.

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

\c         call    (seg procedure):procedure
\c         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 \I\c{CALL FAR}\c{call far procedure} as a
synonym for the first of the above usages. \c{JMP} works identically
to \c{CALL} in these examples.

To declare a \i{far pointer} to a data item in a data segment, you
must code

\c         dw      symbol, seg symbol

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


\H{strict} \i\c{STRICT}: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see
\k{opt-On}), NASM will use size specifiers (\c{BYTE}, \c{WORD},
\c{DWORD}, \c{QWORD}, \c{TWORD} or \c{OWORD}), but will give them the
smallest possible size. The keyword \c{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 \c{BITS 16}
mode,

\c         push dword 33

is encoded in three bytes \c{66 6A 21}, whereas

\c         push strict dword 33

is encoded in six bytes, with a full dword immediate operand \c{66 68
21 00 00 00}.

With the optimizer off, the same code (six bytes) is generated whether
the \c{STRICT} keyword was used or not.


\H{crit} \i{Critical Expressions}

A limitation of NASM is that it is a \i{two-pass assembler}; unlike
TASM and others, it will always do exactly two \I{passes}\i{assembly
passes}. Therefore it is unable to cope with source files that are
complex enough to require three or more passes.

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,

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

The argument to \i\c{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 \c{TIMES} line when it first sees it.
It will just as firmly reject the slightly \I{paradox}paradoxical
code

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

in which \e{any} value for the \c{TIMES} argument is by definition
wrong!

NASM rejects these examples by means of a concept called a
\e{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 \c{TIMES} prefix is a critical expression; for the same reason,
the arguments to the \i\c{RESB} family of pseudo-instructions are
also critical expressions.

Critical expressions can crop up in other contexts as well: consider
the following code.

\c                 mov     ax,symbol1
\c symbol1         equ     symbol2
\c symbol2:

On the first pass, NASM cannot determine the value of \c{symbol1},
because \c{symbol1} is defined to be equal to \c{symbol2} which NASM
hasn't seen yet. On the second pass, therefore, when it encounters
the line \c{mov ax,symbol1}, it is unable to generate the code for
it because it still doesn't know the value of \c{symbol1}. On the
next line, it would see the \i\c{EQU} again and be able to determine
the value of \c{symbol1}, but by then it would be too late.

NASM avoids this problem by defining the right-hand side of an
\c{EQU} statement to be a critical expression, so the definition of
\c{symbol1} would be rejected in the first pass.

There is a related issue involving \i{forward references}: consider
this code fragment.

\c         mov     eax,[ebx+offset]
\c offset  equ     10

NASM, on pass one, must calculate the size of the instruction \c{mov
eax,[ebx+offset]} without knowing the value of \c{offset}. It has no
way of knowing that \c{offset} is small enough to fit into a
one-byte offset field and that it could therefore get away with
generating a shorter form of the \i{effective-address} encoding; for
all it knows, in pass one, \c{offset} could be a symbol in the code
segment, and it might need the full four-byte form. So it is forced
to compute the size of the instruction to accommodate a four-byte
address part. In pass two, having made this decision, it is now
forced to honour it and keep the instruction large, so the code
generated in this case is not as small as it could have been. This
problem can be solved by defining \c{offset} before using it, or by
forcing byte size in the effective address by coding \c{[byte
ebx+offset]}.

Note that use of the \c{-On} switch (with n>=2) makes some of the above
no longer true (see \k{opt-On}).

\H{locallab} \i{Local Labels}

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

\c label1  ; some code
\c
\c .loop
\c         ; some more code
\c
\c         jne     .loop
\c         ret
\c
\c label2  ; some code
\c
\c .loop
\c         ; some more code
\c
\c         jne     .loop
\c         ret

In the above code fragment, each \c{JNE} instruction jumps to the
line immediately before it, because the two definitions of \c{.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 \i{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 \e{defining} a local label in terms of the
previous non-local label: the first definition of \c{.loop} above is
really defining a symbol called \c{label1.loop}, and the second
defines a symbol called \c{label2.loop}. So, if you really needed
to, you could write

\c label3  ; some more code
\c         ; and some more
\c
\c         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 \I{label prefix}special prefix \i\c{..@}, then it does nothing
to the local label mechanism. So you could code

\c label1:                         ; a non-local label
\c .local:                         ; this is really label1.local
\c ..@foo:                         ; this is a special symbol
\c label2:                         ; another non-local label
\c .local:                         ; this is really label2.local
\c
\c         jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with
a double period: for example, \c{..start} is used to specify the
entry point in the \c{obj} output format (see \k{dotdotstart}).


\C{preproc} The NASM \i{Preprocessor}

NASM contains a powerful \i{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 \c{%}
sign.

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

\c %define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \\
\c         THIS_VALUE

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

\H{slmacro} \i{Single-Line Macros}

\S{define} The Normal Way: \I\c{%idefine}\i\c{%define}

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

\c %define ctrl    0x1F &
\c %define param(a,b) ((a)+(a)*(b))
\c
\c         mov     byte [param(2,ebx)], ctrl 'D'

which will expand to

\c         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

\c %define a(x)    1+b(x)
\c %define b(x)    2*x
\c
\c         mov     ax,a(8)

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

Macros defined with \c{%define} are \i{case sensitive}: after
\c{%define foo bar}, only \c{foo} will expand to \c{bar}: \c{Foo} or
\c{FOO} will not. By using \c{%idefine} instead of \c{%define} (the
`i' stands for `insensitive') you can define all the case variants
of a macro at once, so that \c{%idefine foo bar} would cause
\c{foo}, \c{Foo}, \c{FOO}, \c{fOO} and so on all to expand to
\c{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
\i{circular references} and infinite loops. If this happens, the
preprocessor will only expand the first occurrence of the macro.
Hence, if you code

\c %define a(x)    1+a(x)
\c
\c         mov     ax,a(3)

the macro \c{a(3)} will expand once, becoming \c{1+a(3)}, and will
then expand no further. This behaviour can be useful: see \k{32c}
for an example of its use.

You can \I{overloading, single-line macros}overload single-line
macros: if you write

\c %define foo(x)   1+x
\c %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 \c{foo(3)} will become
\c{1+3} whereas \c{foo(ebx,2)} will become \c{1+ebx*2}. However, if
you define

\c %define foo bar

then no other definition of \c{foo} will be accepted: a macro with
no parameters prohibits the definition of the same name as a macro
\e{with} parameters, and vice versa.

This doesn't prevent single-line macros being \e{redefined}: you can
perfectly well define a macro with

\c %define foo bar

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

\c %define foo baz

Then everywhere the macro \c{foo} is invoked, it will be expanded
according to the most recent definition. This is particularly useful
when defining single-line macros with \c{%assign} (see \k{assign}).

You can \i{pre-define} single-line macros using the `-d' option on
the NASM command line: see \k{opt-d}.


\S{xdefine} Enhancing %define: \I\c{%xidefine}\i\c{%xdefine}

To have a reference to an embedded single-line macro resolved at the
time that it is embedded, as opposed to when the calling macro is
expanded, you need a different mechanism to the one offered by
\c{%define}. The solution is to use \c{%xdefine}, or it's
\I{case sensitive}case-insensitive counterpart \c{%xidefine}.

Suppose you have the following code:

\c %define  isTrue  1
\c %define  isFalse isTrue
\c %define  isTrue  0
\c
\c val1:    db      isFalse
\c
\c %define  isTrue  1
\c
\c val2:    db      isFalse

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

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

\c %xdefine isTrue  1
\c %xdefine isFalse isTrue
\c %xdefine isTrue  0
\c
\c val1:    db      isFalse
\c
\c %xdefine isTrue  1
\c
\c val2:    db      isFalse

Now, each time that \c{isFalse} is called, it expands to 1,
as that is what the embedded macro \c{isTrue} expanded to at
the time that \c{isFalse} was defined.


\S{concat%+} Concatenating Single Line Macro Tokens: \i\c{%+}

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.

As an example, consider the following:

\c %define BDASTART 400h                ; Start of BIOS data area

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

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

\c         mov     ax,BDASTART + tBIOSDA.COM1addr
\c         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:

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

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

Now the above code can be written as:

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

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


\S{undef} Undefining macros: \i\c{%undef}

Single-line macros can be removed with the \c{%undef} command.  For
example, the following sequence:

\c %define foo bar
\c %undef  foo
\c
\c         mov     eax, foo

will expand to the instruction \c{mov eax, foo}, since after
\c{%undef} the macro \c{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
\k{opt-u}.


\S{assign} \i{Preprocessor Variables}: \i\c{%assign}

An alternative way to define single-line macros is by means of the
\c{%assign} command (and its \I{case sensitive}case-insensitive
counterpart \i\c{%iassign}, which differs from \c{%assign} in
exactly the same way that \c{%idefine} differs from \c{%define}).

\c{%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
\c{%assign} directive is processed.

Like \c{%define}, macros defined using \c{%assign} can be re-defined
later, so you can do things like

\c %assign i i+1

to increment the numeric value of a macro.

\c{%assign} is useful for controlling the termination of \c{%rep}
preprocessor loops: see \k{rep} for an example of this. Another
use for \c{%assign} is given in \k{16c} and \k{32c}.

The expression passed to \c{%assign} is a \i{critical expression}
(see \k{crit}), 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).


\H{strlen} \i{String Handling in Macros}: \i\c{%strlen} and \i\c{%substr}

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.


\S{strlen} \i{String Length}: \i\c{%strlen}

The \c{%strlen} macro is like \c{%assign} macro in that it creates
(or redefines) a numeric value to a macro. The difference is that
with \c{%strlen}, the numeric value is the length of a string. An
example of the use of this would be:

\c %strlen charcnt 'my string'

In this example, \c{charcnt} would receive the value 8, just as
if an \c{%assign} had been used. In this example, \c{'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:

\c %define sometext 'my string'
\c %strlen charcnt sometext

As in the first case, this would result in \c{charcnt} being
assigned the value of 8.


\S{substr} \i{Sub-strings}: \i\c{%substr}

Individual letters in strings can be extracted using \c{%substr}.
An example of its use is probably more useful than the description:

\c %substr mychar  'xyz' 1         ; equivalent to %define mychar 'x'
\c %substr mychar  'xyz' 2         ; equivalent to %define mychar 'y'
\c %substr mychar  'xyz' 3         ; equivalent to %define mychar 'z'

In this example, mychar gets the value of 'y'. As with \c{%strlen}
(see \k{strlen}), the first parameter is the single-line macro to
be created and the second is the string. The third parameter
specifies which character is to be selected. Note that the first
index is 1, not 0 and the last index is equal to the value that
\c{%strlen} would assign given the same string. Index values out
of range result in an empty string.


\H{mlmacro} \i{Multi-Line Macros}: \I\c{%imacro}\i\c{%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.

\c %macro  prologue 1
\c
\c         push    ebp
\c         mov     ebp,esp
\c         sub     esp,%1
\c
\c %endmacro

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

\c myfunc:   prologue 12

which would expand to the three lines of code

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

The number \c{1} after the macro name in the \c{%macro} line defines
the number of parameters the macro \c{prologue} expects to receive.
The use of \c{%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 \c{%2},
\c{%3} and so on.

Multi-line macros, like single-line macros, are \i{case-sensitive},
unless you define them using the alternative directive \c{%imacro}.

If you need to pass a comma as \e{part} of a parameter to a
multi-line macro, you can do that by enclosing the entire parameter
in \I{braces, around macro parameters}braces. So you could code
things like

\c %macro  silly 2
\c
\c     %2: db      %1
\c
\c %endmacro
\c
\c         silly 'a', letter_a             ; letter_a:  db 'a'
\c         silly 'ab', string_ab           ; string_ab: db 'ab'
\c         silly {13,10}, crlf             ; crlf:      db 13,10


\S{mlmacover} Overloading Multi-Line Macros\I{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

\c %macro  prologue 0
\c
\c         push    ebp
\c         mov     ebp,esp
\c
\c %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

\c %macro  push 2
\c
\c         push    %1
\c         push    %2
\c
\c %endmacro

so that you could code

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

Ordinarily, NASM will give a warning for the first of the above two
lines, since \c{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
\c{-w-macro-params} command-line option (see \k{opt-w}).


\S{maclocal} \i{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 \i\c{%%} to the label name. So
you can invent an instruction which executes a \c{RET} if the \c{Z}
flag is set by doing this:

\c %macro  retz 0
\c
\c         jnz     %%skip
\c         ret
\c     %%skip:
\c
\c %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 \c{%%skip}. The names NASM invents are of the form
\c{..@2345.skip}, where the number 2345 changes with every macro
call. The \i\c{..@} prefix prevents macro-local labels from
interfering with the local label mechanism, as described in
\k{locallab}. You should avoid defining your own labels in this form
(the \c{..@} prefix, then a number, then another period) in case
they interfere with macro-local labels.


\S{mlmacgre} \i{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

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

NASM allows you to define the last parameter of a macro to be
\e{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:

\c %macro  writefile 2+
\c
\c         jmp     %%endstr
\c   %%str:        db      %2
\c   %%endstr:
\c         mov     dx,%%str
\c         mov     cx,%%endstr-%%str
\c         mov     bx,%1
\c         mov     ah,0x40
\c         int     0x21
\c
\c %endmacro

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

The greedy nature of the macro is indicated to NASM by the use of
the \I{+ modifier}\c{+} sign after the parameter count on the
\c{%macro} line.

If you define a greedy macro, you are effectively telling NASM how
it should expand the macro given \e{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
\c{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 \c{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

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

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

See \k{sectmac} for a better way to write the above macro.


\S{mlmacdef} \i{Default Macro Parameters}

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

\c %macro  die 0-1 "Painful program death has occurred."
\c
\c         writefile 2,%1
\c         mov     ax,0x4c01
\c         int     0x21
\c
\c %endmacro

This macro (which makes use of the \c{writefile} macro defined in
\k{mlmacgre}) 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

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

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

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
\i\c{%0} token (see \k{percent0}) 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 \c{die} macro above could be made more powerful,
and more useful, by changing the first line of the definition to

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

The maximum parameter count can be infinite, denoted by \c{*}. In
this case, of course, it is impossible to provide a \e{full} set of
default parameters. Examples of this usage are shown in \k{rotate}.


\S{percent0} \i\c{%0}: \I{counting macro parameters}Macro Parameter Counter

For a macro which can take a variable number of parameters, the
parameter reference \c{%0} will return a numeric constant giving the
number of parameters passed to the macro. This can be used as an
argument to \c{%rep} (see \k{rep}) in order to iterate through all
the parameters of a macro. Examples are given in \k{rotate}.


\S{rotate} \i\c{%rotate}: \i{Rotating Macro Parameters}

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

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

\c{%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 \c{%rotate} is negative, the macro
parameters are rotated to the right.

\I{iterating over macro parameters}So a pair of macros to save and
restore a set of registers might work as follows:

\c %macro  multipush 1-*
\c
\c   %rep  %0
\c         push    %1
\c   %rotate 1
\c   %endrep
\c
\c %endmacro

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

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

It would be convenient, when using this macro, to have a \c{POP}
equivalent, which \e{didn't} require the arguments to be given in
reverse order. Ideally, you would write the \c{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 \c{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:

\c %macro  multipop 1-*
\c
\c   %rep %0
\c   %rotate -1
\c         pop     %1
\c   %endrep
\c
\c %endmacro

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


\S{concat} \i{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

\c %macro keytab_entry 2
\c
\c     keypos%1    equ     $-keytab
\c                 db      %2
\c
\c %endmacro
\c
\c keytab:
\c           keytab_entry F1,128+1
\c           keytab_entry F2,128+2
\c           keytab_entry Return,13

which would expand to

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

You can just as easily concatenate text on to the other end of a
macro parameter, by writing \c{%1foo}.

If you need to append a \e{digit} to a macro parameter, for example
defining labels \c{foo1} and \c{foo2} when passed the parameter
\c{foo}, you can't code \c{%11} because that would be taken as the
eleventh macro parameter. Instead, you must code
\I{braces, after % sign}\c{%\{1\}1}, which will separate the first
\c{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 (\k{maclocal}) and context-local
labels (\k{ctxlocal}). In all cases, ambiguities in syntax can be
resolved by enclosing everything after the \c{%} sign and before the
literal text in braces: so \c{%\{%foo\}bar} concatenates the text
\c{bar} to the end of the real name of the macro-local label
\c{%%foo}. (This is unnecessary, since the form NASM uses for the
real names of macro-local labels means that the two usages
\c{%\{%foo\}bar} and \c{%%foobar} would both expand to the same
thing anyway; nevertheless, the capability is there.)


\S{mlmaccc} \i{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
\c{%1} by means of the alternative syntax \i\c{%+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 \e{not} a valid
condition code.

Far more usefully, though, you can refer to the macro parameter by
means of \i\c{%-1}, which NASM will expand as the \e{inverse}
condition code. So the \c{retz} macro defined in \k{maclocal} can be
replaced by a general \i{conditional-return macro} like this:

\c %macro  retc 1
\c
\c         j%-1    %%skip
\c         ret
\c   %%skip:
\c
\c %endmacro

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

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


\S{nolist} \i{Disabling Listing Expansion}\I\c{.nolist}

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 \c{.nolist} qualifier, which you can
include in a macro definition to inhibit the expansion of the macro
in the listing file. The \c{.nolist} qualifier comes directly after
the number of parameters, like this:

\c %macro foo 1.nolist

Or like this:

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

\H{condasm} \i{Conditional Assembly}\I\c{%if}

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:

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

The \i\c{%else} clause is optional, as is the \i\c{%elif} clause.
You can have more than one \c{%elif} clause as well.


\S{ifdef} \i\c{%ifdef}: Testing Single-Line Macro Existence\I{testing,
single-line macro existence}

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

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

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

Then you could use the command-line option \c{-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 \e{not} being defined by using
\i\c{%ifndef} instead of \c{%ifdef}. You can also test for macro
definitions in \c{%elif} blocks by using \i\c{%elifdef} and
\i\c{%elifndef}.


\S{ifmacro} \i\c{ifmacro}: Testing Multi-Line Macro
Existence\I{testing, multi-line macro existence}

The \c{%ifmacro} directive operates in the same way as the \c{%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 \c{%ifmacro} is considered true if defining a macro with the given name
and number of arguments would cause a definitions conflict. For example:

\c %ifmacro MyMacro 1-3
\c
\c      %error "MyMacro 1-3" causes a conflict with an existing macro.
\c
\c %else
\c
\c      %macro MyMacro 1-3
\c
\c              ; insert code to define the macro
\c
\c      %endmacro
\c
\c %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 \i\c{%ifnmacro} instead
of \c{%ifmacro}. Additional tests can be performed in \c{%elif} blocks by using
\i\c{%elifmacro} and \i\c{%elifnmacro}.


\S{ifctx} \i\c{%ifctx}: Testing the Context Stack\I{testing, context
stack}

The conditional-assembly construct \c{%ifctx ctxname} will cause the
subsequent code to be assembled if and only if the top context on
the preprocessor's context stack has the name \c{ctxname}. As with
\c{%ifdef}, the inverse and \c{%elif} forms \i\c{%ifnctx},
\i\c{%elifctx} and \i\c{%elifnctx} are also supported.

For more details of the context stack, see \k{ctxstack}. For a
sample use of \c{%ifctx}, see \k{blockif}.


\S{if} \i\c{%if}: Testing Arbitrary Numeric Expressions\I{testing,
arbitrary numeric expressions}

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

The expression given to \c{%if}, and its counterpart \i\c{%elif}, is
a critical expression (see \k{crit}).

\c{%if} extends the normal NASM expression syntax, by providing a
set of \i{relational operators} which are not normally available in
expressions. The operators \i\c{=}, \i\c{<}, \i\c{>}, \i\c{<=},
\i\c{>=} and \i\c{<>} test equality, less-than, greater-than,
less-or-equal, greater-or-equal and not-equal respectively. The
C-like forms \i\c{==} and \i\c{!=} are supported as alternative
forms of \c{=} and \c{<>}. In addition, low-priority logical
operators \i\c{&&}, \i\c{^^} and \i\c{||} are provided, supplying
\i{logical AND}, \i{logical XOR} and \i{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 \c{^^}, 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 most other \c{%if} constructs, \c{%if} has a counterpart
\i\c{%elif}, and negative forms \i\c{%ifn} and \i\c{%elifn}.

\S{ifidn} \i\c{%ifidn} and \i\c{%ifidni}: Testing Exact Text
Identity\I{testing, exact text identity}

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

\c{%ifidni} is similar to \c{%ifidn}, but is \i{case-insensitive}.

For example, the following macro pushes a register or number on the
stack, and allows you to treat \c{IP} as a real register:

\c %macro  pushparam 1
\c
\c   %ifidni %1,ip
\c         call    %%label
\c   %%label:
\c   %else
\c         push    %1
\c   %endif
\c
\c %endmacro

Like most other \c{%if} constructs, \c{%ifidn} has a counterpart
\i\c{%elifidn}, and negative forms \i\c{%ifnidn} and \i\c{%elifnidn}.
Similarly, \c{%ifidni} has counterparts \i\c{%elifidni},
\i\c{%ifnidni} and \i\c{%elifnidni}.


\S{iftyp} \i\c{%ifid}, \i\c{%ifnum}, \i\c{%ifstr}: Testing Token
Types\I{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 \c{%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.
\c{%ifnum} works similarly, but tests for the token being a numeric
constant; \c{%ifstr} tests for it being a string.

For example, the \c{writefile} macro defined in \k{mlmacgre} can be
extended to take advantage of \c{%ifstr} in the following fashion:

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

Then the \c{writefile} macro can cope with being called in either of
the following two ways:

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

In the first, \c{strpointer} is used as the address of an
already-declared string, and \c{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 \c{%if} inside the \c{%ifstr}: this is to detect
whether the macro was passed two arguments (so the string would be a
single string constant, and \c{db %2} would be adequate) or more (in
which case, all but the first two would be lumped together into
\c{%3}, and \c{db %2,%3} would be required).

\I\c{%ifnid}\I\c{%elifid}\I\c{%elifnid}\I\c{%ifnnum}\I\c{%elifnum}
\I\c{%elifnnum}\I\c{%ifnstr}\I\c{%elifstr}\I\c{%elifnstr}
The usual \c{%elifXXX}, \c{%ifnXXX} and \c{%elifnXXX} versions exist
for each of \c{%ifid}, \c{%ifnum} and \c{%ifstr}.


\S{pperror} \i\c{%error}: Reporting \i{User-Defined Errors}

The preprocessor directive \c{%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:

\c %ifdef SOME_MACRO
\c     ; do some setup
\c %elifdef SOME_OTHER_MACRO
\c     ; do some different setup
\c %else
\c     %error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.
\c %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.


\H{rep} \i{Preprocessor Loops}\I{repeating code}: \i\c{%rep}

NASM's \c{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: \c{%rep}.

The directives \c{%rep} and \i\c{%endrep} (\c{%rep} takes a numeric
argument, which can be an expression; \c{%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:

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

This will generate a sequence of 64 \c{INC} instructions,
incrementing every word of memory from \c{[table]} to
\c{[table+126]}.

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

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


\H{include} \i{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 \i\c{%include} directive:

\c %include "macros.mac"

will include the contents of the file \c{macros.mac} into the source
file containing the \c{%include} directive.

Include files are \I{searching for include files}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 \c{-i} option.

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

\c %ifndef MACROS_MAC
\c     %define MACROS_MAC
\c     ; now define some macros
\c %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 \c{MACROS_MAC} will already be defined.

You can force a file to be included even if there is no \c{%include}
directive that explicitly includes it, by using the \i\c{-p} option
on the NASM command line (see \k{opt-p}).


\H{ctxstack} The \i{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 \c{REPEAT} ...
\c{UNTIL} loop, in which the expansion of the \c{REPEAT} macro
would need to be able to refer to a label which the \c{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 \e{context stack}.
The preprocessor maintains a stack of \e{contexts}, each of which is
characterized by a name. You add a new context to the stack using
the \i\c{%push} directive, and remove one using \i\c{%pop}. You can
define labels that are local to a particular context on the stack.


\S{pushpop} \i\c{%push} and \i\c{%pop}: \I{creating
contexts}\I{removing contexts}Creating and Removing Contexts

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

\c %push    foobar

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

The directive \c{%pop}, requiring no arguments, removes the top
context from the context stack and destroys it, along with any
labels associated with it.


\S{ctxlocal} \i{Context-Local Labels}

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

\c %macro repeat 0
\c
\c     %push   repeat
\c     %$begin:
\c
\c %endmacro
\c
\c %macro until 1
\c
\c         j%-1    %$begin
\c     %pop
\c
\c %endmacro

and invoked by means of, for example,

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

which would scan every fourth byte of a string in search of the byte
in \c{AL}.

If you need to define, or access, labels local to the context
\e{below} the top one on the stack, you can use \I{%$$}\c{%$$foo}, or
\c{%$$$foo} for the context below that, and so on.


\S{ctxdefine} \i{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:

\c %define %$localmac 3

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


\S{ctxrepl} \i\c{%repl}: \I{renaming contexts}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 \c{%ifctx}),
you can execute a \c{%pop} followed by a \c{%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 \c{%repl}, which \e{replaces} a context
with a different name, without touching the associated macros and
labels. So you could replace the destructive code

\c %pop
\c %push   newname

with the non-destructive version \c{%repl newname}.


\S{blockif} Example Use of the \i{Context Stack}: \i{Block IFs}

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

\c %macro if 1
\c
\c     %push if
\c     j%-1  %$ifnot
\c
\c %endmacro
\c
\c %macro else 0
\c
\c   %ifctx if
\c         %repl   else
\c         jmp     %$ifend
\c         %$ifnot:
\c   %else
\c         %error  "expected `if' before `else'"
\c   %endif
\c
\c %endmacro
\c
\c %macro endif 0
\c
\c   %ifctx if
\c         %$ifnot:
\c         %pop
\c   %elifctx      else
\c         %$ifend:
\c         %pop
\c   %else
\c         %error  "expected `if' or `else' before `endif'"
\c   %endif
\c
\c %endmacro

This code is more robust than the \c{REPEAT} and \c{UNTIL} macros
given in \k{ctxlocal}, because it uses conditional assembly to check
that the macros are issued in the right order (for example, not
calling \c{endif} before \c{if}) and issues a \c{%error} if they're
not.

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

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

A sample usage of these macros might look like:

\c         cmp     ax,bx
\c
\c         if ae
\c                cmp     bx,cx
\c
\c                if ae
\c                        mov     ax,cx
\c                else
\c                        mov     ax,bx
\c                endif
\c
\c         else
\c                cmp     ax,cx
\c
\c                if ae
\c                        mov     ax,cx
\c                endif
\c
\c         endif

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


\H{stdmac} \i{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
\i\c{%clear} directive to empty the preprocessor of everything but
context-local preprocessor variables and single-line macros.

Most \i{user-level assembler directives} (see \k{directive}) are
implemented as macros which invoke primitive directives; these are
described in \k{directive}. The rest of the standard macro set is
described here.


\S{stdmacver} \i\c{__NASM_MAJOR__}, \i\c{__NASM_MINOR__},
\i\c{__NASM_SUBMINOR__} and \i\c{___NASM_PATCHLEVEL__}: \i{NASM Version}

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


\S{stdmacverid} \i\c{__NASM_VERSION_ID__}: \i{NASM Version ID}

The single-line macro \c{__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 \c{__NASM_MAJOR__}, \c{__NASM_MINOR__},
\c{__NASM_SUBMINOR__} and \c{___NASM_PATCHLEVEL__} concatenated to
produce a single doubleword. Hence, for 0.98.32p1, the returned number
would be equivalent to:

\c         dd      0x00622001

or

\c         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.


\S{stdmacverstr} \i\c{__NASM_VER__}: \i{NASM Version string}

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

\c         db      __NASM_VER__

would expand to

\c         db      "0.98.32"


\S{fileline} \i\c{__FILE__} and \i\c{__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
\c{__FILE__} expands to a string constant giving the name of the
current input file (which may change through the course of assembly
if \c{%include} directives are used), and \c{__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 \c{__LINE__} inside a macro
definition (either single-line or multi-line) will return the line
number of the macro \e{call}, rather than \e{definition}. So to
determine where in a piece of code a crash is occurring, for
example, one could write a routine \c{stillhere}, which is passed a
line number in \c{EAX} and outputs something like `line 155: still
here'. You could then write a macro

\c %macro  notdeadyet 0
\c
\c         push    eax
\c         mov     eax,__LINE__
\c         call    stillhere
\c         pop     eax
\c
\c %endmacro

and then pepper your code with calls to \c{notdeadyet} until you
find the crash point.

\S{bitsm} \i\c{__BITS__}: Current BITS Mode

The \c{__BITS__} standard macro is updated every time that the BITS mode is
set using the \c{BITS XX} or \c{[BITS XX]} directive, where XX is a valid mode
number of 16, 32 or 64. \c{__BITS__} receives the specified mode number and
makes it globally available. This can be very useful for those who utilize
mode-dependent macros.


\S{struc} \i\c{STRUC} and \i\c{ENDSTRUC}: \i{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
\c{STRUC} and \c{ENDSTRUC} are used to define a structure data type.

\c{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 \c{_size} appended to it and is then defined as an
\c{EQU} giving the size of the structure. Once \c{STRUC} has been
issued, you are defining the structure, and should define fields
using the \c{RESB} family of pseudo-instructions, and then invoke
\c{ENDSTRUC} to finish the definition.

For example, to define a structure called \c{mytype} containing a
longword, a word, a byte and a string of bytes, you might code

\c struc   mytype
\c
\c   mt_long:      resd    1
\c   mt_word:      resw    1
\c   mt_byte:      resb    1
\c   mt_str:       resb    32
\c
\c endstruc

The above code defines six symbols: \c{mt_long} as 0 (the offset
from the beginning of a \c{mytype} structure to the longword field),
\c{mt_word} as 4, \c{mt_byte} as 6, \c{mt_str} as 7, \c{mytype_size}
as 39, and \c{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:

\c struc mytype
\c
\c   .long:        resd    1
\c   .word:        resw    1
\c   .byte:        resb    1
\c   .str:         resb    32
\c
\c endstruc

This defines the offsets to the structure fields as \c{mytype.long},
\c{mytype.word}, \c{mytype.byte} and \c{mytype.str}.

NASM, since it has no \e{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 \c{mov ax,[mystruc.mt_word]} is not valid.
\c{mt_word} is a constant just like any other constant, so the
correct syntax is \c{mov ax,[mystruc+mt_word]} or \c{mov
ax,[mystruc+mytype.word]}.


\S{istruc} \i\c{ISTRUC}, \i\c{AT} and \i\c{IEND}: Declaring
\i{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 \c{ISTRUC}
mechanism. To declare a structure of type \c{mytype} in a program,
you code something like this:

\c mystruc:
\c     istruc mytype
\c
\c         at mt_long, dd      123456
\c         at mt_word, dw      1024
\c         at mt_byte, db      'x'
\c         at mt_str,  db      'hello, world', 13, 10, 0
\c
\c     iend

The function of the \c{AT} macro is to make use of the \c{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 \c{AT} line. For example:

\c         at mt_str,  db      123,134,145,156,167,178,189
\c                     db      190,100,0

Depending on personal taste, you can also omit the code part of the
\c{AT} line completely, and start the structure field on the next
line:

\c         at mt_str
\c                 db      'hello, world'
\c                 db      13,10,0


\S{align} \i\c{ALIGN} and \i\c{ALIGNB}: Data Alignment

The \c{ALIGN} and \c{ALIGNB} macros provides a convenient way to
align code or data on a word, longword, paragraph or other boundary.
(Some assemblers call this directive \i\c{EVEN}.) The syntax of the
\c{ALIGN} and \c{ALIGNB} macros is

\c         align   4               ; align on 4-byte boundary
\c         align   16              ; align on 16-byte boundary
\c         align   8,db 0          ; pad with 0s rather than NOPs
\c         align   4,resb 1        ; align to 4 in the BSS
\c         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 \c{TIMES} prefix to their second argument to
perform the alignment.

If the second argument is not specified, the default for \c{ALIGN}
is \c{NOP}, and the default for \c{ALIGNB} is \c{RESB 1}. So if the
second argument is specified, the two macros are equivalent.
Normally, you can just use \c{ALIGN} in code and data sections and
\c{ALIGNB} in BSS sections, and never need the second argument
except for special purposes.

\c{ALIGN} and \c{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.

\c{ALIGNB} (or \c{ALIGN} with a second argument of \c{RESB 1}) can
be used within structure definitions:

\c struc mytype2
\c
\c   mt_byte:
\c         resb 1
\c         alignb 2
\c   mt_word:
\c         resw 1
\c         alignb 4
\c   mt_long:
\c         resd 1
\c   mt_str:
\c         resb 32
\c
\c endstruc

This will ensure that the structure members are sensibly aligned
relative to the base of the structure.

A final caveat: \c{ALIGN} and \c{ALIGNB} work relative to the
beginning of the \e{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 \c{ALIGN} or \c{ALIGNB}.


\H{tasmcompat} \i{TASM Compatible Preprocessor Directives}

The following preprocessor directives may only be used when TASM
compatibility is turned on using the \c{-t} command line switch
(This switch is described in \k{opt-t}.)

\b\c{%arg}  (see \k{arg})

\b\c{%stacksize}  (see \k{stacksize})

\b\c{%local}  (see \k{local})


\S{arg} \i\c{%arg} Directive

The \c{%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 comes with macros which attempt to duplicate this
functionality (see \k{16cmacro}), the syntax is not particularly
convenient to use and is not TASM compatible. Here is an example
which shows the use of \c{%arg} without any external macros:

\c some_function:
\c
\c     %push     mycontext        ; save the current context
\c     %stacksize large           ; tell NASM to use bp
\c     %arg      i:word, j_ptr:word
\c
\c         mov     ax,[i]
\c         mov     bx,[j_ptr]
\c         add     ax,[bx]
\c         ret
\c
\c     %pop                       ; restore original context

This is similar to the procedure defined in \k{16cmacro} and adds
the value in i to the value pointed to by j_ptr and returns the
sum in the ax register. See \k{pushpop} for an explanation of
\c{push} and \c{pop} and the use of context stacks.


\S{stacksize} \i\c{%stacksize} Directive

The \c{%stacksize} directive is used in conjunction with the
\c{%arg} (see \k{arg}) and the \c{%local} (see \k{local}) directives.
It tells NASM the default size to use for subsequent \c{%arg} and
\c{%local} directives. The \c{%stacksize} directive takes one
required argument which is one of \c{flat}, \c{large} or \c{small}.

\c %stacksize flat

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

\c %stacksize large

This form uses \c{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 \c{ip} and \c{cs} are on the stack).

\c %stacksize small

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


\S{local} \i\c{%local} Directive

The \c{%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
\c{%local} directive is most useful when used with the \c{%stacksize}
(see \k{stacksize} and is also compatible with the \c{%arg} directive
(see \k{arg}). It allows simplified reference to variables on the
stack which have been allocated typically by using the \c{ENTER}
instruction.
\# (see \k{insENTER} for a description of that instruction).
An example of its use is the following:

\c silly_swap:
\c
\c     %push mycontext             ; save the current context
\c     %stacksize small            ; tell NASM to use bp
\c     %assign %$localsize 0       ; see text for explanation
\c     %local old_ax:word, old_dx:word
\c
\c         enter   %$localsize,0   ; see text for explanation
\c         mov     [old_ax],ax     ; swap ax & bx
\c         mov     [old_dx],dx     ; and swap dx & cx
\c         mov     ax,bx
\c         mov     dx,cx
\c         mov     bx,[old_ax]
\c         mov     cx,[old_dx]
\c         leave                   ; restore old bp
\c         ret                     ;
\c
\c     %pop                        ; restore original context

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

\H{otherpreproc} \i{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.

\b\c{%line} enables NAsM to correctly handle the output of the cpp
C language preprocessor (see \k{line}).

\b\c{%!} enables NASM to read in the value of an environment variable,
which can then be used in your program (see \k{getenv}).

\S{line} \i\c{%line} Directive

The \c{%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 \c{%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
\c{%line} preprocessor directive is as follows:

\c %line nnn[+mmm] [filename]

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

After reading a \c{%line} preprocessor directive, NASM will report
all file name and line numbers relative to the values specified
therein.


\S{getenv} \i\c{%!}\c{<env>}: Read an environment variable.

The \c{%!<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 \c{FOO}, and
you want the contents of \c{FOO} to be embedded in your program. You
could do that as follows:

\c %define FOO    %!FOO
\c %define quote   '
\c
\c tmpstr  db      quote FOO quote

At the time of writing, this will generate an "unterminated string"
warning at the time of defining "quote", and it will add a space
before and after the string that is read in. I was unable to find
a simple workaround (although a workaround can be created using a
multi-line macro), so I believe that you will need to either learn how
to create more complex macros, or allow for the extra spaces if you
make use of this feature in that way.


\C{directive} \i{Assembler Directives}

NASM, though it attempts to avoid the bureaucracy of assemblers like
MASM and TASM, is nevertheless forced to support a \e{few}
directives. These are described in this chapter.

NASM's directives come in two types: \I{user-level
directives}\e{user-level} directives and \I{primitive
directives}\e{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
\I{format-specific directives}\e{format-specific} directives are
documented along with the formats that implement them, in \k{outfmt}.


\H{bits} \i\c{BITS}: Specifying Target \i{Processor Mode}

The \c{BITS} directive specifies whether NASM should generate code
\I{16-bit mode, versus 32-bit mode}designed to run on a processor
operating in 16-bit mode, 32-bit mode or 64-bit mode. The syntax is
\c{BITS XX}, where XX is 16, 32 or 64.

In most cases, you should not need to use \c{BITS} explicitly. The
\c{aout}, \c{coff}, \c{elf}, \c{macho}, \c{win32} and \c{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 \c{obj} object format allows you
to specify each segment you define as either \c{USE16} or \c{USE32},
and NASM will set its operating mode accordingly, so the use of the
\c{BITS} directive is once again unnecessary.

The most likely reason for using the \c{BITS} directive is to write
32-bit or 64-bit code in a flat binary file; this is because the \c{bin}
output format defaults to 16-bit mode in anticipation of it being
used most frequently to write DOS \c{.COM} programs, DOS \c{.SYS}
device drivers and boot loader software.

You do \e{not} need to specify \c{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 \c{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 \c{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 \c{BITS 64} mode, most instructions operate the same
as they do for \c{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 \c{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 \c{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 \c{BITS} directive has an exactly equivalent primitive form,
\c{[BITS 16]}, \c{[BITS 32]} and \c{[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. \c{BITS32} will \e{not} work!

\S{USE16 & USE32} \i\c{USE16} & \i\c{USE32}: Aliases for BITS

The `\c{USE16}' and `\c{USE32}' directives can be used in place of
`\c{BITS 16}' and `\c{BITS 32}', for compatibility with other assemblers.


\H{default} \i\c{DEFAULT}: Change the assembler defaults

The \c{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 \c{DEFAULT} that is settable is whether or not
registerless instructions in 64-bit mode are \c{RIP}-relative or not.
By default, they are absolute unless overridden with the \i\c{REL}
specifier (see \k{effaddr}).  However, if \c{DEFAULT REL} is
specified, \c{REL} is default, unless overridden with the \c{ABS}
specifier, \e{except when used with an FS or GS segment override}.

The special handling of \c{FS} and \c{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
\c{RIP}-relative addresses would be extremely confusing.

\c{DEFAULT REL} is disabled with \c{DEFAULT ABS}.

\H{section} \i\c{SECTION} or \i\c{SEGMENT}: Changing and \i{Defining
Sections}

\I{changing sections}\I{switching between sections}The \c{SECTION}
directive (\c{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 \c{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 \c{bin} object format (but see
\k{multisec}, all support
the \i{standardized section names} \c{.text}, \c{.data} and \c{.bss}
for the code, data and uninitialized-data sections. The \c{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.


\S{sectmac} The \i\c{__SECT__} Macro

The \c{SECTION} directive is unusual in that its user-level form
functions differently from its primitive form. The primitive form,
\c{[SECTION xyz]}, simply switches the current target section to the
one given. The user-level form, \c{SECTION xyz}, however, first
defines the single-line macro \c{__SECT__} to be the primitive
\c{[SECTION]} directive which it is about to issue, and then issues
it. So the user-level directive

\c         SECTION .text

expands to the two lines

\c %define __SECT__        [SECTION .text]
\c         [SECTION .text]

Users may find it useful to make use of this in their own macros.
For example, the \c{writefile} macro defined in \k{mlmacgre} can be
usefully rewritten in the following more sophisticated form:

\c %macro  writefile 2+
\c
\c         [section .data]
\c
\c   %%str:        db      %2
\c   %%endstr:
\c
\c         __SECT__
\c
\c         mov     dx,%%str
\c         mov     cx,%%endstr-%%str
\c         mov     bx,%1
\c         mov     ah,0x40
\c         int     0x21
\c
\c %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 \c{SECTION} directive so as not to modify
\c{__SECT__}. It then declares its string in the data section, and
then invokes \c{__SECT__} to switch back to \e{whichever} section
the user was previously working in. It thus avoids the need, in the
previous version of the macro, to include a \c{JMP} instruction to
jump over the data, and also does not fail if, in a complicated
\c{OBJ} format module, the user could potentially be assembling the
code in any of several separate code sections.


\H{absolute} \i\c{ABSOLUTE}: Defining Absolute Labels

The \c{ABSOLUTE} directive can be thought of as an alternative form
of \c{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 \c{RESB} family.

\c{ABSOLUTE} is used as follows:

\c absolute 0x1A
\c
\c     kbuf_chr    resw    1
\c     kbuf_free   resw    1
\c     kbuf        resw    16

This example describes a section of the PC BIOS data area, at
segment address 0x40: the above code defines \c{kbuf_chr} to be
0x1A, \c{kbuf_free} to be 0x1C, and \c{kbuf} to be 0x1E.

The user-level form of \c{ABSOLUTE}, like that of \c{SECTION},
redefines the \i\c{__SECT__} macro when it is invoked.

\i\c{STRUC} and \i\c{ENDSTRUC} are defined as macros which use
\c{ABSOLUTE} (and also \c{__SECT__}).

\c{ABSOLUTE} doesn't have to take an absolute constant as an
argument: it can take an expression (actually, a \i{critical
expression}: see \k{crit}) 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:

\c         org     100h               ; it's a .COM program
\c
\c         jmp     setup              ; setup code comes last
\c
\c         ; the resident part of the TSR goes here
\c setup:
\c         ; now write the code that installs the TSR here
\c
\c absolute setup
\c
\c runtimevar1     resw    1
\c runtimevar2     resd    20
\c
\c 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.


\H{extern} \i\c{EXTERN}: \i{Importing Symbols} from Other Modules

\c{EXTERN} is similar to the MASM directive \c{EXTRN} and the C
keyword \c{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 \c{bin} format cannot.

The \c{EXTERN} directive takes as many arguments as you like. Each
argument is the name of a symbol:

\c extern  _printf
\c extern  _sscanf,_fscanf

Some object-file formats provide extra features to the \c{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 \c{obj} format allows you to declare that the
default segment base of an external should be the group \c{dgroup}
by means of the directive

\c extern  _variable:wrt dgroup

The primitive form of \c{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 \c{EXTERN} more than once: NASM
will quietly ignore the second and later redeclarations. You can't
declare a variable as \c{EXTERN} as well as something else, though.


\H{global} \i\c{GLOBAL}: \i{Exporting Symbols} to Other Modules

\c{GLOBAL} is the other end of \c{EXTERN}: if one module declares a
symbol as \c{EXTERN} and refers to it, then in order to prevent
linker errors, some other module must actually \e{define} the
symbol and declare it as \c{GLOBAL}. Some assemblers use the name
\i\c{PUBLIC} for this purpose.

The \c{GLOBAL} directive applying to a symbol must appear \e{before}
the definition of the symbol.

\c{GLOBAL} uses the same syntax as \c{EXTERN}, except that it must
refer to symbols which \e{are} defined in the same module as the
\c{GLOBAL} directive. For example:

\c global _main
\c _main:
\c         ; some code

\c{GLOBAL}, like \c{EXTERN}, allows object formats to define private
extensions by means of a colon. The \c{elf} object format, for
example, lets you specify whether global data items are functions or
data:

\c global  hashlookup:function, hashtable:data

Like \c{EXTERN}, the primitive form of \c{GLOBAL} differs from the
user-level form only in that it can take only one argument at a
time.


\H{common} \i\c{COMMON}: Defining Common Data Areas

The \c{COMMON} directive is used to declare \i\e{common variables}.
A common variable is much like a global variable declared in the
uninitialized data section, so that

\c common  intvar  4

is similar in function to

\c global  intvar
\c section .bss
\c
\c 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
\e{merged}, and references to \c{intvar} in all modules will point
at the same piece of memory.

Like \c{GLOBAL} and \c{EXTERN}, \c{COMMON} supports object-format
specific extensions. For example, the \c{obj} format allows common
variables to be NEAR or FAR, and the \c{elf} format allows you to
specify the alignment requirements of a common variable:

\c common  commvar  4:near  ; works in OBJ
\c common  intarray 100:4   ; works in ELF: 4 byte aligned

Once again, like \c{EXTERN} and \c{GLOBAL}, the primitive form of
\c{COMMON} differs from the user-level form only in that it can take
only one argument at a time.


\H{CPU} \i\c{CPU}: Defining CPU Dependencies

The \i\c{CPU} directive restricts assembly to those instructions which
are available on the specified CPU.

Options are:

\b\c{CPU 8086}          Assemble only 8086 instruction set

\b\c{CPU 186}           Assemble instructions up to the 80186 instruction set

\b\c{CPU 286}           Assemble instructions up to the 286 instruction set

\b\c{CPU 386}           Assemble instructions up to the 386 instruction set

\b\c{CPU 486}           486 instruction set

\b\c{CPU 586}           Pentium instruction set

\b\c{CPU PENTIUM}       Same as 586

\b\c{CPU 686}           P6 instruction set

\b\c{CPU PPRO}          Same as 686

\b\c{CPU P2}            Same as 686

\b\c{CPU P3}            Pentium III (Katmai) instruction sets

\b\c{CPU KATMAI}        Same as P3

\b\c{CPU P4}            Pentium 4 (Willamette) instruction set

\b\c{CPU WILLAMETTE}    Same as P4

\b\c{CPU PRESCOTT}      Prescott instruction set

\b\c{CPU X64}           x86-64 (x64/AMD64/EM64T) instruction set

\b\c{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.


\C{outfmt} \i{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 \i\c{-f} option on
the NASM \i{command line}. Each of these formats, along with its
extensions to the base NASM syntax, is detailed in this chapter.

As stated in \k{opt-o}, NASM chooses a \i{default name} for your
output file based on the input file name and the chosen output
format. This will be generated by removing the \i{extension}
(\c{.asm}, \c{.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.


\H{binfmt} \i\c{bin}: \i{Flat-Form Binary}\I{pure binary} Output

The \c{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 \i{MS-DOS}: \i\c{.COM} executables and
\i\c{.SYS} device drivers are pure binary files. Pure binary output
is also useful for \i{operating system} and \i{boot loader}
development.

The \c{bin} format supports \i{multiple section names}. For details of
how nasm handles sections in the \c{bin} format, see \k{multisec}.

Using the \c{bin} format puts NASM by default into 16-bit mode (see
\k{bits}). In order to use \c{bin} to write 32-bit or 64-bit code,
such as an OS kernel, you need to explicitly issue the \I\c{BITS}\c{BITS 32}
or \I\c{BITS}\c{BITS 64} directive.

\c{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 \c{binprog.asm}
into a binary file called \c{binprog}.


\S{org} \i\c{ORG}: Binary File \i{Program Origin}

The \c{bin} format provides an additional directive to the list
given in \k{directive}: \c{ORG}. The function of the \c{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
\c{0x00000104}:

\c         org     0x100
\c         dd      label
\c label:

Unlike the \c{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 \c{ORG} does exactly what
the directive says: \e{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 \k{proborg} for further comments.


\S{binseg} \c{bin} Extensions to the \c{SECTION}
Directive\I{SECTION, bin extensions to}

The \c{bin} output format extends the \c{SECTION} (or \c{SEGMENT})
directive to allow you to specify the alignment requirements of
segments. This is done by appending the \i\c{ALIGN} qualifier to the
end of the section-definition line. For example,

\c section .data   align=16

switches to the section \c{.data} and also specifies that it must be
aligned on a 16-byte boundary.

The parameter to \c{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.\I{section alignment, in
bin}\I{segment alignment, in bin}\I{alignment, in bin sections}


\S{multisec} \i\c{Multisection}\I{bin, multisection} support for the BIN format.

The \c{bin} format allows the use of multiple sections, of arbitrary names, 
besides the "known" \c{.text}, \c{.data}, and \c{.bss} names.

\b Sections may be designated \i\c{progbits} or \i\c{nobits}. Default 
is \c{progbits} (except \c{.bss}, which defaults to \c{nobits}, 
of course).

\b Sections can be aligned at a specified boundary following the previous 
section with \c{align=}, or at an arbitrary byte-granular position with 
\i\c{start=}.

\b Sections can be given a virtual start address, which will be used 
for the calculation of all memory references within that section 
with \i\c{vstart=}.

\b Sections can be ordered using \i\c{follows=}\c{<section>} or 
\i\c{vfollows=}\c{<section>} as an alternative to specifying an explicit 
start address.

\b Arguments to \c{org}, \c{start}, \c{vstart}, and \c{align=} are 
critical expressions. See \k{crit}. E.g. \c{align=(1 << ALIGN_SHIFT)} 
- \c{ALIGN_SHIFT} must be defined before it is used here.

\b Any code which comes before an explicit \c{SECTION} directive
is directed by default into the \c{.text} section.

\b If an \c{ORG} statement is not given, \c{ORG 0} is used 
by default.

\b The \c{.bss} section will be placed after the last \c{progbits} 
section, unless \c{start=}, \c{vstart=}, \c{follows=}, or \c{vfollows=} 
has been specified.

\b All sections are aligned on dword boundaries, unless a different 
alignment has been specified.

\b Sections may not overlap.

\b Nasm creates the \c{section.<secname>.start} for each section, 
which may be used in your code.

\S{map}\i{Map files}

Map files can be generated in \c{-f bin} format by means of the \c{[map]} 
option. Map types of \c{all} (default), \c{brief}, \c{sections}, \c{segments}, 
or \c{symbols} may be specified. Output may be directed to \c{stdout} 
(default), \c{stderr}, or a specified file. E.g.
\c{[map symbols myfile.map]}. No "user form" exists, the square
brackets must be used.


\H{objfmt} \i\c{obj}: \i{Microsoft OMF}\I{OMF} Object Files

The \c{obj} file format (NASM calls it \c{obj} rather than \c{omf}
for historical reasons) is the one produced by \i{MASM} and
\i{TASM}, which is typically fed to 16-bit DOS linkers to produce
\i\c{.EXE} files. It is also the format used by \i{OS/2}.

\c{obj} provides a default output file-name extension of \c{.obj}.

\c{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 \c{obj} format files are used by \i{Borland's Win32
compilers}, instead of using Microsoft's newer \i\c{win32} object
file format.

The \c{obj} format does not define any special segment names: you
can call your segments anything you like. Typical names for segments
in \c{obj} format files are \c{CODE}, \c{DATA} and \c{BSS}.

If your source file contains code before specifying an explicit
\c{SEGMENT} directive, then NASM will invent its own segment called
\i\c{__NASMDEFSEG} for you.

When you define a segment in an \c{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:

\c segment data
\c
\c dvar:   dw      1234
\c
\c segment code
\c
\c function:
\c         mov     ax,data         ; get segment address of data
\c         mov     ds,ax           ; and move it into DS
\c         inc     word [dvar]     ; now this reference will work
\c         ret

The \c{obj} format also enables the use of the \i\c{SEG} and
\i\c{WRT} operators, so that you can write code which does things
like

\c extern  foo
\c
\c       mov   ax,seg foo            ; get preferred segment of foo
\c       mov   ds,ax
\c       mov   ax,data               ; a different segment
\c       mov   es,ax
\c       mov   ax,[ds:foo]           ; this accesses `foo'
\c       mov   [es:foo wrt data],bx  ; so does this


\S{objseg} \c{obj} Extensions to the \c{SEGMENT}
Directive\I{SEGMENT, obj extensions to}

The \c{obj} output format extends the \c{SEGMENT} (or \c{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,

\c segment code private align=16

defines the segment \c{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:

\b \i\c{PRIVATE}, \i\c{PUBLIC}, \i\c{COMMON} and \i\c{STACK} specify
the combination characteristics of the segment. \c{PRIVATE} segments
do not get combined with any others by the linker; \c{PUBLIC} and
\c{STACK} segments get concatenated together at link time; and
\c{COMMON} segments all get overlaid on top of each other rather
than stuck end-to-end.

\b \i\c{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 \i{PharLap} extension to the format and may not be
supported by all linkers.\I{section alignment, in OBJ}\I{segment
alignment, in OBJ}\I{alignment, in OBJ sections}

\b \i\c{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. \c{CLASS=CODE}.

\b \i\c{OVERLAY}, like \c{CLASS}, is specified with an arbitrary word
as an argument, and provides overlay information to an
overlay-capable linker.

\b Segments can be declared as \i\c{USE16} or \i\c{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.

\b When writing \i{OS/2} object files, you should declare 32-bit
segments as \i\c{FLAT}, which causes the default segment base for
anything in the segment to be the special group \c{FLAT}, and also
defines the group if it is not already defined.

\b The \c{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
\c{SEGMENT SCREEN ABSOLUTE=0xB800} if you need to. The \i\c{ABSOLUTE}
and \c{ALIGN} keywords are mutually exclusive.

NASM's default segment attributes are \c{PUBLIC}, \c{ALIGN=1}, no
class, no overlay, and \c{USE16}.


\S{group} \i\c{GROUP}: Defining Groups of Segments\I{segments, groups of}

The \c{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 \c{GROUP} directive, whereby
you can code

\c segment data
\c
\c         ; some data
\c
\c segment bss
\c
\c         ; some uninitialized data
\c
\c group dgroup data bss

which will define a group called \c{dgroup} to contain the segments
\c{data} and \c{bss}. Like \c{SEGMENT}, \c{GROUP} causes the group
name to be defined as a symbol, so that you can refer to a variable
\c{var} in the \c{data} segment as \c{var wrt data} or as \c{var wrt
dgroup}, depending on which segment value is currently in your
segment register.

If you just refer to \c{var}, however, and \c{var} is declared in a
segment which is part of a group, then NASM will default to giving
you the offset of \c{var} from the beginning of the \e{group}, not
the \e{segment}. Therefore \c{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
\c{WRT} references to a group which does not contain the variable
you are referring to. OS/2, for example, defines the special group
\c{FLAT} with no segments in it.


\S{uppercase} \i\c{UPPERCASE}: Disabling Case Sensitivity in Output

Although NASM itself is \i{case sensitive}, some OMF linkers are
not; therefore it can be useful for NASM to output single-case
object files. The \c{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.

\c{UPPERCASE} is used alone on a line; it requires no parameters.


\S{import} \i\c{IMPORT}: Importing DLL Symbols\I{DLL symbols,
importing}\I{symbols, importing from DLLs}

The \c{IMPORT} format-specific directive defines a symbol to be
imported from a DLL, for use if you are writing a DLL's \i{import
library} in NASM. You still need to declare the symbol as \c{EXTERN}
as well as using the \c{IMPORT} directive.

The \c{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:

\c     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:

\c     import  asyncsel wsock32.dll WSAAsyncSelect


\S{export} \i\c{EXPORT}: Exporting DLL Symbols\I{DLL symbols,
exporting}\I{symbols, exporting from DLLs}

The \c{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 \c{GLOBAL} as well as
using the \c{EXPORT} directive.

\c{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 \e{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:

\b \c{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.

\b \c{nodata} indicates that the exported symbol is a function which
does not make use of any initialized data.

\b \c{parm=NNN}, where \c{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.

\b 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:

\c     export  myfunc
\c     export  myfunc TheRealMoreFormalLookingFunctionName
\c     export  myfunc myfunc 1234  ; export by ordinal
\c     export  myfunc myfunc resident parm=23 nodata


\S{dotdotstart} \i\c{..start}: Defining the \i{Program Entry
Point}

\c{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 \c{..start} at the point where you wish execution to
begin.


\S{objextern} \c{obj} Extensions to the \c{EXTERN}
Directive\I{EXTERN, obj extensions to}

If you declare an external symbol with the directive

\c     extern  foo

then references such as \c{mov ax,foo} will give you the offset of
\c{foo} from its preferred segment base (as specified in whichever
module \c{foo} is actually defined in). So to access the contents of
\c{foo} you will usually need to do something like

\c         mov     ax,seg foo      ; get preferred segment base
\c         mov     es,ax           ; move it into ES
\c         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
\c{dgroup}. So if \c{DS} already contained \c{dgroup}, you could
simply code

\c         mov     ax,[foo wrt dgroup]

However, having to type this every time you want to access \c{foo}
can be a pain; so NASM allows you to declare \c{foo} in the
alternative form

\c     extern  foo:wrt dgroup

This form causes NASM to pretend that the preferred segment base of
\c{foo} is in fact \c{dgroup}; so the expression \c{seg foo} will
now return \c{dgroup}, and the expression \c{foo} is equivalent to
\c{foo wrt dgroup}.

This \I{default-WRT mechanism}default-\c{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
\k{objcommon}.


\S{objcommon} \c{obj} Extensions to the \c{COMMON}
Directive\I{COMMON, obj extensions to}

The \c{obj} format allows common variables to be either near\I{near
common variables} or far\I{far common variables}; NASM allows you to
specify which your variables should be by the use of the syntax

\c common  nearvar 2:near   ; `nearvar' is a near common
\c 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
\e{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 \c{OMF} linkers require the \I{element size, in common
variables}\I{common variables, element size}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:

\c common  c_5by2  10:far 5        ; two five-byte elements
\c common  c_2by5  10:far 2        ; five two-byte elements

If no element size is specified, the default is 1. Also, the \c{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

\c common  c_5by2  10:5            ; two five-byte elements
\c common  c_2by5  10:2            ; five two-byte elements

In addition to these extensions, the \c{COMMON} directive in \c{obj}
also supports default-\c{WRT} specification like \c{EXTERN} does
(explained in \k{objextern}). So you can also declare things like

\c common  foo     10:wrt dgroup
\c common  bar     16:far 2:wrt data
\c common  baz     24:wrt data:6


\H{win32fmt} \i\c{win32}: Microsoft Win32 Object Files

The \c{win32} output format generates Microsoft Win32 object files,
suitable for passing to Microsoft linkers such as \i{Visual C++}.
Note that Borland Win32 compilers do not use this format, but use
\c{obj} instead (see \k{objfmt}).

\c{win32} provides a default output file-name extension of \c{.obj}.

Note that although Microsoft say that Win32 object files follow the
\c{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 \c{coff} output
format; conversely, the \c{coff} format does not produce object
files that Win32 linkers can generate correct output from.


\S{win32sect} \c{win32} Extensions to the \c{SECTION}
Directive\I{SECTION, win32 extensions to}

Like the \c{obj} format, \c{win32} allows you to specify additional
information on the \c{SECTION} directive line, to control the type
and properties of sections you declare. Section types and properties
are generated automatically by NASM for the \i{standard section names}
\c{.text}, \c{.data} and \c{.bss}, but may still be overridden by
these qualifiers.

The available qualifiers are:

\b \c{code}, or equivalently \c{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.

\b \c{data} and \c{bss} define the section to be a data section,
analogously to \c{code}. Data sections are marked as readable and
writable, but not executable. \c{data} declares an initialized data
section, whereas \c{bss} declares an uninitialized data section.

\b \c{rdata} declares an initialized data section that is readable
but not writable. Microsoft compilers use this section to place
constants in it.

\b \c{info} defines the section to be an \i{informational section},
which is not included in the executable file by the linker, but may
(for example) pass information \e{to} the linker. For example,
declaring an \c{info}-type section called \i\c{.drectve} causes the
linker to interpret the contents of the section as command-line
options.

\b \c{align=}, used with a trailing number as in \c{obj}, gives the
\I{section alignment, in win32}\I{alignment, in win32
sections}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:

\c section .text    code  align=16
\c section .data    data  align=4
\c section .rdata   rdata align=8
\c section .bss     bss   align=4

Any other section name is treated by default like \c{.text}.


\H{win64fmt} \i\c{win64}: Microsoft Win64 Object Files

The \c{win64} output format generates Microsoft Win64 object files, 
which is nearly 100% indentical to the \c{win32} object format (\k{win32fmt})
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 \c{win32}
object format (\k{win32fmt}), in NASM, with regard to this exception.


\H{cofffmt} \i\c{coff}: \i{Common Object File Format}

The \c{coff} output type produces \c{COFF} object files suitable for
linking with the \i{DJGPP} linker.

\c{coff} provides a default output file-name extension of \c{.o}.

The \c{coff} format supports the same extensions to the \c{SECTION}
directive as \c{win32} does, except that the \c{align} qualifier and
the \c{info} section type are not supported.

\H{machofmt} \i\c{macho}: \i{Mach Object File Format}

The \c{macho} output type produces \c{Mach-O} object files suitable for
linking with the \i{Mac OSX} linker.

\c{macho} provides a default output file-name extension of \c{.o}.

\H{elffmt} \i\c{elf, elf32, and elf64}: \I{ELF}\I{linux, elf}\i{Executable and Linkable
Format} Object Files

The \c{elf32} and \c{elf64} output formats generate \c{ELF32 and ELF64} (Executable and Linkable Format) object files, as used by Linux as well as \i{Unix System V},
including \i{Solaris x86}, \i{UnixWare} and \i{SCO Unix}. \c{elf}
provides a default output file-name extension of \c{.o}. \c{elf} is a synonym for \c{elf32}.


\S{elfsect} \c{elf} Extensions to the \c{SECTION}
Directive\I{SECTION, elf extensions to}

Like the \c{obj} format, \c{elf} allows you to specify additional
information on the \c{SECTION} directive line, to control the type
and properties of sections you declare. Section types and properties
are generated automatically by NASM for the \i{standard section
names} \i\c{.text}, \i\c{.data} and \i\c{.bss}, but may still be
overridden by these qualifiers.

The available qualifiers are:

\b \i\c{alloc} defines the section to be one which is loaded into
memory when the program is run. \i\c{noalloc} defines it to be one
which is not, such as an informational or comment section.

\b \i\c{exec} defines the section to be one which should have execute
permission when the program is run. \i\c{noexec} defines it as one
which should not.

\b \i\c{write} defines the section to be one which should be writable
when the program is run. \i\c{nowrite} defines it as one which should
not.

\b \i\c{progbits} defines the section to be one with explicit contents
stored in the object file: an ordinary code or data section, for
example, \i\c{nobits} defines the section to be one with no explicit
contents given, such as a BSS section.

\b \c{align=}, used with a trailing number as in \c{obj}, gives the
\I{section alignment, in elf}\I{alignment, in elf sections}alignment
requirements of the section.

The defaults assumed by NASM if you do not specify the above
qualifiers are:

\c section .text    progbits  alloc  exec    nowrite  align=16
\c section .rodata  progbits  alloc  noexec  nowrite  align=4
\c section .data    progbits  alloc  noexec  write    align=4
\c section .bss     nobits    alloc  noexec  write    align=4
\c section other    progbits  alloc  noexec  nowrite  align=1

(Any section name other than \c{.text}, \c{.rodata}, \c{.data} and
\c{.bss} is treated by default like \c{other} in the above code.)


\S{elfwrt} \i{Position-Independent Code}\I{PIC}: \c{elf} Special
Symbols and \i\c{WRT}

The \c{ELF} specification contains enough features to allow
position-independent code (PIC) to be written, which makes \i{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 \c{ELF} does not support segment-base references, the \c{WRT}
operator is not used for its normal purpose; therefore NASM's
\c{elf} output format makes use of \c{WRT} for a different purpose,
namely the PIC-specific \I{relocations, PIC-specific}relocation
types.

\c{elf} defines five special symbols which you can use as the
right-hand side of the \c{WRT} operator to obtain PIC relocation
types. They are \i\c{..gotpc}, \i\c{..gotoff}, \i\c{..got},
\i\c{..plt} and \i\c{..sym}. Their functions are summarized here:

\b Referring to the symbol marking the global offset table base
using \c{wrt ..gotpc} will end up giving the distance from the
beginning of the current section to the global offset table.
(\i\c{_GLOBAL_OFFSET_TABLE_} is the standard symbol name used to
refer to the \i{GOT}.) So you would then need to add \i\c{$$} to the
result to get the real address of the GOT.

\b Referring to a location in one of your own sections using \c{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.

\b Referring to an external or global symbol using \c{wrt ..got}
causes the linker to build an entry \e{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.

\b Referring to a procedure name using \c{wrt ..plt} causes the
linker to build a \i{procedure linkage table} entry for the symbol,
and the reference gives the address of the \i{PLT} entry. You can
only use this in contexts which would generate a PC-relative
relocation normally (i.e. as the destination for \c{CALL} or
\c{JMP}), since ELF contains no relocation type to refer to PLT
entries absolutely.

\b Referring to a symbol name using \c{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 \k{picdll}.


\S{elfglob} \c{elf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL,
elf extensions to}\I{GLOBAL, aoutb extensions to}

\c{ELF} object files can contain more information about a global symbol
than just its address: they can contain the \I{symbol sizes,
specifying}\I{size, of symbols}size of the symbol and its \I{symbol
types, specifying}\I{type, of symbols}type as well. These are not
merely debugger conveniences, but are actually necessary when the
program being written is a \i{shared library}. NASM therefore
supports some extensions to the \c{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
\i\c{function} or \i\c{data}. (\i\c{object} is a synonym for
\c{data}.) For example:

\c global   hashlookup:function, hashtable:data

exports the global symbol \c{hashlookup} as a function and
\c{hashtable} as a data object.

Optionally, you can control the ELF visibility of the symbol.  Just
add one of the visibility keywords: \i\c{default}, \i\c{internal},
\i\c{hidden}, or \i\c{protected}.  The default is \i\c{default} of
course.  For example, to make \c{hashlookup} hidden:

\c 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:

\c global  hashtable:data (hashtable.end - hashtable)
\c
\c hashtable:
\c         db this,that,theother  ; some data here
\c .end:

This makes NASM automatically calculate the length of the table and
place that information into the \c{ELF} symbol table.

Declaring the type and size of global symbols is necessary when
writing shared library code. For more information, see
\k{picglobal}.


\S{elfcomm} \c{elf} Extensions to the \c{COMMON} Directive
\I{COMMON, elf extensions to}

\c{ELF} also allows you to specify alignment requirements \I{common
variables, alignment in elf}\I{alignment, of elf common variables}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:

\c 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.


\S{elf16} 16-bit code and ELF
\I{ELF, 16-bit code and}

The \c{ELF32} specification doesn't provide relocations for 8- and
16-bit values, but the GNU \c{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 \c{ld}. If NASM is used with the
\c{-w+gnu-elf-extensions} option, a warning is issued when one of
these relocations is generated.

\H{aoutfmt} \i\c{aout}: Linux \I{a.out, Linux version}\I{linux, a.out}\c{a.out} Object Files

The \c{aout} format generates \c{a.out} object files, in the form used
by early Linux systems (current Linux systems use ELF, see
\k{elffmt}.) These differ from other \c{a.out} object files in that
the magic number in the first four bytes of the file is
different; also, some implementations of \c{a.out}, for example
NetBSD's, support position-independent code, which Linux's
implementation does not.

\c{a.out} provides a default output file-name extension of \c{.o}.

\c{a.out} is a very simple object format. It supports no special
directives, no special symbols, no use of \c{SEG} or \c{WRT}, and no
extensions to any standard directives. It supports only the three
\i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}.


\H{aoutfmt} \i\c{aoutb}: \i{NetBSD}/\i{FreeBSD}/\i{OpenBSD}
\I{a.out, BSD version}\c{a.out} Object Files

The \c{aoutb} format generates \c{a.out} object files, in the form
used by the various free \c{BSD Unix} clones, \c{NetBSD}, \c{FreeBSD}
and \c{OpenBSD}. For simple object files, this object format is exactly
the same as \c{aout} except for the magic number in the first four bytes
of the file. However, the \c{aoutb} format supports
\I{PIC}\i{position-independent code} in the same way as the \c{elf}
format, so you can use it to write \c{BSD} \i{shared libraries}.

\c{aoutb} provides a default output file-name extension of \c{.o}.

\c{aoutb} supports no special directives, no special symbols, and
only the three \i{standard section names} \i\c{.text}, \i\c{.data}
and \i\c{.bss}. However, it also supports the same use of \i\c{WRT} as
\c{elf} does, to provide position-independent code relocation types.
See \k{elfwrt} for full documentation of this feature.

\c{aoutb} also supports the same extensions to the \c{GLOBAL}
directive as \c{elf} does: see \k{elfglob} for documentation of
this.


\H{as86fmt} \c{as86}: \i{Minix}/Linux\I{linux, as86} \i\c{as86} Object Files

The Minix/Linux 16-bit assembler \c{as86} has its own non-standard
object file format. Although its companion linker \i\c{ld86} produces
something close to ordinary \c{a.out} binaries as output, the object
file format used to communicate between \c{as86} and \c{ld86} is not
itself \c{a.out}.

NASM supports this format, just in case it is useful, as \c{as86}.
\c{as86} provides a default output file-name extension of \c{.o}.

\c{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 \c{SEG} or \c{WRT}, and no extensions to any standard
directives. It supports only the three \i{standard section names}
\i\c{.text}, \i\c{.data} and \i\c{.bss}.


\H{rdffmt} \I{RDOFF}\i\c{rdf}: \i{Relocatable Dynamic Object File
Format}

The \c{rdf} output format produces \c{RDOFF} object files. \c{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.

\c{RDOFF} is not used by any well-known operating systems. Those
writing their own systems, however, may well wish to use \c{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 \I{rdoff subdirectory}\c{rdoff} subdirectory holding
a set of RDOFF utilities: an RDF linker, an \c{RDF} static-library
manager, an RDF file dump utility, and a program which will load and
execute an RDF executable under Linux.

\c{rdf} supports only the \i{standard section names} \i\c{.text},
\i\c{.data} and \i\c{.bss}.


\S{rdflib} Requiring a Library: The \i\c{LIBRARY} Directive

\c{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 \c{LIBRARY} directive, which takes one argument
which is the name of the module:

\c     library  mylib.rdl


\S{rdfmod} Specifying a Module Name: The \i\c{MODULE} Directive

Special \c{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. \c{MODULE} directive takes one argument which is the name
of current module:

\c     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 \I{$, prefix}\c{$}, like:

\c     module  $kernel.core


\S{rdfglob} \c{rdf} Extensions to the \c{GLOBAL} directive\I{GLOBAL,
rdf extensions to}

\c{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 \c{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 \i\c{export} you make the
symbol exported:

\c     global  sys_open:export

To specify that exported symbol is a procedure (function), you add the
word \i\c{proc} or \i\c{function} after declaration:

\c     global  sys_open:export proc

Similarly, to specify exported data object, add the word \i\c{data}
or \i\c{object} to the directive:

\c     global  kernel_ticks:export data


\S{rdfimpt} \c{rdf} Extensions to the \c{EXTERN} directive\I{EXTERN,
rdf extensions to}

By default the \c{EXTERN} directive in \c{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, \c{RDOFF} offers an additional \c{import} modifier. As in
\c{GLOBAL}, you can also specify whether an imported symbol is a procedure
(function) or data object. For example:

\c     library $libc
\c     extern  _open:import
\c     extern  _printf:import proc
\c     extern  _errno:import data

Here the directive \c{LIBRARY} is also included, which gives the dynamic linker
a hint as to where to find requested symbols.


\H{dbgfmt} \i\c{dbg}: Debugging Format

The \c{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 \i\c{OF_DBG} in \c{outform.h} or on the
compiler command line, and obtain the \c{dbg} output format.

The \c{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 \c{dbg} format like this:

\c nasm -f dbg filename.asm

which will generate a diagnostic file called \c{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 \c{dbg} format. Therefore it can be
useful to run NASM twice, in order to do the preprocessing with the
native object format selected:

\c nasm -e -f rdf -o rdfprog.i rdfprog.asm
\c nasm -a -f dbg rdfprog.i

This preprocesses \c{rdfprog.asm} into \c{rdfprog.i}, keeping the
\c{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 \c{dbg} format to generate
the final diagnostic output.

This workaround will still typically not work for programs intended
for \c{obj} format, because the \c{obj} \c{SEGMENT} and \c{GROUP}
directives have side effects of defining the segment and group names
as symbols; \c{dbg} will not do this, so the program will not
assemble. You will have to work around that by defining the symbols
yourself (using \c{EXTERN}, for example) if you really need to get a
\c{dbg} trace of an \c{obj}-specific source file.

\c{dbg} accepts any section name and any directives at all, and logs
them all to its output file.


\C{16bit} 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 \c{MS-DOS} or \c{Windows 3.x}. It
covers how to link programs to produce \c{.EXE} or \c{.COM} files,
how to write \c{.SYS} device drivers, and how to interface assembly
language code with 16-bit C compilers and with Borland Pascal.


\H{exefiles} Producing \i\c{.EXE} Files

Any large program written under DOS needs to be built as a \c{.EXE}
file: only \c{.EXE} files have the necessary internal structure
required to span more than one 64K segment. \i{Windows} programs,
also, have to be built as \c{.EXE} files, since Windows does not
support the \c{.COM} format.

In general, you generate \c{.EXE} files by using the \c{obj} output
format to produce one or more \i\c{.OBJ} files, and then linking
them together using a linker. However, NASM also supports the direct
generation of simple DOS \c{.EXE} files using the \c{bin} output
format (by using \c{DB} and \c{DW} to construct the \c{.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 \c{.EXE} natively as another output format in
future releases.


\S{objexe} Using the \c{obj} Format To Generate \c{.EXE} Files

This section describes the usual method of generating \c{.EXE} files
by linking \c{.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
\i{VAL}\I{linker, free}, available in \c{LZH} archive format from
\W{ftp://x2ftp.oulu.fi/pub/msdos/programming/lang/}\i\c{x2ftp.oulu.fi}.
An LZH archiver can be found at
\W{ftp://ftp.simtel.net/pub/simtelnet/msdos/arcers}\i\c{ftp.simtel.net}.
There is another `free' linker (though this one doesn't come with
sources) called \i{FREELINK}, available from
\W{http://www.pcorner.com/tpc/old/3-101.html}\i\c{www.pcorner.com}.
A third, \i\c{djlink}, written by DJ Delorie, is available at
\W{http://www.delorie.com/djgpp/16bit/djlink/}\i\c{www.delorie.com}.
A fourth linker, \i\c{ALINK}, written by Anthony A.J. Williams, is
available at \W{http://alink.sourceforge.net}\i\c{alink.sourceforge.net}.

When linking several \c{.OBJ} files into a \c{.EXE} file, you should
ensure that exactly one of them has a start point defined (using the
\I{program entry point}\i\c{..start} special symbol defined by the
\c{obj} format: see \k{dotdotstart}). 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 \e{which} value to use.

An example of a NASM source file which can be assembled to a
\c{.OBJ} file and linked on its own to a \c{.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 \I{test subdirectory}\c{test} subdirectory of
the NASM archives, under the name \c{objexe.asm}.

\c segment code
\c
\c ..start:
\c         mov     ax,data
\c         mov     ds,ax
\c         mov     ax,stack
\c         mov     ss,ax
\c         mov     sp,stacktop

This initial piece of code sets up \c{DS} to point to the data
segment, and initializes \c{SS} and \c{SP} to point to the top of
the provided stack. Notice that interrupts are implicitly disabled
for one instruction after a move into \c{SS}, precisely for this
situation, so that there's no chance of an interrupt occurring
between the loads of \c{SS} and \c{SP} and not having a stack to
execute on.

Note also that the special symbol \c{..start} is defined at the
beginning of this code, which means that will be the entry point
into the resulting executable file.

\c         mov     dx,hello
\c         mov     ah,9
\c         int     0x21

The above is the main program: load \c{DS:DX} with a pointer to the
greeting message (\c{hello} is implicitly relative to the segment
\c{data}, which was loaded into \c{DS} in the setup code, so the
full pointer is valid), and call the DOS print-string function.

\c         mov     ax,0x4c00
\c         int     0x21

This terminates the program using another DOS system call.

\c segment data
\c
\c hello:  db      'hello, world', 13, 10, '$'

The data segment contains the string we want to display.

\c segment stack stack
\c         resb 64
\c stacktop:

The above code declares a stack segment containing 64 bytes of
uninitialized stack space, and points \c{stacktop} at the top of it.
The directive \c{segment stack stack} defines a segment \e{called}
\c{stack}, and also of \e{type} \c{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 \c{STACK}.

The above file, when assembled into a \c{.OBJ} file, will link on
its own to a valid \c{.EXE} file, which when run will print `hello,
world' and then exit.


\S{binexe} Using the \c{bin} Format To Generate \c{.EXE} Files

The \c{.EXE} file format is simple enough that it's possible to
build a \c{.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 \c{DB} and \c{DW} commands by NASM itself, so
that you can use the \c{bin} output format to directly generate
\c{.EXE} files.

Included in the NASM archives, in the \I{misc subdirectory}\c{misc}
subdirectory, is a file \i\c{exebin.mac} of macros. It defines three
macros: \i\c{EXE_begin}, \i\c{EXE_stack} and \i\c{EXE_end}.

To produce a \c{.EXE} file using this method, you should start by
using \c{%include} to load the \c{exebin.mac} macro package into
your source file. You should then issue the \c{EXE_begin} macro call
(which takes no arguments) to generate the file header data. Then
write code as normal for the \c{bin} format - you can use all three
standard sections \c{.text}, \c{.data} and \c{.bss}. At the end of
the file you should call the \c{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 \c{EXE_begin}.

In this model, the code you end up writing starts at \c{0x100}, just
like a \c{.COM} file - in fact, if you strip off the 32-byte header
from the resulting \c{.EXE} file, you will have a valid \c{.COM}
program. All the segment bases are the same, so you are limited to a
64K program, again just like a \c{.COM} file. Note that an \c{ORG}
directive is issued by the \c{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 \c{CS} instead.

On entry to your \c{.EXE} file, \c{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 \c{EXE_stack} macro. For example, to
change the stack size of your program to 64 bytes, you would call
\c{EXE_stack 64}.

A sample program which generates a \c{.EXE} file in this way is
given in the \c{test} subdirectory of the NASM archive, as
\c{binexe.asm}.


\H{comfiles} Producing \i\c{.COM} Files

While large DOS programs must be written as \c{.EXE} files, small
ones are often better written as \c{.COM} files. \c{.COM} files are
pure binary, and therefore most easily produced using the \c{bin}
output format.


\S{combinfmt} Using the \c{bin} Format To Generate \c{.COM} Files

\c{.COM} files expect to be loaded at offset \c{100h} into their
segment (though the segment may change). Execution then begins at
\I\c{ORG}\c{100h}, i.e. right at the start of the program. So to
write a \c{.COM} program, you would create a source file looking
like

\c         org 100h
\c
\c section .text
\c
\c start:
\c         ; put your code here
\c
\c section .data
\c
\c         ; put data items here
\c
\c section .bss
\c
\c         ; put uninitialized data here

The \c{bin} format puts the \c{.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
\c{.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

\c nasm myprog.asm -fbin -o myprog.com

The \c{bin} format would produce a file called \c{myprog} if no
explicit output file name were specified, so you have to override it
and give the desired file name.


\S{comobjfmt} Using the \c{obj} Format To Generate \c{.COM} Files

If you are writing a \c{.COM} program as more than one module, you
may wish to assemble several \c{.OBJ} files and link them together
into a \c{.COM} program. You can do this, provided you have a linker
capable of outputting \c{.COM} files directly (\i{TLINK} does this),
or alternatively a converter program such as \i\c{EXE2BIN} to
transform the \c{.EXE} file output from the linker into a \c{.COM}
file.

If you do this, you need to take care of several things:

\b The first object file containing code should start its code
segment with a line like \c{RESB 100h}. This is to ensure that the
code begins at offset \c{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
\c{.COM} file. Other assemblers use an \i\c{ORG} directive for this
purpose, but \c{ORG} in NASM is a format-specific directive to the
\c{bin} output format, and does not mean the same thing as it does
in MASM-compatible assemblers.

\b You don't need to define a stack segment.

\b 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 \c{.COM}
file is loaded, all the segment registers contain the same value.


\H{sysfiles} Producing \i\c{.SYS} Files

\i{MS-DOS device drivers} - \c{.SYS} files - are pure binary files,
similar to \c{.COM} files, except that they start at origin zero
rather than \c{100h}. Therefore, if you are writing a device driver
using the \c{bin} format, you do not need the \c{ORG} directive,
since the default origin for \c{bin} is zero. Similarly, if you are
using \c{obj}, you do not need the \c{RESB 100h} at the start of
your code segment.

\c{.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 \c{.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
\W{news:comp.os.msdos.programmer}\i\c{comp.os.msdos.programmer}.


\H{16c} 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 \c{.OBJ} file, and link it
with your C modules to produce a \i{mixed-language program}.


\S{16cunder} External Symbol Names

\I{C symbol names}\I{underscore, in C symbols}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 \c{printf} appears to an assembly language
programmer as \c{_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 \c{GLOBAL} and \c{EXTERN} directives as follows:

\c %macro  cglobal 1
\c
\c   global  _%1
\c   %define %1 _%1
\c
\c %endmacro
\c
\c %macro  cextern 1
\c
\c   extern  _%1
\c   %define %1 _%1
\c
\c %endmacro

(These forms of the macros only take one argument at a time; a
\c{%rep} construct could solve this.)

If you then declare an external like this:

\c cextern printf

then the macro will expand it as

\c extern  _printf
\c %define printf _printf

Thereafter, you can reference \c{printf} as if it was a symbol, and
the preprocessor will put the leading underscore on where necessary.

The \c{cglobal} macro works similarly. You must use \c{cglobal}
before defining the symbol in question, but you would have had to do
that anyway if you used \c{GLOBAL}.

Also see \k{opt-pfix}.

\S{16cmodels} \i{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:

\b 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 \c{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 \c{CALL} instructions and return using \c{RETN} (which, in
NASM, is synonymous with \c{RET} anyway). This means both that you
should write your own routines to return with \c{RETN}, and that you
should call external C routines with near \c{CALL} instructions.

\b 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 \c{CALL FAR} (or
\c{CALL seg:offset}) and return using \c{RETF}. Again, you should
therefore write your own routines to return with \c{RETF} and use
\c{CALL FAR} to call external routines.

\b In models using a single data segment (tiny, small and medium),
data pointers are 16 bits long, containing only an offset field (the
\c{DS} register doesn't change its value, and always gives the
segment part of the full data item address).

\b 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 \c{DS} in your routines without restoring it afterwards, but
\c{ES} is free for you to use to access the contents of 32-bit data
pointers you are passed.

\b 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.

\b In most memory models, there is a \e{default} data segment, whose
segment address is kept in \c{DS} throughout the program. This data
segment is typically the same segment as the stack, kept in \c{SS},
so that functions' local variables (which are stored on the stack)
and global data items can both be accessed easily without changing
\c{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 \c{SS} and \c{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
\i\c{_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 \i\c{_DATA}.


\S{16cfunc} Function Definitions and Function Calls

\I{functions, C calling convention}The \i{C calling convention} in
16-bit programs is as follows. In the following description, the
words \e{caller} and \e{callee} are used to denote the function
doing the calling and the function which gets called.

\b 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).

\b The caller then executes a \c{CALL} instruction to pass control
to the callee. This \c{CALL} is either near or far depending on the
memory model.

\b 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 \c{SP} in \c{BP} so as to
be able to use \c{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 \c{BP} must be preserved by
any C function. Hence the callee, if it is going to set up \c{BP} as
a \i\e{frame pointer}, must push the previous value first.

\b The callee may then access its parameters relative to \c{BP}.
The word at \c{[BP]} holds the previous value of \c{BP} as it was
pushed; the next word, at \c{[BP+2]}, holds the offset part of the
return address, pushed implicitly by \c{CALL}. In a small-model
(near) function, the parameters start after that, at \c{[BP+4]}; in
a large-model (far) function, the segment part of the return address
lives at \c{[BP+4]}, and the parameters begin at \c{[BP+6]}. The
leftmost parameter of the function, since it was pushed last, is
accessible at this offset from \c{BP}; the others follow, at
successively greater offsets. Thus, in a function such as \c{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.

\b The callee may also wish to decrease \c{SP} further, so as to
allocate space on the stack for local variables, which will then be
accessible at negative offsets from \c{BP}.

\b The callee, if it wishes to return a value to the caller, should
leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size
of the value. Floating-point results are sometimes (depending on the
compiler) returned in \c{ST0}.

\b Once the callee has finished processing, it restores \c{SP} from
\c{BP} if it had allocated local stack space, then pops the previous
value of \c{BP}, and returns via \c{RETN} or \c{RETF} depending on
memory model.

\b When the caller regains control from the callee, the function
parameters are still on the stack, so it typically adds an immediate
constant to \c{SP} to remove them (instead of executing a number of
slow \c{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 \e{knows} how many parameters it pushed, does the
removing.

It is instructive to compare this calling convention with that for
Pascal programs (described in \k{16bpfunc}). 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 \c{RET} or \c{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:

\c global  _myfunc
\c
\c _myfunc:
\c         push    bp
\c         mov     bp,sp
\c         sub     sp,0x40         ; 64 bytes of local stack space
\c         mov     bx,[bp+4]       ; first parameter to function
\c
\c         ; some more code
\c
\c         mov     sp,bp           ; undo "sub sp,0x40" above
\c         pop     bp
\c         ret

For a large-model function, you would replace \c{RET} by \c{RETF},
and look for the first parameter at \c{[BP+6]} instead of
\c{[BP+4]}. Of course, if one of the parameters is a pointer, then
the offsets of \e{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:

\c extern  _printf
\c
\c       ; and then, further down...
\c
\c       push    word [myint]        ; one of my integer variables
\c       push    word mystring       ; pointer into my data segment
\c       call    _printf
\c       add     sp,byte 4           ; `byte' saves space
\c
\c       ; then those data items...
\c
\c segment _DATA
\c
\c myint         dw    1234
\c mystring      db    'This number -> %d <- should be 1234',10,0

This piece of code is the small-model assembly equivalent of the C
code

\c     int myint = 1234;
\c     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 \c{DS} already holds the segment
base of the segment \c{_DATA}. If not, you would have to initialize
it first.

\c       push    word [myint]
\c       push    word seg mystring   ; Now push the segment, and...
\c       push    word mystring       ; ... offset of "mystring"
\c       call    far _printf
\c       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 \c{int} data type. The first
argument (pushed last) to \c{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, \c{PUSH DS} would have been a shorter instruction
than \c{PUSH WORD SEG mystring}, if \c{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 \c{SP} has to be
increased by 6 rather than 4 afterwards to make up for the extra
word of parameters.


\S{16cdata} 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 \c{GLOBAL} or
\c{EXTERN}. (Again, the names require leading underscores, as stated
in \k{16cunder}.) Thus, a C variable declared as \c{int i} can be
accessed from assembler as

\c extern _i
\c
\c         mov ax,[_i]

And to declare your own integer variable which C programs can access
as \c{extern int j}, you do this (making sure you are assembling in
the \c{_DATA} segment, if necessary):

\c global  _j
\c
\c _j      dw      0

To access a C array, you need to know the size of the components of
the array. For example, \c{int} variables are two bytes long, so if
a C program declares an array as \c{int a[10]}, you can access
\c{a[3]} by coding \c{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 \c{char}, 2 for \c{short} and \c{int}, 4 for \c{long} and
\c{float}, and 8 for \c{double}.

To access a C \i{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 \i\c{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 \c{STRUC} macro, so you
have to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like

\c struct {
\c     char c;
\c     int i;
\c } foo;

might be four bytes long rather than three, since the \c{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 \c{#pragma} lines, so you have to find
out how your own compiler does it.


\S{16cmacro} \i\c{c16.mac}: Helper Macros for the 16-bit C Interface

Included in the NASM archives, in the \I{misc subdirectory}\c{misc}
directory, is a file \c{c16.mac} of macros. It defines three macros:
\i\c{proc}, \i\c{arg} and \i\c{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 \c{arg} is also now built
into NASM's preprocessor. See \k{tasmcompat} for details.)

An example of an assembly function using the macro set is given
here:

\c proc    _nearproc
\c
\c %$i     arg
\c %$j     arg
\c         mov     ax,[bp + %$i]
\c         mov     bx,[bp + %$j]
\c         add     ax,[bx]
\c
\c endproc

This defines \c{_nearproc} to be a procedure taking two arguments,
the first (\c{i}) an integer and the second (\c{j}) a pointer to an
integer. It returns \c{i + *j}.

Note that the \c{arg} macro has an \c{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 \c{EQU} works, defining
\c{%$i} to be an offset from \c{BP}. A context-local variable is
used, local to the context pushed by the \c{proc} macro and popped
by the \c{endproc} macro, so that the same argument name can be used
in later procedures. Of course, you don't \e{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
\I\c{FARCODE}\c{%define FARCODE}. This changes the kind of return
instruction generated by \c{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.

\c{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 \c{int}.

The large-model equivalent of the above function would look like this:

\c %define FARCODE
\c
\c proc    _farproc
\c
\c %$i     arg
\c %$j     arg     4
\c         mov     ax,[bp + %$i]
\c         mov     bx,[bp + %$j]
\c         mov     es,[bp + %$j + 2]
\c         add     ax,[bx]
\c
\c endproc

This makes use of the argument to the \c{arg} macro to define a
parameter of size 4, because \c{j} is now a far pointer. When we
load from \c{j}, we must load a segment and an offset.


\H{16bp} Interfacing to \i{Borland Pascal} Programs

Interfacing to Borland Pascal programs is similar in concept to
interfacing to 16-bit C programs. The differences are:

\b The leading underscore required for interfacing to C programs is
not required for Pascal.

\b 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 \c{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 \e{pointers}, however, are far.

\b The function calling convention is different - described below.

\b Some data types, such as strings, are stored differently.

\b 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.


\S{16bpfunc} The Pascal Calling Convention

\I{functions, Pascal calling convention}\I{Pascal calling
convention}The 16-bit Pascal calling convention is as follows. In
the following description, the words \e{caller} and \e{callee} are
used to denote the function doing the calling and the function which
gets called.

\b 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).

\b The caller then executes a far \c{CALL} instruction to pass
control to the callee.

\b 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 \c{SP} in \c{BP} so as to
be able to use \c{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 \c{BP} must be preserved by
any function. Hence the callee, if it is going to set up \c{BP} as a
\i{frame pointer}, must push the previous value first.

\b The callee may then access its parameters relative to \c{BP}.
The word at \c{[BP]} holds the previous value of \c{BP} as it was
pushed. The next word, at \c{[BP+2]}, holds the offset part of the
return address, and the next one at \c{[BP+4]} the segment part. The
parameters begin at \c{[BP+6]}. The rightmost parameter of the
function, since it was pushed last, is accessible at this offset
from \c{BP}; the others follow, at successively greater offsets.

\b The callee may also wish to decrease \c{SP} further, so as to
allocate space on the stack for local variables, which will then be
accessible at negative offsets from \c{BP}.

\b The callee, if it wishes to return a value to the caller, should
leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size
of the value. Floating-point results are returned in \c{ST0}.
Results of type \c{Real} (Borland's own custom floating-point data
type, not handled directly by the FPU) are returned in \c{DX:BX:AX}.
To return a result of type \c{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
\c{RETF} instruction.

\b Once the callee has finished processing, it restores \c{SP} from
\c{BP} if it had allocated local stack space, then pops the previous
value of \c{BP}, and returns via \c{RETF}. It uses the form of
\c{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.

\b 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
\c{Integer}-type parameters, in the following way:

\c global  myfunc
\c
\c myfunc: push    bp
\c         mov     bp,sp
\c         sub     sp,0x40         ; 64 bytes of local stack space
\c         mov     bx,[bp+8]       ; first parameter to function
\c         mov     bx,[bp+6]       ; second parameter to function
\c
\c         ; some more code
\c
\c         mov     sp,bp           ; undo "sub sp,0x40" above
\c         pop     bp
\c         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:

\c extern  SomeFunc
\c
\c        ; and then, further down...
\c
\c        push   word seg mystring   ; Now push the segment, and...
\c        push   word mystring       ; ... offset of "mystring"
\c        push   word [myint]        ; one of my variables
\c        call   far SomeFunc

This is equivalent to the Pascal code

\c procedure SomeFunc(String: PChar; Int: Integer);
\c     SomeFunc(@mystring, myint);


\S{16bpseg} Borland Pascal \I{segment names, Borland Pascal}Segment
Name Restrictions

Since Borland Pascal's internal unit file format is completely
different from \c{OBJ}, it only makes a very sketchy job of actually
reading and understanding the various information contained in a
real \c{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:

\b Procedures and functions must be in a segment whose name is
either \c{CODE}, \c{CSEG}, or something ending in \c{_TEXT}.

\b initialized data must be in a segment whose name is either
\c{CONST} or something ending in \c{_DATA}.

\b Uninitialized data must be in a segment whose name is either
\c{DATA}, \c{DSEG}, or something ending in \c{_BSS}.

\b Any other segments in the object file are completely ignored.
\c{GROUP} directives and segment attributes are also ignored.


\S{16bpmacro} Using \i\c{c16.mac} With Pascal Programs

The \c{c16.mac} macro package, described in \k{16cmacro}, can also
be used to simplify writing functions to be called from Pascal
programs, if you code \I\c{PASCAL}\c{%define PASCAL}. This
definition ensures that functions are far (it implies
\i\c{FARCODE}), and also causes procedure return instructions to be
generated with an operand.

Defining \c{PASCAL} does not change the code which calculates the
argument offsets; you must declare your function's arguments in
reverse order. For example:

\c %define PASCAL
\c
\c proc    _pascalproc
\c
\c %$j     arg 4
\c %$i     arg
\c         mov     ax,[bp + %$i]
\c         mov     bx,[bp + %$j]
\c         mov     es,[bp + %$j + 2]
\c         add     ax,[bx]
\c
\c endproc

This defines the same routine, conceptually, as the example in
\k{16cmacro}: 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 \c{PASCAL} is defined
instead of \c{FARCODE}, and that the arguments are declared in
reverse order.


\C{32bit} 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 \i{Win32} or Unix, or to be
linked with C code generated by a Unix-style C compiler such as
\i{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
\c{Win32}, \c{DJGPP} or any of the PC Unix variants, runs in \I{flat
memory model}\e{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 \c{CALL} and \c{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.


\H{32c} Interfacing to 32-bit C Programs

A lot of the discussion in \k{16c}, 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.


\S{32cunder} 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 \c{ELF}
specification states that C symbols do \e{not} have a leading
underscore on their assembly-language names.

The older Linux \c{a.out} C compiler, all \c{Win32} compilers,
\c{DJGPP}, and \c{NetBSD} and \c{FreeBSD}, all use the leading
underscore; for these compilers, the macros \c{cextern} and
\c{cglobal}, as given in \k{16cunder}, will still work. For \c{ELF},
though, the leading underscore should not be used.

See also \k{opt-pfix}.

\S{32cfunc} Function Definitions and Function Calls

\I{functions, C calling convention}The \i{C calling convention}The C
calling convention in 32-bit programs is as follows. In the
following description, the words \e{caller} and \e{callee} are used
to denote the function doing the calling and the function which gets
called.

\b 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).

\b The caller then executes a near \c{CALL} instruction to pass
control to the callee.

\b 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 \c{ESP} in \c{EBP} so as
to be able to use \c{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 \c{EBP} must be preserved
by any C function. Hence the callee, if it is going to set up
\c{EBP} as a \i{frame pointer}, must push the previous value first.

\b The callee may then access its parameters relative to \c{EBP}.
The doubleword at \c{[EBP]} holds the previous value of \c{EBP} as
it was pushed; the next doubleword, at \c{[EBP+4]}, holds the return
address, pushed implicitly by \c{CALL}. The parameters start after
that, at \c{[EBP+8]}. The leftmost parameter of the function, since
it was pushed last, is accessible at this offset from \c{EBP}; the
others follow, at successively greater offsets. Thus, in a function
such as \c{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.

\b The callee may also wish to decrease \c{ESP} further, so as to
allocate space on the stack for local variables, which will then be
accessible at negative offsets from \c{EBP}.

\b The callee, if it wishes to return a value to the caller, should
leave the value in \c{AL}, \c{AX} or \c{EAX} depending on the size
of the value. Floating-point results are typically returned in
\c{ST0}.

\b Once the callee has finished processing, it restores \c{ESP} from
\c{EBP} if it had allocated local stack space, then pops the previous
value of \c{EBP}, and returns via \c{RET} (equivalently, \c{RETN}).

\b When the caller regains control from the callee, the function
parameters are still on the stack, so it typically adds an immediate
constant to \c{ESP} to remove them (instead of executing a number of
slow \c{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 \e{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 \e{by} the
Windows API such as window procedures: they follow what Microsoft
calls the \c{__stdcall} convention. This is slightly closer to the
Pascal convention, in that the callee clears the stack by passing a
parameter to the \c{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:

\c global  _myfunc
\c
\c _myfunc:
\c         push    ebp
\c         mov     ebp,esp
\c         sub     esp,0x40        ; 64 bytes of local stack space
\c         mov     ebx,[ebp+8]     ; first parameter to function
\c
\c         ; some more code
\c
\c         leave                   ; mov esp,ebp / pop ebp
\c         ret

At the other end of the process, to call a C function from your
assembly code, you would do something like this:

\c extern  _printf
\c
\c         ; and then, further down...
\c
\c         push    dword [myint]   ; one of my integer variables
\c         push    dword mystring  ; pointer into my data segment
\c         call    _printf
\c         add     esp,byte 8      ; `byte' saves space
\c
\c         ; then those data items...
\c
\c segment _DATA
\c
\c myint       dd   1234
\c mystring    db   'This number -> %d <- should be 1234',10,0

This piece of code is the assembly equivalent of the C code

\c     int myint = 1234;
\c     printf("This number -> %d <- should be 1234\n", myint);


\S{32cdata} 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 \c{GLOBAL} or
\c{EXTERN}. (Again, the names require leading underscores, as stated
in \k{32cunder}.) Thus, a C variable declared as \c{int i} can be
accessed from assembler as

\c           extern _i
\c           mov eax,[_i]

And to declare your own integer variable which C programs can access
as \c{extern int j}, you do this (making sure you are assembling in
the \c{_DATA} segment, if necessary):

\c           global _j
\c _j        dd 0

To access a C array, you need to know the size of the components of
the array. For example, \c{int} variables are four bytes long, so if
a C program declares an array as \c{int a[10]}, you can access
\c{a[3]} by coding \c{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 \c{char}, 2 for \c{short}, 4 for \c{int}, \c{long} and
\c{float}, and 8 for \c{double}. Pointers, being 32-bit addresses,
are also 4 bytes long.

To access a C \i{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 \c{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 \i\c{STRUC} macro, so you
have to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like

\c struct {
\c     char c;
\c     int i;
\c } foo;

might be eight bytes long rather than five, since the \c{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 \c{#pragma} lines, so you have to find
out how your own compiler does it.


\S{32cmacro} \i\c{c32.mac}: Helper Macros for the 32-bit C Interface

Included in the NASM archives, in the \I{misc directory}\c{misc}
directory, is a file \c{c32.mac} of macros. It defines three macros:
\i\c{proc}, \i\c{arg} and \i\c{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:

\c proc    _proc32
\c
\c %$i     arg
\c %$j     arg
\c         mov     eax,[ebp + %$i]
\c         mov     ebx,[ebp + %$j]
\c         add     eax,[ebx]
\c
\c endproc

This defines \c{_proc32} to be a procedure taking two arguments, the
first (\c{i}) an integer and the second (\c{j}) a pointer to an
integer. It returns \c{i + *j}.

Note that the \c{arg} macro has an \c{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 \c{EQU} works, defining
\c{%$i} to be an offset from \c{BP}. A context-local variable is
used, local to the context pushed by the \c{proc} macro and popped
by the \c{endproc} macro, so that the same argument name can be used
in later procedures. Of course, you don't \e{have} to do that.

\c{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 \c{int} or pointers.


\H{picdll} Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF \i{Shared
Libraries}

\c{ELF} replaced the older \c{a.out} object file format under Linux
because it contains support for \i{position-independent code}
(\i{PIC}), which makes writing shared libraries much easier. NASM
supports the \c{ELF} position-independent code features, so you can
write Linux \c{ELF} shared libraries in NASM.

\i{NetBSD}, and its close cousins \i{FreeBSD} and \i{OpenBSD}, take
a different approach by hacking PIC support into the \c{a.out}
format. NASM supports this as the \i\c{aoutb} output format, so you
can write \i{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:

\c         mov     eax,[myvar]             ; WRONG

Instead, the linker provides an area of memory called the
\i\e{global offset table}, or \i{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
\c{CALL} and \c{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 \e{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 \k{picglobal} for a caveat).


\S{picgot} Obtaining the Address of the GOT

Each code module in your shared library should define the GOT as an
external symbol:

\c extern  _GLOBAL_OFFSET_TABLE_   ; in ELF
\c 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:

\c func:   push    ebp
\c         mov     ebp,esp
\c         push    ebx
\c         call    .get_GOT
\c .get_GOT:
\c         pop     ebx
\c         add     ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
\c
\c         ; the function body comes here
\c
\c         mov     ebx,[ebp-4]
\c         mov     esp,ebp
\c         pop     ebp
\c         ret

(For BSD, again, the symbol \c{_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 \c{EBX} register, because PIC shared
libraries use this register to store the address of the GOT.

The interesting bit is the \c{CALL} instruction and the following
two lines. The \c{CALL} and \c{POP} combination obtains the address
of the label \c{.get_GOT}, without having to know in advance where
the program was loaded (since the \c{CALL} instruction is encoded
relative to the current position). The \c{ADD} instruction makes use
of one of the special PIC relocation types: \i{GOTPC relocation}.
With the \i\c{WRT ..gotpc} qualifier specified, the symbol
referenced (here \c{_GLOBAL_OFFSET_TABLE_}, the special symbol
assigned to the GOT) is given as an offset from the beginning of the
section. (Actually, \c{ELF} encodes it as the offset from the operand
field of the \c{ADD} instruction, but NASM simplifies this
deliberately, so you do things the same way for both \c{ELF} and
\c{BSD}.) So the instruction then \e{adds} the beginning of the section,
to get the real address of the GOT, and subtracts the value of
\c{.get_GOT} which it knows is in \c{EBX}. Therefore, by the time
that instruction has finished, \c{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:

\c %macro  get_GOT 0
\c
\c         call    %%getgot
\c   %%getgot:
\c         pop     ebx
\c         add     ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
\c
\c %endmacro

\S{piclocal} 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 \I{GOTOFF
relocation}\c{..gotoff} special \I\c{WRT ..gotoff}\c{WRT} type. The
way this works is like this:

\c         lea     eax,[ebx+myvar wrt ..gotoff]

The expression \c{myvar wrt ..gotoff} is calculated, when the shared
library is linked, to be the offset to the local variable \c{myvar}
from the beginning of the GOT. Therefore, adding it to \c{EBX} as
above will place the real address of \c{myvar} in \c{EAX}.

If you declare variables as \c{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
\c{..gotoff} mechanism.

Note that due to a peculiarity of the way BSD \c{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.


\S{picextern} Finding External and Common Data Items

If your library needs to get at an external variable (external to
the \e{library}, not just to one of the modules within it), you must
use the \I{GOT relocations}\I\c{WRT ..got}\c{..got} type to get at
it. The \c{..got} type, instead of giving you the offset from the
GOT base to the variable, gives you the offset from the GOT base to
a GOT \e{entry} containing the address of the variable. The linker
will set up this GOT entry when it builds the library, and the
dynamic linker will place the correct address in it at load time. So
to obtain the address of an external variable \c{extvar} in \c{EAX},
you would code

\c         mov     eax,[ebx+extvar wrt ..got]

This loads the address of \c{extvar} out of an entry in the GOT. The
linker, when it builds the shared library, collects together every
relocation of type \c{..got}, and builds the GOT so as to ensure it
has every necessary entry present.

Common variables must also be accessed in this way.


\S{picglobal} Exporting Symbols to the Library User

If you want to export symbols to the user of the library, you have
to declare whether they are functions or data, and if they are data,
you have to give the size of the data item. This is because the
dynamic linker has to build \I{PLT}\i{procedure linkage table}
entries for any exported functions, and also moves exported data
items away from the library's data section in which they were
declared.

So to export a function to users of the library, you must use

\c global  func:function           ; declare it as a function
\c
\c func:   push    ebp
\c
\c         ; etc.

And to export a data item such as an array, you would have to code

\c global  array:data array.end-array      ; give the size too
\c
\c array:  resd    128
\c .end:

Be careful: If you export a variable to the library user, by
declaring it as \c{GLOBAL} and supplying a size, the variable will
end up living in the data section of the main program, rather than
in your library's data section, where you declared it. So you will
have to access your own global variable with the \c{..got} mechanism
rather than \c{..gotoff}, as if it were external (which,
effectively, it has become).

Equally, if you need to store the address of an exported global in
one of your data sections, you can't do it by means of the standard
sort of code:

\c dataptr:        dd      global_data_item        ; WRONG

NASM will interpret this code as an ordinary relocation, in which
\c{global_data_item} is merely an offset from the beginning of the
\c{.data} section (or whatever); so this reference will end up
pointing at your data section instead of at the exported global
which resides elsewhere.

Instead of the above code, then, you must write

\c dataptr:        dd      global_data_item wrt ..sym

which makes use of the special \c{WRT} type \I\c{WRT ..sym}\c{..sym}
to instruct NASM to search the symbol table for a particular symbol
at that address, rather than just relocating by section base.

Either method will work for functions: referring to one of your
functions by means of

\c funcptr:        dd      my_function

will give the user the address of the code you wrote, whereas

\c funcptr:        dd      my_function wrt .sym

will give the address of the procedure linkage table for the
function, which is where the calling program will \e{believe} the
function lives. Either address is a valid way to call the function.


\S{picproc} Calling Procedures Outside the Library

Calling procedures outside your shared library has to be done by
means of a \i\e{procedure linkage table}, or \i{PLT}. The PLT is
placed at a known offset from where the library is loaded, so the
library code can make calls to the PLT in a position-independent
way. Within the PLT there is code to jump to offsets contained in
the GOT, so function calls to other shared libraries or to routines
in the main program can be transparently passed off to their real
destinations.

To call an external routine, you must use another special PIC
relocation type, \I{PLT relocations}\i\c{WRT ..plt}. This is much
easier than the GOT-based ones: you simply replace calls such as
\c{CALL printf} with the PLT-relative version \c{CALL printf WRT
..plt}.


\S{link} Generating the Library File

Having written some code modules and assembled them to \c{.o} files,
you then generate your shared library with a command such as

\c ld -shared -o library.so module1.o module2.o       # for ELF
\c ld -Bshareable -o library.so module1.o module2.o   # for BSD

For ELF, if your shared library is going to reside in system
directories such as \c{/usr/lib} or \c{/lib}, it is usually worth
using the \i\c{-soname} flag to the linker, to store the final
library file name, with a version number, into the library:

\c ld -shared -soname library.so.1 -o library.so.1.2 *.o

You would then copy \c{library.so.1.2} into the library directory,
and create \c{library.so.1} as a symbolic link to it.


\C{mixsize} Mixing 16 and 32 Bit Code

This chapter tries to cover some of the issues, largely related to
unusual forms of addressing and jump instructions, encountered when
writing operating system code such as protected-mode initialisation
routines, which require code that operates in mixed segment sizes,
such as code in a 16-bit segment trying to modify data in a 32-bit
one, or jumps between different-size segments.


\H{mixjump} Mixed-Size Jumps\I{jumps, mixed-size}

\I{operating system, writing}\I{writing operating systems}The most
common form of \i{mixed-size instruction} is the one used when
writing a 32-bit OS: having done your setup in 16-bit mode, such as
loading the kernel, you then have to boot it by switching into
protected mode and jumping to the 32-bit kernel start address. In a
fully 32-bit OS, this tends to be the \e{only} mixed-size
instruction you need, since everything before it can be done in pure
16-bit code, and everything after it can be pure 32-bit.

This jump must specify a 48-bit far address, since the target
segment is a 32-bit one. However, it must be assembled in a 16-bit
segment, so just coding, for example,

\c         jmp     0x1234:0x56789ABC       ; wrong!

will not work, since the offset part of the address will be
truncated to \c{0x9ABC} and the jump will be an ordinary 16-bit far
one.

The Linux kernel setup code gets round the inability of \c{as86} to
generate the required instruction by coding it manually, using
\c{DB} instructions. NASM can go one better than that, by actually
generating the right instruction itself. Here's how to do it right:

\c         jmp     dword 0x1234:0x56789ABC         ; right

\I\c{JMP DWORD}The \c{DWORD} prefix (strictly speaking, it should
come \e{after} the colon, since it is declaring the \e{offset} field
to be a doubleword; but NASM will accept either form, since both are
unambiguous) forces the offset part to be treated as far, in the
assumption that you are deliberately writing a jump from a 16-bit
segment to a 32-bit one.

You can do the reverse operation, jumping from a 32-bit segment to a
16-bit one, by means of the \c{WORD} prefix:

\c         jmp     word 0x8765:0x4321      ; 32 to 16 bit

If the \c{WORD} prefix is specified in 16-bit mode, or the \c{DWORD}
prefix in 32-bit mode, they will be ignored, since each is
explicitly forcing NASM into a mode it was in anyway.


\H{mixaddr} Addressing Between Different-Size Segments\I{addressing,
mixed-size}\I{mixed-size addressing}

If your OS is mixed 16 and 32-bit, or if you are writing a DOS
extender, you are likely to have to deal with some 16-bit segments
and some 32-bit ones. At some point, you will probably end up
writing code in a 16-bit segment which has to access data in a
32-bit segment, or vice versa.

If the data you are trying to access in a 32-bit segment lies within
the first 64K of the segment, you may be able to get away with using
an ordinary 16-bit addressing operation for the purpose; but sooner
or later, you will want to do 32-bit addressing from 16-bit mode.

The easiest way to do this is to make sure you use a register for
the address, since any effective address containing a 32-bit
register is forced to be a 32-bit address. So you can do

\c         mov     eax,offset_into_32_bit_segment_specified_by_fs
\c         mov     dword [fs:eax],0x11223344

This is fine, but slightly cumbersome (since it wastes an
instruction and a register) if you already know the precise offset
you are aiming at. The x86 architecture does allow 32-bit effective
addresses to specify nothing but a 4-byte offset, so why shouldn't
NASM be able to generate the best instruction for the purpose?

It can. As in \k{mixjump}, you need only prefix the address with the
\c{DWORD} keyword, and it will be forced to be a 32-bit address:

\c         mov     dword [fs:dword my_offset],0x11223344

Also as in \k{mixjump}, NASM is not fussy about whether the
\c{DWORD} prefix comes before or after the segment override, so
arguably a nicer-looking way to code the above instruction is

\c         mov     dword [dword fs:my_offset],0x11223344

Don't confuse the \c{DWORD} prefix \e{outside} the square brackets,
which controls the size of the data stored at the address, with the
one \c{inside} the square brackets which controls the length of the
address itself. The two can quite easily be different:

\c         mov     word [dword 0x12345678],0x9ABC

This moves 16 bits of data to an address specified by a 32-bit
offset.

You can also specify \c{WORD} or \c{DWORD} prefixes along with the
\c{FAR} prefix to indirect far jumps or calls. For example:

\c         call    dword far [fs:word 0x4321]

This instruction contains an address specified by a 16-bit offset;
it loads a 48-bit far pointer from that (16-bit segment and 32-bit
offset), and calls that address.


\H{mixother} Other Mixed-Size Instructions

The other way you might want to access data might be using the
string instructions (\c{LODSx}, \c{STOSx} and so on) or the
\c{XLATB} instruction. These instructions, since they take no
parameters, might seem to have no easy way to make them perform
32-bit addressing when assembled in a 16-bit segment.

This is the purpose of NASM's \i\c{a16} and \i\c{a32} prefixes. If
you are coding \c{LODSB} in a 16-bit segment but it is supposed to
be accessing a string in a 32-bit segment, you should load the
desired address into \c{ESI} and then code

\c         a32     lodsb

The prefix forces the addressing size to 32 bits, meaning that
\c{LODSB} loads from \c{[DS:ESI]} instead of \c{[DS:SI]}. To access
a string in a 16-bit segment when coding in a 32-bit one, the
corresponding \c{a16} prefix can be used.

The \c{a16} and \c{a32} prefixes can be applied to any instruction
in NASM's instruction table, but most of them can generate all the
useful forms without them. The prefixes are necessary only for
instructions with implicit addressing:
\# \c{CMPSx} (\k{insCMPSB}),
\# \c{SCASx} (\k{insSCASB}), \c{LODSx} (\k{insLODSB}), \c{STOSx}
\# (\k{insSTOSB}), \c{MOVSx} (\k{insMOVSB}), \c{INSx} (\k{insINSB}),
\# \c{OUTSx} (\k{insOUTSB}), and \c{XLATB} (\k{insXLATB}).
\c{CMPSx}, \c{SCASx}, \c{LODSx}, \c{STOSx}, \c{MOVSx}, \c{INSx},
\c{OUTSx}, and \c{XLATB}.
Also, the
various push and pop instructions (\c{PUSHA} and \c{POPF} as well as
the more usual \c{PUSH} and \c{POP}) can accept \c{a16} or \c{a32}
prefixes to force a particular one of \c{SP} or \c{ESP} to be used
as a stack pointer, in case the stack segment in use is a different
size from the code segment.

\c{PUSH} and \c{POP}, when applied to segment registers in 32-bit
mode, also have the slightly odd behaviour that they push and pop 4
bytes at a time, of which the top two are ignored and the bottom two
give the value of the segment register being manipulated. To force
the 16-bit behaviour of segment-register push and pop instructions,
you can use the operand-size prefix \i\c{o16}:

\c         o16 push    ss
\c         o16 push    ds

This code saves a doubleword of stack space by fitting two segment
registers into the space which would normally be consumed by pushing
one.

(You can also use the \i\c{o32} prefix to force the 32-bit behaviour
when in 16-bit mode, but this seems less useful.)


\C{64bit} Writing 64-bit Code (Unix, Win64)

This chapter attempts to cover some of the common issues involved when
writing 64-bit code, to run under \i{Win64} or Unix.  It covers how to
write assembly code to interface with 64-bit C routines, and how to
write position-independent code for shared libraries.

All 64-bit code uses a flat memory model, since segmentation is not
available in 64-bit mode.  The one exception is the \c{FS} and \c{GS}
registers, which still add their bases.

Position independence in 64-bit mode is significantly simpler, since
the processor supports \c{RIP}-relative addressing directly; see the
\c{REL} keyword (\k{effaddr}).  On most 64-bit platforms, it is
probably desirable to make that the default, using the directive
\c{DEFAULT REL} (\k{default}).

64-bit programming is relatively similar to 32-bit programming, but
of course pointers are 64 bits long; additionally, all existing
platforms pass arguments in registers rather than on the stack.
Furthermore, 64-bit platforms use SSE2 by default for floating point.
Please see the ABI documentation for your platform.

64-bit platforms differ in the sizes of the fundamental datatypes, not
just from 32-bit platforms but from each other.  If a specific size
data type is desired, it is probably best to use the types defined in
the Standard C header \c{<inttypes.h>}.

\H{unix64} Interfacing to 64-bit C Programs (Unix)

On Unix, the 64-bit ABI is defined by the document:

\W{http://www.x86-64.org/documentation/abi.pdf}\c{http://www.x86-64.org/documentation/abi.pdf}

Although written for AT&T-syntax assembly, the concepts apply equally
well for NASM-style assembly.  What follows is a simplified summary.

The first six integer arguments (from the left) are passed in \c{RDI},
\c{RSI}, \c{RDX}, \c{RCX}, \c{R8}, and \c{R9}, in that order.
Additional integer arguments are passed on the stack.  These
registers, plus \c{RAX}, \c{R10} and \c{R11} are destroyed by function
calls, and thus are available for use by the function without saving.

Integer return values are passed in \c{RAX} and \c{RDX}, in that order.

Floating point is done using SSE registers, except for \c{long
double}.  Floating-point arguments are passed in \c{XMM0} to \c{XMM7};
return is \c{XMM0} and \c{XMM1}.  \c{long double} are passed on the
stack, and returned in \c{ST(0)} and \c{ST(1)}.

All SSE and x87 registers are destroyed by function calls.

On 64-bit Unix, \c{long} is 64 bits.

\H{win64} Interfacing to 64-bit C Programs (Win64)

The Win64 ABI is described at:

\W{http://msdn2.microsoft.com/en-gb/library/ms794533.aspx}\c{http://msdn2.microsoft.com/en-gb/library/ms794533.aspx}

What follows is a simplified summary.

The first four integer arguments are passwd in \c{RCX}, \c{RDX},
\c{R8} and \c{R9}, in that order.  Additional integer arguments are
passed on the stack.  These registers, plus \c{RAX}, \c{R10} and
\c{R11} are destroyed by function calls, and thus are available for
use by the function without saving.

Integer return values are passed in \c{RAX} only.

Floating point is done using SSE registers, except for \c{long
double}.  Floating-point arguments are passed in \c{XMM0} to \c{XMM3};
return is \c{XMM0} only.

On Win64, \c{long} is 32 bits; \c{long long} or \c{_int64} is 64 bits.

\C{trouble} Troubleshooting

This chapter describes some of the common problems that users have
been known to encounter with NASM, and answers them. It also gives
instructions for reporting bugs in NASM if you find a difficulty
that isn't listed here.


\H{problems} Common Problems

\S{inefficient} NASM Generates \i{Inefficient Code}

We sometimes get `bug' reports about NASM generating inefficient, or
even `wrong', code on instructions such as \c{ADD ESP,8}. This is a
deliberate design feature, connected to predictability of output:
NASM, on seeing \c{ADD ESP,8}, will generate the form of the
instruction which leaves room for a 32-bit offset. You need to code
\I\c{BYTE}\c{ADD ESP,BYTE 8} if you want the space-efficient form of
the instruction. This isn't a bug, it's user error: if you prefer to
have NASM produce the more efficient code automatically enable
optimization with the \c{-On} option (see \k{opt-On}).


\S{jmprange} My Jumps are Out of Range\I{out of range, jumps}

Similarly, people complain that when they issue \i{conditional
jumps} (which are \c{SHORT} by default) that try to jump too far,
NASM reports `short jump out of range' instead of making the jumps
longer.

This, again, is partly a predictability issue, but in fact has a
more practical reason as well. NASM has no means of being told what
type of processor the code it is generating will be run on; so it
cannot decide for itself that it should generate \i\c{Jcc NEAR} type
instructions, because it doesn't know that it's working for a 386 or
above. Alternatively, it could replace the out-of-range short
\c{JNE} instruction with a very short \c{JE} instruction that jumps
over a \c{JMP NEAR}; this is a sensible solution for processors
below a 386, but hardly efficient on processors which have good
branch prediction \e{and} could have used \c{JNE NEAR} instead. So,
once again, it's up to the user, not the assembler, to decide what
instructions should be generated. See \k{opt-On}.


\S{proborg} \i\c{ORG} Doesn't Work

People writing \i{boot sector} programs in the \c{bin} format often
complain that \c{ORG} doesn't work the way they'd like: in order to
place the \c{0xAA55} signature word at the end of a 512-byte boot
sector, people who are used to MASM tend to code

\c         ORG 0
\c
\c         ; some boot sector code
\c
\c         ORG 510
\c         DW 0xAA55

This is not the intended use of the \c{ORG} directive in NASM, and
will not work. The correct way to solve this problem in NASM is to
use the \i\c{TIMES} directive, like this:

\c         ORG 0
\c
\c         ; some boot sector code
\c
\c         TIMES 510-($-$$) DB 0
\c         DW 0xAA55

The \c{TIMES} directive will insert exactly enough zero bytes into
the output to move the assembly point up to 510. This method also
has the advantage that if you accidentally fill your boot sector too
full, NASM will catch the problem at assembly time and report it, so
you won't end up with a boot sector that you have to disassemble to
find out what's wrong with it.


\S{probtimes} \i\c{TIMES} Doesn't Work

The other common problem with the above code is people who write the
\c{TIMES} line as

\c         TIMES 510-$ DB 0

by reasoning that \c{$} should be a pure number, just like 510, so
the difference between them is also a pure number and can happily be
fed to \c{TIMES}.

NASM is a \e{modular} assembler: the various component parts are
designed to be easily separable for re-use, so they don't exchange
information unnecessarily. In consequence, the \c{bin} output
format, even though it has been told by the \c{ORG} directive that
the \c{.text} section should start at 0, does not pass that
information back to the expression evaluator. So from the
evaluator's point of view, \c{$} isn't a pure number: it's an offset
from a section base. Therefore the difference between \c{$} and 510
is also not a pure number, but involves a section base. Values
involving section bases cannot be passed as arguments to \c{TIMES}.

The solution, as in the previous section, is to code the \c{TIMES}
line in the form

\c         TIMES 510-($-$$) DB 0

in which \c{$} and \c{$$} are offsets from the same section base,
and so their difference is a pure number. This will solve the
problem and generate sensible code.


\H{bugs} \i{Bugs}\I{reporting bugs}

We have never yet released a version of NASM with any \e{known}
bugs. That doesn't usually stop there being plenty we didn't know
about, though. Any that you find should be reported firstly via the
\i\c{bugtracker} at
\W{https://sourceforge.net/projects/nasm/}\c{https://sourceforge.net/projects/nasm/}
(click on "Bugs"), or if that fails then through one of the
contacts in \k{contact}.

Please read \k{qstart} first, and don't report the bug if it's
listed in there as a deliberate feature. (If you think the feature
is badly thought out, feel free to send us reasons why you think it
should be changed, but don't just send us mail saying `This is a
bug' if the documentation says we did it on purpose.) Then read
\k{problems}, and don't bother reporting the bug if it's listed
there.

If you do report a bug, \e{please} give us all of the following
information:

\b What operating system you're running NASM under. DOS, Linux,
NetBSD, Win16, Win32, VMS (I'd be impressed), whatever.

\b If you're running NASM under DOS or Win32, tell us whether you've
compiled your own executable from the DOS source archive, or whether
you were using the standard distribution binaries out of the
archive. If you were using a locally built executable, try to
reproduce the problem using one of the standard binaries, as this
will make it easier for us to reproduce your problem prior to fixing
it.

\b Which version of NASM you're using, and exactly how you invoked
it. Give us the precise command line, and the contents of the
\c{NASMENV} environment variable if any.

\b Which versions of any supplementary programs you're using, and
how you invoked them. If the problem only becomes visible at link
time, tell us what linker you're using, what version of it you've
got, and the exact linker command line. If the problem involves
linking against object files generated by a compiler, tell us what
compiler, what version, and what command line or options you used.
(If you're compiling in an IDE, please try to reproduce the problem
with the command-line version of the compiler.)

\b If at all possible, send us a NASM source file which exhibits the
problem. If this causes copyright problems (e.g. you can only
reproduce the bug in restricted-distribution code) then bear in mind
the following two points: firstly, we guarantee that any source code
sent to us for the purposes of debugging NASM will be used \e{only}
for the purposes of debugging NASM, and that we will delete all our
copies of it as soon as we have found and fixed the bug or bugs in
question; and secondly, we would prefer \e{not} to be mailed large
chunks of code anyway. The smaller the file, the better. A
three-line sample file that does nothing useful \e{except}
demonstrate the problem is much easier to work with than a
fully fledged ten-thousand-line program. (Of course, some errors
\e{do} only crop up in large files, so this may not be possible.)

\b A description of what the problem actually \e{is}. `It doesn't
work' is \e{not} a helpful description! Please describe exactly what
is happening that shouldn't be, or what isn't happening that should.
Examples might be: `NASM generates an error message saying Line 3
for an error that's actually on Line 5'; `NASM generates an error
message that I believe it shouldn't be generating at all'; `NASM
fails to generate an error message that I believe it \e{should} be
generating'; `the object file produced from this source code crashes
my linker'; `the ninth byte of the output file is 66 and I think it
should be 77 instead'.

\b If you believe the output file from NASM to be faulty, send it to
us. That allows us to determine whether our own copy of NASM
generates the same file, or whether the problem is related to
portability issues between our development platforms and yours. We
can handle binary files mailed to us as MIME attachments, uuencoded,
and even BinHex. Alternatively, we may be able to provide an FTP
site you can upload the suspect files to; but mailing them is easier
for us.

\b Any other information or data files that might be helpful. If,
for example, the problem involves NASM failing to generate an object
file while TASM can generate an equivalent file without trouble,
then send us \e{both} object files, so we can see what TASM is doing
differently from us.


\A{ndisasm} \i{Ndisasm}

                  The Netwide Disassembler, NDISASM

\H{ndisintro} Introduction


The Netwide Disassembler is a small companion program to the Netwide
Assembler, NASM. It seemed a shame to have an x86 assembler,
complete with a full instruction table, and not make as much use of
it as possible, so here's a disassembler which shares the
instruction table (and some other bits of code) with NASM.

The Netwide Disassembler does nothing except to produce
disassemblies of \e{binary} source files. NDISASM does not have any
understanding of object file formats, like \c{objdump}, and it will
not understand \c{DOS .EXE} files like \c{debug} will. It just
disassembles.


\H{ndisstart} Getting Started: Installation

See \k{install} for installation instructions. NDISASM, like NASM,
has a \c{man page} which you may want to put somewhere useful, if you
are on a Unix system.


\H{ndisrun} Running NDISASM

To disassemble a file, you will typically use a command of the form

\c        ndisasm -b {16|32|64} filename

NDISASM can disassemble 16-, 32- or 64-bit code equally easily,
provided of course that you remember to specify which it is to work
with. If no \i\c{-b} switch is present, NDISASM works in 16-bit mode
by default. The \i\c{-u} switch (for USE32) also invokes 32-bit mode.

Two more command line options are \i\c{-r} which reports the version
number of NDISASM you are running, and \i\c{-h} which gives a short
summary of command line options.


\S{ndiscom} COM Files: Specifying an Origin

To disassemble a \c{DOS .COM} file correctly, a disassembler must assume
that the first instruction in the file is loaded at address \c{0x100},
rather than at zero. NDISASM, which assumes by default that any file
you give it is loaded at zero, will therefore need to be informed of
this.

The \i\c{-o} option allows you to declare a different origin for the
file you are disassembling. Its argument may be expressed in any of
the NASM numeric formats: decimal by default, if it begins with `\c{$}'
or `\c{0x}' or ends in `\c{H}' it's \c{hex}, if it ends in `\c{Q}' it's
\c{octal}, and if it ends in `\c{B}' it's \c{binary}.

Hence, to disassemble a \c{.COM} file:

\c        ndisasm -o100h filename.com

will do the trick.


\S{ndissync} Code Following Data: Synchronisation

Suppose you are disassembling a file which contains some data which
isn't machine code, and \e{then} contains some machine code. NDISASM
will faithfully plough through the data section, producing machine
instructions wherever it can (although most of them will look
bizarre, and some may have unusual prefixes, e.g. `\c{FS OR AX,0x240A}'),
and generating `DB' instructions ever so often if it's totally stumped.
Then it will reach the code section.

Supposing NDISASM has just finished generating a strange machine
instruction from part of the data section, and its file position is
now one byte \e{before} the beginning of the code section. It's
entirely possible that another spurious instruction will get
generated, starting with the final byte of the data section, and
then the correct first instruction in the code section will not be
seen because the starting point skipped over it. This isn't really
ideal.

To avoid this, you can specify a `\i\c{synchronisation}' point, or indeed
as many synchronisation points as you like (although NDISASM can
only handle 8192 sync points internally). The definition of a sync
point is this: NDISASM guarantees to hit sync points exactly during
disassembly. If it is thinking about generating an instruction which
would cause it to jump over a sync point, it will discard that
instruction and output a `\c{db}' instead. So it \e{will} start
disassembly exactly from the sync point, and so you \e{will} see all
the instructions in your code section.

Sync points are specified using the \i\c{-s} option: they are measured
in terms of the program origin, not the file position. So if you
want to synchronize after 32 bytes of a \c{.COM} file, you would have to
do

\c        ndisasm -o100h -s120h file.com

rather than

\c        ndisasm -o100h -s20h file.com

As stated above, you can specify multiple sync markers if you need
to, just by repeating the \c{-s} option.


\S{ndisisync} Mixed Code and Data: Automatic (Intelligent) Synchronisation
\I\c{auto-sync}

Suppose you are disassembling the boot sector of a \c{DOS} floppy (maybe
it has a virus, and you need to understand the virus so that you
know what kinds of damage it might have done you). Typically, this
will contain a \c{JMP} instruction, then some data, then the rest of the
code. So there is a very good chance of NDISASM being \e{misaligned}
when the data ends and the code begins. Hence a sync point is
needed.

On the other hand, why should you have to specify the sync point
manually? What you'd do in order to find where the sync point would
be, surely, would be to read the \c{JMP} instruction, and then to use
its target address as a sync point. So can NDISASM do that for you?

The answer, of course, is yes: using either of the synonymous
switches \i\c{-a} (for automatic sync) or \i\c{-i} (for intelligent
sync) will enable \c{auto-sync} mode. Auto-sync mode automatically
generates a sync point for any forward-referring PC-relative jump or
call instruction that NDISASM encounters. (Since NDISASM is one-pass,
if it encounters a PC-relative jump whose target has already been
processed, there isn't much it can do about it...)

Only PC-relative jumps are processed, since an absolute jump is
either through a register (in which case NDISASM doesn't know what
the register contains) or involves a segment address (in which case
the target code isn't in the same segment that NDISASM is working
in, and so the sync point can't be placed anywhere useful).

For some kinds of file, this mechanism will automatically put sync
points in all the right places, and save you from having to place
any sync points manually. However, it should be stressed that
auto-sync mode is \e{not} guaranteed to catch all the sync points, and
you may still have to place some manually.

Auto-sync mode doesn't prevent you from declaring manual sync
points: it just adds automatically generated ones to the ones you
provide. It's perfectly feasible to specify \c{-i} \e{and} some \c{-s}
options.

Another caveat with auto-sync mode is that if, by some unpleasant
fluke, something in your data section should disassemble to a
PC-relative call or jump instruction, NDISASM may obediently place a
sync point in a totally random place, for example in the middle of
one of the instructions in your code section. So you may end up with
a wrong disassembly even if you use auto-sync. Again, there isn't
much I can do about this. If you have problems, you'll have to use
manual sync points, or use the \c{-k} option (documented below) to
suppress disassembly of the data area.


\S{ndisother} Other Options

The \i\c{-e} option skips a header on the file, by ignoring the first N
bytes. This means that the header is \e{not} counted towards the
disassembly offset: if you give \c{-e10 -o10}, disassembly will start
at byte 10 in the file, and this will be given offset 10, not 20.

The \i\c{-k} option is provided with two comma-separated numeric
arguments, the first of which is an assembly offset and the second
is a number of bytes to skip. This \e{will} count the skipped bytes
towards the assembly offset: its use is to suppress disassembly of a
data section which wouldn't contain anything you wanted to see
anyway.


\H{ndisbugs} Bugs and Improvements

There are no known bugs. However, any you find, with patches if
possible, should be sent to
\W{mailto:nasm-bugs@lists.sourceforge.net}\c{nasm-bugs@lists.sourceforge.net}, or to the
developer's site at
\W{https://sourceforge.net/projects/nasm/}\c{https://sourceforge.net/projects/nasm/}
and we'll try to fix them. Feel free to send contributions and
new features as well.

Future plans include awareness of which processors certain
instructions will run on, and marking of instructions that are too
advanced for some processor (or are \c{FPU} instructions, or are
undocumented opcodes, or are privileged protected-mode instructions,
or whatever).

That's All Folks!

I hope NDISASM is of some use to somebody. Including me. :-)

I don't recommend taking NDISASM apart to see how an efficient
disassembler works, because as far as I know, it isn't an efficient
one anyway. You have been warned.