1 \# --------------------------------------------------------------------------
3 \# Copyright 1996-2010 The NASM Authors - All Rights Reserved
4 \# See the file AUTHORS included with the NASM distribution for
5 \# the specific copyright holders.
7 \# Redistribution and use in source and binary forms, with or without
8 \# modification, are permitted provided that the following
11 \# * Redistributions of source code must retain the above copyright
12 \# notice, this list of conditions and the following disclaimer.
13 \# * Redistributions in binary form must reproduce the above
14 \# copyright notice, this list of conditions and the following
15 \# disclaimer in the documentation and/or other materials provided
16 \# with the distribution.
18 \# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
19 \# CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
20 \# INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
21 \# MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
22 \# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
23 \# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
24 \# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
25 \# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
26 \# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27 \# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
28 \# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
29 \# OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
30 \# EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
32 \# --------------------------------------------------------------------------
34 \# Source code to NASM documentation
36 \M{category}{Programming}
37 \M{title}{NASM - The Netwide Assembler}
39 \M{author}{The NASM Development Team}
40 \M{copyright_tail}{-- All Rights Reserved}
41 \M{license}{This document is redistributable under the license given in the file "LICENSE" distributed in the NASM archive.}
42 \M{summary}{This file documents NASM, the Netwide Assembler: an assembler targetting the Intel x86 series of processors, with portable source.}
45 \M{infotitle}{The Netwide Assembler for x86}
46 \M{epslogo}{nasmlogo.eps}
52 \IR{-MD} \c{-MD} option
53 \IR{-MF} \c{-MF} option
54 \IR{-MG} \c{-MG} option
55 \IR{-MP} \c{-MP} option
56 \IR{-MQ} \c{-MQ} option
57 \IR{-MT} \c{-MT} option
78 \IR{!=} \c{!=} operator
79 \IR{$, here} \c{$}, Here token
80 \IR{$, prefix} \c{$}, prefix
83 \IR{%%} \c{%%} operator
84 \IR{%+1} \c{%+1} and \c{%-1} syntax
86 \IR{%0} \c{%0} parameter count
88 \IR{&&} \c{&&} operator
90 \IR{..@} \c{..@} symbol prefix
92 \IR{//} \c{//} operator
94 \IR{<<} \c{<<} operator
95 \IR{<=} \c{<=} operator
96 \IR{<>} \c{<>} operator
98 \IR{==} \c{==} operator
100 \IR{>=} \c{>=} operator
101 \IR{>>} \c{>>} operator
102 \IR{?} \c{?} MASM syntax
103 \IR{^} \c{^} operator
104 \IR{^^} \c{^^} operator
105 \IR{|} \c{|} operator
106 \IR{||} \c{||} operator
107 \IR{~} \c{~} operator
108 \IR{%$} \c{%$} and \c{%$$} prefixes
110 \IR{+ opaddition} \c{+} operator, binary
111 \IR{+ opunary} \c{+} operator, unary
112 \IR{+ modifier} \c{+} modifier
113 \IR{- opsubtraction} \c{-} operator, binary
114 \IR{- opunary} \c{-} operator, unary
115 \IR{! opunary} \c{!} operator, unary
116 \IR{alignment, in bin sections} alignment, in \c{bin} sections
117 \IR{alignment, in elf sections} alignment, in \c{elf} sections
118 \IR{alignment, in win32 sections} alignment, in \c{win32} sections
119 \IR{alignment, of elf common variables} alignment, of \c{elf} common
121 \IR{alignment, in obj sections} alignment, in \c{obj} sections
122 \IR{a.out, bsd version} \c{a.out}, BSD version
123 \IR{a.out, linux version} \c{a.out}, Linux version
124 \IR{autoconf} Autoconf
126 \IR{bitwise and} bitwise AND
127 \IR{bitwise or} bitwise OR
128 \IR{bitwise xor} bitwise XOR
129 \IR{block ifs} block IFs
130 \IR{borland pascal} Borland, Pascal
131 \IR{borland's win32 compilers} Borland, Win32 compilers
132 \IR{braces, after % sign} braces, after \c{%} sign
134 \IR{c calling convention} C calling convention
135 \IR{c symbol names} C symbol names
136 \IA{critical expressions}{critical expression}
137 \IA{command line}{command-line}
138 \IA{case sensitivity}{case sensitive}
139 \IA{case-sensitive}{case sensitive}
140 \IA{case-insensitive}{case sensitive}
141 \IA{character constants}{character constant}
142 \IR{common object file format} Common Object File Format
143 \IR{common variables, alignment in elf} common variables, alignment
145 \IR{common, elf extensions to} \c{COMMON}, \c{elf} extensions to
146 \IR{common, obj extensions to} \c{COMMON}, \c{obj} extensions to
147 \IR{declaring structure} declaring structures
148 \IR{default-wrt mechanism} default-\c{WRT} mechanism
151 \IR{dll symbols, exporting} DLL symbols, exporting
152 \IR{dll symbols, importing} DLL symbols, importing
154 \IR{dos archive} DOS archive
155 \IR{dos source archive} DOS source archive
156 \IA{effective address}{effective addresses}
157 \IA{effective-address}{effective addresses}
159 \IR{elf, 16-bit code and} ELF, 16-bit code and
160 \IR{elf shared libraries} ELF, shared libraries
163 \IR{executable and linkable format} Executable and Linkable Format
164 \IR{extern, obj extensions to} \c{EXTERN}, \c{obj} extensions to
165 \IR{extern, rdf extensions to} \c{EXTERN}, \c{rdf} extensions to
166 \IR{floating-point, constants} floating-point, constants
167 \IR{floating-point, packed bcd constants} floating-point, packed BCD constants
169 \IR{freelink} FreeLink
170 \IR{functions, c calling convention} functions, C calling convention
171 \IR{functions, pascal calling convention} functions, Pascal calling
173 \IR{global, aoutb extensions to} \c{GLOBAL}, \c{aoutb} extensions to
174 \IR{global, elf extensions to} \c{GLOBAL}, \c{elf} extensions to
175 \IR{global, rdf extensions to} \c{GLOBAL}, \c{rdf} extensions to
177 \IR{got relocations} \c{GOT} relocations
178 \IR{gotoff relocation} \c{GOTOFF} relocations
179 \IR{gotpc relocation} \c{GOTPC} relocations
180 \IR{intel number formats} Intel number formats
181 \IR{linux, elf} Linux, ELF
182 \IR{linux, a.out} Linux, \c{a.out}
183 \IR{linux, as86} Linux, \c{as86}
184 \IR{logical and} logical AND
185 \IR{logical or} logical OR
186 \IR{logical xor} logical XOR
187 \IR{mach object file format} Mach, object file format
189 \IR{macho32} \c{macho32}
190 \IR{macho64} \c{macho64}
193 \IA{memory reference}{memory references}
195 \IA{misc directory}{misc subdirectory}
196 \IR{misc subdirectory} \c{misc} subdirectory
197 \IR{microsoft omf} Microsoft OMF
198 \IR{mmx registers} MMX registers
199 \IA{modr/m}{modr/m byte}
200 \IR{modr/m byte} ModR/M byte
202 \IR{ms-dos device drivers} MS-DOS device drivers
203 \IR{multipush} \c{multipush} macro
205 \IR{nasm version} NASM version
209 \IR{operating system} operating system
211 \IR{pascal calling convention}Pascal calling convention
212 \IR{passes} passes, assembly
217 \IR{plt} \c{PLT} relocations
218 \IA{pre-defining macros}{pre-define}
219 \IA{preprocessor expressions}{preprocessor, expressions}
220 \IA{preprocessor loops}{preprocessor, loops}
221 \IA{preprocessor variables}{preprocessor, variables}
222 \IA{rdoff subdirectory}{rdoff}
223 \IR{rdoff} \c{rdoff} subdirectory
224 \IR{relocatable dynamic object file format} Relocatable Dynamic
226 \IR{relocations, pic-specific} relocations, PIC-specific
227 \IA{repeating}{repeating code}
228 \IR{section alignment, in elf} section alignment, in \c{elf}
229 \IR{section alignment, in bin} section alignment, in \c{bin}
230 \IR{section alignment, in obj} section alignment, in \c{obj}
231 \IR{section alignment, in win32} section alignment, in \c{win32}
232 \IR{section, elf extensions to} \c{SECTION}, \c{elf} extensions to
233 \IR{section, win32 extensions to} \c{SECTION}, \c{win32} extensions to
234 \IR{segment alignment, in bin} segment alignment, in \c{bin}
235 \IR{segment alignment, in obj} segment alignment, in \c{obj}
236 \IR{segment, obj extensions to} \c{SEGMENT}, \c{elf} extensions to
237 \IR{segment names, borland pascal} segment names, Borland Pascal
238 \IR{shift command} \c{shift} command
240 \IR{sib byte} SIB byte
241 \IR{align, smart} \c{ALIGN}, smart
242 \IA{sectalign}{sectalign}
243 \IR{solaris x86} Solaris x86
244 \IA{standard section names}{standardized section names}
245 \IR{symbols, exporting from dlls} symbols, exporting from DLLs
246 \IR{symbols, importing from dlls} symbols, importing from DLLs
247 \IR{test subdirectory} \c{test} subdirectory
249 \IR{underscore, in c symbols} underscore, in C symbols
255 \IA{sco unix}{unix, sco}
256 \IR{unix, sco} Unix, SCO
257 \IA{unix source archive}{unix, source archive}
258 \IR{unix, source archive} Unix, source archive
259 \IA{unix system v}{unix, system v}
260 \IR{unix, system v} Unix, System V
261 \IR{unixware} UnixWare
263 \IR{version number of nasm} version number of NASM
264 \IR{visual c++} Visual C++
265 \IR{www page} WWW page
269 \IR{windows 95} Windows 95
270 \IR{windows nt} Windows NT
271 \# \IC{program entry point}{entry point, program}
272 \# \IC{program entry point}{start point, program}
273 \# \IC{MS-DOS device drivers}{device drivers, MS-DOS}
274 \# \IC{16-bit mode, versus 32-bit mode}{32-bit mode, versus 16-bit mode}
275 \# \IC{c symbol names}{symbol names, in C}
278 \C{intro} Introduction
280 \H{whatsnasm} What Is NASM?
282 The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed
283 for portability and modularity. It supports a range of object file
284 formats, including Linux and \c{*BSD} \c{a.out}, \c{ELF}, \c{COFF},
285 \c{Mach-O}, Microsoft 16-bit \c{OBJ}, \c{Win32} and \c{Win64}. It will
286 also output plain binary files. Its syntax is designed to be simple
287 and easy to understand, similar to Intel's but less complex. It
288 supports all currently known x86 architectural extensions, and has
289 strong support for macros.
292 \S{yaasm} Why Yet Another Assembler?
294 The Netwide Assembler grew out of an idea on \i\c{comp.lang.asm.x86}
295 (or possibly \i\c{alt.lang.asm} - I forget which), which was
296 essentially that there didn't seem to be a good \e{free} x86-series
297 assembler around, and that maybe someone ought to write one.
299 \b \i\c{a86} is good, but not free, and in particular you don't get any
300 32-bit capability until you pay. It's DOS only, too.
302 \b \i\c{gas} is free, and ports over to DOS and Unix, but it's not
303 very good, since it's designed to be a back end to \i\c{gcc}, which
304 always feeds it correct code. So its error checking is minimal. Also,
305 its syntax is horrible, from the point of view of anyone trying to
306 actually \e{write} anything in it. Plus you can't write 16-bit code in
309 \b \i\c{as86} is specific to Minix and Linux, and (my version at least)
310 doesn't seem to have much (or any) documentation.
312 \b \i\c{MASM} isn't very good, and it's (was) expensive, and it runs only under
315 \b \i\c{TASM} is better, but still strives for MASM compatibility,
316 which means millions of directives and tons of red tape. And its syntax
317 is essentially MASM's, with the contradictions and quirks that
318 entails (although it sorts out some of those by means of Ideal mode.)
319 It's expensive too. And it's DOS-only.
321 So here, for your coding pleasure, is NASM. At present it's
322 still in prototype stage - we don't promise that it can outperform
323 any of these assemblers. But please, \e{please} send us bug reports,
324 fixes, helpful information, and anything else you can get your hands
325 on (and thanks to the many people who've done this already! You all
326 know who you are), and we'll improve it out of all recognition.
330 \S{legal} \i{License} Conditions
332 Please see the file \c{LICENSE}, supplied as part of any NASM
333 distribution archive, for the license conditions under which you may
334 use NASM. NASM is now under the so-called 2-clause BSD license, also
335 known as the simplified BSD license.
337 Copyright 1996-2010 the NASM Authors - All rights reserved.
339 Redistribution and use in source and binary forms, with or without
340 modification, are permitted provided that the following conditions are
343 \b Redistributions of source code must retain the above copyright
344 notice, this list of conditions and the following disclaimer.
346 \b Redistributions in binary form must reproduce the above copyright
347 notice, this list of conditions and the following disclaimer in the
348 documentation and/or other materials provided with the distribution.
350 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
351 CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
352 INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
353 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
354 DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
355 CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
356 SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
357 NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
358 LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
359 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
360 CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
361 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
362 EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
365 \H{contact} Contact Information
367 The current version of NASM (since about 0.98.08) is maintained by a
368 team of developers, accessible through the \c{nasm-devel} mailing list
369 (see below for the link).
370 If you want to report a bug, please read \k{bugs} first.
372 NASM has a \i{website} at
373 \W{http://www.nasm.us/}\c{http://www.nasm.us/}. If it's not there,
376 \i{New releases}, \i{release candidates}, and \I{snapshots, daily
377 development}\i{daily development snapshots} of NASM are available from
378 the official web site.
380 Announcements are posted to
381 \W{news:comp.lang.asm.x86}\i\c{comp.lang.asm.x86},
383 \W{http://www.freshmeat.net/}\c{http://www.freshmeat.net/}.
385 If you want information about the current development status, please
386 subscribe to the \i\c{nasm-devel} email list; see link from the
390 \H{install} Installation
392 \S{instdos} \i{Installing} NASM under MS-\i{DOS} or Windows
394 Once you've obtained the appropriate archive for NASM,
395 \i\c{nasm-XXX-dos.zip} or \i\c{nasm-XXX-win32.zip} (where \c{XXX}
396 denotes the version number of NASM contained in the archive), unpack
397 it into its own directory (for example \c{c:\\nasm}).
399 The archive will contain a set of executable files: the NASM
400 executable file \i\c{nasm.exe}, the NDISASM executable file
401 \i\c{ndisasm.exe}, and possibly additional utilities to handle the
404 The only file NASM needs to run is its own executable, so copy
405 \c{nasm.exe} to a directory on your PATH, or alternatively edit
406 \i\c{autoexec.bat} to add the \c{nasm} directory to your
407 \i\c{PATH} (to do that under Windows XP, go to Start > Control Panel >
408 System > Advanced > Environment Variables; these instructions may work
409 under other versions of Windows as well.)
411 That's it - NASM is installed. You don't need the nasm directory
412 to be present to run NASM (unless you've added it to your \c{PATH}),
413 so you can delete it if you need to save space; however, you may
414 want to keep the documentation or test programs.
416 If you've downloaded the \i{DOS source archive}, \i\c{nasm-XXX.zip},
417 the \c{nasm} directory will also contain the full NASM \i{source
418 code}, and a selection of \i{Makefiles} you can (hopefully) use to
419 rebuild your copy of NASM from scratch. See the file \c{INSTALL} in
422 Note that a number of files are generated from other files by Perl
423 scripts. Although the NASM source distribution includes these
424 generated files, you will need to rebuild them (and hence, will need a
425 Perl interpreter) if you change insns.dat, standard.mac or the
426 documentation. It is possible future source distributions may not
427 include these files at all. Ports of \i{Perl} for a variety of
428 platforms, including DOS and Windows, are available from
429 \W{http://www.cpan.org/ports/}\i{www.cpan.org}.
432 \S{instdos} Installing NASM under \i{Unix}
434 Once you've obtained the \i{Unix source archive} for NASM,
435 \i\c{nasm-XXX.tar.gz} (where \c{XXX} denotes the version number of
436 NASM contained in the archive), unpack it into a directory such
437 as \c{/usr/local/src}. The archive, when unpacked, will create its
438 own subdirectory \c{nasm-XXX}.
440 NASM is an \I{Autoconf}\I\c{configure}auto-configuring package: once
441 you've unpacked it, \c{cd} to the directory it's been unpacked into
442 and type \c{./configure}. This shell script will find the best C
443 compiler to use for building NASM and set up \i{Makefiles}
446 Once NASM has auto-configured, you can type \i\c{make} to build the
447 \c{nasm} and \c{ndisasm} binaries, and then \c{make install} to
448 install them in \c{/usr/local/bin} and install the \i{man pages}
449 \i\c{nasm.1} and \i\c{ndisasm.1} in \c{/usr/local/man/man1}.
450 Alternatively, you can give options such as \c{--prefix} to the
451 configure script (see the file \i\c{INSTALL} for more details), or
452 install the programs yourself.
454 NASM also comes with a set of utilities for handling the \c{RDOFF}
455 custom object-file format, which are in the \i\c{rdoff} subdirectory
456 of the NASM archive. You can build these with \c{make rdf} and
457 install them with \c{make rdf_install}, if you want them.
460 \C{running} Running NASM
462 \H{syntax} NASM \i{Command-Line} Syntax
464 To assemble a file, you issue a command of the form
466 \c nasm -f <format> <filename> [-o <output>]
470 \c nasm -f elf myfile.asm
472 will assemble \c{myfile.asm} into an \c{ELF} object file \c{myfile.o}. And
474 \c nasm -f bin myfile.asm -o myfile.com
476 will assemble \c{myfile.asm} into a raw binary file \c{myfile.com}.
478 To produce a listing file, with the hex codes output from NASM
479 displayed on the left of the original sources, use the \c{-l} option
480 to give a listing file name, for example:
482 \c nasm -f coff myfile.asm -l myfile.lst
484 To get further usage instructions from NASM, try typing
488 As \c{-hf}, this will also list the available output file formats, and what they
491 If you use Linux but aren't sure whether your system is \c{a.out}
496 (in the directory in which you put the NASM binary when you
497 installed it). If it says something like
499 \c nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1
501 then your system is \c{ELF}, and you should use the option \c{-f elf}
502 when you want NASM to produce Linux object files. If it says
504 \c nasm: Linux/i386 demand-paged executable (QMAGIC)
506 or something similar, your system is \c{a.out}, and you should use
507 \c{-f aout} instead (Linux \c{a.out} systems have long been obsolete,
508 and are rare these days.)
510 Like Unix compilers and assemblers, NASM is silent unless it
511 goes wrong: you won't see any output at all, unless it gives error
515 \S{opt-o} The \i\c{-o} Option: Specifying the Output File Name
517 NASM will normally choose the name of your output file for you;
518 precisely how it does this is dependent on the object file format.
519 For Microsoft object file formats (\c{obj}, \c{win32} and \c{win64}),
520 it will remove the \c{.asm} \i{extension} (or whatever extension you
521 like to use - NASM doesn't care) from your source file name and
522 substitute \c{.obj}. For Unix object file formats (\c{aout}, \c{as86},
523 \c{coff}, \c{elf32}, \c{elf64}, \c{ieee}, \c{macho32} and \c{macho64})
524 it will substitute \c{.o}. For \c{dbg}, \c{rdf}, \c{ith} and \c{srec},
525 it will use \c{.dbg}, \c{.rdf}, \c{.ith} and \c{.srec}, respectively,
526 and for the \c{bin} format it will simply remove the extension, so
527 that \c{myfile.asm} produces the output file \c{myfile}.
529 If the output file already exists, NASM will overwrite it, unless it
530 has the same name as the input file, in which case it will give a
531 warning and use \i\c{nasm.out} as the output file name instead.
533 For situations in which this behaviour is unacceptable, NASM
534 provides the \c{-o} command-line option, which allows you to specify
535 your desired output file name. You invoke \c{-o} by following it
536 with the name you wish for the output file, either with or without
537 an intervening space. For example:
539 \c nasm -f bin program.asm -o program.com
540 \c nasm -f bin driver.asm -odriver.sys
542 Note that this is a small o, and is different from a capital O , which
543 is used to specify the number of optimisation passes required. See \k{opt-O}.
546 \S{opt-f} The \i\c{-f} Option: Specifying the \i{Output File Format}
548 If you do not supply the \c{-f} option to NASM, it will choose an
549 output file format for you itself. In the distribution versions of
550 NASM, the default is always \i\c{bin}; if you've compiled your own
551 copy of NASM, you can redefine \i\c{OF_DEFAULT} at compile time and
552 choose what you want the default to be.
554 Like \c{-o}, the intervening space between \c{-f} and the output
555 file format is optional; so \c{-f elf} and \c{-felf} are both valid.
557 A complete list of the available output file formats can be given by
558 issuing the command \i\c{nasm -hf}.
561 \S{opt-l} The \i\c{-l} Option: Generating a \i{Listing File}
563 If you supply the \c{-l} option to NASM, followed (with the usual
564 optional space) by a file name, NASM will generate a
565 \i{source-listing file} for you, in which addresses and generated
566 code are listed on the left, and the actual source code, with
567 expansions of multi-line macros (except those which specifically
568 request no expansion in source listings: see \k{nolist}) on the
571 \c nasm -f elf myfile.asm -l myfile.lst
573 If a list file is selected, you may turn off listing for a
574 section of your source with \c{[list -]}, and turn it back on
575 with \c{[list +]}, (the default, obviously). There is no "user
576 form" (without the brackets). This can be used to list only
577 sections of interest, avoiding excessively long listings.
580 \S{opt-M} The \i\c{-M} Option: Generate \i{Makefile Dependencies}
582 This option can be used to generate makefile dependencies on stdout.
583 This can be redirected to a file for further processing. For example:
585 \c nasm -M myfile.asm > myfile.dep
588 \S{opt-MG} The \i\c{-MG} Option: Generate \i{Makefile Dependencies}
590 This option can be used to generate makefile dependencies on stdout.
591 This differs from the \c{-M} option in that if a nonexisting file is
592 encountered, it is assumed to be a generated file and is added to the
593 dependency list without a prefix.
596 \S{opt-MF} The \i\c\{-MF} Option: Set Makefile Dependency File
598 This option can be used with the \c{-M} or \c{-MG} options to send the
599 output to a file, rather than to stdout. For example:
601 \c nasm -M -MF myfile.dep myfile.asm
604 \S{opt-MD} The \i\c{-MD} Option: Assemble and Generate Dependencies
606 The \c{-MD} option acts as the combination of the \c{-M} and \c{-MF}
607 options (i.e. a filename has to be specified.) However, unlike the
608 \c{-M} or \c{-MG} options, \c{-MD} does \e{not} inhibit the normal
609 operation of the assembler. Use this to automatically generate
610 updated dependencies with every assembly session. For example:
612 \c nasm -f elf -o myfile.o -MD myfile.dep myfile.asm
615 \S{opt-MT} The \i\c{-MT} Option: Dependency Target Name
617 The \c{-MT} option can be used to override the default name of the
618 dependency target. This is normally the same as the output filename,
619 specified by the \c{-o} option.
622 \S{opt-MQ} The \i\c{-MQ} Option: Dependency Target Name (Quoted)
624 The \c{-MQ} option acts as the \c{-MT} option, except it tries to
625 quote characters that have special meaning in Makefile syntax. This
626 is not foolproof, as not all characters with special meaning are
630 \S{opt-MP} The \i\c{-MP} Option: Emit phony targets
632 When used with any of the dependency generation options, the \c{-MP}
633 option causes NASM to emit a phony target without dependencies for
634 each header file. This prevents Make from complaining if a header
635 file has been removed.
638 \S{opt-F} The \i\c{-F} Option: Selecting a \i{Debug Information Format}
640 This option is used to select the format of the debug information
641 emitted into the output file, to be used by a debugger (or \e{will}
642 be). Prior to version 2.03.01, the use of this switch did \e{not} enable
643 output of the selected debug info format. Use \c{-g}, see \k{opt-g},
644 to enable output. Versions 2.03.01 and later automatically enable \c{-g}
645 if \c{-F} is specified.
647 A complete list of the available debug file formats for an output
648 format can be seen by issuing the command \c{nasm -f <format> -y}. Not
649 all output formats currently support debugging output. See \k{opt-y}.
651 This should not be confused with the \c{-f dbg} output format option which
652 is not built into NASM by default. For information on how
653 to enable it when building from the sources, see \k{dbgfmt}.
656 \S{opt-g} The \i\c{-g} Option: Enabling \i{Debug Information}.
658 This option can be used to generate debugging information in the specified
659 format. See \k{opt-F}. Using \c{-g} without \c{-F} results in emitting
660 debug info in the default format, if any, for the selected output format.
661 If no debug information is currently implemented in the selected output
662 format, \c{-g} is \e{silently ignored}.
665 \S{opt-X} The \i\c{-X} Option: Selecting an \i{Error Reporting Format}
667 This option can be used to select an error reporting format for any
668 error messages that might be produced by NASM.
670 Currently, two error reporting formats may be selected. They are
671 the \c{-Xvc} option and the \c{-Xgnu} option. The GNU format is
672 the default and looks like this:
674 \c filename.asm:65: error: specific error message
676 where \c{filename.asm} is the name of the source file in which the
677 error was detected, \c{65} is the source file line number on which
678 the error was detected, \c{error} is the severity of the error (this
679 could be \c{warning}), and \c{specific error message} is a more
680 detailed text message which should help pinpoint the exact problem.
682 The other format, specified by \c{-Xvc} is the style used by Microsoft
683 Visual C++ and some other programs. It looks like this:
685 \c filename.asm(65) : error: specific error message
687 where the only difference is that the line number is in parentheses
688 instead of being delimited by colons.
690 See also the \c{Visual C++} output format, \k{win32fmt}.
692 \S{opt-Z} The \i\c{-Z} Option: Send Errors to a File
694 Under \I{DOS}\c{MS-DOS} it can be difficult (though there are ways) to
695 redirect the standard-error output of a program to a file. Since
696 NASM usually produces its warning and \i{error messages} on
697 \i\c{stderr}, this can make it hard to capture the errors if (for
698 example) you want to load them into an editor.
700 NASM therefore provides the \c{-Z} option, taking a filename argument
701 which causes errors to be sent to the specified files rather than
702 standard error. Therefore you can \I{redirecting errors}redirect
703 the errors into a file by typing
705 \c nasm -Z myfile.err -f obj myfile.asm
707 In earlier versions of NASM, this option was called \c{-E}, but it was
708 changed since \c{-E} is an option conventionally used for
709 preprocessing only, with disastrous results. See \k{opt-E}.
711 \S{opt-s} The \i\c{-s} Option: Send Errors to \i\c{stdout}
713 The \c{-s} option redirects \i{error messages} to \c{stdout} rather
714 than \c{stderr}, so it can be redirected under \I{DOS}\c{MS-DOS}. To
715 assemble the file \c{myfile.asm} and pipe its output to the \c{more}
716 program, you can type:
718 \c nasm -s -f obj myfile.asm | more
720 See also the \c{-Z} option, \k{opt-Z}.
723 \S{opt-i} The \i\c{-i}\I\c{-I} Option: Include File Search Directories
725 When NASM sees the \i\c{%include} or \i\c{%pathsearch} directive in a
726 source file (see \k{include}, \k{pathsearch} or \k{incbin}), it will
727 search for the given file not only in the current directory, but also
728 in any directories specified on the command line by the use of the
729 \c{-i} option. Therefore you can include files from a \i{macro
730 library}, for example, by typing
732 \c nasm -ic:\macrolib\ -f obj myfile.asm
734 (As usual, a space between \c{-i} and the path name is allowed, and
737 NASM, in the interests of complete source-code portability, does not
738 understand the file naming conventions of the OS it is running on;
739 the string you provide as an argument to the \c{-i} option will be
740 prepended exactly as written to the name of the include file.
741 Therefore the trailing backslash in the above example is necessary.
742 Under Unix, a trailing forward slash is similarly necessary.
744 (You can use this to your advantage, if you're really \i{perverse},
745 by noting that the option \c{-ifoo} will cause \c{%include "bar.i"}
746 to search for the file \c{foobar.i}...)
748 If you want to define a \e{standard} \i{include search path},
749 similar to \c{/usr/include} on Unix systems, you should place one or
750 more \c{-i} directives in the \c{NASMENV} environment variable (see
753 For Makefile compatibility with many C compilers, this option can also
754 be specified as \c{-I}.
757 \S{opt-p} The \i\c{-p}\I\c{-P} Option: \I{pre-including files}Pre-Include a File
759 \I\c{%include}NASM allows you to specify files to be
760 \e{pre-included} into your source file, by the use of the \c{-p}
763 \c nasm myfile.asm -p myinc.inc
765 is equivalent to running \c{nasm myfile.asm} and placing the
766 directive \c{%include "myinc.inc"} at the start of the file.
768 For consistency with the \c{-I}, \c{-D} and \c{-U} options, this
769 option can also be specified as \c{-P}.
772 \S{opt-d} The \i\c{-d}\I\c{-D} Option: \I{pre-defining macros}Pre-Define a Macro
774 \I\c{%define}Just as the \c{-p} option gives an alternative to placing
775 \c{%include} directives at the start of a source file, the \c{-d}
776 option gives an alternative to placing a \c{%define} directive. You
779 \c nasm myfile.asm -dFOO=100
781 as an alternative to placing the directive
785 at the start of the file. You can miss off the macro value, as well:
786 the option \c{-dFOO} is equivalent to coding \c{%define FOO}. This
787 form of the directive may be useful for selecting \i{assembly-time
788 options} which are then tested using \c{%ifdef}, for example
791 For Makefile compatibility with many C compilers, this option can also
792 be specified as \c{-D}.
795 \S{opt-u} The \i\c{-u}\I\c{-U} Option: \I{Undefining macros}Undefine a Macro
797 \I\c{%undef}The \c{-u} option undefines a macro that would otherwise
798 have been pre-defined, either automatically or by a \c{-p} or \c{-d}
799 option specified earlier on the command lines.
801 For example, the following command line:
803 \c nasm myfile.asm -dFOO=100 -uFOO
805 would result in \c{FOO} \e{not} being a predefined macro in the
806 program. This is useful to override options specified at a different
809 For Makefile compatibility with many C compilers, this option can also
810 be specified as \c{-U}.
813 \S{opt-E} The \i\c{-E}\I{-e} Option: Preprocess Only
815 NASM allows the \i{preprocessor} to be run on its own, up to a
816 point. Using the \c{-E} option (which requires no arguments) will
817 cause NASM to preprocess its input file, expand all the macro
818 references, remove all the comments and preprocessor directives, and
819 print the resulting file on standard output (or save it to a file,
820 if the \c{-o} option is also used).
822 This option cannot be applied to programs which require the
823 preprocessor to evaluate \I{preprocessor expressions}\i{expressions}
824 which depend on the values of symbols: so code such as
826 \c %assign tablesize ($-tablestart)
828 will cause an error in \i{preprocess-only mode}.
830 For compatiblity with older version of NASM, this option can also be
831 written \c{-e}. \c{-E} in older versions of NASM was the equivalent
832 of the current \c{-Z} option, \k{opt-Z}.
834 \S{opt-a} The \i\c{-a} Option: Don't Preprocess At All
836 If NASM is being used as the back end to a compiler, it might be
837 desirable to \I{suppressing preprocessing}suppress preprocessing
838 completely and assume the compiler has already done it, to save time
839 and increase compilation speeds. The \c{-a} option, requiring no
840 argument, instructs NASM to replace its powerful \i{preprocessor}
841 with a \i{stub preprocessor} which does nothing.
844 \S{opt-O} The \i\c{-O} Option: Specifying \i{Multipass Optimization}
846 NASM defaults to not optimizing operands which can fit into a signed byte.
847 This means that if you want the shortest possible object code,
848 you have to enable optimization.
850 Using the \c{-O} option, you can tell NASM to carry out different
851 levels of optimization. The syntax is:
853 \b \c{-O0}: No optimization. All operands take their long forms,
854 if a short form is not specified, except conditional jumps.
855 This is intended to match NASM 0.98 behavior.
857 \b \c{-O1}: Minimal optimization. As above, but immediate operands
858 which will fit in a signed byte are optimized,
859 unless the long form is specified. Conditional jumps default
860 to the long form unless otherwise specified.
862 \b \c{-Ox} (where \c{x} is the actual letter \c{x}): Multipass optimization.
863 Minimize branch offsets and signed immediate bytes,
864 overriding size specification unless the \c{strict} keyword
865 has been used (see \k{strict}). For compatability with earlier
866 releases, the letter \c{x} may also be any number greater than
867 one. This number has no effect on the actual number of passes.
869 The \c{-Ox} mode is recommended for most uses, and is the default
872 Note that this is a capital \c{O}, and is different from a small \c{o}, which
873 is used to specify the output file name. See \k{opt-o}.
876 \S{opt-t} The \i\c{-t} Option: Enable TASM Compatibility Mode
878 NASM includes a limited form of compatibility with Borland's \i\c{TASM}.
879 When NASM's \c{-t} option is used, the following changes are made:
881 \b local labels may be prefixed with \c{@@} instead of \c{.}
883 \b size override is supported within brackets. In TASM compatible mode,
884 a size override inside square brackets changes the size of the operand,
885 and not the address type of the operand as it does in NASM syntax. E.g.
886 \c{mov eax,[DWORD val]} is valid syntax in TASM compatibility mode.
887 Note that you lose the ability to override the default address type for
890 \b unprefixed forms of some directives supported (\c{arg}, \c{elif},
891 \c{else}, \c{endif}, \c{if}, \c{ifdef}, \c{ifdifi}, \c{ifndef},
892 \c{include}, \c{local})
894 \S{opt-w} The \i\c{-w} and \i\c{-W} Options: Enable or Disable Assembly \i{Warnings}
896 NASM can observe many conditions during the course of assembly which
897 are worth mentioning to the user, but not a sufficiently severe
898 error to justify NASM refusing to generate an output file. These
899 conditions are reported like errors, but come up with the word
900 `warning' before the message. Warnings do not prevent NASM from
901 generating an output file and returning a success status to the
904 Some conditions are even less severe than that: they are only
905 sometimes worth mentioning to the user. Therefore NASM supports the
906 \c{-w} command-line option, which enables or disables certain
907 classes of assembly warning. Such warning classes are described by a
908 name, for example \c{orphan-labels}; you can enable warnings of
909 this class by the command-line option \c{-w+orphan-labels} and
910 disable it by \c{-w-orphan-labels}.
912 The \i{suppressible warning} classes are:
914 \b \i\c{macro-params} covers warnings about \i{multi-line macros}
915 being invoked with the wrong number of parameters. This warning
916 class is enabled by default; see \k{mlmacover} for an example of why
917 you might want to disable it.
919 \b \i\c{macro-selfref} warns if a macro references itself. This
920 warning class is disabled by default.
922 \b\i\c{macro-defaults} warns when a macro has more default
923 parameters than optional parameters. This warning class
924 is enabled by default; see \k{mlmacdef} for why you might want to disable it.
926 \b \i\c{orphan-labels} covers warnings about source lines which
927 contain no instruction but define a label without a trailing colon.
928 NASM warns about this somewhat obscure condition by default;
929 see \k{syntax} for more information.
931 \b \i\c{number-overflow} covers warnings about numeric constants which
932 don't fit in 64 bits. This warning class is enabled by default.
934 \b \i\c{gnu-elf-extensions} warns if 8-bit or 16-bit relocations
935 are used in \c{-f elf} format. The GNU extensions allow this.
936 This warning class is disabled by default.
938 \b \i\c{float-overflow} warns about floating point overflow.
941 \b \i\c{float-denorm} warns about floating point denormals.
944 \b \i\c{float-underflow} warns about floating point underflow.
947 \b \i\c{float-toolong} warns about too many digits in floating-point numbers.
950 \b \i\c{user} controls \c{%warning} directives (see \k{pperror}).
953 \b \i\c{error} causes warnings to be treated as errors. Disabled by
956 \b \i\c{all} is an alias for \e{all} suppressible warning classes (not
957 including \c{error}). Thus, \c{-w+all} enables all available warnings.
959 In addition, you can set warning classes across sections.
960 Warning classes may be enabled with \i\c{[warning +warning-name]},
961 disabled with \i\c{[warning -warning-name]} or reset to their
962 original value with \i\c{[warning *warning-name]}. No "user form"
963 (without the brackets) exists.
965 Since version 2.00, NASM has also supported the gcc-like syntax
966 \c{-Wwarning} and \c{-Wno-warning} instead of \c{-w+warning} and
967 \c{-w-warning}, respectively.
970 \S{opt-v} The \i\c{-v} Option: Display \i{Version} Info
972 Typing \c{NASM -v} will display the version of NASM which you are using,
973 and the date on which it was compiled.
975 You will need the version number if you report a bug.
977 \S{opt-y} The \i\c{-y} Option: Display Available Debug Info Formats
979 Typing \c{nasm -f <option> -y} will display a list of the available
980 debug info formats for the given output format. The default format
981 is indicated by an asterisk. For example:
985 \c valid debug formats for 'elf32' output format are
986 \c ('*' denotes default):
987 \c * stabs ELF32 (i386) stabs debug format for Linux
988 \c dwarf elf32 (i386) dwarf debug format for Linux
991 \S{opt-pfix} The \i\c{--prefix} and \i\c{--postfix} Options.
993 The \c{--prefix} and \c{--postfix} options prepend or append
994 (respectively) the given argument to all \c{global} or
995 \c{extern} variables. E.g. \c{--prefix _} will prepend the
996 underscore to all global and external variables, as C sometimes
997 (but not always) likes it.
1000 \S{nasmenv} The \i\c{NASMENV} \i{Environment} Variable
1002 If you define an environment variable called \c{NASMENV}, the program
1003 will interpret it as a list of extra command-line options, which are
1004 processed before the real command line. You can use this to define
1005 standard search directories for include files, by putting \c{-i}
1006 options in the \c{NASMENV} variable.
1008 The value of the variable is split up at white space, so that the
1009 value \c{-s -ic:\\nasmlib\\} will be treated as two separate options.
1010 However, that means that the value \c{-dNAME="my name"} won't do
1011 what you might want, because it will be split at the space and the
1012 NASM command-line processing will get confused by the two
1013 nonsensical words \c{-dNAME="my} and \c{name"}.
1015 To get round this, NASM provides a feature whereby, if you begin the
1016 \c{NASMENV} environment variable with some character that isn't a minus
1017 sign, then NASM will treat this character as the \i{separator
1018 character} for options. So setting the \c{NASMENV} variable to the
1019 value \c{!-s!-ic:\\nasmlib\\} is equivalent to setting it to \c{-s
1020 -ic:\\nasmlib\\}, but \c{!-dNAME="my name"} will work.
1022 This environment variable was previously called \c{NASM}. This was
1023 changed with version 0.98.31.
1026 \H{qstart} \i{Quick Start} for \i{MASM} Users
1028 If you're used to writing programs with MASM, or with \i{TASM} in
1029 MASM-compatible (non-Ideal) mode, or with \i\c{a86}, this section
1030 attempts to outline the major differences between MASM's syntax and
1031 NASM's. If you're not already used to MASM, it's probably worth
1032 skipping this section.
1035 \S{qscs} NASM Is \I{case sensitivity}Case-Sensitive
1037 One simple difference is that NASM is case-sensitive. It makes a
1038 difference whether you call your label \c{foo}, \c{Foo} or \c{FOO}.
1039 If you're assembling to \c{DOS} or \c{OS/2} \c{.OBJ} files, you can
1040 invoke the \i\c{UPPERCASE} directive (documented in \k{objfmt}) to
1041 ensure that all symbols exported to other code modules are forced
1042 to be upper case; but even then, \e{within} a single module, NASM
1043 will distinguish between labels differing only in case.
1046 \S{qsbrackets} NASM Requires \i{Square Brackets} For \i{Memory References}
1048 NASM was designed with simplicity of syntax in mind. One of the
1049 \i{design goals} of NASM is that it should be possible, as far as is
1050 practical, for the user to look at a single line of NASM code
1051 and tell what opcode is generated by it. You can't do this in MASM:
1052 if you declare, for example,
1057 then the two lines of code
1062 generate completely different opcodes, despite having
1063 identical-looking syntaxes.
1065 NASM avoids this undesirable situation by having a much simpler
1066 syntax for memory references. The rule is simply that any access to
1067 the \e{contents} of a memory location requires square brackets
1068 around the address, and any access to the \e{address} of a variable
1069 doesn't. So an instruction of the form \c{mov ax,foo} will
1070 \e{always} refer to a compile-time constant, whether it's an \c{EQU}
1071 or the address of a variable; and to access the \e{contents} of the
1072 variable \c{bar}, you must code \c{mov ax,[bar]}.
1074 This also means that NASM has no need for MASM's \i\c{OFFSET}
1075 keyword, since the MASM code \c{mov ax,offset bar} means exactly the
1076 same thing as NASM's \c{mov ax,bar}. If you're trying to get
1077 large amounts of MASM code to assemble sensibly under NASM, you
1078 can always code \c{%idefine offset} to make the preprocessor treat
1079 the \c{OFFSET} keyword as a no-op.
1081 This issue is even more confusing in \i\c{a86}, where declaring a
1082 label with a trailing colon defines it to be a `label' as opposed to
1083 a `variable' and causes \c{a86} to adopt NASM-style semantics; so in
1084 \c{a86}, \c{mov ax,var} has different behaviour depending on whether
1085 \c{var} was declared as \c{var: dw 0} (a label) or \c{var dw 0} (a
1086 word-size variable). NASM is very simple by comparison:
1087 \e{everything} is a label.
1089 NASM, in the interests of simplicity, also does not support the
1090 \i{hybrid syntaxes} supported by MASM and its clones, such as
1091 \c{mov ax,table[bx]}, where a memory reference is denoted by one
1092 portion outside square brackets and another portion inside. The
1093 correct syntax for the above is \c{mov ax,[table+bx]}. Likewise,
1094 \c{mov ax,es:[di]} is wrong and \c{mov ax,[es:di]} is right.
1097 \S{qstypes} NASM Doesn't Store \i{Variable Types}
1099 NASM, by design, chooses not to remember the types of variables you
1100 declare. Whereas MASM will remember, on seeing \c{var dw 0}, that
1101 you declared \c{var} as a word-size variable, and will then be able
1102 to fill in the \i{ambiguity} in the size of the instruction \c{mov
1103 var,2}, NASM will deliberately remember nothing about the symbol
1104 \c{var} except where it begins, and so you must explicitly code
1105 \c{mov word [var],2}.
1107 For this reason, NASM doesn't support the \c{LODS}, \c{MOVS},
1108 \c{STOS}, \c{SCAS}, \c{CMPS}, \c{INS}, or \c{OUTS} instructions,
1109 but only supports the forms such as \c{LODSB}, \c{MOVSW}, and
1110 \c{SCASD}, which explicitly specify the size of the components of
1111 the strings being manipulated.
1114 \S{qsassume} NASM Doesn't \i\c{ASSUME}
1116 As part of NASM's drive for simplicity, it also does not support the
1117 \c{ASSUME} directive. NASM will not keep track of what values you
1118 choose to put in your segment registers, and will never
1119 \e{automatically} generate a \i{segment override} prefix.
1122 \S{qsmodel} NASM Doesn't Support \i{Memory Models}
1124 NASM also does not have any directives to support different 16-bit
1125 memory models. The programmer has to keep track of which functions
1126 are supposed to be called with a \i{far call} and which with a
1127 \i{near call}, and is responsible for putting the correct form of
1128 \c{RET} instruction (\c{RETN} or \c{RETF}; NASM accepts \c{RET}
1129 itself as an alternate form for \c{RETN}); in addition, the
1130 programmer is responsible for coding CALL FAR instructions where
1131 necessary when calling \e{external} functions, and must also keep
1132 track of which external variable definitions are far and which are
1136 \S{qsfpu} \i{Floating-Point} Differences
1138 NASM uses different names to refer to floating-point registers from
1139 MASM: where MASM would call them \c{ST(0)}, \c{ST(1)} and so on, and
1140 \i\c{a86} would call them simply \c{0}, \c{1} and so on, NASM
1141 chooses to call them \c{st0}, \c{st1} etc.
1143 As of version 0.96, NASM now treats the instructions with
1144 \i{`nowait'} forms in the same way as MASM-compatible assemblers.
1145 The idiosyncratic treatment employed by 0.95 and earlier was based
1146 on a misunderstanding by the authors.
1149 \S{qsother} Other Differences
1151 For historical reasons, NASM uses the keyword \i\c{TWORD} where MASM
1152 and compatible assemblers use \i\c{TBYTE}.
1154 NASM does not declare \i{uninitialized storage} in the same way as
1155 MASM: where a MASM programmer might use \c{stack db 64 dup (?)},
1156 NASM requires \c{stack resb 64}, intended to be read as `reserve 64
1157 bytes'. For a limited amount of compatibility, since NASM treats
1158 \c{?} as a valid character in symbol names, you can code \c{? equ 0}
1159 and then writing \c{dw ?} will at least do something vaguely useful.
1160 \I\c{RESB}\i\c{DUP} is still not a supported syntax, however.
1162 In addition to all of this, macros and directives work completely
1163 differently to MASM. See \k{preproc} and \k{directive} for further
1167 \C{lang} The NASM Language
1169 \H{syntax} Layout of a NASM Source Line
1171 Like most assemblers, each NASM source line contains (unless it
1172 is a macro, a preprocessor directive or an assembler directive: see
1173 \k{preproc} and \k{directive}) some combination of the four fields
1175 \c label: instruction operands ; comment
1177 As usual, most of these fields are optional; the presence or absence
1178 of any combination of a label, an instruction and a comment is allowed.
1179 Of course, the operand field is either required or forbidden by the
1180 presence and nature of the instruction field.
1182 NASM uses backslash (\\) as the line continuation character; if a line
1183 ends with backslash, the next line is considered to be a part of the
1184 backslash-ended line.
1186 NASM places no restrictions on white space within a line: labels may
1187 have white space before them, or instructions may have no space
1188 before them, or anything. The \i{colon} after a label is also
1189 optional. (Note that this means that if you intend to code \c{lodsb}
1190 alone on a line, and type \c{lodab} by accident, then that's still a
1191 valid source line which does nothing but define a label. Running
1192 NASM with the command-line option
1193 \I{orphan-labels}\c{-w+orphan-labels} will cause it to warn you if
1194 you define a label alone on a line without a \i{trailing colon}.)
1196 \i{Valid characters} in labels are letters, numbers, \c{_}, \c{$},
1197 \c{#}, \c{@}, \c{~}, \c{.}, and \c{?}. The only characters which may
1198 be used as the \e{first} character of an identifier are letters,
1199 \c{.} (with special meaning: see \k{locallab}), \c{_} and \c{?}.
1200 An identifier may also be prefixed with a \I{$, prefix}\c{$} to
1201 indicate that it is intended to be read as an identifier and not a
1202 reserved word; thus, if some other module you are linking with
1203 defines a symbol called \c{eax}, you can refer to \c{$eax} in NASM
1204 code to distinguish the symbol from the register. Maximum length of
1205 an identifier is 4095 characters.
1207 The instruction field may contain any machine instruction: Pentium
1208 and P6 instructions, FPU instructions, MMX instructions and even
1209 undocumented instructions are all supported. The instruction may be
1210 prefixed by \c{LOCK}, \c{REP}, \c{REPE}/\c{REPZ} or
1211 \c{REPNE}/\c{REPNZ}, in the usual way. Explicit \I{address-size
1212 prefixes}address-size and \i{operand-size prefixes} \i\c{A16},
1213 \i\c{A32}, \i\c{A64}, \i\c{O16} and \i\c{O32}, \i\c{O64} are provided - one example of their use
1214 is given in \k{mixsize}. You can also use the name of a \I{segment
1215 override}segment register as an instruction prefix: coding
1216 \c{es mov [bx],ax} is equivalent to coding \c{mov [es:bx],ax}. We
1217 recommend the latter syntax, since it is consistent with other
1218 syntactic features of the language, but for instructions such as
1219 \c{LODSB}, which has no operands and yet can require a segment
1220 override, there is no clean syntactic way to proceed apart from
1223 An instruction is not required to use a prefix: prefixes such as
1224 \c{CS}, \c{A32}, \c{LOCK} or \c{REPE} can appear on a line by
1225 themselves, and NASM will just generate the prefix bytes.
1227 In addition to actual machine instructions, NASM also supports a
1228 number of pseudo-instructions, described in \k{pseudop}.
1230 Instruction \i{operands} may take a number of forms: they can be
1231 registers, described simply by the register name (e.g. \c{ax},
1232 \c{bp}, \c{ebx}, \c{cr0}: NASM does not use the \c{gas}-style
1233 syntax in which register names must be prefixed by a \c{%} sign), or
1234 they can be \i{effective addresses} (see \k{effaddr}), constants
1235 (\k{const}) or expressions (\k{expr}).
1237 For x87 \i{floating-point} instructions, NASM accepts a wide range of
1238 syntaxes: you can use two-operand forms like MASM supports, or you
1239 can use NASM's native single-operand forms in most cases.
1241 \# all forms of each supported instruction are given in
1243 For example, you can code:
1245 \c fadd st1 ; this sets st0 := st0 + st1
1246 \c fadd st0,st1 ; so does this
1248 \c fadd st1,st0 ; this sets st1 := st1 + st0
1249 \c fadd to st1 ; so does this
1251 Almost any x87 floating-point instruction that references memory must
1252 use one of the prefixes \i\c{DWORD}, \i\c{QWORD} or \i\c{TWORD} to
1253 indicate what size of \i{memory operand} it refers to.
1256 \H{pseudop} \i{Pseudo-Instructions}
1258 Pseudo-instructions are things which, though not real x86 machine
1259 instructions, are used in the instruction field anyway because that's
1260 the most convenient place to put them. The current pseudo-instructions
1261 are \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, \i\c{DO} and
1262 \i\c{DY}; their \i{uninitialized} counterparts \i\c{RESB}, \i\c{RESW},
1263 \i\c{RESD}, \i\c{RESQ}, \i\c{REST}, \i\c{RESO} and \i\c{RESY}; the
1264 \i\c{INCBIN} command, the \i\c{EQU} command, and the \i\c{TIMES}
1268 \S{db} \c{DB} and Friends: Declaring Initialized Data
1270 \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, \i\c{DO} and
1271 \i\c{DY} are used, much as in MASM, to declare initialized data in the
1272 output file. They can be invoked in a wide range of ways:
1273 \I{floating-point}\I{character constant}\I{string constant}
1275 \c db 0x55 ; just the byte 0x55
1276 \c db 0x55,0x56,0x57 ; three bytes in succession
1277 \c db 'a',0x55 ; character constants are OK
1278 \c db 'hello',13,10,'$' ; so are string constants
1279 \c dw 0x1234 ; 0x34 0x12
1280 \c dw 'a' ; 0x61 0x00 (it's just a number)
1281 \c dw 'ab' ; 0x61 0x62 (character constant)
1282 \c dw 'abc' ; 0x61 0x62 0x63 0x00 (string)
1283 \c dd 0x12345678 ; 0x78 0x56 0x34 0x12
1284 \c dd 1.234567e20 ; floating-point constant
1285 \c dq 0x123456789abcdef0 ; eight byte constant
1286 \c dq 1.234567e20 ; double-precision float
1287 \c dt 1.234567e20 ; extended-precision float
1289 \c{DT}, \c{DO} and \c{DY} do not accept \i{numeric constants} as operands.
1292 \S{resb} \c{RESB} and Friends: Declaring \i{Uninitialized} Data
1294 \i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST}, \i\c{RESO}
1295 and \i\c{RESY} are designed to be used in the BSS section of a module:
1296 they declare \e{uninitialized} storage space. Each takes a single
1297 operand, which is the number of bytes, words, doublewords or whatever
1298 to reserve. As stated in \k{qsother}, NASM does not support the
1299 MASM/TASM syntax of reserving uninitialized space by writing
1300 \I\c{?}\c{DW ?} or similar things: this is what it does instead. The
1301 operand to a \c{RESB}-type pseudo-instruction is a \i\e{critical
1302 expression}: see \k{crit}.
1306 \c buffer: resb 64 ; reserve 64 bytes
1307 \c wordvar: resw 1 ; reserve a word
1308 \c realarray resq 10 ; array of ten reals
1309 \c ymmval: resy 1 ; one YMM register
1311 \S{incbin} \i\c{INCBIN}: Including External \i{Binary Files}
1313 \c{INCBIN} is borrowed from the old Amiga assembler \i{DevPac}: it
1314 includes a binary file verbatim into the output file. This can be
1315 handy for (for example) including \i{graphics} and \i{sound} data
1316 directly into a game executable file. It can be called in one of
1319 \c incbin "file.dat" ; include the whole file
1320 \c incbin "file.dat",1024 ; skip the first 1024 bytes
1321 \c incbin "file.dat",1024,512 ; skip the first 1024, and
1322 \c ; actually include at most 512
1324 \c{INCBIN} is both a directive and a standard macro; the standard
1325 macro version searches for the file in the include file search path
1326 and adds the file to the dependency lists. This macro can be
1327 overridden if desired.
1330 \S{equ} \i\c{EQU}: Defining Constants
1332 \c{EQU} defines a symbol to a given constant value: when \c{EQU} is
1333 used, the source line must contain a label. The action of \c{EQU} is
1334 to define the given label name to the value of its (only) operand.
1335 This definition is absolute, and cannot change later. So, for
1338 \c message db 'hello, world'
1339 \c msglen equ $-message
1341 defines \c{msglen} to be the constant 12. \c{msglen} may not then be
1342 redefined later. This is not a \i{preprocessor} definition either:
1343 the value of \c{msglen} is evaluated \e{once}, using the value of
1344 \c{$} (see \k{expr} for an explanation of \c{$}) at the point of
1345 definition, rather than being evaluated wherever it is referenced
1346 and using the value of \c{$} at the point of reference.
1349 \S{times} \i\c{TIMES}: \i{Repeating} Instructions or Data
1351 The \c{TIMES} prefix causes the instruction to be assembled multiple
1352 times. This is partly present as NASM's equivalent of the \i\c{DUP}
1353 syntax supported by \i{MASM}-compatible assemblers, in that you can
1356 \c zerobuf: times 64 db 0
1358 or similar things; but \c{TIMES} is more versatile than that. The
1359 argument to \c{TIMES} is not just a numeric constant, but a numeric
1360 \e{expression}, so you can do things like
1362 \c buffer: db 'hello, world'
1363 \c times 64-$+buffer db ' '
1365 which will store exactly enough spaces to make the total length of
1366 \c{buffer} up to 64. Finally, \c{TIMES} can be applied to ordinary
1367 instructions, so you can code trivial \i{unrolled loops} in it:
1371 Note that there is no effective difference between \c{times 100 resb
1372 1} and \c{resb 100}, except that the latter will be assembled about
1373 100 times faster due to the internal structure of the assembler.
1375 The operand to \c{TIMES} is a critical expression (\k{crit}).
1377 Note also that \c{TIMES} can't be applied to \i{macros}: the reason
1378 for this is that \c{TIMES} is processed after the macro phase, which
1379 allows the argument to \c{TIMES} to contain expressions such as
1380 \c{64-$+buffer} as above. To repeat more than one line of code, or a
1381 complex macro, use the preprocessor \i\c{%rep} directive.
1384 \H{effaddr} Effective Addresses
1386 An \i{effective address} is any operand to an instruction which
1387 \I{memory reference}references memory. Effective addresses, in NASM,
1388 have a very simple syntax: they consist of an expression evaluating
1389 to the desired address, enclosed in \i{square brackets}. For
1394 \c mov ax,[wordvar+1]
1395 \c mov ax,[es:wordvar+bx]
1397 Anything not conforming to this simple system is not a valid memory
1398 reference in NASM, for example \c{es:wordvar[bx]}.
1400 More complicated effective addresses, such as those involving more
1401 than one register, work in exactly the same way:
1403 \c mov eax,[ebx*2+ecx+offset]
1406 NASM is capable of doing \i{algebra} on these effective addresses,
1407 so that things which don't necessarily \e{look} legal are perfectly
1410 \c mov eax,[ebx*5] ; assembles as [ebx*4+ebx]
1411 \c mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]
1413 Some forms of effective address have more than one assembled form;
1414 in most such cases NASM will generate the smallest form it can. For
1415 example, there are distinct assembled forms for the 32-bit effective
1416 addresses \c{[eax*2+0]} and \c{[eax+eax]}, and NASM will generally
1417 generate the latter on the grounds that the former requires four
1418 bytes to store a zero offset.
1420 NASM has a hinting mechanism which will cause \c{[eax+ebx]} and
1421 \c{[ebx+eax]} to generate different opcodes; this is occasionally
1422 useful because \c{[esi+ebp]} and \c{[ebp+esi]} have different
1423 default segment registers.
1425 However, you can force NASM to generate an effective address in a
1426 particular form by the use of the keywords \c{BYTE}, \c{WORD},
1427 \c{DWORD} and \c{NOSPLIT}. If you need \c{[eax+3]} to be assembled
1428 using a double-word offset field instead of the one byte NASM will
1429 normally generate, you can code \c{[dword eax+3]}. Similarly, you
1430 can force NASM to use a byte offset for a small value which it
1431 hasn't seen on the first pass (see \k{crit} for an example of such a
1432 code fragment) by using \c{[byte eax+offset]}. As special cases,
1433 \c{[byte eax]} will code \c{[eax+0]} with a byte offset of zero, and
1434 \c{[dword eax]} will code it with a double-word offset of zero. The
1435 normal form, \c{[eax]}, will be coded with no offset field.
1437 The form described in the previous paragraph is also useful if you
1438 are trying to access data in a 32-bit segment from within 16 bit code.
1439 For more information on this see the section on mixed-size addressing
1440 (\k{mixaddr}). In particular, if you need to access data with a known
1441 offset that is larger than will fit in a 16-bit value, if you don't
1442 specify that it is a dword offset, nasm will cause the high word of
1443 the offset to be lost.
1445 Similarly, NASM will split \c{[eax*2]} into \c{[eax+eax]} because
1446 that allows the offset field to be absent and space to be saved; in
1447 fact, it will also split \c{[eax*2+offset]} into
1448 \c{[eax+eax+offset]}. You can combat this behaviour by the use of
1449 the \c{NOSPLIT} keyword: \c{[nosplit eax*2]} will force
1450 \c{[eax*2+0]} to be generated literally.
1452 In 64-bit mode, NASM will by default generate absolute addresses. The
1453 \i\c{REL} keyword makes it produce \c{RIP}-relative addresses. Since
1454 this is frequently the normally desired behaviour, see the \c{DEFAULT}
1455 directive (\k{default}). The keyword \i\c{ABS} overrides \i\c{REL}.
1458 \H{const} \i{Constants}
1460 NASM understands four different types of constant: numeric,
1461 character, string and floating-point.
1464 \S{numconst} \i{Numeric Constants}
1466 A numeric constant is simply a number. NASM allows you to specify
1467 numbers in a variety of number bases, in a variety of ways: you can
1468 suffix \c{H} or \c{X}, \c{D} or \c{T}, \c{Q} or \c{O}, and \c{B} or
1469 \c{Y} for \i{hexadecimal}, \i{decimal}, \i{octal} and \i{binary}
1470 respectively, or you can prefix \c{0x}, for hexadecimal in the style
1471 of C, or you can prefix \c{$} for hexadecimal in the style of Borland
1472 Pascal or Motorola Assemblers. Note, though, that the \I{$,
1473 prefix}\c{$} prefix does double duty as a prefix on identifiers (see
1474 \k{syntax}), so a hex number prefixed with a \c{$} sign must have a
1475 digit after the \c{$} rather than a letter. In addition, current
1476 versions of NASM accept the prefix \c{0h} for hexadecimal, \c{0d} or
1477 \c{0t} for decimal, \c{0o} or \c{0q} for octal, and \c{0b} or \c{0y}
1478 for binary. Please note that unlike C, a \c{0} prefix by itself does
1479 \e{not} imply an octal constant!
1481 Numeric constants can have underscores (\c{_}) interspersed to break
1484 Some examples (all producing exactly the same code):
1486 \c mov ax,200 ; decimal
1487 \c mov ax,0200 ; still decimal
1488 \c mov ax,0200d ; explicitly decimal
1489 \c mov ax,0d200 ; also decimal
1490 \c mov ax,0c8h ; hex
1491 \c mov ax,$0c8 ; hex again: the 0 is required
1492 \c mov ax,0xc8 ; hex yet again
1493 \c mov ax,0hc8 ; still hex
1494 \c mov ax,310q ; octal
1495 \c mov ax,310o ; octal again
1496 \c mov ax,0o310 ; octal yet again
1497 \c mov ax,0q310 ; hex yet again
1498 \c mov ax,11001000b ; binary
1499 \c mov ax,1100_1000b ; same binary constant
1500 \c mov ax,1100_1000y ; same binary constant once more
1501 \c mov ax,0b1100_1000 ; same binary constant yet again
1502 \c mov ax,0y1100_1000 ; same binary constant yet again
1504 \S{strings} \I{Strings}\i{Character Strings}
1506 A character string consists of up to eight characters enclosed in
1507 either single quotes (\c{'...'}), double quotes (\c{"..."}) or
1508 backquotes (\c{`...`}). Single or double quotes are equivalent to
1509 NASM (except of course that surrounding the constant with single
1510 quotes allows double quotes to appear within it and vice versa); the
1511 contents of those are represented verbatim. Strings enclosed in
1512 backquotes support C-style \c{\\}-escapes for special characters.
1515 The following \i{escape sequences} are recognized by backquoted strings:
1517 \c \' single quote (')
1518 \c \" double quote (")
1520 \c \\\ backslash (\)
1521 \c \? question mark (?)
1529 \c \e ESC (ASCII 27)
1530 \c \377 Up to 3 octal digits - literal byte
1531 \c \xFF Up to 2 hexadecimal digits - literal byte
1532 \c \u1234 4 hexadecimal digits - Unicode character
1533 \c \U12345678 8 hexadecimal digits - Unicode character
1535 All other escape sequences are reserved. Note that \c{\\0}, meaning a
1536 \c{NUL} character (ASCII 0), is a special case of the octal escape
1539 \i{Unicode} characters specified with \c{\\u} or \c{\\U} are converted to
1540 \i{UTF-8}. For example, the following lines are all equivalent:
1542 \c db `\u263a` ; UTF-8 smiley face
1543 \c db `\xe2\x98\xba` ; UTF-8 smiley face
1544 \c db 0E2h, 098h, 0BAh ; UTF-8 smiley face
1547 \S{chrconst} \i{Character Constants}
1549 A character constant consists of a string up to eight bytes long, used
1550 in an expression context. It is treated as if it was an integer.
1552 A character constant with more than one byte will be arranged
1553 with \i{little-endian} order in mind: if you code
1557 then the constant generated is not \c{0x61626364}, but
1558 \c{0x64636261}, so that if you were then to store the value into
1559 memory, it would read \c{abcd} rather than \c{dcba}. This is also
1560 the sense of character constants understood by the Pentium's
1561 \i\c{CPUID} instruction.
1564 \S{strconst} \i{String Constants}
1566 String constants are character strings used in the context of some
1567 pseudo-instructions, namely the
1568 \I\c{DW}\I\c{DD}\I\c{DQ}\I\c{DT}\I\c{DO}\I\c{DY}\i\c{DB} family and
1569 \i\c{INCBIN} (where it represents a filename.) They are also used in
1570 certain preprocessor directives.
1572 A string constant looks like a character constant, only longer. It
1573 is treated as a concatenation of maximum-size character constants
1574 for the conditions. So the following are equivalent:
1576 \c db 'hello' ; string constant
1577 \c db 'h','e','l','l','o' ; equivalent character constants
1579 And the following are also equivalent:
1581 \c dd 'ninechars' ; doubleword string constant
1582 \c dd 'nine','char','s' ; becomes three doublewords
1583 \c db 'ninechars',0,0,0 ; and really looks like this
1585 Note that when used in a string-supporting context, quoted strings are
1586 treated as a string constants even if they are short enough to be a
1587 character constant, because otherwise \c{db 'ab'} would have the same
1588 effect as \c{db 'a'}, which would be silly. Similarly, three-character
1589 or four-character constants are treated as strings when they are
1590 operands to \c{DW}, and so forth.
1592 \S{unicode} \I{UTF-16}\I{UTF-32}\i{Unicode} Strings
1594 The special operators \i\c{__utf16__} and \i\c{__utf32__} allows
1595 definition of Unicode strings. They take a string in UTF-8 format and
1596 converts it to (littleendian) UTF-16 or UTF-32, respectively.
1600 \c %define u(x) __utf16__(x)
1601 \c %define w(x) __utf32__(x)
1603 \c dw u('C:\WINDOWS'), 0 ; Pathname in UTF-16
1604 \c dd w(`A + B = \u206a`), 0 ; String in UTF-32
1606 \c{__utf16__} and \c{__utf32__} can be applied either to strings
1607 passed to the \c{DB} family instructions, or to character constants in
1608 an expression context.
1610 \S{fltconst} \I{floating-point, constants}Floating-Point Constants
1612 \i{Floating-point} constants are acceptable only as arguments to
1613 \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, and \i\c{DO}, or as
1614 arguments to the special operators \i\c{__float8__},
1615 \i\c{__float16__}, \i\c{__float32__}, \i\c{__float64__},
1616 \i\c{__float80m__}, \i\c{__float80e__}, \i\c{__float128l__}, and
1617 \i\c{__float128h__}.
1619 Floating-point constants are expressed in the traditional form:
1620 digits, then a period, then optionally more digits, then optionally an
1621 \c{E} followed by an exponent. The period is mandatory, so that NASM
1622 can distinguish between \c{dd 1}, which declares an integer constant,
1623 and \c{dd 1.0} which declares a floating-point constant.
1625 NASM also support C99-style hexadecimal floating-point: \c{0x},
1626 hexadecimal digits, period, optionally more hexadeximal digits, then
1627 optionally a \c{P} followed by a \e{binary} (not hexadecimal) exponent
1628 in decimal notation. As an extension, NASM additionally supports the
1629 \c{0h} and \c{$} prefixes for hexadecimal, as well binary and octal
1630 floating-point, using the \c{0b} or \c{0y} and \c{0o} or \c{0q}
1631 prefixes, respectively.
1633 Underscores to break up groups of digits are permitted in
1634 floating-point constants as well.
1638 \c db -0.2 ; "Quarter precision"
1639 \c dw -0.5 ; IEEE 754r/SSE5 half precision
1640 \c dd 1.2 ; an easy one
1641 \c dd 1.222_222_222 ; underscores are permitted
1642 \c dd 0x1p+2 ; 1.0x2^2 = 4.0
1643 \c dq 0x1p+32 ; 1.0x2^32 = 4 294 967 296.0
1644 \c dq 1.e10 ; 10 000 000 000.0
1645 \c dq 1.e+10 ; synonymous with 1.e10
1646 \c dq 1.e-10 ; 0.000 000 000 1
1647 \c dt 3.141592653589793238462 ; pi
1648 \c do 1.e+4000 ; IEEE 754r quad precision
1650 The 8-bit "quarter-precision" floating-point format is
1651 sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This
1652 appears to be the most frequently used 8-bit floating-point format,
1653 although it is not covered by any formal standard. This is sometimes
1654 called a "\i{minifloat}."
1656 The special operators are used to produce floating-point numbers in
1657 other contexts. They produce the binary representation of a specific
1658 floating-point number as an integer, and can use anywhere integer
1659 constants are used in an expression. \c{__float80m__} and
1660 \c{__float80e__} produce the 64-bit mantissa and 16-bit exponent of an
1661 80-bit floating-point number, and \c{__float128l__} and
1662 \c{__float128h__} produce the lower and upper 64-bit halves of a 128-bit
1663 floating-point number, respectively.
1667 \c mov rax,__float64__(3.141592653589793238462)
1669 ... would assign the binary representation of pi as a 64-bit floating
1670 point number into \c{RAX}. This is exactly equivalent to:
1672 \c mov rax,0x400921fb54442d18
1674 NASM cannot do compile-time arithmetic on floating-point constants.
1675 This is because NASM is designed to be portable - although it always
1676 generates code to run on x86 processors, the assembler itself can
1677 run on any system with an ANSI C compiler. Therefore, the assembler
1678 cannot guarantee the presence of a floating-point unit capable of
1679 handling the \i{Intel number formats}, and so for NASM to be able to
1680 do floating arithmetic it would have to include its own complete set
1681 of floating-point routines, which would significantly increase the
1682 size of the assembler for very little benefit.
1684 The special tokens \i\c{__Infinity__}, \i\c{__QNaN__} (or
1685 \i\c{__NaN__}) and \i\c{__SNaN__} can be used to generate
1686 \I{infinity}infinities, quiet \i{NaN}s, and signalling NaNs,
1687 respectively. These are normally used as macros:
1689 \c %define Inf __Infinity__
1690 \c %define NaN __QNaN__
1692 \c dq +1.5, -Inf, NaN ; Double-precision constants
1694 \S{bcdconst} \I{floating-point, packed BCD constants}Packed BCD Constants
1696 x87-style packed BCD constants can be used in the same contexts as
1697 80-bit floating-point numbers. They are suffixed with \c{p} or
1698 prefixed with \c{0p}, and can include up to 18 decimal digits.
1700 As with other numeric constants, underscores can be used to separate
1705 \c dt 12_345_678_901_245_678p
1706 \c dt -12_345_678_901_245_678p
1711 \H{expr} \i{Expressions}
1713 Expressions in NASM are similar in syntax to those in C. Expressions
1714 are evaluated as 64-bit integers which are then adjusted to the
1717 NASM supports two special tokens in expressions, allowing
1718 calculations to involve the current assembly position: the
1719 \I{$, here}\c{$} and \i\c{$$} tokens. \c{$} evaluates to the assembly
1720 position at the beginning of the line containing the expression; so
1721 you can code an \i{infinite loop} using \c{JMP $}. \c{$$} evaluates
1722 to the beginning of the current section; so you can tell how far
1723 into the section you are by using \c{($-$$)}.
1725 The arithmetic \i{operators} provided by NASM are listed here, in
1726 increasing order of \i{precedence}.
1729 \S{expor} \i\c{|}: \i{Bitwise OR} Operator
1731 The \c{|} operator gives a bitwise OR, exactly as performed by the
1732 \c{OR} machine instruction. Bitwise OR is the lowest-priority
1733 arithmetic operator supported by NASM.
1736 \S{expxor} \i\c{^}: \i{Bitwise XOR} Operator
1738 \c{^} provides the bitwise XOR operation.
1741 \S{expand} \i\c{&}: \i{Bitwise AND} Operator
1743 \c{&} provides the bitwise AND operation.
1746 \S{expshift} \i\c{<<} and \i\c{>>}: \i{Bit Shift} Operators
1748 \c{<<} gives a bit-shift to the left, just as it does in C. So \c{5<<3}
1749 evaluates to 5 times 8, or 40. \c{>>} gives a bit-shift to the
1750 right; in NASM, such a shift is \e{always} unsigned, so that
1751 the bits shifted in from the left-hand end are filled with zero
1752 rather than a sign-extension of the previous highest bit.
1755 \S{expplmi} \I{+ opaddition}\c{+} and \I{- opsubtraction}\c{-}:
1756 \i{Addition} and \i{Subtraction} Operators
1758 The \c{+} and \c{-} operators do perfectly ordinary addition and
1762 \S{expmul} \i\c{*}, \i\c{/}, \i\c{//}, \i\c{%} and \i\c{%%}:
1763 \i{Multiplication} and \i{Division}
1765 \c{*} is the multiplication operator. \c{/} and \c{//} are both
1766 division operators: \c{/} is \i{unsigned division} and \c{//} is
1767 \i{signed division}. Similarly, \c{%} and \c{%%} provide \I{unsigned
1768 modulo}\I{modulo operators}unsigned and
1769 \i{signed modulo} operators respectively.
1771 NASM, like ANSI C, provides no guarantees about the sensible
1772 operation of the signed modulo operator.
1774 Since the \c{%} character is used extensively by the macro
1775 \i{preprocessor}, you should ensure that both the signed and unsigned
1776 modulo operators are followed by white space wherever they appear.
1779 \S{expmul} \i{Unary Operators}: \I{+ opunary}\c{+}, \I{- opunary}\c{-},
1780 \i\c{~}, \I{! opunary}\c{!} and \i\c{SEG}
1782 The highest-priority operators in NASM's expression grammar are
1783 those which only apply to one argument. \c{-} negates its operand,
1784 \c{+} does nothing (it's provided for symmetry with \c{-}), \c{~}
1785 computes the \i{one's complement} of its operand, \c{!} is the
1786 \i{logical negation} operator, and \c{SEG} provides the \i{segment address}
1787 of its operand (explained in more detail in \k{segwrt}).
1790 \H{segwrt} \i\c{SEG} and \i\c{WRT}
1792 When writing large 16-bit programs, which must be split into
1793 multiple \i{segments}, it is often necessary to be able to refer to
1794 the \I{segment address}segment part of the address of a symbol. NASM
1795 supports the \c{SEG} operator to perform this function.
1797 The \c{SEG} operator returns the \i\e{preferred} segment base of a
1798 symbol, defined as the segment base relative to which the offset of
1799 the symbol makes sense. So the code
1801 \c mov ax,seg symbol
1805 will load \c{ES:BX} with a valid pointer to the symbol \c{symbol}.
1807 Things can be more complex than this: since 16-bit segments and
1808 \i{groups} may \I{overlapping segments}overlap, you might occasionally
1809 want to refer to some symbol using a different segment base from the
1810 preferred one. NASM lets you do this, by the use of the \c{WRT}
1811 (With Reference To) keyword. So you can do things like
1813 \c mov ax,weird_seg ; weird_seg is a segment base
1815 \c mov bx,symbol wrt weird_seg
1817 to load \c{ES:BX} with a different, but functionally equivalent,
1818 pointer to the symbol \c{symbol}.
1820 NASM supports far (inter-segment) calls and jumps by means of the
1821 syntax \c{call segment:offset}, where \c{segment} and \c{offset}
1822 both represent immediate values. So to call a far procedure, you
1823 could code either of
1825 \c call (seg procedure):procedure
1826 \c call weird_seg:(procedure wrt weird_seg)
1828 (The parentheses are included for clarity, to show the intended
1829 parsing of the above instructions. They are not necessary in
1832 NASM supports the syntax \I\c{CALL FAR}\c{call far procedure} as a
1833 synonym for the first of the above usages. \c{JMP} works identically
1834 to \c{CALL} in these examples.
1836 To declare a \i{far pointer} to a data item in a data segment, you
1839 \c dw symbol, seg symbol
1841 NASM supports no convenient synonym for this, though you can always
1842 invent one using the macro processor.
1845 \H{strict} \i\c{STRICT}: Inhibiting Optimization
1847 When assembling with the optimizer set to level 2 or higher (see
1848 \k{opt-O}), NASM will use size specifiers (\c{BYTE}, \c{WORD},
1849 \c{DWORD}, \c{QWORD}, \c{TWORD}, \c{OWORD} or \c{YWORD}), but will
1850 give them the smallest possible size. The keyword \c{STRICT} can be
1851 used to inhibit optimization and force a particular operand to be
1852 emitted in the specified size. For example, with the optimizer on, and
1853 in \c{BITS 16} mode,
1857 is encoded in three bytes \c{66 6A 21}, whereas
1859 \c push strict dword 33
1861 is encoded in six bytes, with a full dword immediate operand \c{66 68
1864 With the optimizer off, the same code (six bytes) is generated whether
1865 the \c{STRICT} keyword was used or not.
1868 \H{crit} \i{Critical Expressions}
1870 Although NASM has an optional multi-pass optimizer, there are some
1871 expressions which must be resolvable on the first pass. These are
1872 called \e{Critical Expressions}.
1874 The first pass is used to determine the size of all the assembled
1875 code and data, so that the second pass, when generating all the
1876 code, knows all the symbol addresses the code refers to. So one
1877 thing NASM can't handle is code whose size depends on the value of a
1878 symbol declared after the code in question. For example,
1880 \c times (label-$) db 0
1881 \c label: db 'Where am I?'
1883 The argument to \i\c{TIMES} in this case could equally legally
1884 evaluate to anything at all; NASM will reject this example because
1885 it cannot tell the size of the \c{TIMES} line when it first sees it.
1886 It will just as firmly reject the slightly \I{paradox}paradoxical
1889 \c times (label-$+1) db 0
1890 \c label: db 'NOW where am I?'
1892 in which \e{any} value for the \c{TIMES} argument is by definition
1895 NASM rejects these examples by means of a concept called a
1896 \e{critical expression}, which is defined to be an expression whose
1897 value is required to be computable in the first pass, and which must
1898 therefore depend only on symbols defined before it. The argument to
1899 the \c{TIMES} prefix is a critical expression.
1901 \H{locallab} \i{Local Labels}
1903 NASM gives special treatment to symbols beginning with a \i{period}.
1904 A label beginning with a single period is treated as a \e{local}
1905 label, which means that it is associated with the previous non-local
1906 label. So, for example:
1908 \c label1 ; some code
1916 \c label2 ; some code
1924 In the above code fragment, each \c{JNE} instruction jumps to the
1925 line immediately before it, because the two definitions of \c{.loop}
1926 are kept separate by virtue of each being associated with the
1927 previous non-local label.
1929 This form of local label handling is borrowed from the old Amiga
1930 assembler \i{DevPac}; however, NASM goes one step further, in
1931 allowing access to local labels from other parts of the code. This
1932 is achieved by means of \e{defining} a local label in terms of the
1933 previous non-local label: the first definition of \c{.loop} above is
1934 really defining a symbol called \c{label1.loop}, and the second
1935 defines a symbol called \c{label2.loop}. So, if you really needed
1938 \c label3 ; some more code
1943 Sometimes it is useful - in a macro, for instance - to be able to
1944 define a label which can be referenced from anywhere but which
1945 doesn't interfere with the normal local-label mechanism. Such a
1946 label can't be non-local because it would interfere with subsequent
1947 definitions of, and references to, local labels; and it can't be
1948 local because the macro that defined it wouldn't know the label's
1949 full name. NASM therefore introduces a third type of label, which is
1950 probably only useful in macro definitions: if a label begins with
1951 the \I{label prefix}special prefix \i\c{..@}, then it does nothing
1952 to the local label mechanism. So you could code
1954 \c label1: ; a non-local label
1955 \c .local: ; this is really label1.local
1956 \c ..@foo: ; this is a special symbol
1957 \c label2: ; another non-local label
1958 \c .local: ; this is really label2.local
1960 \c jmp ..@foo ; this will jump three lines up
1962 NASM has the capacity to define other special symbols beginning with
1963 a double period: for example, \c{..start} is used to specify the
1964 entry point in the \c{obj} output format (see \k{dotdotstart}),
1965 \c{..imagebase} is used to find out the offset from a base address
1966 of the current image in the \c{win64} output format (see \k{win64pic}).
1967 So just keep in mind that symbols beginning with a double period are
1971 \C{preproc} The NASM \i{Preprocessor}
1973 NASM contains a powerful \i{macro processor}, which supports
1974 conditional assembly, multi-level file inclusion, two forms of macro
1975 (single-line and multi-line), and a `context stack' mechanism for
1976 extra macro power. Preprocessor directives all begin with a \c{%}
1979 The preprocessor collapses all lines which end with a backslash (\\)
1980 character into a single line. Thus:
1982 \c %define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \\
1985 will work like a single-line macro without the backslash-newline
1988 \H{slmacro} \i{Single-Line Macros}
1990 \S{define} The Normal Way: \I\c{%idefine}\i\c{%define}
1992 Single-line macros are defined using the \c{%define} preprocessor
1993 directive. The definitions work in a similar way to C; so you can do
1996 \c %define ctrl 0x1F &
1997 \c %define param(a,b) ((a)+(a)*(b))
1999 \c mov byte [param(2,ebx)], ctrl 'D'
2001 which will expand to
2003 \c mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
2005 When the expansion of a single-line macro contains tokens which
2006 invoke another macro, the expansion is performed at invocation time,
2007 not at definition time. Thus the code
2009 \c %define a(x) 1+b(x)
2014 will evaluate in the expected way to \c{mov ax,1+2*8}, even though
2015 the macro \c{b} wasn't defined at the time of definition of \c{a}.
2017 Macros defined with \c{%define} are \i{case sensitive}: after
2018 \c{%define foo bar}, only \c{foo} will expand to \c{bar}: \c{Foo} or
2019 \c{FOO} will not. By using \c{%idefine} instead of \c{%define} (the
2020 `i' stands for `insensitive') you can define all the case variants
2021 of a macro at once, so that \c{%idefine foo bar} would cause
2022 \c{foo}, \c{Foo}, \c{FOO}, \c{fOO} and so on all to expand to
2025 There is a mechanism which detects when a macro call has occurred as
2026 a result of a previous expansion of the same macro, to guard against
2027 \i{circular references} and infinite loops. If this happens, the
2028 preprocessor will only expand the first occurrence of the macro.
2031 \c %define a(x) 1+a(x)
2035 the macro \c{a(3)} will expand once, becoming \c{1+a(3)}, and will
2036 then expand no further. This behaviour can be useful: see \k{32c}
2037 for an example of its use.
2039 You can \I{overloading, single-line macros}overload single-line
2040 macros: if you write
2042 \c %define foo(x) 1+x
2043 \c %define foo(x,y) 1+x*y
2045 the preprocessor will be able to handle both types of macro call,
2046 by counting the parameters you pass; so \c{foo(3)} will become
2047 \c{1+3} whereas \c{foo(ebx,2)} will become \c{1+ebx*2}. However, if
2052 then no other definition of \c{foo} will be accepted: a macro with
2053 no parameters prohibits the definition of the same name as a macro
2054 \e{with} parameters, and vice versa.
2056 This doesn't prevent single-line macros being \e{redefined}: you can
2057 perfectly well define a macro with
2061 and then re-define it later in the same source file with
2065 Then everywhere the macro \c{foo} is invoked, it will be expanded
2066 according to the most recent definition. This is particularly useful
2067 when defining single-line macros with \c{%assign} (see \k{assign}).
2069 You can \i{pre-define} single-line macros using the `-d' option on
2070 the NASM command line: see \k{opt-d}.
2073 \S{xdefine} Resolving \c{%define}: \I\c{%ixdefine}\i\c{%xdefine}
2075 To have a reference to an embedded single-line macro resolved at the
2076 time that the embedding macro is \e{defined}, as opposed to when the
2077 embedding macro is \e{expanded}, you need a different mechanism to the
2078 one offered by \c{%define}. The solution is to use \c{%xdefine}, or
2079 it's \I{case sensitive}case-insensitive counterpart \c{%ixdefine}.
2081 Suppose you have the following code:
2084 \c %define isFalse isTrue
2093 In this case, \c{val1} is equal to 0, and \c{val2} is equal to 1.
2094 This is because, when a single-line macro is defined using
2095 \c{%define}, it is expanded only when it is called. As \c{isFalse}
2096 expands to \c{isTrue}, the expansion will be the current value of
2097 \c{isTrue}. The first time it is called that is 0, and the second
2100 If you wanted \c{isFalse} to expand to the value assigned to the
2101 embedded macro \c{isTrue} at the time that \c{isFalse} was defined,
2102 you need to change the above code to use \c{%xdefine}.
2104 \c %xdefine isTrue 1
2105 \c %xdefine isFalse isTrue
2106 \c %xdefine isTrue 0
2110 \c %xdefine isTrue 1
2114 Now, each time that \c{isFalse} is called, it expands to 1,
2115 as that is what the embedded macro \c{isTrue} expanded to at
2116 the time that \c{isFalse} was defined.
2119 \S{indmacro} \i{Macro Indirection}: \I\c{%[}\c{%[...]}
2121 The \c{%[...]} construct can be used to expand macros in contexts
2122 where macro expansion would otherwise not occur, including in the
2123 names other macros. For example, if you have a set of macros named
2124 \c{Foo16}, \c{Foo32} and \c{Foo64}, you could write:
2126 \c mov ax,Foo%[__BITS__] ; The Foo value
2128 to use the builtin macro \c{__BITS__} (see \k{bitsm}) to automatically
2129 select between them. Similarly, the two statements:
2131 \c %xdefine Bar Quux ; Expands due to %xdefine
2132 \c %define Bar %[Quux] ; Expands due to %[...]
2134 have, in fact, exactly the same effect.
2136 \c{%[...]} concatenates to adjacent tokens in the same way that
2137 multi-line macro parameters do, see \k{concat} for details.
2140 \S{concat%+} Concatenating Single Line Macro Tokens: \i\c{%+}
2142 Individual tokens in single line macros can be concatenated, to produce
2143 longer tokens for later processing. This can be useful if there are
2144 several similar macros that perform similar functions.
2146 Please note that a space is required after \c{%+}, in order to
2147 disambiguate it from the syntax \c{%+1} used in multiline macros.
2149 As an example, consider the following:
2151 \c %define BDASTART 400h ; Start of BIOS data area
2153 \c struc tBIOSDA ; its structure
2159 Now, if we need to access the elements of tBIOSDA in different places,
2162 \c mov ax,BDASTART + tBIOSDA.COM1addr
2163 \c mov bx,BDASTART + tBIOSDA.COM2addr
2165 This will become pretty ugly (and tedious) if used in many places, and
2166 can be reduced in size significantly by using the following macro:
2168 \c ; Macro to access BIOS variables by their names (from tBDA):
2170 \c %define BDA(x) BDASTART + tBIOSDA. %+ x
2172 Now the above code can be written as:
2174 \c mov ax,BDA(COM1addr)
2175 \c mov bx,BDA(COM2addr)
2177 Using this feature, we can simplify references to a lot of macros (and,
2178 in turn, reduce typing errors).
2181 \S{selfref%?} The Macro Name Itself: \i\c{%?} and \i\c{%??}
2183 The special symbols \c{%?} and \c{%??} can be used to reference the
2184 macro name itself inside a macro expansion, this is supported for both
2185 single-and multi-line macros. \c{%?} refers to the macro name as
2186 \e{invoked}, whereas \c{%??} refers to the macro name as
2187 \e{declared}. The two are always the same for case-sensitive
2188 macros, but for case-insensitive macros, they can differ.
2192 \c %idefine Foo mov %?,%??
2204 \c %idefine keyword $%?
2206 can be used to make a keyword "disappear", for example in case a new
2207 instruction has been used as a label in older code. For example:
2209 \c %idefine pause $%? ; Hide the PAUSE instruction
2212 \S{undef} Undefining Single-Line Macros: \i\c{%undef}
2214 Single-line macros can be removed with the \c{%undef} directive. For
2215 example, the following sequence:
2222 will expand to the instruction \c{mov eax, foo}, since after
2223 \c{%undef} the macro \c{foo} is no longer defined.
2225 Macros that would otherwise be pre-defined can be undefined on the
2226 command-line using the `-u' option on the NASM command line: see
2230 \S{assign} \i{Preprocessor Variables}: \i\c{%assign}
2232 An alternative way to define single-line macros is by means of the
2233 \c{%assign} command (and its \I{case sensitive}case-insensitive
2234 counterpart \i\c{%iassign}, which differs from \c{%assign} in
2235 exactly the same way that \c{%idefine} differs from \c{%define}).
2237 \c{%assign} is used to define single-line macros which take no
2238 parameters and have a numeric value. This value can be specified in
2239 the form of an expression, and it will be evaluated once, when the
2240 \c{%assign} directive is processed.
2242 Like \c{%define}, macros defined using \c{%assign} can be re-defined
2243 later, so you can do things like
2247 to increment the numeric value of a macro.
2249 \c{%assign} is useful for controlling the termination of \c{%rep}
2250 preprocessor loops: see \k{rep} for an example of this. Another
2251 use for \c{%assign} is given in \k{16c} and \k{32c}.
2253 The expression passed to \c{%assign} is a \i{critical expression}
2254 (see \k{crit}), and must also evaluate to a pure number (rather than
2255 a relocatable reference such as a code or data address, or anything
2256 involving a register).
2259 \S{defstr} Defining Strings: \I\c{%idefstr}\i\c{%defstr}
2261 \c{%defstr}, and its case-insensitive counterpart \c{%idefstr}, define
2262 or redefine a single-line macro without parameters but converts the
2263 entire right-hand side, after macro expansion, to a quoted string
2268 \c %defstr test TEST
2272 \c %define test 'TEST'
2274 This can be used, for example, with the \c{%!} construct (see
2277 \c %defstr PATH %!PATH ; The operating system PATH variable
2280 \S{deftok} Defining Tokens: \I\c{%ideftok}\i\c{%deftok}
2282 \c{%deftok}, and its case-insensitive counterpart \c{%ideftok}, define
2283 or redefine a single-line macro without parameters but converts the
2284 second parameter, after string conversion, to a sequence of tokens.
2288 \c %deftok test 'TEST'
2292 \c %define test TEST
2295 \H{strlen} \i{String Manipulation in Macros}
2297 It's often useful to be able to handle strings in macros. NASM
2298 supports a few simple string handling macro operators from which
2299 more complex operations can be constructed.
2301 All the string operators define or redefine a value (either a string
2302 or a numeric value) to a single-line macro. When producing a string
2303 value, it may change the style of quoting of the input string or
2304 strings, and possibly use \c{\\}-escapes inside \c{`}-quoted strings.
2306 \S{strcat} \i{Concatenating Strings}: \i\c{%strcat}
2308 The \c{%strcat} operator concatenates quoted strings and assign them to
2309 a single-line macro.
2313 \c %strcat alpha "Alpha: ", '12" screen'
2315 ... would assign the value \c{'Alpha: 12" screen'} to \c{alpha}.
2318 \c %strcat beta '"foo"\', "'bar'"
2320 ... would assign the value \c{`"foo"\\\\'bar'`} to \c{beta}.
2322 The use of commas to separate strings is permitted but optional.
2325 \S{strlen} \i{String Length}: \i\c{%strlen}
2327 The \c{%strlen} operator assigns the length of a string to a macro.
2330 \c %strlen charcnt 'my string'
2332 In this example, \c{charcnt} would receive the value 9, just as
2333 if an \c{%assign} had been used. In this example, \c{'my string'}
2334 was a literal string but it could also have been a single-line
2335 macro that expands to a string, as in the following example:
2337 \c %define sometext 'my string'
2338 \c %strlen charcnt sometext
2340 As in the first case, this would result in \c{charcnt} being
2341 assigned the value of 9.
2344 \S{substr} \i{Extracting Substrings}: \i\c{%substr}
2346 Individual letters or substrings in strings can be extracted using the
2347 \c{%substr} operator. An example of its use is probably more useful
2348 than the description:
2350 \c %substr mychar 'xyzw' 1 ; equivalent to %define mychar 'x'
2351 \c %substr mychar 'xyzw' 2 ; equivalent to %define mychar 'y'
2352 \c %substr mychar 'xyzw' 3 ; equivalent to %define mychar 'z'
2353 \c %substr mychar 'xyzw' 2,2 ; equivalent to %define mychar 'yz'
2354 \c %substr mychar 'xyzw' 2,-1 ; equivalent to %define mychar 'yzw'
2355 \c %substr mychar 'xyzw' 2,-2 ; equivalent to %define mychar 'yz'
2357 As with \c{%strlen} (see \k{strlen}), the first parameter is the
2358 single-line macro to be created and the second is the string. The
2359 third parameter specifies the first character to be selected, and the
2360 optional fourth parameter preceeded by comma) is the length. Note
2361 that the first index is 1, not 0 and the last index is equal to the
2362 value that \c{%strlen} would assign given the same string. Index
2363 values out of range result in an empty string. A negative length
2364 means "until N-1 characters before the end of string", i.e. \c{-1}
2365 means until end of string, \c{-2} until one character before, etc.
2368 \H{mlmacro} \i{Multi-Line Macros}: \I\c{%imacro}\i\c{%macro}
2370 Multi-line macros are much more like the type of macro seen in MASM
2371 and TASM: a multi-line macro definition in NASM looks something like
2374 \c %macro prologue 1
2382 This defines a C-like function prologue as a macro: so you would
2383 invoke the macro with a call such as
2385 \c myfunc: prologue 12
2387 which would expand to the three lines of code
2393 The number \c{1} after the macro name in the \c{%macro} line defines
2394 the number of parameters the macro \c{prologue} expects to receive.
2395 The use of \c{%1} inside the macro definition refers to the first
2396 parameter to the macro call. With a macro taking more than one
2397 parameter, subsequent parameters would be referred to as \c{%2},
2400 Multi-line macros, like single-line macros, are \i{case-sensitive},
2401 unless you define them using the alternative directive \c{%imacro}.
2403 If you need to pass a comma as \e{part} of a parameter to a
2404 multi-line macro, you can do that by enclosing the entire parameter
2405 in \I{braces, around macro parameters}braces. So you could code
2414 \c silly 'a', letter_a ; letter_a: db 'a'
2415 \c silly 'ab', string_ab ; string_ab: db 'ab'
2416 \c silly {13,10}, crlf ; crlf: db 13,10
2419 \#\S{mlrmacro} \i{Recursive Multi-Line Macros}: \I\c{%irmacro}\i\c{%rmacro}
2421 \#A multi-line macro cannot be referenced within itself, in order to
2422 \#prevent accidental infinite recursion.
2424 \#Recursive multi-line macros allow for self-referencing, with the
2425 \#caveat that the user is aware of the existence, use and purpose of
2426 \#recursive multi-line macros. There is also a generous, but sane, upper
2427 \#limit to the number of recursions, in order to prevent run-away memory
2428 \#consumption in case of accidental infinite recursion.
2430 \#As with non-recursive multi-line macros, recursive multi-line macros are
2431 \#\i{case-sensitive}, unless you define them using the alternative
2432 \#directive \c{%irmacro}.
2434 \S{mlmacover} Overloading Multi-Line Macros\I{overloading, multi-line macros}
2436 As with single-line macros, multi-line macros can be overloaded by
2437 defining the same macro name several times with different numbers of
2438 parameters. This time, no exception is made for macros with no
2439 parameters at all. So you could define
2441 \c %macro prologue 0
2448 to define an alternative form of the function prologue which
2449 allocates no local stack space.
2451 Sometimes, however, you might want to `overload' a machine
2452 instruction; for example, you might want to define
2461 so that you could code
2463 \c push ebx ; this line is not a macro call
2464 \c push eax,ecx ; but this one is
2466 Ordinarily, NASM will give a warning for the first of the above two
2467 lines, since \c{push} is now defined to be a macro, and is being
2468 invoked with a number of parameters for which no definition has been
2469 given. The correct code will still be generated, but the assembler
2470 will give a warning. This warning can be disabled by the use of the
2471 \c{-w-macro-params} command-line option (see \k{opt-w}).
2474 \S{maclocal} \i{Macro-Local Labels}
2476 NASM allows you to define labels within a multi-line macro
2477 definition in such a way as to make them local to the macro call: so
2478 calling the same macro multiple times will use a different label
2479 each time. You do this by prefixing \i\c{%%} to the label name. So
2480 you can invent an instruction which executes a \c{RET} if the \c{Z}
2481 flag is set by doing this:
2491 You can call this macro as many times as you want, and every time
2492 you call it NASM will make up a different `real' name to substitute
2493 for the label \c{%%skip}. The names NASM invents are of the form
2494 \c{..@2345.skip}, where the number 2345 changes with every macro
2495 call. The \i\c{..@} prefix prevents macro-local labels from
2496 interfering with the local label mechanism, as described in
2497 \k{locallab}. You should avoid defining your own labels in this form
2498 (the \c{..@} prefix, then a number, then another period) in case
2499 they interfere with macro-local labels.
2502 \S{mlmacgre} \i{Greedy Macro Parameters}
2504 Occasionally it is useful to define a macro which lumps its entire
2505 command line into one parameter definition, possibly after
2506 extracting one or two smaller parameters from the front. An example
2507 might be a macro to write a text string to a file in MS-DOS, where
2508 you might want to be able to write
2510 \c writefile [filehandle],"hello, world",13,10
2512 NASM allows you to define the last parameter of a macro to be
2513 \e{greedy}, meaning that if you invoke the macro with more
2514 parameters than it expects, all the spare parameters get lumped into
2515 the last defined one along with the separating commas. So if you
2518 \c %macro writefile 2+
2524 \c mov cx,%%endstr-%%str
2531 then the example call to \c{writefile} above will work as expected:
2532 the text before the first comma, \c{[filehandle]}, is used as the
2533 first macro parameter and expanded when \c{%1} is referred to, and
2534 all the subsequent text is lumped into \c{%2} and placed after the
2537 The greedy nature of the macro is indicated to NASM by the use of
2538 the \I{+ modifier}\c{+} sign after the parameter count on the
2541 If you define a greedy macro, you are effectively telling NASM how
2542 it should expand the macro given \e{any} number of parameters from
2543 the actual number specified up to infinity; in this case, for
2544 example, NASM now knows what to do when it sees a call to
2545 \c{writefile} with 2, 3, 4 or more parameters. NASM will take this
2546 into account when overloading macros, and will not allow you to
2547 define another form of \c{writefile} taking 4 parameters (for
2550 Of course, the above macro could have been implemented as a
2551 non-greedy macro, in which case the call to it would have had to
2554 \c writefile [filehandle], {"hello, world",13,10}
2556 NASM provides both mechanisms for putting \i{commas in macro
2557 parameters}, and you choose which one you prefer for each macro
2560 See \k{sectmac} for a better way to write the above macro.
2562 \S{mlmacrange} \i{Macro Parameters Range}
2564 NASM allows you to expand parameters via special construction \c{%\{x:y\}}
2565 where \c{x} is the first parameter index and \c{y} is the last. Any index can
2566 be either negative or positive but must never be zero.
2576 expands to \c{3,4,5} range.
2578 Even more, the parameters can be reversed so that
2586 expands to \c{5,4,3} range.
2588 But even this is not the last. The parameters can be addressed via negative
2589 indices so NASM will count them reversed. The ones who know Python may see
2598 expands to \c{6,5,4} range.
2600 Note that NASM uses \i{comma} to separate parameters being expanded.
2602 By the way, here is a trick - you might use the index \c{%{-1:-1}}
2603 which gives you the \i{last} argument passed to a macro.
2605 \S{mlmacdef} \i{Default Macro Parameters}
2607 NASM also allows you to define a multi-line macro with a \e{range}
2608 of allowable parameter counts. If you do this, you can specify
2609 defaults for \i{omitted parameters}. So, for example:
2611 \c %macro die 0-1 "Painful program death has occurred."
2619 This macro (which makes use of the \c{writefile} macro defined in
2620 \k{mlmacgre}) can be called with an explicit error message, which it
2621 will display on the error output stream before exiting, or it can be
2622 called with no parameters, in which case it will use the default
2623 error message supplied in the macro definition.
2625 In general, you supply a minimum and maximum number of parameters
2626 for a macro of this type; the minimum number of parameters are then
2627 required in the macro call, and then you provide defaults for the
2628 optional ones. So if a macro definition began with the line
2630 \c %macro foobar 1-3 eax,[ebx+2]
2632 then it could be called with between one and three parameters, and
2633 \c{%1} would always be taken from the macro call. \c{%2}, if not
2634 specified by the macro call, would default to \c{eax}, and \c{%3} if
2635 not specified would default to \c{[ebx+2]}.
2637 You can provide extra information to a macro by providing
2638 too many default parameters:
2640 \c %macro quux 1 something
2642 This will trigger a warning by default; see \k{opt-w} for
2644 When \c{quux} is invoked, it receives not one but two parameters.
2645 \c{something} can be referred to as \c{%2}. The difference
2646 between passing \c{something} this way and writing \c{something}
2647 in the macro body is that with this way \c{something} is evaluated
2648 when the macro is defined, not when it is expanded.
2650 You may omit parameter defaults from the macro definition, in which
2651 case the parameter default is taken to be blank. This can be useful
2652 for macros which can take a variable number of parameters, since the
2653 \i\c{%0} token (see \k{percent0}) allows you to determine how many
2654 parameters were really passed to the macro call.
2656 This defaulting mechanism can be combined with the greedy-parameter
2657 mechanism; so the \c{die} macro above could be made more powerful,
2658 and more useful, by changing the first line of the definition to
2660 \c %macro die 0-1+ "Painful program death has occurred.",13,10
2662 The maximum parameter count can be infinite, denoted by \c{*}. In
2663 this case, of course, it is impossible to provide a \e{full} set of
2664 default parameters. Examples of this usage are shown in \k{rotate}.
2667 \S{percent0} \i\c{%0}: \I{counting macro parameters}Macro Parameter Counter
2669 The parameter reference \c{%0} will return a numeric constant giving the
2670 number of parameters received, that is, if \c{%0} is n then \c{%}n is the
2671 last parameter. \c{%0} is mostly useful for macros that can take a variable
2672 number of parameters. It can be used as an argument to \c{%rep}
2673 (see \k{rep}) in order to iterate through all the parameters of a macro.
2674 Examples are given in \k{rotate}.
2677 \S{rotate} \i\c{%rotate}: \i{Rotating Macro Parameters}
2679 Unix shell programmers will be familiar with the \I{shift
2680 command}\c{shift} shell command, which allows the arguments passed
2681 to a shell script (referenced as \c{$1}, \c{$2} and so on) to be
2682 moved left by one place, so that the argument previously referenced
2683 as \c{$2} becomes available as \c{$1}, and the argument previously
2684 referenced as \c{$1} is no longer available at all.
2686 NASM provides a similar mechanism, in the form of \c{%rotate}. As
2687 its name suggests, it differs from the Unix \c{shift} in that no
2688 parameters are lost: parameters rotated off the left end of the
2689 argument list reappear on the right, and vice versa.
2691 \c{%rotate} is invoked with a single numeric argument (which may be
2692 an expression). The macro parameters are rotated to the left by that
2693 many places. If the argument to \c{%rotate} is negative, the macro
2694 parameters are rotated to the right.
2696 \I{iterating over macro parameters}So a pair of macros to save and
2697 restore a set of registers might work as follows:
2699 \c %macro multipush 1-*
2708 This macro invokes the \c{PUSH} instruction on each of its arguments
2709 in turn, from left to right. It begins by pushing its first
2710 argument, \c{%1}, then invokes \c{%rotate} to move all the arguments
2711 one place to the left, so that the original second argument is now
2712 available as \c{%1}. Repeating this procedure as many times as there
2713 were arguments (achieved by supplying \c{%0} as the argument to
2714 \c{%rep}) causes each argument in turn to be pushed.
2716 Note also the use of \c{*} as the maximum parameter count,
2717 indicating that there is no upper limit on the number of parameters
2718 you may supply to the \i\c{multipush} macro.
2720 It would be convenient, when using this macro, to have a \c{POP}
2721 equivalent, which \e{didn't} require the arguments to be given in
2722 reverse order. Ideally, you would write the \c{multipush} macro
2723 call, then cut-and-paste the line to where the pop needed to be
2724 done, and change the name of the called macro to \c{multipop}, and
2725 the macro would take care of popping the registers in the opposite
2726 order from the one in which they were pushed.
2728 This can be done by the following definition:
2730 \c %macro multipop 1-*
2739 This macro begins by rotating its arguments one place to the
2740 \e{right}, so that the original \e{last} argument appears as \c{%1}.
2741 This is then popped, and the arguments are rotated right again, so
2742 the second-to-last argument becomes \c{%1}. Thus the arguments are
2743 iterated through in reverse order.
2746 \S{concat} \i{Concatenating Macro Parameters}
2748 NASM can concatenate macro parameters and macro indirection constructs
2749 on to other text surrounding them. This allows you to declare a family
2750 of symbols, for example, in a macro definition. If, for example, you
2751 wanted to generate a table of key codes along with offsets into the
2752 table, you could code something like
2754 \c %macro keytab_entry 2
2756 \c keypos%1 equ $-keytab
2762 \c keytab_entry F1,128+1
2763 \c keytab_entry F2,128+2
2764 \c keytab_entry Return,13
2766 which would expand to
2769 \c keyposF1 equ $-keytab
2771 \c keyposF2 equ $-keytab
2773 \c keyposReturn equ $-keytab
2776 You can just as easily concatenate text on to the other end of a
2777 macro parameter, by writing \c{%1foo}.
2779 If you need to append a \e{digit} to a macro parameter, for example
2780 defining labels \c{foo1} and \c{foo2} when passed the parameter
2781 \c{foo}, you can't code \c{%11} because that would be taken as the
2782 eleventh macro parameter. Instead, you must code
2783 \I{braces, after % sign}\c{%\{1\}1}, which will separate the first
2784 \c{1} (giving the number of the macro parameter) from the second
2785 (literal text to be concatenated to the parameter).
2787 This concatenation can also be applied to other preprocessor in-line
2788 objects, such as macro-local labels (\k{maclocal}) and context-local
2789 labels (\k{ctxlocal}). In all cases, ambiguities in syntax can be
2790 resolved by enclosing everything after the \c{%} sign and before the
2791 literal text in braces: so \c{%\{%foo\}bar} concatenates the text
2792 \c{bar} to the end of the real name of the macro-local label
2793 \c{%%foo}. (This is unnecessary, since the form NASM uses for the
2794 real names of macro-local labels means that the two usages
2795 \c{%\{%foo\}bar} and \c{%%foobar} would both expand to the same
2796 thing anyway; nevertheless, the capability is there.)
2798 The single-line macro indirection construct, \c{%[...]}
2799 (\k{indmacro}), behaves the same way as macro parameters for the
2800 purpose of concatenation.
2802 See also the \c{%+} operator, \k{concat%+}.
2805 \S{mlmaccc} \i{Condition Codes as Macro Parameters}
2807 NASM can give special treatment to a macro parameter which contains
2808 a condition code. For a start, you can refer to the macro parameter
2809 \c{%1} by means of the alternative syntax \i\c{%+1}, which informs
2810 NASM that this macro parameter is supposed to contain a condition
2811 code, and will cause the preprocessor to report an error message if
2812 the macro is called with a parameter which is \e{not} a valid
2815 Far more usefully, though, you can refer to the macro parameter by
2816 means of \i\c{%-1}, which NASM will expand as the \e{inverse}
2817 condition code. So the \c{retz} macro defined in \k{maclocal} can be
2818 replaced by a general \i{conditional-return macro} like this:
2828 This macro can now be invoked using calls like \c{retc ne}, which
2829 will cause the conditional-jump instruction in the macro expansion
2830 to come out as \c{JE}, or \c{retc po} which will make the jump a
2833 The \c{%+1} macro-parameter reference is quite happy to interpret
2834 the arguments \c{CXZ} and \c{ECXZ} as valid condition codes;
2835 however, \c{%-1} will report an error if passed either of these,
2836 because no inverse condition code exists.
2839 \S{nolist} \i{Disabling Listing Expansion}\I\c{.nolist}
2841 When NASM is generating a listing file from your program, it will
2842 generally expand multi-line macros by means of writing the macro
2843 call and then listing each line of the expansion. This allows you to
2844 see which instructions in the macro expansion are generating what
2845 code; however, for some macros this clutters the listing up
2848 NASM therefore provides the \c{.nolist} qualifier, which you can
2849 include in a macro definition to inhibit the expansion of the macro
2850 in the listing file. The \c{.nolist} qualifier comes directly after
2851 the number of parameters, like this:
2853 \c %macro foo 1.nolist
2857 \c %macro bar 1-5+.nolist a,b,c,d,e,f,g,h
2859 \S{unmacro} Undefining Multi-Line Macros: \i\c{%unmacro}
2861 Multi-line macros can be removed with the \c{%unmacro} directive.
2862 Unlike the \c{%undef} directive, however, \c{%unmacro} takes an
2863 argument specification, and will only remove \i{exact matches} with
2864 that argument specification.
2873 removes the previously defined macro \c{foo}, but
2880 does \e{not} remove the macro \c{bar}, since the argument
2881 specification does not match exactly.
2884 \#\S{exitmacro} Exiting Multi-Line Macros: \i\c{%exitmacro}
2886 \#Multi-line macro expansions can be arbitrarily terminated with
2887 \#the \c{%exitmacro} directive.
2899 \H{condasm} \i{Conditional Assembly}\I\c{%if}
2901 Similarly to the C preprocessor, NASM allows sections of a source
2902 file to be assembled only if certain conditions are met. The general
2903 syntax of this feature looks like this:
2906 \c ; some code which only appears if <condition> is met
2907 \c %elif<condition2>
2908 \c ; only appears if <condition> is not met but <condition2> is
2910 \c ; this appears if neither <condition> nor <condition2> was met
2913 The inverse forms \i\c{%ifn} and \i\c{%elifn} are also supported.
2915 The \i\c{%else} clause is optional, as is the \i\c{%elif} clause.
2916 You can have more than one \c{%elif} clause as well.
2918 There are a number of variants of the \c{%if} directive. Each has its
2919 corresponding \c{%elif}, \c{%ifn}, and \c{%elifn} directives; for
2920 example, the equivalents to the \c{%ifdef} directive are \c{%elifdef},
2921 \c{%ifndef}, and \c{%elifndef}.
2923 \S{ifdef} \i\c{%ifdef}: Testing Single-Line Macro Existence\I{testing,
2924 single-line macro existence}
2926 Beginning a conditional-assembly block with the line \c{%ifdef
2927 MACRO} will assemble the subsequent code if, and only if, a
2928 single-line macro called \c{MACRO} is defined. If not, then the
2929 \c{%elif} and \c{%else} blocks (if any) will be processed instead.
2931 For example, when debugging a program, you might want to write code
2934 \c ; perform some function
2936 \c writefile 2,"Function performed successfully",13,10
2938 \c ; go and do something else
2940 Then you could use the command-line option \c{-dDEBUG} to create a
2941 version of the program which produced debugging messages, and remove
2942 the option to generate the final release version of the program.
2944 You can test for a macro \e{not} being defined by using
2945 \i\c{%ifndef} instead of \c{%ifdef}. You can also test for macro
2946 definitions in \c{%elif} blocks by using \i\c{%elifdef} and
2950 \S{ifmacro} \i\c{%ifmacro}: Testing Multi-Line Macro
2951 Existence\I{testing, multi-line macro existence}
2953 The \c{%ifmacro} directive operates in the same way as the \c{%ifdef}
2954 directive, except that it checks for the existence of a multi-line macro.
2956 For example, you may be working with a large project and not have control
2957 over the macros in a library. You may want to create a macro with one
2958 name if it doesn't already exist, and another name if one with that name
2961 The \c{%ifmacro} is considered true if defining a macro with the given name
2962 and number of arguments would cause a definitions conflict. For example:
2964 \c %ifmacro MyMacro 1-3
2966 \c %error "MyMacro 1-3" causes a conflict with an existing macro.
2970 \c %macro MyMacro 1-3
2972 \c ; insert code to define the macro
2978 This will create the macro "MyMacro 1-3" if no macro already exists which
2979 would conflict with it, and emits a warning if there would be a definition
2982 You can test for the macro not existing by using the \i\c{%ifnmacro} instead
2983 of \c{%ifmacro}. Additional tests can be performed in \c{%elif} blocks by using
2984 \i\c{%elifmacro} and \i\c{%elifnmacro}.
2987 \S{ifctx} \i\c{%ifctx}: Testing the Context Stack\I{testing, context
2990 The conditional-assembly construct \c{%ifctx} will cause the
2991 subsequent code to be assembled if and only if the top context on
2992 the preprocessor's context stack has the same name as one of the arguments.
2993 As with \c{%ifdef}, the inverse and \c{%elif} forms \i\c{%ifnctx},
2994 \i\c{%elifctx} and \i\c{%elifnctx} are also supported.
2996 For more details of the context stack, see \k{ctxstack}. For a
2997 sample use of \c{%ifctx}, see \k{blockif}.
3000 \S{if} \i\c{%if}: Testing Arbitrary Numeric Expressions\I{testing,
3001 arbitrary numeric expressions}
3003 The conditional-assembly construct \c{%if expr} will cause the
3004 subsequent code to be assembled if and only if the value of the
3005 numeric expression \c{expr} is non-zero. An example of the use of
3006 this feature is in deciding when to break out of a \c{%rep}
3007 preprocessor loop: see \k{rep} for a detailed example.
3009 The expression given to \c{%if}, and its counterpart \i\c{%elif}, is
3010 a critical expression (see \k{crit}).
3012 \c{%if} extends the normal NASM expression syntax, by providing a
3013 set of \i{relational operators} which are not normally available in
3014 expressions. The operators \i\c{=}, \i\c{<}, \i\c{>}, \i\c{<=},
3015 \i\c{>=} and \i\c{<>} test equality, less-than, greater-than,
3016 less-or-equal, greater-or-equal and not-equal respectively. The
3017 C-like forms \i\c{==} and \i\c{!=} are supported as alternative
3018 forms of \c{=} and \c{<>}. In addition, low-priority logical
3019 operators \i\c{&&}, \i\c{^^} and \i\c{||} are provided, supplying
3020 \i{logical AND}, \i{logical XOR} and \i{logical OR}. These work like
3021 the C logical operators (although C has no logical XOR), in that
3022 they always return either 0 or 1, and treat any non-zero input as 1
3023 (so that \c{^^}, for example, returns 1 if exactly one of its inputs
3024 is zero, and 0 otherwise). The relational operators also return 1
3025 for true and 0 for false.
3027 Like other \c{%if} constructs, \c{%if} has a counterpart
3028 \i\c{%elif}, and negative forms \i\c{%ifn} and \i\c{%elifn}.
3030 \S{ifidn} \i\c{%ifidn} and \i\c{%ifidni}: Testing Exact Text
3031 Identity\I{testing, exact text identity}
3033 The construct \c{%ifidn text1,text2} will cause the subsequent code
3034 to be assembled if and only if \c{text1} and \c{text2}, after
3035 expanding single-line macros, are identical pieces of text.
3036 Differences in white space are not counted.
3038 \c{%ifidni} is similar to \c{%ifidn}, but is \i{case-insensitive}.
3040 For example, the following macro pushes a register or number on the
3041 stack, and allows you to treat \c{IP} as a real register:
3043 \c %macro pushparam 1
3054 Like other \c{%if} constructs, \c{%ifidn} has a counterpart
3055 \i\c{%elifidn}, and negative forms \i\c{%ifnidn} and \i\c{%elifnidn}.
3056 Similarly, \c{%ifidni} has counterparts \i\c{%elifidni},
3057 \i\c{%ifnidni} and \i\c{%elifnidni}.
3059 \S{iftyp} \i\c{%ifid}, \i\c{%ifnum}, \i\c{%ifstr}: Testing Token
3060 Types\I{testing, token types}
3062 Some macros will want to perform different tasks depending on
3063 whether they are passed a number, a string, or an identifier. For
3064 example, a string output macro might want to be able to cope with
3065 being passed either a string constant or a pointer to an existing
3068 The conditional assembly construct \c{%ifid}, taking one parameter
3069 (which may be blank), assembles the subsequent code if and only if
3070 the first token in the parameter exists and is an identifier.
3071 \c{%ifnum} works similarly, but tests for the token being a numeric
3072 constant; \c{%ifstr} tests for it being a string.
3074 For example, the \c{writefile} macro defined in \k{mlmacgre} can be
3075 extended to take advantage of \c{%ifstr} in the following fashion:
3077 \c %macro writefile 2-3+
3086 \c %%endstr: mov dx,%%str
3087 \c mov cx,%%endstr-%%str
3098 Then the \c{writefile} macro can cope with being called in either of
3099 the following two ways:
3101 \c writefile [file], strpointer, length
3102 \c writefile [file], "hello", 13, 10
3104 In the first, \c{strpointer} is used as the address of an
3105 already-declared string, and \c{length} is used as its length; in
3106 the second, a string is given to the macro, which therefore declares
3107 it itself and works out the address and length for itself.
3109 Note the use of \c{%if} inside the \c{%ifstr}: this is to detect
3110 whether the macro was passed two arguments (so the string would be a
3111 single string constant, and \c{db %2} would be adequate) or more (in
3112 which case, all but the first two would be lumped together into
3113 \c{%3}, and \c{db %2,%3} would be required).
3115 The usual \I\c{%elifid}\I\c{%elifnum}\I\c{%elifstr}\c{%elif}...,
3116 \I\c{%ifnid}\I\c{%ifnnum}\I\c{%ifnstr}\c{%ifn}..., and
3117 \I\c{%elifnid}\I\c{%elifnnum}\I\c{%elifnstr}\c{%elifn}... versions
3118 exist for each of \c{%ifid}, \c{%ifnum} and \c{%ifstr}.
3120 \S{iftoken} \i\c{%iftoken}: Test for a Single Token
3122 Some macros will want to do different things depending on if it is
3123 passed a single token (e.g. paste it to something else using \c{%+})
3124 versus a multi-token sequence.
3126 The conditional assembly construct \c{%iftoken} assembles the
3127 subsequent code if and only if the expanded parameters consist of
3128 exactly one token, possibly surrounded by whitespace.
3134 will assemble the subsequent code, but
3138 will not, since \c{-1} contains two tokens: the unary minus operator
3139 \c{-}, and the number \c{1}.
3141 The usual \i\c{%eliftoken}, \i\c\{%ifntoken}, and \i\c{%elifntoken}
3142 variants are also provided.
3144 \S{ifempty} \i\c{%ifempty}: Test for Empty Expansion
3146 The conditional assembly construct \c{%ifempty} assembles the
3147 subsequent code if and only if the expanded parameters do not contain
3148 any tokens at all, whitespace excepted.
3150 The usual \i\c{%elifempty}, \i\c\{%ifnempty}, and \i\c{%elifnempty}
3151 variants are also provided.
3153 \S{ifenv} \i\c{%ifenv}: Test If Environment Variable Exists
3155 The conditional assembly construct \c{%ifenv} assembles the
3156 subsequent code if and only if the environment variable referenced by
3157 the \c{%!<env>} directive exists.
3159 The usual \i\c{%elifenv}, \i\c\{%ifnenv}, and \i\c{%elifnenv}
3160 variants are also provided.
3162 Just as for \c{%!<env>} the argument should be written as a string if
3163 it contains characters that would not be legal in an identifier. See
3166 \H{rep} \i{Preprocessor Loops}\I{repeating code}: \i\c{%rep}
3168 NASM's \c{TIMES} prefix, though useful, cannot be used to invoke a
3169 multi-line macro multiple times, because it is processed by NASM
3170 after macros have already been expanded. Therefore NASM provides
3171 another form of loop, this time at the preprocessor level: \c{%rep}.
3173 The directives \c{%rep} and \i\c{%endrep} (\c{%rep} takes a numeric
3174 argument, which can be an expression; \c{%endrep} takes no
3175 arguments) can be used to enclose a chunk of code, which is then
3176 replicated as many times as specified by the preprocessor:
3180 \c inc word [table+2*i]
3184 This will generate a sequence of 64 \c{INC} instructions,
3185 incrementing every word of memory from \c{[table]} to
3188 For more complex termination conditions, or to break out of a repeat
3189 loop part way along, you can use the \i\c{%exitrep} directive to
3190 terminate the loop, like this:
3205 \c fib_number equ ($-fibonacci)/2
3207 This produces a list of all the Fibonacci numbers that will fit in
3208 16 bits. Note that a maximum repeat count must still be given to
3209 \c{%rep}. This is to prevent the possibility of NASM getting into an
3210 infinite loop in the preprocessor, which (on multitasking or
3211 multi-user systems) would typically cause all the system memory to
3212 be gradually used up and other applications to start crashing.
3214 Note a maximum repeat count is limited by 62 bit number, though it
3215 is hardly possible that you ever need anything bigger.
3218 \H{files} Source Files and Dependencies
3220 These commands allow you to split your sources into multiple files.
3222 \S{include} \i\c{%include}: \i{Including Other Files}
3224 Using, once again, a very similar syntax to the C preprocessor,
3225 NASM's preprocessor lets you include other source files into your
3226 code. This is done by the use of the \i\c{%include} directive:
3228 \c %include "macros.mac"
3230 will include the contents of the file \c{macros.mac} into the source
3231 file containing the \c{%include} directive.
3233 Include files are \I{searching for include files}searched for in the
3234 current directory (the directory you're in when you run NASM, as
3235 opposed to the location of the NASM executable or the location of
3236 the source file), plus any directories specified on the NASM command
3237 line using the \c{-i} option.
3239 The standard C idiom for preventing a file being included more than
3240 once is just as applicable in NASM: if the file \c{macros.mac} has
3243 \c %ifndef MACROS_MAC
3244 \c %define MACROS_MAC
3245 \c ; now define some macros
3248 then including the file more than once will not cause errors,
3249 because the second time the file is included nothing will happen
3250 because the macro \c{MACROS_MAC} will already be defined.
3252 You can force a file to be included even if there is no \c{%include}
3253 directive that explicitly includes it, by using the \i\c{-p} option
3254 on the NASM command line (see \k{opt-p}).
3257 \S{pathsearch} \i\c{%pathsearch}: Search the Include Path
3259 The \c{%pathsearch} directive takes a single-line macro name and a
3260 filename, and declare or redefines the specified single-line macro to
3261 be the include-path-resolved version of the filename, if the file
3262 exists (otherwise, it is passed unchanged.)
3266 \c %pathsearch MyFoo "foo.bin"
3268 ... with \c{-Ibins/} in the include path may end up defining the macro
3269 \c{MyFoo} to be \c{"bins/foo.bin"}.
3272 \S{depend} \i\c{%depend}: Add Dependent Files
3274 The \c{%depend} directive takes a filename and adds it to the list of
3275 files to be emitted as dependency generation when the \c{-M} options
3276 and its relatives (see \k{opt-M}) are used. It produces no output.
3278 This is generally used in conjunction with \c{%pathsearch}. For
3279 example, a simplified version of the standard macro wrapper for the
3280 \c{INCBIN} directive looks like:
3282 \c %imacro incbin 1-2+ 0
3283 \c %pathsearch dep %1
3288 This first resolves the location of the file into the macro \c{dep},
3289 then adds it to the dependency lists, and finally issues the
3290 assembler-level \c{INCBIN} directive.
3293 \S{use} \i\c{%use}: Include Standard Macro Package
3295 The \c{%use} directive is similar to \c{%include}, but rather than
3296 including the contents of a file, it includes a named standard macro
3297 package. The standard macro packages are part of NASM, and are
3298 described in \k{macropkg}.
3300 Unlike the \c{%include} directive, package names for the \c{%use}
3301 directive do not require quotes, but quotes are permitted. In NASM
3302 2.04 and 2.05 the unquoted form would be macro-expanded; this is no
3303 longer true. Thus, the following lines are equivalent:
3308 Standard macro packages are protected from multiple inclusion. When a
3309 standard macro package is used, a testable single-line macro of the
3310 form \c{__USE_}\e{package}\c{__} is also defined, see \k{use_def}.
3312 \H{ctxstack} The \i{Context Stack}
3314 Having labels that are local to a macro definition is sometimes not
3315 quite powerful enough: sometimes you want to be able to share labels
3316 between several macro calls. An example might be a \c{REPEAT} ...
3317 \c{UNTIL} loop, in which the expansion of the \c{REPEAT} macro
3318 would need to be able to refer to a label which the \c{UNTIL} macro
3319 had defined. However, for such a macro you would also want to be
3320 able to nest these loops.
3322 NASM provides this level of power by means of a \e{context stack}.
3323 The preprocessor maintains a stack of \e{contexts}, each of which is
3324 characterized by a name. You add a new context to the stack using
3325 the \i\c{%push} directive, and remove one using \i\c{%pop}. You can
3326 define labels that are local to a particular context on the stack.
3329 \S{pushpop} \i\c{%push} and \i\c{%pop}: \I{creating
3330 contexts}\I{removing contexts}Creating and Removing Contexts
3332 The \c{%push} directive is used to create a new context and place it
3333 on the top of the context stack. \c{%push} takes an optional argument,
3334 which is the name of the context. For example:
3338 This pushes a new context called \c{foobar} on the stack. You can have
3339 several contexts on the stack with the same name: they can still be
3340 distinguished. If no name is given, the context is unnamed (this is
3341 normally used when both the \c{%push} and the \c{%pop} are inside a
3342 single macro definition.)
3344 The directive \c{%pop}, taking one optional argument, removes the top
3345 context from the context stack and destroys it, along with any
3346 labels associated with it. If an argument is given, it must match the
3347 name of the current context, otherwise it will issue an error.
3350 \S{ctxlocal} \i{Context-Local Labels}
3352 Just as the usage \c{%%foo} defines a label which is local to the
3353 particular macro call in which it is used, the usage \I{%$}\c{%$foo}
3354 is used to define a label which is local to the context on the top
3355 of the context stack. So the \c{REPEAT} and \c{UNTIL} example given
3356 above could be implemented by means of:
3372 and invoked by means of, for example,
3380 which would scan every fourth byte of a string in search of the byte
3383 If you need to define, or access, labels local to the context
3384 \e{below} the top one on the stack, you can use \I{%$$}\c{%$$foo}, or
3385 \c{%$$$foo} for the context below that, and so on.
3388 \S{ctxdefine} \i{Context-Local Single-Line Macros}
3390 NASM also allows you to define single-line macros which are local to
3391 a particular context, in just the same way:
3393 \c %define %$localmac 3
3395 will define the single-line macro \c{%$localmac} to be local to the
3396 top context on the stack. Of course, after a subsequent \c{%push},
3397 it can then still be accessed by the name \c{%$$localmac}.
3400 \S{ctxrepl} \i\c{%repl}: \I{renaming contexts}Renaming a Context
3402 If you need to change the name of the top context on the stack (in
3403 order, for example, to have it respond differently to \c{%ifctx}),
3404 you can execute a \c{%pop} followed by a \c{%push}; but this will
3405 have the side effect of destroying all context-local labels and
3406 macros associated with the context that was just popped.
3408 NASM provides the directive \c{%repl}, which \e{replaces} a context
3409 with a different name, without touching the associated macros and
3410 labels. So you could replace the destructive code
3415 with the non-destructive version \c{%repl newname}.
3418 \S{blockif} Example Use of the \i{Context Stack}: \i{Block IFs}
3420 This example makes use of almost all the context-stack features,
3421 including the conditional-assembly construct \i\c{%ifctx}, to
3422 implement a block IF statement as a set of macros.
3438 \c %error "expected `if' before `else'"
3452 \c %error "expected `if' or `else' before `endif'"
3457 This code is more robust than the \c{REPEAT} and \c{UNTIL} macros
3458 given in \k{ctxlocal}, because it uses conditional assembly to check
3459 that the macros are issued in the right order (for example, not
3460 calling \c{endif} before \c{if}) and issues a \c{%error} if they're
3463 In addition, the \c{endif} macro has to be able to cope with the two
3464 distinct cases of either directly following an \c{if}, or following
3465 an \c{else}. It achieves this, again, by using conditional assembly
3466 to do different things depending on whether the context on top of
3467 the stack is \c{if} or \c{else}.
3469 The \c{else} macro has to preserve the context on the stack, in
3470 order to have the \c{%$ifnot} referred to by the \c{if} macro be the
3471 same as the one defined by the \c{endif} macro, but has to change
3472 the context's name so that \c{endif} will know there was an
3473 intervening \c{else}. It does this by the use of \c{%repl}.
3475 A sample usage of these macros might look like:
3497 The block-\c{IF} macros handle nesting quite happily, by means of
3498 pushing another context, describing the inner \c{if}, on top of the
3499 one describing the outer \c{if}; thus \c{else} and \c{endif} always
3500 refer to the last unmatched \c{if} or \c{else}.
3503 \H{stackrel} \i{Stack Relative Preprocessor Directives}
3505 The following preprocessor directives provide a way to use
3506 labels to refer to local variables allocated on the stack.
3508 \b\c{%arg} (see \k{arg})
3510 \b\c{%stacksize} (see \k{stacksize})
3512 \b\c{%local} (see \k{local})
3515 \S{arg} \i\c{%arg} Directive
3517 The \c{%arg} directive is used to simplify the handling of
3518 parameters passed on the stack. Stack based parameter passing
3519 is used by many high level languages, including C, C++ and Pascal.
3521 While NASM has macros which attempt to duplicate this
3522 functionality (see \k{16cmacro}), the syntax is not particularly
3523 convenient to use and is not TASM compatible. Here is an example
3524 which shows the use of \c{%arg} without any external macros:
3528 \c %push mycontext ; save the current context
3529 \c %stacksize large ; tell NASM to use bp
3530 \c %arg i:word, j_ptr:word
3537 \c %pop ; restore original context
3539 This is similar to the procedure defined in \k{16cmacro} and adds
3540 the value in i to the value pointed to by j_ptr and returns the
3541 sum in the ax register. See \k{pushpop} for an explanation of
3542 \c{push} and \c{pop} and the use of context stacks.
3545 \S{stacksize} \i\c{%stacksize} Directive
3547 The \c{%stacksize} directive is used in conjunction with the
3548 \c{%arg} (see \k{arg}) and the \c{%local} (see \k{local}) directives.
3549 It tells NASM the default size to use for subsequent \c{%arg} and
3550 \c{%local} directives. The \c{%stacksize} directive takes one
3551 required argument which is one of \c{flat}, \c{flat64}, \c{large} or \c{small}.
3555 This form causes NASM to use stack-based parameter addressing
3556 relative to \c{ebp} and it assumes that a near form of call was used
3557 to get to this label (i.e. that \c{eip} is on the stack).
3559 \c %stacksize flat64
3561 This form causes NASM to use stack-based parameter addressing
3562 relative to \c{rbp} and it assumes that a near form of call was used
3563 to get to this label (i.e. that \c{rip} is on the stack).
3567 This form uses \c{bp} to do stack-based parameter addressing and
3568 assumes that a far form of call was used to get to this address
3569 (i.e. that \c{ip} and \c{cs} are on the stack).
3573 This form also uses \c{bp} to address stack parameters, but it is
3574 different from \c{large} because it also assumes that the old value
3575 of bp is pushed onto the stack (i.e. it expects an \c{ENTER}
3576 instruction). In other words, it expects that \c{bp}, \c{ip} and
3577 \c{cs} are on the top of the stack, underneath any local space which
3578 may have been allocated by \c{ENTER}. This form is probably most
3579 useful when used in combination with the \c{%local} directive
3583 \S{local} \i\c{%local} Directive
3585 The \c{%local} directive is used to simplify the use of local
3586 temporary stack variables allocated in a stack frame. Automatic
3587 local variables in C are an example of this kind of variable. The
3588 \c{%local} directive is most useful when used with the \c{%stacksize}
3589 (see \k{stacksize} and is also compatible with the \c{%arg} directive
3590 (see \k{arg}). It allows simplified reference to variables on the
3591 stack which have been allocated typically by using the \c{ENTER}
3593 \# (see \k{insENTER} for a description of that instruction).
3594 An example of its use is the following:
3598 \c %push mycontext ; save the current context
3599 \c %stacksize small ; tell NASM to use bp
3600 \c %assign %$localsize 0 ; see text for explanation
3601 \c %local old_ax:word, old_dx:word
3603 \c enter %$localsize,0 ; see text for explanation
3604 \c mov [old_ax],ax ; swap ax & bx
3605 \c mov [old_dx],dx ; and swap dx & cx
3610 \c leave ; restore old bp
3613 \c %pop ; restore original context
3615 The \c{%$localsize} variable is used internally by the
3616 \c{%local} directive and \e{must} be defined within the
3617 current context before the \c{%local} directive may be used.
3618 Failure to do so will result in one expression syntax error for
3619 each \c{%local} variable declared. It then may be used in
3620 the construction of an appropriately sized ENTER instruction
3621 as shown in the example.
3624 \H{pperror} Reporting \i{User-Defined Errors}: \i\c{%error}, \i\c{%warning}, \i\c{%fatal}
3626 The preprocessor directive \c{%error} will cause NASM to report an
3627 error if it occurs in assembled code. So if other users are going to
3628 try to assemble your source files, you can ensure that they define the
3629 right macros by means of code like this:
3634 \c ; do some different setup
3636 \c %error "Neither F1 nor F2 was defined."
3639 Then any user who fails to understand the way your code is supposed
3640 to be assembled will be quickly warned of their mistake, rather than
3641 having to wait until the program crashes on being run and then not
3642 knowing what went wrong.
3644 Similarly, \c{%warning} issues a warning, but allows assembly to continue:
3649 \c ; do some different setup
3651 \c %warning "Neither F1 nor F2 was defined, assuming F1."
3655 \c{%error} and \c{%warning} are issued only on the final assembly
3656 pass. This makes them safe to use in conjunction with tests that
3657 depend on symbol values.
3659 \c{%fatal} terminates assembly immediately, regardless of pass. This
3660 is useful when there is no point in continuing the assembly further,
3661 and doing so is likely just going to cause a spew of confusing error
3664 It is optional for the message string after \c{%error}, \c{%warning}
3665 or \c{%fatal} to be quoted. If it is \e{not}, then single-line macros
3666 are expanded in it, which can be used to display more information to
3667 the user. For example:
3670 \c %assign foo_over foo-64
3671 \c %error foo is foo_over bytes too large
3675 \H{otherpreproc} \i{Other Preprocessor Directives}
3677 NASM also has preprocessor directives which allow access to
3678 information from external sources. Currently they include:
3680 \b\c{%line} enables NASM to correctly handle the output of another
3681 preprocessor (see \k{line}).
3683 \b\c{%!} enables NASM to read in the value of an environment variable,
3684 which can then be used in your program (see \k{getenv}).
3686 \S{line} \i\c{%line} Directive
3688 The \c{%line} directive is used to notify NASM that the input line
3689 corresponds to a specific line number in another file. Typically
3690 this other file would be an original source file, with the current
3691 NASM input being the output of a pre-processor. The \c{%line}
3692 directive allows NASM to output messages which indicate the line
3693 number of the original source file, instead of the file that is being
3696 This preprocessor directive is not generally of use to programmers,
3697 by may be of interest to preprocessor authors. The usage of the
3698 \c{%line} preprocessor directive is as follows:
3700 \c %line nnn[+mmm] [filename]
3702 In this directive, \c{nnn} identifies the line of the original source
3703 file which this line corresponds to. \c{mmm} is an optional parameter
3704 which specifies a line increment value; each line of the input file
3705 read in is considered to correspond to \c{mmm} lines of the original
3706 source file. Finally, \c{filename} is an optional parameter which
3707 specifies the file name of the original source file.
3709 After reading a \c{%line} preprocessor directive, NASM will report
3710 all file name and line numbers relative to the values specified
3714 \S{getenv} \i\c{%!}\c{<env>}: Read an environment variable.
3716 The \c{%!<env>} directive makes it possible to read the value of an
3717 environment variable at assembly time. This could, for example, be used
3718 to store the contents of an environment variable into a string, which
3719 could be used at some other point in your code.
3721 For example, suppose that you have an environment variable \c{FOO}, and
3722 you want the contents of \c{FOO} to be embedded in your program. You
3723 could do that as follows:
3725 \c %defstr FOO %!FOO
3727 See \k{defstr} for notes on the \c{%defstr} directive.
3729 If the name of the environment variable contains non-identifier
3730 characters, you can use string quotes to surround the name of the
3731 variable, for example:
3733 \c %defstr C_colon %!'C:'
3736 \H{stdmac} \i{Standard Macros}
3738 NASM defines a set of standard macros, which are already defined
3739 when it starts to process any source file. If you really need a
3740 program to be assembled with no pre-defined macros, you can use the
3741 \i\c{%clear} directive to empty the preprocessor of everything but
3742 context-local preprocessor variables and single-line macros.
3744 Most \i{user-level assembler directives} (see \k{directive}) are
3745 implemented as macros which invoke primitive directives; these are
3746 described in \k{directive}. The rest of the standard macro set is
3750 \S{stdmacver} \i{NASM Version} Macros
3752 The single-line macros \i\c{__NASM_MAJOR__}, \i\c{__NASM_MINOR__},
3753 \i\c{__NASM_SUBMINOR__} and \i\c{___NASM_PATCHLEVEL__} expand to the
3754 major, minor, subminor and patch level parts of the \i{version
3755 number of NASM} being used. So, under NASM 0.98.32p1 for
3756 example, \c{__NASM_MAJOR__} would be defined to be 0, \c{__NASM_MINOR__}
3757 would be defined as 98, \c{__NASM_SUBMINOR__} would be defined to 32,
3758 and \c{___NASM_PATCHLEVEL__} would be defined as 1.
3760 Additionally, the macro \i\c{__NASM_SNAPSHOT__} is defined for
3761 automatically generated snapshot releases \e{only}.
3764 \S{stdmacverid} \i\c{__NASM_VERSION_ID__}: \i{NASM Version ID}
3766 The single-line macro \c{__NASM_VERSION_ID__} expands to a dword integer
3767 representing the full version number of the version of nasm being used.
3768 The value is the equivalent to \c{__NASM_MAJOR__}, \c{__NASM_MINOR__},
3769 \c{__NASM_SUBMINOR__} and \c{___NASM_PATCHLEVEL__} concatenated to
3770 produce a single doubleword. Hence, for 0.98.32p1, the returned number
3771 would be equivalent to:
3779 Note that the above lines are generate exactly the same code, the second
3780 line is used just to give an indication of the order that the separate
3781 values will be present in memory.
3784 \S{stdmacverstr} \i\c{__NASM_VER__}: \i{NASM Version string}
3786 The single-line macro \c{__NASM_VER__} expands to a string which defines
3787 the version number of nasm being used. So, under NASM 0.98.32 for example,
3796 \S{fileline} \i\c{__FILE__} and \i\c{__LINE__}: File Name and Line Number
3798 Like the C preprocessor, NASM allows the user to find out the file
3799 name and line number containing the current instruction. The macro
3800 \c{__FILE__} expands to a string constant giving the name of the
3801 current input file (which may change through the course of assembly
3802 if \c{%include} directives are used), and \c{__LINE__} expands to a
3803 numeric constant giving the current line number in the input file.
3805 These macros could be used, for example, to communicate debugging
3806 information to a macro, since invoking \c{__LINE__} inside a macro
3807 definition (either single-line or multi-line) will return the line
3808 number of the macro \e{call}, rather than \e{definition}. So to
3809 determine where in a piece of code a crash is occurring, for
3810 example, one could write a routine \c{stillhere}, which is passed a
3811 line number in \c{EAX} and outputs something like `line 155: still
3812 here'. You could then write a macro
3814 \c %macro notdeadyet 0
3823 and then pepper your code with calls to \c{notdeadyet} until you
3824 find the crash point.
3827 \S{bitsm} \i\c{__BITS__}: Current BITS Mode
3829 The \c{__BITS__} standard macro is updated every time that the BITS mode is
3830 set using the \c{BITS XX} or \c{[BITS XX]} directive, where XX is a valid mode
3831 number of 16, 32 or 64. \c{__BITS__} receives the specified mode number and
3832 makes it globally available. This can be very useful for those who utilize
3833 mode-dependent macros.
3835 \S{ofmtm} \i\c{__OUTPUT_FORMAT__}: Current Output Format
3837 The \c{__OUTPUT_FORMAT__} standard macro holds the current Output Format,
3838 as given by the \c{-f} option or NASM's default. Type \c{nasm -hf} for a
3841 \c %ifidn __OUTPUT_FORMAT__, win32
3842 \c %define NEWLINE 13, 10
3843 \c %elifidn __OUTPUT_FORMAT__, elf32
3844 \c %define NEWLINE 10
3848 \S{datetime} Assembly Date and Time Macros
3850 NASM provides a variety of macros that represent the timestamp of the
3853 \b The \i\c{__DATE__} and \i\c{__TIME__} macros give the assembly date and
3854 time as strings, in ISO 8601 format (\c{"YYYY-MM-DD"} and \c{"HH:MM:SS"},
3857 \b The \i\c{__DATE_NUM__} and \i\c{__TIME_NUM__} macros give the assembly
3858 date and time in numeric form; in the format \c{YYYYMMDD} and
3859 \c{HHMMSS} respectively.
3861 \b The \i\c{__UTC_DATE__} and \i\c{__UTC_TIME__} macros give the assembly
3862 date and time in universal time (UTC) as strings, in ISO 8601 format
3863 (\c{"YYYY-MM-DD"} and \c{"HH:MM:SS"}, respectively.) If the host
3864 platform doesn't provide UTC time, these macros are undefined.
3866 \b The \i\c{__UTC_DATE_NUM__} and \i\c{__UTC_TIME_NUM__} macros give the
3867 assembly date and time universal time (UTC) in numeric form; in the
3868 format \c{YYYYMMDD} and \c{HHMMSS} respectively. If the
3869 host platform doesn't provide UTC time, these macros are
3872 \b The \c{__POSIX_TIME__} macro is defined as a number containing the
3873 number of seconds since the POSIX epoch, 1 January 1970 00:00:00 UTC;
3874 excluding any leap seconds. This is computed using UTC time if
3875 available on the host platform, otherwise it is computed using the
3876 local time as if it was UTC.
3878 All instances of time and date macros in the same assembly session
3879 produce consistent output. For example, in an assembly session
3880 started at 42 seconds after midnight on January 1, 2010 in Moscow
3881 (timezone UTC+3) these macros would have the following values,
3882 assuming, of course, a properly configured environment with a correct
3885 \c __DATE__ "2010-01-01"
3886 \c __TIME__ "00:00:42"
3887 \c __DATE_NUM__ 20100101
3888 \c __TIME_NUM__ 000042
3889 \c __UTC_DATE__ "2009-12-31"
3890 \c __UTC_TIME__ "21:00:42"
3891 \c __UTC_DATE_NUM__ 20091231
3892 \c __UTC_TIME_NUM__ 210042
3893 \c __POSIX_TIME__ 1262293242
3896 \S{use_def} \I\c{__USE_*__}\c{__USE_}\e{package}\c{__}: Package
3899 When a standard macro package (see \k{macropkg}) is included with the
3900 \c{%use} directive (see \k{use}), a single-line macro of the form
3901 \c{__USE_}\e{package}\c{__} is automatically defined. This allows
3902 testing if a particular package is invoked or not.
3904 For example, if the \c{altreg} package is included (see
3905 \k{pkg_altreg}), then the macro \c{__USE_ALTREG__} is defined.
3908 \S{pass_macro} \i\c{__PASS__}: Assembly Pass
3910 The macro \c{__PASS__} is defined to be \c{1} on preparatory passes,
3911 and \c{2} on the final pass. In preprocess-only mode, it is set to
3912 \c{3}, and when running only to generate dependencies (due to the
3913 \c{-M} or \c{-MG} option, see \k{opt-M}) it is set to \c{0}.
3915 \e{Avoid using this macro if at all possible. It is tremendously easy
3916 to generate very strange errors by misusing it, and the semantics may
3917 change in future versions of NASM.}
3920 \S{struc} \i\c{STRUC} and \i\c{ENDSTRUC}: \i{Declaring Structure} Data Types
3922 The core of NASM contains no intrinsic means of defining data
3923 structures; instead, the preprocessor is sufficiently powerful that
3924 data structures can be implemented as a set of macros. The macros
3925 \c{STRUC} and \c{ENDSTRUC} are used to define a structure data type.
3927 \c{STRUC} takes one or two parameters. The first parameter is the name
3928 of the data type. The second, optional parameter is the base offset of
3929 the structure. The name of the data type is defined as a symbol with
3930 the value of the base offset, and the name of the data type with the
3931 suffix \c{_size} appended to it is defined as an \c{EQU} giving the
3932 size of the structure. Once \c{STRUC} has been issued, you are
3933 defining the structure, and should define fields using the \c{RESB}
3934 family of pseudo-instructions, and then invoke \c{ENDSTRUC} to finish
3937 For example, to define a structure called \c{mytype} containing a
3938 longword, a word, a byte and a string of bytes, you might code
3949 The above code defines six symbols: \c{mt_long} as 0 (the offset
3950 from the beginning of a \c{mytype} structure to the longword field),
3951 \c{mt_word} as 4, \c{mt_byte} as 6, \c{mt_str} as 7, \c{mytype_size}
3952 as 39, and \c{mytype} itself as zero.
3954 The reason why the structure type name is defined at zero by default
3955 is a side effect of allowing structures to work with the local label
3956 mechanism: if your structure members tend to have the same names in
3957 more than one structure, you can define the above structure like this:
3968 This defines the offsets to the structure fields as \c{mytype.long},
3969 \c{mytype.word}, \c{mytype.byte} and \c{mytype.str}.
3971 NASM, since it has no \e{intrinsic} structure support, does not
3972 support any form of period notation to refer to the elements of a
3973 structure once you have one (except the above local-label notation),
3974 so code such as \c{mov ax,[mystruc.mt_word]} is not valid.
3975 \c{mt_word} is a constant just like any other constant, so the
3976 correct syntax is \c{mov ax,[mystruc+mt_word]} or \c{mov
3977 ax,[mystruc+mytype.word]}.
3979 Sometimes you only have the address of the structure displaced by an
3980 offset. For example, consider this standard stack frame setup:
3986 In this case, you could access an element by subtracting the offset:
3988 \c mov [ebp - 40 + mytype.word], ax
3990 However, if you do not want to repeat this offset, you can use -40 as
3993 \c struc mytype, -40
3995 And access an element this way:
3997 \c mov [ebp + mytype.word], ax
4000 \S{istruc} \i\c{ISTRUC}, \i\c{AT} and \i\c{IEND}: Declaring
4001 \i{Instances of Structures}
4003 Having defined a structure type, the next thing you typically want
4004 to do is to declare instances of that structure in your data
4005 segment. NASM provides an easy way to do this in the \c{ISTRUC}
4006 mechanism. To declare a structure of type \c{mytype} in a program,
4007 you code something like this:
4012 \c at mt_long, dd 123456
4013 \c at mt_word, dw 1024
4014 \c at mt_byte, db 'x'
4015 \c at mt_str, db 'hello, world', 13, 10, 0
4019 The function of the \c{AT} macro is to make use of the \c{TIMES}
4020 prefix to advance the assembly position to the correct point for the
4021 specified structure field, and then to declare the specified data.
4022 Therefore the structure fields must be declared in the same order as
4023 they were specified in the structure definition.
4025 If the data to go in a structure field requires more than one source
4026 line to specify, the remaining source lines can easily come after
4027 the \c{AT} line. For example:
4029 \c at mt_str, db 123,134,145,156,167,178,189
4032 Depending on personal taste, you can also omit the code part of the
4033 \c{AT} line completely, and start the structure field on the next
4037 \c db 'hello, world'
4041 \S{align} \i\c{ALIGN} and \i\c{ALIGNB}: Data Alignment
4043 The \c{ALIGN} and \c{ALIGNB} macros provides a convenient way to
4044 align code or data on a word, longword, paragraph or other boundary.
4045 (Some assemblers call this directive \i\c{EVEN}.) The syntax of the
4046 \c{ALIGN} and \c{ALIGNB} macros is
4048 \c align 4 ; align on 4-byte boundary
4049 \c align 16 ; align on 16-byte boundary
4050 \c align 8,db 0 ; pad with 0s rather than NOPs
4051 \c align 4,resb 1 ; align to 4 in the BSS
4052 \c alignb 4 ; equivalent to previous line
4054 Both macros require their first argument to be a power of two; they
4055 both compute the number of additional bytes required to bring the
4056 length of the current section up to a multiple of that power of two,
4057 and then apply the \c{TIMES} prefix to their second argument to
4058 perform the alignment.
4060 If the second argument is not specified, the default for \c{ALIGN}
4061 is \c{NOP}, and the default for \c{ALIGNB} is \c{RESB 1}. So if the
4062 second argument is specified, the two macros are equivalent.
4063 Normally, you can just use \c{ALIGN} in code and data sections and
4064 \c{ALIGNB} in BSS sections, and never need the second argument
4065 except for special purposes.
4067 \c{ALIGN} and \c{ALIGNB}, being simple macros, perform no error
4068 checking: they cannot warn you if their first argument fails to be a
4069 power of two, or if their second argument generates more than one
4070 byte of code. In each of these cases they will silently do the wrong
4073 \c{ALIGNB} (or \c{ALIGN} with a second argument of \c{RESB 1}) can
4074 be used within structure definitions:
4091 This will ensure that the structure members are sensibly aligned
4092 relative to the base of the structure.
4094 A final caveat: \c{ALIGN} and \c{ALIGNB} work relative to the
4095 beginning of the \e{section}, not the beginning of the address space
4096 in the final executable. Aligning to a 16-byte boundary when the
4097 section you're in is only guaranteed to be aligned to a 4-byte
4098 boundary, for example, is a waste of effort. Again, NASM does not
4099 check that the section's alignment characteristics are sensible for
4100 the use of \c{ALIGN} or \c{ALIGNB}.
4102 See also the \c{smartalign} standard macro package, \k{pkg_smartalign}.
4105 \S{sectalign} \i\c{SECTALIGN}: Section Alignment
4107 The \c{SECTALIGN} macros provides a way to modify alignment attribute
4108 of output file section. Unlike the \c{align=} attribute allowed at section
4109 definition only the \c{SECTALIGN} macro may be used any time you need it.
4111 For example the statement
4117 sets a section alignment requirement to 16 bytes. Note that once increased
4118 the section alignment can not be decreased, the magnitude can grow only.
4120 It must be also noted that \c{SECTALIGN} gets called implicitly inside \c{ALIGN}
4121 handler and as result \c{ALIGN} may update section alignment.
4124 \C{macropkg} \i{Standard Macro Packages}
4126 The \i\c{%use} directive (see \k{use}) includes one of the standard
4127 macro packages included with the NASM distribution and compiled into
4128 the NASM binary. It operates like the \c{%include} directive (see
4129 \k{include}), but the included contents is provided by NASM itself.
4131 The names of standard macro packages are case insensitive, and can be
4135 \H{pkg_altreg} \i\c{altreg}: \i{Alternate Register Names}
4137 The \c{altreg} standard macro package provides alternate register
4138 names. It provides numeric register names for all registers (not just
4139 \c{R8}-\c{R15}), the Intel-defined aliases \c{R8L}-\c{R15L} for the
4140 low bytes of register (as opposed to the NASM/AMD standard names
4141 \c{R8B}-\c{R15B}), and the names \c{R0H}-\c{R3H} (by analogy with
4142 \c{R0L}-\c{R3L}) for \c{AH}, \c{CH}, \c{DH}, and \c{BH}.
4149 \c mov r0l,r3h ; mov al,bh
4155 \H{pkg_smartalign} \i\c{smartalign}\I{align, smart}: Smart \c{ALIGN} Macro
4157 The \c{smartalign} standard macro package provides for an \i\c{ALIGN}
4158 macro which is more powerful than the default (and
4159 backwards-compatible) one (see \k{align}). When the \c{smartalign}
4160 package is enabled, when \c{ALIGN} is used without a second argument,
4161 NASM will generate a sequence of instructions more efficient than a
4162 series of \c{NOP}. Furthermore, if the padding exceeds a specific
4163 threshold, then NASM will generate a jump over the entire padding
4166 The specific instructions generated can be controlled with the
4167 new \i\c{ALIGNMODE} macro. This macro takes two parameters: one mode,
4168 and an optional jump threshold override. If (for any reason) you need
4169 to turn off the jump completely just set jump threshold value to -1
4170 (or set it to \c{nojmp}). The following modes are possible:
4172 \b \c{generic}: Works on all x86 CPUs and should have reasonable
4173 performance. The default jump threshold is 8. This is the
4176 \b \c{nop}: Pad out with \c{NOP} instructions. The only difference
4177 compared to the standard \c{ALIGN} macro is that NASM can still jump
4178 over a large padding area. The default jump threshold is 16.
4180 \b \c{k7}: Optimize for the AMD K7 (Athlon/Althon XP). These
4181 instructions should still work on all x86 CPUs. The default jump
4184 \b \c{k8}: Optimize for the AMD K8 (Opteron/Althon 64). These
4185 instructions should still work on all x86 CPUs. The default jump
4188 \b \c{p6}: Optimize for Intel CPUs. This uses the long \c{NOP}
4189 instructions first introduced in Pentium Pro. This is incompatible
4190 with all CPUs of family 5 or lower, as well as some VIA CPUs and
4191 several virtualization solutions. The default jump threshold is 16.
4193 The macro \i\c{__ALIGNMODE__} is defined to contain the current
4194 alignment mode. A number of other macros beginning with \c{__ALIGN_}
4195 are used internally by this macro package.
4198 \C{directive} \i{Assembler Directives}
4200 NASM, though it attempts to avoid the bureaucracy of assemblers like
4201 MASM and TASM, is nevertheless forced to support a \e{few}
4202 directives. These are described in this chapter.
4204 NASM's directives come in two types: \I{user-level
4205 directives}\e{user-level} directives and \I{primitive
4206 directives}\e{primitive} directives. Typically, each directive has a
4207 user-level form and a primitive form. In almost all cases, we
4208 recommend that users use the user-level forms of the directives,
4209 which are implemented as macros which call the primitive forms.
4211 Primitive directives are enclosed in square brackets; user-level
4214 In addition to the universal directives described in this chapter,
4215 each object file format can optionally supply extra directives in
4216 order to control particular features of that file format. These
4217 \I{format-specific directives}\e{format-specific} directives are
4218 documented along with the formats that implement them, in \k{outfmt}.
4221 \H{bits} \i\c{BITS}: Specifying Target \i{Processor Mode}
4223 The \c{BITS} directive specifies whether NASM should generate code
4224 \I{16-bit mode, versus 32-bit mode}designed to run on a processor
4225 operating in 16-bit mode, 32-bit mode or 64-bit mode. The syntax is
4226 \c{BITS XX}, where XX is 16, 32 or 64.
4228 In most cases, you should not need to use \c{BITS} explicitly. The
4229 \c{aout}, \c{coff}, \c{elf}, \c{macho}, \c{win32} and \c{win64}
4230 object formats, which are designed for use in 32-bit or 64-bit
4231 operating systems, all cause NASM to select 32-bit or 64-bit mode,
4232 respectively, by default. The \c{obj} object format allows you
4233 to specify each segment you define as either \c{USE16} or \c{USE32},
4234 and NASM will set its operating mode accordingly, so the use of the
4235 \c{BITS} directive is once again unnecessary.
4237 The most likely reason for using the \c{BITS} directive is to write
4238 32-bit or 64-bit code in a flat binary file; this is because the \c{bin}
4239 output format defaults to 16-bit mode in anticipation of it being
4240 used most frequently to write DOS \c{.COM} programs, DOS \c{.SYS}
4241 device drivers and boot loader software.
4243 You do \e{not} need to specify \c{BITS 32} merely in order to use
4244 32-bit instructions in a 16-bit DOS program; if you do, the
4245 assembler will generate incorrect code because it will be writing
4246 code targeted at a 32-bit platform, to be run on a 16-bit one.
4248 When NASM is in \c{BITS 16} mode, instructions which use 32-bit
4249 data are prefixed with an 0x66 byte, and those referring to 32-bit
4250 addresses have an 0x67 prefix. In \c{BITS 32} mode, the reverse is
4251 true: 32-bit instructions require no prefixes, whereas instructions
4252 using 16-bit data need an 0x66 and those working on 16-bit addresses
4255 When NASM is in \c{BITS 64} mode, most instructions operate the same
4256 as they do for \c{BITS 32} mode. However, there are 8 more general and
4257 SSE registers, and 16-bit addressing is no longer supported.
4259 The default address size is 64 bits; 32-bit addressing can be selected
4260 with the 0x67 prefix. The default operand size is still 32 bits,
4261 however, and the 0x66 prefix selects 16-bit operand size. The \c{REX}
4262 prefix is used both to select 64-bit operand size, and to access the
4263 new registers. NASM automatically inserts REX prefixes when
4266 When the \c{REX} prefix is used, the processor does not know how to
4267 address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead,
4268 it is possible to access the the low 8-bits of the SP, BP SI and DI
4269 registers as SPL, BPL, SIL and DIL, respectively; but only when the
4272 The \c{BITS} directive has an exactly equivalent primitive form,
4273 \c{[BITS 16]}, \c{[BITS 32]} and \c{[BITS 64]}. The user-level form is
4274 a macro which has no function other than to call the primitive form.
4276 Note that the space is neccessary, e.g. \c{BITS32} will \e{not} work!
4278 \S{USE16 & USE32} \i\c{USE16} & \i\c{USE32}: Aliases for BITS
4280 The `\c{USE16}' and `\c{USE32}' directives can be used in place of
4281 `\c{BITS 16}' and `\c{BITS 32}', for compatibility with other assemblers.
4284 \H{default} \i\c{DEFAULT}: Change the assembler defaults
4286 The \c{DEFAULT} directive changes the assembler defaults. Normally,
4287 NASM defaults to a mode where the programmer is expected to explicitly
4288 specify most features directly. However, this is occationally
4289 obnoxious, as the explicit form is pretty much the only one one wishes
4292 Currently, the only \c{DEFAULT} that is settable is whether or not
4293 registerless instructions in 64-bit mode are \c{RIP}-relative or not.
4294 By default, they are absolute unless overridden with the \i\c{REL}
4295 specifier (see \k{effaddr}). However, if \c{DEFAULT REL} is
4296 specified, \c{REL} is default, unless overridden with the \c{ABS}
4297 specifier, \e{except when used with an FS or GS segment override}.
4299 The special handling of \c{FS} and \c{GS} overrides are due to the
4300 fact that these registers are generally used as thread pointers or
4301 other special functions in 64-bit mode, and generating
4302 \c{RIP}-relative addresses would be extremely confusing.
4304 \c{DEFAULT REL} is disabled with \c{DEFAULT ABS}.
4306 \H{section} \i\c{SECTION} or \i\c{SEGMENT}: Changing and \i{Defining
4309 \I{changing sections}\I{switching between sections}The \c{SECTION}
4310 directive (\c{SEGMENT} is an exactly equivalent synonym) changes
4311 which section of the output file the code you write will be
4312 assembled into. In some object file formats, the number and names of
4313 sections are fixed; in others, the user may make up as many as they
4314 wish. Hence \c{SECTION} may sometimes give an error message, or may
4315 define a new section, if you try to switch to a section that does
4318 The Unix object formats, and the \c{bin} object format (but see
4319 \k{multisec}, all support
4320 the \i{standardized section names} \c{.text}, \c{.data} and \c{.bss}
4321 for the code, data and uninitialized-data sections. The \c{obj}
4322 format, by contrast, does not recognize these section names as being
4323 special, and indeed will strip off the leading period of any section
4327 \S{sectmac} The \i\c{__SECT__} Macro
4329 The \c{SECTION} directive is unusual in that its user-level form
4330 functions differently from its primitive form. The primitive form,
4331 \c{[SECTION xyz]}, simply switches the current target section to the
4332 one given. The user-level form, \c{SECTION xyz}, however, first
4333 defines the single-line macro \c{__SECT__} to be the primitive
4334 \c{[SECTION]} directive which it is about to issue, and then issues
4335 it. So the user-level directive
4339 expands to the two lines
4341 \c %define __SECT__ [SECTION .text]
4344 Users may find it useful to make use of this in their own macros.
4345 For example, the \c{writefile} macro defined in \k{mlmacgre} can be
4346 usefully rewritten in the following more sophisticated form:
4348 \c %macro writefile 2+
4358 \c mov cx,%%endstr-%%str
4365 This form of the macro, once passed a string to output, first
4366 switches temporarily to the data section of the file, using the
4367 primitive form of the \c{SECTION} directive so as not to modify
4368 \c{__SECT__}. It then declares its string in the data section, and
4369 then invokes \c{__SECT__} to switch back to \e{whichever} section
4370 the user was previously working in. It thus avoids the need, in the
4371 previous version of the macro, to include a \c{JMP} instruction to
4372 jump over the data, and also does not fail if, in a complicated
4373 \c{OBJ} format module, the user could potentially be assembling the
4374 code in any of several separate code sections.
4377 \H{absolute} \i\c{ABSOLUTE}: Defining Absolute Labels
4379 The \c{ABSOLUTE} directive can be thought of as an alternative form
4380 of \c{SECTION}: it causes the subsequent code to be directed at no
4381 physical section, but at the hypothetical section starting at the
4382 given absolute address. The only instructions you can use in this
4383 mode are the \c{RESB} family.
4385 \c{ABSOLUTE} is used as follows:
4393 This example describes a section of the PC BIOS data area, at
4394 segment address 0x40: the above code defines \c{kbuf_chr} to be
4395 0x1A, \c{kbuf_free} to be 0x1C, and \c{kbuf} to be 0x1E.
4397 The user-level form of \c{ABSOLUTE}, like that of \c{SECTION},
4398 redefines the \i\c{__SECT__} macro when it is invoked.
4400 \i\c{STRUC} and \i\c{ENDSTRUC} are defined as macros which use
4401 \c{ABSOLUTE} (and also \c{__SECT__}).
4403 \c{ABSOLUTE} doesn't have to take an absolute constant as an
4404 argument: it can take an expression (actually, a \i{critical
4405 expression}: see \k{crit}) and it can be a value in a segment. For
4406 example, a TSR can re-use its setup code as run-time BSS like this:
4408 \c org 100h ; it's a .COM program
4410 \c jmp setup ; setup code comes last
4412 \c ; the resident part of the TSR goes here
4414 \c ; now write the code that installs the TSR here
4418 \c runtimevar1 resw 1
4419 \c runtimevar2 resd 20
4423 This defines some variables `on top of' the setup code, so that
4424 after the setup has finished running, the space it took up can be
4425 re-used as data storage for the running TSR. The symbol `tsr_end'
4426 can be used to calculate the total size of the part of the TSR that
4427 needs to be made resident.
4430 \H{extern} \i\c{EXTERN}: \i{Importing Symbols} from Other Modules
4432 \c{EXTERN} is similar to the MASM directive \c{EXTRN} and the C
4433 keyword \c{extern}: it is used to declare a symbol which is not
4434 defined anywhere in the module being assembled, but is assumed to be
4435 defined in some other module and needs to be referred to by this
4436 one. Not every object-file format can support external variables:
4437 the \c{bin} format cannot.
4439 The \c{EXTERN} directive takes as many arguments as you like. Each
4440 argument is the name of a symbol:
4443 \c extern _sscanf,_fscanf
4445 Some object-file formats provide extra features to the \c{EXTERN}
4446 directive. In all cases, the extra features are used by suffixing a
4447 colon to the symbol name followed by object-format specific text.
4448 For example, the \c{obj} format allows you to declare that the
4449 default segment base of an external should be the group \c{dgroup}
4450 by means of the directive
4452 \c extern _variable:wrt dgroup
4454 The primitive form of \c{EXTERN} differs from the user-level form
4455 only in that it can take only one argument at a time: the support
4456 for multiple arguments is implemented at the preprocessor level.
4458 You can declare the same variable as \c{EXTERN} more than once: NASM
4459 will quietly ignore the second and later redeclarations. You can't
4460 declare a variable as \c{EXTERN} as well as something else, though.
4463 \H{global} \i\c{GLOBAL}: \i{Exporting Symbols} to Other Modules
4465 \c{GLOBAL} is the other end of \c{EXTERN}: if one module declares a
4466 symbol as \c{EXTERN} and refers to it, then in order to prevent
4467 linker errors, some other module must actually \e{define} the
4468 symbol and declare it as \c{GLOBAL}. Some assemblers use the name
4469 \i\c{PUBLIC} for this purpose.
4471 The \c{GLOBAL} directive applying to a symbol must appear \e{before}
4472 the definition of the symbol.
4474 \c{GLOBAL} uses the same syntax as \c{EXTERN}, except that it must
4475 refer to symbols which \e{are} defined in the same module as the
4476 \c{GLOBAL} directive. For example:
4482 \c{GLOBAL}, like \c{EXTERN}, allows object formats to define private
4483 extensions by means of a colon. The \c{elf} object format, for
4484 example, lets you specify whether global data items are functions or
4487 \c global hashlookup:function, hashtable:data
4489 Like \c{EXTERN}, the primitive form of \c{GLOBAL} differs from the
4490 user-level form only in that it can take only one argument at a
4494 \H{common} \i\c{COMMON}: Defining Common Data Areas
4496 The \c{COMMON} directive is used to declare \i\e{common variables}.
4497 A common variable is much like a global variable declared in the
4498 uninitialized data section, so that
4502 is similar in function to
4509 The difference is that if more than one module defines the same
4510 common variable, then at link time those variables will be
4511 \e{merged}, and references to \c{intvar} in all modules will point
4512 at the same piece of memory.
4514 Like \c{GLOBAL} and \c{EXTERN}, \c{COMMON} supports object-format
4515 specific extensions. For example, the \c{obj} format allows common
4516 variables to be NEAR or FAR, and the \c{elf} format allows you to
4517 specify the alignment requirements of a common variable:
4519 \c common commvar 4:near ; works in OBJ
4520 \c common intarray 100:4 ; works in ELF: 4 byte aligned
4522 Once again, like \c{EXTERN} and \c{GLOBAL}, the primitive form of
4523 \c{COMMON} differs from the user-level form only in that it can take
4524 only one argument at a time.
4527 \H{CPU} \i\c{CPU}: Defining CPU Dependencies
4529 The \i\c{CPU} directive restricts assembly to those instructions which
4530 are available on the specified CPU.
4534 \b\c{CPU 8086} Assemble only 8086 instruction set
4536 \b\c{CPU 186} Assemble instructions up to the 80186 instruction set
4538 \b\c{CPU 286} Assemble instructions up to the 286 instruction set
4540 \b\c{CPU 386} Assemble instructions up to the 386 instruction set
4542 \b\c{CPU 486} 486 instruction set
4544 \b\c{CPU 586} Pentium instruction set
4546 \b\c{CPU PENTIUM} Same as 586
4548 \b\c{CPU 686} P6 instruction set
4550 \b\c{CPU PPRO} Same as 686
4552 \b\c{CPU P2} Same as 686
4554 \b\c{CPU P3} Pentium III (Katmai) instruction sets
4556 \b\c{CPU KATMAI} Same as P3
4558 \b\c{CPU P4} Pentium 4 (Willamette) instruction set
4560 \b\c{CPU WILLAMETTE} Same as P4
4562 \b\c{CPU PRESCOTT} Prescott instruction set
4564 \b\c{CPU X64} x86-64 (x64/AMD64/Intel 64) instruction set
4566 \b\c{CPU IA64} IA64 CPU (in x86 mode) instruction set
4568 All options are case insensitive. All instructions will be selected
4569 only if they apply to the selected CPU or lower. By default, all
4570 instructions are available.
4573 \H{FLOAT} \i\c{FLOAT}: Handling of \I{floating-point, constants}floating-point constants
4575 By default, floating-point constants are rounded to nearest, and IEEE
4576 denormals are supported. The following options can be set to alter
4579 \b\c{FLOAT DAZ} Flush denormals to zero
4581 \b\c{FLOAT NODAZ} Do not flush denormals to zero (default)
4583 \b\c{FLOAT NEAR} Round to nearest (default)
4585 \b\c{FLOAT UP} Round up (toward +Infinity)
4587 \b\c{FLOAT DOWN} Round down (toward -Infinity)
4589 \b\c{FLOAT ZERO} Round toward zero
4591 \b\c{FLOAT DEFAULT} Restore default settings
4593 The standard macros \i\c{__FLOAT_DAZ__}, \i\c{__FLOAT_ROUND__}, and
4594 \i\c{__FLOAT__} contain the current state, as long as the programmer
4595 has avoided the use of the brackeded primitive form, (\c{[FLOAT]}).
4597 \c{__FLOAT__} contains the full set of floating-point settings; this
4598 value can be saved away and invoked later to restore the setting.
4601 \C{outfmt} \i{Output Formats}
4603 NASM is a portable assembler, designed to be able to compile on any
4604 ANSI C-supporting platform and produce output to run on a variety of
4605 Intel x86 operating systems. For this reason, it has a large number
4606 of available output formats, selected using the \i\c{-f} option on
4607 the NASM \i{command line}. Each of these formats, along with its
4608 extensions to the base NASM syntax, is detailed in this chapter.
4610 As stated in \k{opt-o}, NASM chooses a \i{default name} for your
4611 output file based on the input file name and the chosen output
4612 format. This will be generated by removing the \i{extension}
4613 (\c{.asm}, \c{.s}, or whatever you like to use) from the input file
4614 name, and substituting an extension defined by the output format.
4615 The extensions are given with each format below.
4618 \H{binfmt} \i\c{bin}: \i{Flat-Form Binary}\I{pure binary} Output
4620 The \c{bin} format does not produce object files: it generates
4621 nothing in the output file except the code you wrote. Such `pure
4622 binary' files are used by \i{MS-DOS}: \i\c{.COM} executables and
4623 \i\c{.SYS} device drivers are pure binary files. Pure binary output
4624 is also useful for \i{operating system} and \i{boot loader}
4627 The \c{bin} format supports \i{multiple section names}. For details of
4628 how NASM handles sections in the \c{bin} format, see \k{multisec}.
4630 Using the \c{bin} format puts NASM by default into 16-bit mode (see
4631 \k{bits}). In order to use \c{bin} to write 32-bit or 64-bit code,
4632 such as an OS kernel, you need to explicitly issue the \I\c{BITS}\c{BITS 32}
4633 or \I\c{BITS}\c{BITS 64} directive.
4635 \c{bin} has no default output file name extension: instead, it
4636 leaves your file name as it is once the original extension has been
4637 removed. Thus, the default is for NASM to assemble \c{binprog.asm}
4638 into a binary file called \c{binprog}.
4641 \S{org} \i\c{ORG}: Binary File \i{Program Origin}
4643 The \c{bin} format provides an additional directive to the list
4644 given in \k{directive}: \c{ORG}. The function of the \c{ORG}
4645 directive is to specify the origin address which NASM will assume
4646 the program begins at when it is loaded into memory.
4648 For example, the following code will generate the longword
4655 Unlike the \c{ORG} directive provided by MASM-compatible assemblers,
4656 which allows you to jump around in the object file and overwrite
4657 code you have already generated, NASM's \c{ORG} does exactly what
4658 the directive says: \e{origin}. Its sole function is to specify one
4659 offset which is added to all internal address references within the
4660 section; it does not permit any of the trickery that MASM's version
4661 does. See \k{proborg} for further comments.
4664 \S{binseg} \c{bin} Extensions to the \c{SECTION}
4665 Directive\I{SECTION, bin extensions to}
4667 The \c{bin} output format extends the \c{SECTION} (or \c{SEGMENT})
4668 directive to allow you to specify the alignment requirements of
4669 segments. This is done by appending the \i\c{ALIGN} qualifier to the
4670 end of the section-definition line. For example,
4672 \c section .data align=16
4674 switches to the section \c{.data} and also specifies that it must be
4675 aligned on a 16-byte boundary.
4677 The parameter to \c{ALIGN} specifies how many low bits of the
4678 section start address must be forced to zero. The alignment value
4679 given may be any power of two.\I{section alignment, in
4680 bin}\I{segment alignment, in bin}\I{alignment, in bin sections}
4683 \S{multisec} \i{Multisection}\I{bin, multisection} Support for the \c{bin} Format
4685 The \c{bin} format allows the use of multiple sections, of arbitrary names,
4686 besides the "known" \c{.text}, \c{.data}, and \c{.bss} names.
4688 \b Sections may be designated \i\c{progbits} or \i\c{nobits}. Default
4689 is \c{progbits} (except \c{.bss}, which defaults to \c{nobits},
4692 \b Sections can be aligned at a specified boundary following the previous
4693 section with \c{align=}, or at an arbitrary byte-granular position with
4696 \b Sections can be given a virtual start address, which will be used
4697 for the calculation of all memory references within that section
4700 \b Sections can be ordered using \i\c{follows=}\c{<section>} or
4701 \i\c{vfollows=}\c{<section>} as an alternative to specifying an explicit
4704 \b Arguments to \c{org}, \c{start}, \c{vstart}, and \c{align=} are
4705 critical expressions. See \k{crit}. E.g. \c{align=(1 << ALIGN_SHIFT)}
4706 - \c{ALIGN_SHIFT} must be defined before it is used here.
4708 \b Any code which comes before an explicit \c{SECTION} directive
4709 is directed by default into the \c{.text} section.
4711 \b If an \c{ORG} statement is not given, \c{ORG 0} is used
4714 \b The \c{.bss} section will be placed after the last \c{progbits}
4715 section, unless \c{start=}, \c{vstart=}, \c{follows=}, or \c{vfollows=}
4718 \b All sections are aligned on dword boundaries, unless a different
4719 alignment has been specified.
4721 \b Sections may not overlap.
4723 \b NASM creates the \c{section.<secname>.start} for each section,
4724 which may be used in your code.
4726 \S{map}\i{Map Files}
4728 Map files can be generated in \c{-f bin} format by means of the \c{[map]}
4729 option. Map types of \c{all} (default), \c{brief}, \c{sections}, \c{segments},
4730 or \c{symbols} may be specified. Output may be directed to \c{stdout}
4731 (default), \c{stderr}, or a specified file. E.g.
4732 \c{[map symbols myfile.map]}. No "user form" exists, the square
4733 brackets must be used.
4736 \H{ithfmt} \i\c{ith}: \i{Intel Hex} Output
4738 The \c{ith} file format produces Intel hex-format files. Just as the
4739 \c{bin} format, this is a flat memory image format with no support for
4740 relocation or linking. It is usually used with ROM programmers and
4743 All extensions supported by the \c{bin} file format is also supported by
4744 the \c{ith} file format.
4746 \c{ith} provides a default output file-name extension of \c{.ith}.
4749 \H{srecfmt} \i\c{srec}: \i{Motorola S-Records} Output
4751 The \c{srec} file format produces Motorola S-records files. Just as the
4752 \c{bin} format, this is a flat memory image format with no support for
4753 relocation or linking. It is usually used with ROM programmers and
4756 All extensions supported by the \c{bin} file format is also supported by
4757 the \c{srec} file format.
4759 \c{srec} provides a default output file-name extension of \c{.srec}.
4762 \H{objfmt} \i\c{obj}: \i{Microsoft OMF}\I{OMF} Object Files
4764 The \c{obj} file format (NASM calls it \c{obj} rather than \c{omf}
4765 for historical reasons) is the one produced by \i{MASM} and
4766 \i{TASM}, which is typically fed to 16-bit DOS linkers to produce
4767 \i\c{.EXE} files. It is also the format used by \i{OS/2}.
4769 \c{obj} provides a default output file-name extension of \c{.obj}.
4771 \c{obj} is not exclusively a 16-bit format, though: NASM has full
4772 support for the 32-bit extensions to the format. In particular,
4773 32-bit \c{obj} format files are used by \i{Borland's Win32
4774 compilers}, instead of using Microsoft's newer \i\c{win32} object
4777 The \c{obj} format does not define any special segment names: you
4778 can call your segments anything you like. Typical names for segments
4779 in \c{obj} format files are \c{CODE}, \c{DATA} and \c{BSS}.
4781 If your source file contains code before specifying an explicit
4782 \c{SEGMENT} directive, then NASM will invent its own segment called
4783 \i\c{__NASMDEFSEG} for you.
4785 When you define a segment in an \c{obj} file, NASM defines the
4786 segment name as a symbol as well, so that you can access the segment
4787 address of the segment. So, for example:
4796 \c mov ax,data ; get segment address of data
4797 \c mov ds,ax ; and move it into DS
4798 \c inc word [dvar] ; now this reference will work
4801 The \c{obj} format also enables the use of the \i\c{SEG} and
4802 \i\c{WRT} operators, so that you can write code which does things
4807 \c mov ax,seg foo ; get preferred segment of foo
4809 \c mov ax,data ; a different segment
4811 \c mov ax,[ds:foo] ; this accesses `foo'
4812 \c mov [es:foo wrt data],bx ; so does this
4815 \S{objseg} \c{obj} Extensions to the \c{SEGMENT}
4816 Directive\I{SEGMENT, obj extensions to}
4818 The \c{obj} output format extends the \c{SEGMENT} (or \c{SECTION})
4819 directive to allow you to specify various properties of the segment
4820 you are defining. This is done by appending extra qualifiers to the
4821 end of the segment-definition line. For example,
4823 \c segment code private align=16
4825 defines the segment \c{code}, but also declares it to be a private
4826 segment, and requires that the portion of it described in this code
4827 module must be aligned on a 16-byte boundary.
4829 The available qualifiers are:
4831 \b \i\c{PRIVATE}, \i\c{PUBLIC}, \i\c{COMMON} and \i\c{STACK} specify
4832 the combination characteristics of the segment. \c{PRIVATE} segments
4833 do not get combined with any others by the linker; \c{PUBLIC} and
4834 \c{STACK} segments get concatenated together at link time; and
4835 \c{COMMON} segments all get overlaid on top of each other rather
4836 than stuck end-to-end.
4838 \b \i\c{ALIGN} is used, as shown above, to specify how many low bits
4839 of the segment start address must be forced to zero. The alignment
4840 value given may be any power of two from 1 to 4096; in reality, the
4841 only values supported are 1, 2, 4, 16, 256 and 4096, so if 8 is
4842 specified it will be rounded up to 16, and 32, 64 and 128 will all
4843 be rounded up to 256, and so on. Note that alignment to 4096-byte
4844 boundaries is a \i{PharLap} extension to the format and may not be
4845 supported by all linkers.\I{section alignment, in OBJ}\I{segment
4846 alignment, in OBJ}\I{alignment, in OBJ sections}
4848 \b \i\c{CLASS} can be used to specify the segment class; this feature
4849 indicates to the linker that segments of the same class should be
4850 placed near each other in the output file. The class name can be any
4851 word, e.g. \c{CLASS=CODE}.
4853 \b \i\c{OVERLAY}, like \c{CLASS}, is specified with an arbitrary word
4854 as an argument, and provides overlay information to an
4855 overlay-capable linker.
4857 \b Segments can be declared as \i\c{USE16} or \i\c{USE32}, which has
4858 the effect of recording the choice in the object file and also
4859 ensuring that NASM's default assembly mode when assembling in that
4860 segment is 16-bit or 32-bit respectively.
4862 \b When writing \i{OS/2} object files, you should declare 32-bit
4863 segments as \i\c{FLAT}, which causes the default segment base for
4864 anything in the segment to be the special group \c{FLAT}, and also
4865 defines the group if it is not already defined.
4867 \b The \c{obj} file format also allows segments to be declared as
4868 having a pre-defined absolute segment address, although no linkers
4869 are currently known to make sensible use of this feature;
4870 nevertheless, NASM allows you to declare a segment such as
4871 \c{SEGMENT SCREEN ABSOLUTE=0xB800} if you need to. The \i\c{ABSOLUTE}
4872 and \c{ALIGN} keywords are mutually exclusive.
4874 NASM's default segment attributes are \c{PUBLIC}, \c{ALIGN=1}, no
4875 class, no overlay, and \c{USE16}.
4878 \S{group} \i\c{GROUP}: Defining Groups of Segments\I{segments, groups of}
4880 The \c{obj} format also allows segments to be grouped, so that a
4881 single segment register can be used to refer to all the segments in
4882 a group. NASM therefore supplies the \c{GROUP} directive, whereby
4891 \c ; some uninitialized data
4893 \c group dgroup data bss
4895 which will define a group called \c{dgroup} to contain the segments
4896 \c{data} and \c{bss}. Like \c{SEGMENT}, \c{GROUP} causes the group
4897 name to be defined as a symbol, so that you can refer to a variable
4898 \c{var} in the \c{data} segment as \c{var wrt data} or as \c{var wrt
4899 dgroup}, depending on which segment value is currently in your
4902 If you just refer to \c{var}, however, and \c{var} is declared in a
4903 segment which is part of a group, then NASM will default to giving
4904 you the offset of \c{var} from the beginning of the \e{group}, not
4905 the \e{segment}. Therefore \c{SEG var}, also, will return the group
4906 base rather than the segment base.
4908 NASM will allow a segment to be part of more than one group, but
4909 will generate a warning if you do this. Variables declared in a
4910 segment which is part of more than one group will default to being
4911 relative to the first group that was defined to contain the segment.
4913 A group does not have to contain any segments; you can still make
4914 \c{WRT} references to a group which does not contain the variable
4915 you are referring to. OS/2, for example, defines the special group
4916 \c{FLAT} with no segments in it.
4919 \S{uppercase} \i\c{UPPERCASE}: Disabling Case Sensitivity in Output
4921 Although NASM itself is \i{case sensitive}, some OMF linkers are
4922 not; therefore it can be useful for NASM to output single-case
4923 object files. The \c{UPPERCASE} format-specific directive causes all
4924 segment, group and symbol names that are written to the object file
4925 to be forced to upper case just before being written. Within a
4926 source file, NASM is still case-sensitive; but the object file can
4927 be written entirely in upper case if desired.
4929 \c{UPPERCASE} is used alone on a line; it requires no parameters.
4932 \S{import} \i\c{IMPORT}: Importing DLL Symbols\I{DLL symbols,
4933 importing}\I{symbols, importing from DLLs}
4935 The \c{IMPORT} format-specific directive defines a symbol to be
4936 imported from a DLL, for use if you are writing a DLL's \i{import
4937 library} in NASM. You still need to declare the symbol as \c{EXTERN}
4938 as well as using the \c{IMPORT} directive.
4940 The \c{IMPORT} directive takes two required parameters, separated by
4941 white space, which are (respectively) the name of the symbol you
4942 wish to import and the name of the library you wish to import it
4945 \c import WSAStartup wsock32.dll
4947 A third optional parameter gives the name by which the symbol is
4948 known in the library you are importing it from, in case this is not
4949 the same as the name you wish the symbol to be known by to your code
4950 once you have imported it. For example:
4952 \c import asyncsel wsock32.dll WSAAsyncSelect
4955 \S{export} \i\c{EXPORT}: Exporting DLL Symbols\I{DLL symbols,
4956 exporting}\I{symbols, exporting from DLLs}
4958 The \c{EXPORT} format-specific directive defines a global symbol to
4959 be exported as a DLL symbol, for use if you are writing a DLL in
4960 NASM. You still need to declare the symbol as \c{GLOBAL} as well as
4961 using the \c{EXPORT} directive.
4963 \c{EXPORT} takes one required parameter, which is the name of the
4964 symbol you wish to export, as it was defined in your source file. An
4965 optional second parameter (separated by white space from the first)
4966 gives the \e{external} name of the symbol: the name by which you
4967 wish the symbol to be known to programs using the DLL. If this name
4968 is the same as the internal name, you may leave the second parameter
4971 Further parameters can be given to define attributes of the exported
4972 symbol. These parameters, like the second, are separated by white
4973 space. If further parameters are given, the external name must also
4974 be specified, even if it is the same as the internal name. The
4975 available attributes are:
4977 \b \c{resident} indicates that the exported name is to be kept
4978 resident by the system loader. This is an optimisation for
4979 frequently used symbols imported by name.
4981 \b \c{nodata} indicates that the exported symbol is a function which
4982 does not make use of any initialized data.
4984 \b \c{parm=NNN}, where \c{NNN} is an integer, sets the number of
4985 parameter words for the case in which the symbol is a call gate
4986 between 32-bit and 16-bit segments.
4988 \b An attribute which is just a number indicates that the symbol
4989 should be exported with an identifying number (ordinal), and gives
4995 \c export myfunc TheRealMoreFormalLookingFunctionName
4996 \c export myfunc myfunc 1234 ; export by ordinal
4997 \c export myfunc myfunc resident parm=23 nodata
5000 \S{dotdotstart} \i\c{..start}: Defining the \i{Program Entry
5003 \c{OMF} linkers require exactly one of the object files being linked to
5004 define the program entry point, where execution will begin when the
5005 program is run. If the object file that defines the entry point is
5006 assembled using NASM, you specify the entry point by declaring the
5007 special symbol \c{..start} at the point where you wish execution to
5011 \S{objextern} \c{obj} Extensions to the \c{EXTERN}
5012 Directive\I{EXTERN, obj extensions to}
5014 If you declare an external symbol with the directive
5018 then references such as \c{mov ax,foo} will give you the offset of
5019 \c{foo} from its preferred segment base (as specified in whichever
5020 module \c{foo} is actually defined in). So to access the contents of
5021 \c{foo} you will usually need to do something like
5023 \c mov ax,seg foo ; get preferred segment base
5024 \c mov es,ax ; move it into ES
5025 \c mov ax,[es:foo] ; and use offset `foo' from it
5027 This is a little unwieldy, particularly if you know that an external
5028 is going to be accessible from a given segment or group, say
5029 \c{dgroup}. So if \c{DS} already contained \c{dgroup}, you could
5032 \c mov ax,[foo wrt dgroup]
5034 However, having to type this every time you want to access \c{foo}
5035 can be a pain; so NASM allows you to declare \c{foo} in the
5038 \c extern foo:wrt dgroup
5040 This form causes NASM to pretend that the preferred segment base of
5041 \c{foo} is in fact \c{dgroup}; so the expression \c{seg foo} will
5042 now return \c{dgroup}, and the expression \c{foo} is equivalent to
5045 This \I{default-WRT mechanism}default-\c{WRT} mechanism can be used
5046 to make externals appear to be relative to any group or segment in
5047 your program. It can also be applied to common variables: see
5051 \S{objcommon} \c{obj} Extensions to the \c{COMMON}
5052 Directive\I{COMMON, obj extensions to}
5054 The \c{obj} format allows common variables to be either near\I{near
5055 common variables} or far\I{far common variables}; NASM allows you to
5056 specify which your variables should be by the use of the syntax
5058 \c common nearvar 2:near ; `nearvar' is a near common
5059 \c common farvar 10:far ; and `farvar' is far
5061 Far common variables may be greater in size than 64Kb, and so the
5062 OMF specification says that they are declared as a number of
5063 \e{elements} of a given size. So a 10-byte far common variable could
5064 be declared as ten one-byte elements, five two-byte elements, two
5065 five-byte elements or one ten-byte element.
5067 Some \c{OMF} linkers require the \I{element size, in common
5068 variables}\I{common variables, element size}element size, as well as
5069 the variable size, to match when resolving common variables declared
5070 in more than one module. Therefore NASM must allow you to specify
5071 the element size on your far common variables. This is done by the
5074 \c common c_5by2 10:far 5 ; two five-byte elements
5075 \c common c_2by5 10:far 2 ; five two-byte elements
5077 If no element size is specified, the default is 1. Also, the \c{FAR}
5078 keyword is not required when an element size is specified, since
5079 only far commons may have element sizes at all. So the above
5080 declarations could equivalently be
5082 \c common c_5by2 10:5 ; two five-byte elements
5083 \c common c_2by5 10:2 ; five two-byte elements
5085 In addition to these extensions, the \c{COMMON} directive in \c{obj}
5086 also supports default-\c{WRT} specification like \c{EXTERN} does
5087 (explained in \k{objextern}). So you can also declare things like
5089 \c common foo 10:wrt dgroup
5090 \c common bar 16:far 2:wrt data
5091 \c common baz 24:wrt data:6
5094 \H{win32fmt} \i\c{win32}: Microsoft Win32 Object Files
5096 The \c{win32} output format generates Microsoft Win32 object files,
5097 suitable for passing to Microsoft linkers such as \i{Visual C++}.
5098 Note that Borland Win32 compilers do not use this format, but use
5099 \c{obj} instead (see \k{objfmt}).
5101 \c{win32} provides a default output file-name extension of \c{.obj}.
5103 Note that although Microsoft say that Win32 object files follow the
5104 \c{COFF} (Common Object File Format) standard, the object files produced
5105 by Microsoft Win32 compilers are not compatible with COFF linkers
5106 such as DJGPP's, and vice versa. This is due to a difference of
5107 opinion over the precise semantics of PC-relative relocations. To
5108 produce COFF files suitable for DJGPP, use NASM's \c{coff} output
5109 format; conversely, the \c{coff} format does not produce object
5110 files that Win32 linkers can generate correct output from.
5113 \S{win32sect} \c{win32} Extensions to the \c{SECTION}
5114 Directive\I{SECTION, win32 extensions to}
5116 Like the \c{obj} format, \c{win32} allows you to specify additional
5117 information on the \c{SECTION} directive line, to control the type
5118 and properties of sections you declare. Section types and properties
5119 are generated automatically by NASM for the \i{standard section names}
5120 \c{.text}, \c{.data} and \c{.bss}, but may still be overridden by
5123 The available qualifiers are:
5125 \b \c{code}, or equivalently \c{text}, defines the section to be a
5126 code section. This marks the section as readable and executable, but
5127 not writable, and also indicates to the linker that the type of the
5130 \b \c{data} and \c{bss} define the section to be a data section,
5131 analogously to \c{code}. Data sections are marked as readable and
5132 writable, but not executable. \c{data} declares an initialized data
5133 section, whereas \c{bss} declares an uninitialized data section.
5135 \b \c{rdata} declares an initialized data section that is readable
5136 but not writable. Microsoft compilers use this section to place
5139 \b \c{info} defines the section to be an \i{informational section},
5140 which is not included in the executable file by the linker, but may
5141 (for example) pass information \e{to} the linker. For example,
5142 declaring an \c{info}-type section called \i\c{.drectve} causes the
5143 linker to interpret the contents of the section as command-line
5146 \b \c{align=}, used with a trailing number as in \c{obj}, gives the
5147 \I{section alignment, in win32}\I{alignment, in win32
5148 sections}alignment requirements of the section. The maximum you may
5149 specify is 64: the Win32 object file format contains no means to
5150 request a greater section alignment than this. If alignment is not
5151 explicitly specified, the defaults are 16-byte alignment for code
5152 sections, 8-byte alignment for rdata sections and 4-byte alignment
5153 for data (and BSS) sections.
5154 Informational sections get a default alignment of 1 byte (no
5155 alignment), though the value does not matter.
5157 The defaults assumed by NASM if you do not specify the above
5160 \c section .text code align=16
5161 \c section .data data align=4
5162 \c section .rdata rdata align=8
5163 \c section .bss bss align=4
5165 Any other section name is treated by default like \c{.text}.
5167 \S{win32safeseh} \c{win32}: Safe Structured Exception Handling
5169 Among other improvements in Windows XP SP2 and Windows Server 2003
5170 Microsoft has introduced concept of "safe structured exception
5171 handling." General idea is to collect handlers' entry points in
5172 designated read-only table and have alleged entry point verified
5173 against this table prior exception control is passed to the handler. In
5174 order for an executable module to be equipped with such "safe exception
5175 handler table," all object modules on linker command line has to comply
5176 with certain criteria. If one single module among them does not, then
5177 the table in question is omitted and above mentioned run-time checks
5178 will not be performed for application in question. Table omission is by
5179 default silent and therefore can be easily overlooked. One can instruct
5180 linker to refuse to produce binary without such table by passing
5181 \c{/safeseh} command line option.
5183 Without regard to this run-time check merits it's natural to expect
5184 NASM to be capable of generating modules suitable for \c{/safeseh}
5185 linking. From developer's viewpoint the problem is two-fold:
5187 \b how to adapt modules not deploying exception handlers of their own;
5189 \b how to adapt/develop modules utilizing custom exception handling;
5191 Former can be easily achieved with any NASM version by adding following
5192 line to source code:
5196 As of version 2.03 NASM adds this absolute symbol automatically. If
5197 it's not already present to be precise. I.e. if for whatever reason
5198 developer would choose to assign another value in source file, it would
5199 still be perfectly possible.
5201 Registering custom exception handler on the other hand requires certain
5202 "magic." As of version 2.03 additional directive is implemented,
5203 \c{safeseh}, which instructs the assembler to produce appropriately
5204 formatted input data for above mentioned "safe exception handler
5205 table." Its typical use would be:
5208 \c extern _MessageBoxA@16
5209 \c %if __NASM_VERSION_ID__ >= 0x02030000
5210 \c safeseh handler ; register handler as "safe handler"
5213 \c push DWORD 1 ; MB_OKCANCEL
5214 \c push DWORD caption
5217 \c call _MessageBoxA@16
5218 \c sub eax,1 ; incidentally suits as return value
5219 \c ; for exception handler
5223 \c push DWORD handler
5224 \c push DWORD [fs:0]
5225 \c mov DWORD [fs:0],esp ; engage exception handler
5227 \c mov eax,DWORD[eax] ; cause exception
5228 \c pop DWORD [fs:0] ; disengage exception handler
5231 \c text: db 'OK to rethrow, CANCEL to generate core dump',0
5232 \c caption:db 'SEGV',0
5234 \c section .drectve info
5235 \c db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
5237 As you might imagine, it's perfectly possible to produce .exe binary
5238 with "safe exception handler table" and yet engage unregistered
5239 exception handler. Indeed, handler is engaged by simply manipulating
5240 \c{[fs:0]} location at run-time, something linker has no power over,
5241 run-time that is. It should be explicitly mentioned that such failure
5242 to register handler's entry point with \c{safeseh} directive has
5243 undesired side effect at run-time. If exception is raised and
5244 unregistered handler is to be executed, the application is abruptly
5245 terminated without any notification whatsoever. One can argue that
5246 system could at least have logged some kind "non-safe exception
5247 handler in x.exe at address n" message in event log, but no, literally
5248 no notification is provided and user is left with no clue on what
5249 caused application failure.
5251 Finally, all mentions of linker in this paragraph refer to Microsoft
5252 linker version 7.x and later. Presence of \c{@feat.00} symbol and input
5253 data for "safe exception handler table" causes no backward
5254 incompatibilities and "safeseh" modules generated by NASM 2.03 and
5255 later can still be linked by earlier versions or non-Microsoft linkers.
5258 \H{win64fmt} \i\c{win64}: Microsoft Win64 Object Files
5260 The \c{win64} output format generates Microsoft Win64 object files,
5261 which is nearly 100% identical to the \c{win32} object format (\k{win32fmt})
5262 with the exception that it is meant to target 64-bit code and the x86-64
5263 platform altogether. This object file is used exactly the same as the \c{win32}
5264 object format (\k{win32fmt}), in NASM, with regard to this exception.
5266 \S{win64pic} \c{win64}: Writing Position-Independent Code
5268 While \c{REL} takes good care of RIP-relative addressing, there is one
5269 aspect that is easy to overlook for a Win64 programmer: indirect
5270 references. Consider a switch dispatch table:
5272 \c jmp QWORD[dsptch+rax*8]
5278 Even novice Win64 assembler programmer will soon realize that the code
5279 is not 64-bit savvy. Most notably linker will refuse to link it with
5280 "\c{'ADDR32' relocation to '.text' invalid without
5281 /LARGEADDRESSAWARE:NO}". So [s]he will have to split jmp instruction as
5284 \c lea rbx,[rel dsptch]
5285 \c jmp QWORD[rbx+rax*8]
5287 What happens behind the scene is that effective address in \c{lea} is
5288 encoded relative to instruction pointer, or in perfectly
5289 position-independent manner. But this is only part of the problem!
5290 Trouble is that in .dll context \c{caseN} relocations will make their
5291 way to the final module and might have to be adjusted at .dll load
5292 time. To be specific when it can't be loaded at preferred address. And
5293 when this occurs, pages with such relocations will be rendered private
5294 to current process, which kind of undermines the idea of sharing .dll.
5295 But no worry, it's trivial to fix:
5297 \c lea rbx,[rel dsptch]
5298 \c add rbx,QWORD[rbx+rax*8]
5301 \c dsptch: dq case0-dsptch
5305 NASM version 2.03 and later provides another alternative, \c{wrt
5306 ..imagebase} operator, which returns offset from base address of the
5307 current image, be it .exe or .dll module, therefore the name. For those
5308 acquainted with PE-COFF format base address denotes start of
5309 \c{IMAGE_DOS_HEADER} structure. Here is how to implement switch with
5310 these image-relative references:
5312 \c lea rbx,[rel dsptch]
5313 \c mov eax,DWORD[rbx+rax*4]
5314 \c sub rbx,dsptch wrt ..imagebase
5318 \c dsptch: dd case0 wrt ..imagebase
5319 \c dd case1 wrt ..imagebase
5321 One can argue that the operator is redundant. Indeed, snippet before
5322 last works just fine with any NASM version and is not even Windows
5323 specific... The real reason for implementing \c{wrt ..imagebase} will
5324 become apparent in next paragraph.
5326 It should be noted that \c{wrt ..imagebase} is defined as 32-bit
5329 \c dd label wrt ..imagebase ; ok
5330 \c dq label wrt ..imagebase ; bad
5331 \c mov eax,label wrt ..imagebase ; ok
5332 \c mov rax,label wrt ..imagebase ; bad
5334 \S{win64seh} \c{win64}: Structured Exception Handling
5336 Structured exception handing in Win64 is completely different matter
5337 from Win32. Upon exception program counter value is noted, and
5338 linker-generated table comprising start and end addresses of all the
5339 functions [in given executable module] is traversed and compared to the
5340 saved program counter. Thus so called \c{UNWIND_INFO} structure is
5341 identified. If it's not found, then offending subroutine is assumed to
5342 be "leaf" and just mentioned lookup procedure is attempted for its
5343 caller. In Win64 leaf function is such function that does not call any
5344 other function \e{nor} modifies any Win64 non-volatile registers,
5345 including stack pointer. The latter ensures that it's possible to
5346 identify leaf function's caller by simply pulling the value from the
5349 While majority of subroutines written in assembler are not calling any
5350 other function, requirement for non-volatile registers' immutability
5351 leaves developer with not more than 7 registers and no stack frame,
5352 which is not necessarily what [s]he counted with. Customarily one would
5353 meet the requirement by saving non-volatile registers on stack and
5354 restoring them upon return, so what can go wrong? If [and only if] an
5355 exception is raised at run-time and no \c{UNWIND_INFO} structure is
5356 associated with such "leaf" function, the stack unwind procedure will
5357 expect to find caller's return address on the top of stack immediately
5358 followed by its frame. Given that developer pushed caller's
5359 non-volatile registers on stack, would the value on top point at some
5360 code segment or even addressable space? Well, developer can attempt
5361 copying caller's return address to the top of stack and this would
5362 actually work in some very specific circumstances. But unless developer
5363 can guarantee that these circumstances are always met, it's more
5364 appropriate to assume worst case scenario, i.e. stack unwind procedure
5365 going berserk. Relevant question is what happens then? Application is
5366 abruptly terminated without any notification whatsoever. Just like in
5367 Win32 case, one can argue that system could at least have logged
5368 "unwind procedure went berserk in x.exe at address n" in event log, but
5369 no, no trace of failure is left.
5371 Now, when we understand significance of the \c{UNWIND_INFO} structure,
5372 let's discuss what's in it and/or how it's processed. First of all it
5373 is checked for presence of reference to custom language-specific
5374 exception handler. If there is one, then it's invoked. Depending on the
5375 return value, execution flow is resumed (exception is said to be
5376 "handled"), \e{or} rest of \c{UNWIND_INFO} structure is processed as
5377 following. Beside optional reference to custom handler, it carries
5378 information about current callee's stack frame and where non-volatile
5379 registers are saved. Information is detailed enough to be able to
5380 reconstruct contents of caller's non-volatile registers upon call to
5381 current callee. And so caller's context is reconstructed, and then
5382 unwind procedure is repeated, i.e. another \c{UNWIND_INFO} structure is
5383 associated, this time, with caller's instruction pointer, which is then
5384 checked for presence of reference to language-specific handler, etc.
5385 The procedure is recursively repeated till exception is handled. As
5386 last resort system "handles" it by generating memory core dump and
5387 terminating the application.
5389 As for the moment of this writing NASM unfortunately does not
5390 facilitate generation of above mentioned detailed information about
5391 stack frame layout. But as of version 2.03 it implements building
5392 blocks for generating structures involved in stack unwinding. As
5393 simplest example, here is how to deploy custom exception handler for
5398 \c extern MessageBoxA
5404 \c mov r9,1 ; MB_OKCANCEL
5406 \c sub eax,1 ; incidentally suits as return value
5407 \c ; for exception handler
5413 \c mov rax,QWORD[rax] ; cause exception
5416 \c text: db 'OK to rethrow, CANCEL to generate core dump',0
5417 \c caption:db 'SEGV',0
5419 \c section .pdata rdata align=4
5420 \c dd main wrt ..imagebase
5421 \c dd main_end wrt ..imagebase
5422 \c dd xmain wrt ..imagebase
5423 \c section .xdata rdata align=8
5424 \c xmain: db 9,0,0,0
5425 \c dd handler wrt ..imagebase
5426 \c section .drectve info
5427 \c db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
5429 What you see in \c{.pdata} section is element of the "table comprising
5430 start and end addresses of function" along with reference to associated
5431 \c{UNWIND_INFO} structure. And what you see in \c{.xdata} section is
5432 \c{UNWIND_INFO} structure describing function with no frame, but with
5433 designated exception handler. References are \e{required} to be
5434 image-relative (which is the real reason for implementing \c{wrt
5435 ..imagebase} operator). It should be noted that \c{rdata align=n}, as
5436 well as \c{wrt ..imagebase}, are optional in these two segments'
5437 contexts, i.e. can be omitted. Latter means that \e{all} 32-bit
5438 references, not only above listed required ones, placed into these two
5439 segments turn out image-relative. Why is it important to understand?
5440 Developer is allowed to append handler-specific data to \c{UNWIND_INFO}
5441 structure, and if [s]he adds a 32-bit reference, then [s]he will have
5442 to remember to adjust its value to obtain the real pointer.
5444 As already mentioned, in Win64 terms leaf function is one that does not
5445 call any other function \e{nor} modifies any non-volatile register,
5446 including stack pointer. But it's not uncommon that assembler
5447 programmer plans to utilize every single register and sometimes even
5448 have variable stack frame. Is there anything one can do with bare
5449 building blocks? I.e. besides manually composing fully-fledged
5450 \c{UNWIND_INFO} structure, which would surely be considered
5451 error-prone? Yes, there is. Recall that exception handler is called
5452 first, before stack layout is analyzed. As it turned out, it's
5453 perfectly possible to manipulate current callee's context in custom
5454 handler in manner that permits further stack unwinding. General idea is
5455 that handler would not actually "handle" the exception, but instead
5456 restore callee's context, as it was at its entry point and thus mimic
5457 leaf function. In other words, handler would simply undertake part of
5458 unwinding procedure. Consider following example:
5461 \c mov rax,rsp ; copy rsp to volatile register
5462 \c push r15 ; save non-volatile registers
5465 \c mov r11,rsp ; prepare variable stack frame
5468 \c mov QWORD[r11],rax ; check for exceptions
5469 \c mov rsp,r11 ; allocate stack frame
5470 \c mov QWORD[rsp],rax ; save original rsp value
5473 \c mov r11,QWORD[rsp] ; pull original rsp value
5474 \c mov rbp,QWORD[r11-24]
5475 \c mov rbx,QWORD[r11-16]
5476 \c mov r15,QWORD[r11-8]
5477 \c mov rsp,r11 ; destroy frame
5480 The keyword is that up to \c{magic_point} original \c{rsp} value
5481 remains in chosen volatile register and no non-volatile register,
5482 except for \c{rsp}, is modified. While past \c{magic_point} \c{rsp}
5483 remains constant till the very end of the \c{function}. In this case
5484 custom language-specific exception handler would look like this:
5486 \c EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame,
5487 \c CONTEXT *context,DISPATCHER_CONTEXT *disp)
5489 \c if (context->Rip<(ULONG64)magic_point)
5490 \c rsp = (ULONG64 *)context->Rax;
5492 \c { rsp = ((ULONG64 **)context->Rsp)[0];
5493 \c context->Rbp = rsp[-3];
5494 \c context->Rbx = rsp[-2];
5495 \c context->R15 = rsp[-1];
5497 \c context->Rsp = (ULONG64)rsp;
5499 \c memcpy (disp->ContextRecord,context,sizeof(CONTEXT));
5500 \c RtlVirtualUnwind(UNW_FLAG_NHANDLER,disp->ImageBase,
5501 \c dips->ControlPc,disp->FunctionEntry,disp->ContextRecord,
5502 \c &disp->HandlerData,&disp->EstablisherFrame,NULL);
5503 \c return ExceptionContinueSearch;
5506 As custom handler mimics leaf function, corresponding \c{UNWIND_INFO}
5507 structure does not have to contain any information about stack frame
5510 \H{cofffmt} \i\c{coff}: \i{Common Object File Format}
5512 The \c{coff} output type produces \c{COFF} object files suitable for
5513 linking with the \i{DJGPP} linker.
5515 \c{coff} provides a default output file-name extension of \c{.o}.
5517 The \c{coff} format supports the same extensions to the \c{SECTION}
5518 directive as \c{win32} does, except that the \c{align} qualifier and
5519 the \c{info} section type are not supported.
5521 \H{machofmt} \I{Mach-O}\i\c{macho32} and \i\c{macho64}: \i{Mach Object File Format}
5523 The \c{macho32} and \c{macho64} output formts produces \c{Mach-O}
5524 object files suitable for linking with the \i{MacOS X} linker.
5525 \i\c{macho} is a synonym for \c{macho32}.
5527 \c{macho} provides a default output file-name extension of \c{.o}.
5529 \H{elffmt} \i\c{elf32} and \i\c{elf64}: \I{ELF}\I{linux, elf}\i{Executable and Linkable
5530 Format} Object Files
5532 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},
5533 including \i{Solaris x86}, \i{UnixWare} and \i{SCO Unix}. \c{elf}
5534 provides a default output file-name extension of \c{.o}.
5535 \c{elf} is a synonym for \c{elf32}.
5537 \S{abisect} ELF specific directive \i\c{osabi}
5539 The ELF header specifies the application binary interface for the target operating system (OSABI).
5540 This field can be set by using the \c{osabi} directive with the numeric value (0-255) of the target
5541 system. If this directive is not used, the default value will be "UNIX System V ABI" (0) which will work on
5542 most systems which support ELF.
5544 \S{elfsect} \c{elf} Extensions to the \c{SECTION}
5545 Directive\I{SECTION, elf extensions to}
5547 Like the \c{obj} format, \c{elf} allows you to specify additional
5548 information on the \c{SECTION} directive line, to control the type
5549 and properties of sections you declare. Section types and properties
5550 are generated automatically by NASM for the \i{standard section
5551 names}, but may still be
5552 overridden by these qualifiers.
5554 The available qualifiers are:
5556 \b \i\c{alloc} defines the section to be one which is loaded into
5557 memory when the program is run. \i\c{noalloc} defines it to be one
5558 which is not, such as an informational or comment section.
5560 \b \i\c{exec} defines the section to be one which should have execute
5561 permission when the program is run. \i\c{noexec} defines it as one
5564 \b \i\c{write} defines the section to be one which should be writable
5565 when the program is run. \i\c{nowrite} defines it as one which should
5568 \b \i\c{progbits} defines the section to be one with explicit contents
5569 stored in the object file: an ordinary code or data section, for
5570 example, \i\c{nobits} defines the section to be one with no explicit
5571 contents given, such as a BSS section.
5573 \b \c{align=}, used with a trailing number as in \c{obj}, gives the
5574 \I{section alignment, in elf}\I{alignment, in elf sections}alignment
5575 requirements of the section.
5577 \b \i\c{tls} defines the section to be one which contains
5578 thread local variables.
5580 The defaults assumed by NASM if you do not specify the above
5583 \I\c{.text} \I\c{.rodata} \I\c{.lrodata} \I\c{.data} \I\c{.ldata}
5584 \I\c{.bss} \I\c{.lbss} \I\c{.tdata} \I\c{.tbss} \I\c\{.comment}
5586 \c section .text progbits alloc exec nowrite align=16
5587 \c section .rodata progbits alloc noexec nowrite align=4
5588 \c section .lrodata progbits alloc noexec nowrite align=4
5589 \c section .data progbits alloc noexec write align=4
5590 \c section .ldata progbits alloc noexec write align=4
5591 \c section .bss nobits alloc noexec write align=4
5592 \c section .lbss nobits alloc noexec write align=4
5593 \c section .tdata progbits alloc noexec write align=4 tls
5594 \c section .tbss nobits alloc noexec write align=4 tls
5595 \c section .comment progbits noalloc noexec nowrite align=1
5596 \c section other progbits alloc noexec nowrite align=1
5598 (Any section name other than those in the above table
5599 is treated by default like \c{other} in the above table.
5600 Please note that section names are case sensitive.)
5603 \S{elfwrt} \i{Position-Independent Code}\I{PIC}: \c{elf} Special
5604 Symbols and \i\c{WRT}
5606 The \c{ELF} specification contains enough features to allow
5607 position-independent code (PIC) to be written, which makes \i{ELF
5608 shared libraries} very flexible. However, it also means NASM has to
5609 be able to generate a variety of ELF specific relocation types in ELF
5610 object files, if it is to be an assembler which can write PIC.
5612 Since \c{ELF} does not support segment-base references, the \c{WRT}
5613 operator is not used for its normal purpose; therefore NASM's
5614 \c{elf} output format makes use of \c{WRT} for a different purpose,
5615 namely the PIC-specific \I{relocations, PIC-specific}relocation
5618 \c{elf} defines five special symbols which you can use as the
5619 right-hand side of the \c{WRT} operator to obtain PIC relocation
5620 types. They are \i\c{..gotpc}, \i\c{..gotoff}, \i\c{..got},
5621 \i\c{..plt} and \i\c{..sym}. Their functions are summarized here:
5623 \b Referring to the symbol marking the global offset table base
5624 using \c{wrt ..gotpc} will end up giving the distance from the
5625 beginning of the current section to the global offset table.
5626 (\i\c{_GLOBAL_OFFSET_TABLE_} is the standard symbol name used to
5627 refer to the \i{GOT}.) So you would then need to add \i\c{$$} to the
5628 result to get the real address of the GOT.
5630 \b Referring to a location in one of your own sections using \c{wrt
5631 ..gotoff} will give the distance from the beginning of the GOT to
5632 the specified location, so that adding on the address of the GOT
5633 would give the real address of the location you wanted.
5635 \b Referring to an external or global symbol using \c{wrt ..got}
5636 causes the linker to build an entry \e{in} the GOT containing the
5637 address of the symbol, and the reference gives the distance from the
5638 beginning of the GOT to the entry; so you can add on the address of
5639 the GOT, load from the resulting address, and end up with the
5640 address of the symbol.
5642 \b Referring to a procedure name using \c{wrt ..plt} causes the
5643 linker to build a \i{procedure linkage table} entry for the symbol,
5644 and the reference gives the address of the \i{PLT} entry. You can
5645 only use this in contexts which would generate a PC-relative
5646 relocation normally (i.e. as the destination for \c{CALL} or
5647 \c{JMP}), since ELF contains no relocation type to refer to PLT
5650 \b Referring to a symbol name using \c{wrt ..sym} causes NASM to
5651 write an ordinary relocation, but instead of making the relocation
5652 relative to the start of the section and then adding on the offset
5653 to the symbol, it will write a relocation record aimed directly at
5654 the symbol in question. The distinction is a necessary one due to a
5655 peculiarity of the dynamic linker.
5657 A fuller explanation of how to use these relocation types to write
5658 shared libraries entirely in NASM is given in \k{picdll}.
5660 \S{elftls} \i{Thread Local Storage}\I{TLS}: \c{elf} Special
5661 Symbols and \i\c{WRT}
5663 \b In ELF32 mode, referring to an external or global symbol using
5664 \c{wrt ..tlsie} \I\c{..tlsie}
5665 causes the linker to build an entry \e{in} the GOT containing the
5666 offset of the symbol within the TLS block, so you can access the value
5667 of the symbol with code such as:
5669 \c mov eax,[tid wrt ..tlsie]
5673 \b In ELF64 mode, referring to an external or global symbol using
5674 \c{wrt ..gottpoff} \I\c{..gottpoff}
5675 causes the linker to build an entry \e{in} the GOT containing the
5676 offset of the symbol within the TLS block, so you can access the value
5677 of the symbol with code such as:
5679 \c mov rax,[rel tid wrt ..gottpoff]
5683 \S{elfglob} \c{elf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL,
5684 elf extensions to}\I{GLOBAL, aoutb extensions to}
5686 \c{ELF} object files can contain more information about a global symbol
5687 than just its address: they can contain the \I{symbol sizes,
5688 specifying}\I{size, of symbols}size of the symbol and its \I{symbol
5689 types, specifying}\I{type, of symbols}type as well. These are not
5690 merely debugger conveniences, but are actually necessary when the
5691 program being written is a \i{shared library}. NASM therefore
5692 supports some extensions to the \c{GLOBAL} directive, allowing you
5693 to specify these features.
5695 You can specify whether a global variable is a function or a data
5696 object by suffixing the name with a colon and the word
5697 \i\c{function} or \i\c{data}. (\i\c{object} is a synonym for
5698 \c{data}.) For example:
5700 \c global hashlookup:function, hashtable:data
5702 exports the global symbol \c{hashlookup} as a function and
5703 \c{hashtable} as a data object.
5705 Optionally, you can control the ELF visibility of the symbol. Just
5706 add one of the visibility keywords: \i\c{default}, \i\c{internal},
5707 \i\c{hidden}, or \i\c{protected}. The default is \i\c{default} of
5708 course. For example, to make \c{hashlookup} hidden:
5710 \c global hashlookup:function hidden
5712 You can also specify the size of the data associated with the
5713 symbol, as a numeric expression (which may involve labels, and even
5714 forward references) after the type specifier. Like this:
5716 \c global hashtable:data (hashtable.end - hashtable)
5719 \c db this,that,theother ; some data here
5722 This makes NASM automatically calculate the length of the table and
5723 place that information into the \c{ELF} symbol table.
5725 Declaring the type and size of global symbols is necessary when
5726 writing shared library code. For more information, see
5730 \S{elfcomm} \c{elf} Extensions to the \c{COMMON} Directive
5731 \I{COMMON, elf extensions to}
5733 \c{ELF} also allows you to specify alignment requirements \I{common
5734 variables, alignment in elf}\I{alignment, of elf common variables}on
5735 common variables. This is done by putting a number (which must be a
5736 power of two) after the name and size of the common variable,
5737 separated (as usual) by a colon. For example, an array of
5738 doublewords would benefit from 4-byte alignment:
5740 \c common dwordarray 128:4
5742 This declares the total size of the array to be 128 bytes, and
5743 requires that it be aligned on a 4-byte boundary.
5746 \S{elf16} 16-bit code and ELF
5747 \I{ELF, 16-bit code and}
5749 The \c{ELF32} specification doesn't provide relocations for 8- and
5750 16-bit values, but the GNU \c{ld} linker adds these as an extension.
5751 NASM can generate GNU-compatible relocations, to allow 16-bit code to
5752 be linked as ELF using GNU \c{ld}. If NASM is used with the
5753 \c{-w+gnu-elf-extensions} option, a warning is issued when one of
5754 these relocations is generated.
5756 \S{elfdbg} Debug formats and ELF
5757 \I{ELF, Debug formats and}
5759 \c{ELF32} and \c{ELF64} provide debug information in \c{STABS} and \c{DWARF} formats.
5760 Line number information is generated for all executable sections, but please
5761 note that only the ".text" section is executable by default.
5763 \H{aoutfmt} \i\c{aout}: Linux \I{a.out, Linux version}\I{linux, a.out}\c{a.out} Object Files
5765 The \c{aout} format generates \c{a.out} object files, in the form used
5766 by early Linux systems (current Linux systems use ELF, see
5767 \k{elffmt}.) These differ from other \c{a.out} object files in that
5768 the magic number in the first four bytes of the file is
5769 different; also, some implementations of \c{a.out}, for example
5770 NetBSD's, support position-independent code, which Linux's
5771 implementation does not.
5773 \c{a.out} provides a default output file-name extension of \c{.o}.
5775 \c{a.out} is a very simple object format. It supports no special
5776 directives, no special symbols, no use of \c{SEG} or \c{WRT}, and no
5777 extensions to any standard directives. It supports only the three
5778 \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}.
5781 \H{aoutfmt} \i\c{aoutb}: \i{NetBSD}/\i{FreeBSD}/\i{OpenBSD}
5782 \I{a.out, BSD version}\c{a.out} Object Files
5784 The \c{aoutb} format generates \c{a.out} object files, in the form
5785 used by the various free \c{BSD Unix} clones, \c{NetBSD}, \c{FreeBSD}
5786 and \c{OpenBSD}. For simple object files, this object format is exactly
5787 the same as \c{aout} except for the magic number in the first four bytes
5788 of the file. However, the \c{aoutb} format supports
5789 \I{PIC}\i{position-independent code} in the same way as the \c{elf}
5790 format, so you can use it to write \c{BSD} \i{shared libraries}.
5792 \c{aoutb} provides a default output file-name extension of \c{.o}.
5794 \c{aoutb} supports no special directives, no special symbols, and
5795 only the three \i{standard section names} \i\c{.text}, \i\c{.data}
5796 and \i\c{.bss}. However, it also supports the same use of \i\c{WRT} as
5797 \c{elf} does, to provide position-independent code relocation types.
5798 See \k{elfwrt} for full documentation of this feature.
5800 \c{aoutb} also supports the same extensions to the \c{GLOBAL}
5801 directive as \c{elf} does: see \k{elfglob} for documentation of
5805 \H{as86fmt} \c{as86}: \i{Minix}/Linux\I{linux, as86} \i\c{as86} Object Files
5807 The Minix/Linux 16-bit assembler \c{as86} has its own non-standard
5808 object file format. Although its companion linker \i\c{ld86} produces
5809 something close to ordinary \c{a.out} binaries as output, the object
5810 file format used to communicate between \c{as86} and \c{ld86} is not
5813 NASM supports this format, just in case it is useful, as \c{as86}.
5814 \c{as86} provides a default output file-name extension of \c{.o}.
5816 \c{as86} is a very simple object format (from the NASM user's point
5817 of view). It supports no special directives, no use of \c{SEG} or \c{WRT},
5818 and no extensions to any standard directives. It supports only the three
5819 \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}. The
5820 only special symbol supported is \c{..start}.
5823 \H{rdffmt} \I{RDOFF}\i\c{rdf}: \i{Relocatable Dynamic Object File
5826 The \c{rdf} output format produces \c{RDOFF} object files. \c{RDOFF}
5827 (Relocatable Dynamic Object File Format) is a home-grown object-file
5828 format, designed alongside NASM itself and reflecting in its file
5829 format the internal structure of the assembler.
5831 \c{RDOFF} is not used by any well-known operating systems. Those
5832 writing their own systems, however, may well wish to use \c{RDOFF}
5833 as their object format, on the grounds that it is designed primarily
5834 for simplicity and contains very little file-header bureaucracy.
5836 The Unix NASM archive, and the DOS archive which includes sources,
5837 both contain an \I{rdoff subdirectory}\c{rdoff} subdirectory holding
5838 a set of RDOFF utilities: an RDF linker, an \c{RDF} static-library
5839 manager, an RDF file dump utility, and a program which will load and
5840 execute an RDF executable under Linux.
5842 \c{rdf} supports only the \i{standard section names} \i\c{.text},
5843 \i\c{.data} and \i\c{.bss}.
5846 \S{rdflib} Requiring a Library: The \i\c{LIBRARY} Directive
5848 \c{RDOFF} contains a mechanism for an object file to demand a given
5849 library to be linked to the module, either at load time or run time.
5850 This is done by the \c{LIBRARY} directive, which takes one argument
5851 which is the name of the module:
5853 \c library mylib.rdl
5856 \S{rdfmod} Specifying a Module Name: The \i\c{MODULE} Directive
5858 Special \c{RDOFF} header record is used to store the name of the module.
5859 It can be used, for example, by run-time loader to perform dynamic
5860 linking. \c{MODULE} directive takes one argument which is the name
5865 Note that when you statically link modules and tell linker to strip
5866 the symbols from output file, all module names will be stripped too.
5867 To avoid it, you should start module names with \I{$, prefix}\c{$}, like:
5869 \c module $kernel.core
5872 \S{rdfglob} \c{rdf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL,
5875 \c{RDOFF} global symbols can contain additional information needed by
5876 the static linker. You can mark a global symbol as exported, thus
5877 telling the linker do not strip it from target executable or library
5878 file. Like in \c{ELF}, you can also specify whether an exported symbol
5879 is a procedure (function) or data object.
5881 Suffixing the name with a colon and the word \i\c{export} you make the
5884 \c global sys_open:export
5886 To specify that exported symbol is a procedure (function), you add the
5887 word \i\c{proc} or \i\c{function} after declaration:
5889 \c global sys_open:export proc
5891 Similarly, to specify exported data object, add the word \i\c{data}
5892 or \i\c{object} to the directive:
5894 \c global kernel_ticks:export data
5897 \S{rdfimpt} \c{rdf} Extensions to the \c{EXTERN} Directive\I{EXTERN,
5900 By default the \c{EXTERN} directive in \c{RDOFF} declares a "pure external"
5901 symbol (i.e. the static linker will complain if such a symbol is not resolved).
5902 To declare an "imported" symbol, which must be resolved later during a dynamic
5903 linking phase, \c{RDOFF} offers an additional \c{import} modifier. As in
5904 \c{GLOBAL}, you can also specify whether an imported symbol is a procedure
5905 (function) or data object. For example:
5908 \c extern _open:import
5909 \c extern _printf:import proc
5910 \c extern _errno:import data
5912 Here the directive \c{LIBRARY} is also included, which gives the dynamic linker
5913 a hint as to where to find requested symbols.
5916 \H{dbgfmt} \i\c{dbg}: Debugging Format
5918 The \c{dbg} output format is not built into NASM in the default
5919 configuration. If you are building your own NASM executable from the
5920 sources, you can define \i\c{OF_DBG} in \c{output/outform.h} or on the
5921 compiler command line, and obtain the \c{dbg} output format.
5923 The \c{dbg} format does not output an object file as such; instead,
5924 it outputs a text file which contains a complete list of all the
5925 transactions between the main body of NASM and the output-format
5926 back end module. It is primarily intended to aid people who want to
5927 write their own output drivers, so that they can get a clearer idea
5928 of the various requests the main program makes of the output driver,
5929 and in what order they happen.
5931 For simple files, one can easily use the \c{dbg} format like this:
5933 \c nasm -f dbg filename.asm
5935 which will generate a diagnostic file called \c{filename.dbg}.
5936 However, this will not work well on files which were designed for a
5937 different object format, because each object format defines its own
5938 macros (usually user-level forms of directives), and those macros
5939 will not be defined in the \c{dbg} format. Therefore it can be
5940 useful to run NASM twice, in order to do the preprocessing with the
5941 native object format selected:
5943 \c nasm -e -f rdf -o rdfprog.i rdfprog.asm
5944 \c nasm -a -f dbg rdfprog.i
5946 This preprocesses \c{rdfprog.asm} into \c{rdfprog.i}, keeping the
5947 \c{rdf} object format selected in order to make sure RDF special
5948 directives are converted into primitive form correctly. Then the
5949 preprocessed source is fed through the \c{dbg} format to generate
5950 the final diagnostic output.
5952 This workaround will still typically not work for programs intended
5953 for \c{obj} format, because the \c{obj} \c{SEGMENT} and \c{GROUP}
5954 directives have side effects of defining the segment and group names
5955 as symbols; \c{dbg} will not do this, so the program will not
5956 assemble. You will have to work around that by defining the symbols
5957 yourself (using \c{EXTERN}, for example) if you really need to get a
5958 \c{dbg} trace of an \c{obj}-specific source file.
5960 \c{dbg} accepts any section name and any directives at all, and logs
5961 them all to its output file.
5964 \C{16bit} Writing 16-bit Code (DOS, Windows 3/3.1)
5966 This chapter attempts to cover some of the common issues encountered
5967 when writing 16-bit code to run under \c{MS-DOS} or \c{Windows 3.x}. It
5968 covers how to link programs to produce \c{.EXE} or \c{.COM} files,
5969 how to write \c{.SYS} device drivers, and how to interface assembly
5970 language code with 16-bit C compilers and with Borland Pascal.
5973 \H{exefiles} Producing \i\c{.EXE} Files
5975 Any large program written under DOS needs to be built as a \c{.EXE}
5976 file: only \c{.EXE} files have the necessary internal structure
5977 required to span more than one 64K segment. \i{Windows} programs,
5978 also, have to be built as \c{.EXE} files, since Windows does not
5979 support the \c{.COM} format.
5981 In general, you generate \c{.EXE} files by using the \c{obj} output
5982 format to produce one or more \i\c{.OBJ} files, and then linking
5983 them together using a linker. However, NASM also supports the direct
5984 generation of simple DOS \c{.EXE} files using the \c{bin} output
5985 format (by using \c{DB} and \c{DW} to construct the \c{.EXE} file
5986 header), and a macro package is supplied to do this. Thanks to
5987 Yann Guidon for contributing the code for this.
5989 NASM may also support \c{.EXE} natively as another output format in
5993 \S{objexe} Using the \c{obj} Format To Generate \c{.EXE} Files
5995 This section describes the usual method of generating \c{.EXE} files
5996 by linking \c{.OBJ} files together.
5998 Most 16-bit programming language packages come with a suitable
5999 linker; if you have none of these, there is a free linker called
6000 \i{VAL}\I{linker, free}, available in \c{LZH} archive format from
6001 \W{ftp://x2ftp.oulu.fi/pub/msdos/programming/lang/}\i\c{x2ftp.oulu.fi}.
6002 An LZH archiver can be found at
6003 \W{ftp://ftp.simtel.net/pub/simtelnet/msdos/arcers}\i\c{ftp.simtel.net}.
6004 There is another `free' linker (though this one doesn't come with
6005 sources) called \i{FREELINK}, available from
6006 \W{http://www.pcorner.com/tpc/old/3-101.html}\i\c{www.pcorner.com}.
6007 A third, \i\c{djlink}, written by DJ Delorie, is available at
6008 \W{http://www.delorie.com/djgpp/16bit/djlink/}\i\c{www.delorie.com}.
6009 A fourth linker, \i\c{ALINK}, written by Anthony A.J. Williams, is
6010 available at \W{http://alink.sourceforge.net}\i\c{alink.sourceforge.net}.
6012 When linking several \c{.OBJ} files into a \c{.EXE} file, you should
6013 ensure that exactly one of them has a start point defined (using the
6014 \I{program entry point}\i\c{..start} special symbol defined by the
6015 \c{obj} format: see \k{dotdotstart}). If no module defines a start
6016 point, the linker will not know what value to give the entry-point
6017 field in the output file header; if more than one defines a start
6018 point, the linker will not know \e{which} value to use.
6020 An example of a NASM source file which can be assembled to a
6021 \c{.OBJ} file and linked on its own to a \c{.EXE} is given here. It
6022 demonstrates the basic principles of defining a stack, initialising
6023 the segment registers, and declaring a start point. This file is
6024 also provided in the \I{test subdirectory}\c{test} subdirectory of
6025 the NASM archives, under the name \c{objexe.asm}.
6036 This initial piece of code sets up \c{DS} to point to the data
6037 segment, and initializes \c{SS} and \c{SP} to point to the top of
6038 the provided stack. Notice that interrupts are implicitly disabled
6039 for one instruction after a move into \c{SS}, precisely for this
6040 situation, so that there's no chance of an interrupt occurring
6041 between the loads of \c{SS} and \c{SP} and not having a stack to
6044 Note also that the special symbol \c{..start} is defined at the
6045 beginning of this code, which means that will be the entry point
6046 into the resulting executable file.
6052 The above is the main program: load \c{DS:DX} with a pointer to the
6053 greeting message (\c{hello} is implicitly relative to the segment
6054 \c{data}, which was loaded into \c{DS} in the setup code, so the
6055 full pointer is valid), and call the DOS print-string function.
6060 This terminates the program using another DOS system call.
6064 \c hello: db 'hello, world', 13, 10, '$'
6066 The data segment contains the string we want to display.
6068 \c segment stack stack
6072 The above code declares a stack segment containing 64 bytes of
6073 uninitialized stack space, and points \c{stacktop} at the top of it.
6074 The directive \c{segment stack stack} defines a segment \e{called}
6075 \c{stack}, and also of \e{type} \c{STACK}. The latter is not
6076 necessary to the correct running of the program, but linkers are
6077 likely to issue warnings or errors if your program has no segment of
6080 The above file, when assembled into a \c{.OBJ} file, will link on
6081 its own to a valid \c{.EXE} file, which when run will print `hello,
6082 world' and then exit.
6085 \S{binexe} Using the \c{bin} Format To Generate \c{.EXE} Files
6087 The \c{.EXE} file format is simple enough that it's possible to
6088 build a \c{.EXE} file by writing a pure-binary program and sticking
6089 a 32-byte header on the front. This header is simple enough that it
6090 can be generated using \c{DB} and \c{DW} commands by NASM itself, so
6091 that you can use the \c{bin} output format to directly generate
6094 Included in the NASM archives, in the \I{misc subdirectory}\c{misc}
6095 subdirectory, is a file \i\c{exebin.mac} of macros. It defines three
6096 macros: \i\c{EXE_begin}, \i\c{EXE_stack} and \i\c{EXE_end}.
6098 To produce a \c{.EXE} file using this method, you should start by
6099 using \c{%include} to load the \c{exebin.mac} macro package into
6100 your source file. You should then issue the \c{EXE_begin} macro call
6101 (which takes no arguments) to generate the file header data. Then
6102 write code as normal for the \c{bin} format - you can use all three
6103 standard sections \c{.text}, \c{.data} and \c{.bss}. At the end of
6104 the file you should call the \c{EXE_end} macro (again, no arguments),
6105 which defines some symbols to mark section sizes, and these symbols
6106 are referred to in the header code generated by \c{EXE_begin}.
6108 In this model, the code you end up writing starts at \c{0x100}, just
6109 like a \c{.COM} file - in fact, if you strip off the 32-byte header
6110 from the resulting \c{.EXE} file, you will have a valid \c{.COM}
6111 program. All the segment bases are the same, so you are limited to a
6112 64K program, again just like a \c{.COM} file. Note that an \c{ORG}
6113 directive is issued by the \c{EXE_begin} macro, so you should not
6114 explicitly issue one of your own.
6116 You can't directly refer to your segment base value, unfortunately,
6117 since this would require a relocation in the header, and things
6118 would get a lot more complicated. So you should get your segment
6119 base by copying it out of \c{CS} instead.
6121 On entry to your \c{.EXE} file, \c{SS:SP} are already set up to
6122 point to the top of a 2Kb stack. You can adjust the default stack
6123 size of 2Kb by calling the \c{EXE_stack} macro. For example, to
6124 change the stack size of your program to 64 bytes, you would call
6127 A sample program which generates a \c{.EXE} file in this way is
6128 given in the \c{test} subdirectory of the NASM archive, as
6132 \H{comfiles} Producing \i\c{.COM} Files
6134 While large DOS programs must be written as \c{.EXE} files, small
6135 ones are often better written as \c{.COM} files. \c{.COM} files are
6136 pure binary, and therefore most easily produced using the \c{bin}
6140 \S{combinfmt} Using the \c{bin} Format To Generate \c{.COM} Files
6142 \c{.COM} files expect to be loaded at offset \c{100h} into their
6143 segment (though the segment may change). Execution then begins at
6144 \I\c{ORG}\c{100h}, i.e. right at the start of the program. So to
6145 write a \c{.COM} program, you would create a source file looking
6153 \c ; put your code here
6157 \c ; put data items here
6161 \c ; put uninitialized data here
6163 The \c{bin} format puts the \c{.text} section first in the file, so
6164 you can declare data or BSS items before beginning to write code if
6165 you want to and the code will still end up at the front of the file
6168 The BSS (uninitialized data) section does not take up space in the
6169 \c{.COM} file itself: instead, addresses of BSS items are resolved
6170 to point at space beyond the end of the file, on the grounds that
6171 this will be free memory when the program is run. Therefore you
6172 should not rely on your BSS being initialized to all zeros when you
6175 To assemble the above program, you should use a command line like
6177 \c nasm myprog.asm -fbin -o myprog.com
6179 The \c{bin} format would produce a file called \c{myprog} if no
6180 explicit output file name were specified, so you have to override it
6181 and give the desired file name.
6184 \S{comobjfmt} Using the \c{obj} Format To Generate \c{.COM} Files
6186 If you are writing a \c{.COM} program as more than one module, you
6187 may wish to assemble several \c{.OBJ} files and link them together
6188 into a \c{.COM} program. You can do this, provided you have a linker
6189 capable of outputting \c{.COM} files directly (\i{TLINK} does this),
6190 or alternatively a converter program such as \i\c{EXE2BIN} to
6191 transform the \c{.EXE} file output from the linker into a \c{.COM}
6194 If you do this, you need to take care of several things:
6196 \b The first object file containing code should start its code
6197 segment with a line like \c{RESB 100h}. This is to ensure that the
6198 code begins at offset \c{100h} relative to the beginning of the code
6199 segment, so that the linker or converter program does not have to
6200 adjust address references within the file when generating the
6201 \c{.COM} file. Other assemblers use an \i\c{ORG} directive for this
6202 purpose, but \c{ORG} in NASM is a format-specific directive to the
6203 \c{bin} output format, and does not mean the same thing as it does
6204 in MASM-compatible assemblers.
6206 \b You don't need to define a stack segment.
6208 \b All your segments should be in the same group, so that every time
6209 your code or data references a symbol offset, all offsets are
6210 relative to the same segment base. This is because, when a \c{.COM}
6211 file is loaded, all the segment registers contain the same value.
6214 \H{sysfiles} Producing \i\c{.SYS} Files
6216 \i{MS-DOS device drivers} - \c{.SYS} files - are pure binary files,
6217 similar to \c{.COM} files, except that they start at origin zero
6218 rather than \c{100h}. Therefore, if you are writing a device driver
6219 using the \c{bin} format, you do not need the \c{ORG} directive,
6220 since the default origin for \c{bin} is zero. Similarly, if you are
6221 using \c{obj}, you do not need the \c{RESB 100h} at the start of
6224 \c{.SYS} files start with a header structure, containing pointers to
6225 the various routines inside the driver which do the work. This
6226 structure should be defined at the start of the code segment, even
6227 though it is not actually code.
6229 For more information on the format of \c{.SYS} files, and the data
6230 which has to go in the header structure, a list of books is given in
6231 the Frequently Asked Questions list for the newsgroup
6232 \W{news:comp.os.msdos.programmer}\i\c{comp.os.msdos.programmer}.
6235 \H{16c} Interfacing to 16-bit C Programs
6237 This section covers the basics of writing assembly routines that
6238 call, or are called from, C programs. To do this, you would
6239 typically write an assembly module as a \c{.OBJ} file, and link it
6240 with your C modules to produce a \i{mixed-language program}.
6243 \S{16cunder} External Symbol Names
6245 \I{C symbol names}\I{underscore, in C symbols}C compilers have the
6246 convention that the names of all global symbols (functions or data)
6247 they define are formed by prefixing an underscore to the name as it
6248 appears in the C program. So, for example, the function a C
6249 programmer thinks of as \c{printf} appears to an assembly language
6250 programmer as \c{_printf}. This means that in your assembly
6251 programs, you can define symbols without a leading underscore, and
6252 not have to worry about name clashes with C symbols.
6254 If you find the underscores inconvenient, you can define macros to
6255 replace the \c{GLOBAL} and \c{EXTERN} directives as follows:
6271 (These forms of the macros only take one argument at a time; a
6272 \c{%rep} construct could solve this.)
6274 If you then declare an external like this:
6278 then the macro will expand it as
6281 \c %define printf _printf
6283 Thereafter, you can reference \c{printf} as if it was a symbol, and
6284 the preprocessor will put the leading underscore on where necessary.
6286 The \c{cglobal} macro works similarly. You must use \c{cglobal}
6287 before defining the symbol in question, but you would have had to do
6288 that anyway if you used \c{GLOBAL}.
6290 Also see \k{opt-pfix}.
6292 \S{16cmodels} \i{Memory Models}
6294 NASM contains no mechanism to support the various C memory models
6295 directly; you have to keep track yourself of which one you are
6296 writing for. This means you have to keep track of the following
6299 \b In models using a single code segment (tiny, small and compact),
6300 functions are near. This means that function pointers, when stored
6301 in data segments or pushed on the stack as function arguments, are
6302 16 bits long and contain only an offset field (the \c{CS} register
6303 never changes its value, and always gives the segment part of the
6304 full function address), and that functions are called using ordinary
6305 near \c{CALL} instructions and return using \c{RETN} (which, in
6306 NASM, is synonymous with \c{RET} anyway). This means both that you
6307 should write your own routines to return with \c{RETN}, and that you
6308 should call external C routines with near \c{CALL} instructions.
6310 \b In models using more than one code segment (medium, large and
6311 huge), functions are far. This means that function pointers are 32
6312 bits long (consisting of a 16-bit offset followed by a 16-bit
6313 segment), and that functions are called using \c{CALL FAR} (or
6314 \c{CALL seg:offset}) and return using \c{RETF}. Again, you should
6315 therefore write your own routines to return with \c{RETF} and use
6316 \c{CALL FAR} to call external routines.
6318 \b In models using a single data segment (tiny, small and medium),
6319 data pointers are 16 bits long, containing only an offset field (the
6320 \c{DS} register doesn't change its value, and always gives the
6321 segment part of the full data item address).
6323 \b In models using more than one data segment (compact, large and
6324 huge), data pointers are 32 bits long, consisting of a 16-bit offset
6325 followed by a 16-bit segment. You should still be careful not to
6326 modify \c{DS} in your routines without restoring it afterwards, but
6327 \c{ES} is free for you to use to access the contents of 32-bit data
6328 pointers you are passed.
6330 \b The huge memory model allows single data items to exceed 64K in
6331 size. In all other memory models, you can access the whole of a data
6332 item just by doing arithmetic on the offset field of the pointer you
6333 are given, whether a segment field is present or not; in huge model,
6334 you have to be more careful of your pointer arithmetic.
6336 \b In most memory models, there is a \e{default} data segment, whose
6337 segment address is kept in \c{DS} throughout the program. This data
6338 segment is typically the same segment as the stack, kept in \c{SS},
6339 so that functions' local variables (which are stored on the stack)
6340 and global data items can both be accessed easily without changing
6341 \c{DS}. Particularly large data items are typically stored in other
6342 segments. However, some memory models (though not the standard
6343 ones, usually) allow the assumption that \c{SS} and \c{DS} hold the
6344 same value to be removed. Be careful about functions' local
6345 variables in this latter case.
6347 In models with a single code segment, the segment is called
6348 \i\c{_TEXT}, so your code segment must also go by this name in order
6349 to be linked into the same place as the main code segment. In models
6350 with a single data segment, or with a default data segment, it is
6354 \S{16cfunc} Function Definitions and Function Calls
6356 \I{functions, C calling convention}The \i{C calling convention} in
6357 16-bit programs is as follows. In the following description, the
6358 words \e{caller} and \e{callee} are used to denote the function
6359 doing the calling and the function which gets called.
6361 \b The caller pushes the function's parameters on the stack, one
6362 after another, in reverse order (right to left, so that the first
6363 argument specified to the function is pushed last).
6365 \b The caller then executes a \c{CALL} instruction to pass control
6366 to the callee. This \c{CALL} is either near or far depending on the
6369 \b The callee receives control, and typically (although this is not
6370 actually necessary, in functions which do not need to access their
6371 parameters) starts by saving the value of \c{SP} in \c{BP} so as to
6372 be able to use \c{BP} as a base pointer to find its parameters on
6373 the stack. However, the caller was probably doing this too, so part
6374 of the calling convention states that \c{BP} must be preserved by
6375 any C function. Hence the callee, if it is going to set up \c{BP} as
6376 a \i\e{frame pointer}, must push the previous value first.
6378 \b The callee may then access its parameters relative to \c{BP}.
6379 The word at \c{[BP]} holds the previous value of \c{BP} as it was
6380 pushed; the next word, at \c{[BP+2]}, holds the offset part of the
6381 return address, pushed implicitly by \c{CALL}. In a small-model
6382 (near) function, the parameters start after that, at \c{[BP+4]}; in
6383 a large-model (far) function, the segment part of the return address
6384 lives at \c{[BP+4]}, and the parameters begin at \c{[BP+6]}. The
6385 leftmost parameter of the function, since it was pushed last, is
6386 accessible at this offset from \c{BP}; the others follow, at
6387 successively greater offsets. Thus, in a function such as \c{printf}
6388 which takes a variable number of parameters, the pushing of the
6389 parameters in reverse order means that the function knows where to
6390 find its first parameter, which tells it the number and type of the
6393 \b The callee may also wish to decrease \c{SP} further, so as to
6394 allocate space on the stack for local variables, which will then be
6395 accessible at negative offsets from \c{BP}.
6397 \b The callee, if it wishes to return a value to the caller, should
6398 leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size
6399 of the value. Floating-point results are sometimes (depending on the
6400 compiler) returned in \c{ST0}.
6402 \b Once the callee has finished processing, it restores \c{SP} from
6403 \c{BP} if it had allocated local stack space, then pops the previous
6404 value of \c{BP}, and returns via \c{RETN} or \c{RETF} depending on
6407 \b When the caller regains control from the callee, the function
6408 parameters are still on the stack, so it typically adds an immediate
6409 constant to \c{SP} to remove them (instead of executing a number of
6410 slow \c{POP} instructions). Thus, if a function is accidentally
6411 called with the wrong number of parameters due to a prototype
6412 mismatch, the stack will still be returned to a sensible state since
6413 the caller, which \e{knows} how many parameters it pushed, does the
6416 It is instructive to compare this calling convention with that for
6417 Pascal programs (described in \k{16bpfunc}). Pascal has a simpler
6418 convention, since no functions have variable numbers of parameters.
6419 Therefore the callee knows how many parameters it should have been
6420 passed, and is able to deallocate them from the stack itself by
6421 passing an immediate argument to the \c{RET} or \c{RETF}
6422 instruction, so the caller does not have to do it. Also, the
6423 parameters are pushed in left-to-right order, not right-to-left,
6424 which means that a compiler can give better guarantees about
6425 sequence points without performance suffering.
6427 Thus, you would define a function in C style in the following way.
6428 The following example is for small model:
6435 \c sub sp,0x40 ; 64 bytes of local stack space
6436 \c mov bx,[bp+4] ; first parameter to function
6440 \c mov sp,bp ; undo "sub sp,0x40" above
6444 For a large-model function, you would replace \c{RET} by \c{RETF},
6445 and look for the first parameter at \c{[BP+6]} instead of
6446 \c{[BP+4]}. Of course, if one of the parameters is a pointer, then
6447 the offsets of \e{subsequent} parameters will change depending on
6448 the memory model as well: far pointers take up four bytes on the
6449 stack when passed as a parameter, whereas near pointers take up two.
6451 At the other end of the process, to call a C function from your
6452 assembly code, you would do something like this:
6456 \c ; and then, further down...
6458 \c push word [myint] ; one of my integer variables
6459 \c push word mystring ; pointer into my data segment
6461 \c add sp,byte 4 ; `byte' saves space
6463 \c ; then those data items...
6468 \c mystring db 'This number -> %d <- should be 1234',10,0
6470 This piece of code is the small-model assembly equivalent of the C
6473 \c int myint = 1234;
6474 \c printf("This number -> %d <- should be 1234\n", myint);
6476 In large model, the function-call code might look more like this. In
6477 this example, it is assumed that \c{DS} already holds the segment
6478 base of the segment \c{_DATA}. If not, you would have to initialize
6481 \c push word [myint]
6482 \c push word seg mystring ; Now push the segment, and...
6483 \c push word mystring ; ... offset of "mystring"
6487 The integer value still takes up one word on the stack, since large
6488 model does not affect the size of the \c{int} data type. The first
6489 argument (pushed last) to \c{printf}, however, is a data pointer,
6490 and therefore has to contain a segment and offset part. The segment
6491 should be stored second in memory, and therefore must be pushed
6492 first. (Of course, \c{PUSH DS} would have been a shorter instruction
6493 than \c{PUSH WORD SEG mystring}, if \c{DS} was set up as the above
6494 example assumed.) Then the actual call becomes a far call, since
6495 functions expect far calls in large model; and \c{SP} has to be
6496 increased by 6 rather than 4 afterwards to make up for the extra
6500 \S{16cdata} Accessing Data Items
6502 To get at the contents of C variables, or to declare variables which
6503 C can access, you need only declare the names as \c{GLOBAL} or
6504 \c{EXTERN}. (Again, the names require leading underscores, as stated
6505 in \k{16cunder}.) Thus, a C variable declared as \c{int i} can be
6506 accessed from assembler as
6512 And to declare your own integer variable which C programs can access
6513 as \c{extern int j}, you do this (making sure you are assembling in
6514 the \c{_DATA} segment, if necessary):
6520 To access a C array, you need to know the size of the components of
6521 the array. For example, \c{int} variables are two bytes long, so if
6522 a C program declares an array as \c{int a[10]}, you can access
6523 \c{a[3]} by coding \c{mov ax,[_a+6]}. (The byte offset 6 is obtained
6524 by multiplying the desired array index, 3, by the size of the array
6525 element, 2.) The sizes of the C base types in 16-bit compilers are:
6526 1 for \c{char}, 2 for \c{short} and \c{int}, 4 for \c{long} and
6527 \c{float}, and 8 for \c{double}.
6529 To access a C \i{data structure}, you need to know the offset from
6530 the base of the structure to the field you are interested in. You
6531 can either do this by converting the C structure definition into a
6532 NASM structure definition (using \i\c{STRUC}), or by calculating the
6533 one offset and using just that.
6535 To do either of these, you should read your C compiler's manual to
6536 find out how it organizes data structures. NASM gives no special
6537 alignment to structure members in its own \c{STRUC} macro, so you
6538 have to specify alignment yourself if the C compiler generates it.
6539 Typically, you might find that a structure like
6546 might be four bytes long rather than three, since the \c{int} field
6547 would be aligned to a two-byte boundary. However, this sort of
6548 feature tends to be a configurable option in the C compiler, either
6549 using command-line options or \c{#pragma} lines, so you have to find
6550 out how your own compiler does it.
6553 \S{16cmacro} \i\c{c16.mac}: Helper Macros for the 16-bit C Interface
6555 Included in the NASM archives, in the \I{misc subdirectory}\c{misc}
6556 directory, is a file \c{c16.mac} of macros. It defines three macros:
6557 \i\c{proc}, \i\c{arg} and \i\c{endproc}. These are intended to be
6558 used for C-style procedure definitions, and they automate a lot of
6559 the work involved in keeping track of the calling convention.
6561 (An alternative, TASM compatible form of \c{arg} is also now built
6562 into NASM's preprocessor. See \k{stackrel} for details.)
6564 An example of an assembly function using the macro set is given
6571 \c mov ax,[bp + %$i]
6572 \c mov bx,[bp + %$j]
6577 This defines \c{_nearproc} to be a procedure taking two arguments,
6578 the first (\c{i}) an integer and the second (\c{j}) a pointer to an
6579 integer. It returns \c{i + *j}.
6581 Note that the \c{arg} macro has an \c{EQU} as the first line of its
6582 expansion, and since the label before the macro call gets prepended
6583 to the first line of the expanded macro, the \c{EQU} works, defining
6584 \c{%$i} to be an offset from \c{BP}. A context-local variable is
6585 used, local to the context pushed by the \c{proc} macro and popped
6586 by the \c{endproc} macro, so that the same argument name can be used
6587 in later procedures. Of course, you don't \e{have} to do that.
6589 The macro set produces code for near functions (tiny, small and
6590 compact-model code) by default. You can have it generate far
6591 functions (medium, large and huge-model code) by means of coding
6592 \I\c{FARCODE}\c{%define FARCODE}. This changes the kind of return
6593 instruction generated by \c{endproc}, and also changes the starting
6594 point for the argument offsets. The macro set contains no intrinsic
6595 dependency on whether data pointers are far or not.
6597 \c{arg} can take an optional parameter, giving the size of the
6598 argument. If no size is given, 2 is assumed, since it is likely that
6599 many function parameters will be of type \c{int}.
6601 The large-model equivalent of the above function would look like this:
6609 \c mov ax,[bp + %$i]
6610 \c mov bx,[bp + %$j]
6611 \c mov es,[bp + %$j + 2]
6616 This makes use of the argument to the \c{arg} macro to define a
6617 parameter of size 4, because \c{j} is now a far pointer. When we
6618 load from \c{j}, we must load a segment and an offset.
6621 \H{16bp} Interfacing to \i{Borland Pascal} Programs
6623 Interfacing to Borland Pascal programs is similar in concept to
6624 interfacing to 16-bit C programs. The differences are:
6626 \b The leading underscore required for interfacing to C programs is
6627 not required for Pascal.
6629 \b The memory model is always large: functions are far, data
6630 pointers are far, and no data item can be more than 64K long.
6631 (Actually, some functions are near, but only those functions that
6632 are local to a Pascal unit and never called from outside it. All
6633 assembly functions that Pascal calls, and all Pascal functions that
6634 assembly routines are able to call, are far.) However, all static
6635 data declared in a Pascal program goes into the default data
6636 segment, which is the one whose segment address will be in \c{DS}
6637 when control is passed to your assembly code. The only things that
6638 do not live in the default data segment are local variables (they
6639 live in the stack segment) and dynamically allocated variables. All
6640 data \e{pointers}, however, are far.
6642 \b The function calling convention is different - described below.
6644 \b Some data types, such as strings, are stored differently.
6646 \b There are restrictions on the segment names you are allowed to
6647 use - Borland Pascal will ignore code or data declared in a segment
6648 it doesn't like the name of. The restrictions are described below.
6651 \S{16bpfunc} The Pascal Calling Convention
6653 \I{functions, Pascal calling convention}\I{Pascal calling
6654 convention}The 16-bit Pascal calling convention is as follows. In
6655 the following description, the words \e{caller} and \e{callee} are
6656 used to denote the function doing the calling and the function which
6659 \b The caller pushes the function's parameters on the stack, one
6660 after another, in normal order (left to right, so that the first
6661 argument specified to the function is pushed first).
6663 \b The caller then executes a far \c{CALL} instruction to pass
6664 control to the callee.
6666 \b The callee receives control, and typically (although this is not
6667 actually necessary, in functions which do not need to access their
6668 parameters) starts by saving the value of \c{SP} in \c{BP} so as to
6669 be able to use \c{BP} as a base pointer to find its parameters on
6670 the stack. However, the caller was probably doing this too, so part
6671 of the calling convention states that \c{BP} must be preserved by
6672 any function. Hence the callee, if it is going to set up \c{BP} as a
6673 \i{frame pointer}, must push the previous value first.
6675 \b The callee may then access its parameters relative to \c{BP}.
6676 The word at \c{[BP]} holds the previous value of \c{BP} as it was
6677 pushed. The next word, at \c{[BP+2]}, holds the offset part of the
6678 return address, and the next one at \c{[BP+4]} the segment part. The
6679 parameters begin at \c{[BP+6]}. The rightmost parameter of the
6680 function, since it was pushed last, is accessible at this offset
6681 from \c{BP}; the others follow, at successively greater offsets.
6683 \b The callee may also wish to decrease \c{SP} further, so as to
6684 allocate space on the stack for local variables, which will then be
6685 accessible at negative offsets from \c{BP}.
6687 \b The callee, if it wishes to return a value to the caller, should
6688 leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size
6689 of the value. Floating-point results are returned in \c{ST0}.
6690 Results of type \c{Real} (Borland's own custom floating-point data
6691 type, not handled directly by the FPU) are returned in \c{DX:BX:AX}.
6692 To return a result of type \c{String}, the caller pushes a pointer
6693 to a temporary string before pushing the parameters, and the callee
6694 places the returned string value at that location. The pointer is
6695 not a parameter, and should not be removed from the stack by the
6696 \c{RETF} instruction.
6698 \b Once the callee has finished processing, it restores \c{SP} from
6699 \c{BP} if it had allocated local stack space, then pops the previous
6700 value of \c{BP}, and returns via \c{RETF}. It uses the form of
6701 \c{RETF} with an immediate parameter, giving the number of bytes
6702 taken up by the parameters on the stack. This causes the parameters
6703 to be removed from the stack as a side effect of the return
6706 \b When the caller regains control from the callee, the function
6707 parameters have already been removed from the stack, so it needs to
6710 Thus, you would define a function in Pascal style, taking two
6711 \c{Integer}-type parameters, in the following way:
6717 \c sub sp,0x40 ; 64 bytes of local stack space
6718 \c mov bx,[bp+8] ; first parameter to function
6719 \c mov bx,[bp+6] ; second parameter to function
6723 \c mov sp,bp ; undo "sub sp,0x40" above
6725 \c retf 4 ; total size of params is 4
6727 At the other end of the process, to call a Pascal function from your
6728 assembly code, you would do something like this:
6732 \c ; and then, further down...
6734 \c push word seg mystring ; Now push the segment, and...
6735 \c push word mystring ; ... offset of "mystring"
6736 \c push word [myint] ; one of my variables
6737 \c call far SomeFunc
6739 This is equivalent to the Pascal code
6741 \c procedure SomeFunc(String: PChar; Int: Integer);
6742 \c SomeFunc(@mystring, myint);
6745 \S{16bpseg} Borland Pascal \I{segment names, Borland Pascal}Segment
6748 Since Borland Pascal's internal unit file format is completely
6749 different from \c{OBJ}, it only makes a very sketchy job of actually
6750 reading and understanding the various information contained in a
6751 real \c{OBJ} file when it links that in. Therefore an object file
6752 intended to be linked to a Pascal program must obey a number of
6755 \b Procedures and functions must be in a segment whose name is
6756 either \c{CODE}, \c{CSEG}, or something ending in \c{_TEXT}.
6758 \b initialized data must be in a segment whose name is either
6759 \c{CONST} or something ending in \c{_DATA}.
6761 \b Uninitialized data must be in a segment whose name is either
6762 \c{DATA}, \c{DSEG}, or something ending in \c{_BSS}.
6764 \b Any other segments in the object file are completely ignored.
6765 \c{GROUP} directives and segment attributes are also ignored.
6768 \S{16bpmacro} Using \i\c{c16.mac} With Pascal Programs
6770 The \c{c16.mac} macro package, described in \k{16cmacro}, can also
6771 be used to simplify writing functions to be called from Pascal
6772 programs, if you code \I\c{PASCAL}\c{%define PASCAL}. This
6773 definition ensures that functions are far (it implies
6774 \i\c{FARCODE}), and also causes procedure return instructions to be
6775 generated with an operand.
6777 Defining \c{PASCAL} does not change the code which calculates the
6778 argument offsets; you must declare your function's arguments in
6779 reverse order. For example:
6787 \c mov ax,[bp + %$i]
6788 \c mov bx,[bp + %$j]
6789 \c mov es,[bp + %$j + 2]
6794 This defines the same routine, conceptually, as the example in
6795 \k{16cmacro}: it defines a function taking two arguments, an integer
6796 and a pointer to an integer, which returns the sum of the integer
6797 and the contents of the pointer. The only difference between this
6798 code and the large-model C version is that \c{PASCAL} is defined
6799 instead of \c{FARCODE}, and that the arguments are declared in
6803 \C{32bit} Writing 32-bit Code (Unix, Win32, DJGPP)
6805 This chapter attempts to cover some of the common issues involved
6806 when writing 32-bit code, to run under \i{Win32} or Unix, or to be
6807 linked with C code generated by a Unix-style C compiler such as
6808 \i{DJGPP}. It covers how to write assembly code to interface with
6809 32-bit C routines, and how to write position-independent code for
6812 Almost all 32-bit code, and in particular all code running under
6813 \c{Win32}, \c{DJGPP} or any of the PC Unix variants, runs in \I{flat
6814 memory model}\e{flat} memory model. This means that the segment registers
6815 and paging have already been set up to give you the same 32-bit 4Gb
6816 address space no matter what segment you work relative to, and that
6817 you should ignore all segment registers completely. When writing
6818 flat-model application code, you never need to use a segment
6819 override or modify any segment register, and the code-section
6820 addresses you pass to \c{CALL} and \c{JMP} live in the same address
6821 space as the data-section addresses you access your variables by and
6822 the stack-section addresses you access local variables and procedure
6823 parameters by. Every address is 32 bits long and contains only an
6827 \H{32c} Interfacing to 32-bit C Programs
6829 A lot of the discussion in \k{16c}, about interfacing to 16-bit C
6830 programs, still applies when working in 32 bits. The absence of
6831 memory models or segmentation worries simplifies things a lot.
6834 \S{32cunder} External Symbol Names
6836 Most 32-bit C compilers share the convention used by 16-bit
6837 compilers, that the names of all global symbols (functions or data)
6838 they define are formed by prefixing an underscore to the name as it
6839 appears in the C program. However, not all of them do: the \c{ELF}
6840 specification states that C symbols do \e{not} have a leading
6841 underscore on their assembly-language names.
6843 The older Linux \c{a.out} C compiler, all \c{Win32} compilers,
6844 \c{DJGPP}, and \c{NetBSD} and \c{FreeBSD}, all use the leading
6845 underscore; for these compilers, the macros \c{cextern} and
6846 \c{cglobal}, as given in \k{16cunder}, will still work. For \c{ELF},
6847 though, the leading underscore should not be used.
6849 See also \k{opt-pfix}.
6851 \S{32cfunc} Function Definitions and Function Calls
6853 \I{functions, C calling convention}The \i{C calling convention}
6854 in 32-bit programs is as follows. In the following description,
6855 the words \e{caller} and \e{callee} are used to denote
6856 the function doing the calling and the function which gets called.
6858 \b The caller pushes the function's parameters on the stack, one
6859 after another, in reverse order (right to left, so that the first
6860 argument specified to the function is pushed last).
6862 \b The caller then executes a near \c{CALL} instruction to pass
6863 control to the callee.
6865 \b The callee receives control, and typically (although this is not
6866 actually necessary, in functions which do not need to access their
6867 parameters) starts by saving the value of \c{ESP} in \c{EBP} so as
6868 to be able to use \c{EBP} as a base pointer to find its parameters
6869 on the stack. However, the caller was probably doing this too, so
6870 part of the calling convention states that \c{EBP} must be preserved
6871 by any C function. Hence the callee, if it is going to set up
6872 \c{EBP} as a \i{frame pointer}, must push the previous value first.
6874 \b The callee may then access its parameters relative to \c{EBP}.
6875 The doubleword at \c{[EBP]} holds the previous value of \c{EBP} as
6876 it was pushed; the next doubleword, at \c{[EBP+4]}, holds the return
6877 address, pushed implicitly by \c{CALL}. The parameters start after
6878 that, at \c{[EBP+8]}. The leftmost parameter of the function, since
6879 it was pushed last, is accessible at this offset from \c{EBP}; the
6880 others follow, at successively greater offsets. Thus, in a function
6881 such as \c{printf} which takes a variable number of parameters, the
6882 pushing of the parameters in reverse order means that the function
6883 knows where to find its first parameter, which tells it the number
6884 and type of the remaining ones.
6886 \b The callee may also wish to decrease \c{ESP} further, so as to
6887 allocate space on the stack for local variables, which will then be
6888 accessible at negative offsets from \c{EBP}.
6890 \b The callee, if it wishes to return a value to the caller, should
6891 leave the value in \c{AL}, \c{AX} or \c{EAX} depending on the size
6892 of the value. Floating-point results are typically returned in
6895 \b Once the callee has finished processing, it restores \c{ESP} from
6896 \c{EBP} if it had allocated local stack space, then pops the previous
6897 value of \c{EBP}, and returns via \c{RET} (equivalently, \c{RETN}).
6899 \b When the caller regains control from the callee, the function
6900 parameters are still on the stack, so it typically adds an immediate
6901 constant to \c{ESP} to remove them (instead of executing a number of
6902 slow \c{POP} instructions). Thus, if a function is accidentally
6903 called with the wrong number of parameters due to a prototype
6904 mismatch, the stack will still be returned to a sensible state since
6905 the caller, which \e{knows} how many parameters it pushed, does the
6908 There is an alternative calling convention used by Win32 programs
6909 for Windows API calls, and also for functions called \e{by} the
6910 Windows API such as window procedures: they follow what Microsoft
6911 calls the \c{__stdcall} convention. This is slightly closer to the
6912 Pascal convention, in that the callee clears the stack by passing a
6913 parameter to the \c{RET} instruction. However, the parameters are
6914 still pushed in right-to-left order.
6916 Thus, you would define a function in C style in the following way:
6923 \c sub esp,0x40 ; 64 bytes of local stack space
6924 \c mov ebx,[ebp+8] ; first parameter to function
6928 \c leave ; mov esp,ebp / pop ebp
6931 At the other end of the process, to call a C function from your
6932 assembly code, you would do something like this:
6936 \c ; and then, further down...
6938 \c push dword [myint] ; one of my integer variables
6939 \c push dword mystring ; pointer into my data segment
6941 \c add esp,byte 8 ; `byte' saves space
6943 \c ; then those data items...
6948 \c mystring db 'This number -> %d <- should be 1234',10,0
6950 This piece of code is the assembly equivalent of the C code
6952 \c int myint = 1234;
6953 \c printf("This number -> %d <- should be 1234\n", myint);
6956 \S{32cdata} Accessing Data Items
6958 To get at the contents of C variables, or to declare variables which
6959 C can access, you need only declare the names as \c{GLOBAL} or
6960 \c{EXTERN}. (Again, the names require leading underscores, as stated
6961 in \k{32cunder}.) Thus, a C variable declared as \c{int i} can be
6962 accessed from assembler as
6967 And to declare your own integer variable which C programs can access
6968 as \c{extern int j}, you do this (making sure you are assembling in
6969 the \c{_DATA} segment, if necessary):
6974 To access a C array, you need to know the size of the components of
6975 the array. For example, \c{int} variables are four bytes long, so if
6976 a C program declares an array as \c{int a[10]}, you can access
6977 \c{a[3]} by coding \c{mov ax,[_a+12]}. (The byte offset 12 is obtained
6978 by multiplying the desired array index, 3, by the size of the array
6979 element, 4.) The sizes of the C base types in 32-bit compilers are:
6980 1 for \c{char}, 2 for \c{short}, 4 for \c{int}, \c{long} and
6981 \c{float}, and 8 for \c{double}. Pointers, being 32-bit addresses,
6982 are also 4 bytes long.
6984 To access a C \i{data structure}, you need to know the offset from
6985 the base of the structure to the field you are interested in. You
6986 can either do this by converting the C structure definition into a
6987 NASM structure definition (using \c{STRUC}), or by calculating the
6988 one offset and using just that.
6990 To do either of these, you should read your C compiler's manual to
6991 find out how it organizes data structures. NASM gives no special
6992 alignment to structure members in its own \i\c{STRUC} macro, so you
6993 have to specify alignment yourself if the C compiler generates it.
6994 Typically, you might find that a structure like
7001 might be eight bytes long rather than five, since the \c{int} field
7002 would be aligned to a four-byte boundary. However, this sort of
7003 feature is sometimes a configurable option in the C compiler, either
7004 using command-line options or \c{#pragma} lines, so you have to find
7005 out how your own compiler does it.
7008 \S{32cmacro} \i\c{c32.mac}: Helper Macros for the 32-bit C Interface
7010 Included in the NASM archives, in the \I{misc directory}\c{misc}
7011 directory, is a file \c{c32.mac} of macros. It defines three macros:
7012 \i\c{proc}, \i\c{arg} and \i\c{endproc}. These are intended to be
7013 used for C-style procedure definitions, and they automate a lot of
7014 the work involved in keeping track of the calling convention.
7016 An example of an assembly function using the macro set is given
7023 \c mov eax,[ebp + %$i]
7024 \c mov ebx,[ebp + %$j]
7029 This defines \c{_proc32} to be a procedure taking two arguments, the
7030 first (\c{i}) an integer and the second (\c{j}) a pointer to an
7031 integer. It returns \c{i + *j}.
7033 Note that the \c{arg} macro has an \c{EQU} as the first line of its
7034 expansion, and since the label before the macro call gets prepended
7035 to the first line of the expanded macro, the \c{EQU} works, defining
7036 \c{%$i} to be an offset from \c{BP}. A context-local variable is
7037 used, local to the context pushed by the \c{proc} macro and popped
7038 by the \c{endproc} macro, so that the same argument name can be used
7039 in later procedures. Of course, you don't \e{have} to do that.
7041 \c{arg} can take an optional parameter, giving the size of the
7042 argument. If no size is given, 4 is assumed, since it is likely that
7043 many function parameters will be of type \c{int} or pointers.
7046 \H{picdll} Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF \i{Shared
7049 \c{ELF} replaced the older \c{a.out} object file format under Linux
7050 because it contains support for \i{position-independent code}
7051 (\i{PIC}), which makes writing shared libraries much easier. NASM
7052 supports the \c{ELF} position-independent code features, so you can
7053 write Linux \c{ELF} shared libraries in NASM.
7055 \i{NetBSD}, and its close cousins \i{FreeBSD} and \i{OpenBSD}, take
7056 a different approach by hacking PIC support into the \c{a.out}
7057 format. NASM supports this as the \i\c{aoutb} output format, so you
7058 can write \i{BSD} shared libraries in NASM too.
7060 The operating system loads a PIC shared library by memory-mapping
7061 the library file at an arbitrarily chosen point in the address space
7062 of the running process. The contents of the library's code section
7063 must therefore not depend on where it is loaded in memory.
7065 Therefore, you cannot get at your variables by writing code like
7068 \c mov eax,[myvar] ; WRONG
7070 Instead, the linker provides an area of memory called the
7071 \i\e{global offset table}, or \i{GOT}; the GOT is situated at a
7072 constant distance from your library's code, so if you can find out
7073 where your library is loaded (which is typically done using a
7074 \c{CALL} and \c{POP} combination), you can obtain the address of the
7075 GOT, and you can then load the addresses of your variables out of
7076 linker-generated entries in the GOT.
7078 The \e{data} section of a PIC shared library does not have these
7079 restrictions: since the data section is writable, it has to be
7080 copied into memory anyway rather than just paged in from the library
7081 file, so as long as it's being copied it can be relocated too. So
7082 you can put ordinary types of relocation in the data section without
7083 too much worry (but see \k{picglobal} for a caveat).
7086 \S{picgot} Obtaining the Address of the GOT
7088 Each code module in your shared library should define the GOT as an
7091 \c extern _GLOBAL_OFFSET_TABLE_ ; in ELF
7092 \c extern __GLOBAL_OFFSET_TABLE_ ; in BSD a.out
7094 At the beginning of any function in your shared library which plans
7095 to access your data or BSS sections, you must first calculate the
7096 address of the GOT. This is typically done by writing the function
7105 \c add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
7107 \c ; the function body comes here
7114 (For BSD, again, the symbol \c{_GLOBAL_OFFSET_TABLE} requires a
7115 second leading underscore.)
7117 The first two lines of this function are simply the standard C
7118 prologue to set up a stack frame, and the last three lines are
7119 standard C function epilogue. The third line, and the fourth to last
7120 line, save and restore the \c{EBX} register, because PIC shared
7121 libraries use this register to store the address of the GOT.
7123 The interesting bit is the \c{CALL} instruction and the following
7124 two lines. The \c{CALL} and \c{POP} combination obtains the address
7125 of the label \c{.get_GOT}, without having to know in advance where
7126 the program was loaded (since the \c{CALL} instruction is encoded
7127 relative to the current position). The \c{ADD} instruction makes use
7128 of one of the special PIC relocation types: \i{GOTPC relocation}.
7129 With the \i\c{WRT ..gotpc} qualifier specified, the symbol
7130 referenced (here \c{_GLOBAL_OFFSET_TABLE_}, the special symbol
7131 assigned to the GOT) is given as an offset from the beginning of the
7132 section. (Actually, \c{ELF} encodes it as the offset from the operand
7133 field of the \c{ADD} instruction, but NASM simplifies this
7134 deliberately, so you do things the same way for both \c{ELF} and
7135 \c{BSD}.) So the instruction then \e{adds} the beginning of the section,
7136 to get the real address of the GOT, and subtracts the value of
7137 \c{.get_GOT} which it knows is in \c{EBX}. Therefore, by the time
7138 that instruction has finished, \c{EBX} contains the address of the GOT.
7140 If you didn't follow that, don't worry: it's never necessary to
7141 obtain the address of the GOT by any other means, so you can put
7142 those three instructions into a macro and safely ignore them:
7149 \c add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
7153 \S{piclocal} Finding Your Local Data Items
7155 Having got the GOT, you can then use it to obtain the addresses of
7156 your data items. Most variables will reside in the sections you have
7157 declared; they can be accessed using the \I{GOTOFF
7158 relocation}\c{..gotoff} special \I\c{WRT ..gotoff}\c{WRT} type. The
7159 way this works is like this:
7161 \c lea eax,[ebx+myvar wrt ..gotoff]
7163 The expression \c{myvar wrt ..gotoff} is calculated, when the shared
7164 library is linked, to be the offset to the local variable \c{myvar}
7165 from the beginning of the GOT. Therefore, adding it to \c{EBX} as
7166 above will place the real address of \c{myvar} in \c{EAX}.
7168 If you declare variables as \c{GLOBAL} without specifying a size for
7169 them, they are shared between code modules in the library, but do
7170 not get exported from the library to the program that loaded it.
7171 They will still be in your ordinary data and BSS sections, so you
7172 can access them in the same way as local variables, using the above
7173 \c{..gotoff} mechanism.
7175 Note that due to a peculiarity of the way BSD \c{a.out} format
7176 handles this relocation type, there must be at least one non-local
7177 symbol in the same section as the address you're trying to access.
7180 \S{picextern} Finding External and Common Data Items
7182 If your library needs to get at an external variable (external to
7183 the \e{library}, not just to one of the modules within it), you must
7184 use the \I{GOT relocations}\I\c{WRT ..got}\c{..got} type to get at
7185 it. The \c{..got} type, instead of giving you the offset from the
7186 GOT base to the variable, gives you the offset from the GOT base to
7187 a GOT \e{entry} containing the address of the variable. The linker
7188 will set up this GOT entry when it builds the library, and the
7189 dynamic linker will place the correct address in it at load time. So
7190 to obtain the address of an external variable \c{extvar} in \c{EAX},
7193 \c mov eax,[ebx+extvar wrt ..got]
7195 This loads the address of \c{extvar} out of an entry in the GOT. The
7196 linker, when it builds the shared library, collects together every
7197 relocation of type \c{..got}, and builds the GOT so as to ensure it
7198 has every necessary entry present.
7200 Common variables must also be accessed in this way.
7203 \S{picglobal} Exporting Symbols to the Library User
7205 If you want to export symbols to the user of the library, you have
7206 to declare whether they are functions or data, and if they are data,
7207 you have to give the size of the data item. This is because the
7208 dynamic linker has to build \I{PLT}\i{procedure linkage table}
7209 entries for any exported functions, and also moves exported data
7210 items away from the library's data section in which they were
7213 So to export a function to users of the library, you must use
7215 \c global func:function ; declare it as a function
7221 And to export a data item such as an array, you would have to code
7223 \c global array:data array.end-array ; give the size too
7228 Be careful: If you export a variable to the library user, by
7229 declaring it as \c{GLOBAL} and supplying a size, the variable will
7230 end up living in the data section of the main program, rather than
7231 in your library's data section, where you declared it. So you will
7232 have to access your own global variable with the \c{..got} mechanism
7233 rather than \c{..gotoff}, as if it were external (which,
7234 effectively, it has become).
7236 Equally, if you need to store the address of an exported global in
7237 one of your data sections, you can't do it by means of the standard
7240 \c dataptr: dd global_data_item ; WRONG
7242 NASM will interpret this code as an ordinary relocation, in which
7243 \c{global_data_item} is merely an offset from the beginning of the
7244 \c{.data} section (or whatever); so this reference will end up
7245 pointing at your data section instead of at the exported global
7246 which resides elsewhere.
7248 Instead of the above code, then, you must write
7250 \c dataptr: dd global_data_item wrt ..sym
7252 which makes use of the special \c{WRT} type \I\c{WRT ..sym}\c{..sym}
7253 to instruct NASM to search the symbol table for a particular symbol
7254 at that address, rather than just relocating by section base.
7256 Either method will work for functions: referring to one of your
7257 functions by means of
7259 \c funcptr: dd my_function
7261 will give the user the address of the code you wrote, whereas
7263 \c funcptr: dd my_function wrt ..sym
7265 will give the address of the procedure linkage table for the
7266 function, which is where the calling program will \e{believe} the
7267 function lives. Either address is a valid way to call the function.
7270 \S{picproc} Calling Procedures Outside the Library
7272 Calling procedures outside your shared library has to be done by
7273 means of a \i\e{procedure linkage table}, or \i{PLT}. The PLT is
7274 placed at a known offset from where the library is loaded, so the
7275 library code can make calls to the PLT in a position-independent
7276 way. Within the PLT there is code to jump to offsets contained in
7277 the GOT, so function calls to other shared libraries or to routines
7278 in the main program can be transparently passed off to their real
7281 To call an external routine, you must use another special PIC
7282 relocation type, \I{PLT relocations}\i\c{WRT ..plt}. This is much
7283 easier than the GOT-based ones: you simply replace calls such as
7284 \c{CALL printf} with the PLT-relative version \c{CALL printf WRT
7288 \S{link} Generating the Library File
7290 Having written some code modules and assembled them to \c{.o} files,
7291 you then generate your shared library with a command such as
7293 \c ld -shared -o library.so module1.o module2.o # for ELF
7294 \c ld -Bshareable -o library.so module1.o module2.o # for BSD
7296 For ELF, if your shared library is going to reside in system
7297 directories such as \c{/usr/lib} or \c{/lib}, it is usually worth
7298 using the \i\c{-soname} flag to the linker, to store the final
7299 library file name, with a version number, into the library:
7301 \c ld -shared -soname library.so.1 -o library.so.1.2 *.o
7303 You would then copy \c{library.so.1.2} into the library directory,
7304 and create \c{library.so.1} as a symbolic link to it.
7307 \C{mixsize} Mixing 16 and 32 Bit Code
7309 This chapter tries to cover some of the issues, largely related to
7310 unusual forms of addressing and jump instructions, encountered when
7311 writing operating system code such as protected-mode initialisation
7312 routines, which require code that operates in mixed segment sizes,
7313 such as code in a 16-bit segment trying to modify data in a 32-bit
7314 one, or jumps between different-size segments.
7317 \H{mixjump} Mixed-Size Jumps\I{jumps, mixed-size}
7319 \I{operating system, writing}\I{writing operating systems}The most
7320 common form of \i{mixed-size instruction} is the one used when
7321 writing a 32-bit OS: having done your setup in 16-bit mode, such as
7322 loading the kernel, you then have to boot it by switching into
7323 protected mode and jumping to the 32-bit kernel start address. In a
7324 fully 32-bit OS, this tends to be the \e{only} mixed-size
7325 instruction you need, since everything before it can be done in pure
7326 16-bit code, and everything after it can be pure 32-bit.
7328 This jump must specify a 48-bit far address, since the target
7329 segment is a 32-bit one. However, it must be assembled in a 16-bit
7330 segment, so just coding, for example,
7332 \c jmp 0x1234:0x56789ABC ; wrong!
7334 will not work, since the offset part of the address will be
7335 truncated to \c{0x9ABC} and the jump will be an ordinary 16-bit far
7338 The Linux kernel setup code gets round the inability of \c{as86} to
7339 generate the required instruction by coding it manually, using
7340 \c{DB} instructions. NASM can go one better than that, by actually
7341 generating the right instruction itself. Here's how to do it right:
7343 \c jmp dword 0x1234:0x56789ABC ; right
7345 \I\c{JMP DWORD}The \c{DWORD} prefix (strictly speaking, it should
7346 come \e{after} the colon, since it is declaring the \e{offset} field
7347 to be a doubleword; but NASM will accept either form, since both are
7348 unambiguous) forces the offset part to be treated as far, in the
7349 assumption that you are deliberately writing a jump from a 16-bit
7350 segment to a 32-bit one.
7352 You can do the reverse operation, jumping from a 32-bit segment to a
7353 16-bit one, by means of the \c{WORD} prefix:
7355 \c jmp word 0x8765:0x4321 ; 32 to 16 bit
7357 If the \c{WORD} prefix is specified in 16-bit mode, or the \c{DWORD}
7358 prefix in 32-bit mode, they will be ignored, since each is
7359 explicitly forcing NASM into a mode it was in anyway.
7362 \H{mixaddr} Addressing Between Different-Size Segments\I{addressing,
7363 mixed-size}\I{mixed-size addressing}
7365 If your OS is mixed 16 and 32-bit, or if you are writing a DOS
7366 extender, you are likely to have to deal with some 16-bit segments
7367 and some 32-bit ones. At some point, you will probably end up
7368 writing code in a 16-bit segment which has to access data in a
7369 32-bit segment, or vice versa.
7371 If the data you are trying to access in a 32-bit segment lies within
7372 the first 64K of the segment, you may be able to get away with using
7373 an ordinary 16-bit addressing operation for the purpose; but sooner
7374 or later, you will want to do 32-bit addressing from 16-bit mode.
7376 The easiest way to do this is to make sure you use a register for
7377 the address, since any effective address containing a 32-bit
7378 register is forced to be a 32-bit address. So you can do
7380 \c mov eax,offset_into_32_bit_segment_specified_by_fs
7381 \c mov dword [fs:eax],0x11223344
7383 This is fine, but slightly cumbersome (since it wastes an
7384 instruction and a register) if you already know the precise offset
7385 you are aiming at. The x86 architecture does allow 32-bit effective
7386 addresses to specify nothing but a 4-byte offset, so why shouldn't
7387 NASM be able to generate the best instruction for the purpose?
7389 It can. As in \k{mixjump}, you need only prefix the address with the
7390 \c{DWORD} keyword, and it will be forced to be a 32-bit address:
7392 \c mov dword [fs:dword my_offset],0x11223344
7394 Also as in \k{mixjump}, NASM is not fussy about whether the
7395 \c{DWORD} prefix comes before or after the segment override, so
7396 arguably a nicer-looking way to code the above instruction is
7398 \c mov dword [dword fs:my_offset],0x11223344
7400 Don't confuse the \c{DWORD} prefix \e{outside} the square brackets,
7401 which controls the size of the data stored at the address, with the
7402 one \c{inside} the square brackets which controls the length of the
7403 address itself. The two can quite easily be different:
7405 \c mov word [dword 0x12345678],0x9ABC
7407 This moves 16 bits of data to an address specified by a 32-bit
7410 You can also specify \c{WORD} or \c{DWORD} prefixes along with the
7411 \c{FAR} prefix to indirect far jumps or calls. For example:
7413 \c call dword far [fs:word 0x4321]
7415 This instruction contains an address specified by a 16-bit offset;
7416 it loads a 48-bit far pointer from that (16-bit segment and 32-bit
7417 offset), and calls that address.
7420 \H{mixother} Other Mixed-Size Instructions
7422 The other way you might want to access data might be using the
7423 string instructions (\c{LODSx}, \c{STOSx} and so on) or the
7424 \c{XLATB} instruction. These instructions, since they take no
7425 parameters, might seem to have no easy way to make them perform
7426 32-bit addressing when assembled in a 16-bit segment.
7428 This is the purpose of NASM's \i\c{a16}, \i\c{a32} and \i\c{a64} prefixes. If
7429 you are coding \c{LODSB} in a 16-bit segment but it is supposed to
7430 be accessing a string in a 32-bit segment, you should load the
7431 desired address into \c{ESI} and then code
7435 The prefix forces the addressing size to 32 bits, meaning that
7436 \c{LODSB} loads from \c{[DS:ESI]} instead of \c{[DS:SI]}. To access
7437 a string in a 16-bit segment when coding in a 32-bit one, the
7438 corresponding \c{a16} prefix can be used.
7440 The \c{a16}, \c{a32} and \c{a64} prefixes can be applied to any instruction
7441 in NASM's instruction table, but most of them can generate all the
7442 useful forms without them. The prefixes are necessary only for
7443 instructions with implicit addressing:
7444 \# \c{CMPSx} (\k{insCMPSB}),
7445 \# \c{SCASx} (\k{insSCASB}), \c{LODSx} (\k{insLODSB}), \c{STOSx}
7446 \# (\k{insSTOSB}), \c{MOVSx} (\k{insMOVSB}), \c{INSx} (\k{insINSB}),
7447 \# \c{OUTSx} (\k{insOUTSB}), and \c{XLATB} (\k{insXLATB}).
7448 \c{CMPSx}, \c{SCASx}, \c{LODSx}, \c{STOSx}, \c{MOVSx}, \c{INSx},
7449 \c{OUTSx}, and \c{XLATB}.
7451 various push and pop instructions (\c{PUSHA} and \c{POPF} as well as
7452 the more usual \c{PUSH} and \c{POP}) can accept \c{a16}, \c{a32} or \c{a64}
7453 prefixes to force a particular one of \c{SP}, \c{ESP} or \c{RSP} to be used
7454 as a stack pointer, in case the stack segment in use is a different
7455 size from the code segment.
7457 \c{PUSH} and \c{POP}, when applied to segment registers in 32-bit
7458 mode, also have the slightly odd behaviour that they push and pop 4
7459 bytes at a time, of which the top two are ignored and the bottom two
7460 give the value of the segment register being manipulated. To force
7461 the 16-bit behaviour of segment-register push and pop instructions,
7462 you can use the operand-size prefix \i\c{o16}:
7467 This code saves a doubleword of stack space by fitting two segment
7468 registers into the space which would normally be consumed by pushing
7471 (You can also use the \i\c{o32} prefix to force the 32-bit behaviour
7472 when in 16-bit mode, but this seems less useful.)
7475 \C{64bit} Writing 64-bit Code (Unix, Win64)
7477 This chapter attempts to cover some of the common issues involved when
7478 writing 64-bit code, to run under \i{Win64} or Unix. It covers how to
7479 write assembly code to interface with 64-bit C routines, and how to
7480 write position-independent code for shared libraries.
7482 All 64-bit code uses a flat memory model, since segmentation is not
7483 available in 64-bit mode. The one exception is the \c{FS} and \c{GS}
7484 registers, which still add their bases.
7486 Position independence in 64-bit mode is significantly simpler, since
7487 the processor supports \c{RIP}-relative addressing directly; see the
7488 \c{REL} keyword (\k{effaddr}). On most 64-bit platforms, it is
7489 probably desirable to make that the default, using the directive
7490 \c{DEFAULT REL} (\k{default}).
7492 64-bit programming is relatively similar to 32-bit programming, but
7493 of course pointers are 64 bits long; additionally, all existing
7494 platforms pass arguments in registers rather than on the stack.
7495 Furthermore, 64-bit platforms use SSE2 by default for floating point.
7496 Please see the ABI documentation for your platform.
7498 64-bit platforms differ in the sizes of the fundamental datatypes, not
7499 just from 32-bit platforms but from each other. If a specific size
7500 data type is desired, it is probably best to use the types defined in
7501 the Standard C header \c{<inttypes.h>}.
7503 In 64-bit mode, the default instruction size is still 32 bits. When
7504 loading a value into a 32-bit register (but not an 8- or 16-bit
7505 register), the upper 32 bits of the corresponding 64-bit register are
7508 \H{reg64} Register Names in 64-bit Mode
7510 NASM uses the following names for general-purpose registers in 64-bit
7511 mode, for 8-, 16-, 32- and 64-bit references, respecitively:
7513 \c AL/AH, CL/CH, DL/DH, BL/BH, SPL, BPL, SIL, DIL, R8B-R15B
7514 \c AX, CX, DX, BX, SP, BP, SI, DI, R8W-R15W
7515 \c EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, R8D-R15D
7516 \c RAX, RCX, RDX, RBX, RSP, RBP, RSI, RDI, R8-R15
7518 This is consistent with the AMD documentation and most other
7519 assemblers. The Intel documentation, however, uses the names
7520 \c{R8L-R15L} for 8-bit references to the higher registers. It is
7521 possible to use those names by definiting them as macros; similarly,
7522 if one wants to use numeric names for the low 8 registers, define them
7523 as macros. The standard macro package \c{altreg} (see \k{pkg_altreg})
7524 can be used for this purpose.
7526 \H{id64} Immediates and Displacements in 64-bit Mode
7528 In 64-bit mode, immediates and displacements are generally only 32
7529 bits wide. NASM will therefore truncate most displacements and
7530 immediates to 32 bits.
7532 The only instruction which takes a full \i{64-bit immediate} is:
7536 NASM will produce this instruction whenever the programmer uses
7537 \c{MOV} with an immediate into a 64-bit register. If this is not
7538 desirable, simply specify the equivalent 32-bit register, which will
7539 be automatically zero-extended by the processor, or specify the
7540 immediate as \c{DWORD}:
7542 \c mov rax,foo ; 64-bit immediate
7543 \c mov rax,qword foo ; (identical)
7544 \c mov eax,foo ; 32-bit immediate, zero-extended
7545 \c mov rax,dword foo ; 32-bit immediate, sign-extended
7547 The length of these instructions are 10, 5 and 7 bytes, respectively.
7549 The only instructions which take a full \I{64-bit displacement}64-bit
7550 \e{displacement} is loading or storing, using \c{MOV}, \c{AL}, \c{AX},
7551 \c{EAX} or \c{RAX} (but no other registers) to an absolute 64-bit address.
7552 Since this is a relatively rarely used instruction (64-bit code generally uses
7553 relative addressing), the programmer has to explicitly declare the
7554 displacement size as \c{QWORD}:
7558 \c mov eax,[foo] ; 32-bit absolute disp, sign-extended
7559 \c mov eax,[a32 foo] ; 32-bit absolute disp, zero-extended
7560 \c mov eax,[qword foo] ; 64-bit absolute disp
7564 \c mov eax,[foo] ; 32-bit relative disp
7565 \c mov eax,[a32 foo] ; d:o, address truncated to 32 bits(!)
7566 \c mov eax,[qword foo] ; error
7567 \c mov eax,[abs qword foo] ; 64-bit absolute disp
7569 A sign-extended absolute displacement can access from -2 GB to +2 GB;
7570 a zero-extended absolute displacement can access from 0 to 4 GB.
7572 \H{unix64} Interfacing to 64-bit C Programs (Unix)
7574 On Unix, the 64-bit ABI is defined by the document:
7576 \W{http://www.nasm.us/links/unix64abi}\c{http://www.nasm.us/links/unix64abi}
7578 Although written for AT&T-syntax assembly, the concepts apply equally
7579 well for NASM-style assembly. What follows is a simplified summary.
7581 The first six integer arguments (from the left) are passed in \c{RDI},
7582 \c{RSI}, \c{RDX}, \c{RCX}, \c{R8}, and \c{R9}, in that order.
7583 Additional integer arguments are passed on the stack. These
7584 registers, plus \c{RAX}, \c{R10} and \c{R11} are destroyed by function
7585 calls, and thus are available for use by the function without saving.
7587 Integer return values are passed in \c{RAX} and \c{RDX}, in that order.
7589 Floating point is done using SSE registers, except for \c{long
7590 double}. Floating-point arguments are passed in \c{XMM0} to \c{XMM7};
7591 return is \c{XMM0} and \c{XMM1}. \c{long double} are passed on the
7592 stack, and returned in \c{ST0} and \c{ST1}.
7594 All SSE and x87 registers are destroyed by function calls.
7596 On 64-bit Unix, \c{long} is 64 bits.
7598 Integer and SSE register arguments are counted separately, so for the case of
7600 \c void foo(long a, double b, int c)
7602 \c{a} is passed in \c{RDI}, \c{b} in \c{XMM0}, and \c{c} in \c{ESI}.
7604 \H{win64} Interfacing to 64-bit C Programs (Win64)
7606 The Win64 ABI is described at:
7608 \W{http://www.nasm.us/links/win64abi}\c{http://www.nasm.us/links/win64abi}
7610 What follows is a simplified summary.
7612 The first four integer arguments are passed in \c{RCX}, \c{RDX},
7613 \c{R8} and \c{R9}, in that order. Additional integer arguments are
7614 passed on the stack. These registers, plus \c{RAX}, \c{R10} and
7615 \c{R11} are destroyed by function calls, and thus are available for
7616 use by the function without saving.
7618 Integer return values are passed in \c{RAX} only.
7620 Floating point is done using SSE registers, except for \c{long
7621 double}. Floating-point arguments are passed in \c{XMM0} to \c{XMM3};
7622 return is \c{XMM0} only.
7624 On Win64, \c{long} is 32 bits; \c{long long} or \c{_int64} is 64 bits.
7626 Integer and SSE register arguments are counted together, so for the case of
7628 \c void foo(long long a, double b, int c)
7630 \c{a} is passed in \c{RCX}, \c{b} in \c{XMM1}, and \c{c} in \c{R8D}.
7632 \C{trouble} Troubleshooting
7634 This chapter describes some of the common problems that users have
7635 been known to encounter with NASM, and answers them. It also gives
7636 instructions for reporting bugs in NASM if you find a difficulty
7637 that isn't listed here.
7640 \H{problems} Common Problems
7642 \S{inefficient} NASM Generates \i{Inefficient Code}
7644 We sometimes get `bug' reports about NASM generating inefficient, or
7645 even `wrong', code on instructions such as \c{ADD ESP,8}. This is a
7646 deliberate design feature, connected to predictability of output:
7647 NASM, on seeing \c{ADD ESP,8}, will generate the form of the
7648 instruction which leaves room for a 32-bit offset. You need to code
7649 \I\c{BYTE}\c{ADD ESP,BYTE 8} if you want the space-efficient form of
7650 the instruction. This isn't a bug, it's user error: if you prefer to
7651 have NASM produce the more efficient code automatically enable
7652 optimization with the \c{-O} option (see \k{opt-O}).
7655 \S{jmprange} My Jumps are Out of Range\I{out of range, jumps}
7657 Similarly, people complain that when they issue \i{conditional
7658 jumps} (which are \c{SHORT} by default) that try to jump too far,
7659 NASM reports `short jump out of range' instead of making the jumps
7662 This, again, is partly a predictability issue, but in fact has a
7663 more practical reason as well. NASM has no means of being told what
7664 type of processor the code it is generating will be run on; so it
7665 cannot decide for itself that it should generate \i\c{Jcc NEAR} type
7666 instructions, because it doesn't know that it's working for a 386 or
7667 above. Alternatively, it could replace the out-of-range short
7668 \c{JNE} instruction with a very short \c{JE} instruction that jumps
7669 over a \c{JMP NEAR}; this is a sensible solution for processors
7670 below a 386, but hardly efficient on processors which have good
7671 branch prediction \e{and} could have used \c{JNE NEAR} instead. So,
7672 once again, it's up to the user, not the assembler, to decide what
7673 instructions should be generated. See \k{opt-O}.
7676 \S{proborg} \i\c{ORG} Doesn't Work
7678 People writing \i{boot sector} programs in the \c{bin} format often
7679 complain that \c{ORG} doesn't work the way they'd like: in order to
7680 place the \c{0xAA55} signature word at the end of a 512-byte boot
7681 sector, people who are used to MASM tend to code
7685 \c ; some boot sector code
7690 This is not the intended use of the \c{ORG} directive in NASM, and
7691 will not work. The correct way to solve this problem in NASM is to
7692 use the \i\c{TIMES} directive, like this:
7696 \c ; some boot sector code
7698 \c TIMES 510-($-$$) DB 0
7701 The \c{TIMES} directive will insert exactly enough zero bytes into
7702 the output to move the assembly point up to 510. This method also
7703 has the advantage that if you accidentally fill your boot sector too
7704 full, NASM will catch the problem at assembly time and report it, so
7705 you won't end up with a boot sector that you have to disassemble to
7706 find out what's wrong with it.
7709 \S{probtimes} \i\c{TIMES} Doesn't Work
7711 The other common problem with the above code is people who write the
7716 by reasoning that \c{$} should be a pure number, just like 510, so
7717 the difference between them is also a pure number and can happily be
7720 NASM is a \e{modular} assembler: the various component parts are
7721 designed to be easily separable for re-use, so they don't exchange
7722 information unnecessarily. In consequence, the \c{bin} output
7723 format, even though it has been told by the \c{ORG} directive that
7724 the \c{.text} section should start at 0, does not pass that
7725 information back to the expression evaluator. So from the
7726 evaluator's point of view, \c{$} isn't a pure number: it's an offset
7727 from a section base. Therefore the difference between \c{$} and 510
7728 is also not a pure number, but involves a section base. Values
7729 involving section bases cannot be passed as arguments to \c{TIMES}.
7731 The solution, as in the previous section, is to code the \c{TIMES}
7734 \c TIMES 510-($-$$) DB 0
7736 in which \c{$} and \c{$$} are offsets from the same section base,
7737 and so their difference is a pure number. This will solve the
7738 problem and generate sensible code.
7741 \H{bugs} \i{Bugs}\I{reporting bugs}
7743 We have never yet released a version of NASM with any \e{known}
7744 bugs. That doesn't usually stop there being plenty we didn't know
7745 about, though. Any that you find should be reported firstly via the
7747 \W{http://www.nasm.us/}\c{http://www.nasm.us/}
7748 (click on "Bug Tracker"), or if that fails then through one of the
7749 contacts in \k{contact}.
7751 Please read \k{qstart} first, and don't report the bug if it's
7752 listed in there as a deliberate feature. (If you think the feature
7753 is badly thought out, feel free to send us reasons why you think it
7754 should be changed, but don't just send us mail saying `This is a
7755 bug' if the documentation says we did it on purpose.) Then read
7756 \k{problems}, and don't bother reporting the bug if it's listed
7759 If you do report a bug, \e{please} give us all of the following
7762 \b What operating system you're running NASM under. DOS, Linux,
7763 NetBSD, Win16, Win32, VMS (I'd be impressed), whatever.
7765 \b If you're running NASM under DOS or Win32, tell us whether you've
7766 compiled your own executable from the DOS source archive, or whether
7767 you were using the standard distribution binaries out of the
7768 archive. If you were using a locally built executable, try to
7769 reproduce the problem using one of the standard binaries, as this
7770 will make it easier for us to reproduce your problem prior to fixing
7773 \b Which version of NASM you're using, and exactly how you invoked
7774 it. Give us the precise command line, and the contents of the
7775 \c{NASMENV} environment variable if any.
7777 \b Which versions of any supplementary programs you're using, and
7778 how you invoked them. If the problem only becomes visible at link
7779 time, tell us what linker you're using, what version of it you've
7780 got, and the exact linker command line. If the problem involves
7781 linking against object files generated by a compiler, tell us what
7782 compiler, what version, and what command line or options you used.
7783 (If you're compiling in an IDE, please try to reproduce the problem
7784 with the command-line version of the compiler.)
7786 \b If at all possible, send us a NASM source file which exhibits the
7787 problem. If this causes copyright problems (e.g. you can only
7788 reproduce the bug in restricted-distribution code) then bear in mind
7789 the following two points: firstly, we guarantee that any source code
7790 sent to us for the purposes of debugging NASM will be used \e{only}
7791 for the purposes of debugging NASM, and that we will delete all our
7792 copies of it as soon as we have found and fixed the bug or bugs in
7793 question; and secondly, we would prefer \e{not} to be mailed large
7794 chunks of code anyway. The smaller the file, the better. A
7795 three-line sample file that does nothing useful \e{except}
7796 demonstrate the problem is much easier to work with than a
7797 fully fledged ten-thousand-line program. (Of course, some errors
7798 \e{do} only crop up in large files, so this may not be possible.)
7800 \b A description of what the problem actually \e{is}. `It doesn't
7801 work' is \e{not} a helpful description! Please describe exactly what
7802 is happening that shouldn't be, or what isn't happening that should.
7803 Examples might be: `NASM generates an error message saying Line 3
7804 for an error that's actually on Line 5'; `NASM generates an error
7805 message that I believe it shouldn't be generating at all'; `NASM
7806 fails to generate an error message that I believe it \e{should} be
7807 generating'; `the object file produced from this source code crashes
7808 my linker'; `the ninth byte of the output file is 66 and I think it
7809 should be 77 instead'.
7811 \b If you believe the output file from NASM to be faulty, send it to
7812 us. That allows us to determine whether our own copy of NASM
7813 generates the same file, or whether the problem is related to
7814 portability issues between our development platforms and yours. We
7815 can handle binary files mailed to us as MIME attachments, uuencoded,
7816 and even BinHex. Alternatively, we may be able to provide an FTP
7817 site you can upload the suspect files to; but mailing them is easier
7820 \b Any other information or data files that might be helpful. If,
7821 for example, the problem involves NASM failing to generate an object
7822 file while TASM can generate an equivalent file without trouble,
7823 then send us \e{both} object files, so we can see what TASM is doing
7824 differently from us.
7827 \A{ndisasm} \i{Ndisasm}
7829 The Netwide Disassembler, NDISASM
7831 \H{ndisintro} Introduction
7834 The Netwide Disassembler is a small companion program to the Netwide
7835 Assembler, NASM. It seemed a shame to have an x86 assembler,
7836 complete with a full instruction table, and not make as much use of
7837 it as possible, so here's a disassembler which shares the
7838 instruction table (and some other bits of code) with NASM.
7840 The Netwide Disassembler does nothing except to produce
7841 disassemblies of \e{binary} source files. NDISASM does not have any
7842 understanding of object file formats, like \c{objdump}, and it will
7843 not understand \c{DOS .EXE} files like \c{debug} will. It just
7847 \H{ndisstart} Getting Started: Installation
7849 See \k{install} for installation instructions. NDISASM, like NASM,
7850 has a \c{man page} which you may want to put somewhere useful, if you
7851 are on a Unix system.
7854 \H{ndisrun} Running NDISASM
7856 To disassemble a file, you will typically use a command of the form
7858 \c ndisasm -b {16|32|64} filename
7860 NDISASM can disassemble 16-, 32- or 64-bit code equally easily,
7861 provided of course that you remember to specify which it is to work
7862 with. If no \i\c{-b} switch is present, NDISASM works in 16-bit mode
7863 by default. The \i\c{-u} switch (for USE32) also invokes 32-bit mode.
7865 Two more command line options are \i\c{-r} which reports the version
7866 number of NDISASM you are running, and \i\c{-h} which gives a short
7867 summary of command line options.
7870 \S{ndiscom} COM Files: Specifying an Origin
7872 To disassemble a \c{DOS .COM} file correctly, a disassembler must assume
7873 that the first instruction in the file is loaded at address \c{0x100},
7874 rather than at zero. NDISASM, which assumes by default that any file
7875 you give it is loaded at zero, will therefore need to be informed of
7878 The \i\c{-o} option allows you to declare a different origin for the
7879 file you are disassembling. Its argument may be expressed in any of
7880 the NASM numeric formats: decimal by default, if it begins with `\c{$}'
7881 or `\c{0x}' or ends in `\c{H}' it's \c{hex}, if it ends in `\c{Q}' it's
7882 \c{octal}, and if it ends in `\c{B}' it's \c{binary}.
7884 Hence, to disassemble a \c{.COM} file:
7886 \c ndisasm -o100h filename.com
7891 \S{ndissync} Code Following Data: Synchronisation
7893 Suppose you are disassembling a file which contains some data which
7894 isn't machine code, and \e{then} contains some machine code. NDISASM
7895 will faithfully plough through the data section, producing machine
7896 instructions wherever it can (although most of them will look
7897 bizarre, and some may have unusual prefixes, e.g. `\c{FS OR AX,0x240A}'),
7898 and generating `DB' instructions ever so often if it's totally stumped.
7899 Then it will reach the code section.
7901 Supposing NDISASM has just finished generating a strange machine
7902 instruction from part of the data section, and its file position is
7903 now one byte \e{before} the beginning of the code section. It's
7904 entirely possible that another spurious instruction will get
7905 generated, starting with the final byte of the data section, and
7906 then the correct first instruction in the code section will not be
7907 seen because the starting point skipped over it. This isn't really
7910 To avoid this, you can specify a `\i\c{synchronisation}' point, or indeed
7911 as many synchronisation points as you like (although NDISASM can
7912 only handle 2147483647 sync points internally). The definition of a sync
7913 point is this: NDISASM guarantees to hit sync points exactly during
7914 disassembly. If it is thinking about generating an instruction which
7915 would cause it to jump over a sync point, it will discard that
7916 instruction and output a `\c{db}' instead. So it \e{will} start
7917 disassembly exactly from the sync point, and so you \e{will} see all
7918 the instructions in your code section.
7920 Sync points are specified using the \i\c{-s} option: they are measured
7921 in terms of the program origin, not the file position. So if you
7922 want to synchronize after 32 bytes of a \c{.COM} file, you would have to
7925 \c ndisasm -o100h -s120h file.com
7929 \c ndisasm -o100h -s20h file.com
7931 As stated above, you can specify multiple sync markers if you need
7932 to, just by repeating the \c{-s} option.
7935 \S{ndisisync} Mixed Code and Data: Automatic (Intelligent) Synchronisation
7938 Suppose you are disassembling the boot sector of a \c{DOS} floppy (maybe
7939 it has a virus, and you need to understand the virus so that you
7940 know what kinds of damage it might have done you). Typically, this
7941 will contain a \c{JMP} instruction, then some data, then the rest of the
7942 code. So there is a very good chance of NDISASM being \e{misaligned}
7943 when the data ends and the code begins. Hence a sync point is
7946 On the other hand, why should you have to specify the sync point
7947 manually? What you'd do in order to find where the sync point would
7948 be, surely, would be to read the \c{JMP} instruction, and then to use
7949 its target address as a sync point. So can NDISASM do that for you?
7951 The answer, of course, is yes: using either of the synonymous
7952 switches \i\c{-a} (for automatic sync) or \i\c{-i} (for intelligent
7953 sync) will enable \c{auto-sync} mode. Auto-sync mode automatically
7954 generates a sync point for any forward-referring PC-relative jump or
7955 call instruction that NDISASM encounters. (Since NDISASM is one-pass,
7956 if it encounters a PC-relative jump whose target has already been
7957 processed, there isn't much it can do about it...)
7959 Only PC-relative jumps are processed, since an absolute jump is
7960 either through a register (in which case NDISASM doesn't know what
7961 the register contains) or involves a segment address (in which case
7962 the target code isn't in the same segment that NDISASM is working
7963 in, and so the sync point can't be placed anywhere useful).
7965 For some kinds of file, this mechanism will automatically put sync
7966 points in all the right places, and save you from having to place
7967 any sync points manually. However, it should be stressed that
7968 auto-sync mode is \e{not} guaranteed to catch all the sync points, and
7969 you may still have to place some manually.
7971 Auto-sync mode doesn't prevent you from declaring manual sync
7972 points: it just adds automatically generated ones to the ones you
7973 provide. It's perfectly feasible to specify \c{-i} \e{and} some \c{-s}
7976 Another caveat with auto-sync mode is that if, by some unpleasant
7977 fluke, something in your data section should disassemble to a
7978 PC-relative call or jump instruction, NDISASM may obediently place a
7979 sync point in a totally random place, for example in the middle of
7980 one of the instructions in your code section. So you may end up with
7981 a wrong disassembly even if you use auto-sync. Again, there isn't
7982 much I can do about this. If you have problems, you'll have to use
7983 manual sync points, or use the \c{-k} option (documented below) to
7984 suppress disassembly of the data area.
7987 \S{ndisother} Other Options
7989 The \i\c{-e} option skips a header on the file, by ignoring the first N
7990 bytes. This means that the header is \e{not} counted towards the
7991 disassembly offset: if you give \c{-e10 -o10}, disassembly will start
7992 at byte 10 in the file, and this will be given offset 10, not 20.
7994 The \i\c{-k} option is provided with two comma-separated numeric
7995 arguments, the first of which is an assembly offset and the second
7996 is a number of bytes to skip. This \e{will} count the skipped bytes
7997 towards the assembly offset: its use is to suppress disassembly of a
7998 data section which wouldn't contain anything you wanted to see
8002 \H{ndisbugs} Bugs and Improvements
8004 There are no known bugs. However, any you find, with patches if
8005 possible, should be sent to
8006 \W{mailto:nasm-bugs@lists.sourceforge.net}\c{nasm-bugs@lists.sourceforge.net}, or to the
8008 \W{http://www.nasm.us/}\c{http://www.nasm.us/}
8009 and we'll try to fix them. Feel free to send contributions and
8010 new features as well.
8012 \A{inslist} \i{Instruction List}
8014 \H{inslistintro} Introduction
8016 The following sections show the instructions which NASM currently supports. For each
8017 instruction, there is a separate entry for each supported addressing mode. The third
8018 column shows the processor type in which the instruction was introduced and,
8019 when appropriate, one or more usage flags.
8023 \A{changelog} \i{NASM Version History}