1 \# --------------------------------------------------------------------------
3 \# Copyright 1996-2013 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}
53 \IR{-MD} \c{-MD} option
54 \IR{-MF} \c{-MF} option
55 \IR{-MG} \c{-MG} option
56 \IR{-MP} \c{-MP} option
57 \IR{-MQ} \c{-MQ} option
58 \IR{-MT} \c{-MT} option
79 \IR{!=} \c{!=} operator
80 \IR{$, here} \c{$}, Here token
81 \IR{$, prefix} \c{$}, prefix
84 \IR{%%} \c{%%} operator
85 \IR{%+1} \c{%+1} and \c{%-1} syntax
87 \IR{%0} \c{%0} parameter count
89 \IR{&&} \c{&&} operator
91 \IR{..@} \c{..@} symbol prefix
93 \IR{//} \c{//} operator
95 \IR{<<} \c{<<} operator
96 \IR{<=} \c{<=} operator
97 \IR{<>} \c{<>} operator
99 \IR{==} \c{==} operator
100 \IR{>} \c{>} operator
101 \IR{>=} \c{>=} operator
102 \IR{>>} \c{>>} operator
103 \IR{?} \c{?} MASM syntax
104 \IR{^} \c{^} operator
105 \IR{^^} \c{^^} operator
106 \IR{|} \c{|} operator
107 \IR{||} \c{||} operator
108 \IR{~} \c{~} operator
109 \IR{%$} \c{%$} and \c{%$$} prefixes
111 \IR{+ opaddition} \c{+} operator, binary
112 \IR{+ opunary} \c{+} operator, unary
113 \IR{+ modifier} \c{+} modifier
114 \IR{- opsubtraction} \c{-} operator, binary
115 \IR{- opunary} \c{-} operator, unary
116 \IR{! opunary} \c{!} operator, unary
117 \IR{alignment, in bin sections} alignment, in \c{bin} sections
118 \IR{alignment, in elf sections} alignment, in \c{elf} sections
119 \IR{alignment, in win32 sections} alignment, in \c{win32} sections
120 \IR{alignment, of elf common variables} alignment, of \c{elf} common
122 \IR{alignment, in obj sections} alignment, in \c{obj} sections
123 \IR{a.out, bsd version} \c{a.out}, BSD version
124 \IR{a.out, linux version} \c{a.out}, Linux version
125 \IR{autoconf} Autoconf
127 \IR{bitwise and} bitwise AND
128 \IR{bitwise or} bitwise OR
129 \IR{bitwise xor} bitwise XOR
130 \IR{block ifs} block IFs
131 \IR{borland pascal} Borland, Pascal
132 \IR{borland's win32 compilers} Borland, Win32 compilers
133 \IR{braces, after % sign} braces, after \c{%} sign
135 \IR{c calling convention} C calling convention
136 \IR{c symbol names} C symbol names
137 \IA{critical expressions}{critical expression}
138 \IA{command line}{command-line}
139 \IA{case sensitivity}{case sensitive}
140 \IA{case-sensitive}{case sensitive}
141 \IA{case-insensitive}{case sensitive}
142 \IA{character constants}{character constant}
143 \IR{common object file format} Common Object File Format
144 \IR{common variables, alignment in elf} common variables, alignment
146 \IR{common, elf extensions to} \c{COMMON}, \c{elf} extensions to
147 \IR{common, obj extensions to} \c{COMMON}, \c{obj} extensions to
148 \IR{declaring structure} declaring structures
149 \IR{default-wrt mechanism} default-\c{WRT} mechanism
152 \IR{dll symbols, exporting} DLL symbols, exporting
153 \IR{dll symbols, importing} DLL symbols, importing
155 \IR{dos archive} DOS archive
156 \IR{dos source archive} DOS source archive
157 \IA{effective address}{effective addresses}
158 \IA{effective-address}{effective addresses}
160 \IR{elf, 16-bit code and} ELF, 16-bit code and
161 \IR{elf shared libraries} ELF, shared libraries
164 \IR{elfx32} \c{elfx32}
165 \IR{executable and linkable format} Executable and Linkable Format
166 \IR{extern, obj extensions to} \c{EXTERN}, \c{obj} extensions to
167 \IR{extern, rdf extensions to} \c{EXTERN}, \c{rdf} extensions to
168 \IR{floating-point, constants} floating-point, constants
169 \IR{floating-point, packed bcd constants} floating-point, packed BCD constants
171 \IR{freelink} FreeLink
172 \IR{functions, c calling convention} functions, C calling convention
173 \IR{functions, pascal calling convention} functions, Pascal calling
175 \IR{global, aoutb extensions to} \c{GLOBAL}, \c{aoutb} extensions to
176 \IR{global, elf extensions to} \c{GLOBAL}, \c{elf} extensions to
177 \IR{global, rdf extensions to} \c{GLOBAL}, \c{rdf} extensions to
179 \IR{got relocations} \c{GOT} relocations
180 \IR{gotoff relocation} \c{GOTOFF} relocations
181 \IR{gotpc relocation} \c{GOTPC} relocations
182 \IR{intel number formats} Intel number formats
183 \IR{linux, elf} Linux, ELF
184 \IR{linux, a.out} Linux, \c{a.out}
185 \IR{linux, as86} Linux, \c{as86}
186 \IR{logical and} logical AND
187 \IR{logical or} logical OR
188 \IR{logical xor} logical XOR
189 \IR{mach object file format} Mach, object file format
191 \IR{macho32} \c{macho32}
192 \IR{macho64} \c{macho64}
195 \IA{memory reference}{memory references}
197 \IA{misc directory}{misc subdirectory}
198 \IR{misc subdirectory} \c{misc} subdirectory
199 \IR{microsoft omf} Microsoft OMF
200 \IR{mmx registers} MMX registers
201 \IA{modr/m}{modr/m byte}
202 \IR{modr/m byte} ModR/M byte
204 \IR{ms-dos device drivers} MS-DOS device drivers
205 \IR{multipush} \c{multipush} macro
207 \IR{nasm version} NASM version
211 \IR{operating system} operating system
213 \IR{pascal calling convention}Pascal calling convention
214 \IR{passes} passes, assembly
219 \IR{plt} \c{PLT} relocations
220 \IA{pre-defining macros}{pre-define}
221 \IA{preprocessor expressions}{preprocessor, expressions}
222 \IA{preprocessor loops}{preprocessor, loops}
223 \IA{preprocessor variables}{preprocessor, variables}
224 \IA{rdoff subdirectory}{rdoff}
225 \IR{rdoff} \c{rdoff} subdirectory
226 \IR{relocatable dynamic object file format} Relocatable Dynamic
228 \IR{relocations, pic-specific} relocations, PIC-specific
229 \IA{repeating}{repeating code}
230 \IR{section alignment, in elf} section alignment, in \c{elf}
231 \IR{section alignment, in bin} section alignment, in \c{bin}
232 \IR{section alignment, in obj} section alignment, in \c{obj}
233 \IR{section alignment, in win32} section alignment, in \c{win32}
234 \IR{section, elf extensions to} \c{SECTION}, \c{elf} extensions to
235 \IR{section, win32 extensions to} \c{SECTION}, \c{win32} extensions to
236 \IR{segment alignment, in bin} segment alignment, in \c{bin}
237 \IR{segment alignment, in obj} segment alignment, in \c{obj}
238 \IR{segment, obj extensions to} \c{SEGMENT}, \c{elf} extensions to
239 \IR{segment names, borland pascal} segment names, Borland Pascal
240 \IR{shift command} \c{shift} command
242 \IR{sib byte} SIB byte
243 \IR{align, smart} \c{ALIGN}, smart
244 \IA{sectalign}{sectalign}
245 \IR{solaris x86} Solaris x86
246 \IA{standard section names}{standardized section names}
247 \IR{symbols, exporting from dlls} symbols, exporting from DLLs
248 \IR{symbols, importing from dlls} symbols, importing from DLLs
249 \IR{test subdirectory} \c{test} subdirectory
251 \IR{underscore, in c symbols} underscore, in C symbols
257 \IA{sco unix}{unix, sco}
258 \IR{unix, sco} Unix, SCO
259 \IA{unix source archive}{unix, source archive}
260 \IR{unix, source archive} Unix, source archive
261 \IA{unix system v}{unix, system v}
262 \IR{unix, system v} Unix, System V
263 \IR{unixware} UnixWare
265 \IR{version number of nasm} version number of NASM
266 \IR{visual c++} Visual C++
267 \IR{www page} WWW page
271 \IR{windows 95} Windows 95
272 \IR{windows nt} Windows NT
273 \# \IC{program entry point}{entry point, program}
274 \# \IC{program entry point}{start point, program}
275 \# \IC{MS-DOS device drivers}{device drivers, MS-DOS}
276 \# \IC{16-bit mode, versus 32-bit mode}{32-bit mode, versus 16-bit mode}
277 \# \IC{c symbol names}{symbol names, in C}
280 \C{intro} Introduction
282 \H{whatsnasm} What Is NASM?
284 The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed
285 for portability and modularity. It supports a range of object file
286 formats, including Linux and \c{*BSD} \c{a.out}, \c{ELF}, \c{COFF},
287 \c{Mach-O}, Microsoft 16-bit \c{OBJ}, \c{Win32} and \c{Win64}. It will
288 also output plain binary files. Its syntax is designed to be simple
289 and easy to understand, similar to Intel's but less complex. It
290 supports all currently known x86 architectural extensions, and has
291 strong support for macros.
294 \S{yaasm} Why Yet Another Assembler?
296 The Netwide Assembler grew out of an idea on \i\c{comp.lang.asm.x86}
297 (or possibly \i\c{alt.lang.asm} - I forget which), which was
298 essentially that there didn't seem to be a good \e{free} x86-series
299 assembler around, and that maybe someone ought to write one.
301 \b \i\c{a86} is good, but not free, and in particular you don't get any
302 32-bit capability until you pay. It's DOS only, too.
304 \b \i\c{gas} is free, and ports over to DOS and Unix, but it's not
305 very good, since it's designed to be a back end to \i\c{gcc}, which
306 always feeds it correct code. So its error checking is minimal. Also,
307 its syntax is horrible, from the point of view of anyone trying to
308 actually \e{write} anything in it. Plus you can't write 16-bit code in
311 \b \i\c{as86} is specific to Minix and Linux, and (my version at least)
312 doesn't seem to have much (or any) documentation.
314 \b \i\c{MASM} isn't very good, and it's (was) expensive, and it runs only under
317 \b \i\c{TASM} is better, but still strives for MASM compatibility,
318 which means millions of directives and tons of red tape. And its syntax
319 is essentially MASM's, with the contradictions and quirks that
320 entails (although it sorts out some of those by means of Ideal mode.)
321 It's expensive too. And it's DOS-only.
323 So here, for your coding pleasure, is NASM. At present it's
324 still in prototype stage - we don't promise that it can outperform
325 any of these assemblers. But please, \e{please} send us bug reports,
326 fixes, helpful information, and anything else you can get your hands
327 on (and thanks to the many people who've done this already! You all
328 know who you are), and we'll improve it out of all recognition.
332 \S{legal} \i{License} Conditions
334 Please see the file \c{LICENSE}, supplied as part of any NASM
335 distribution archive, for the license conditions under which you may
336 use NASM. NASM is now under the so-called 2-clause BSD license, also
337 known as the simplified BSD license.
339 Copyright 1996-2011 the NASM Authors - All rights reserved.
341 Redistribution and use in source and binary forms, with or without
342 modification, are permitted provided that the following conditions are
345 \b Redistributions of source code must retain the above copyright
346 notice, this list of conditions and the following disclaimer.
348 \b Redistributions in binary form must reproduce the above copyright
349 notice, this list of conditions and the following disclaimer in the
350 documentation and/or other materials provided with the distribution.
352 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
353 CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
354 INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
355 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
356 DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
357 CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
358 SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
359 NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
360 LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
361 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
362 CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
363 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
364 EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
367 \H{contact} Contact Information
369 The current version of NASM (since about 0.98.08) is maintained by a
370 team of developers, accessible through the \c{nasm-devel} mailing list
371 (see below for the link).
372 If you want to report a bug, please read \k{bugs} first.
374 NASM has a \i{website} at
375 \W{http://www.nasm.us/}\c{http://www.nasm.us/}. If it's not there,
378 \i{New releases}, \i{release candidates}, and \I{snapshots, daily
379 development}\i{daily development snapshots} of NASM are available from
380 the official web site.
382 Announcements are posted to
383 \W{news:comp.lang.asm.x86}\i\c{comp.lang.asm.x86},
385 \W{http://www.freshmeat.net/}\c{http://www.freshmeat.net/}.
387 If you want information about the current development status, please
388 subscribe to the \i\c{nasm-devel} email list; see link from the
392 \H{install} Installation
394 \S{instdos} \i{Installing} NASM under MS-\i{DOS} or Windows
396 Once you've obtained the appropriate archive for NASM,
397 \i\c{nasm-XXX-dos.zip} or \i\c{nasm-XXX-win32.zip} (where \c{XXX}
398 denotes the version number of NASM contained in the archive), unpack
399 it into its own directory (for example \c{c:\\nasm}).
401 The archive will contain a set of executable files: the NASM
402 executable file \i\c{nasm.exe}, the NDISASM executable file
403 \i\c{ndisasm.exe}, and possibly additional utilities to handle the
406 The only file NASM needs to run is its own executable, so copy
407 \c{nasm.exe} to a directory on your PATH, or alternatively edit
408 \i\c{autoexec.bat} to add the \c{nasm} directory to your
409 \i\c{PATH} (to do that under Windows XP, go to Start > Control Panel >
410 System > Advanced > Environment Variables; these instructions may work
411 under other versions of Windows as well.)
413 That's it - NASM is installed. You don't need the nasm directory
414 to be present to run NASM (unless you've added it to your \c{PATH}),
415 so you can delete it if you need to save space; however, you may
416 want to keep the documentation or test programs.
418 If you've downloaded the \i{DOS source archive}, \i\c{nasm-XXX.zip},
419 the \c{nasm} directory will also contain the full NASM \i{source
420 code}, and a selection of \i{Makefiles} you can (hopefully) use to
421 rebuild your copy of NASM from scratch. See the file \c{INSTALL} in
424 Note that a number of files are generated from other files by Perl
425 scripts. Although the NASM source distribution includes these
426 generated files, you will need to rebuild them (and hence, will need a
427 Perl interpreter) if you change insns.dat, standard.mac or the
428 documentation. It is possible future source distributions may not
429 include these files at all. Ports of \i{Perl} for a variety of
430 platforms, including DOS and Windows, are available from
431 \W{http://www.cpan.org/ports/}\i{www.cpan.org}.
434 \S{instdos} Installing NASM under \i{Unix}
436 Once you've obtained the \i{Unix source archive} for NASM,
437 \i\c{nasm-XXX.tar.gz} (where \c{XXX} denotes the version number of
438 NASM contained in the archive), unpack it into a directory such
439 as \c{/usr/local/src}. The archive, when unpacked, will create its
440 own subdirectory \c{nasm-XXX}.
442 NASM is an \I{Autoconf}\I\c{configure}auto-configuring package: once
443 you've unpacked it, \c{cd} to the directory it's been unpacked into
444 and type \c{./configure}. This shell script will find the best C
445 compiler to use for building NASM and set up \i{Makefiles}
448 Once NASM has auto-configured, you can type \i\c{make} to build the
449 \c{nasm} and \c{ndisasm} binaries, and then \c{make install} to
450 install them in \c{/usr/local/bin} and install the \i{man pages}
451 \i\c{nasm.1} and \i\c{ndisasm.1} in \c{/usr/local/man/man1}.
452 Alternatively, you can give options such as \c{--prefix} to the
453 configure script (see the file \i\c{INSTALL} for more details), or
454 install the programs yourself.
456 NASM also comes with a set of utilities for handling the \c{RDOFF}
457 custom object-file format, which are in the \i\c{rdoff} subdirectory
458 of the NASM archive. You can build these with \c{make rdf} and
459 install them with \c{make rdf_install}, if you want them.
462 \C{running} Running NASM
464 \H{syntax} NASM \i{Command-Line} Syntax
466 To assemble a file, you issue a command of the form
468 \c nasm -f <format> <filename> [-o <output>]
472 \c nasm -f elf myfile.asm
474 will assemble \c{myfile.asm} into an \c{ELF} object file \c{myfile.o}. And
476 \c nasm -f bin myfile.asm -o myfile.com
478 will assemble \c{myfile.asm} into a raw binary file \c{myfile.com}.
480 To produce a listing file, with the hex codes output from NASM
481 displayed on the left of the original sources, use the \c{-l} option
482 to give a listing file name, for example:
484 \c nasm -f coff myfile.asm -l myfile.lst
486 To get further usage instructions from NASM, try typing
490 As \c{-hf}, this will also list the available output file formats, and what they
493 If you use Linux but aren't sure whether your system is \c{a.out}
498 (in the directory in which you put the NASM binary when you
499 installed it). If it says something like
501 \c nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1
503 then your system is \c{ELF}, and you should use the option \c{-f elf}
504 when you want NASM to produce Linux object files. If it says
506 \c nasm: Linux/i386 demand-paged executable (QMAGIC)
508 or something similar, your system is \c{a.out}, and you should use
509 \c{-f aout} instead (Linux \c{a.out} systems have long been obsolete,
510 and are rare these days.)
512 Like Unix compilers and assemblers, NASM is silent unless it
513 goes wrong: you won't see any output at all, unless it gives error
517 \S{opt-o} The \i\c{-o} Option: Specifying the Output File Name
519 NASM will normally choose the name of your output file for you;
520 precisely how it does this is dependent on the object file format.
521 For Microsoft object file formats (\c{obj}, \c{win32} and \c{win64}),
522 it will remove the \c{.asm} \i{extension} (or whatever extension you
523 like to use - NASM doesn't care) from your source file name and
524 substitute \c{.obj}. For Unix object file formats (\c{aout}, \c{as86},
525 \c{coff}, \c{elf32}, \c{elf64}, \c{elfx32}, \c{ieee}, \c{macho32} and
526 \c{macho64}) it will substitute \c{.o}. For \c{dbg}, \c{rdf}, \c{ith}
527 and \c{srec}, it will use \c{.dbg}, \c{.rdf}, \c{.ith} and \c{.srec},
528 respectively, and for the \c{bin} format it will simply remove the
529 extension, so that \c{myfile.asm} produces the output file \c{myfile}.
531 If the output file already exists, NASM will overwrite it, unless it
532 has the same name as the input file, in which case it will give a
533 warning and use \i\c{nasm.out} as the output file name instead.
535 For situations in which this behaviour is unacceptable, NASM
536 provides the \c{-o} command-line option, which allows you to specify
537 your desired output file name. You invoke \c{-o} by following it
538 with the name you wish for the output file, either with or without
539 an intervening space. For example:
541 \c nasm -f bin program.asm -o program.com
542 \c nasm -f bin driver.asm -odriver.sys
544 Note that this is a small o, and is different from a capital O , which
545 is used to specify the number of optimisation passes required. See \k{opt-O}.
548 \S{opt-f} The \i\c{-f} Option: Specifying the \i{Output File Format}
550 If you do not supply the \c{-f} option to NASM, it will choose an
551 output file format for you itself. In the distribution versions of
552 NASM, the default is always \i\c{bin}; if you've compiled your own
553 copy of NASM, you can redefine \i\c{OF_DEFAULT} at compile time and
554 choose what you want the default to be.
556 Like \c{-o}, the intervening space between \c{-f} and the output
557 file format is optional; so \c{-f elf} and \c{-felf} are both valid.
559 A complete list of the available output file formats can be given by
560 issuing the command \i\c{nasm -hf}.
563 \S{opt-l} The \i\c{-l} Option: Generating a \i{Listing File}
565 If you supply the \c{-l} option to NASM, followed (with the usual
566 optional space) by a file name, NASM will generate a
567 \i{source-listing file} for you, in which addresses and generated
568 code are listed on the left, and the actual source code, with
569 expansions of multi-line macros (except those which specifically
570 request no expansion in source listings: see \k{nolist}) on the
573 \c nasm -f elf myfile.asm -l myfile.lst
575 If a list file is selected, you may turn off listing for a
576 section of your source with \c{[list -]}, and turn it back on
577 with \c{[list +]}, (the default, obviously). There is no "user
578 form" (without the brackets). This can be used to list only
579 sections of interest, avoiding excessively long listings.
582 \S{opt-M} The \i\c{-M} Option: Generate \i{Makefile Dependencies}
584 This option can be used to generate makefile dependencies on stdout.
585 This can be redirected to a file for further processing. For example:
587 \c nasm -M myfile.asm > myfile.dep
590 \S{opt-MG} The \i\c{-MG} Option: Generate \i{Makefile Dependencies}
592 This option can be used to generate makefile dependencies on stdout.
593 This differs from the \c{-M} option in that if a nonexisting file is
594 encountered, it is assumed to be a generated file and is added to the
595 dependency list without a prefix.
598 \S{opt-MF} The \i\c\{-MF} Option: Set Makefile Dependency File
600 This option can be used with the \c{-M} or \c{-MG} options to send the
601 output to a file, rather than to stdout. For example:
603 \c nasm -M -MF myfile.dep myfile.asm
606 \S{opt-MD} The \i\c{-MD} Option: Assemble and Generate Dependencies
608 The \c{-MD} option acts as the combination of the \c{-M} and \c{-MF}
609 options (i.e. a filename has to be specified.) However, unlike the
610 \c{-M} or \c{-MG} options, \c{-MD} does \e{not} inhibit the normal
611 operation of the assembler. Use this to automatically generate
612 updated dependencies with every assembly session. For example:
614 \c nasm -f elf -o myfile.o -MD myfile.dep myfile.asm
617 \S{opt-MT} The \i\c{-MT} Option: Dependency Target Name
619 The \c{-MT} option can be used to override the default name of the
620 dependency target. This is normally the same as the output filename,
621 specified by the \c{-o} option.
624 \S{opt-MQ} The \i\c{-MQ} Option: Dependency Target Name (Quoted)
626 The \c{-MQ} option acts as the \c{-MT} option, except it tries to
627 quote characters that have special meaning in Makefile syntax. This
628 is not foolproof, as not all characters with special meaning are
629 quotable in Make. The default output (if no \c{-MT} or \c{-MQ} option
630 is specified) is automatically quoted.
633 \S{opt-MP} The \i\c{-MP} Option: Emit phony targets
635 When used with any of the dependency generation options, the \c{-MP}
636 option causes NASM to emit a phony target without dependencies for
637 each header file. This prevents Make from complaining if a header
638 file has been removed.
641 \S{opt-F} The \i\c{-F} Option: Selecting a \i{Debug Information Format}
643 This option is used to select the format of the debug information
644 emitted into the output file, to be used by a debugger (or \e{will}
645 be). Prior to version 2.03.01, the use of this switch did \e{not} enable
646 output of the selected debug info format. Use \c{-g}, see \k{opt-g},
647 to enable output. Versions 2.03.01 and later automatically enable \c{-g}
648 if \c{-F} is specified.
650 A complete list of the available debug file formats for an output
651 format can be seen by issuing the command \c{nasm -f <format> -y}. Not
652 all output formats currently support debugging output. See \k{opt-y}.
654 This should not be confused with the \c{-f dbg} output format option which
655 is not built into NASM by default. For information on how
656 to enable it when building from the sources, see \k{dbgfmt}.
659 \S{opt-g} The \i\c{-g} Option: Enabling \i{Debug Information}.
661 This option can be used to generate debugging information in the specified
662 format. See \k{opt-F}. Using \c{-g} without \c{-F} results in emitting
663 debug info in the default format, if any, for the selected output format.
664 If no debug information is currently implemented in the selected output
665 format, \c{-g} is \e{silently ignored}.
668 \S{opt-X} The \i\c{-X} Option: Selecting an \i{Error Reporting Format}
670 This option can be used to select an error reporting format for any
671 error messages that might be produced by NASM.
673 Currently, two error reporting formats may be selected. They are
674 the \c{-Xvc} option and the \c{-Xgnu} option. The GNU format is
675 the default and looks like this:
677 \c filename.asm:65: error: specific error message
679 where \c{filename.asm} is the name of the source file in which the
680 error was detected, \c{65} is the source file line number on which
681 the error was detected, \c{error} is the severity of the error (this
682 could be \c{warning}), and \c{specific error message} is a more
683 detailed text message which should help pinpoint the exact problem.
685 The other format, specified by \c{-Xvc} is the style used by Microsoft
686 Visual C++ and some other programs. It looks like this:
688 \c filename.asm(65) : error: specific error message
690 where the only difference is that the line number is in parentheses
691 instead of being delimited by colons.
693 See also the \c{Visual C++} output format, \k{win32fmt}.
695 \S{opt-Z} The \i\c{-Z} Option: Send Errors to a File
697 Under \I{DOS}\c{MS-DOS} it can be difficult (though there are ways) to
698 redirect the standard-error output of a program to a file. Since
699 NASM usually produces its warning and \i{error messages} on
700 \i\c{stderr}, this can make it hard to capture the errors if (for
701 example) you want to load them into an editor.
703 NASM therefore provides the \c{-Z} option, taking a filename argument
704 which causes errors to be sent to the specified files rather than
705 standard error. Therefore you can \I{redirecting errors}redirect
706 the errors into a file by typing
708 \c nasm -Z myfile.err -f obj myfile.asm
710 In earlier versions of NASM, this option was called \c{-E}, but it was
711 changed since \c{-E} is an option conventionally used for
712 preprocessing only, with disastrous results. See \k{opt-E}.
714 \S{opt-s} The \i\c{-s} Option: Send Errors to \i\c{stdout}
716 The \c{-s} option redirects \i{error messages} to \c{stdout} rather
717 than \c{stderr}, so it can be redirected under \I{DOS}\c{MS-DOS}. To
718 assemble the file \c{myfile.asm} and pipe its output to the \c{more}
719 program, you can type:
721 \c nasm -s -f obj myfile.asm | more
723 See also the \c{-Z} option, \k{opt-Z}.
726 \S{opt-i} The \i\c{-i}\I\c{-I} Option: Include File Search Directories
728 When NASM sees the \i\c{%include} or \i\c{%pathsearch} directive in a
729 source file (see \k{include}, \k{pathsearch} or \k{incbin}), it will
730 search for the given file not only in the current directory, but also
731 in any directories specified on the command line by the use of the
732 \c{-i} option. Therefore you can include files from a \i{macro
733 library}, for example, by typing
735 \c nasm -ic:\macrolib\ -f obj myfile.asm
737 (As usual, a space between \c{-i} and the path name is allowed, and
740 NASM, in the interests of complete source-code portability, does not
741 understand the file naming conventions of the OS it is running on;
742 the string you provide as an argument to the \c{-i} option will be
743 prepended exactly as written to the name of the include file.
744 Therefore the trailing backslash in the above example is necessary.
745 Under Unix, a trailing forward slash is similarly necessary.
747 (You can use this to your advantage, if you're really \i{perverse},
748 by noting that the option \c{-ifoo} will cause \c{%include "bar.i"}
749 to search for the file \c{foobar.i}...)
751 If you want to define a \e{standard} \i{include search path},
752 similar to \c{/usr/include} on Unix systems, you should place one or
753 more \c{-i} directives in the \c{NASMENV} environment variable (see
756 For Makefile compatibility with many C compilers, this option can also
757 be specified as \c{-I}.
760 \S{opt-p} The \i\c{-p}\I\c{-P} Option: \I{pre-including files}Pre-Include a File
762 \I\c{%include}NASM allows you to specify files to be
763 \e{pre-included} into your source file, by the use of the \c{-p}
766 \c nasm myfile.asm -p myinc.inc
768 is equivalent to running \c{nasm myfile.asm} and placing the
769 directive \c{%include "myinc.inc"} at the start of the file.
771 For consistency with the \c{-I}, \c{-D} and \c{-U} options, this
772 option can also be specified as \c{-P}.
775 \S{opt-d} The \i\c{-d}\I\c{-D} Option: \I{pre-defining macros}Pre-Define a Macro
777 \I\c{%define}Just as the \c{-p} option gives an alternative to placing
778 \c{%include} directives at the start of a source file, the \c{-d}
779 option gives an alternative to placing a \c{%define} directive. You
782 \c nasm myfile.asm -dFOO=100
784 as an alternative to placing the directive
788 at the start of the file. You can miss off the macro value, as well:
789 the option \c{-dFOO} is equivalent to coding \c{%define FOO}. This
790 form of the directive may be useful for selecting \i{assembly-time
791 options} which are then tested using \c{%ifdef}, for example
794 For Makefile compatibility with many C compilers, this option can also
795 be specified as \c{-D}.
798 \S{opt-u} The \i\c{-u}\I\c{-U} Option: \I{Undefining macros}Undefine a Macro
800 \I\c{%undef}The \c{-u} option undefines a macro that would otherwise
801 have been pre-defined, either automatically or by a \c{-p} or \c{-d}
802 option specified earlier on the command lines.
804 For example, the following command line:
806 \c nasm myfile.asm -dFOO=100 -uFOO
808 would result in \c{FOO} \e{not} being a predefined macro in the
809 program. This is useful to override options specified at a different
812 For Makefile compatibility with many C compilers, this option can also
813 be specified as \c{-U}.
816 \S{opt-E} The \i\c{-E}\I{-e} Option: Preprocess Only
818 NASM allows the \i{preprocessor} to be run on its own, up to a
819 point. Using the \c{-E} option (which requires no arguments) will
820 cause NASM to preprocess its input file, expand all the macro
821 references, remove all the comments and preprocessor directives, and
822 print the resulting file on standard output (or save it to a file,
823 if the \c{-o} option is also used).
825 This option cannot be applied to programs which require the
826 preprocessor to evaluate \I{preprocessor expressions}\i{expressions}
827 which depend on the values of symbols: so code such as
829 \c %assign tablesize ($-tablestart)
831 will cause an error in \i{preprocess-only mode}.
833 For compatiblity with older version of NASM, this option can also be
834 written \c{-e}. \c{-E} in older versions of NASM was the equivalent
835 of the current \c{-Z} option, \k{opt-Z}.
837 \S{opt-a} The \i\c{-a} Option: Don't Preprocess At All
839 If NASM is being used as the back end to a compiler, it might be
840 desirable to \I{suppressing preprocessing}suppress preprocessing
841 completely and assume the compiler has already done it, to save time
842 and increase compilation speeds. The \c{-a} option, requiring no
843 argument, instructs NASM to replace its powerful \i{preprocessor}
844 with a \i{stub preprocessor} which does nothing.
847 \S{opt-O} The \i\c{-O} Option: Specifying \i{Multipass Optimization}
849 Using the \c{-O} option, you can tell NASM to carry out different
850 levels of optimization. The syntax is:
852 \b \c{-O0}: No optimization. All operands take their long forms,
853 if a short form is not specified, except conditional jumps.
854 This is intended to match NASM 0.98 behavior.
856 \b \c{-O1}: Minimal optimization. As above, but immediate operands
857 which will fit in a signed byte are optimized,
858 unless the long form is specified. Conditional jumps default
859 to the long form unless otherwise specified.
861 \b \c{-Ox} (where \c{x} is the actual letter \c{x}): Multipass optimization.
862 Minimize branch offsets and signed immediate bytes,
863 overriding size specification unless the \c{strict} keyword
864 has been used (see \k{strict}). For compatibility with earlier
865 releases, the letter \c{x} may also be any number greater than
866 one. This number has no effect on the actual number of passes.
868 The \c{-Ox} mode is recommended for most uses, and is the default
871 Note that this is a capital \c{O}, and is different from a small \c{o}, which
872 is used to specify the output file name. See \k{opt-o}.
875 \S{opt-t} The \i\c{-t} Option: Enable TASM Compatibility Mode
877 NASM includes a limited form of compatibility with Borland's \i\c{TASM}.
878 When NASM's \c{-t} option is used, the following changes are made:
880 \b local labels may be prefixed with \c{@@} instead of \c{.}
882 \b size override is supported within brackets. In TASM compatible mode,
883 a size override inside square brackets changes the size of the operand,
884 and not the address type of the operand as it does in NASM syntax. E.g.
885 \c{mov eax,[DWORD val]} is valid syntax in TASM compatibility mode.
886 Note that you lose the ability to override the default address type for
889 \b unprefixed forms of some directives supported (\c{arg}, \c{elif},
890 \c{else}, \c{endif}, \c{if}, \c{ifdef}, \c{ifdifi}, \c{ifndef},
891 \c{include}, \c{local})
893 \S{opt-w} The \i\c{-w} and \i\c{-W} Options: Enable or Disable Assembly \i{Warnings}
895 NASM can observe many conditions during the course of assembly which
896 are worth mentioning to the user, but not a sufficiently severe
897 error to justify NASM refusing to generate an output file. These
898 conditions are reported like errors, but come up with the word
899 `warning' before the message. Warnings do not prevent NASM from
900 generating an output file and returning a success status to the
903 Some conditions are even less severe than that: they are only
904 sometimes worth mentioning to the user. Therefore NASM supports the
905 \c{-w} command-line option, which enables or disables certain
906 classes of assembly warning. Such warning classes are described by a
907 name, for example \c{orphan-labels}; you can enable warnings of
908 this class by the command-line option \c{-w+orphan-labels} and
909 disable it by \c{-w-orphan-labels}.
911 The \i{suppressible warning} classes are:
913 \b \i\c{macro-params} covers warnings about \i{multi-line macros}
914 being invoked with the wrong number of parameters. This warning
915 class is enabled by default; see \k{mlmacover} for an example of why
916 you might want to disable it.
918 \b \i\c{macro-selfref} warns if a macro references itself. This
919 warning class is disabled by default.
921 \b\i\c{macro-defaults} warns when a macro has more default
922 parameters than optional parameters. This warning class
923 is enabled by default; see \k{mlmacdef} for why you might want to disable it.
925 \b \i\c{orphan-labels} covers warnings about source lines which
926 contain no instruction but define a label without a trailing colon.
927 NASM warns about this somewhat obscure condition by default;
928 see \k{syntax} for more information.
930 \b \i\c{number-overflow} covers warnings about numeric constants which
931 don't fit in 64 bits. This warning class is enabled by default.
933 \b \i\c{gnu-elf-extensions} warns if 8-bit or 16-bit relocations
934 are used in \c{-f elf} format. The GNU extensions allow this.
935 This warning class is disabled by default.
937 \b \i\c{float-overflow} warns about floating point overflow.
940 \b \i\c{float-denorm} warns about floating point denormals.
943 \b \i\c{float-underflow} warns about floating point underflow.
946 \b \i\c{float-toolong} warns about too many digits in floating-point numbers.
949 \b \i\c{user} controls \c{%warning} directives (see \k{pperror}).
952 \b \i\c{lock} warns about \c{LOCK} prefixes on unlockable instructions.
955 \b \i\c{hle} warns about invalid use of the HLE \c{XACQUIRE} or \c{XRELEASE}
959 \b \i\c{bnd} warns about ineffective use of the \c{BND} prefix when a relaxed
960 form of jmp instruction becomes jmp short form.
963 \b \i\c{error} causes warnings to be treated as errors. Disabled by
966 \b \i\c{all} is an alias for \e{all} suppressible warning classes (not
967 including \c{error}). Thus, \c{-w+all} enables all available warnings.
969 In addition, you can set warning classes across sections.
970 Warning classes may be enabled with \i\c{[warning +warning-name]},
971 disabled with \i\c{[warning -warning-name]} or reset to their
972 original value with \i\c{[warning *warning-name]}. No "user form"
973 (without the brackets) exists.
975 Since version 2.00, NASM has also supported the gcc-like syntax
976 \c{-Wwarning} and \c{-Wno-warning} instead of \c{-w+warning} and
977 \c{-w-warning}, respectively.
980 \S{opt-v} The \i\c{-v} Option: Display \i{Version} Info
982 Typing \c{NASM -v} will display the version of NASM which you are using,
983 and the date on which it was compiled.
985 You will need the version number if you report a bug.
987 \S{opt-y} The \i\c{-y} Option: Display Available Debug Info Formats
989 Typing \c{nasm -f <option> -y} will display a list of the available
990 debug info formats for the given output format. The default format
991 is indicated by an asterisk. For example:
995 \c valid debug formats for 'elf32' output format are
996 \c ('*' denotes default):
997 \c * stabs ELF32 (i386) stabs debug format for Linux
998 \c dwarf elf32 (i386) dwarf debug format for Linux
1001 \S{opt-pfix} The \i\c{--prefix} and \i\c{--postfix} Options.
1003 The \c{--prefix} and \c{--postfix} options prepend or append
1004 (respectively) the given argument to all \c{global} or
1005 \c{extern} variables. E.g. \c{--prefix _} will prepend the
1006 underscore to all global and external variables, as C sometimes
1007 (but not always) likes it.
1010 \S{nasmenv} The \i\c{NASMENV} \i{Environment} Variable
1012 If you define an environment variable called \c{NASMENV}, the program
1013 will interpret it as a list of extra command-line options, which are
1014 processed before the real command line. You can use this to define
1015 standard search directories for include files, by putting \c{-i}
1016 options in the \c{NASMENV} variable.
1018 The value of the variable is split up at white space, so that the
1019 value \c{-s -ic:\\nasmlib\\} will be treated as two separate options.
1020 However, that means that the value \c{-dNAME="my name"} won't do
1021 what you might want, because it will be split at the space and the
1022 NASM command-line processing will get confused by the two
1023 nonsensical words \c{-dNAME="my} and \c{name"}.
1025 To get round this, NASM provides a feature whereby, if you begin the
1026 \c{NASMENV} environment variable with some character that isn't a minus
1027 sign, then NASM will treat this character as the \i{separator
1028 character} for options. So setting the \c{NASMENV} variable to the
1029 value \c{!-s!-ic:\\nasmlib\\} is equivalent to setting it to \c{-s
1030 -ic:\\nasmlib\\}, but \c{!-dNAME="my name"} will work.
1032 This environment variable was previously called \c{NASM}. This was
1033 changed with version 0.98.31.
1036 \H{qstart} \i{Quick Start} for \i{MASM} Users
1038 If you're used to writing programs with MASM, or with \i{TASM} in
1039 MASM-compatible (non-Ideal) mode, or with \i\c{a86}, this section
1040 attempts to outline the major differences between MASM's syntax and
1041 NASM's. If you're not already used to MASM, it's probably worth
1042 skipping this section.
1045 \S{qscs} NASM Is \I{case sensitivity}Case-Sensitive
1047 One simple difference is that NASM is case-sensitive. It makes a
1048 difference whether you call your label \c{foo}, \c{Foo} or \c{FOO}.
1049 If you're assembling to \c{DOS} or \c{OS/2} \c{.OBJ} files, you can
1050 invoke the \i\c{UPPERCASE} directive (documented in \k{objfmt}) to
1051 ensure that all symbols exported to other code modules are forced
1052 to be upper case; but even then, \e{within} a single module, NASM
1053 will distinguish between labels differing only in case.
1056 \S{qsbrackets} NASM Requires \i{Square Brackets} For \i{Memory References}
1058 NASM was designed with simplicity of syntax in mind. One of the
1059 \i{design goals} of NASM is that it should be possible, as far as is
1060 practical, for the user to look at a single line of NASM code
1061 and tell what opcode is generated by it. You can't do this in MASM:
1062 if you declare, for example,
1067 then the two lines of code
1072 generate completely different opcodes, despite having
1073 identical-looking syntaxes.
1075 NASM avoids this undesirable situation by having a much simpler
1076 syntax for memory references. The rule is simply that any access to
1077 the \e{contents} of a memory location requires square brackets
1078 around the address, and any access to the \e{address} of a variable
1079 doesn't. So an instruction of the form \c{mov ax,foo} will
1080 \e{always} refer to a compile-time constant, whether it's an \c{EQU}
1081 or the address of a variable; and to access the \e{contents} of the
1082 variable \c{bar}, you must code \c{mov ax,[bar]}.
1084 This also means that NASM has no need for MASM's \i\c{OFFSET}
1085 keyword, since the MASM code \c{mov ax,offset bar} means exactly the
1086 same thing as NASM's \c{mov ax,bar}. If you're trying to get
1087 large amounts of MASM code to assemble sensibly under NASM, you
1088 can always code \c{%idefine offset} to make the preprocessor treat
1089 the \c{OFFSET} keyword as a no-op.
1091 This issue is even more confusing in \i\c{a86}, where declaring a
1092 label with a trailing colon defines it to be a `label' as opposed to
1093 a `variable' and causes \c{a86} to adopt NASM-style semantics; so in
1094 \c{a86}, \c{mov ax,var} has different behaviour depending on whether
1095 \c{var} was declared as \c{var: dw 0} (a label) or \c{var dw 0} (a
1096 word-size variable). NASM is very simple by comparison:
1097 \e{everything} is a label.
1099 NASM, in the interests of simplicity, also does not support the
1100 \i{hybrid syntaxes} supported by MASM and its clones, such as
1101 \c{mov ax,table[bx]}, where a memory reference is denoted by one
1102 portion outside square brackets and another portion inside. The
1103 correct syntax for the above is \c{mov ax,[table+bx]}. Likewise,
1104 \c{mov ax,es:[di]} is wrong and \c{mov ax,[es:di]} is right.
1107 \S{qstypes} NASM Doesn't Store \i{Variable Types}
1109 NASM, by design, chooses not to remember the types of variables you
1110 declare. Whereas MASM will remember, on seeing \c{var dw 0}, that
1111 you declared \c{var} as a word-size variable, and will then be able
1112 to fill in the \i{ambiguity} in the size of the instruction \c{mov
1113 var,2}, NASM will deliberately remember nothing about the symbol
1114 \c{var} except where it begins, and so you must explicitly code
1115 \c{mov word [var],2}.
1117 For this reason, NASM doesn't support the \c{LODS}, \c{MOVS},
1118 \c{STOS}, \c{SCAS}, \c{CMPS}, \c{INS}, or \c{OUTS} instructions,
1119 but only supports the forms such as \c{LODSB}, \c{MOVSW}, and
1120 \c{SCASD}, which explicitly specify the size of the components of
1121 the strings being manipulated.
1124 \S{qsassume} NASM Doesn't \i\c{ASSUME}
1126 As part of NASM's drive for simplicity, it also does not support the
1127 \c{ASSUME} directive. NASM will not keep track of what values you
1128 choose to put in your segment registers, and will never
1129 \e{automatically} generate a \i{segment override} prefix.
1132 \S{qsmodel} NASM Doesn't Support \i{Memory Models}
1134 NASM also does not have any directives to support different 16-bit
1135 memory models. The programmer has to keep track of which functions
1136 are supposed to be called with a \i{far call} and which with a
1137 \i{near call}, and is responsible for putting the correct form of
1138 \c{RET} instruction (\c{RETN} or \c{RETF}; NASM accepts \c{RET}
1139 itself as an alternate form for \c{RETN}); in addition, the
1140 programmer is responsible for coding CALL FAR instructions where
1141 necessary when calling \e{external} functions, and must also keep
1142 track of which external variable definitions are far and which are
1146 \S{qsfpu} \i{Floating-Point} Differences
1148 NASM uses different names to refer to floating-point registers from
1149 MASM: where MASM would call them \c{ST(0)}, \c{ST(1)} and so on, and
1150 \i\c{a86} would call them simply \c{0}, \c{1} and so on, NASM
1151 chooses to call them \c{st0}, \c{st1} etc.
1153 As of version 0.96, NASM now treats the instructions with
1154 \i{`nowait'} forms in the same way as MASM-compatible assemblers.
1155 The idiosyncratic treatment employed by 0.95 and earlier was based
1156 on a misunderstanding by the authors.
1159 \S{qsother} Other Differences
1161 For historical reasons, NASM uses the keyword \i\c{TWORD} where MASM
1162 and compatible assemblers use \i\c{TBYTE}.
1164 NASM does not declare \i{uninitialized storage} in the same way as
1165 MASM: where a MASM programmer might use \c{stack db 64 dup (?)},
1166 NASM requires \c{stack resb 64}, intended to be read as `reserve 64
1167 bytes'. For a limited amount of compatibility, since NASM treats
1168 \c{?} as a valid character in symbol names, you can code \c{? equ 0}
1169 and then writing \c{dw ?} will at least do something vaguely useful.
1170 \I\c{RESB}\i\c{DUP} is still not a supported syntax, however.
1172 In addition to all of this, macros and directives work completely
1173 differently to MASM. See \k{preproc} and \k{directive} for further
1177 \C{lang} The NASM Language
1179 \H{syntax} Layout of a NASM Source Line
1181 Like most assemblers, each NASM source line contains (unless it
1182 is a macro, a preprocessor directive or an assembler directive: see
1183 \k{preproc} and \k{directive}) some combination of the four fields
1185 \c label: instruction operands ; comment
1187 As usual, most of these fields are optional; the presence or absence
1188 of any combination of a label, an instruction and a comment is allowed.
1189 Of course, the operand field is either required or forbidden by the
1190 presence and nature of the instruction field.
1192 NASM uses backslash (\\) as the line continuation character; if a line
1193 ends with backslash, the next line is considered to be a part of the
1194 backslash-ended line.
1196 NASM places no restrictions on white space within a line: labels may
1197 have white space before them, or instructions may have no space
1198 before them, or anything. The \i{colon} after a label is also
1199 optional. (Note that this means that if you intend to code \c{lodsb}
1200 alone on a line, and type \c{lodab} by accident, then that's still a
1201 valid source line which does nothing but define a label. Running
1202 NASM with the command-line option
1203 \I{orphan-labels}\c{-w+orphan-labels} will cause it to warn you if
1204 you define a label alone on a line without a \i{trailing colon}.)
1206 \i{Valid characters} in labels are letters, numbers, \c{_}, \c{$},
1207 \c{#}, \c{@}, \c{~}, \c{.}, and \c{?}. The only characters which may
1208 be used as the \e{first} character of an identifier are letters,
1209 \c{.} (with special meaning: see \k{locallab}), \c{_} and \c{?}.
1210 An identifier may also be prefixed with a \I{$, prefix}\c{$} to
1211 indicate that it is intended to be read as an identifier and not a
1212 reserved word; thus, if some other module you are linking with
1213 defines a symbol called \c{eax}, you can refer to \c{$eax} in NASM
1214 code to distinguish the symbol from the register. Maximum length of
1215 an identifier is 4095 characters.
1217 The instruction field may contain any machine instruction: Pentium
1218 and P6 instructions, FPU instructions, MMX instructions and even
1219 undocumented instructions are all supported. The instruction may be
1220 prefixed by \c{LOCK}, \c{REP}, \c{REPE}/\c{REPZ}, \c{REPNE}/\c{REPNZ},
1221 \c{XACQUIRE}/\c{XRELEASE} or \c{BND}/\c{NOBND}, in the usual way. Explicit
1222 \I{address-size prefixes}address-size and \i{operand-size prefixes} \i\c{A16},
1223 \i\c{A32}, \i\c{A64}, \i\c{O16} and \i\c{O32}, \i\c{O64} are provided - one example of their use
1224 is given in \k{mixsize}. You can also use the name of a \I{segment
1225 override}segment register as an instruction prefix: coding
1226 \c{es mov [bx],ax} is equivalent to coding \c{mov [es:bx],ax}. We
1227 recommend the latter syntax, since it is consistent with other
1228 syntactic features of the language, but for instructions such as
1229 \c{LODSB}, which has no operands and yet can require a segment
1230 override, there is no clean syntactic way to proceed apart from
1233 An instruction is not required to use a prefix: prefixes such as
1234 \c{CS}, \c{A32}, \c{LOCK} or \c{REPE} can appear on a line by
1235 themselves, and NASM will just generate the prefix bytes.
1237 In addition to actual machine instructions, NASM also supports a
1238 number of pseudo-instructions, described in \k{pseudop}.
1240 Instruction \i{operands} may take a number of forms: they can be
1241 registers, described simply by the register name (e.g. \c{ax},
1242 \c{bp}, \c{ebx}, \c{cr0}: NASM does not use the \c{gas}-style
1243 syntax in which register names must be prefixed by a \c{%} sign), or
1244 they can be \i{effective addresses} (see \k{effaddr}), constants
1245 (\k{const}) or expressions (\k{expr}).
1247 For x87 \i{floating-point} instructions, NASM accepts a wide range of
1248 syntaxes: you can use two-operand forms like MASM supports, or you
1249 can use NASM's native single-operand forms in most cases.
1251 \# all forms of each supported instruction are given in
1253 For example, you can code:
1255 \c fadd st1 ; this sets st0 := st0 + st1
1256 \c fadd st0,st1 ; so does this
1258 \c fadd st1,st0 ; this sets st1 := st1 + st0
1259 \c fadd to st1 ; so does this
1261 Almost any x87 floating-point instruction that references memory must
1262 use one of the prefixes \i\c{DWORD}, \i\c{QWORD} or \i\c{TWORD} to
1263 indicate what size of \i{memory operand} it refers to.
1266 \H{pseudop} \i{Pseudo-Instructions}
1268 Pseudo-instructions are things which, though not real x86 machine
1269 instructions, are used in the instruction field anyway because that's
1270 the most convenient place to put them. The current pseudo-instructions
1271 are \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, \i\c{DO},
1272 \i\c{DY} and \i\c\{DZ}; their \i{uninitialized} counterparts
1273 \i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST},
1274 \i\c{RESO}, \i\c{RESY} and \i\c\{RESZ}; the \i\c{INCBIN} command, the
1275 \i\c{EQU} command, and the \i\c{TIMES} prefix.
1278 \S{db} \c{DB} and Friends: Declaring Initialized Data
1280 \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, \i\c{DO}, \i\c{DY}
1281 and \i\c{DZ} are used, much as in MASM, to declare initialized data in
1282 the output file. They can be invoked in a wide range of ways:
1283 \I{floating-point}\I{character constant}\I{string constant}
1285 \c db 0x55 ; just the byte 0x55
1286 \c db 0x55,0x56,0x57 ; three bytes in succession
1287 \c db 'a',0x55 ; character constants are OK
1288 \c db 'hello',13,10,'$' ; so are string constants
1289 \c dw 0x1234 ; 0x34 0x12
1290 \c dw 'a' ; 0x61 0x00 (it's just a number)
1291 \c dw 'ab' ; 0x61 0x62 (character constant)
1292 \c dw 'abc' ; 0x61 0x62 0x63 0x00 (string)
1293 \c dd 0x12345678 ; 0x78 0x56 0x34 0x12
1294 \c dd 1.234567e20 ; floating-point constant
1295 \c dq 0x123456789abcdef0 ; eight byte constant
1296 \c dq 1.234567e20 ; double-precision float
1297 \c dt 1.234567e20 ; extended-precision float
1299 \c{DT}, \c{DO}, \c{DY} and \c{DZ} do not accept \i{numeric constants}
1303 \S{resb} \c{RESB} and Friends: Declaring \i{Uninitialized} Data
1305 \i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST},
1306 \i\c{RESO}, \i\c{RESY} and \i\c\{RESZ} are designed to be used in the
1307 BSS section of a module: they declare \e{uninitialized} storage
1308 space. Each takes a single operand, which is the number of bytes,
1309 words, doublewords or whatever to reserve. As stated in \k{qsother},
1310 NASM does not support the MASM/TASM syntax of reserving uninitialized
1311 space by writing \I\c{?}\c{DW ?} or similar things: this is what it
1312 does instead. The operand to a \c{RESB}-type pseudo-instruction is a
1313 \i\e{critical expression}: see \k{crit}.
1317 \c buffer: resb 64 ; reserve 64 bytes
1318 \c wordvar: resw 1 ; reserve a word
1319 \c realarray resq 10 ; array of ten reals
1320 \c ymmval: resy 1 ; one YMM register
1321 \c zmmvals: resz 32 ; 32 ZMM registers
1323 \S{incbin} \i\c{INCBIN}: Including External \i{Binary Files}
1325 \c{INCBIN} is borrowed from the old Amiga assembler \i{DevPac}: it
1326 includes a binary file verbatim into the output file. This can be
1327 handy for (for example) including \i{graphics} and \i{sound} data
1328 directly into a game executable file. It can be called in one of
1331 \c incbin "file.dat" ; include the whole file
1332 \c incbin "file.dat",1024 ; skip the first 1024 bytes
1333 \c incbin "file.dat",1024,512 ; skip the first 1024, and
1334 \c ; actually include at most 512
1336 \c{INCBIN} is both a directive and a standard macro; the standard
1337 macro version searches for the file in the include file search path
1338 and adds the file to the dependency lists. This macro can be
1339 overridden if desired.
1342 \S{equ} \i\c{EQU}: Defining Constants
1344 \c{EQU} defines a symbol to a given constant value: when \c{EQU} is
1345 used, the source line must contain a label. The action of \c{EQU} is
1346 to define the given label name to the value of its (only) operand.
1347 This definition is absolute, and cannot change later. So, for
1350 \c message db 'hello, world'
1351 \c msglen equ $-message
1353 defines \c{msglen} to be the constant 12. \c{msglen} may not then be
1354 redefined later. This is not a \i{preprocessor} definition either:
1355 the value of \c{msglen} is evaluated \e{once}, using the value of
1356 \c{$} (see \k{expr} for an explanation of \c{$}) at the point of
1357 definition, rather than being evaluated wherever it is referenced
1358 and using the value of \c{$} at the point of reference.
1361 \S{times} \i\c{TIMES}: \i{Repeating} Instructions or Data
1363 The \c{TIMES} prefix causes the instruction to be assembled multiple
1364 times. This is partly present as NASM's equivalent of the \i\c{DUP}
1365 syntax supported by \i{MASM}-compatible assemblers, in that you can
1368 \c zerobuf: times 64 db 0
1370 or similar things; but \c{TIMES} is more versatile than that. The
1371 argument to \c{TIMES} is not just a numeric constant, but a numeric
1372 \e{expression}, so you can do things like
1374 \c buffer: db 'hello, world'
1375 \c times 64-$+buffer db ' '
1377 which will store exactly enough spaces to make the total length of
1378 \c{buffer} up to 64. Finally, \c{TIMES} can be applied to ordinary
1379 instructions, so you can code trivial \i{unrolled loops} in it:
1383 Note that there is no effective difference between \c{times 100 resb
1384 1} and \c{resb 100}, except that the latter will be assembled about
1385 100 times faster due to the internal structure of the assembler.
1387 The operand to \c{TIMES} is a critical expression (\k{crit}).
1389 Note also that \c{TIMES} can't be applied to \i{macros}: the reason
1390 for this is that \c{TIMES} is processed after the macro phase, which
1391 allows the argument to \c{TIMES} to contain expressions such as
1392 \c{64-$+buffer} as above. To repeat more than one line of code, or a
1393 complex macro, use the preprocessor \i\c{%rep} directive.
1396 \H{effaddr} Effective Addresses
1398 An \i{effective address} is any operand to an instruction which
1399 \I{memory reference}references memory. Effective addresses, in NASM,
1400 have a very simple syntax: they consist of an expression evaluating
1401 to the desired address, enclosed in \i{square brackets}. For
1406 \c mov ax,[wordvar+1]
1407 \c mov ax,[es:wordvar+bx]
1409 Anything not conforming to this simple system is not a valid memory
1410 reference in NASM, for example \c{es:wordvar[bx]}.
1412 More complicated effective addresses, such as those involving more
1413 than one register, work in exactly the same way:
1415 \c mov eax,[ebx*2+ecx+offset]
1418 NASM is capable of doing \i{algebra} on these effective addresses,
1419 so that things which don't necessarily \e{look} legal are perfectly
1422 \c mov eax,[ebx*5] ; assembles as [ebx*4+ebx]
1423 \c mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]
1425 Some forms of effective address have more than one assembled form;
1426 in most such cases NASM will generate the smallest form it can. For
1427 example, there are distinct assembled forms for the 32-bit effective
1428 addresses \c{[eax*2+0]} and \c{[eax+eax]}, and NASM will generally
1429 generate the latter on the grounds that the former requires four
1430 bytes to store a zero offset.
1432 NASM has a hinting mechanism which will cause \c{[eax+ebx]} and
1433 \c{[ebx+eax]} to generate different opcodes; this is occasionally
1434 useful because \c{[esi+ebp]} and \c{[ebp+esi]} have different
1435 default segment registers.
1437 However, you can force NASM to generate an effective address in a
1438 particular form by the use of the keywords \c{BYTE}, \c{WORD},
1439 \c{DWORD} and \c{NOSPLIT}. If you need \c{[eax+3]} to be assembled
1440 using a double-word offset field instead of the one byte NASM will
1441 normally generate, you can code \c{[dword eax+3]}. Similarly, you
1442 can force NASM to use a byte offset for a small value which it
1443 hasn't seen on the first pass (see \k{crit} for an example of such a
1444 code fragment) by using \c{[byte eax+offset]}. As special cases,
1445 \c{[byte eax]} will code \c{[eax+0]} with a byte offset of zero, and
1446 \c{[dword eax]} will code it with a double-word offset of zero. The
1447 normal form, \c{[eax]}, will be coded with no offset field.
1449 The form described in the previous paragraph is also useful if you
1450 are trying to access data in a 32-bit segment from within 16 bit code.
1451 For more information on this see the section on mixed-size addressing
1452 (\k{mixaddr}). In particular, if you need to access data with a known
1453 offset that is larger than will fit in a 16-bit value, if you don't
1454 specify that it is a dword offset, nasm will cause the high word of
1455 the offset to be lost.
1457 Similarly, NASM will split \c{[eax*2]} into \c{[eax+eax]} because
1458 that allows the offset field to be absent and space to be saved; in
1459 fact, it will also split \c{[eax*2+offset]} into
1460 \c{[eax+eax+offset]}. You can combat this behaviour by the use of
1461 the \c{NOSPLIT} keyword: \c{[nosplit eax*2]} will force
1462 \c{[eax*2+0]} to be generated literally.
1463 However, \c{NOSPLIT} in \c{[nosplit eax+eax]} will be ignored because user's
1464 intention here is considered as \c{[eax+eax]}.
1466 In 64-bit mode, NASM will by default generate absolute addresses. The
1467 \i\c{REL} keyword makes it produce \c{RIP}-relative addresses. Since
1468 this is frequently the normally desired behaviour, see the \c{DEFAULT}
1469 directive (\k{default}). The keyword \i\c{ABS} overrides \i\c{REL}.
1471 A new form of split effective addres syntax is also supported. This is
1472 mainly intended for mib operands as used by MPX instructions, but can
1473 be used for any memory reference. The basic concept of this form is
1474 splitting base and index.
1476 \c mov eax,[ebx+8,ecx*4] ; ebx=base, ecx=index, 4=scale, 8=disp
1478 For mib operands, there are several ways of writing effective address depending
1479 on the tools. NASM supports all currently possible ways of mib syntax:
1482 \c ; next 5 lines are parsed same
1483 \c ; base=rax, index=rbx, scale=1, displacement=3
1484 \c bndstx [rax+0x3,rbx], bnd0 ; NASM - split EA
1485 \c bndstx [rbx*1+rax+0x3], bnd0 ; GAS - '*1' indecates an index reg
1486 \c bndstx [rax+rbx+3], bnd0 ; GAS - without hints
1487 \c bndstx [rax+0x3], bnd0, rbx ; ICC-1
1488 \c bndstx [rax+0x3], rbx, bnd0 ; ICC-2
1490 When broadcasting decorator is used, the opsize keyword should match
1491 the size of each element.
1493 \c VDIVPS zmm4, zmm5, dword [rbx]{1to16} ; single-precision float
1494 \c VDIVPS zmm4, zmm5, zword [rbx] ; packed 512 bit memory
1497 \H{const} \i{Constants}
1499 NASM understands four different types of constant: numeric,
1500 character, string and floating-point.
1503 \S{numconst} \i{Numeric Constants}
1505 A numeric constant is simply a number. NASM allows you to specify
1506 numbers in a variety of number bases, in a variety of ways: you can
1507 suffix \c{H} or \c{X}, \c{D} or \c{T}, \c{Q} or \c{O}, and \c{B} or
1508 \c{Y} for \i{hexadecimal}, \i{decimal}, \i{octal} and \i{binary}
1509 respectively, or you can prefix \c{0x}, for hexadecimal in the style
1510 of C, or you can prefix \c{$} for hexadecimal in the style of Borland
1511 Pascal or Motorola Assemblers. Note, though, that the \I{$,
1512 prefix}\c{$} prefix does double duty as a prefix on identifiers (see
1513 \k{syntax}), so a hex number prefixed with a \c{$} sign must have a
1514 digit after the \c{$} rather than a letter. In addition, current
1515 versions of NASM accept the prefix \c{0h} for hexadecimal, \c{0d} or
1516 \c{0t} for decimal, \c{0o} or \c{0q} for octal, and \c{0b} or \c{0y}
1517 for binary. Please note that unlike C, a \c{0} prefix by itself does
1518 \e{not} imply an octal constant!
1520 Numeric constants can have underscores (\c{_}) interspersed to break
1523 Some examples (all producing exactly the same code):
1525 \c mov ax,200 ; decimal
1526 \c mov ax,0200 ; still decimal
1527 \c mov ax,0200d ; explicitly decimal
1528 \c mov ax,0d200 ; also decimal
1529 \c mov ax,0c8h ; hex
1530 \c mov ax,$0c8 ; hex again: the 0 is required
1531 \c mov ax,0xc8 ; hex yet again
1532 \c mov ax,0hc8 ; still hex
1533 \c mov ax,310q ; octal
1534 \c mov ax,310o ; octal again
1535 \c mov ax,0o310 ; octal yet again
1536 \c mov ax,0q310 ; octal yet again
1537 \c mov ax,11001000b ; binary
1538 \c mov ax,1100_1000b ; same binary constant
1539 \c mov ax,1100_1000y ; same binary constant once more
1540 \c mov ax,0b1100_1000 ; same binary constant yet again
1541 \c mov ax,0y1100_1000 ; same binary constant yet again
1543 \S{strings} \I{Strings}\i{Character Strings}
1545 A character string consists of up to eight characters enclosed in
1546 either single quotes (\c{'...'}), double quotes (\c{"..."}) or
1547 backquotes (\c{`...`}). Single or double quotes are equivalent to
1548 NASM (except of course that surrounding the constant with single
1549 quotes allows double quotes to appear within it and vice versa); the
1550 contents of those are represented verbatim. Strings enclosed in
1551 backquotes support C-style \c{\\}-escapes for special characters.
1554 The following \i{escape sequences} are recognized by backquoted strings:
1556 \c \' single quote (')
1557 \c \" double quote (")
1559 \c \\\ backslash (\)
1560 \c \? question mark (?)
1568 \c \e ESC (ASCII 27)
1569 \c \377 Up to 3 octal digits - literal byte
1570 \c \xFF Up to 2 hexadecimal digits - literal byte
1571 \c \u1234 4 hexadecimal digits - Unicode character
1572 \c \U12345678 8 hexadecimal digits - Unicode character
1574 All other escape sequences are reserved. Note that \c{\\0}, meaning a
1575 \c{NUL} character (ASCII 0), is a special case of the octal escape
1578 \i{Unicode} characters specified with \c{\\u} or \c{\\U} are converted to
1579 \i{UTF-8}. For example, the following lines are all equivalent:
1581 \c db `\u263a` ; UTF-8 smiley face
1582 \c db `\xe2\x98\xba` ; UTF-8 smiley face
1583 \c db 0E2h, 098h, 0BAh ; UTF-8 smiley face
1586 \S{chrconst} \i{Character Constants}
1588 A character constant consists of a string up to eight bytes long, used
1589 in an expression context. It is treated as if it was an integer.
1591 A character constant with more than one byte will be arranged
1592 with \i{little-endian} order in mind: if you code
1596 then the constant generated is not \c{0x61626364}, but
1597 \c{0x64636261}, so that if you were then to store the value into
1598 memory, it would read \c{abcd} rather than \c{dcba}. This is also
1599 the sense of character constants understood by the Pentium's
1600 \i\c{CPUID} instruction.
1603 \S{strconst} \i{String Constants}
1605 String constants are character strings used in the context of some
1606 pseudo-instructions, namely the
1607 \I\c{DW}\I\c{DD}\I\c{DQ}\I\c{DT}\I\c{DO}\I\c{DY}\i\c{DB} family and
1608 \i\c{INCBIN} (where it represents a filename.) They are also used in
1609 certain preprocessor directives.
1611 A string constant looks like a character constant, only longer. It
1612 is treated as a concatenation of maximum-size character constants
1613 for the conditions. So the following are equivalent:
1615 \c db 'hello' ; string constant
1616 \c db 'h','e','l','l','o' ; equivalent character constants
1618 And the following are also equivalent:
1620 \c dd 'ninechars' ; doubleword string constant
1621 \c dd 'nine','char','s' ; becomes three doublewords
1622 \c db 'ninechars',0,0,0 ; and really looks like this
1624 Note that when used in a string-supporting context, quoted strings are
1625 treated as a string constants even if they are short enough to be a
1626 character constant, because otherwise \c{db 'ab'} would have the same
1627 effect as \c{db 'a'}, which would be silly. Similarly, three-character
1628 or four-character constants are treated as strings when they are
1629 operands to \c{DW}, and so forth.
1631 \S{unicode} \I{UTF-16}\I{UTF-32}\i{Unicode} Strings
1633 The special operators \i\c{__utf16__}, \i\c{__utf16le__},
1634 \i\c{__utf16be__}, \i\c{__utf32__}, \i\c{__utf32le__} and
1635 \i\c{__utf32be__} allows definition of Unicode strings. They take a
1636 string in UTF-8 format and converts it to UTF-16 or UTF-32,
1637 respectively. Unless the \c{be} forms are specified, the output is
1642 \c %define u(x) __utf16__(x)
1643 \c %define w(x) __utf32__(x)
1645 \c dw u('C:\WINDOWS'), 0 ; Pathname in UTF-16
1646 \c dd w(`A + B = \u206a`), 0 ; String in UTF-32
1648 The UTF operators can be applied either to strings passed to the
1649 \c{DB} family instructions, or to character constants in an expression
1652 \S{fltconst} \I{floating-point, constants}Floating-Point Constants
1654 \i{Floating-point} constants are acceptable only as arguments to
1655 \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, and \i\c{DO}, or as
1656 arguments to the special operators \i\c{__float8__},
1657 \i\c{__float16__}, \i\c{__float32__}, \i\c{__float64__},
1658 \i\c{__float80m__}, \i\c{__float80e__}, \i\c{__float128l__}, and
1659 \i\c{__float128h__}.
1661 Floating-point constants are expressed in the traditional form:
1662 digits, then a period, then optionally more digits, then optionally an
1663 \c{E} followed by an exponent. The period is mandatory, so that NASM
1664 can distinguish between \c{dd 1}, which declares an integer constant,
1665 and \c{dd 1.0} which declares a floating-point constant.
1667 NASM also support C99-style hexadecimal floating-point: \c{0x},
1668 hexadecimal digits, period, optionally more hexadeximal digits, then
1669 optionally a \c{P} followed by a \e{binary} (not hexadecimal) exponent
1670 in decimal notation. As an extension, NASM additionally supports the
1671 \c{0h} and \c{$} prefixes for hexadecimal, as well binary and octal
1672 floating-point, using the \c{0b} or \c{0y} and \c{0o} or \c{0q}
1673 prefixes, respectively.
1675 Underscores to break up groups of digits are permitted in
1676 floating-point constants as well.
1680 \c db -0.2 ; "Quarter precision"
1681 \c dw -0.5 ; IEEE 754r/SSE5 half precision
1682 \c dd 1.2 ; an easy one
1683 \c dd 1.222_222_222 ; underscores are permitted
1684 \c dd 0x1p+2 ; 1.0x2^2 = 4.0
1685 \c dq 0x1p+32 ; 1.0x2^32 = 4 294 967 296.0
1686 \c dq 1.e10 ; 10 000 000 000.0
1687 \c dq 1.e+10 ; synonymous with 1.e10
1688 \c dq 1.e-10 ; 0.000 000 000 1
1689 \c dt 3.141592653589793238462 ; pi
1690 \c do 1.e+4000 ; IEEE 754r quad precision
1692 The 8-bit "quarter-precision" floating-point format is
1693 sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This
1694 appears to be the most frequently used 8-bit floating-point format,
1695 although it is not covered by any formal standard. This is sometimes
1696 called a "\i{minifloat}."
1698 The special operators are used to produce floating-point numbers in
1699 other contexts. They produce the binary representation of a specific
1700 floating-point number as an integer, and can use anywhere integer
1701 constants are used in an expression. \c{__float80m__} and
1702 \c{__float80e__} produce the 64-bit mantissa and 16-bit exponent of an
1703 80-bit floating-point number, and \c{__float128l__} and
1704 \c{__float128h__} produce the lower and upper 64-bit halves of a 128-bit
1705 floating-point number, respectively.
1709 \c mov rax,__float64__(3.141592653589793238462)
1711 ... would assign the binary representation of pi as a 64-bit floating
1712 point number into \c{RAX}. This is exactly equivalent to:
1714 \c mov rax,0x400921fb54442d18
1716 NASM cannot do compile-time arithmetic on floating-point constants.
1717 This is because NASM is designed to be portable - although it always
1718 generates code to run on x86 processors, the assembler itself can
1719 run on any system with an ANSI C compiler. Therefore, the assembler
1720 cannot guarantee the presence of a floating-point unit capable of
1721 handling the \i{Intel number formats}, and so for NASM to be able to
1722 do floating arithmetic it would have to include its own complete set
1723 of floating-point routines, which would significantly increase the
1724 size of the assembler for very little benefit.
1726 The special tokens \i\c{__Infinity__}, \i\c{__QNaN__} (or
1727 \i\c{__NaN__}) and \i\c{__SNaN__} can be used to generate
1728 \I{infinity}infinities, quiet \i{NaN}s, and signalling NaNs,
1729 respectively. These are normally used as macros:
1731 \c %define Inf __Infinity__
1732 \c %define NaN __QNaN__
1734 \c dq +1.5, -Inf, NaN ; Double-precision constants
1736 The \c{%use fp} standard macro package contains a set of convenience
1737 macros. See \k{pkg_fp}.
1739 \S{bcdconst} \I{floating-point, packed BCD constants}Packed BCD Constants
1741 x87-style packed BCD constants can be used in the same contexts as
1742 80-bit floating-point numbers. They are suffixed with \c{p} or
1743 prefixed with \c{0p}, and can include up to 18 decimal digits.
1745 As with other numeric constants, underscores can be used to separate
1750 \c dt 12_345_678_901_245_678p
1751 \c dt -12_345_678_901_245_678p
1756 \H{expr} \i{Expressions}
1758 Expressions in NASM are similar in syntax to those in C. Expressions
1759 are evaluated as 64-bit integers which are then adjusted to the
1762 NASM supports two special tokens in expressions, allowing
1763 calculations to involve the current assembly position: the
1764 \I{$, here}\c{$} and \i\c{$$} tokens. \c{$} evaluates to the assembly
1765 position at the beginning of the line containing the expression; so
1766 you can code an \i{infinite loop} using \c{JMP $}. \c{$$} evaluates
1767 to the beginning of the current section; so you can tell how far
1768 into the section you are by using \c{($-$$)}.
1770 The arithmetic \i{operators} provided by NASM are listed here, in
1771 increasing order of \i{precedence}.
1774 \S{expor} \i\c{|}: \i{Bitwise OR} Operator
1776 The \c{|} operator gives a bitwise OR, exactly as performed by the
1777 \c{OR} machine instruction. Bitwise OR is the lowest-priority
1778 arithmetic operator supported by NASM.
1781 \S{expxor} \i\c{^}: \i{Bitwise XOR} Operator
1783 \c{^} provides the bitwise XOR operation.
1786 \S{expand} \i\c{&}: \i{Bitwise AND} Operator
1788 \c{&} provides the bitwise AND operation.
1791 \S{expshift} \i\c{<<} and \i\c{>>}: \i{Bit Shift} Operators
1793 \c{<<} gives a bit-shift to the left, just as it does in C. So \c{5<<3}
1794 evaluates to 5 times 8, or 40. \c{>>} gives a bit-shift to the
1795 right; in NASM, such a shift is \e{always} unsigned, so that
1796 the bits shifted in from the left-hand end are filled with zero
1797 rather than a sign-extension of the previous highest bit.
1800 \S{expplmi} \I{+ opaddition}\c{+} and \I{- opsubtraction}\c{-}:
1801 \i{Addition} and \i{Subtraction} Operators
1803 The \c{+} and \c{-} operators do perfectly ordinary addition and
1807 \S{expmul} \i\c{*}, \i\c{/}, \i\c{//}, \i\c{%} and \i\c{%%}:
1808 \i{Multiplication} and \i{Division}
1810 \c{*} is the multiplication operator. \c{/} and \c{//} are both
1811 division operators: \c{/} is \i{unsigned division} and \c{//} is
1812 \i{signed division}. Similarly, \c{%} and \c{%%} provide \I{unsigned
1813 modulo}\I{modulo operators}unsigned and
1814 \i{signed modulo} operators respectively.
1816 NASM, like ANSI C, provides no guarantees about the sensible
1817 operation of the signed modulo operator.
1819 Since the \c{%} character is used extensively by the macro
1820 \i{preprocessor}, you should ensure that both the signed and unsigned
1821 modulo operators are followed by white space wherever they appear.
1824 \S{expmul} \i{Unary Operators}
1826 The highest-priority operators in NASM's expression grammar are those
1827 which only apply to one argument. These are \I{+ opunary}\c{+}, \I{-
1828 opunary}\c{-}, \i\c{~}, \I{! opunary}\c{!}, \i\c{SEG}, and the
1829 \i{integer functions} operators.
1831 \c{-} negates its operand, \c{+} does nothing (it's provided for
1832 symmetry with \c{-}), \c{~} computes the \i{one's complement} of its
1833 operand, \c{!} is the \i{logical negation} operator.
1835 \c{SEG} provides the \i{segment address}
1836 of its operand (explained in more detail in \k{segwrt}).
1838 A set of additional operators with leading and trailing double
1839 underscores are used to implement the integer functions of the
1840 \c{ifunc} macro package, see \k{pkg_ifunc}.
1843 \H{segwrt} \i\c{SEG} and \i\c{WRT}
1845 When writing large 16-bit programs, which must be split into
1846 multiple \i{segments}, it is often necessary to be able to refer to
1847 the \I{segment address}segment part of the address of a symbol. NASM
1848 supports the \c{SEG} operator to perform this function.
1850 The \c{SEG} operator returns the \i\e{preferred} segment base of a
1851 symbol, defined as the segment base relative to which the offset of
1852 the symbol makes sense. So the code
1854 \c mov ax,seg symbol
1858 will load \c{ES:BX} with a valid pointer to the symbol \c{symbol}.
1860 Things can be more complex than this: since 16-bit segments and
1861 \i{groups} may \I{overlapping segments}overlap, you might occasionally
1862 want to refer to some symbol using a different segment base from the
1863 preferred one. NASM lets you do this, by the use of the \c{WRT}
1864 (With Reference To) keyword. So you can do things like
1866 \c mov ax,weird_seg ; weird_seg is a segment base
1868 \c mov bx,symbol wrt weird_seg
1870 to load \c{ES:BX} with a different, but functionally equivalent,
1871 pointer to the symbol \c{symbol}.
1873 NASM supports far (inter-segment) calls and jumps by means of the
1874 syntax \c{call segment:offset}, where \c{segment} and \c{offset}
1875 both represent immediate values. So to call a far procedure, you
1876 could code either of
1878 \c call (seg procedure):procedure
1879 \c call weird_seg:(procedure wrt weird_seg)
1881 (The parentheses are included for clarity, to show the intended
1882 parsing of the above instructions. They are not necessary in
1885 NASM supports the syntax \I\c{CALL FAR}\c{call far procedure} as a
1886 synonym for the first of the above usages. \c{JMP} works identically
1887 to \c{CALL} in these examples.
1889 To declare a \i{far pointer} to a data item in a data segment, you
1892 \c dw symbol, seg symbol
1894 NASM supports no convenient synonym for this, though you can always
1895 invent one using the macro processor.
1898 \H{strict} \i\c{STRICT}: Inhibiting Optimization
1900 When assembling with the optimizer set to level 2 or higher (see
1901 \k{opt-O}), NASM will use size specifiers (\c{BYTE}, \c{WORD},
1902 \c{DWORD}, \c{QWORD}, \c{TWORD}, \c{OWORD}, \c{YWORD} or \c{ZWORD}),
1903 but will give them the smallest possible size. The keyword \c{STRICT}
1904 can be used to inhibit optimization and force a particular operand to
1905 be emitted in the specified size. For example, with the optimizer on,
1906 and in \c{BITS 16} mode,
1910 is encoded in three bytes \c{66 6A 21}, whereas
1912 \c push strict dword 33
1914 is encoded in six bytes, with a full dword immediate operand \c{66 68
1917 With the optimizer off, the same code (six bytes) is generated whether
1918 the \c{STRICT} keyword was used or not.
1921 \H{crit} \i{Critical Expressions}
1923 Although NASM has an optional multi-pass optimizer, there are some
1924 expressions which must be resolvable on the first pass. These are
1925 called \e{Critical Expressions}.
1927 The first pass is used to determine the size of all the assembled
1928 code and data, so that the second pass, when generating all the
1929 code, knows all the symbol addresses the code refers to. So one
1930 thing NASM can't handle is code whose size depends on the value of a
1931 symbol declared after the code in question. For example,
1933 \c times (label-$) db 0
1934 \c label: db 'Where am I?'
1936 The argument to \i\c{TIMES} in this case could equally legally
1937 evaluate to anything at all; NASM will reject this example because
1938 it cannot tell the size of the \c{TIMES} line when it first sees it.
1939 It will just as firmly reject the slightly \I{paradox}paradoxical
1942 \c times (label-$+1) db 0
1943 \c label: db 'NOW where am I?'
1945 in which \e{any} value for the \c{TIMES} argument is by definition
1948 NASM rejects these examples by means of a concept called a
1949 \e{critical expression}, which is defined to be an expression whose
1950 value is required to be computable in the first pass, and which must
1951 therefore depend only on symbols defined before it. The argument to
1952 the \c{TIMES} prefix is a critical expression.
1954 \H{locallab} \i{Local Labels}
1956 NASM gives special treatment to symbols beginning with a \i{period}.
1957 A label beginning with a single period is treated as a \e{local}
1958 label, which means that it is associated with the previous non-local
1959 label. So, for example:
1961 \c label1 ; some code
1969 \c label2 ; some code
1977 In the above code fragment, each \c{JNE} instruction jumps to the
1978 line immediately before it, because the two definitions of \c{.loop}
1979 are kept separate by virtue of each being associated with the
1980 previous non-local label.
1982 This form of local label handling is borrowed from the old Amiga
1983 assembler \i{DevPac}; however, NASM goes one step further, in
1984 allowing access to local labels from other parts of the code. This
1985 is achieved by means of \e{defining} a local label in terms of the
1986 previous non-local label: the first definition of \c{.loop} above is
1987 really defining a symbol called \c{label1.loop}, and the second
1988 defines a symbol called \c{label2.loop}. So, if you really needed
1991 \c label3 ; some more code
1996 Sometimes it is useful - in a macro, for instance - to be able to
1997 define a label which can be referenced from anywhere but which
1998 doesn't interfere with the normal local-label mechanism. Such a
1999 label can't be non-local because it would interfere with subsequent
2000 definitions of, and references to, local labels; and it can't be
2001 local because the macro that defined it wouldn't know the label's
2002 full name. NASM therefore introduces a third type of label, which is
2003 probably only useful in macro definitions: if a label begins with
2004 the \I{label prefix}special prefix \i\c{..@}, then it does nothing
2005 to the local label mechanism. So you could code
2007 \c label1: ; a non-local label
2008 \c .local: ; this is really label1.local
2009 \c ..@foo: ; this is a special symbol
2010 \c label2: ; another non-local label
2011 \c .local: ; this is really label2.local
2013 \c jmp ..@foo ; this will jump three lines up
2015 NASM has the capacity to define other special symbols beginning with
2016 a double period: for example, \c{..start} is used to specify the
2017 entry point in the \c{obj} output format (see \k{dotdotstart}),
2018 \c{..imagebase} is used to find out the offset from a base address
2019 of the current image in the \c{win64} output format (see \k{win64pic}).
2020 So just keep in mind that symbols beginning with a double period are
2024 \C{preproc} The NASM \i{Preprocessor}
2026 NASM contains a powerful \i{macro processor}, which supports
2027 conditional assembly, multi-level file inclusion, two forms of macro
2028 (single-line and multi-line), and a `context stack' mechanism for
2029 extra macro power. Preprocessor directives all begin with a \c{%}
2032 The preprocessor collapses all lines which end with a backslash (\\)
2033 character into a single line. Thus:
2035 \c %define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \\
2038 will work like a single-line macro without the backslash-newline
2041 \H{slmacro} \i{Single-Line Macros}
2043 \S{define} The Normal Way: \I\c{%idefine}\i\c{%define}
2045 Single-line macros are defined using the \c{%define} preprocessor
2046 directive. The definitions work in a similar way to C; so you can do
2049 \c %define ctrl 0x1F &
2050 \c %define param(a,b) ((a)+(a)*(b))
2052 \c mov byte [param(2,ebx)], ctrl 'D'
2054 which will expand to
2056 \c mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
2058 When the expansion of a single-line macro contains tokens which
2059 invoke another macro, the expansion is performed at invocation time,
2060 not at definition time. Thus the code
2062 \c %define a(x) 1+b(x)
2067 will evaluate in the expected way to \c{mov ax,1+2*8}, even though
2068 the macro \c{b} wasn't defined at the time of definition of \c{a}.
2070 Macros defined with \c{%define} are \i{case sensitive}: after
2071 \c{%define foo bar}, only \c{foo} will expand to \c{bar}: \c{Foo} or
2072 \c{FOO} will not. By using \c{%idefine} instead of \c{%define} (the
2073 `i' stands for `insensitive') you can define all the case variants
2074 of a macro at once, so that \c{%idefine foo bar} would cause
2075 \c{foo}, \c{Foo}, \c{FOO}, \c{fOO} and so on all to expand to
2078 There is a mechanism which detects when a macro call has occurred as
2079 a result of a previous expansion of the same macro, to guard against
2080 \i{circular references} and infinite loops. If this happens, the
2081 preprocessor will only expand the first occurrence of the macro.
2084 \c %define a(x) 1+a(x)
2088 the macro \c{a(3)} will expand once, becoming \c{1+a(3)}, and will
2089 then expand no further. This behaviour can be useful: see \k{32c}
2090 for an example of its use.
2092 You can \I{overloading, single-line macros}overload single-line
2093 macros: if you write
2095 \c %define foo(x) 1+x
2096 \c %define foo(x,y) 1+x*y
2098 the preprocessor will be able to handle both types of macro call,
2099 by counting the parameters you pass; so \c{foo(3)} will become
2100 \c{1+3} whereas \c{foo(ebx,2)} will become \c{1+ebx*2}. However, if
2105 then no other definition of \c{foo} will be accepted: a macro with
2106 no parameters prohibits the definition of the same name as a macro
2107 \e{with} parameters, and vice versa.
2109 This doesn't prevent single-line macros being \e{redefined}: you can
2110 perfectly well define a macro with
2114 and then re-define it later in the same source file with
2118 Then everywhere the macro \c{foo} is invoked, it will be expanded
2119 according to the most recent definition. This is particularly useful
2120 when defining single-line macros with \c{%assign} (see \k{assign}).
2122 You can \i{pre-define} single-line macros using the `-d' option on
2123 the NASM command line: see \k{opt-d}.
2126 \S{xdefine} Resolving \c{%define}: \I\c{%ixdefine}\i\c{%xdefine}
2128 To have a reference to an embedded single-line macro resolved at the
2129 time that the embedding macro is \e{defined}, as opposed to when the
2130 embedding macro is \e{expanded}, you need a different mechanism to the
2131 one offered by \c{%define}. The solution is to use \c{%xdefine}, or
2132 it's \I{case sensitive}case-insensitive counterpart \c{%ixdefine}.
2134 Suppose you have the following code:
2137 \c %define isFalse isTrue
2146 In this case, \c{val1} is equal to 0, and \c{val2} is equal to 1.
2147 This is because, when a single-line macro is defined using
2148 \c{%define}, it is expanded only when it is called. As \c{isFalse}
2149 expands to \c{isTrue}, the expansion will be the current value of
2150 \c{isTrue}. The first time it is called that is 0, and the second
2153 If you wanted \c{isFalse} to expand to the value assigned to the
2154 embedded macro \c{isTrue} at the time that \c{isFalse} was defined,
2155 you need to change the above code to use \c{%xdefine}.
2157 \c %xdefine isTrue 1
2158 \c %xdefine isFalse isTrue
2159 \c %xdefine isTrue 0
2163 \c %xdefine isTrue 1
2167 Now, each time that \c{isFalse} is called, it expands to 1,
2168 as that is what the embedded macro \c{isTrue} expanded to at
2169 the time that \c{isFalse} was defined.
2172 \S{indmacro} \i{Macro Indirection}: \I\c{%[}\c{%[...]}
2174 The \c{%[...]} construct can be used to expand macros in contexts
2175 where macro expansion would otherwise not occur, including in the
2176 names other macros. For example, if you have a set of macros named
2177 \c{Foo16}, \c{Foo32} and \c{Foo64}, you could write:
2179 \c mov ax,Foo%[__BITS__] ; The Foo value
2181 to use the builtin macro \c{__BITS__} (see \k{bitsm}) to automatically
2182 select between them. Similarly, the two statements:
2184 \c %xdefine Bar Quux ; Expands due to %xdefine
2185 \c %define Bar %[Quux] ; Expands due to %[...]
2187 have, in fact, exactly the same effect.
2189 \c{%[...]} concatenates to adjacent tokens in the same way that
2190 multi-line macro parameters do, see \k{concat} for details.
2193 \S{concat%+} Concatenating Single Line Macro Tokens: \i\c{%+}
2195 Individual tokens in single line macros can be concatenated, to produce
2196 longer tokens for later processing. This can be useful if there are
2197 several similar macros that perform similar functions.
2199 Please note that a space is required after \c{%+}, in order to
2200 disambiguate it from the syntax \c{%+1} used in multiline macros.
2202 As an example, consider the following:
2204 \c %define BDASTART 400h ; Start of BIOS data area
2206 \c struc tBIOSDA ; its structure
2212 Now, if we need to access the elements of tBIOSDA in different places,
2215 \c mov ax,BDASTART + tBIOSDA.COM1addr
2216 \c mov bx,BDASTART + tBIOSDA.COM2addr
2218 This will become pretty ugly (and tedious) if used in many places, and
2219 can be reduced in size significantly by using the following macro:
2221 \c ; Macro to access BIOS variables by their names (from tBDA):
2223 \c %define BDA(x) BDASTART + tBIOSDA. %+ x
2225 Now the above code can be written as:
2227 \c mov ax,BDA(COM1addr)
2228 \c mov bx,BDA(COM2addr)
2230 Using this feature, we can simplify references to a lot of macros (and,
2231 in turn, reduce typing errors).
2234 \S{selfref%?} The Macro Name Itself: \i\c{%?} and \i\c{%??}
2236 The special symbols \c{%?} and \c{%??} can be used to reference the
2237 macro name itself inside a macro expansion, this is supported for both
2238 single-and multi-line macros. \c{%?} refers to the macro name as
2239 \e{invoked}, whereas \c{%??} refers to the macro name as
2240 \e{declared}. The two are always the same for case-sensitive
2241 macros, but for case-insensitive macros, they can differ.
2245 \c %idefine Foo mov %?,%??
2257 \c %idefine keyword $%?
2259 can be used to make a keyword "disappear", for example in case a new
2260 instruction has been used as a label in older code. For example:
2262 \c %idefine pause $%? ; Hide the PAUSE instruction
2265 \S{undef} Undefining Single-Line Macros: \i\c{%undef}
2267 Single-line macros can be removed with the \c{%undef} directive. For
2268 example, the following sequence:
2275 will expand to the instruction \c{mov eax, foo}, since after
2276 \c{%undef} the macro \c{foo} is no longer defined.
2278 Macros that would otherwise be pre-defined can be undefined on the
2279 command-line using the `-u' option on the NASM command line: see
2283 \S{assign} \i{Preprocessor Variables}: \i\c{%assign}
2285 An alternative way to define single-line macros is by means of the
2286 \c{%assign} command (and its \I{case sensitive}case-insensitive
2287 counterpart \i\c{%iassign}, which differs from \c{%assign} in
2288 exactly the same way that \c{%idefine} differs from \c{%define}).
2290 \c{%assign} is used to define single-line macros which take no
2291 parameters and have a numeric value. This value can be specified in
2292 the form of an expression, and it will be evaluated once, when the
2293 \c{%assign} directive is processed.
2295 Like \c{%define}, macros defined using \c{%assign} can be re-defined
2296 later, so you can do things like
2300 to increment the numeric value of a macro.
2302 \c{%assign} is useful for controlling the termination of \c{%rep}
2303 preprocessor loops: see \k{rep} for an example of this. Another
2304 use for \c{%assign} is given in \k{16c} and \k{32c}.
2306 The expression passed to \c{%assign} is a \i{critical expression}
2307 (see \k{crit}), and must also evaluate to a pure number (rather than
2308 a relocatable reference such as a code or data address, or anything
2309 involving a register).
2312 \S{defstr} Defining Strings: \I\c{%idefstr}\i\c{%defstr}
2314 \c{%defstr}, and its case-insensitive counterpart \c{%idefstr}, define
2315 or redefine a single-line macro without parameters but converts the
2316 entire right-hand side, after macro expansion, to a quoted string
2321 \c %defstr test TEST
2325 \c %define test 'TEST'
2327 This can be used, for example, with the \c{%!} construct (see
2330 \c %defstr PATH %!PATH ; The operating system PATH variable
2333 \S{deftok} Defining Tokens: \I\c{%ideftok}\i\c{%deftok}
2335 \c{%deftok}, and its case-insensitive counterpart \c{%ideftok}, define
2336 or redefine a single-line macro without parameters but converts the
2337 second parameter, after string conversion, to a sequence of tokens.
2341 \c %deftok test 'TEST'
2345 \c %define test TEST
2348 \H{strlen} \i{String Manipulation in Macros}
2350 It's often useful to be able to handle strings in macros. NASM
2351 supports a few simple string handling macro operators from which
2352 more complex operations can be constructed.
2354 All the string operators define or redefine a value (either a string
2355 or a numeric value) to a single-line macro. When producing a string
2356 value, it may change the style of quoting of the input string or
2357 strings, and possibly use \c{\\}-escapes inside \c{`}-quoted strings.
2359 \S{strcat} \i{Concatenating Strings}: \i\c{%strcat}
2361 The \c{%strcat} operator concatenates quoted strings and assign them to
2362 a single-line macro.
2366 \c %strcat alpha "Alpha: ", '12" screen'
2368 ... would assign the value \c{'Alpha: 12" screen'} to \c{alpha}.
2371 \c %strcat beta '"foo"\', "'bar'"
2373 ... would assign the value \c{`"foo"\\\\'bar'`} to \c{beta}.
2375 The use of commas to separate strings is permitted but optional.
2378 \S{strlen} \i{String Length}: \i\c{%strlen}
2380 The \c{%strlen} operator assigns the length of a string to a macro.
2383 \c %strlen charcnt 'my string'
2385 In this example, \c{charcnt} would receive the value 9, just as
2386 if an \c{%assign} had been used. In this example, \c{'my string'}
2387 was a literal string but it could also have been a single-line
2388 macro that expands to a string, as in the following example:
2390 \c %define sometext 'my string'
2391 \c %strlen charcnt sometext
2393 As in the first case, this would result in \c{charcnt} being
2394 assigned the value of 9.
2397 \S{substr} \i{Extracting Substrings}: \i\c{%substr}
2399 Individual letters or substrings in strings can be extracted using the
2400 \c{%substr} operator. An example of its use is probably more useful
2401 than the description:
2403 \c %substr mychar 'xyzw' 1 ; equivalent to %define mychar 'x'
2404 \c %substr mychar 'xyzw' 2 ; equivalent to %define mychar 'y'
2405 \c %substr mychar 'xyzw' 3 ; equivalent to %define mychar 'z'
2406 \c %substr mychar 'xyzw' 2,2 ; equivalent to %define mychar 'yz'
2407 \c %substr mychar 'xyzw' 2,-1 ; equivalent to %define mychar 'yzw'
2408 \c %substr mychar 'xyzw' 2,-2 ; equivalent to %define mychar 'yz'
2410 As with \c{%strlen} (see \k{strlen}), the first parameter is the
2411 single-line macro to be created and the second is the string. The
2412 third parameter specifies the first character to be selected, and the
2413 optional fourth parameter preceeded by comma) is the length. Note
2414 that the first index is 1, not 0 and the last index is equal to the
2415 value that \c{%strlen} would assign given the same string. Index
2416 values out of range result in an empty string. A negative length
2417 means "until N-1 characters before the end of string", i.e. \c{-1}
2418 means until end of string, \c{-2} until one character before, etc.
2421 \H{mlmacro} \i{Multi-Line Macros}: \I\c{%imacro}\i\c{%macro}
2423 Multi-line macros are much more like the type of macro seen in MASM
2424 and TASM: a multi-line macro definition in NASM looks something like
2427 \c %macro prologue 1
2435 This defines a C-like function prologue as a macro: so you would
2436 invoke the macro with a call such as
2438 \c myfunc: prologue 12
2440 which would expand to the three lines of code
2446 The number \c{1} after the macro name in the \c{%macro} line defines
2447 the number of parameters the macro \c{prologue} expects to receive.
2448 The use of \c{%1} inside the macro definition refers to the first
2449 parameter to the macro call. With a macro taking more than one
2450 parameter, subsequent parameters would be referred to as \c{%2},
2453 Multi-line macros, like single-line macros, are \i{case-sensitive},
2454 unless you define them using the alternative directive \c{%imacro}.
2456 If you need to pass a comma as \e{part} of a parameter to a
2457 multi-line macro, you can do that by enclosing the entire parameter
2458 in \I{braces, around macro parameters}braces. So you could code
2467 \c silly 'a', letter_a ; letter_a: db 'a'
2468 \c silly 'ab', string_ab ; string_ab: db 'ab'
2469 \c silly {13,10}, crlf ; crlf: db 13,10
2472 \S{mlmacover} Overloading Multi-Line Macros\I{overloading, multi-line macros}
2474 As with single-line macros, multi-line macros can be overloaded by
2475 defining the same macro name several times with different numbers of
2476 parameters. This time, no exception is made for macros with no
2477 parameters at all. So you could define
2479 \c %macro prologue 0
2486 to define an alternative form of the function prologue which
2487 allocates no local stack space.
2489 Sometimes, however, you might want to `overload' a machine
2490 instruction; for example, you might want to define
2499 so that you could code
2501 \c push ebx ; this line is not a macro call
2502 \c push eax,ecx ; but this one is
2504 Ordinarily, NASM will give a warning for the first of the above two
2505 lines, since \c{push} is now defined to be a macro, and is being
2506 invoked with a number of parameters for which no definition has been
2507 given. The correct code will still be generated, but the assembler
2508 will give a warning. This warning can be disabled by the use of the
2509 \c{-w-macro-params} command-line option (see \k{opt-w}).
2512 \S{maclocal} \i{Macro-Local Labels}
2514 NASM allows you to define labels within a multi-line macro
2515 definition in such a way as to make them local to the macro call: so
2516 calling the same macro multiple times will use a different label
2517 each time. You do this by prefixing \i\c{%%} to the label name. So
2518 you can invent an instruction which executes a \c{RET} if the \c{Z}
2519 flag is set by doing this:
2529 You can call this macro as many times as you want, and every time
2530 you call it NASM will make up a different `real' name to substitute
2531 for the label \c{%%skip}. The names NASM invents are of the form
2532 \c{..@2345.skip}, where the number 2345 changes with every macro
2533 call. The \i\c{..@} prefix prevents macro-local labels from
2534 interfering with the local label mechanism, as described in
2535 \k{locallab}. You should avoid defining your own labels in this form
2536 (the \c{..@} prefix, then a number, then another period) in case
2537 they interfere with macro-local labels.
2540 \S{mlmacgre} \i{Greedy Macro Parameters}
2542 Occasionally it is useful to define a macro which lumps its entire
2543 command line into one parameter definition, possibly after
2544 extracting one or two smaller parameters from the front. An example
2545 might be a macro to write a text string to a file in MS-DOS, where
2546 you might want to be able to write
2548 \c writefile [filehandle],"hello, world",13,10
2550 NASM allows you to define the last parameter of a macro to be
2551 \e{greedy}, meaning that if you invoke the macro with more
2552 parameters than it expects, all the spare parameters get lumped into
2553 the last defined one along with the separating commas. So if you
2556 \c %macro writefile 2+
2562 \c mov cx,%%endstr-%%str
2569 then the example call to \c{writefile} above will work as expected:
2570 the text before the first comma, \c{[filehandle]}, is used as the
2571 first macro parameter and expanded when \c{%1} is referred to, and
2572 all the subsequent text is lumped into \c{%2} and placed after the
2575 The greedy nature of the macro is indicated to NASM by the use of
2576 the \I{+ modifier}\c{+} sign after the parameter count on the
2579 If you define a greedy macro, you are effectively telling NASM how
2580 it should expand the macro given \e{any} number of parameters from
2581 the actual number specified up to infinity; in this case, for
2582 example, NASM now knows what to do when it sees a call to
2583 \c{writefile} with 2, 3, 4 or more parameters. NASM will take this
2584 into account when overloading macros, and will not allow you to
2585 define another form of \c{writefile} taking 4 parameters (for
2588 Of course, the above macro could have been implemented as a
2589 non-greedy macro, in which case the call to it would have had to
2592 \c writefile [filehandle], {"hello, world",13,10}
2594 NASM provides both mechanisms for putting \i{commas in macro
2595 parameters}, and you choose which one you prefer for each macro
2598 See \k{sectmac} for a better way to write the above macro.
2600 \S{mlmacrange} \i{Macro Parameters Range}
2602 NASM allows you to expand parameters via special construction \c{%\{x:y\}}
2603 where \c{x} is the first parameter index and \c{y} is the last. Any index can
2604 be either negative or positive but must never be zero.
2614 expands to \c{3,4,5} range.
2616 Even more, the parameters can be reversed so that
2624 expands to \c{5,4,3} range.
2626 But even this is not the last. The parameters can be addressed via negative
2627 indices so NASM will count them reversed. The ones who know Python may see
2636 expands to \c{6,5,4} range.
2638 Note that NASM uses \i{comma} to separate parameters being expanded.
2640 By the way, here is a trick - you might use the index \c{%{-1:-1}}
2641 which gives you the \i{last} argument passed to a macro.
2643 \S{mlmacdef} \i{Default Macro Parameters}
2645 NASM also allows you to define a multi-line macro with a \e{range}
2646 of allowable parameter counts. If you do this, you can specify
2647 defaults for \i{omitted parameters}. So, for example:
2649 \c %macro die 0-1 "Painful program death has occurred."
2657 This macro (which makes use of the \c{writefile} macro defined in
2658 \k{mlmacgre}) can be called with an explicit error message, which it
2659 will display on the error output stream before exiting, or it can be
2660 called with no parameters, in which case it will use the default
2661 error message supplied in the macro definition.
2663 In general, you supply a minimum and maximum number of parameters
2664 for a macro of this type; the minimum number of parameters are then
2665 required in the macro call, and then you provide defaults for the
2666 optional ones. So if a macro definition began with the line
2668 \c %macro foobar 1-3 eax,[ebx+2]
2670 then it could be called with between one and three parameters, and
2671 \c{%1} would always be taken from the macro call. \c{%2}, if not
2672 specified by the macro call, would default to \c{eax}, and \c{%3} if
2673 not specified would default to \c{[ebx+2]}.
2675 You can provide extra information to a macro by providing
2676 too many default parameters:
2678 \c %macro quux 1 something
2680 This will trigger a warning by default; see \k{opt-w} for
2682 When \c{quux} is invoked, it receives not one but two parameters.
2683 \c{something} can be referred to as \c{%2}. The difference
2684 between passing \c{something} this way and writing \c{something}
2685 in the macro body is that with this way \c{something} is evaluated
2686 when the macro is defined, not when it is expanded.
2688 You may omit parameter defaults from the macro definition, in which
2689 case the parameter default is taken to be blank. This can be useful
2690 for macros which can take a variable number of parameters, since the
2691 \i\c{%0} token (see \k{percent0}) allows you to determine how many
2692 parameters were really passed to the macro call.
2694 This defaulting mechanism can be combined with the greedy-parameter
2695 mechanism; so the \c{die} macro above could be made more powerful,
2696 and more useful, by changing the first line of the definition to
2698 \c %macro die 0-1+ "Painful program death has occurred.",13,10
2700 The maximum parameter count can be infinite, denoted by \c{*}. In
2701 this case, of course, it is impossible to provide a \e{full} set of
2702 default parameters. Examples of this usage are shown in \k{rotate}.
2705 \S{percent0} \i\c{%0}: \I{counting macro parameters}Macro Parameter Counter
2707 The parameter reference \c{%0} will return a numeric constant giving the
2708 number of parameters received, that is, if \c{%0} is n then \c{%}n is the
2709 last parameter. \c{%0} is mostly useful for macros that can take a variable
2710 number of parameters. It can be used as an argument to \c{%rep}
2711 (see \k{rep}) in order to iterate through all the parameters of a macro.
2712 Examples are given in \k{rotate}.
2715 \S{percent00} \i\c{%00}: \I{label preceeding macro}Label Preceeding Macro
2717 \c{%00} will return the label preceeding the macro invocation, if any. The
2718 label must be on the same line as the macro invocation, may be a local label
2719 (see \k{locallab}), and need not end in a colon.
2722 \S{rotate} \i\c{%rotate}: \i{Rotating Macro Parameters}
2724 Unix shell programmers will be familiar with the \I{shift
2725 command}\c{shift} shell command, which allows the arguments passed
2726 to a shell script (referenced as \c{$1}, \c{$2} and so on) to be
2727 moved left by one place, so that the argument previously referenced
2728 as \c{$2} becomes available as \c{$1}, and the argument previously
2729 referenced as \c{$1} is no longer available at all.
2731 NASM provides a similar mechanism, in the form of \c{%rotate}. As
2732 its name suggests, it differs from the Unix \c{shift} in that no
2733 parameters are lost: parameters rotated off the left end of the
2734 argument list reappear on the right, and vice versa.
2736 \c{%rotate} is invoked with a single numeric argument (which may be
2737 an expression). The macro parameters are rotated to the left by that
2738 many places. If the argument to \c{%rotate} is negative, the macro
2739 parameters are rotated to the right.
2741 \I{iterating over macro parameters}So a pair of macros to save and
2742 restore a set of registers might work as follows:
2744 \c %macro multipush 1-*
2753 This macro invokes the \c{PUSH} instruction on each of its arguments
2754 in turn, from left to right. It begins by pushing its first
2755 argument, \c{%1}, then invokes \c{%rotate} to move all the arguments
2756 one place to the left, so that the original second argument is now
2757 available as \c{%1}. Repeating this procedure as many times as there
2758 were arguments (achieved by supplying \c{%0} as the argument to
2759 \c{%rep}) causes each argument in turn to be pushed.
2761 Note also the use of \c{*} as the maximum parameter count,
2762 indicating that there is no upper limit on the number of parameters
2763 you may supply to the \i\c{multipush} macro.
2765 It would be convenient, when using this macro, to have a \c{POP}
2766 equivalent, which \e{didn't} require the arguments to be given in
2767 reverse order. Ideally, you would write the \c{multipush} macro
2768 call, then cut-and-paste the line to where the pop needed to be
2769 done, and change the name of the called macro to \c{multipop}, and
2770 the macro would take care of popping the registers in the opposite
2771 order from the one in which they were pushed.
2773 This can be done by the following definition:
2775 \c %macro multipop 1-*
2784 This macro begins by rotating its arguments one place to the
2785 \e{right}, so that the original \e{last} argument appears as \c{%1}.
2786 This is then popped, and the arguments are rotated right again, so
2787 the second-to-last argument becomes \c{%1}. Thus the arguments are
2788 iterated through in reverse order.
2791 \S{concat} \i{Concatenating Macro Parameters}
2793 NASM can concatenate macro parameters and macro indirection constructs
2794 on to other text surrounding them. This allows you to declare a family
2795 of symbols, for example, in a macro definition. If, for example, you
2796 wanted to generate a table of key codes along with offsets into the
2797 table, you could code something like
2799 \c %macro keytab_entry 2
2801 \c keypos%1 equ $-keytab
2807 \c keytab_entry F1,128+1
2808 \c keytab_entry F2,128+2
2809 \c keytab_entry Return,13
2811 which would expand to
2814 \c keyposF1 equ $-keytab
2816 \c keyposF2 equ $-keytab
2818 \c keyposReturn equ $-keytab
2821 You can just as easily concatenate text on to the other end of a
2822 macro parameter, by writing \c{%1foo}.
2824 If you need to append a \e{digit} to a macro parameter, for example
2825 defining labels \c{foo1} and \c{foo2} when passed the parameter
2826 \c{foo}, you can't code \c{%11} because that would be taken as the
2827 eleventh macro parameter. Instead, you must code
2828 \I{braces, after % sign}\c{%\{1\}1}, which will separate the first
2829 \c{1} (giving the number of the macro parameter) from the second
2830 (literal text to be concatenated to the parameter).
2832 This concatenation can also be applied to other preprocessor in-line
2833 objects, such as macro-local labels (\k{maclocal}) and context-local
2834 labels (\k{ctxlocal}). In all cases, ambiguities in syntax can be
2835 resolved by enclosing everything after the \c{%} sign and before the
2836 literal text in braces: so \c{%\{%foo\}bar} concatenates the text
2837 \c{bar} to the end of the real name of the macro-local label
2838 \c{%%foo}. (This is unnecessary, since the form NASM uses for the
2839 real names of macro-local labels means that the two usages
2840 \c{%\{%foo\}bar} and \c{%%foobar} would both expand to the same
2841 thing anyway; nevertheless, the capability is there.)
2843 The single-line macro indirection construct, \c{%[...]}
2844 (\k{indmacro}), behaves the same way as macro parameters for the
2845 purpose of concatenation.
2847 See also the \c{%+} operator, \k{concat%+}.
2850 \S{mlmaccc} \i{Condition Codes as Macro Parameters}
2852 NASM can give special treatment to a macro parameter which contains
2853 a condition code. For a start, you can refer to the macro parameter
2854 \c{%1} by means of the alternative syntax \i\c{%+1}, which informs
2855 NASM that this macro parameter is supposed to contain a condition
2856 code, and will cause the preprocessor to report an error message if
2857 the macro is called with a parameter which is \e{not} a valid
2860 Far more usefully, though, you can refer to the macro parameter by
2861 means of \i\c{%-1}, which NASM will expand as the \e{inverse}
2862 condition code. So the \c{retz} macro defined in \k{maclocal} can be
2863 replaced by a general \i{conditional-return macro} like this:
2873 This macro can now be invoked using calls like \c{retc ne}, which
2874 will cause the conditional-jump instruction in the macro expansion
2875 to come out as \c{JE}, or \c{retc po} which will make the jump a
2878 The \c{%+1} macro-parameter reference is quite happy to interpret
2879 the arguments \c{CXZ} and \c{ECXZ} as valid condition codes;
2880 however, \c{%-1} will report an error if passed either of these,
2881 because no inverse condition code exists.
2884 \S{nolist} \i{Disabling Listing Expansion}\I\c{.nolist}
2886 When NASM is generating a listing file from your program, it will
2887 generally expand multi-line macros by means of writing the macro
2888 call and then listing each line of the expansion. This allows you to
2889 see which instructions in the macro expansion are generating what
2890 code; however, for some macros this clutters the listing up
2893 NASM therefore provides the \c{.nolist} qualifier, which you can
2894 include in a macro definition to inhibit the expansion of the macro
2895 in the listing file. The \c{.nolist} qualifier comes directly after
2896 the number of parameters, like this:
2898 \c %macro foo 1.nolist
2902 \c %macro bar 1-5+.nolist a,b,c,d,e,f,g,h
2904 \S{unmacro} Undefining Multi-Line Macros: \i\c{%unmacro}
2906 Multi-line macros can be removed with the \c{%unmacro} directive.
2907 Unlike the \c{%undef} directive, however, \c{%unmacro} takes an
2908 argument specification, and will only remove \i{exact matches} with
2909 that argument specification.
2918 removes the previously defined macro \c{foo}, but
2925 does \e{not} remove the macro \c{bar}, since the argument
2926 specification does not match exactly.
2929 \H{condasm} \i{Conditional Assembly}\I\c{%if}
2931 Similarly to the C preprocessor, NASM allows sections of a source
2932 file to be assembled only if certain conditions are met. The general
2933 syntax of this feature looks like this:
2936 \c ; some code which only appears if <condition> is met
2937 \c %elif<condition2>
2938 \c ; only appears if <condition> is not met but <condition2> is
2940 \c ; this appears if neither <condition> nor <condition2> was met
2943 The inverse forms \i\c{%ifn} and \i\c{%elifn} are also supported.
2945 The \i\c{%else} clause is optional, as is the \i\c{%elif} clause.
2946 You can have more than one \c{%elif} clause as well.
2948 There are a number of variants of the \c{%if} directive. Each has its
2949 corresponding \c{%elif}, \c{%ifn}, and \c{%elifn} directives; for
2950 example, the equivalents to the \c{%ifdef} directive are \c{%elifdef},
2951 \c{%ifndef}, and \c{%elifndef}.
2953 \S{ifdef} \i\c{%ifdef}: Testing Single-Line Macro Existence\I{testing,
2954 single-line macro existence}
2956 Beginning a conditional-assembly block with the line \c{%ifdef
2957 MACRO} will assemble the subsequent code if, and only if, a
2958 single-line macro called \c{MACRO} is defined. If not, then the
2959 \c{%elif} and \c{%else} blocks (if any) will be processed instead.
2961 For example, when debugging a program, you might want to write code
2964 \c ; perform some function
2966 \c writefile 2,"Function performed successfully",13,10
2968 \c ; go and do something else
2970 Then you could use the command-line option \c{-dDEBUG} to create a
2971 version of the program which produced debugging messages, and remove
2972 the option to generate the final release version of the program.
2974 You can test for a macro \e{not} being defined by using
2975 \i\c{%ifndef} instead of \c{%ifdef}. You can also test for macro
2976 definitions in \c{%elif} blocks by using \i\c{%elifdef} and
2980 \S{ifmacro} \i\c{%ifmacro}: Testing Multi-Line Macro
2981 Existence\I{testing, multi-line macro existence}
2983 The \c{%ifmacro} directive operates in the same way as the \c{%ifdef}
2984 directive, except that it checks for the existence of a multi-line macro.
2986 For example, you may be working with a large project and not have control
2987 over the macros in a library. You may want to create a macro with one
2988 name if it doesn't already exist, and another name if one with that name
2991 The \c{%ifmacro} is considered true if defining a macro with the given name
2992 and number of arguments would cause a definitions conflict. For example:
2994 \c %ifmacro MyMacro 1-3
2996 \c %error "MyMacro 1-3" causes a conflict with an existing macro.
3000 \c %macro MyMacro 1-3
3002 \c ; insert code to define the macro
3008 This will create the macro "MyMacro 1-3" if no macro already exists which
3009 would conflict with it, and emits a warning if there would be a definition
3012 You can test for the macro not existing by using the \i\c{%ifnmacro} instead
3013 of \c{%ifmacro}. Additional tests can be performed in \c{%elif} blocks by using
3014 \i\c{%elifmacro} and \i\c{%elifnmacro}.
3017 \S{ifctx} \i\c{%ifctx}: Testing the Context Stack\I{testing, context
3020 The conditional-assembly construct \c{%ifctx} will cause the
3021 subsequent code to be assembled if and only if the top context on
3022 the preprocessor's context stack has the same name as one of the arguments.
3023 As with \c{%ifdef}, the inverse and \c{%elif} forms \i\c{%ifnctx},
3024 \i\c{%elifctx} and \i\c{%elifnctx} are also supported.
3026 For more details of the context stack, see \k{ctxstack}. For a
3027 sample use of \c{%ifctx}, see \k{blockif}.
3030 \S{if} \i\c{%if}: Testing Arbitrary Numeric Expressions\I{testing,
3031 arbitrary numeric expressions}
3033 The conditional-assembly construct \c{%if expr} will cause the
3034 subsequent code to be assembled if and only if the value of the
3035 numeric expression \c{expr} is non-zero. An example of the use of
3036 this feature is in deciding when to break out of a \c{%rep}
3037 preprocessor loop: see \k{rep} for a detailed example.
3039 The expression given to \c{%if}, and its counterpart \i\c{%elif}, is
3040 a critical expression (see \k{crit}).
3042 \c{%if} extends the normal NASM expression syntax, by providing a
3043 set of \i{relational operators} which are not normally available in
3044 expressions. The operators \i\c{=}, \i\c{<}, \i\c{>}, \i\c{<=},
3045 \i\c{>=} and \i\c{<>} test equality, less-than, greater-than,
3046 less-or-equal, greater-or-equal and not-equal respectively. The
3047 C-like forms \i\c{==} and \i\c{!=} are supported as alternative
3048 forms of \c{=} and \c{<>}. In addition, low-priority logical
3049 operators \i\c{&&}, \i\c{^^} and \i\c{||} are provided, supplying
3050 \i{logical AND}, \i{logical XOR} and \i{logical OR}. These work like
3051 the C logical operators (although C has no logical XOR), in that
3052 they always return either 0 or 1, and treat any non-zero input as 1
3053 (so that \c{^^}, for example, returns 1 if exactly one of its inputs
3054 is zero, and 0 otherwise). The relational operators also return 1
3055 for true and 0 for false.
3057 Like other \c{%if} constructs, \c{%if} has a counterpart
3058 \i\c{%elif}, and negative forms \i\c{%ifn} and \i\c{%elifn}.
3060 \S{ifidn} \i\c{%ifidn} and \i\c{%ifidni}: Testing Exact Text
3061 Identity\I{testing, exact text identity}
3063 The construct \c{%ifidn text1,text2} will cause the subsequent code
3064 to be assembled if and only if \c{text1} and \c{text2}, after
3065 expanding single-line macros, are identical pieces of text.
3066 Differences in white space are not counted.
3068 \c{%ifidni} is similar to \c{%ifidn}, but is \i{case-insensitive}.
3070 For example, the following macro pushes a register or number on the
3071 stack, and allows you to treat \c{IP} as a real register:
3073 \c %macro pushparam 1
3084 Like other \c{%if} constructs, \c{%ifidn} has a counterpart
3085 \i\c{%elifidn}, and negative forms \i\c{%ifnidn} and \i\c{%elifnidn}.
3086 Similarly, \c{%ifidni} has counterparts \i\c{%elifidni},
3087 \i\c{%ifnidni} and \i\c{%elifnidni}.
3089 \S{iftyp} \i\c{%ifid}, \i\c{%ifnum}, \i\c{%ifstr}: Testing Token
3090 Types\I{testing, token types}
3092 Some macros will want to perform different tasks depending on
3093 whether they are passed a number, a string, or an identifier. For
3094 example, a string output macro might want to be able to cope with
3095 being passed either a string constant or a pointer to an existing
3098 The conditional assembly construct \c{%ifid}, taking one parameter
3099 (which may be blank), assembles the subsequent code if and only if
3100 the first token in the parameter exists and is an identifier.
3101 \c{%ifnum} works similarly, but tests for the token being a numeric
3102 constant; \c{%ifstr} tests for it being a string.
3104 For example, the \c{writefile} macro defined in \k{mlmacgre} can be
3105 extended to take advantage of \c{%ifstr} in the following fashion:
3107 \c %macro writefile 2-3+
3116 \c %%endstr: mov dx,%%str
3117 \c mov cx,%%endstr-%%str
3128 Then the \c{writefile} macro can cope with being called in either of
3129 the following two ways:
3131 \c writefile [file], strpointer, length
3132 \c writefile [file], "hello", 13, 10
3134 In the first, \c{strpointer} is used as the address of an
3135 already-declared string, and \c{length} is used as its length; in
3136 the second, a string is given to the macro, which therefore declares
3137 it itself and works out the address and length for itself.
3139 Note the use of \c{%if} inside the \c{%ifstr}: this is to detect
3140 whether the macro was passed two arguments (so the string would be a
3141 single string constant, and \c{db %2} would be adequate) or more (in
3142 which case, all but the first two would be lumped together into
3143 \c{%3}, and \c{db %2,%3} would be required).
3145 The usual \I\c{%elifid}\I\c{%elifnum}\I\c{%elifstr}\c{%elif}...,
3146 \I\c{%ifnid}\I\c{%ifnnum}\I\c{%ifnstr}\c{%ifn}..., and
3147 \I\c{%elifnid}\I\c{%elifnnum}\I\c{%elifnstr}\c{%elifn}... versions
3148 exist for each of \c{%ifid}, \c{%ifnum} and \c{%ifstr}.
3150 \S{iftoken} \i\c{%iftoken}: Test for a Single Token
3152 Some macros will want to do different things depending on if it is
3153 passed a single token (e.g. paste it to something else using \c{%+})
3154 versus a multi-token sequence.
3156 The conditional assembly construct \c{%iftoken} assembles the
3157 subsequent code if and only if the expanded parameters consist of
3158 exactly one token, possibly surrounded by whitespace.
3164 will assemble the subsequent code, but
3168 will not, since \c{-1} contains two tokens: the unary minus operator
3169 \c{-}, and the number \c{1}.
3171 The usual \i\c{%eliftoken}, \i\c\{%ifntoken}, and \i\c{%elifntoken}
3172 variants are also provided.
3174 \S{ifempty} \i\c{%ifempty}: Test for Empty Expansion
3176 The conditional assembly construct \c{%ifempty} assembles the
3177 subsequent code if and only if the expanded parameters do not contain
3178 any tokens at all, whitespace excepted.
3180 The usual \i\c{%elifempty}, \i\c\{%ifnempty}, and \i\c{%elifnempty}
3181 variants are also provided.
3183 \S{ifenv} \i\c{%ifenv}: Test If Environment Variable Exists
3185 The conditional assembly construct \c{%ifenv} assembles the
3186 subsequent code if and only if the environment variable referenced by
3187 the \c{%!<env>} directive exists.
3189 The usual \i\c{%elifenv}, \i\c\{%ifnenv}, and \i\c{%elifnenv}
3190 variants are also provided.
3192 Just as for \c{%!<env>} the argument should be written as a string if
3193 it contains characters that would not be legal in an identifier. See
3196 \H{rep} \i{Preprocessor Loops}\I{repeating code}: \i\c{%rep}
3198 NASM's \c{TIMES} prefix, though useful, cannot be used to invoke a
3199 multi-line macro multiple times, because it is processed by NASM
3200 after macros have already been expanded. Therefore NASM provides
3201 another form of loop, this time at the preprocessor level: \c{%rep}.
3203 The directives \c{%rep} and \i\c{%endrep} (\c{%rep} takes a numeric
3204 argument, which can be an expression; \c{%endrep} takes no
3205 arguments) can be used to enclose a chunk of code, which is then
3206 replicated as many times as specified by the preprocessor:
3210 \c inc word [table+2*i]
3214 This will generate a sequence of 64 \c{INC} instructions,
3215 incrementing every word of memory from \c{[table]} to
3218 For more complex termination conditions, or to break out of a repeat
3219 loop part way along, you can use the \i\c{%exitrep} directive to
3220 terminate the loop, like this:
3235 \c fib_number equ ($-fibonacci)/2
3237 This produces a list of all the Fibonacci numbers that will fit in
3238 16 bits. Note that a maximum repeat count must still be given to
3239 \c{%rep}. This is to prevent the possibility of NASM getting into an
3240 infinite loop in the preprocessor, which (on multitasking or
3241 multi-user systems) would typically cause all the system memory to
3242 be gradually used up and other applications to start crashing.
3244 Note a maximum repeat count is limited by 62 bit number, though it
3245 is hardly possible that you ever need anything bigger.
3248 \H{files} Source Files and Dependencies
3250 These commands allow you to split your sources into multiple files.
3252 \S{include} \i\c{%include}: \i{Including Other Files}
3254 Using, once again, a very similar syntax to the C preprocessor,
3255 NASM's preprocessor lets you include other source files into your
3256 code. This is done by the use of the \i\c{%include} directive:
3258 \c %include "macros.mac"
3260 will include the contents of the file \c{macros.mac} into the source
3261 file containing the \c{%include} directive.
3263 Include files are \I{searching for include files}searched for in the
3264 current directory (the directory you're in when you run NASM, as
3265 opposed to the location of the NASM executable or the location of
3266 the source file), plus any directories specified on the NASM command
3267 line using the \c{-i} option.
3269 The standard C idiom for preventing a file being included more than
3270 once is just as applicable in NASM: if the file \c{macros.mac} has
3273 \c %ifndef MACROS_MAC
3274 \c %define MACROS_MAC
3275 \c ; now define some macros
3278 then including the file more than once will not cause errors,
3279 because the second time the file is included nothing will happen
3280 because the macro \c{MACROS_MAC} will already be defined.
3282 You can force a file to be included even if there is no \c{%include}
3283 directive that explicitly includes it, by using the \i\c{-p} option
3284 on the NASM command line (see \k{opt-p}).
3287 \S{pathsearch} \i\c{%pathsearch}: Search the Include Path
3289 The \c{%pathsearch} directive takes a single-line macro name and a
3290 filename, and declare or redefines the specified single-line macro to
3291 be the include-path-resolved version of the filename, if the file
3292 exists (otherwise, it is passed unchanged.)
3296 \c %pathsearch MyFoo "foo.bin"
3298 ... with \c{-Ibins/} in the include path may end up defining the macro
3299 \c{MyFoo} to be \c{"bins/foo.bin"}.
3302 \S{depend} \i\c{%depend}: Add Dependent Files
3304 The \c{%depend} directive takes a filename and adds it to the list of
3305 files to be emitted as dependency generation when the \c{-M} options
3306 and its relatives (see \k{opt-M}) are used. It produces no output.
3308 This is generally used in conjunction with \c{%pathsearch}. For
3309 example, a simplified version of the standard macro wrapper for the
3310 \c{INCBIN} directive looks like:
3312 \c %imacro incbin 1-2+ 0
3313 \c %pathsearch dep %1
3318 This first resolves the location of the file into the macro \c{dep},
3319 then adds it to the dependency lists, and finally issues the
3320 assembler-level \c{INCBIN} directive.
3323 \S{use} \i\c{%use}: Include Standard Macro Package
3325 The \c{%use} directive is similar to \c{%include}, but rather than
3326 including the contents of a file, it includes a named standard macro
3327 package. The standard macro packages are part of NASM, and are
3328 described in \k{macropkg}.
3330 Unlike the \c{%include} directive, package names for the \c{%use}
3331 directive do not require quotes, but quotes are permitted. In NASM
3332 2.04 and 2.05 the unquoted form would be macro-expanded; this is no
3333 longer true. Thus, the following lines are equivalent:
3338 Standard macro packages are protected from multiple inclusion. When a
3339 standard macro package is used, a testable single-line macro of the
3340 form \c{__USE_}\e{package}\c{__} is also defined, see \k{use_def}.
3342 \H{ctxstack} The \i{Context Stack}
3344 Having labels that are local to a macro definition is sometimes not
3345 quite powerful enough: sometimes you want to be able to share labels
3346 between several macro calls. An example might be a \c{REPEAT} ...
3347 \c{UNTIL} loop, in which the expansion of the \c{REPEAT} macro
3348 would need to be able to refer to a label which the \c{UNTIL} macro
3349 had defined. However, for such a macro you would also want to be
3350 able to nest these loops.
3352 NASM provides this level of power by means of a \e{context stack}.
3353 The preprocessor maintains a stack of \e{contexts}, each of which is
3354 characterized by a name. You add a new context to the stack using
3355 the \i\c{%push} directive, and remove one using \i\c{%pop}. You can
3356 define labels that are local to a particular context on the stack.
3359 \S{pushpop} \i\c{%push} and \i\c{%pop}: \I{creating
3360 contexts}\I{removing contexts}Creating and Removing Contexts
3362 The \c{%push} directive is used to create a new context and place it
3363 on the top of the context stack. \c{%push} takes an optional argument,
3364 which is the name of the context. For example:
3368 This pushes a new context called \c{foobar} on the stack. You can have
3369 several contexts on the stack with the same name: they can still be
3370 distinguished. If no name is given, the context is unnamed (this is
3371 normally used when both the \c{%push} and the \c{%pop} are inside a
3372 single macro definition.)
3374 The directive \c{%pop}, taking one optional argument, removes the top
3375 context from the context stack and destroys it, along with any
3376 labels associated with it. If an argument is given, it must match the
3377 name of the current context, otherwise it will issue an error.
3380 \S{ctxlocal} \i{Context-Local Labels}
3382 Just as the usage \c{%%foo} defines a label which is local to the
3383 particular macro call in which it is used, the usage \I{%$}\c{%$foo}
3384 is used to define a label which is local to the context on the top
3385 of the context stack. So the \c{REPEAT} and \c{UNTIL} example given
3386 above could be implemented by means of:
3402 and invoked by means of, for example,
3410 which would scan every fourth byte of a string in search of the byte
3413 If you need to define, or access, labels local to the context
3414 \e{below} the top one on the stack, you can use \I{%$$}\c{%$$foo}, or
3415 \c{%$$$foo} for the context below that, and so on.
3418 \S{ctxdefine} \i{Context-Local Single-Line Macros}
3420 NASM also allows you to define single-line macros which are local to
3421 a particular context, in just the same way:
3423 \c %define %$localmac 3
3425 will define the single-line macro \c{%$localmac} to be local to the
3426 top context on the stack. Of course, after a subsequent \c{%push},
3427 it can then still be accessed by the name \c{%$$localmac}.
3430 \S{ctxfallthrough} \i{Context Fall-Through Lookup}
3432 Context fall-through lookup (automatic searching of outer contexts)
3433 is a feature that was added in NASM version 0.98.03. Unfortunately,
3434 this feature is unintuitive and can result in buggy code that would
3435 have otherwise been prevented by NASM's error reporting. As a result,
3436 this feature has been \e{deprecated}. NASM version 2.09 will issue a
3437 warning when usage of this \e{deprecated} feature is detected. Starting
3438 with NASM version 2.10, usage of this \e{deprecated} feature will simply
3439 result in an \e{expression syntax error}.
3441 An example usage of this \e{deprecated} feature follows:
3445 \c %assign %$external 1
3447 \c %assign %$internal 1
3448 \c mov eax, %$external
3449 \c mov eax, %$internal
3454 As demonstrated, \c{%$external} is being defined in the \c{ctx1}
3455 context and referenced within the \c{ctx2} context. With context
3456 fall-through lookup, referencing an undefined context-local macro
3457 like this implicitly searches through all outer contexts until a match
3458 is made or isn't found in any context. As a result, \c{%$external}
3459 referenced within the \c{ctx2} context would implicitly use \c{%$external}
3460 as defined in \c{ctx1}. Most people would expect NASM to issue an error in
3461 this situation because \c{%$external} was never defined within \c{ctx2} and also
3462 isn't qualified with the proper context depth, \c{%$$external}.
3464 Here is a revision of the above example with proper context depth:
3468 \c %assign %$external 1
3470 \c %assign %$internal 1
3471 \c mov eax, %$$external
3472 \c mov eax, %$internal
3477 As demonstrated, \c{%$external} is still being defined in the \c{ctx1}
3478 context and referenced within the \c{ctx2} context. However, the
3479 reference to \c{%$external} within \c{ctx2} has been fully qualified with
3480 the proper context depth, \c{%$$external}, and thus is no longer ambiguous,
3481 unintuitive or erroneous.
3484 \S{ctxrepl} \i\c{%repl}: \I{renaming contexts}Renaming a Context
3486 If you need to change the name of the top context on the stack (in
3487 order, for example, to have it respond differently to \c{%ifctx}),
3488 you can execute a \c{%pop} followed by a \c{%push}; but this will
3489 have the side effect of destroying all context-local labels and
3490 macros associated with the context that was just popped.
3492 NASM provides the directive \c{%repl}, which \e{replaces} a context
3493 with a different name, without touching the associated macros and
3494 labels. So you could replace the destructive code
3499 with the non-destructive version \c{%repl newname}.
3502 \S{blockif} Example Use of the \i{Context Stack}: \i{Block IFs}
3504 This example makes use of almost all the context-stack features,
3505 including the conditional-assembly construct \i\c{%ifctx}, to
3506 implement a block IF statement as a set of macros.
3522 \c %error "expected `if' before `else'"
3536 \c %error "expected `if' or `else' before `endif'"
3541 This code is more robust than the \c{REPEAT} and \c{UNTIL} macros
3542 given in \k{ctxlocal}, because it uses conditional assembly to check
3543 that the macros are issued in the right order (for example, not
3544 calling \c{endif} before \c{if}) and issues a \c{%error} if they're
3547 In addition, the \c{endif} macro has to be able to cope with the two
3548 distinct cases of either directly following an \c{if}, or following
3549 an \c{else}. It achieves this, again, by using conditional assembly
3550 to do different things depending on whether the context on top of
3551 the stack is \c{if} or \c{else}.
3553 The \c{else} macro has to preserve the context on the stack, in
3554 order to have the \c{%$ifnot} referred to by the \c{if} macro be the
3555 same as the one defined by the \c{endif} macro, but has to change
3556 the context's name so that \c{endif} will know there was an
3557 intervening \c{else}. It does this by the use of \c{%repl}.
3559 A sample usage of these macros might look like:
3581 The block-\c{IF} macros handle nesting quite happily, by means of
3582 pushing another context, describing the inner \c{if}, on top of the
3583 one describing the outer \c{if}; thus \c{else} and \c{endif} always
3584 refer to the last unmatched \c{if} or \c{else}.
3587 \H{stackrel} \i{Stack Relative Preprocessor Directives}
3589 The following preprocessor directives provide a way to use
3590 labels to refer to local variables allocated on the stack.
3592 \b\c{%arg} (see \k{arg})
3594 \b\c{%stacksize} (see \k{stacksize})
3596 \b\c{%local} (see \k{local})
3599 \S{arg} \i\c{%arg} Directive
3601 The \c{%arg} directive is used to simplify the handling of
3602 parameters passed on the stack. Stack based parameter passing
3603 is used by many high level languages, including C, C++ and Pascal.
3605 While NASM has macros which attempt to duplicate this
3606 functionality (see \k{16cmacro}), the syntax is not particularly
3607 convenient to use and is not TASM compatible. Here is an example
3608 which shows the use of \c{%arg} without any external macros:
3612 \c %push mycontext ; save the current context
3613 \c %stacksize large ; tell NASM to use bp
3614 \c %arg i:word, j_ptr:word
3621 \c %pop ; restore original context
3623 This is similar to the procedure defined in \k{16cmacro} and adds
3624 the value in i to the value pointed to by j_ptr and returns the
3625 sum in the ax register. See \k{pushpop} for an explanation of
3626 \c{push} and \c{pop} and the use of context stacks.
3629 \S{stacksize} \i\c{%stacksize} Directive
3631 The \c{%stacksize} directive is used in conjunction with the
3632 \c{%arg} (see \k{arg}) and the \c{%local} (see \k{local}) directives.
3633 It tells NASM the default size to use for subsequent \c{%arg} and
3634 \c{%local} directives. The \c{%stacksize} directive takes one
3635 required argument which is one of \c{flat}, \c{flat64}, \c{large} or \c{small}.
3639 This form causes NASM to use stack-based parameter addressing
3640 relative to \c{ebp} and it assumes that a near form of call was used
3641 to get to this label (i.e. that \c{eip} is on the stack).
3643 \c %stacksize flat64
3645 This form causes NASM to use stack-based parameter addressing
3646 relative to \c{rbp} and it assumes that a near form of call was used
3647 to get to this label (i.e. that \c{rip} is on the stack).
3651 This form uses \c{bp} to do stack-based parameter addressing and
3652 assumes that a far form of call was used to get to this address
3653 (i.e. that \c{ip} and \c{cs} are on the stack).
3657 This form also uses \c{bp} to address stack parameters, but it is
3658 different from \c{large} because it also assumes that the old value
3659 of bp is pushed onto the stack (i.e. it expects an \c{ENTER}
3660 instruction). In other words, it expects that \c{bp}, \c{ip} and
3661 \c{cs} are on the top of the stack, underneath any local space which
3662 may have been allocated by \c{ENTER}. This form is probably most
3663 useful when used in combination with the \c{%local} directive
3667 \S{local} \i\c{%local} Directive
3669 The \c{%local} directive is used to simplify the use of local
3670 temporary stack variables allocated in a stack frame. Automatic
3671 local variables in C are an example of this kind of variable. The
3672 \c{%local} directive is most useful when used with the \c{%stacksize}
3673 (see \k{stacksize} and is also compatible with the \c{%arg} directive
3674 (see \k{arg}). It allows simplified reference to variables on the
3675 stack which have been allocated typically by using the \c{ENTER}
3677 \# (see \k{insENTER} for a description of that instruction).
3678 An example of its use is the following:
3682 \c %push mycontext ; save the current context
3683 \c %stacksize small ; tell NASM to use bp
3684 \c %assign %$localsize 0 ; see text for explanation
3685 \c %local old_ax:word, old_dx:word
3687 \c enter %$localsize,0 ; see text for explanation
3688 \c mov [old_ax],ax ; swap ax & bx
3689 \c mov [old_dx],dx ; and swap dx & cx
3694 \c leave ; restore old bp
3697 \c %pop ; restore original context
3699 The \c{%$localsize} variable is used internally by the
3700 \c{%local} directive and \e{must} be defined within the
3701 current context before the \c{%local} directive may be used.
3702 Failure to do so will result in one expression syntax error for
3703 each \c{%local} variable declared. It then may be used in
3704 the construction of an appropriately sized ENTER instruction
3705 as shown in the example.
3708 \H{pperror} Reporting \i{User-Defined Errors}: \i\c{%error}, \i\c{%warning}, \i\c{%fatal}
3710 The preprocessor directive \c{%error} will cause NASM to report an
3711 error if it occurs in assembled code. So if other users are going to
3712 try to assemble your source files, you can ensure that they define the
3713 right macros by means of code like this:
3718 \c ; do some different setup
3720 \c %error "Neither F1 nor F2 was defined."
3723 Then any user who fails to understand the way your code is supposed
3724 to be assembled will be quickly warned of their mistake, rather than
3725 having to wait until the program crashes on being run and then not
3726 knowing what went wrong.
3728 Similarly, \c{%warning} issues a warning, but allows assembly to continue:
3733 \c ; do some different setup
3735 \c %warning "Neither F1 nor F2 was defined, assuming F1."
3739 \c{%error} and \c{%warning} are issued only on the final assembly
3740 pass. This makes them safe to use in conjunction with tests that
3741 depend on symbol values.
3743 \c{%fatal} terminates assembly immediately, regardless of pass. This
3744 is useful when there is no point in continuing the assembly further,
3745 and doing so is likely just going to cause a spew of confusing error
3748 It is optional for the message string after \c{%error}, \c{%warning}
3749 or \c{%fatal} to be quoted. If it is \e{not}, then single-line macros
3750 are expanded in it, which can be used to display more information to
3751 the user. For example:
3754 \c %assign foo_over foo-64
3755 \c %error foo is foo_over bytes too large
3759 \H{otherpreproc} \i{Other Preprocessor Directives}
3761 NASM also has preprocessor directives which allow access to
3762 information from external sources. Currently they include:
3764 \b\c{%line} enables NASM to correctly handle the output of another
3765 preprocessor (see \k{line}).
3767 \b\c{%!} enables NASM to read in the value of an environment variable,
3768 which can then be used in your program (see \k{getenv}).
3770 \S{line} \i\c{%line} Directive
3772 The \c{%line} directive is used to notify NASM that the input line
3773 corresponds to a specific line number in another file. Typically
3774 this other file would be an original source file, with the current
3775 NASM input being the output of a pre-processor. The \c{%line}
3776 directive allows NASM to output messages which indicate the line
3777 number of the original source file, instead of the file that is being
3780 This preprocessor directive is not generally of use to programmers,
3781 by may be of interest to preprocessor authors. The usage of the
3782 \c{%line} preprocessor directive is as follows:
3784 \c %line nnn[+mmm] [filename]
3786 In this directive, \c{nnn} identifies the line of the original source
3787 file which this line corresponds to. \c{mmm} is an optional parameter
3788 which specifies a line increment value; each line of the input file
3789 read in is considered to correspond to \c{mmm} lines of the original
3790 source file. Finally, \c{filename} is an optional parameter which
3791 specifies the file name of the original source file.
3793 After reading a \c{%line} preprocessor directive, NASM will report
3794 all file name and line numbers relative to the values specified
3798 \S{getenv} \i\c{%!}\c{<env>}: Read an environment variable.
3800 The \c{%!<env>} directive makes it possible to read the value of an
3801 environment variable at assembly time. This could, for example, be used
3802 to store the contents of an environment variable into a string, which
3803 could be used at some other point in your code.
3805 For example, suppose that you have an environment variable \c{FOO}, and
3806 you want the contents of \c{FOO} to be embedded in your program. You
3807 could do that as follows:
3809 \c %defstr FOO %!FOO
3811 See \k{defstr} for notes on the \c{%defstr} directive.
3813 If the name of the environment variable contains non-identifier
3814 characters, you can use string quotes to surround the name of the
3815 variable, for example:
3817 \c %defstr C_colon %!'C:'
3820 \H{comment} Comment Blocks: \i\c{%comment}
3822 The \c{%comment} and \c{%endcomment} directives are used to specify
3823 a block of commented (i.e. unprocessed) code/text. Everything between
3824 \c{%comment} and \c{%endcomment} will be ignored by the preprocessor.
3827 \c ; some code, text or data to be ignored
3831 \H{stdmac} \i{Standard Macros}
3833 NASM defines a set of standard macros, which are already defined
3834 when it starts to process any source file. If you really need a
3835 program to be assembled with no pre-defined macros, you can use the
3836 \i\c{%clear} directive to empty the preprocessor of everything but
3837 context-local preprocessor variables and single-line macros.
3839 Most \i{user-level assembler directives} (see \k{directive}) are
3840 implemented as macros which invoke primitive directives; these are
3841 described in \k{directive}. The rest of the standard macro set is
3845 \S{stdmacver} \i{NASM Version} Macros
3847 The single-line macros \i\c{__NASM_MAJOR__}, \i\c{__NASM_MINOR__},
3848 \i\c{__NASM_SUBMINOR__} and \i\c{___NASM_PATCHLEVEL__} expand to the
3849 major, minor, subminor and patch level parts of the \i{version
3850 number of NASM} being used. So, under NASM 0.98.32p1 for
3851 example, \c{__NASM_MAJOR__} would be defined to be 0, \c{__NASM_MINOR__}
3852 would be defined as 98, \c{__NASM_SUBMINOR__} would be defined to 32,
3853 and \c{___NASM_PATCHLEVEL__} would be defined as 1.
3855 Additionally, the macro \i\c{__NASM_SNAPSHOT__} is defined for
3856 automatically generated snapshot releases \e{only}.
3859 \S{stdmacverid} \i\c{__NASM_VERSION_ID__}: \i{NASM Version ID}
3861 The single-line macro \c{__NASM_VERSION_ID__} expands to a dword integer
3862 representing the full version number of the version of nasm being used.
3863 The value is the equivalent to \c{__NASM_MAJOR__}, \c{__NASM_MINOR__},
3864 \c{__NASM_SUBMINOR__} and \c{___NASM_PATCHLEVEL__} concatenated to
3865 produce a single doubleword. Hence, for 0.98.32p1, the returned number
3866 would be equivalent to:
3874 Note that the above lines are generate exactly the same code, the second
3875 line is used just to give an indication of the order that the separate
3876 values will be present in memory.
3879 \S{stdmacverstr} \i\c{__NASM_VER__}: \i{NASM Version string}
3881 The single-line macro \c{__NASM_VER__} expands to a string which defines
3882 the version number of nasm being used. So, under NASM 0.98.32 for example,
3891 \S{fileline} \i\c{__FILE__} and \i\c{__LINE__}: File Name and Line Number
3893 Like the C preprocessor, NASM allows the user to find out the file
3894 name and line number containing the current instruction. The macro
3895 \c{__FILE__} expands to a string constant giving the name of the
3896 current input file (which may change through the course of assembly
3897 if \c{%include} directives are used), and \c{__LINE__} expands to a
3898 numeric constant giving the current line number in the input file.
3900 These macros could be used, for example, to communicate debugging
3901 information to a macro, since invoking \c{__LINE__} inside a macro
3902 definition (either single-line or multi-line) will return the line
3903 number of the macro \e{call}, rather than \e{definition}. So to
3904 determine where in a piece of code a crash is occurring, for
3905 example, one could write a routine \c{stillhere}, which is passed a
3906 line number in \c{EAX} and outputs something like `line 155: still
3907 here'. You could then write a macro
3909 \c %macro notdeadyet 0
3918 and then pepper your code with calls to \c{notdeadyet} until you
3919 find the crash point.
3922 \S{bitsm} \i\c{__BITS__}: Current BITS Mode
3924 The \c{__BITS__} standard macro is updated every time that the BITS mode is
3925 set using the \c{BITS XX} or \c{[BITS XX]} directive, where XX is a valid mode
3926 number of 16, 32 or 64. \c{__BITS__} receives the specified mode number and
3927 makes it globally available. This can be very useful for those who utilize
3928 mode-dependent macros.
3930 \S{ofmtm} \i\c{__OUTPUT_FORMAT__}: Current Output Format
3932 The \c{__OUTPUT_FORMAT__} standard macro holds the current Output Format,
3933 as given by the \c{-f} option or NASM's default. Type \c{nasm -hf} for a
3936 \c %ifidn __OUTPUT_FORMAT__, win32
3937 \c %define NEWLINE 13, 10
3938 \c %elifidn __OUTPUT_FORMAT__, elf32
3939 \c %define NEWLINE 10
3943 \S{datetime} Assembly Date and Time Macros
3945 NASM provides a variety of macros that represent the timestamp of the
3948 \b The \i\c{__DATE__} and \i\c{__TIME__} macros give the assembly date and
3949 time as strings, in ISO 8601 format (\c{"YYYY-MM-DD"} and \c{"HH:MM:SS"},
3952 \b The \i\c{__DATE_NUM__} and \i\c{__TIME_NUM__} macros give the assembly
3953 date and time in numeric form; in the format \c{YYYYMMDD} and
3954 \c{HHMMSS} respectively.
3956 \b The \i\c{__UTC_DATE__} and \i\c{__UTC_TIME__} macros give the assembly
3957 date and time in universal time (UTC) as strings, in ISO 8601 format
3958 (\c{"YYYY-MM-DD"} and \c{"HH:MM:SS"}, respectively.) If the host
3959 platform doesn't provide UTC time, these macros are undefined.
3961 \b The \i\c{__UTC_DATE_NUM__} and \i\c{__UTC_TIME_NUM__} macros give the
3962 assembly date and time universal time (UTC) in numeric form; in the
3963 format \c{YYYYMMDD} and \c{HHMMSS} respectively. If the
3964 host platform doesn't provide UTC time, these macros are
3967 \b The \c{__POSIX_TIME__} macro is defined as a number containing the
3968 number of seconds since the POSIX epoch, 1 January 1970 00:00:00 UTC;
3969 excluding any leap seconds. This is computed using UTC time if
3970 available on the host platform, otherwise it is computed using the
3971 local time as if it was UTC.
3973 All instances of time and date macros in the same assembly session
3974 produce consistent output. For example, in an assembly session
3975 started at 42 seconds after midnight on January 1, 2010 in Moscow
3976 (timezone UTC+3) these macros would have the following values,
3977 assuming, of course, a properly configured environment with a correct
3980 \c __DATE__ "2010-01-01"
3981 \c __TIME__ "00:00:42"
3982 \c __DATE_NUM__ 20100101
3983 \c __TIME_NUM__ 000042
3984 \c __UTC_DATE__ "2009-12-31"
3985 \c __UTC_TIME__ "21:00:42"
3986 \c __UTC_DATE_NUM__ 20091231
3987 \c __UTC_TIME_NUM__ 210042
3988 \c __POSIX_TIME__ 1262293242
3991 \S{use_def} \I\c{__USE_*__}\c{__USE_}\e{package}\c{__}: Package
3994 When a standard macro package (see \k{macropkg}) is included with the
3995 \c{%use} directive (see \k{use}), a single-line macro of the form
3996 \c{__USE_}\e{package}\c{__} is automatically defined. This allows
3997 testing if a particular package is invoked or not.
3999 For example, if the \c{altreg} package is included (see
4000 \k{pkg_altreg}), then the macro \c{__USE_ALTREG__} is defined.
4003 \S{pass_macro} \i\c{__PASS__}: Assembly Pass
4005 The macro \c{__PASS__} is defined to be \c{1} on preparatory passes,
4006 and \c{2} on the final pass. In preprocess-only mode, it is set to
4007 \c{3}, and when running only to generate dependencies (due to the
4008 \c{-M} or \c{-MG} option, see \k{opt-M}) it is set to \c{0}.
4010 \e{Avoid using this macro if at all possible. It is tremendously easy
4011 to generate very strange errors by misusing it, and the semantics may
4012 change in future versions of NASM.}
4015 \S{struc} \i\c{STRUC} and \i\c{ENDSTRUC}: \i{Declaring Structure} Data Types
4017 The core of NASM contains no intrinsic means of defining data
4018 structures; instead, the preprocessor is sufficiently powerful that
4019 data structures can be implemented as a set of macros. The macros
4020 \c{STRUC} and \c{ENDSTRUC} are used to define a structure data type.
4022 \c{STRUC} takes one or two parameters. The first parameter is the name
4023 of the data type. The second, optional parameter is the base offset of
4024 the structure. The name of the data type is defined as a symbol with
4025 the value of the base offset, and the name of the data type with the
4026 suffix \c{_size} appended to it is defined as an \c{EQU} giving the
4027 size of the structure. Once \c{STRUC} has been issued, you are
4028 defining the structure, and should define fields using the \c{RESB}
4029 family of pseudo-instructions, and then invoke \c{ENDSTRUC} to finish
4032 For example, to define a structure called \c{mytype} containing a
4033 longword, a word, a byte and a string of bytes, you might code
4044 The above code defines six symbols: \c{mt_long} as 0 (the offset
4045 from the beginning of a \c{mytype} structure to the longword field),
4046 \c{mt_word} as 4, \c{mt_byte} as 6, \c{mt_str} as 7, \c{mytype_size}
4047 as 39, and \c{mytype} itself as zero.
4049 The reason why the structure type name is defined at zero by default
4050 is a side effect of allowing structures to work with the local label
4051 mechanism: if your structure members tend to have the same names in
4052 more than one structure, you can define the above structure like this:
4063 This defines the offsets to the structure fields as \c{mytype.long},
4064 \c{mytype.word}, \c{mytype.byte} and \c{mytype.str}.
4066 NASM, since it has no \e{intrinsic} structure support, does not
4067 support any form of period notation to refer to the elements of a
4068 structure once you have one (except the above local-label notation),
4069 so code such as \c{mov ax,[mystruc.mt_word]} is not valid.
4070 \c{mt_word} is a constant just like any other constant, so the
4071 correct syntax is \c{mov ax,[mystruc+mt_word]} or \c{mov
4072 ax,[mystruc+mytype.word]}.
4074 Sometimes you only have the address of the structure displaced by an
4075 offset. For example, consider this standard stack frame setup:
4081 In this case, you could access an element by subtracting the offset:
4083 \c mov [ebp - 40 + mytype.word], ax
4085 However, if you do not want to repeat this offset, you can use -40 as
4088 \c struc mytype, -40
4090 And access an element this way:
4092 \c mov [ebp + mytype.word], ax
4095 \S{istruc} \i\c{ISTRUC}, \i\c{AT} and \i\c{IEND}: Declaring
4096 \i{Instances of Structures}
4098 Having defined a structure type, the next thing you typically want
4099 to do is to declare instances of that structure in your data
4100 segment. NASM provides an easy way to do this in the \c{ISTRUC}
4101 mechanism. To declare a structure of type \c{mytype} in a program,
4102 you code something like this:
4107 \c at mt_long, dd 123456
4108 \c at mt_word, dw 1024
4109 \c at mt_byte, db 'x'
4110 \c at mt_str, db 'hello, world', 13, 10, 0
4114 The function of the \c{AT} macro is to make use of the \c{TIMES}
4115 prefix to advance the assembly position to the correct point for the
4116 specified structure field, and then to declare the specified data.
4117 Therefore the structure fields must be declared in the same order as
4118 they were specified in the structure definition.
4120 If the data to go in a structure field requires more than one source
4121 line to specify, the remaining source lines can easily come after
4122 the \c{AT} line. For example:
4124 \c at mt_str, db 123,134,145,156,167,178,189
4127 Depending on personal taste, you can also omit the code part of the
4128 \c{AT} line completely, and start the structure field on the next
4132 \c db 'hello, world'
4136 \S{align} \i\c{ALIGN} and \i\c{ALIGNB}: Data Alignment
4138 The \c{ALIGN} and \c{ALIGNB} macros provides a convenient way to
4139 align code or data on a word, longword, paragraph or other boundary.
4140 (Some assemblers call this directive \i\c{EVEN}.) The syntax of the
4141 \c{ALIGN} and \c{ALIGNB} macros is
4143 \c align 4 ; align on 4-byte boundary
4144 \c align 16 ; align on 16-byte boundary
4145 \c align 8,db 0 ; pad with 0s rather than NOPs
4146 \c align 4,resb 1 ; align to 4 in the BSS
4147 \c alignb 4 ; equivalent to previous line
4149 Both macros require their first argument to be a power of two; they
4150 both compute the number of additional bytes required to bring the
4151 length of the current section up to a multiple of that power of two,
4152 and then apply the \c{TIMES} prefix to their second argument to
4153 perform the alignment.
4155 If the second argument is not specified, the default for \c{ALIGN}
4156 is \c{NOP}, and the default for \c{ALIGNB} is \c{RESB 1}. So if the
4157 second argument is specified, the two macros are equivalent.
4158 Normally, you can just use \c{ALIGN} in code and data sections and
4159 \c{ALIGNB} in BSS sections, and never need the second argument
4160 except for special purposes.
4162 \c{ALIGN} and \c{ALIGNB}, being simple macros, perform no error
4163 checking: they cannot warn you if their first argument fails to be a
4164 power of two, or if their second argument generates more than one
4165 byte of code. In each of these cases they will silently do the wrong
4168 \c{ALIGNB} (or \c{ALIGN} with a second argument of \c{RESB 1}) can
4169 be used within structure definitions:
4186 This will ensure that the structure members are sensibly aligned
4187 relative to the base of the structure.
4189 A final caveat: \c{ALIGN} and \c{ALIGNB} work relative to the
4190 beginning of the \e{section}, not the beginning of the address space
4191 in the final executable. Aligning to a 16-byte boundary when the
4192 section you're in is only guaranteed to be aligned to a 4-byte
4193 boundary, for example, is a waste of effort. Again, NASM does not
4194 check that the section's alignment characteristics are sensible for
4195 the use of \c{ALIGN} or \c{ALIGNB}.
4197 Both \c{ALIGN} and \c{ALIGNB} do call \c{SECTALIGN} macro implicitly.
4198 See \k{sectalign} for details.
4200 See also the \c{smartalign} standard macro package, \k{pkg_smartalign}.
4203 \S{sectalign} \i\c{SECTALIGN}: Section Alignment
4205 The \c{SECTALIGN} macros provides a way to modify alignment attribute
4206 of output file section. Unlike the \c{align=} attribute (which is allowed
4207 at section definition only) the \c{SECTALIGN} macro may be used at any time.
4209 For example the directive
4213 sets the section alignment requirements to 16 bytes. Once increased it can
4214 not be decreased, the magnitude may grow only.
4216 Note that \c{ALIGN} (see \k{align}) calls the \c{SECTALIGN} macro implicitly
4217 so the active section alignment requirements may be updated. This is by default
4218 behaviour, if for some reason you want the \c{ALIGN} do not call \c{SECTALIGN}
4219 at all use the directive
4223 It is still possible to turn in on again by
4228 \C{macropkg} \i{Standard Macro Packages}
4230 The \i\c{%use} directive (see \k{use}) includes one of the standard
4231 macro packages included with the NASM distribution and compiled into
4232 the NASM binary. It operates like the \c{%include} directive (see
4233 \k{include}), but the included contents is provided by NASM itself.
4235 The names of standard macro packages are case insensitive, and can be
4239 \H{pkg_altreg} \i\c{altreg}: \i{Alternate Register Names}
4241 The \c{altreg} standard macro package provides alternate register
4242 names. It provides numeric register names for all registers (not just
4243 \c{R8}-\c{R15}), the Intel-defined aliases \c{R8L}-\c{R15L} for the
4244 low bytes of register (as opposed to the NASM/AMD standard names
4245 \c{R8B}-\c{R15B}), and the names \c{R0H}-\c{R3H} (by analogy with
4246 \c{R0L}-\c{R3L}) for \c{AH}, \c{CH}, \c{DH}, and \c{BH}.
4253 \c mov r0l,r3h ; mov al,bh
4259 \H{pkg_smartalign} \i\c{smartalign}\I{align, smart}: Smart \c{ALIGN} Macro
4261 The \c{smartalign} standard macro package provides for an \i\c{ALIGN}
4262 macro which is more powerful than the default (and
4263 backwards-compatible) one (see \k{align}). When the \c{smartalign}
4264 package is enabled, when \c{ALIGN} is used without a second argument,
4265 NASM will generate a sequence of instructions more efficient than a
4266 series of \c{NOP}. Furthermore, if the padding exceeds a specific
4267 threshold, then NASM will generate a jump over the entire padding
4270 The specific instructions generated can be controlled with the
4271 new \i\c{ALIGNMODE} macro. This macro takes two parameters: one mode,
4272 and an optional jump threshold override. If (for any reason) you need
4273 to turn off the jump completely just set jump threshold value to -1
4274 (or set it to \c{nojmp}). The following modes are possible:
4276 \b \c{generic}: Works on all x86 CPUs and should have reasonable
4277 performance. The default jump threshold is 8. This is the
4280 \b \c{nop}: Pad out with \c{NOP} instructions. The only difference
4281 compared to the standard \c{ALIGN} macro is that NASM can still jump
4282 over a large padding area. The default jump threshold is 16.
4284 \b \c{k7}: Optimize for the AMD K7 (Athlon/Althon XP). These
4285 instructions should still work on all x86 CPUs. The default jump
4288 \b \c{k8}: Optimize for the AMD K8 (Opteron/Althon 64). These
4289 instructions should still work on all x86 CPUs. The default jump
4292 \b \c{p6}: Optimize for Intel CPUs. This uses the long \c{NOP}
4293 instructions first introduced in Pentium Pro. This is incompatible
4294 with all CPUs of family 5 or lower, as well as some VIA CPUs and
4295 several virtualization solutions. The default jump threshold is 16.
4297 The macro \i\c{__ALIGNMODE__} is defined to contain the current
4298 alignment mode. A number of other macros beginning with \c{__ALIGN_}
4299 are used internally by this macro package.
4302 \H{pkg_fp} \i\c\{fp}: Floating-point macros
4304 This packages contains the following floating-point convenience macros:
4306 \c %define Inf __Infinity__
4307 \c %define NaN __QNaN__
4308 \c %define QNaN __QNaN__
4309 \c %define SNaN __SNaN__
4311 \c %define float8(x) __float8__(x)
4312 \c %define float16(x) __float16__(x)
4313 \c %define float32(x) __float32__(x)
4314 \c %define float64(x) __float64__(x)
4315 \c %define float80m(x) __float80m__(x)
4316 \c %define float80e(x) __float80e__(x)
4317 \c %define float128l(x) __float128l__(x)
4318 \c %define float128h(x) __float128h__(x)
4321 \H{pkg_ifunc} \i\c{ifunc}: \i{Integer functions}
4323 This package contains a set of macros which implement integer
4324 functions. These are actually implemented as special operators, but
4325 are most conveniently accessed via this macro package.
4327 The macros provided are:
4329 \S{ilog2} \i{Integer logarithms}
4331 These functions calculate the integer logarithm base 2 of their
4332 argument, considered as an unsigned integer. The only differences
4333 between the functions is their behavior if the argument provided is
4336 The function \i\c{ilog2e()} (alias \i\c{ilog2()}) generate an error if
4337 the argument is not a power of two.
4339 The function \i\c{ilog2w()} generate a warning if the argument is not
4342 The function \i\c{ilog2f()} rounds the argument down to the nearest
4343 power of two; if the argument is zero it returns zero.
4345 The function \i\c{ilog2c()} rounds the argument up to the nearest
4349 \C{directive} \i{Assembler Directives}
4351 NASM, though it attempts to avoid the bureaucracy of assemblers like
4352 MASM and TASM, is nevertheless forced to support a \e{few}
4353 directives. These are described in this chapter.
4355 NASM's directives come in two types: \I{user-level
4356 directives}\e{user-level} directives and \I{primitive
4357 directives}\e{primitive} directives. Typically, each directive has a
4358 user-level form and a primitive form. In almost all cases, we
4359 recommend that users use the user-level forms of the directives,
4360 which are implemented as macros which call the primitive forms.
4362 Primitive directives are enclosed in square brackets; user-level
4365 In addition to the universal directives described in this chapter,
4366 each object file format can optionally supply extra directives in
4367 order to control particular features of that file format. These
4368 \I{format-specific directives}\e{format-specific} directives are
4369 documented along with the formats that implement them, in \k{outfmt}.
4372 \H{bits} \i\c{BITS}: Specifying Target \i{Processor Mode}
4374 The \c{BITS} directive specifies whether NASM should generate code
4375 \I{16-bit mode, versus 32-bit mode}designed to run on a processor
4376 operating in 16-bit mode, 32-bit mode or 64-bit mode. The syntax is
4377 \c{BITS XX}, where XX is 16, 32 or 64.
4379 In most cases, you should not need to use \c{BITS} explicitly. The
4380 \c{aout}, \c{coff}, \c{elf}, \c{macho}, \c{win32} and \c{win64}
4381 object formats, which are designed for use in 32-bit or 64-bit
4382 operating systems, all cause NASM to select 32-bit or 64-bit mode,
4383 respectively, by default. The \c{obj} object format allows you
4384 to specify each segment you define as either \c{USE16} or \c{USE32},
4385 and NASM will set its operating mode accordingly, so the use of the
4386 \c{BITS} directive is once again unnecessary.
4388 The most likely reason for using the \c{BITS} directive is to write
4389 32-bit or 64-bit code in a flat binary file; this is because the \c{bin}
4390 output format defaults to 16-bit mode in anticipation of it being
4391 used most frequently to write DOS \c{.COM} programs, DOS \c{.SYS}
4392 device drivers and boot loader software.
4394 You do \e{not} need to specify \c{BITS 32} merely in order to use
4395 32-bit instructions in a 16-bit DOS program; if you do, the
4396 assembler will generate incorrect code because it will be writing
4397 code targeted at a 32-bit platform, to be run on a 16-bit one.
4399 When NASM is in \c{BITS 16} mode, instructions which use 32-bit
4400 data are prefixed with an 0x66 byte, and those referring to 32-bit
4401 addresses have an 0x67 prefix. In \c{BITS 32} mode, the reverse is
4402 true: 32-bit instructions require no prefixes, whereas instructions
4403 using 16-bit data need an 0x66 and those working on 16-bit addresses
4406 When NASM is in \c{BITS 64} mode, most instructions operate the same
4407 as they do for \c{BITS 32} mode. However, there are 8 more general and
4408 SSE registers, and 16-bit addressing is no longer supported.
4410 The default address size is 64 bits; 32-bit addressing can be selected
4411 with the 0x67 prefix. The default operand size is still 32 bits,
4412 however, and the 0x66 prefix selects 16-bit operand size. The \c{REX}
4413 prefix is used both to select 64-bit operand size, and to access the
4414 new registers. NASM automatically inserts REX prefixes when
4417 When the \c{REX} prefix is used, the processor does not know how to
4418 address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead,
4419 it is possible to access the the low 8-bits of the SP, BP SI and DI
4420 registers as SPL, BPL, SIL and DIL, respectively; but only when the
4423 The \c{BITS} directive has an exactly equivalent primitive form,
4424 \c{[BITS 16]}, \c{[BITS 32]} and \c{[BITS 64]}. The user-level form is
4425 a macro which has no function other than to call the primitive form.
4427 Note that the space is neccessary, e.g. \c{BITS32} will \e{not} work!
4429 \S{USE16 & USE32} \i\c{USE16} & \i\c{USE32}: Aliases for BITS
4431 The `\c{USE16}' and `\c{USE32}' directives can be used in place of
4432 `\c{BITS 16}' and `\c{BITS 32}', for compatibility with other assemblers.
4435 \H{default} \i\c{DEFAULT}: Change the assembler defaults
4437 The \c{DEFAULT} directive changes the assembler defaults. Normally,
4438 NASM defaults to a mode where the programmer is expected to explicitly
4439 specify most features directly. However, this is occationally
4440 obnoxious, as the explicit form is pretty much the only one one wishes
4443 Currently, \c{DEFAULT} can set \c{REL} & \c{ABS} and \c{BND} & \c{NOBND}.
4445 \S{REL & ABS} \i\c{REL} & \i\c{ABS}: RIP-relative addressing
4447 This sets whether registerless instructions in 64-bit mode are \c{RIP}-relative
4448 or not. By default, they are absolute unless overridden with the \i\c{REL}
4449 specifier (see \k{effaddr}). However, if \c{DEFAULT REL} is
4450 specified, \c{REL} is default, unless overridden with the \c{ABS}
4451 specifier, \e{except when used with an FS or GS segment override}.
4453 The special handling of \c{FS} and \c{GS} overrides are due to the
4454 fact that these registers are generally used as thread pointers or
4455 other special functions in 64-bit mode, and generating
4456 \c{RIP}-relative addresses would be extremely confusing.
4458 \c{DEFAULT REL} is disabled with \c{DEFAULT ABS}.
4460 \S{BND & NOBND} \i\c{BND} & \i\c{NOBND}: \c{BND} prefix
4462 If \c{DEFAULT BND} is set, all bnd-prefix available instructions following
4463 this directive are prefixed with bnd. To override it, \c{NOBND} prefix can
4467 \c call foo ; BND will be prefixed
4468 \c nobnd call foo ; BND will NOT be prefixed
4470 \c{DEFAULT NOBND} can disable \c{DEFAULT BND} and then \c{BND} prefix will be
4471 added only when explicitly specified in code.
4473 \H{section} \i\c{SECTION} or \i\c{SEGMENT}: Changing and \i{Defining
4476 \I{changing sections}\I{switching between sections}The \c{SECTION}
4477 directive (\c{SEGMENT} is an exactly equivalent synonym) changes
4478 which section of the output file the code you write will be
4479 assembled into. In some object file formats, the number and names of
4480 sections are fixed; in others, the user may make up as many as they
4481 wish. Hence \c{SECTION} may sometimes give an error message, or may
4482 define a new section, if you try to switch to a section that does
4485 The Unix object formats, and the \c{bin} object format (but see
4486 \k{multisec}, all support
4487 the \i{standardized section names} \c{.text}, \c{.data} and \c{.bss}
4488 for the code, data and uninitialized-data sections. The \c{obj}
4489 format, by contrast, does not recognize these section names as being
4490 special, and indeed will strip off the leading period of any section
4494 \S{sectmac} The \i\c{__SECT__} Macro
4496 The \c{SECTION} directive is unusual in that its user-level form
4497 functions differently from its primitive form. The primitive form,
4498 \c{[SECTION xyz]}, simply switches the current target section to the
4499 one given. The user-level form, \c{SECTION xyz}, however, first
4500 defines the single-line macro \c{__SECT__} to be the primitive
4501 \c{[SECTION]} directive which it is about to issue, and then issues
4502 it. So the user-level directive
4506 expands to the two lines
4508 \c %define __SECT__ [SECTION .text]
4511 Users may find it useful to make use of this in their own macros.
4512 For example, the \c{writefile} macro defined in \k{mlmacgre} can be
4513 usefully rewritten in the following more sophisticated form:
4515 \c %macro writefile 2+
4525 \c mov cx,%%endstr-%%str
4532 This form of the macro, once passed a string to output, first
4533 switches temporarily to the data section of the file, using the
4534 primitive form of the \c{SECTION} directive so as not to modify
4535 \c{__SECT__}. It then declares its string in the data section, and
4536 then invokes \c{__SECT__} to switch back to \e{whichever} section
4537 the user was previously working in. It thus avoids the need, in the
4538 previous version of the macro, to include a \c{JMP} instruction to
4539 jump over the data, and also does not fail if, in a complicated
4540 \c{OBJ} format module, the user could potentially be assembling the
4541 code in any of several separate code sections.
4544 \H{absolute} \i\c{ABSOLUTE}: Defining Absolute Labels
4546 The \c{ABSOLUTE} directive can be thought of as an alternative form
4547 of \c{SECTION}: it causes the subsequent code to be directed at no
4548 physical section, but at the hypothetical section starting at the
4549 given absolute address. The only instructions you can use in this
4550 mode are the \c{RESB} family.
4552 \c{ABSOLUTE} is used as follows:
4560 This example describes a section of the PC BIOS data area, at
4561 segment address 0x40: the above code defines \c{kbuf_chr} to be
4562 0x1A, \c{kbuf_free} to be 0x1C, and \c{kbuf} to be 0x1E.
4564 The user-level form of \c{ABSOLUTE}, like that of \c{SECTION},
4565 redefines the \i\c{__SECT__} macro when it is invoked.
4567 \i\c{STRUC} and \i\c{ENDSTRUC} are defined as macros which use
4568 \c{ABSOLUTE} (and also \c{__SECT__}).
4570 \c{ABSOLUTE} doesn't have to take an absolute constant as an
4571 argument: it can take an expression (actually, a \i{critical
4572 expression}: see \k{crit}) and it can be a value in a segment. For
4573 example, a TSR can re-use its setup code as run-time BSS like this:
4575 \c org 100h ; it's a .COM program
4577 \c jmp setup ; setup code comes last
4579 \c ; the resident part of the TSR goes here
4581 \c ; now write the code that installs the TSR here
4585 \c runtimevar1 resw 1
4586 \c runtimevar2 resd 20
4590 This defines some variables `on top of' the setup code, so that
4591 after the setup has finished running, the space it took up can be
4592 re-used as data storage for the running TSR. The symbol `tsr_end'
4593 can be used to calculate the total size of the part of the TSR that
4594 needs to be made resident.
4597 \H{extern} \i\c{EXTERN}: \i{Importing Symbols} from Other Modules
4599 \c{EXTERN} is similar to the MASM directive \c{EXTRN} and the C
4600 keyword \c{extern}: it is used to declare a symbol which is not
4601 defined anywhere in the module being assembled, but is assumed to be
4602 defined in some other module and needs to be referred to by this
4603 one. Not every object-file format can support external variables:
4604 the \c{bin} format cannot.
4606 The \c{EXTERN} directive takes as many arguments as you like. Each
4607 argument is the name of a symbol:
4610 \c extern _sscanf,_fscanf
4612 Some object-file formats provide extra features to the \c{EXTERN}
4613 directive. In all cases, the extra features are used by suffixing a
4614 colon to the symbol name followed by object-format specific text.
4615 For example, the \c{obj} format allows you to declare that the
4616 default segment base of an external should be the group \c{dgroup}
4617 by means of the directive
4619 \c extern _variable:wrt dgroup
4621 The primitive form of \c{EXTERN} differs from the user-level form
4622 only in that it can take only one argument at a time: the support
4623 for multiple arguments is implemented at the preprocessor level.
4625 You can declare the same variable as \c{EXTERN} more than once: NASM
4626 will quietly ignore the second and later redeclarations. You can't
4627 declare a variable as \c{EXTERN} as well as something else, though.
4630 \H{global} \i\c{GLOBAL}: \i{Exporting Symbols} to Other Modules
4632 \c{GLOBAL} is the other end of \c{EXTERN}: if one module declares a
4633 symbol as \c{EXTERN} and refers to it, then in order to prevent
4634 linker errors, some other module must actually \e{define} the
4635 symbol and declare it as \c{GLOBAL}. Some assemblers use the name
4636 \i\c{PUBLIC} for this purpose.
4638 The \c{GLOBAL} directive applying to a symbol must appear \e{before}
4639 the definition of the symbol.
4641 \c{GLOBAL} uses the same syntax as \c{EXTERN}, except that it must
4642 refer to symbols which \e{are} defined in the same module as the
4643 \c{GLOBAL} directive. For example:
4649 \c{GLOBAL}, like \c{EXTERN}, allows object formats to define private
4650 extensions by means of a colon. The \c{elf} object format, for
4651 example, lets you specify whether global data items are functions or
4654 \c global hashlookup:function, hashtable:data
4656 Like \c{EXTERN}, the primitive form of \c{GLOBAL} differs from the
4657 user-level form only in that it can take only one argument at a
4661 \H{common} \i\c{COMMON}: Defining Common Data Areas
4663 The \c{COMMON} directive is used to declare \i\e{common variables}.
4664 A common variable is much like a global variable declared in the
4665 uninitialized data section, so that
4669 is similar in function to
4676 The difference is that if more than one module defines the same
4677 common variable, then at link time those variables will be
4678 \e{merged}, and references to \c{intvar} in all modules will point
4679 at the same piece of memory.
4681 Like \c{GLOBAL} and \c{EXTERN}, \c{COMMON} supports object-format
4682 specific extensions. For example, the \c{obj} format allows common
4683 variables to be NEAR or FAR, and the \c{elf} format allows you to
4684 specify the alignment requirements of a common variable:
4686 \c common commvar 4:near ; works in OBJ
4687 \c common intarray 100:4 ; works in ELF: 4 byte aligned
4689 Once again, like \c{EXTERN} and \c{GLOBAL}, the primitive form of
4690 \c{COMMON} differs from the user-level form only in that it can take
4691 only one argument at a time.
4694 \H{CPU} \i\c{CPU}: Defining CPU Dependencies
4696 The \i\c{CPU} directive restricts assembly to those instructions which
4697 are available on the specified CPU.
4701 \b\c{CPU 8086} Assemble only 8086 instruction set
4703 \b\c{CPU 186} Assemble instructions up to the 80186 instruction set
4705 \b\c{CPU 286} Assemble instructions up to the 286 instruction set
4707 \b\c{CPU 386} Assemble instructions up to the 386 instruction set
4709 \b\c{CPU 486} 486 instruction set
4711 \b\c{CPU 586} Pentium instruction set
4713 \b\c{CPU PENTIUM} Same as 586
4715 \b\c{CPU 686} P6 instruction set
4717 \b\c{CPU PPRO} Same as 686
4719 \b\c{CPU P2} Same as 686
4721 \b\c{CPU P3} Pentium III (Katmai) instruction sets
4723 \b\c{CPU KATMAI} Same as P3
4725 \b\c{CPU P4} Pentium 4 (Willamette) instruction set
4727 \b\c{CPU WILLAMETTE} Same as P4
4729 \b\c{CPU PRESCOTT} Prescott instruction set
4731 \b\c{CPU X64} x86-64 (x64/AMD64/Intel 64) instruction set
4733 \b\c{CPU IA64} IA64 CPU (in x86 mode) instruction set
4735 All options are case insensitive. All instructions will be selected
4736 only if they apply to the selected CPU or lower. By default, all
4737 instructions are available.
4740 \H{FLOAT} \i\c{FLOAT}: Handling of \I{floating-point, constants}floating-point constants
4742 By default, floating-point constants are rounded to nearest, and IEEE
4743 denormals are supported. The following options can be set to alter
4746 \b\c{FLOAT DAZ} Flush denormals to zero
4748 \b\c{FLOAT NODAZ} Do not flush denormals to zero (default)
4750 \b\c{FLOAT NEAR} Round to nearest (default)
4752 \b\c{FLOAT UP} Round up (toward +Infinity)
4754 \b\c{FLOAT DOWN} Round down (toward -Infinity)
4756 \b\c{FLOAT ZERO} Round toward zero
4758 \b\c{FLOAT DEFAULT} Restore default settings
4760 The standard macros \i\c{__FLOAT_DAZ__}, \i\c{__FLOAT_ROUND__}, and
4761 \i\c{__FLOAT__} contain the current state, as long as the programmer
4762 has avoided the use of the brackeded primitive form, (\c{[FLOAT]}).
4764 \c{__FLOAT__} contains the full set of floating-point settings; this
4765 value can be saved away and invoked later to restore the setting.
4768 \C{outfmt} \i{Output Formats}
4770 NASM is a portable assembler, designed to be able to compile on any
4771 ANSI C-supporting platform and produce output to run on a variety of
4772 Intel x86 operating systems. For this reason, it has a large number
4773 of available output formats, selected using the \i\c{-f} option on
4774 the NASM \i{command line}. Each of these formats, along with its
4775 extensions to the base NASM syntax, is detailed in this chapter.
4777 As stated in \k{opt-o}, NASM chooses a \i{default name} for your
4778 output file based on the input file name and the chosen output
4779 format. This will be generated by removing the \i{extension}
4780 (\c{.asm}, \c{.s}, or whatever you like to use) from the input file
4781 name, and substituting an extension defined by the output format.
4782 The extensions are given with each format below.
4785 \H{binfmt} \i\c{bin}: \i{Flat-Form Binary}\I{pure binary} Output
4787 The \c{bin} format does not produce object files: it generates
4788 nothing in the output file except the code you wrote. Such `pure
4789 binary' files are used by \i{MS-DOS}: \i\c{.COM} executables and
4790 \i\c{.SYS} device drivers are pure binary files. Pure binary output
4791 is also useful for \i{operating system} and \i{boot loader}
4794 The \c{bin} format supports \i{multiple section names}. For details of
4795 how NASM handles sections in the \c{bin} format, see \k{multisec}.
4797 Using the \c{bin} format puts NASM by default into 16-bit mode (see
4798 \k{bits}). In order to use \c{bin} to write 32-bit or 64-bit code,
4799 such as an OS kernel, you need to explicitly issue the \I\c{BITS}\c{BITS 32}
4800 or \I\c{BITS}\c{BITS 64} directive.
4802 \c{bin} has no default output file name extension: instead, it
4803 leaves your file name as it is once the original extension has been
4804 removed. Thus, the default is for NASM to assemble \c{binprog.asm}
4805 into a binary file called \c{binprog}.
4808 \S{org} \i\c{ORG}: Binary File \i{Program Origin}
4810 The \c{bin} format provides an additional directive to the list
4811 given in \k{directive}: \c{ORG}. The function of the \c{ORG}
4812 directive is to specify the origin address which NASM will assume
4813 the program begins at when it is loaded into memory.
4815 For example, the following code will generate the longword
4822 Unlike the \c{ORG} directive provided by MASM-compatible assemblers,
4823 which allows you to jump around in the object file and overwrite
4824 code you have already generated, NASM's \c{ORG} does exactly what
4825 the directive says: \e{origin}. Its sole function is to specify one
4826 offset which is added to all internal address references within the
4827 section; it does not permit any of the trickery that MASM's version
4828 does. See \k{proborg} for further comments.
4831 \S{binseg} \c{bin} Extensions to the \c{SECTION}
4832 Directive\I{SECTION, bin extensions to}
4834 The \c{bin} output format extends the \c{SECTION} (or \c{SEGMENT})
4835 directive to allow you to specify the alignment requirements of
4836 segments. This is done by appending the \i\c{ALIGN} qualifier to the
4837 end of the section-definition line. For example,
4839 \c section .data align=16
4841 switches to the section \c{.data} and also specifies that it must be
4842 aligned on a 16-byte boundary.
4844 The parameter to \c{ALIGN} specifies how many low bits of the
4845 section start address must be forced to zero. The alignment value
4846 given may be any power of two.\I{section alignment, in
4847 bin}\I{segment alignment, in bin}\I{alignment, in bin sections}
4850 \S{multisec} \i{Multisection}\I{bin, multisection} Support for the \c{bin} Format
4852 The \c{bin} format allows the use of multiple sections, of arbitrary names,
4853 besides the "known" \c{.text}, \c{.data}, and \c{.bss} names.
4855 \b Sections may be designated \i\c{progbits} or \i\c{nobits}. Default
4856 is \c{progbits} (except \c{.bss}, which defaults to \c{nobits},
4859 \b Sections can be aligned at a specified boundary following the previous
4860 section with \c{align=}, or at an arbitrary byte-granular position with
4863 \b Sections can be given a virtual start address, which will be used
4864 for the calculation of all memory references within that section
4867 \b Sections can be ordered using \i\c{follows=}\c{<section>} or
4868 \i\c{vfollows=}\c{<section>} as an alternative to specifying an explicit
4871 \b Arguments to \c{org}, \c{start}, \c{vstart}, and \c{align=} are
4872 critical expressions. See \k{crit}. E.g. \c{align=(1 << ALIGN_SHIFT)}
4873 - \c{ALIGN_SHIFT} must be defined before it is used here.
4875 \b Any code which comes before an explicit \c{SECTION} directive
4876 is directed by default into the \c{.text} section.
4878 \b If an \c{ORG} statement is not given, \c{ORG 0} is used
4881 \b The \c{.bss} section will be placed after the last \c{progbits}
4882 section, unless \c{start=}, \c{vstart=}, \c{follows=}, or \c{vfollows=}
4885 \b All sections are aligned on dword boundaries, unless a different
4886 alignment has been specified.
4888 \b Sections may not overlap.
4890 \b NASM creates the \c{section.<secname>.start} for each section,
4891 which may be used in your code.
4893 \S{map}\i{Map Files}
4895 Map files can be generated in \c{-f bin} format by means of the \c{[map]}
4896 option. Map types of \c{all} (default), \c{brief}, \c{sections}, \c{segments},
4897 or \c{symbols} may be specified. Output may be directed to \c{stdout}
4898 (default), \c{stderr}, or a specified file. E.g.
4899 \c{[map symbols myfile.map]}. No "user form" exists, the square
4900 brackets must be used.
4903 \H{ithfmt} \i\c{ith}: \i{Intel Hex} Output
4905 The \c{ith} file format produces Intel hex-format files. Just as the
4906 \c{bin} format, this is a flat memory image format with no support for
4907 relocation or linking. It is usually used with ROM programmers and
4910 All extensions supported by the \c{bin} file format is also supported by
4911 the \c{ith} file format.
4913 \c{ith} provides a default output file-name extension of \c{.ith}.
4916 \H{srecfmt} \i\c{srec}: \i{Motorola S-Records} Output
4918 The \c{srec} file format produces Motorola S-records files. Just as the
4919 \c{bin} format, this is a flat memory image format with no support for
4920 relocation or linking. It is usually used with ROM programmers and
4923 All extensions supported by the \c{bin} file format is also supported by
4924 the \c{srec} file format.
4926 \c{srec} provides a default output file-name extension of \c{.srec}.
4929 \H{objfmt} \i\c{obj}: \i{Microsoft OMF}\I{OMF} Object Files
4931 The \c{obj} file format (NASM calls it \c{obj} rather than \c{omf}
4932 for historical reasons) is the one produced by \i{MASM} and
4933 \i{TASM}, which is typically fed to 16-bit DOS linkers to produce
4934 \i\c{.EXE} files. It is also the format used by \i{OS/2}.
4936 \c{obj} provides a default output file-name extension of \c{.obj}.
4938 \c{obj} is not exclusively a 16-bit format, though: NASM has full
4939 support for the 32-bit extensions to the format. In particular,
4940 32-bit \c{obj} format files are used by \i{Borland's Win32
4941 compilers}, instead of using Microsoft's newer \i\c{win32} object
4944 The \c{obj} format does not define any special segment names: you
4945 can call your segments anything you like. Typical names for segments
4946 in \c{obj} format files are \c{CODE}, \c{DATA} and \c{BSS}.
4948 If your source file contains code before specifying an explicit
4949 \c{SEGMENT} directive, then NASM will invent its own segment called
4950 \i\c{__NASMDEFSEG} for you.
4952 When you define a segment in an \c{obj} file, NASM defines the
4953 segment name as a symbol as well, so that you can access the segment
4954 address of the segment. So, for example:
4963 \c mov ax,data ; get segment address of data
4964 \c mov ds,ax ; and move it into DS
4965 \c inc word [dvar] ; now this reference will work
4968 The \c{obj} format also enables the use of the \i\c{SEG} and
4969 \i\c{WRT} operators, so that you can write code which does things
4974 \c mov ax,seg foo ; get preferred segment of foo
4976 \c mov ax,data ; a different segment
4978 \c mov ax,[ds:foo] ; this accesses `foo'
4979 \c mov [es:foo wrt data],bx ; so does this
4982 \S{objseg} \c{obj} Extensions to the \c{SEGMENT}
4983 Directive\I{SEGMENT, obj extensions to}
4985 The \c{obj} output format extends the \c{SEGMENT} (or \c{SECTION})
4986 directive to allow you to specify various properties of the segment
4987 you are defining. This is done by appending extra qualifiers to the
4988 end of the segment-definition line. For example,
4990 \c segment code private align=16
4992 defines the segment \c{code}, but also declares it to be a private
4993 segment, and requires that the portion of it described in this code
4994 module must be aligned on a 16-byte boundary.
4996 The available qualifiers are:
4998 \b \i\c{PRIVATE}, \i\c{PUBLIC}, \i\c{COMMON} and \i\c{STACK} specify
4999 the combination characteristics of the segment. \c{PRIVATE} segments
5000 do not get combined with any others by the linker; \c{PUBLIC} and
5001 \c{STACK} segments get concatenated together at link time; and
5002 \c{COMMON} segments all get overlaid on top of each other rather
5003 than stuck end-to-end.
5005 \b \i\c{ALIGN} is used, as shown above, to specify how many low bits
5006 of the segment start address must be forced to zero. The alignment
5007 value given may be any power of two from 1 to 4096; in reality, the
5008 only values supported are 1, 2, 4, 16, 256 and 4096, so if 8 is
5009 specified it will be rounded up to 16, and 32, 64 and 128 will all
5010 be rounded up to 256, and so on. Note that alignment to 4096-byte
5011 boundaries is a \i{PharLap} extension to the format and may not be
5012 supported by all linkers.\I{section alignment, in OBJ}\I{segment
5013 alignment, in OBJ}\I{alignment, in OBJ sections}
5015 \b \i\c{CLASS} can be used to specify the segment class; this feature
5016 indicates to the linker that segments of the same class should be
5017 placed near each other in the output file. The class name can be any
5018 word, e.g. \c{CLASS=CODE}.
5020 \b \i\c{OVERLAY}, like \c{CLASS}, is specified with an arbitrary word
5021 as an argument, and provides overlay information to an
5022 overlay-capable linker.
5024 \b Segments can be declared as \i\c{USE16} or \i\c{USE32}, which has
5025 the effect of recording the choice in the object file and also
5026 ensuring that NASM's default assembly mode when assembling in that
5027 segment is 16-bit or 32-bit respectively.
5029 \b When writing \i{OS/2} object files, you should declare 32-bit
5030 segments as \i\c{FLAT}, which causes the default segment base for
5031 anything in the segment to be the special group \c{FLAT}, and also
5032 defines the group if it is not already defined.
5034 \b The \c{obj} file format also allows segments to be declared as
5035 having a pre-defined absolute segment address, although no linkers
5036 are currently known to make sensible use of this feature;
5037 nevertheless, NASM allows you to declare a segment such as
5038 \c{SEGMENT SCREEN ABSOLUTE=0xB800} if you need to. The \i\c{ABSOLUTE}
5039 and \c{ALIGN} keywords are mutually exclusive.
5041 NASM's default segment attributes are \c{PUBLIC}, \c{ALIGN=1}, no
5042 class, no overlay, and \c{USE16}.
5045 \S{group} \i\c{GROUP}: Defining Groups of Segments\I{segments, groups of}
5047 The \c{obj} format also allows segments to be grouped, so that a
5048 single segment register can be used to refer to all the segments in
5049 a group. NASM therefore supplies the \c{GROUP} directive, whereby
5058 \c ; some uninitialized data
5060 \c group dgroup data bss
5062 which will define a group called \c{dgroup} to contain the segments
5063 \c{data} and \c{bss}. Like \c{SEGMENT}, \c{GROUP} causes the group
5064 name to be defined as a symbol, so that you can refer to a variable
5065 \c{var} in the \c{data} segment as \c{var wrt data} or as \c{var wrt
5066 dgroup}, depending on which segment value is currently in your
5069 If you just refer to \c{var}, however, and \c{var} is declared in a
5070 segment which is part of a group, then NASM will default to giving
5071 you the offset of \c{var} from the beginning of the \e{group}, not
5072 the \e{segment}. Therefore \c{SEG var}, also, will return the group
5073 base rather than the segment base.
5075 NASM will allow a segment to be part of more than one group, but
5076 will generate a warning if you do this. Variables declared in a
5077 segment which is part of more than one group will default to being
5078 relative to the first group that was defined to contain the segment.
5080 A group does not have to contain any segments; you can still make
5081 \c{WRT} references to a group which does not contain the variable
5082 you are referring to. OS/2, for example, defines the special group
5083 \c{FLAT} with no segments in it.
5086 \S{uppercase} \i\c{UPPERCASE}: Disabling Case Sensitivity in Output
5088 Although NASM itself is \i{case sensitive}, some OMF linkers are
5089 not; therefore it can be useful for NASM to output single-case
5090 object files. The \c{UPPERCASE} format-specific directive causes all
5091 segment, group and symbol names that are written to the object file
5092 to be forced to upper case just before being written. Within a
5093 source file, NASM is still case-sensitive; but the object file can
5094 be written entirely in upper case if desired.
5096 \c{UPPERCASE} is used alone on a line; it requires no parameters.
5099 \S{import} \i\c{IMPORT}: Importing DLL Symbols\I{DLL symbols,
5100 importing}\I{symbols, importing from DLLs}
5102 The \c{IMPORT} format-specific directive defines a symbol to be
5103 imported from a DLL, for use if you are writing a DLL's \i{import
5104 library} in NASM. You still need to declare the symbol as \c{EXTERN}
5105 as well as using the \c{IMPORT} directive.
5107 The \c{IMPORT} directive takes two required parameters, separated by
5108 white space, which are (respectively) the name of the symbol you
5109 wish to import and the name of the library you wish to import it
5112 \c import WSAStartup wsock32.dll
5114 A third optional parameter gives the name by which the symbol is
5115 known in the library you are importing it from, in case this is not
5116 the same as the name you wish the symbol to be known by to your code
5117 once you have imported it. For example:
5119 \c import asyncsel wsock32.dll WSAAsyncSelect
5122 \S{export} \i\c{EXPORT}: Exporting DLL Symbols\I{DLL symbols,
5123 exporting}\I{symbols, exporting from DLLs}
5125 The \c{EXPORT} format-specific directive defines a global symbol to
5126 be exported as a DLL symbol, for use if you are writing a DLL in
5127 NASM. You still need to declare the symbol as \c{GLOBAL} as well as
5128 using the \c{EXPORT} directive.
5130 \c{EXPORT} takes one required parameter, which is the name of the
5131 symbol you wish to export, as it was defined in your source file. An
5132 optional second parameter (separated by white space from the first)
5133 gives the \e{external} name of the symbol: the name by which you
5134 wish the symbol to be known to programs using the DLL. If this name
5135 is the same as the internal name, you may leave the second parameter
5138 Further parameters can be given to define attributes of the exported
5139 symbol. These parameters, like the second, are separated by white
5140 space. If further parameters are given, the external name must also
5141 be specified, even if it is the same as the internal name. The
5142 available attributes are:
5144 \b \c{resident} indicates that the exported name is to be kept
5145 resident by the system loader. This is an optimisation for
5146 frequently used symbols imported by name.
5148 \b \c{nodata} indicates that the exported symbol is a function which
5149 does not make use of any initialized data.
5151 \b \c{parm=NNN}, where \c{NNN} is an integer, sets the number of
5152 parameter words for the case in which the symbol is a call gate
5153 between 32-bit and 16-bit segments.
5155 \b An attribute which is just a number indicates that the symbol
5156 should be exported with an identifying number (ordinal), and gives
5162 \c export myfunc TheRealMoreFormalLookingFunctionName
5163 \c export myfunc myfunc 1234 ; export by ordinal
5164 \c export myfunc myfunc resident parm=23 nodata
5167 \S{dotdotstart} \i\c{..start}: Defining the \i{Program Entry
5170 \c{OMF} linkers require exactly one of the object files being linked to
5171 define the program entry point, where execution will begin when the
5172 program is run. If the object file that defines the entry point is
5173 assembled using NASM, you specify the entry point by declaring the
5174 special symbol \c{..start} at the point where you wish execution to
5178 \S{objextern} \c{obj} Extensions to the \c{EXTERN}
5179 Directive\I{EXTERN, obj extensions to}
5181 If you declare an external symbol with the directive
5185 then references such as \c{mov ax,foo} will give you the offset of
5186 \c{foo} from its preferred segment base (as specified in whichever
5187 module \c{foo} is actually defined in). So to access the contents of
5188 \c{foo} you will usually need to do something like
5190 \c mov ax,seg foo ; get preferred segment base
5191 \c mov es,ax ; move it into ES
5192 \c mov ax,[es:foo] ; and use offset `foo' from it
5194 This is a little unwieldy, particularly if you know that an external
5195 is going to be accessible from a given segment or group, say
5196 \c{dgroup}. So if \c{DS} already contained \c{dgroup}, you could
5199 \c mov ax,[foo wrt dgroup]
5201 However, having to type this every time you want to access \c{foo}
5202 can be a pain; so NASM allows you to declare \c{foo} in the
5205 \c extern foo:wrt dgroup
5207 This form causes NASM to pretend that the preferred segment base of
5208 \c{foo} is in fact \c{dgroup}; so the expression \c{seg foo} will
5209 now return \c{dgroup}, and the expression \c{foo} is equivalent to
5212 This \I{default-WRT mechanism}default-\c{WRT} mechanism can be used
5213 to make externals appear to be relative to any group or segment in
5214 your program. It can also be applied to common variables: see
5218 \S{objcommon} \c{obj} Extensions to the \c{COMMON}
5219 Directive\I{COMMON, obj extensions to}
5221 The \c{obj} format allows common variables to be either near\I{near
5222 common variables} or far\I{far common variables}; NASM allows you to
5223 specify which your variables should be by the use of the syntax
5225 \c common nearvar 2:near ; `nearvar' is a near common
5226 \c common farvar 10:far ; and `farvar' is far
5228 Far common variables may be greater in size than 64Kb, and so the
5229 OMF specification says that they are declared as a number of
5230 \e{elements} of a given size. So a 10-byte far common variable could
5231 be declared as ten one-byte elements, five two-byte elements, two
5232 five-byte elements or one ten-byte element.
5234 Some \c{OMF} linkers require the \I{element size, in common
5235 variables}\I{common variables, element size}element size, as well as
5236 the variable size, to match when resolving common variables declared
5237 in more than one module. Therefore NASM must allow you to specify
5238 the element size on your far common variables. This is done by the
5241 \c common c_5by2 10:far 5 ; two five-byte elements
5242 \c common c_2by5 10:far 2 ; five two-byte elements
5244 If no element size is specified, the default is 1. Also, the \c{FAR}
5245 keyword is not required when an element size is specified, since
5246 only far commons may have element sizes at all. So the above
5247 declarations could equivalently be
5249 \c common c_5by2 10:5 ; two five-byte elements
5250 \c common c_2by5 10:2 ; five two-byte elements
5252 In addition to these extensions, the \c{COMMON} directive in \c{obj}
5253 also supports default-\c{WRT} specification like \c{EXTERN} does
5254 (explained in \k{objextern}). So you can also declare things like
5256 \c common foo 10:wrt dgroup
5257 \c common bar 16:far 2:wrt data
5258 \c common baz 24:wrt data:6
5261 \H{win32fmt} \i\c{win32}: Microsoft Win32 Object Files
5263 The \c{win32} output format generates Microsoft Win32 object files,
5264 suitable for passing to Microsoft linkers such as \i{Visual C++}.
5265 Note that Borland Win32 compilers do not use this format, but use
5266 \c{obj} instead (see \k{objfmt}).
5268 \c{win32} provides a default output file-name extension of \c{.obj}.
5270 Note that although Microsoft say that Win32 object files follow the
5271 \c{COFF} (Common Object File Format) standard, the object files produced
5272 by Microsoft Win32 compilers are not compatible with COFF linkers
5273 such as DJGPP's, and vice versa. This is due to a difference of
5274 opinion over the precise semantics of PC-relative relocations. To
5275 produce COFF files suitable for DJGPP, use NASM's \c{coff} output
5276 format; conversely, the \c{coff} format does not produce object
5277 files that Win32 linkers can generate correct output from.
5280 \S{win32sect} \c{win32} Extensions to the \c{SECTION}
5281 Directive\I{SECTION, win32 extensions to}
5283 Like the \c{obj} format, \c{win32} allows you to specify additional
5284 information on the \c{SECTION} directive line, to control the type
5285 and properties of sections you declare. Section types and properties
5286 are generated automatically by NASM for the \i{standard section names}
5287 \c{.text}, \c{.data} and \c{.bss}, but may still be overridden by
5290 The available qualifiers are:
5292 \b \c{code}, or equivalently \c{text}, defines the section to be a
5293 code section. This marks the section as readable and executable, but
5294 not writable, and also indicates to the linker that the type of the
5297 \b \c{data} and \c{bss} define the section to be a data section,
5298 analogously to \c{code}. Data sections are marked as readable and
5299 writable, but not executable. \c{data} declares an initialized data
5300 section, whereas \c{bss} declares an uninitialized data section.
5302 \b \c{rdata} declares an initialized data section that is readable
5303 but not writable. Microsoft compilers use this section to place
5306 \b \c{info} defines the section to be an \i{informational section},
5307 which is not included in the executable file by the linker, but may
5308 (for example) pass information \e{to} the linker. For example,
5309 declaring an \c{info}-type section called \i\c{.drectve} causes the
5310 linker to interpret the contents of the section as command-line
5313 \b \c{align=}, used with a trailing number as in \c{obj}, gives the
5314 \I{section alignment, in win32}\I{alignment, in win32
5315 sections}alignment requirements of the section. The maximum you may
5316 specify is 64: the Win32 object file format contains no means to
5317 request a greater section alignment than this. If alignment is not
5318 explicitly specified, the defaults are 16-byte alignment for code
5319 sections, 8-byte alignment for rdata sections and 4-byte alignment
5320 for data (and BSS) sections.
5321 Informational sections get a default alignment of 1 byte (no
5322 alignment), though the value does not matter.
5324 The defaults assumed by NASM if you do not specify the above
5327 \c section .text code align=16
5328 \c section .data data align=4
5329 \c section .rdata rdata align=8
5330 \c section .bss bss align=4
5332 Any other section name is treated by default like \c{.text}.
5334 \S{win32safeseh} \c{win32}: Safe Structured Exception Handling
5336 Among other improvements in Windows XP SP2 and Windows Server 2003
5337 Microsoft has introduced concept of "safe structured exception
5338 handling." General idea is to collect handlers' entry points in
5339 designated read-only table and have alleged entry point verified
5340 against this table prior exception control is passed to the handler. In
5341 order for an executable module to be equipped with such "safe exception
5342 handler table," all object modules on linker command line has to comply
5343 with certain criteria. If one single module among them does not, then
5344 the table in question is omitted and above mentioned run-time checks
5345 will not be performed for application in question. Table omission is by
5346 default silent and therefore can be easily overlooked. One can instruct
5347 linker to refuse to produce binary without such table by passing
5348 \c{/safeseh} command line option.
5350 Without regard to this run-time check merits it's natural to expect
5351 NASM to be capable of generating modules suitable for \c{/safeseh}
5352 linking. From developer's viewpoint the problem is two-fold:
5354 \b how to adapt modules not deploying exception handlers of their own;
5356 \b how to adapt/develop modules utilizing custom exception handling;
5358 Former can be easily achieved with any NASM version by adding following
5359 line to source code:
5363 As of version 2.03 NASM adds this absolute symbol automatically. If
5364 it's not already present to be precise. I.e. if for whatever reason
5365 developer would choose to assign another value in source file, it would
5366 still be perfectly possible.
5368 Registering custom exception handler on the other hand requires certain
5369 "magic." As of version 2.03 additional directive is implemented,
5370 \c{safeseh}, which instructs the assembler to produce appropriately
5371 formatted input data for above mentioned "safe exception handler
5372 table." Its typical use would be:
5375 \c extern _MessageBoxA@16
5376 \c %if __NASM_VERSION_ID__ >= 0x02030000
5377 \c safeseh handler ; register handler as "safe handler"
5380 \c push DWORD 1 ; MB_OKCANCEL
5381 \c push DWORD caption
5384 \c call _MessageBoxA@16
5385 \c sub eax,1 ; incidentally suits as return value
5386 \c ; for exception handler
5390 \c push DWORD handler
5391 \c push DWORD [fs:0]
5392 \c mov DWORD [fs:0],esp ; engage exception handler
5394 \c mov eax,DWORD[eax] ; cause exception
5395 \c pop DWORD [fs:0] ; disengage exception handler
5398 \c text: db 'OK to rethrow, CANCEL to generate core dump',0
5399 \c caption:db 'SEGV',0
5401 \c section .drectve info
5402 \c db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
5404 As you might imagine, it's perfectly possible to produce .exe binary
5405 with "safe exception handler table" and yet engage unregistered
5406 exception handler. Indeed, handler is engaged by simply manipulating
5407 \c{[fs:0]} location at run-time, something linker has no power over,
5408 run-time that is. It should be explicitly mentioned that such failure
5409 to register handler's entry point with \c{safeseh} directive has
5410 undesired side effect at run-time. If exception is raised and
5411 unregistered handler is to be executed, the application is abruptly
5412 terminated without any notification whatsoever. One can argue that
5413 system could at least have logged some kind "non-safe exception
5414 handler in x.exe at address n" message in event log, but no, literally
5415 no notification is provided and user is left with no clue on what
5416 caused application failure.
5418 Finally, all mentions of linker in this paragraph refer to Microsoft
5419 linker version 7.x and later. Presence of \c{@feat.00} symbol and input
5420 data for "safe exception handler table" causes no backward
5421 incompatibilities and "safeseh" modules generated by NASM 2.03 and
5422 later can still be linked by earlier versions or non-Microsoft linkers.
5425 \H{win64fmt} \i\c{win64}: Microsoft Win64 Object Files
5427 The \c{win64} output format generates Microsoft Win64 object files,
5428 which is nearly 100% identical to the \c{win32} object format (\k{win32fmt})
5429 with the exception that it is meant to target 64-bit code and the x86-64
5430 platform altogether. This object file is used exactly the same as the \c{win32}
5431 object format (\k{win32fmt}), in NASM, with regard to this exception.
5433 \S{win64pic} \c{win64}: Writing Position-Independent Code
5435 While \c{REL} takes good care of RIP-relative addressing, there is one
5436 aspect that is easy to overlook for a Win64 programmer: indirect
5437 references. Consider a switch dispatch table:
5439 \c jmp qword [dsptch+rax*8]
5445 Even a novice Win64 assembler programmer will soon realize that the code
5446 is not 64-bit savvy. Most notably linker will refuse to link it with
5448 \c 'ADDR32' relocation to '.text' invalid without /LARGEADDRESSAWARE:NO
5450 So [s]he will have to split jmp instruction as following:
5452 \c lea rbx,[rel dsptch]
5453 \c jmp qword [rbx+rax*8]
5455 What happens behind the scene is that effective address in \c{lea} is
5456 encoded relative to instruction pointer, or in perfectly
5457 position-independent manner. But this is only part of the problem!
5458 Trouble is that in .dll context \c{caseN} relocations will make their
5459 way to the final module and might have to be adjusted at .dll load
5460 time. To be specific when it can't be loaded at preferred address. And
5461 when this occurs, pages with such relocations will be rendered private
5462 to current process, which kind of undermines the idea of sharing .dll.
5463 But no worry, it's trivial to fix:
5465 \c lea rbx,[rel dsptch]
5466 \c add rbx,[rbx+rax*8]
5469 \c dsptch: dq case0-dsptch
5473 NASM version 2.03 and later provides another alternative, \c{wrt
5474 ..imagebase} operator, which returns offset from base address of the
5475 current image, be it .exe or .dll module, therefore the name. For those
5476 acquainted with PE-COFF format base address denotes start of
5477 \c{IMAGE_DOS_HEADER} structure. Here is how to implement switch with
5478 these image-relative references:
5480 \c lea rbx,[rel dsptch]
5481 \c mov eax,[rbx+rax*4]
5482 \c sub rbx,dsptch wrt ..imagebase
5486 \c dsptch: dd case0 wrt ..imagebase
5487 \c dd case1 wrt ..imagebase
5489 One can argue that the operator is redundant. Indeed, snippet before
5490 last works just fine with any NASM version and is not even Windows
5491 specific... The real reason for implementing \c{wrt ..imagebase} will
5492 become apparent in next paragraph.
5494 It should be noted that \c{wrt ..imagebase} is defined as 32-bit
5497 \c dd label wrt ..imagebase ; ok
5498 \c dq label wrt ..imagebase ; bad
5499 \c mov eax,label wrt ..imagebase ; ok
5500 \c mov rax,label wrt ..imagebase ; bad
5502 \S{win64seh} \c{win64}: Structured Exception Handling
5504 Structured exception handing in Win64 is completely different matter
5505 from Win32. Upon exception program counter value is noted, and
5506 linker-generated table comprising start and end addresses of all the
5507 functions [in given executable module] is traversed and compared to the
5508 saved program counter. Thus so called \c{UNWIND_INFO} structure is
5509 identified. If it's not found, then offending subroutine is assumed to
5510 be "leaf" and just mentioned lookup procedure is attempted for its
5511 caller. In Win64 leaf function is such function that does not call any
5512 other function \e{nor} modifies any Win64 non-volatile registers,
5513 including stack pointer. The latter ensures that it's possible to
5514 identify leaf function's caller by simply pulling the value from the
5517 While majority of subroutines written in assembler are not calling any
5518 other function, requirement for non-volatile registers' immutability
5519 leaves developer with not more than 7 registers and no stack frame,
5520 which is not necessarily what [s]he counted with. Customarily one would
5521 meet the requirement by saving non-volatile registers on stack and
5522 restoring them upon return, so what can go wrong? If [and only if] an
5523 exception is raised at run-time and no \c{UNWIND_INFO} structure is
5524 associated with such "leaf" function, the stack unwind procedure will
5525 expect to find caller's return address on the top of stack immediately
5526 followed by its frame. Given that developer pushed caller's
5527 non-volatile registers on stack, would the value on top point at some
5528 code segment or even addressable space? Well, developer can attempt
5529 copying caller's return address to the top of stack and this would
5530 actually work in some very specific circumstances. But unless developer
5531 can guarantee that these circumstances are always met, it's more
5532 appropriate to assume worst case scenario, i.e. stack unwind procedure
5533 going berserk. Relevant question is what happens then? Application is
5534 abruptly terminated without any notification whatsoever. Just like in
5535 Win32 case, one can argue that system could at least have logged
5536 "unwind procedure went berserk in x.exe at address n" in event log, but
5537 no, no trace of failure is left.
5539 Now, when we understand significance of the \c{UNWIND_INFO} structure,
5540 let's discuss what's in it and/or how it's processed. First of all it
5541 is checked for presence of reference to custom language-specific
5542 exception handler. If there is one, then it's invoked. Depending on the
5543 return value, execution flow is resumed (exception is said to be
5544 "handled"), \e{or} rest of \c{UNWIND_INFO} structure is processed as
5545 following. Beside optional reference to custom handler, it carries
5546 information about current callee's stack frame and where non-volatile
5547 registers are saved. Information is detailed enough to be able to
5548 reconstruct contents of caller's non-volatile registers upon call to
5549 current callee. And so caller's context is reconstructed, and then
5550 unwind procedure is repeated, i.e. another \c{UNWIND_INFO} structure is
5551 associated, this time, with caller's instruction pointer, which is then
5552 checked for presence of reference to language-specific handler, etc.
5553 The procedure is recursively repeated till exception is handled. As
5554 last resort system "handles" it by generating memory core dump and
5555 terminating the application.
5557 As for the moment of this writing NASM unfortunately does not
5558 facilitate generation of above mentioned detailed information about
5559 stack frame layout. But as of version 2.03 it implements building
5560 blocks for generating structures involved in stack unwinding. As
5561 simplest example, here is how to deploy custom exception handler for
5566 \c extern MessageBoxA
5572 \c mov r9,1 ; MB_OKCANCEL
5574 \c sub eax,1 ; incidentally suits as return value
5575 \c ; for exception handler
5581 \c mov rax,QWORD[rax] ; cause exception
5584 \c text: db 'OK to rethrow, CANCEL to generate core dump',0
5585 \c caption:db 'SEGV',0
5587 \c section .pdata rdata align=4
5588 \c dd main wrt ..imagebase
5589 \c dd main_end wrt ..imagebase
5590 \c dd xmain wrt ..imagebase
5591 \c section .xdata rdata align=8
5592 \c xmain: db 9,0,0,0
5593 \c dd handler wrt ..imagebase
5594 \c section .drectve info
5595 \c db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
5597 What you see in \c{.pdata} section is element of the "table comprising
5598 start and end addresses of function" along with reference to associated
5599 \c{UNWIND_INFO} structure. And what you see in \c{.xdata} section is
5600 \c{UNWIND_INFO} structure describing function with no frame, but with
5601 designated exception handler. References are \e{required} to be
5602 image-relative (which is the real reason for implementing \c{wrt
5603 ..imagebase} operator). It should be noted that \c{rdata align=n}, as
5604 well as \c{wrt ..imagebase}, are optional in these two segments'
5605 contexts, i.e. can be omitted. Latter means that \e{all} 32-bit
5606 references, not only above listed required ones, placed into these two
5607 segments turn out image-relative. Why is it important to understand?
5608 Developer is allowed to append handler-specific data to \c{UNWIND_INFO}
5609 structure, and if [s]he adds a 32-bit reference, then [s]he will have
5610 to remember to adjust its value to obtain the real pointer.
5612 As already mentioned, in Win64 terms leaf function is one that does not
5613 call any other function \e{nor} modifies any non-volatile register,
5614 including stack pointer. But it's not uncommon that assembler
5615 programmer plans to utilize every single register and sometimes even
5616 have variable stack frame. Is there anything one can do with bare
5617 building blocks? I.e. besides manually composing fully-fledged
5618 \c{UNWIND_INFO} structure, which would surely be considered
5619 error-prone? Yes, there is. Recall that exception handler is called
5620 first, before stack layout is analyzed. As it turned out, it's
5621 perfectly possible to manipulate current callee's context in custom
5622 handler in manner that permits further stack unwinding. General idea is
5623 that handler would not actually "handle" the exception, but instead
5624 restore callee's context, as it was at its entry point and thus mimic
5625 leaf function. In other words, handler would simply undertake part of
5626 unwinding procedure. Consider following example:
5629 \c mov rax,rsp ; copy rsp to volatile register
5630 \c push r15 ; save non-volatile registers
5633 \c mov r11,rsp ; prepare variable stack frame
5636 \c mov QWORD[r11],rax ; check for exceptions
5637 \c mov rsp,r11 ; allocate stack frame
5638 \c mov QWORD[rsp],rax ; save original rsp value
5641 \c mov r11,QWORD[rsp] ; pull original rsp value
5642 \c mov rbp,QWORD[r11-24]
5643 \c mov rbx,QWORD[r11-16]
5644 \c mov r15,QWORD[r11-8]
5645 \c mov rsp,r11 ; destroy frame
5648 The keyword is that up to \c{magic_point} original \c{rsp} value
5649 remains in chosen volatile register and no non-volatile register,
5650 except for \c{rsp}, is modified. While past \c{magic_point} \c{rsp}
5651 remains constant till the very end of the \c{function}. In this case
5652 custom language-specific exception handler would look like this:
5654 \c EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame,
5655 \c CONTEXT *context,DISPATCHER_CONTEXT *disp)
5657 \c if (context->Rip<(ULONG64)magic_point)
5658 \c rsp = (ULONG64 *)context->Rax;
5660 \c { rsp = ((ULONG64 **)context->Rsp)[0];
5661 \c context->Rbp = rsp[-3];
5662 \c context->Rbx = rsp[-2];
5663 \c context->R15 = rsp[-1];
5665 \c context->Rsp = (ULONG64)rsp;
5667 \c memcpy (disp->ContextRecord,context,sizeof(CONTEXT));
5668 \c RtlVirtualUnwind(UNW_FLAG_NHANDLER,disp->ImageBase,
5669 \c dips->ControlPc,disp->FunctionEntry,disp->ContextRecord,
5670 \c &disp->HandlerData,&disp->EstablisherFrame,NULL);
5671 \c return ExceptionContinueSearch;
5674 As custom handler mimics leaf function, corresponding \c{UNWIND_INFO}
5675 structure does not have to contain any information about stack frame
5678 \H{cofffmt} \i\c{coff}: \i{Common Object File Format}
5680 The \c{coff} output type produces \c{COFF} object files suitable for
5681 linking with the \i{DJGPP} linker.
5683 \c{coff} provides a default output file-name extension of \c{.o}.
5685 The \c{coff} format supports the same extensions to the \c{SECTION}
5686 directive as \c{win32} does, except that the \c{align} qualifier and
5687 the \c{info} section type are not supported.
5689 \H{machofmt} \I{Mach-O}\i\c{macho32} and \i\c{macho64}: \i{Mach Object File Format}
5691 The \c{macho32} and \c{macho64} output formts produces \c{Mach-O}
5692 object files suitable for linking with the \i{MacOS X} linker.
5693 \i\c{macho} is a synonym for \c{macho32}.
5695 \c{macho} provides a default output file-name extension of \c{.o}.
5697 \H{elffmt} \i\c{elf32}, \i\c{elf64}, \i\c{elfx32}: \I{ELF}\I{linux, elf}\i{Executable and Linkable
5698 Format} Object Files
5700 The \c{elf32}, \c{elf64} and \c{elfx32} output formats generate
5701 \c{ELF32 and ELF64} (Executable and Linkable Format) object files, as
5702 used by Linux as well as \i{Unix System V}, including \i{Solaris x86},
5703 \i{UnixWare} and \i{SCO Unix}. \c{elf} provides a default output
5704 file-name extension of \c{.o}. \c{elf} is a synonym for \c{elf32}.
5706 The \c{elfx32} format is used for the \i{x32} ABI, which is a 32-bit
5707 ABI with the CPU in 64-bit mode.
5709 \S{abisect} ELF specific directive \i\c{osabi}
5711 The ELF header specifies the application binary interface for the target operating system (OSABI).
5712 This field can be set by using the \c{osabi} directive with the numeric value (0-255) of the target
5713 system. If this directive is not used, the default value will be "UNIX System V ABI" (0) which will work on
5714 most systems which support ELF.
5716 \S{elfsect} \c{elf} Extensions to the \c{SECTION}
5717 Directive\I{SECTION, elf extensions to}
5719 Like the \c{obj} format, \c{elf} allows you to specify additional
5720 information on the \c{SECTION} directive line, to control the type
5721 and properties of sections you declare. Section types and properties
5722 are generated automatically by NASM for the \i{standard section
5723 names}, but may still be
5724 overridden by these qualifiers.
5726 The available qualifiers are:
5728 \b \i\c{alloc} defines the section to be one which is loaded into
5729 memory when the program is run. \i\c{noalloc} defines it to be one
5730 which is not, such as an informational or comment section.
5732 \b \i\c{exec} defines the section to be one which should have execute
5733 permission when the program is run. \i\c{noexec} defines it as one
5736 \b \i\c{write} defines the section to be one which should be writable
5737 when the program is run. \i\c{nowrite} defines it as one which should
5740 \b \i\c{progbits} defines the section to be one with explicit contents
5741 stored in the object file: an ordinary code or data section, for
5742 example, \i\c{nobits} defines the section to be one with no explicit
5743 contents given, such as a BSS section.
5745 \b \c{align=}, used with a trailing number as in \c{obj}, gives the
5746 \I{section alignment, in elf}\I{alignment, in elf sections}alignment
5747 requirements of the section.
5749 \b \i\c{tls} defines the section to be one which contains
5750 thread local variables.
5752 The defaults assumed by NASM if you do not specify the above
5755 \I\c{.text} \I\c{.rodata} \I\c{.lrodata} \I\c{.data} \I\c{.ldata}
5756 \I\c{.bss} \I\c{.lbss} \I\c{.tdata} \I\c{.tbss} \I\c\{.comment}
5758 \c section .text progbits alloc exec nowrite align=16
5759 \c section .rodata progbits alloc noexec nowrite align=4
5760 \c section .lrodata progbits alloc noexec nowrite align=4
5761 \c section .data progbits alloc noexec write align=4
5762 \c section .ldata progbits alloc noexec write align=4
5763 \c section .bss nobits alloc noexec write align=4
5764 \c section .lbss nobits alloc noexec write align=4
5765 \c section .tdata progbits alloc noexec write align=4 tls
5766 \c section .tbss nobits alloc noexec write align=4 tls
5767 \c section .comment progbits noalloc noexec nowrite align=1
5768 \c section other progbits alloc noexec nowrite align=1
5770 (Any section name other than those in the above table
5771 is treated by default like \c{other} in the above table.
5772 Please note that section names are case sensitive.)
5775 \S{elfwrt} \i{Position-Independent Code}\I{PIC}: \c{elf} Special
5776 Symbols and \i\c{WRT}
5778 The \c{ELF} specification contains enough features to allow
5779 position-independent code (PIC) to be written, which makes \i{ELF
5780 shared libraries} very flexible. However, it also means NASM has to
5781 be able to generate a variety of ELF specific relocation types in ELF
5782 object files, if it is to be an assembler which can write PIC.
5784 Since \c{ELF} does not support segment-base references, the \c{WRT}
5785 operator is not used for its normal purpose; therefore NASM's
5786 \c{elf} output format makes use of \c{WRT} for a different purpose,
5787 namely the PIC-specific \I{relocations, PIC-specific}relocation
5790 \c{elf} defines five special symbols which you can use as the
5791 right-hand side of the \c{WRT} operator to obtain PIC relocation
5792 types. They are \i\c{..gotpc}, \i\c{..gotoff}, \i\c{..got},
5793 \i\c{..plt} and \i\c{..sym}. Their functions are summarized here:
5795 \b Referring to the symbol marking the global offset table base
5796 using \c{wrt ..gotpc} will end up giving the distance from the
5797 beginning of the current section to the global offset table.
5798 (\i\c{_GLOBAL_OFFSET_TABLE_} is the standard symbol name used to
5799 refer to the \i{GOT}.) So you would then need to add \i\c{$$} to the
5800 result to get the real address of the GOT.
5802 \b Referring to a location in one of your own sections using \c{wrt
5803 ..gotoff} will give the distance from the beginning of the GOT to
5804 the specified location, so that adding on the address of the GOT
5805 would give the real address of the location you wanted.
5807 \b Referring to an external or global symbol using \c{wrt ..got}
5808 causes the linker to build an entry \e{in} the GOT containing the
5809 address of the symbol, and the reference gives the distance from the
5810 beginning of the GOT to the entry; so you can add on the address of
5811 the GOT, load from the resulting address, and end up with the
5812 address of the symbol.
5814 \b Referring to a procedure name using \c{wrt ..plt} causes the
5815 linker to build a \i{procedure linkage table} entry for the symbol,
5816 and the reference gives the address of the \i{PLT} entry. You can
5817 only use this in contexts which would generate a PC-relative
5818 relocation normally (i.e. as the destination for \c{CALL} or
5819 \c{JMP}), since ELF contains no relocation type to refer to PLT
5822 \b Referring to a symbol name using \c{wrt ..sym} causes NASM to
5823 write an ordinary relocation, but instead of making the relocation
5824 relative to the start of the section and then adding on the offset
5825 to the symbol, it will write a relocation record aimed directly at
5826 the symbol in question. The distinction is a necessary one due to a
5827 peculiarity of the dynamic linker.
5829 A fuller explanation of how to use these relocation types to write
5830 shared libraries entirely in NASM is given in \k{picdll}.
5832 \S{elftls} \i{Thread Local Storage}\I{TLS}: \c{elf} Special
5833 Symbols and \i\c{WRT}
5835 \b In ELF32 mode, referring to an external or global symbol using
5836 \c{wrt ..tlsie} \I\c{..tlsie}
5837 causes the linker to build an entry \e{in} the GOT containing the
5838 offset of the symbol within the TLS block, so you can access the value
5839 of the symbol with code such as:
5841 \c mov eax,[tid wrt ..tlsie]
5845 \b In ELF64 or ELFx32 mode, referring to an external or global symbol using
5846 \c{wrt ..gottpoff} \I\c{..gottpoff}
5847 causes the linker to build an entry \e{in} the GOT containing the
5848 offset of the symbol within the TLS block, so you can access the value
5849 of the symbol with code such as:
5851 \c mov rax,[rel tid wrt ..gottpoff]
5855 \S{elfglob} \c{elf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL,
5856 elf extensions to}\I{GLOBAL, aoutb extensions to}
5858 \c{ELF} object files can contain more information about a global symbol
5859 than just its address: they can contain the \I{symbol sizes,
5860 specifying}\I{size, of symbols}size of the symbol and its \I{symbol
5861 types, specifying}\I{type, of symbols}type as well. These are not
5862 merely debugger conveniences, but are actually necessary when the
5863 program being written is a \i{shared library}. NASM therefore
5864 supports some extensions to the \c{GLOBAL} directive, allowing you
5865 to specify these features.
5867 You can specify whether a global variable is a function or a data
5868 object by suffixing the name with a colon and the word
5869 \i\c{function} or \i\c{data}. (\i\c{object} is a synonym for
5870 \c{data}.) For example:
5872 \c global hashlookup:function, hashtable:data
5874 exports the global symbol \c{hashlookup} as a function and
5875 \c{hashtable} as a data object.
5877 Optionally, you can control the ELF visibility of the symbol. Just
5878 add one of the visibility keywords: \i\c{default}, \i\c{internal},
5879 \i\c{hidden}, or \i\c{protected}. The default is \i\c{default} of
5880 course. For example, to make \c{hashlookup} hidden:
5882 \c global hashlookup:function hidden
5884 You can also specify the size of the data associated with the
5885 symbol, as a numeric expression (which may involve labels, and even
5886 forward references) after the type specifier. Like this:
5888 \c global hashtable:data (hashtable.end - hashtable)
5891 \c db this,that,theother ; some data here
5894 This makes NASM automatically calculate the length of the table and
5895 place that information into the \c{ELF} symbol table.
5897 Declaring the type and size of global symbols is necessary when
5898 writing shared library code. For more information, see
5902 \S{elfcomm} \c{elf} Extensions to the \c{COMMON} Directive
5903 \I{COMMON, elf extensions to}
5905 \c{ELF} also allows you to specify alignment requirements \I{common
5906 variables, alignment in elf}\I{alignment, of elf common variables}on
5907 common variables. This is done by putting a number (which must be a
5908 power of two) after the name and size of the common variable,
5909 separated (as usual) by a colon. For example, an array of
5910 doublewords would benefit from 4-byte alignment:
5912 \c common dwordarray 128:4
5914 This declares the total size of the array to be 128 bytes, and
5915 requires that it be aligned on a 4-byte boundary.
5918 \S{elf16} 16-bit code and ELF
5919 \I{ELF, 16-bit code and}
5921 The \c{ELF32} specification doesn't provide relocations for 8- and
5922 16-bit values, but the GNU \c{ld} linker adds these as an extension.
5923 NASM can generate GNU-compatible relocations, to allow 16-bit code to
5924 be linked as ELF using GNU \c{ld}. If NASM is used with the
5925 \c{-w+gnu-elf-extensions} option, a warning is issued when one of
5926 these relocations is generated.
5928 \S{elfdbg} Debug formats and ELF
5929 \I{ELF, Debug formats and}
5931 ELF provides debug information in \c{STABS} and \c{DWARF} formats.
5932 Line number information is generated for all executable sections, but please
5933 note that only the ".text" section is executable by default.
5935 \H{aoutfmt} \i\c{aout}: Linux \I{a.out, Linux version}\I{linux, a.out}\c{a.out} Object Files
5937 The \c{aout} format generates \c{a.out} object files, in the form used
5938 by early Linux systems (current Linux systems use ELF, see
5939 \k{elffmt}.) These differ from other \c{a.out} object files in that
5940 the magic number in the first four bytes of the file is
5941 different; also, some implementations of \c{a.out}, for example
5942 NetBSD's, support position-independent code, which Linux's
5943 implementation does not.
5945 \c{a.out} provides a default output file-name extension of \c{.o}.
5947 \c{a.out} is a very simple object format. It supports no special
5948 directives, no special symbols, no use of \c{SEG} or \c{WRT}, and no
5949 extensions to any standard directives. It supports only the three
5950 \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}.
5953 \H{aoutfmt} \i\c{aoutb}: \i{NetBSD}/\i{FreeBSD}/\i{OpenBSD}
5954 \I{a.out, BSD version}\c{a.out} Object Files
5956 The \c{aoutb} format generates \c{a.out} object files, in the form
5957 used by the various free \c{BSD Unix} clones, \c{NetBSD}, \c{FreeBSD}
5958 and \c{OpenBSD}. For simple object files, this object format is exactly
5959 the same as \c{aout} except for the magic number in the first four bytes
5960 of the file. However, the \c{aoutb} format supports
5961 \I{PIC}\i{position-independent code} in the same way as the \c{elf}
5962 format, so you can use it to write \c{BSD} \i{shared libraries}.
5964 \c{aoutb} provides a default output file-name extension of \c{.o}.
5966 \c{aoutb} supports no special directives, no special symbols, and
5967 only the three \i{standard section names} \i\c{.text}, \i\c{.data}
5968 and \i\c{.bss}. However, it also supports the same use of \i\c{WRT} as
5969 \c{elf} does, to provide position-independent code relocation types.
5970 See \k{elfwrt} for full documentation of this feature.
5972 \c{aoutb} also supports the same extensions to the \c{GLOBAL}
5973 directive as \c{elf} does: see \k{elfglob} for documentation of
5977 \H{as86fmt} \c{as86}: \i{Minix}/Linux\I{linux, as86} \i\c{as86} Object Files
5979 The Minix/Linux 16-bit assembler \c{as86} has its own non-standard
5980 object file format. Although its companion linker \i\c{ld86} produces
5981 something close to ordinary \c{a.out} binaries as output, the object
5982 file format used to communicate between \c{as86} and \c{ld86} is not
5985 NASM supports this format, just in case it is useful, as \c{as86}.
5986 \c{as86} provides a default output file-name extension of \c{.o}.
5988 \c{as86} is a very simple object format (from the NASM user's point
5989 of view). It supports no special directives, no use of \c{SEG} or \c{WRT},
5990 and no extensions to any standard directives. It supports only the three
5991 \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}. The
5992 only special symbol supported is \c{..start}.
5995 \H{rdffmt} \I{RDOFF}\i\c{rdf}: \i{Relocatable Dynamic Object File
5998 The \c{rdf} output format produces \c{RDOFF} object files. \c{RDOFF}
5999 (Relocatable Dynamic Object File Format) is a home-grown object-file
6000 format, designed alongside NASM itself and reflecting in its file
6001 format the internal structure of the assembler.
6003 \c{RDOFF} is not used by any well-known operating systems. Those
6004 writing their own systems, however, may well wish to use \c{RDOFF}
6005 as their object format, on the grounds that it is designed primarily
6006 for simplicity and contains very little file-header bureaucracy.
6008 The Unix NASM archive, and the DOS archive which includes sources,
6009 both contain an \I{rdoff subdirectory}\c{rdoff} subdirectory holding
6010 a set of RDOFF utilities: an RDF linker, an \c{RDF} static-library
6011 manager, an RDF file dump utility, and a program which will load and
6012 execute an RDF executable under Linux.
6014 \c{rdf} supports only the \i{standard section names} \i\c{.text},
6015 \i\c{.data} and \i\c{.bss}.
6018 \S{rdflib} Requiring a Library: The \i\c{LIBRARY} Directive
6020 \c{RDOFF} contains a mechanism for an object file to demand a given
6021 library to be linked to the module, either at load time or run time.
6022 This is done by the \c{LIBRARY} directive, which takes one argument
6023 which is the name of the module:
6025 \c library mylib.rdl
6028 \S{rdfmod} Specifying a Module Name: The \i\c{MODULE} Directive
6030 Special \c{RDOFF} header record is used to store the name of the module.
6031 It can be used, for example, by run-time loader to perform dynamic
6032 linking. \c{MODULE} directive takes one argument which is the name
6037 Note that when you statically link modules and tell linker to strip
6038 the symbols from output file, all module names will be stripped too.
6039 To avoid it, you should start module names with \I{$, prefix}\c{$}, like:
6041 \c module $kernel.core
6044 \S{rdfglob} \c{rdf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL,
6047 \c{RDOFF} global symbols can contain additional information needed by
6048 the static linker. You can mark a global symbol as exported, thus
6049 telling the linker do not strip it from target executable or library
6050 file. Like in \c{ELF}, you can also specify whether an exported symbol
6051 is a procedure (function) or data object.
6053 Suffixing the name with a colon and the word \i\c{export} you make the
6056 \c global sys_open:export
6058 To specify that exported symbol is a procedure (function), you add the
6059 word \i\c{proc} or \i\c{function} after declaration:
6061 \c global sys_open:export proc
6063 Similarly, to specify exported data object, add the word \i\c{data}
6064 or \i\c{object} to the directive:
6066 \c global kernel_ticks:export data
6069 \S{rdfimpt} \c{rdf} Extensions to the \c{EXTERN} Directive\I{EXTERN,
6072 By default the \c{EXTERN} directive in \c{RDOFF} declares a "pure external"
6073 symbol (i.e. the static linker will complain if such a symbol is not resolved).
6074 To declare an "imported" symbol, which must be resolved later during a dynamic
6075 linking phase, \c{RDOFF} offers an additional \c{import} modifier. As in
6076 \c{GLOBAL}, you can also specify whether an imported symbol is a procedure
6077 (function) or data object. For example:
6080 \c extern _open:import
6081 \c extern _printf:import proc
6082 \c extern _errno:import data
6084 Here the directive \c{LIBRARY} is also included, which gives the dynamic linker
6085 a hint as to where to find requested symbols.
6088 \H{dbgfmt} \i\c{dbg}: Debugging Format
6090 The \c{dbg} output format is not built into NASM in the default
6091 configuration. If you are building your own NASM executable from the
6092 sources, you can define \i\c{OF_DBG} in \c{output/outform.h} or on the
6093 compiler command line, and obtain the \c{dbg} output format.
6095 The \c{dbg} format does not output an object file as such; instead,
6096 it outputs a text file which contains a complete list of all the
6097 transactions between the main body of NASM and the output-format
6098 back end module. It is primarily intended to aid people who want to
6099 write their own output drivers, so that they can get a clearer idea
6100 of the various requests the main program makes of the output driver,
6101 and in what order they happen.
6103 For simple files, one can easily use the \c{dbg} format like this:
6105 \c nasm -f dbg filename.asm
6107 which will generate a diagnostic file called \c{filename.dbg}.
6108 However, this will not work well on files which were designed for a
6109 different object format, because each object format defines its own
6110 macros (usually user-level forms of directives), and those macros
6111 will not be defined in the \c{dbg} format. Therefore it can be
6112 useful to run NASM twice, in order to do the preprocessing with the
6113 native object format selected:
6115 \c nasm -e -f rdf -o rdfprog.i rdfprog.asm
6116 \c nasm -a -f dbg rdfprog.i
6118 This preprocesses \c{rdfprog.asm} into \c{rdfprog.i}, keeping the
6119 \c{rdf} object format selected in order to make sure RDF special
6120 directives are converted into primitive form correctly. Then the
6121 preprocessed source is fed through the \c{dbg} format to generate
6122 the final diagnostic output.
6124 This workaround will still typically not work for programs intended
6125 for \c{obj} format, because the \c{obj} \c{SEGMENT} and \c{GROUP}
6126 directives have side effects of defining the segment and group names
6127 as symbols; \c{dbg} will not do this, so the program will not
6128 assemble. You will have to work around that by defining the symbols
6129 yourself (using \c{EXTERN}, for example) if you really need to get a
6130 \c{dbg} trace of an \c{obj}-specific source file.
6132 \c{dbg} accepts any section name and any directives at all, and logs
6133 them all to its output file.
6136 \C{16bit} Writing 16-bit Code (DOS, Windows 3/3.1)
6138 This chapter attempts to cover some of the common issues encountered
6139 when writing 16-bit code to run under \c{MS-DOS} or \c{Windows 3.x}. It
6140 covers how to link programs to produce \c{.EXE} or \c{.COM} files,
6141 how to write \c{.SYS} device drivers, and how to interface assembly
6142 language code with 16-bit C compilers and with Borland Pascal.
6145 \H{exefiles} Producing \i\c{.EXE} Files
6147 Any large program written under DOS needs to be built as a \c{.EXE}
6148 file: only \c{.EXE} files have the necessary internal structure
6149 required to span more than one 64K segment. \i{Windows} programs,
6150 also, have to be built as \c{.EXE} files, since Windows does not
6151 support the \c{.COM} format.
6153 In general, you generate \c{.EXE} files by using the \c{obj} output
6154 format to produce one or more \i\c{.OBJ} files, and then linking
6155 them together using a linker. However, NASM also supports the direct
6156 generation of simple DOS \c{.EXE} files using the \c{bin} output
6157 format (by using \c{DB} and \c{DW} to construct the \c{.EXE} file
6158 header), and a macro package is supplied to do this. Thanks to
6159 Yann Guidon for contributing the code for this.
6161 NASM may also support \c{.EXE} natively as another output format in
6165 \S{objexe} Using the \c{obj} Format To Generate \c{.EXE} Files
6167 This section describes the usual method of generating \c{.EXE} files
6168 by linking \c{.OBJ} files together.
6170 Most 16-bit programming language packages come with a suitable
6171 linker; if you have none of these, there is a free linker called
6172 \i{VAL}\I{linker, free}, available in \c{LZH} archive format from
6173 \W{ftp://x2ftp.oulu.fi/pub/msdos/programming/lang/}\i\c{x2ftp.oulu.fi}.
6174 An LZH archiver can be found at
6175 \W{ftp://ftp.simtel.net/pub/simtelnet/msdos/arcers}\i\c{ftp.simtel.net}.
6176 There is another `free' linker (though this one doesn't come with
6177 sources) called \i{FREELINK}, available from
6178 \W{http://www.pcorner.com/tpc/old/3-101.html}\i\c{www.pcorner.com}.
6179 A third, \i\c{djlink}, written by DJ Delorie, is available at
6180 \W{http://www.delorie.com/djgpp/16bit/djlink/}\i\c{www.delorie.com}.
6181 A fourth linker, \i\c{ALINK}, written by Anthony A.J. Williams, is
6182 available at \W{http://alink.sourceforge.net}\i\c{alink.sourceforge.net}.
6184 When linking several \c{.OBJ} files into a \c{.EXE} file, you should
6185 ensure that exactly one of them has a start point defined (using the
6186 \I{program entry point}\i\c{..start} special symbol defined by the
6187 \c{obj} format: see \k{dotdotstart}). If no module defines a start
6188 point, the linker will not know what value to give the entry-point
6189 field in the output file header; if more than one defines a start
6190 point, the linker will not know \e{which} value to use.
6192 An example of a NASM source file which can be assembled to a
6193 \c{.OBJ} file and linked on its own to a \c{.EXE} is given here. It
6194 demonstrates the basic principles of defining a stack, initialising
6195 the segment registers, and declaring a start point. This file is
6196 also provided in the \I{test subdirectory}\c{test} subdirectory of
6197 the NASM archives, under the name \c{objexe.asm}.
6208 This initial piece of code sets up \c{DS} to point to the data
6209 segment, and initializes \c{SS} and \c{SP} to point to the top of
6210 the provided stack. Notice that interrupts are implicitly disabled
6211 for one instruction after a move into \c{SS}, precisely for this
6212 situation, so that there's no chance of an interrupt occurring
6213 between the loads of \c{SS} and \c{SP} and not having a stack to
6216 Note also that the special symbol \c{..start} is defined at the
6217 beginning of this code, which means that will be the entry point
6218 into the resulting executable file.
6224 The above is the main program: load \c{DS:DX} with a pointer to the
6225 greeting message (\c{hello} is implicitly relative to the segment
6226 \c{data}, which was loaded into \c{DS} in the setup code, so the
6227 full pointer is valid), and call the DOS print-string function.
6232 This terminates the program using another DOS system call.
6236 \c hello: db 'hello, world', 13, 10, '$'
6238 The data segment contains the string we want to display.
6240 \c segment stack stack
6244 The above code declares a stack segment containing 64 bytes of
6245 uninitialized stack space, and points \c{stacktop} at the top of it.
6246 The directive \c{segment stack stack} defines a segment \e{called}
6247 \c{stack}, and also of \e{type} \c{STACK}. The latter is not
6248 necessary to the correct running of the program, but linkers are
6249 likely to issue warnings or errors if your program has no segment of
6252 The above file, when assembled into a \c{.OBJ} file, will link on
6253 its own to a valid \c{.EXE} file, which when run will print `hello,
6254 world' and then exit.
6257 \S{binexe} Using the \c{bin} Format To Generate \c{.EXE} Files
6259 The \c{.EXE} file format is simple enough that it's possible to
6260 build a \c{.EXE} file by writing a pure-binary program and sticking
6261 a 32-byte header on the front. This header is simple enough that it
6262 can be generated using \c{DB} and \c{DW} commands by NASM itself, so
6263 that you can use the \c{bin} output format to directly generate
6266 Included in the NASM archives, in the \I{misc subdirectory}\c{misc}
6267 subdirectory, is a file \i\c{exebin.mac} of macros. It defines three
6268 macros: \i\c{EXE_begin}, \i\c{EXE_stack} and \i\c{EXE_end}.
6270 To produce a \c{.EXE} file using this method, you should start by
6271 using \c{%include} to load the \c{exebin.mac} macro package into
6272 your source file. You should then issue the \c{EXE_begin} macro call
6273 (which takes no arguments) to generate the file header data. Then
6274 write code as normal for the \c{bin} format - you can use all three
6275 standard sections \c{.text}, \c{.data} and \c{.bss}. At the end of
6276 the file you should call the \c{EXE_end} macro (again, no arguments),
6277 which defines some symbols to mark section sizes, and these symbols
6278 are referred to in the header code generated by \c{EXE_begin}.
6280 In this model, the code you end up writing starts at \c{0x100}, just
6281 like a \c{.COM} file - in fact, if you strip off the 32-byte header
6282 from the resulting \c{.EXE} file, you will have a valid \c{.COM}
6283 program. All the segment bases are the same, so you are limited to a
6284 64K program, again just like a \c{.COM} file. Note that an \c{ORG}
6285 directive is issued by the \c{EXE_begin} macro, so you should not
6286 explicitly issue one of your own.
6288 You can't directly refer to your segment base value, unfortunately,
6289 since this would require a relocation in the header, and things
6290 would get a lot more complicated. So you should get your segment
6291 base by copying it out of \c{CS} instead.
6293 On entry to your \c{.EXE} file, \c{SS:SP} are already set up to
6294 point to the top of a 2Kb stack. You can adjust the default stack
6295 size of 2Kb by calling the \c{EXE_stack} macro. For example, to
6296 change the stack size of your program to 64 bytes, you would call
6299 A sample program which generates a \c{.EXE} file in this way is
6300 given in the \c{test} subdirectory of the NASM archive, as
6304 \H{comfiles} Producing \i\c{.COM} Files
6306 While large DOS programs must be written as \c{.EXE} files, small
6307 ones are often better written as \c{.COM} files. \c{.COM} files are
6308 pure binary, and therefore most easily produced using the \c{bin}
6312 \S{combinfmt} Using the \c{bin} Format To Generate \c{.COM} Files
6314 \c{.COM} files expect to be loaded at offset \c{100h} into their
6315 segment (though the segment may change). Execution then begins at
6316 \I\c{ORG}\c{100h}, i.e. right at the start of the program. So to
6317 write a \c{.COM} program, you would create a source file looking
6325 \c ; put your code here
6329 \c ; put data items here
6333 \c ; put uninitialized data here
6335 The \c{bin} format puts the \c{.text} section first in the file, so
6336 you can declare data or BSS items before beginning to write code if
6337 you want to and the code will still end up at the front of the file
6340 The BSS (uninitialized data) section does not take up space in the
6341 \c{.COM} file itself: instead, addresses of BSS items are resolved
6342 to point at space beyond the end of the file, on the grounds that
6343 this will be free memory when the program is run. Therefore you
6344 should not rely on your BSS being initialized to all zeros when you
6347 To assemble the above program, you should use a command line like
6349 \c nasm myprog.asm -fbin -o myprog.com
6351 The \c{bin} format would produce a file called \c{myprog} if no
6352 explicit output file name were specified, so you have to override it
6353 and give the desired file name.
6356 \S{comobjfmt} Using the \c{obj} Format To Generate \c{.COM} Files
6358 If you are writing a \c{.COM} program as more than one module, you
6359 may wish to assemble several \c{.OBJ} files and link them together
6360 into a \c{.COM} program. You can do this, provided you have a linker
6361 capable of outputting \c{.COM} files directly (\i{TLINK} does this),
6362 or alternatively a converter program such as \i\c{EXE2BIN} to
6363 transform the \c{.EXE} file output from the linker into a \c{.COM}
6366 If you do this, you need to take care of several things:
6368 \b The first object file containing code should start its code
6369 segment with a line like \c{RESB 100h}. This is to ensure that the
6370 code begins at offset \c{100h} relative to the beginning of the code
6371 segment, so that the linker or converter program does not have to
6372 adjust address references within the file when generating the
6373 \c{.COM} file. Other assemblers use an \i\c{ORG} directive for this
6374 purpose, but \c{ORG} in NASM is a format-specific directive to the
6375 \c{bin} output format, and does not mean the same thing as it does
6376 in MASM-compatible assemblers.
6378 \b You don't need to define a stack segment.
6380 \b All your segments should be in the same group, so that every time
6381 your code or data references a symbol offset, all offsets are
6382 relative to the same segment base. This is because, when a \c{.COM}
6383 file is loaded, all the segment registers contain the same value.
6386 \H{sysfiles} Producing \i\c{.SYS} Files
6388 \i{MS-DOS device drivers} - \c{.SYS} files - are pure binary files,
6389 similar to \c{.COM} files, except that they start at origin zero
6390 rather than \c{100h}. Therefore, if you are writing a device driver
6391 using the \c{bin} format, you do not need the \c{ORG} directive,
6392 since the default origin for \c{bin} is zero. Similarly, if you are
6393 using \c{obj}, you do not need the \c{RESB 100h} at the start of
6396 \c{.SYS} files start with a header structure, containing pointers to
6397 the various routines inside the driver which do the work. This
6398 structure should be defined at the start of the code segment, even
6399 though it is not actually code.
6401 For more information on the format of \c{.SYS} files, and the data
6402 which has to go in the header structure, a list of books is given in
6403 the Frequently Asked Questions list for the newsgroup
6404 \W{news:comp.os.msdos.programmer}\i\c{comp.os.msdos.programmer}.
6407 \H{16c} Interfacing to 16-bit C Programs
6409 This section covers the basics of writing assembly routines that
6410 call, or are called from, C programs. To do this, you would
6411 typically write an assembly module as a \c{.OBJ} file, and link it
6412 with your C modules to produce a \i{mixed-language program}.
6415 \S{16cunder} External Symbol Names
6417 \I{C symbol names}\I{underscore, in C symbols}C compilers have the
6418 convention that the names of all global symbols (functions or data)
6419 they define are formed by prefixing an underscore to the name as it
6420 appears in the C program. So, for example, the function a C
6421 programmer thinks of as \c{printf} appears to an assembly language
6422 programmer as \c{_printf}. This means that in your assembly
6423 programs, you can define symbols without a leading underscore, and
6424 not have to worry about name clashes with C symbols.
6426 If you find the underscores inconvenient, you can define macros to
6427 replace the \c{GLOBAL} and \c{EXTERN} directives as follows:
6443 (These forms of the macros only take one argument at a time; a
6444 \c{%rep} construct could solve this.)
6446 If you then declare an external like this:
6450 then the macro will expand it as
6453 \c %define printf _printf
6455 Thereafter, you can reference \c{printf} as if it was a symbol, and
6456 the preprocessor will put the leading underscore on where necessary.
6458 The \c{cglobal} macro works similarly. You must use \c{cglobal}
6459 before defining the symbol in question, but you would have had to do
6460 that anyway if you used \c{GLOBAL}.
6462 Also see \k{opt-pfix}.
6464 \S{16cmodels} \i{Memory Models}
6466 NASM contains no mechanism to support the various C memory models
6467 directly; you have to keep track yourself of which one you are
6468 writing for. This means you have to keep track of the following
6471 \b In models using a single code segment (tiny, small and compact),
6472 functions are near. This means that function pointers, when stored
6473 in data segments or pushed on the stack as function arguments, are
6474 16 bits long and contain only an offset field (the \c{CS} register
6475 never changes its value, and always gives the segment part of the
6476 full function address), and that functions are called using ordinary
6477 near \c{CALL} instructions and return using \c{RETN} (which, in
6478 NASM, is synonymous with \c{RET} anyway). This means both that you
6479 should write your own routines to return with \c{RETN}, and that you
6480 should call external C routines with near \c{CALL} instructions.
6482 \b In models using more than one code segment (medium, large and
6483 huge), functions are far. This means that function pointers are 32
6484 bits long (consisting of a 16-bit offset followed by a 16-bit
6485 segment), and that functions are called using \c{CALL FAR} (or
6486 \c{CALL seg:offset}) and return using \c{RETF}. Again, you should
6487 therefore write your own routines to return with \c{RETF} and use
6488 \c{CALL FAR} to call external routines.
6490 \b In models using a single data segment (tiny, small and medium),
6491 data pointers are 16 bits long, containing only an offset field (the
6492 \c{DS} register doesn't change its value, and always gives the
6493 segment part of the full data item address).
6495 \b In models using more than one data segment (compact, large and
6496 huge), data pointers are 32 bits long, consisting of a 16-bit offset
6497 followed by a 16-bit segment. You should still be careful not to
6498 modify \c{DS} in your routines without restoring it afterwards, but
6499 \c{ES} is free for you to use to access the contents of 32-bit data
6500 pointers you are passed.
6502 \b The huge memory model allows single data items to exceed 64K in
6503 size. In all other memory models, you can access the whole of a data
6504 item just by doing arithmetic on the offset field of the pointer you
6505 are given, whether a segment field is present or not; in huge model,
6506 you have to be more careful of your pointer arithmetic.
6508 \b In most memory models, there is a \e{default} data segment, whose
6509 segment address is kept in \c{DS} throughout the program. This data
6510 segment is typically the same segment as the stack, kept in \c{SS},
6511 so that functions' local variables (which are stored on the stack)
6512 and global data items can both be accessed easily without changing
6513 \c{DS}. Particularly large data items are typically stored in other
6514 segments. However, some memory models (though not the standard
6515 ones, usually) allow the assumption that \c{SS} and \c{DS} hold the
6516 same value to be removed. Be careful about functions' local
6517 variables in this latter case.
6519 In models with a single code segment, the segment is called
6520 \i\c{_TEXT}, so your code segment must also go by this name in order
6521 to be linked into the same place as the main code segment. In models
6522 with a single data segment, or with a default data segment, it is
6526 \S{16cfunc} Function Definitions and Function Calls
6528 \I{functions, C calling convention}The \i{C calling convention} in
6529 16-bit programs is as follows. In the following description, the
6530 words \e{caller} and \e{callee} are used to denote the function
6531 doing the calling and the function which gets called.
6533 \b The caller pushes the function's parameters on the stack, one
6534 after another, in reverse order (right to left, so that the first
6535 argument specified to the function is pushed last).
6537 \b The caller then executes a \c{CALL} instruction to pass control
6538 to the callee. This \c{CALL} is either near or far depending on the
6541 \b The callee receives control, and typically (although this is not
6542 actually necessary, in functions which do not need to access their
6543 parameters) starts by saving the value of \c{SP} in \c{BP} so as to
6544 be able to use \c{BP} as a base pointer to find its parameters on
6545 the stack. However, the caller was probably doing this too, so part
6546 of the calling convention states that \c{BP} must be preserved by
6547 any C function. Hence the callee, if it is going to set up \c{BP} as
6548 a \i\e{frame pointer}, must push the previous value first.
6550 \b The callee may then access its parameters relative to \c{BP}.
6551 The word at \c{[BP]} holds the previous value of \c{BP} as it was
6552 pushed; the next word, at \c{[BP+2]}, holds the offset part of the
6553 return address, pushed implicitly by \c{CALL}. In a small-model
6554 (near) function, the parameters start after that, at \c{[BP+4]}; in
6555 a large-model (far) function, the segment part of the return address
6556 lives at \c{[BP+4]}, and the parameters begin at \c{[BP+6]}. The
6557 leftmost parameter of the function, since it was pushed last, is
6558 accessible at this offset from \c{BP}; the others follow, at
6559 successively greater offsets. Thus, in a function such as \c{printf}
6560 which takes a variable number of parameters, the pushing of the
6561 parameters in reverse order means that the function knows where to
6562 find its first parameter, which tells it the number and type of the
6565 \b The callee may also wish to decrease \c{SP} further, so as to
6566 allocate space on the stack for local variables, which will then be
6567 accessible at negative offsets from \c{BP}.
6569 \b The callee, if it wishes to return a value to the caller, should
6570 leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size
6571 of the value. Floating-point results are sometimes (depending on the
6572 compiler) returned in \c{ST0}.
6574 \b Once the callee has finished processing, it restores \c{SP} from
6575 \c{BP} if it had allocated local stack space, then pops the previous
6576 value of \c{BP}, and returns via \c{RETN} or \c{RETF} depending on
6579 \b When the caller regains control from the callee, the function
6580 parameters are still on the stack, so it typically adds an immediate
6581 constant to \c{SP} to remove them (instead of executing a number of
6582 slow \c{POP} instructions). Thus, if a function is accidentally
6583 called with the wrong number of parameters due to a prototype
6584 mismatch, the stack will still be returned to a sensible state since
6585 the caller, which \e{knows} how many parameters it pushed, does the
6588 It is instructive to compare this calling convention with that for
6589 Pascal programs (described in \k{16bpfunc}). Pascal has a simpler
6590 convention, since no functions have variable numbers of parameters.
6591 Therefore the callee knows how many parameters it should have been
6592 passed, and is able to deallocate them from the stack itself by
6593 passing an immediate argument to the \c{RET} or \c{RETF}
6594 instruction, so the caller does not have to do it. Also, the
6595 parameters are pushed in left-to-right order, not right-to-left,
6596 which means that a compiler can give better guarantees about
6597 sequence points without performance suffering.
6599 Thus, you would define a function in C style in the following way.
6600 The following example is for small model:
6607 \c sub sp,0x40 ; 64 bytes of local stack space
6608 \c mov bx,[bp+4] ; first parameter to function
6612 \c mov sp,bp ; undo "sub sp,0x40" above
6616 For a large-model function, you would replace \c{RET} by \c{RETF},
6617 and look for the first parameter at \c{[BP+6]} instead of
6618 \c{[BP+4]}. Of course, if one of the parameters is a pointer, then
6619 the offsets of \e{subsequent} parameters will change depending on
6620 the memory model as well: far pointers take up four bytes on the
6621 stack when passed as a parameter, whereas near pointers take up two.
6623 At the other end of the process, to call a C function from your
6624 assembly code, you would do something like this:
6628 \c ; and then, further down...
6630 \c push word [myint] ; one of my integer variables
6631 \c push word mystring ; pointer into my data segment
6633 \c add sp,byte 4 ; `byte' saves space
6635 \c ; then those data items...
6640 \c mystring db 'This number -> %d <- should be 1234',10,0
6642 This piece of code is the small-model assembly equivalent of the C
6645 \c int myint = 1234;
6646 \c printf("This number -> %d <- should be 1234\n", myint);
6648 In large model, the function-call code might look more like this. In
6649 this example, it is assumed that \c{DS} already holds the segment
6650 base of the segment \c{_DATA}. If not, you would have to initialize
6653 \c push word [myint]
6654 \c push word seg mystring ; Now push the segment, and...
6655 \c push word mystring ; ... offset of "mystring"
6659 The integer value still takes up one word on the stack, since large
6660 model does not affect the size of the \c{int} data type. The first
6661 argument (pushed last) to \c{printf}, however, is a data pointer,
6662 and therefore has to contain a segment and offset part. The segment
6663 should be stored second in memory, and therefore must be pushed
6664 first. (Of course, \c{PUSH DS} would have been a shorter instruction
6665 than \c{PUSH WORD SEG mystring}, if \c{DS} was set up as the above
6666 example assumed.) Then the actual call becomes a far call, since
6667 functions expect far calls in large model; and \c{SP} has to be
6668 increased by 6 rather than 4 afterwards to make up for the extra
6672 \S{16cdata} Accessing Data Items
6674 To get at the contents of C variables, or to declare variables which
6675 C can access, you need only declare the names as \c{GLOBAL} or
6676 \c{EXTERN}. (Again, the names require leading underscores, as stated
6677 in \k{16cunder}.) Thus, a C variable declared as \c{int i} can be
6678 accessed from assembler as
6684 And to declare your own integer variable which C programs can access
6685 as \c{extern int j}, you do this (making sure you are assembling in
6686 the \c{_DATA} segment, if necessary):
6692 To access a C array, you need to know the size of the components of
6693 the array. For example, \c{int} variables are two bytes long, so if
6694 a C program declares an array as \c{int a[10]}, you can access
6695 \c{a[3]} by coding \c{mov ax,[_a+6]}. (The byte offset 6 is obtained
6696 by multiplying the desired array index, 3, by the size of the array
6697 element, 2.) The sizes of the C base types in 16-bit compilers are:
6698 1 for \c{char}, 2 for \c{short} and \c{int}, 4 for \c{long} and
6699 \c{float}, and 8 for \c{double}.
6701 To access a C \i{data structure}, you need to know the offset from
6702 the base of the structure to the field you are interested in. You
6703 can either do this by converting the C structure definition into a
6704 NASM structure definition (using \i\c{STRUC}), or by calculating the
6705 one offset and using just that.
6707 To do either of these, you should read your C compiler's manual to
6708 find out how it organizes data structures. NASM gives no special
6709 alignment to structure members in its own \c{STRUC} macro, so you
6710 have to specify alignment yourself if the C compiler generates it.
6711 Typically, you might find that a structure like
6718 might be four bytes long rather than three, since the \c{int} field
6719 would be aligned to a two-byte boundary. However, this sort of
6720 feature tends to be a configurable option in the C compiler, either
6721 using command-line options or \c{#pragma} lines, so you have to find
6722 out how your own compiler does it.
6725 \S{16cmacro} \i\c{c16.mac}: Helper Macros for the 16-bit C Interface
6727 Included in the NASM archives, in the \I{misc subdirectory}\c{misc}
6728 directory, is a file \c{c16.mac} of macros. It defines three macros:
6729 \i\c{proc}, \i\c{arg} and \i\c{endproc}. These are intended to be
6730 used for C-style procedure definitions, and they automate a lot of
6731 the work involved in keeping track of the calling convention.
6733 (An alternative, TASM compatible form of \c{arg} is also now built
6734 into NASM's preprocessor. See \k{stackrel} for details.)
6736 An example of an assembly function using the macro set is given
6743 \c mov ax,[bp + %$i]
6744 \c mov bx,[bp + %$j]
6749 This defines \c{_nearproc} to be a procedure taking two arguments,
6750 the first (\c{i}) an integer and the second (\c{j}) a pointer to an
6751 integer. It returns \c{i + *j}.
6753 Note that the \c{arg} macro has an \c{EQU} as the first line of its
6754 expansion, and since the label before the macro call gets prepended
6755 to the first line of the expanded macro, the \c{EQU} works, defining
6756 \c{%$i} to be an offset from \c{BP}. A context-local variable is
6757 used, local to the context pushed by the \c{proc} macro and popped
6758 by the \c{endproc} macro, so that the same argument name can be used
6759 in later procedures. Of course, you don't \e{have} to do that.
6761 The macro set produces code for near functions (tiny, small and
6762 compact-model code) by default. You can have it generate far
6763 functions (medium, large and huge-model code) by means of coding
6764 \I\c{FARCODE}\c{%define FARCODE}. This changes the kind of return
6765 instruction generated by \c{endproc}, and also changes the starting
6766 point for the argument offsets. The macro set contains no intrinsic
6767 dependency on whether data pointers are far or not.
6769 \c{arg} can take an optional parameter, giving the size of the
6770 argument. If no size is given, 2 is assumed, since it is likely that
6771 many function parameters will be of type \c{int}.
6773 The large-model equivalent of the above function would look like this:
6781 \c mov ax,[bp + %$i]
6782 \c mov bx,[bp + %$j]
6783 \c mov es,[bp + %$j + 2]
6788 This makes use of the argument to the \c{arg} macro to define a
6789 parameter of size 4, because \c{j} is now a far pointer. When we
6790 load from \c{j}, we must load a segment and an offset.
6793 \H{16bp} Interfacing to \i{Borland Pascal} Programs
6795 Interfacing to Borland Pascal programs is similar in concept to
6796 interfacing to 16-bit C programs. The differences are:
6798 \b The leading underscore required for interfacing to C programs is
6799 not required for Pascal.
6801 \b The memory model is always large: functions are far, data
6802 pointers are far, and no data item can be more than 64K long.
6803 (Actually, some functions are near, but only those functions that
6804 are local to a Pascal unit and never called from outside it. All
6805 assembly functions that Pascal calls, and all Pascal functions that
6806 assembly routines are able to call, are far.) However, all static
6807 data declared in a Pascal program goes into the default data
6808 segment, which is the one whose segment address will be in \c{DS}
6809 when control is passed to your assembly code. The only things that
6810 do not live in the default data segment are local variables (they
6811 live in the stack segment) and dynamically allocated variables. All
6812 data \e{pointers}, however, are far.
6814 \b The function calling convention is different - described below.
6816 \b Some data types, such as strings, are stored differently.
6818 \b There are restrictions on the segment names you are allowed to
6819 use - Borland Pascal will ignore code or data declared in a segment
6820 it doesn't like the name of. The restrictions are described below.
6823 \S{16bpfunc} The Pascal Calling Convention
6825 \I{functions, Pascal calling convention}\I{Pascal calling
6826 convention}The 16-bit Pascal calling convention is as follows. In
6827 the following description, the words \e{caller} and \e{callee} are
6828 used to denote the function doing the calling and the function which
6831 \b The caller pushes the function's parameters on the stack, one
6832 after another, in normal order (left to right, so that the first
6833 argument specified to the function is pushed first).
6835 \b The caller then executes a far \c{CALL} instruction to pass
6836 control to the callee.
6838 \b The callee receives control, and typically (although this is not
6839 actually necessary, in functions which do not need to access their
6840 parameters) starts by saving the value of \c{SP} in \c{BP} so as to
6841 be able to use \c{BP} as a base pointer to find its parameters on
6842 the stack. However, the caller was probably doing this too, so part
6843 of the calling convention states that \c{BP} must be preserved by
6844 any function. Hence the callee, if it is going to set up \c{BP} as a
6845 \i{frame pointer}, must push the previous value first.
6847 \b The callee may then access its parameters relative to \c{BP}.
6848 The word at \c{[BP]} holds the previous value of \c{BP} as it was
6849 pushed. The next word, at \c{[BP+2]}, holds the offset part of the
6850 return address, and the next one at \c{[BP+4]} the segment part. The
6851 parameters begin at \c{[BP+6]}. The rightmost parameter of the
6852 function, since it was pushed last, is accessible at this offset
6853 from \c{BP}; the others follow, at successively greater offsets.
6855 \b The callee may also wish to decrease \c{SP} further, so as to
6856 allocate space on the stack for local variables, which will then be
6857 accessible at negative offsets from \c{BP}.
6859 \b The callee, if it wishes to return a value to the caller, should
6860 leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size
6861 of the value. Floating-point results are returned in \c{ST0}.
6862 Results of type \c{Real} (Borland's own custom floating-point data
6863 type, not handled directly by the FPU) are returned in \c{DX:BX:AX}.
6864 To return a result of type \c{String}, the caller pushes a pointer
6865 to a temporary string before pushing the parameters, and the callee
6866 places the returned string value at that location. The pointer is
6867 not a parameter, and should not be removed from the stack by the
6868 \c{RETF} instruction.
6870 \b Once the callee has finished processing, it restores \c{SP} from
6871 \c{BP} if it had allocated local stack space, then pops the previous
6872 value of \c{BP}, and returns via \c{RETF}. It uses the form of
6873 \c{RETF} with an immediate parameter, giving the number of bytes
6874 taken up by the parameters on the stack. This causes the parameters
6875 to be removed from the stack as a side effect of the return
6878 \b When the caller regains control from the callee, the function
6879 parameters have already been removed from the stack, so it needs to
6882 Thus, you would define a function in Pascal style, taking two
6883 \c{Integer}-type parameters, in the following way:
6889 \c sub sp,0x40 ; 64 bytes of local stack space
6890 \c mov bx,[bp+8] ; first parameter to function
6891 \c mov bx,[bp+6] ; second parameter to function
6895 \c mov sp,bp ; undo "sub sp,0x40" above
6897 \c retf 4 ; total size of params is 4
6899 At the other end of the process, to call a Pascal function from your
6900 assembly code, you would do something like this:
6904 \c ; and then, further down...
6906 \c push word seg mystring ; Now push the segment, and...
6907 \c push word mystring ; ... offset of "mystring"
6908 \c push word [myint] ; one of my variables
6909 \c call far SomeFunc
6911 This is equivalent to the Pascal code
6913 \c procedure SomeFunc(String: PChar; Int: Integer);
6914 \c SomeFunc(@mystring, myint);
6917 \S{16bpseg} Borland Pascal \I{segment names, Borland Pascal}Segment
6920 Since Borland Pascal's internal unit file format is completely
6921 different from \c{OBJ}, it only makes a very sketchy job of actually
6922 reading and understanding the various information contained in a
6923 real \c{OBJ} file when it links that in. Therefore an object file
6924 intended to be linked to a Pascal program must obey a number of
6927 \b Procedures and functions must be in a segment whose name is
6928 either \c{CODE}, \c{CSEG}, or something ending in \c{_TEXT}.
6930 \b initialized data must be in a segment whose name is either
6931 \c{CONST} or something ending in \c{_DATA}.
6933 \b Uninitialized data must be in a segment whose name is either
6934 \c{DATA}, \c{DSEG}, or something ending in \c{_BSS}.
6936 \b Any other segments in the object file are completely ignored.
6937 \c{GROUP} directives and segment attributes are also ignored.
6940 \S{16bpmacro} Using \i\c{c16.mac} With Pascal Programs
6942 The \c{c16.mac} macro package, described in \k{16cmacro}, can also
6943 be used to simplify writing functions to be called from Pascal
6944 programs, if you code \I\c{PASCAL}\c{%define PASCAL}. This
6945 definition ensures that functions are far (it implies
6946 \i\c{FARCODE}), and also causes procedure return instructions to be
6947 generated with an operand.
6949 Defining \c{PASCAL} does not change the code which calculates the
6950 argument offsets; you must declare your function's arguments in
6951 reverse order. For example:
6959 \c mov ax,[bp + %$i]
6960 \c mov bx,[bp + %$j]
6961 \c mov es,[bp + %$j + 2]
6966 This defines the same routine, conceptually, as the example in
6967 \k{16cmacro}: it defines a function taking two arguments, an integer
6968 and a pointer to an integer, which returns the sum of the integer
6969 and the contents of the pointer. The only difference between this
6970 code and the large-model C version is that \c{PASCAL} is defined
6971 instead of \c{FARCODE}, and that the arguments are declared in
6975 \C{32bit} Writing 32-bit Code (Unix, Win32, DJGPP)
6977 This chapter attempts to cover some of the common issues involved
6978 when writing 32-bit code, to run under \i{Win32} or Unix, or to be
6979 linked with C code generated by a Unix-style C compiler such as
6980 \i{DJGPP}. It covers how to write assembly code to interface with
6981 32-bit C routines, and how to write position-independent code for
6984 Almost all 32-bit code, and in particular all code running under
6985 \c{Win32}, \c{DJGPP} or any of the PC Unix variants, runs in \I{flat
6986 memory model}\e{flat} memory model. This means that the segment registers
6987 and paging have already been set up to give you the same 32-bit 4Gb
6988 address space no matter what segment you work relative to, and that
6989 you should ignore all segment registers completely. When writing
6990 flat-model application code, you never need to use a segment
6991 override or modify any segment register, and the code-section
6992 addresses you pass to \c{CALL} and \c{JMP} live in the same address
6993 space as the data-section addresses you access your variables by and
6994 the stack-section addresses you access local variables and procedure
6995 parameters by. Every address is 32 bits long and contains only an
6999 \H{32c} Interfacing to 32-bit C Programs
7001 A lot of the discussion in \k{16c}, about interfacing to 16-bit C
7002 programs, still applies when working in 32 bits. The absence of
7003 memory models or segmentation worries simplifies things a lot.
7006 \S{32cunder} External Symbol Names
7008 Most 32-bit C compilers share the convention used by 16-bit
7009 compilers, that the names of all global symbols (functions or data)
7010 they define are formed by prefixing an underscore to the name as it
7011 appears in the C program. However, not all of them do: the \c{ELF}
7012 specification states that C symbols do \e{not} have a leading
7013 underscore on their assembly-language names.
7015 The older Linux \c{a.out} C compiler, all \c{Win32} compilers,
7016 \c{DJGPP}, and \c{NetBSD} and \c{FreeBSD}, all use the leading
7017 underscore; for these compilers, the macros \c{cextern} and
7018 \c{cglobal}, as given in \k{16cunder}, will still work. For \c{ELF},
7019 though, the leading underscore should not be used.
7021 See also \k{opt-pfix}.
7023 \S{32cfunc} Function Definitions and Function Calls
7025 \I{functions, C calling convention}The \i{C calling convention}
7026 in 32-bit programs is as follows. In the following description,
7027 the words \e{caller} and \e{callee} are used to denote
7028 the function doing the calling and the function which gets called.
7030 \b The caller pushes the function's parameters on the stack, one
7031 after another, in reverse order (right to left, so that the first
7032 argument specified to the function is pushed last).
7034 \b The caller then executes a near \c{CALL} instruction to pass
7035 control to the callee.
7037 \b The callee receives control, and typically (although this is not
7038 actually necessary, in functions which do not need to access their
7039 parameters) starts by saving the value of \c{ESP} in \c{EBP} so as
7040 to be able to use \c{EBP} as a base pointer to find its parameters
7041 on the stack. However, the caller was probably doing this too, so
7042 part of the calling convention states that \c{EBP} must be preserved
7043 by any C function. Hence the callee, if it is going to set up
7044 \c{EBP} as a \i{frame pointer}, must push the previous value first.
7046 \b The callee may then access its parameters relative to \c{EBP}.
7047 The doubleword at \c{[EBP]} holds the previous value of \c{EBP} as
7048 it was pushed; the next doubleword, at \c{[EBP+4]}, holds the return
7049 address, pushed implicitly by \c{CALL}. The parameters start after
7050 that, at \c{[EBP+8]}. The leftmost parameter of the function, since
7051 it was pushed last, is accessible at this offset from \c{EBP}; the
7052 others follow, at successively greater offsets. Thus, in a function
7053 such as \c{printf} which takes a variable number of parameters, the
7054 pushing of the parameters in reverse order means that the function
7055 knows where to find its first parameter, which tells it the number
7056 and type of the remaining ones.
7058 \b The callee may also wish to decrease \c{ESP} further, so as to
7059 allocate space on the stack for local variables, which will then be
7060 accessible at negative offsets from \c{EBP}.
7062 \b The callee, if it wishes to return a value to the caller, should
7063 leave the value in \c{AL}, \c{AX} or \c{EAX} depending on the size
7064 of the value. Floating-point results are typically returned in
7067 \b Once the callee has finished processing, it restores \c{ESP} from
7068 \c{EBP} if it had allocated local stack space, then pops the previous
7069 value of \c{EBP}, and returns via \c{RET} (equivalently, \c{RETN}).
7071 \b When the caller regains control from the callee, the function
7072 parameters are still on the stack, so it typically adds an immediate
7073 constant to \c{ESP} to remove them (instead of executing a number of
7074 slow \c{POP} instructions). Thus, if a function is accidentally
7075 called with the wrong number of parameters due to a prototype
7076 mismatch, the stack will still be returned to a sensible state since
7077 the caller, which \e{knows} how many parameters it pushed, does the
7080 There is an alternative calling convention used by Win32 programs
7081 for Windows API calls, and also for functions called \e{by} the
7082 Windows API such as window procedures: they follow what Microsoft
7083 calls the \c{__stdcall} convention. This is slightly closer to the
7084 Pascal convention, in that the callee clears the stack by passing a
7085 parameter to the \c{RET} instruction. However, the parameters are
7086 still pushed in right-to-left order.
7088 Thus, you would define a function in C style in the following way:
7095 \c sub esp,0x40 ; 64 bytes of local stack space
7096 \c mov ebx,[ebp+8] ; first parameter to function
7100 \c leave ; mov esp,ebp / pop ebp
7103 At the other end of the process, to call a C function from your
7104 assembly code, you would do something like this:
7108 \c ; and then, further down...
7110 \c push dword [myint] ; one of my integer variables
7111 \c push dword mystring ; pointer into my data segment
7113 \c add esp,byte 8 ; `byte' saves space
7115 \c ; then those data items...
7120 \c mystring db 'This number -> %d <- should be 1234',10,0
7122 This piece of code is the assembly equivalent of the C code
7124 \c int myint = 1234;
7125 \c printf("This number -> %d <- should be 1234\n", myint);
7128 \S{32cdata} Accessing Data Items
7130 To get at the contents of C variables, or to declare variables which
7131 C can access, you need only declare the names as \c{GLOBAL} or
7132 \c{EXTERN}. (Again, the names require leading underscores, as stated
7133 in \k{32cunder}.) Thus, a C variable declared as \c{int i} can be
7134 accessed from assembler as
7139 And to declare your own integer variable which C programs can access
7140 as \c{extern int j}, you do this (making sure you are assembling in
7141 the \c{_DATA} segment, if necessary):
7146 To access a C array, you need to know the size of the components of
7147 the array. For example, \c{int} variables are four bytes long, so if
7148 a C program declares an array as \c{int a[10]}, you can access
7149 \c{a[3]} by coding \c{mov ax,[_a+12]}. (The byte offset 12 is obtained
7150 by multiplying the desired array index, 3, by the size of the array
7151 element, 4.) The sizes of the C base types in 32-bit compilers are:
7152 1 for \c{char}, 2 for \c{short}, 4 for \c{int}, \c{long} and
7153 \c{float}, and 8 for \c{double}. Pointers, being 32-bit addresses,
7154 are also 4 bytes long.
7156 To access a C \i{data structure}, you need to know the offset from
7157 the base of the structure to the field you are interested in. You
7158 can either do this by converting the C structure definition into a
7159 NASM structure definition (using \c{STRUC}), or by calculating the
7160 one offset and using just that.
7162 To do either of these, you should read your C compiler's manual to
7163 find out how it organizes data structures. NASM gives no special
7164 alignment to structure members in its own \i\c{STRUC} macro, so you
7165 have to specify alignment yourself if the C compiler generates it.
7166 Typically, you might find that a structure like
7173 might be eight bytes long rather than five, since the \c{int} field
7174 would be aligned to a four-byte boundary. However, this sort of
7175 feature is sometimes a configurable option in the C compiler, either
7176 using command-line options or \c{#pragma} lines, so you have to find
7177 out how your own compiler does it.
7180 \S{32cmacro} \i\c{c32.mac}: Helper Macros for the 32-bit C Interface
7182 Included in the NASM archives, in the \I{misc directory}\c{misc}
7183 directory, is a file \c{c32.mac} of macros. It defines three macros:
7184 \i\c{proc}, \i\c{arg} and \i\c{endproc}. These are intended to be
7185 used for C-style procedure definitions, and they automate a lot of
7186 the work involved in keeping track of the calling convention.
7188 An example of an assembly function using the macro set is given
7195 \c mov eax,[ebp + %$i]
7196 \c mov ebx,[ebp + %$j]
7201 This defines \c{_proc32} to be a procedure taking two arguments, the
7202 first (\c{i}) an integer and the second (\c{j}) a pointer to an
7203 integer. It returns \c{i + *j}.
7205 Note that the \c{arg} macro has an \c{EQU} as the first line of its
7206 expansion, and since the label before the macro call gets prepended
7207 to the first line of the expanded macro, the \c{EQU} works, defining
7208 \c{%$i} to be an offset from \c{BP}. A context-local variable is
7209 used, local to the context pushed by the \c{proc} macro and popped
7210 by the \c{endproc} macro, so that the same argument name can be used
7211 in later procedures. Of course, you don't \e{have} to do that.
7213 \c{arg} can take an optional parameter, giving the size of the
7214 argument. If no size is given, 4 is assumed, since it is likely that
7215 many function parameters will be of type \c{int} or pointers.
7218 \H{picdll} Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF \i{Shared
7221 \c{ELF} replaced the older \c{a.out} object file format under Linux
7222 because it contains support for \i{position-independent code}
7223 (\i{PIC}), which makes writing shared libraries much easier. NASM
7224 supports the \c{ELF} position-independent code features, so you can
7225 write Linux \c{ELF} shared libraries in NASM.
7227 \i{NetBSD}, and its close cousins \i{FreeBSD} and \i{OpenBSD}, take
7228 a different approach by hacking PIC support into the \c{a.out}
7229 format. NASM supports this as the \i\c{aoutb} output format, so you
7230 can write \i{BSD} shared libraries in NASM too.
7232 The operating system loads a PIC shared library by memory-mapping
7233 the library file at an arbitrarily chosen point in the address space
7234 of the running process. The contents of the library's code section
7235 must therefore not depend on where it is loaded in memory.
7237 Therefore, you cannot get at your variables by writing code like
7240 \c mov eax,[myvar] ; WRONG
7242 Instead, the linker provides an area of memory called the
7243 \i\e{global offset table}, or \i{GOT}; the GOT is situated at a
7244 constant distance from your library's code, so if you can find out
7245 where your library is loaded (which is typically done using a
7246 \c{CALL} and \c{POP} combination), you can obtain the address of the
7247 GOT, and you can then load the addresses of your variables out of
7248 linker-generated entries in the GOT.
7250 The \e{data} section of a PIC shared library does not have these
7251 restrictions: since the data section is writable, it has to be
7252 copied into memory anyway rather than just paged in from the library
7253 file, so as long as it's being copied it can be relocated too. So
7254 you can put ordinary types of relocation in the data section without
7255 too much worry (but see \k{picglobal} for a caveat).
7258 \S{picgot} Obtaining the Address of the GOT
7260 Each code module in your shared library should define the GOT as an
7263 \c extern _GLOBAL_OFFSET_TABLE_ ; in ELF
7264 \c extern __GLOBAL_OFFSET_TABLE_ ; in BSD a.out
7266 At the beginning of any function in your shared library which plans
7267 to access your data or BSS sections, you must first calculate the
7268 address of the GOT. This is typically done by writing the function
7277 \c add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
7279 \c ; the function body comes here
7286 (For BSD, again, the symbol \c{_GLOBAL_OFFSET_TABLE} requires a
7287 second leading underscore.)
7289 The first two lines of this function are simply the standard C
7290 prologue to set up a stack frame, and the last three lines are
7291 standard C function epilogue. The third line, and the fourth to last
7292 line, save and restore the \c{EBX} register, because PIC shared
7293 libraries use this register to store the address of the GOT.
7295 The interesting bit is the \c{CALL} instruction and the following
7296 two lines. The \c{CALL} and \c{POP} combination obtains the address
7297 of the label \c{.get_GOT}, without having to know in advance where
7298 the program was loaded (since the \c{CALL} instruction is encoded
7299 relative to the current position). The \c{ADD} instruction makes use
7300 of one of the special PIC relocation types: \i{GOTPC relocation}.
7301 With the \i\c{WRT ..gotpc} qualifier specified, the symbol
7302 referenced (here \c{_GLOBAL_OFFSET_TABLE_}, the special symbol
7303 assigned to the GOT) is given as an offset from the beginning of the
7304 section. (Actually, \c{ELF} encodes it as the offset from the operand
7305 field of the \c{ADD} instruction, but NASM simplifies this
7306 deliberately, so you do things the same way for both \c{ELF} and
7307 \c{BSD}.) So the instruction then \e{adds} the beginning of the section,
7308 to get the real address of the GOT, and subtracts the value of
7309 \c{.get_GOT} which it knows is in \c{EBX}. Therefore, by the time
7310 that instruction has finished, \c{EBX} contains the address of the GOT.
7312 If you didn't follow that, don't worry: it's never necessary to
7313 obtain the address of the GOT by any other means, so you can put
7314 those three instructions into a macro and safely ignore them:
7321 \c add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
7325 \S{piclocal} Finding Your Local Data Items
7327 Having got the GOT, you can then use it to obtain the addresses of
7328 your data items. Most variables will reside in the sections you have
7329 declared; they can be accessed using the \I{GOTOFF
7330 relocation}\c{..gotoff} special \I\c{WRT ..gotoff}\c{WRT} type. The
7331 way this works is like this:
7333 \c lea eax,[ebx+myvar wrt ..gotoff]
7335 The expression \c{myvar wrt ..gotoff} is calculated, when the shared
7336 library is linked, to be the offset to the local variable \c{myvar}
7337 from the beginning of the GOT. Therefore, adding it to \c{EBX} as
7338 above will place the real address of \c{myvar} in \c{EAX}.
7340 If you declare variables as \c{GLOBAL} without specifying a size for
7341 them, they are shared between code modules in the library, but do
7342 not get exported from the library to the program that loaded it.
7343 They will still be in your ordinary data and BSS sections, so you
7344 can access them in the same way as local variables, using the above
7345 \c{..gotoff} mechanism.
7347 Note that due to a peculiarity of the way BSD \c{a.out} format
7348 handles this relocation type, there must be at least one non-local
7349 symbol in the same section as the address you're trying to access.
7352 \S{picextern} Finding External and Common Data Items
7354 If your library needs to get at an external variable (external to
7355 the \e{library}, not just to one of the modules within it), you must
7356 use the \I{GOT relocations}\I\c{WRT ..got}\c{..got} type to get at
7357 it. The \c{..got} type, instead of giving you the offset from the
7358 GOT base to the variable, gives you the offset from the GOT base to
7359 a GOT \e{entry} containing the address of the variable. The linker
7360 will set up this GOT entry when it builds the library, and the
7361 dynamic linker will place the correct address in it at load time. So
7362 to obtain the address of an external variable \c{extvar} in \c{EAX},
7365 \c mov eax,[ebx+extvar wrt ..got]
7367 This loads the address of \c{extvar} out of an entry in the GOT. The
7368 linker, when it builds the shared library, collects together every
7369 relocation of type \c{..got}, and builds the GOT so as to ensure it
7370 has every necessary entry present.
7372 Common variables must also be accessed in this way.
7375 \S{picglobal} Exporting Symbols to the Library User
7377 If you want to export symbols to the user of the library, you have
7378 to declare whether they are functions or data, and if they are data,
7379 you have to give the size of the data item. This is because the
7380 dynamic linker has to build \I{PLT}\i{procedure linkage table}
7381 entries for any exported functions, and also moves exported data
7382 items away from the library's data section in which they were
7385 So to export a function to users of the library, you must use
7387 \c global func:function ; declare it as a function
7393 And to export a data item such as an array, you would have to code
7395 \c global array:data array.end-array ; give the size too
7400 Be careful: If you export a variable to the library user, by
7401 declaring it as \c{GLOBAL} and supplying a size, the variable will
7402 end up living in the data section of the main program, rather than
7403 in your library's data section, where you declared it. So you will
7404 have to access your own global variable with the \c{..got} mechanism
7405 rather than \c{..gotoff}, as if it were external (which,
7406 effectively, it has become).
7408 Equally, if you need to store the address of an exported global in
7409 one of your data sections, you can't do it by means of the standard
7412 \c dataptr: dd global_data_item ; WRONG
7414 NASM will interpret this code as an ordinary relocation, in which
7415 \c{global_data_item} is merely an offset from the beginning of the
7416 \c{.data} section (or whatever); so this reference will end up
7417 pointing at your data section instead of at the exported global
7418 which resides elsewhere.
7420 Instead of the above code, then, you must write
7422 \c dataptr: dd global_data_item wrt ..sym
7424 which makes use of the special \c{WRT} type \I\c{WRT ..sym}\c{..sym}
7425 to instruct NASM to search the symbol table for a particular symbol
7426 at that address, rather than just relocating by section base.
7428 Either method will work for functions: referring to one of your
7429 functions by means of
7431 \c funcptr: dd my_function
7433 will give the user the address of the code you wrote, whereas
7435 \c funcptr: dd my_function wrt ..sym
7437 will give the address of the procedure linkage table for the
7438 function, which is where the calling program will \e{believe} the
7439 function lives. Either address is a valid way to call the function.
7442 \S{picproc} Calling Procedures Outside the Library
7444 Calling procedures outside your shared library has to be done by
7445 means of a \i\e{procedure linkage table}, or \i{PLT}. The PLT is
7446 placed at a known offset from where the library is loaded, so the
7447 library code can make calls to the PLT in a position-independent
7448 way. Within the PLT there is code to jump to offsets contained in
7449 the GOT, so function calls to other shared libraries or to routines
7450 in the main program can be transparently passed off to their real
7453 To call an external routine, you must use another special PIC
7454 relocation type, \I{PLT relocations}\i\c{WRT ..plt}. This is much
7455 easier than the GOT-based ones: you simply replace calls such as
7456 \c{CALL printf} with the PLT-relative version \c{CALL printf WRT
7460 \S{link} Generating the Library File
7462 Having written some code modules and assembled them to \c{.o} files,
7463 you then generate your shared library with a command such as
7465 \c ld -shared -o library.so module1.o module2.o # for ELF
7466 \c ld -Bshareable -o library.so module1.o module2.o # for BSD
7468 For ELF, if your shared library is going to reside in system
7469 directories such as \c{/usr/lib} or \c{/lib}, it is usually worth
7470 using the \i\c{-soname} flag to the linker, to store the final
7471 library file name, with a version number, into the library:
7473 \c ld -shared -soname library.so.1 -o library.so.1.2 *.o
7475 You would then copy \c{library.so.1.2} into the library directory,
7476 and create \c{library.so.1} as a symbolic link to it.
7479 \C{mixsize} Mixing 16 and 32 Bit Code
7481 This chapter tries to cover some of the issues, largely related to
7482 unusual forms of addressing and jump instructions, encountered when
7483 writing operating system code such as protected-mode initialisation
7484 routines, which require code that operates in mixed segment sizes,
7485 such as code in a 16-bit segment trying to modify data in a 32-bit
7486 one, or jumps between different-size segments.
7489 \H{mixjump} Mixed-Size Jumps\I{jumps, mixed-size}
7491 \I{operating system, writing}\I{writing operating systems}The most
7492 common form of \i{mixed-size instruction} is the one used when
7493 writing a 32-bit OS: having done your setup in 16-bit mode, such as
7494 loading the kernel, you then have to boot it by switching into
7495 protected mode and jumping to the 32-bit kernel start address. In a
7496 fully 32-bit OS, this tends to be the \e{only} mixed-size
7497 instruction you need, since everything before it can be done in pure
7498 16-bit code, and everything after it can be pure 32-bit.
7500 This jump must specify a 48-bit far address, since the target
7501 segment is a 32-bit one. However, it must be assembled in a 16-bit
7502 segment, so just coding, for example,
7504 \c jmp 0x1234:0x56789ABC ; wrong!
7506 will not work, since the offset part of the address will be
7507 truncated to \c{0x9ABC} and the jump will be an ordinary 16-bit far
7510 The Linux kernel setup code gets round the inability of \c{as86} to
7511 generate the required instruction by coding it manually, using
7512 \c{DB} instructions. NASM can go one better than that, by actually
7513 generating the right instruction itself. Here's how to do it right:
7515 \c jmp dword 0x1234:0x56789ABC ; right
7517 \I\c{JMP DWORD}The \c{DWORD} prefix (strictly speaking, it should
7518 come \e{after} the colon, since it is declaring the \e{offset} field
7519 to be a doubleword; but NASM will accept either form, since both are
7520 unambiguous) forces the offset part to be treated as far, in the
7521 assumption that you are deliberately writing a jump from a 16-bit
7522 segment to a 32-bit one.
7524 You can do the reverse operation, jumping from a 32-bit segment to a
7525 16-bit one, by means of the \c{WORD} prefix:
7527 \c jmp word 0x8765:0x4321 ; 32 to 16 bit
7529 If the \c{WORD} prefix is specified in 16-bit mode, or the \c{DWORD}
7530 prefix in 32-bit mode, they will be ignored, since each is
7531 explicitly forcing NASM into a mode it was in anyway.
7534 \H{mixaddr} Addressing Between Different-Size Segments\I{addressing,
7535 mixed-size}\I{mixed-size addressing}
7537 If your OS is mixed 16 and 32-bit, or if you are writing a DOS
7538 extender, you are likely to have to deal with some 16-bit segments
7539 and some 32-bit ones. At some point, you will probably end up
7540 writing code in a 16-bit segment which has to access data in a
7541 32-bit segment, or vice versa.
7543 If the data you are trying to access in a 32-bit segment lies within
7544 the first 64K of the segment, you may be able to get away with using
7545 an ordinary 16-bit addressing operation for the purpose; but sooner
7546 or later, you will want to do 32-bit addressing from 16-bit mode.
7548 The easiest way to do this is to make sure you use a register for
7549 the address, since any effective address containing a 32-bit
7550 register is forced to be a 32-bit address. So you can do
7552 \c mov eax,offset_into_32_bit_segment_specified_by_fs
7553 \c mov dword [fs:eax],0x11223344
7555 This is fine, but slightly cumbersome (since it wastes an
7556 instruction and a register) if you already know the precise offset
7557 you are aiming at. The x86 architecture does allow 32-bit effective
7558 addresses to specify nothing but a 4-byte offset, so why shouldn't
7559 NASM be able to generate the best instruction for the purpose?
7561 It can. As in \k{mixjump}, you need only prefix the address with the
7562 \c{DWORD} keyword, and it will be forced to be a 32-bit address:
7564 \c mov dword [fs:dword my_offset],0x11223344
7566 Also as in \k{mixjump}, NASM is not fussy about whether the
7567 \c{DWORD} prefix comes before or after the segment override, so
7568 arguably a nicer-looking way to code the above instruction is
7570 \c mov dword [dword fs:my_offset],0x11223344
7572 Don't confuse the \c{DWORD} prefix \e{outside} the square brackets,
7573 which controls the size of the data stored at the address, with the
7574 one \c{inside} the square brackets which controls the length of the
7575 address itself. The two can quite easily be different:
7577 \c mov word [dword 0x12345678],0x9ABC
7579 This moves 16 bits of data to an address specified by a 32-bit
7582 You can also specify \c{WORD} or \c{DWORD} prefixes along with the
7583 \c{FAR} prefix to indirect far jumps or calls. For example:
7585 \c call dword far [fs:word 0x4321]
7587 This instruction contains an address specified by a 16-bit offset;
7588 it loads a 48-bit far pointer from that (16-bit segment and 32-bit
7589 offset), and calls that address.
7592 \H{mixother} Other Mixed-Size Instructions
7594 The other way you might want to access data might be using the
7595 string instructions (\c{LODSx}, \c{STOSx} and so on) or the
7596 \c{XLATB} instruction. These instructions, since they take no
7597 parameters, might seem to have no easy way to make them perform
7598 32-bit addressing when assembled in a 16-bit segment.
7600 This is the purpose of NASM's \i\c{a16}, \i\c{a32} and \i\c{a64} prefixes. If
7601 you are coding \c{LODSB} in a 16-bit segment but it is supposed to
7602 be accessing a string in a 32-bit segment, you should load the
7603 desired address into \c{ESI} and then code
7607 The prefix forces the addressing size to 32 bits, meaning that
7608 \c{LODSB} loads from \c{[DS:ESI]} instead of \c{[DS:SI]}. To access
7609 a string in a 16-bit segment when coding in a 32-bit one, the
7610 corresponding \c{a16} prefix can be used.
7612 The \c{a16}, \c{a32} and \c{a64} prefixes can be applied to any instruction
7613 in NASM's instruction table, but most of them can generate all the
7614 useful forms without them. The prefixes are necessary only for
7615 instructions with implicit addressing:
7616 \# \c{CMPSx} (\k{insCMPSB}),
7617 \# \c{SCASx} (\k{insSCASB}), \c{LODSx} (\k{insLODSB}), \c{STOSx}
7618 \# (\k{insSTOSB}), \c{MOVSx} (\k{insMOVSB}), \c{INSx} (\k{insINSB}),
7619 \# \c{OUTSx} (\k{insOUTSB}), and \c{XLATB} (\k{insXLATB}).
7620 \c{CMPSx}, \c{SCASx}, \c{LODSx}, \c{STOSx}, \c{MOVSx}, \c{INSx},
7621 \c{OUTSx}, and \c{XLATB}.
7623 various push and pop instructions (\c{PUSHA} and \c{POPF} as well as
7624 the more usual \c{PUSH} and \c{POP}) can accept \c{a16}, \c{a32} or \c{a64}
7625 prefixes to force a particular one of \c{SP}, \c{ESP} or \c{RSP} to be used
7626 as a stack pointer, in case the stack segment in use is a different
7627 size from the code segment.
7629 \c{PUSH} and \c{POP}, when applied to segment registers in 32-bit
7630 mode, also have the slightly odd behaviour that they push and pop 4
7631 bytes at a time, of which the top two are ignored and the bottom two
7632 give the value of the segment register being manipulated. To force
7633 the 16-bit behaviour of segment-register push and pop instructions,
7634 you can use the operand-size prefix \i\c{o16}:
7639 This code saves a doubleword of stack space by fitting two segment
7640 registers into the space which would normally be consumed by pushing
7643 (You can also use the \i\c{o32} prefix to force the 32-bit behaviour
7644 when in 16-bit mode, but this seems less useful.)
7647 \C{64bit} Writing 64-bit Code (Unix, Win64)
7649 This chapter attempts to cover some of the common issues involved when
7650 writing 64-bit code, to run under \i{Win64} or Unix. It covers how to
7651 write assembly code to interface with 64-bit C routines, and how to
7652 write position-independent code for shared libraries.
7654 All 64-bit code uses a flat memory model, since segmentation is not
7655 available in 64-bit mode. The one exception is the \c{FS} and \c{GS}
7656 registers, which still add their bases.
7658 Position independence in 64-bit mode is significantly simpler, since
7659 the processor supports \c{RIP}-relative addressing directly; see the
7660 \c{REL} keyword (\k{effaddr}). On most 64-bit platforms, it is
7661 probably desirable to make that the default, using the directive
7662 \c{DEFAULT REL} (\k{default}).
7664 64-bit programming is relatively similar to 32-bit programming, but
7665 of course pointers are 64 bits long; additionally, all existing
7666 platforms pass arguments in registers rather than on the stack.
7667 Furthermore, 64-bit platforms use SSE2 by default for floating point.
7668 Please see the ABI documentation for your platform.
7670 64-bit platforms differ in the sizes of the fundamental datatypes, not
7671 just from 32-bit platforms but from each other. If a specific size
7672 data type is desired, it is probably best to use the types defined in
7673 the Standard C header \c{<inttypes.h>}.
7675 In 64-bit mode, the default instruction size is still 32 bits. When
7676 loading a value into a 32-bit register (but not an 8- or 16-bit
7677 register), the upper 32 bits of the corresponding 64-bit register are
7680 \H{reg64} Register Names in 64-bit Mode
7682 NASM uses the following names for general-purpose registers in 64-bit
7683 mode, for 8-, 16-, 32- and 64-bit references, respectively:
7685 \c AL/AH, CL/CH, DL/DH, BL/BH, SPL, BPL, SIL, DIL, R8B-R15B
7686 \c AX, CX, DX, BX, SP, BP, SI, DI, R8W-R15W
7687 \c EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, R8D-R15D
7688 \c RAX, RCX, RDX, RBX, RSP, RBP, RSI, RDI, R8-R15
7690 This is consistent with the AMD documentation and most other
7691 assemblers. The Intel documentation, however, uses the names
7692 \c{R8L-R15L} for 8-bit references to the higher registers. It is
7693 possible to use those names by definiting them as macros; similarly,
7694 if one wants to use numeric names for the low 8 registers, define them
7695 as macros. The standard macro package \c{altreg} (see \k{pkg_altreg})
7696 can be used for this purpose.
7698 \H{id64} Immediates and Displacements in 64-bit Mode
7700 In 64-bit mode, immediates and displacements are generally only 32
7701 bits wide. NASM will therefore truncate most displacements and
7702 immediates to 32 bits.
7704 The only instruction which takes a full \i{64-bit immediate} is:
7708 NASM will produce this instruction whenever the programmer uses
7709 \c{MOV} with an immediate into a 64-bit register. If this is not
7710 desirable, simply specify the equivalent 32-bit register, which will
7711 be automatically zero-extended by the processor, or specify the
7712 immediate as \c{DWORD}:
7714 \c mov rax,foo ; 64-bit immediate
7715 \c mov rax,qword foo ; (identical)
7716 \c mov eax,foo ; 32-bit immediate, zero-extended
7717 \c mov rax,dword foo ; 32-bit immediate, sign-extended
7719 The length of these instructions are 10, 5 and 7 bytes, respectively.
7721 The only instructions which take a full \I{64-bit displacement}64-bit
7722 \e{displacement} is loading or storing, using \c{MOV}, \c{AL}, \c{AX},
7723 \c{EAX} or \c{RAX} (but no other registers) to an absolute 64-bit address.
7724 Since this is a relatively rarely used instruction (64-bit code generally uses
7725 relative addressing), the programmer has to explicitly declare the
7726 displacement size as \c{QWORD}:
7730 \c mov eax,[foo] ; 32-bit absolute disp, sign-extended
7731 \c mov eax,[a32 foo] ; 32-bit absolute disp, zero-extended
7732 \c mov eax,[qword foo] ; 64-bit absolute disp
7736 \c mov eax,[foo] ; 32-bit relative disp
7737 \c mov eax,[a32 foo] ; d:o, address truncated to 32 bits(!)
7738 \c mov eax,[qword foo] ; error
7739 \c mov eax,[abs qword foo] ; 64-bit absolute disp
7741 A sign-extended absolute displacement can access from -2 GB to +2 GB;
7742 a zero-extended absolute displacement can access from 0 to 4 GB.
7744 \H{unix64} Interfacing to 64-bit C Programs (Unix)
7746 On Unix, the 64-bit ABI is defined by the document:
7748 \W{http://www.nasm.us/links/unix64abi}\c{http://www.nasm.us/links/unix64abi}
7750 Although written for AT&T-syntax assembly, the concepts apply equally
7751 well for NASM-style assembly. What follows is a simplified summary.
7753 The first six integer arguments (from the left) are passed in \c{RDI},
7754 \c{RSI}, \c{RDX}, \c{RCX}, \c{R8}, and \c{R9}, in that order.
7755 Additional integer arguments are passed on the stack. These
7756 registers, plus \c{RAX}, \c{R10} and \c{R11} are destroyed by function
7757 calls, and thus are available for use by the function without saving.
7759 Integer return values are passed in \c{RAX} and \c{RDX}, in that order.
7761 Floating point is done using SSE registers, except for \c{long
7762 double}. Floating-point arguments are passed in \c{XMM0} to \c{XMM7};
7763 return is \c{XMM0} and \c{XMM1}. \c{long double} are passed on the
7764 stack, and returned in \c{ST0} and \c{ST1}.
7766 All SSE and x87 registers are destroyed by function calls.
7768 On 64-bit Unix, \c{long} is 64 bits.
7770 Integer and SSE register arguments are counted separately, so for the case of
7772 \c void foo(long a, double b, int c)
7774 \c{a} is passed in \c{RDI}, \c{b} in \c{XMM0}, and \c{c} in \c{ESI}.
7776 \H{win64} Interfacing to 64-bit C Programs (Win64)
7778 The Win64 ABI is described at:
7780 \W{http://www.nasm.us/links/win64abi}\c{http://www.nasm.us/links/win64abi}
7782 What follows is a simplified summary.
7784 The first four integer arguments are passed in \c{RCX}, \c{RDX},
7785 \c{R8} and \c{R9}, in that order. Additional integer arguments are
7786 passed on the stack. These registers, plus \c{RAX}, \c{R10} and
7787 \c{R11} are destroyed by function calls, and thus are available for
7788 use by the function without saving.
7790 Integer return values are passed in \c{RAX} only.
7792 Floating point is done using SSE registers, except for \c{long
7793 double}. Floating-point arguments are passed in \c{XMM0} to \c{XMM3};
7794 return is \c{XMM0} only.
7796 On Win64, \c{long} is 32 bits; \c{long long} or \c{_int64} is 64 bits.
7798 Integer and SSE register arguments are counted together, so for the case of
7800 \c void foo(long long a, double b, int c)
7802 \c{a} is passed in \c{RCX}, \c{b} in \c{XMM1}, and \c{c} in \c{R8D}.
7804 \C{trouble} Troubleshooting
7806 This chapter describes some of the common problems that users have
7807 been known to encounter with NASM, and answers them. It also gives
7808 instructions for reporting bugs in NASM if you find a difficulty
7809 that isn't listed here.
7812 \H{problems} Common Problems
7814 \S{inefficient} NASM Generates \i{Inefficient Code}
7816 We sometimes get `bug' reports about NASM generating inefficient, or
7817 even `wrong', code on instructions such as \c{ADD ESP,8}. This is a
7818 deliberate design feature, connected to predictability of output:
7819 NASM, on seeing \c{ADD ESP,8}, will generate the form of the
7820 instruction which leaves room for a 32-bit offset. You need to code
7821 \I\c{BYTE}\c{ADD ESP,BYTE 8} if you want the space-efficient form of
7822 the instruction. This isn't a bug, it's user error: if you prefer to
7823 have NASM produce the more efficient code automatically enable
7824 optimization with the \c{-O} option (see \k{opt-O}).
7827 \S{jmprange} My Jumps are Out of Range\I{out of range, jumps}
7829 Similarly, people complain that when they issue \i{conditional
7830 jumps} (which are \c{SHORT} by default) that try to jump too far,
7831 NASM reports `short jump out of range' instead of making the jumps
7834 This, again, is partly a predictability issue, but in fact has a
7835 more practical reason as well. NASM has no means of being told what
7836 type of processor the code it is generating will be run on; so it
7837 cannot decide for itself that it should generate \i\c{Jcc NEAR} type
7838 instructions, because it doesn't know that it's working for a 386 or
7839 above. Alternatively, it could replace the out-of-range short
7840 \c{JNE} instruction with a very short \c{JE} instruction that jumps
7841 over a \c{JMP NEAR}; this is a sensible solution for processors
7842 below a 386, but hardly efficient on processors which have good
7843 branch prediction \e{and} could have used \c{JNE NEAR} instead. So,
7844 once again, it's up to the user, not the assembler, to decide what
7845 instructions should be generated. See \k{opt-O}.
7848 \S{proborg} \i\c{ORG} Doesn't Work
7850 People writing \i{boot sector} programs in the \c{bin} format often
7851 complain that \c{ORG} doesn't work the way they'd like: in order to
7852 place the \c{0xAA55} signature word at the end of a 512-byte boot
7853 sector, people who are used to MASM tend to code
7857 \c ; some boot sector code
7862 This is not the intended use of the \c{ORG} directive in NASM, and
7863 will not work. The correct way to solve this problem in NASM is to
7864 use the \i\c{TIMES} directive, like this:
7868 \c ; some boot sector code
7870 \c TIMES 510-($-$$) DB 0
7873 The \c{TIMES} directive will insert exactly enough zero bytes into
7874 the output to move the assembly point up to 510. This method also
7875 has the advantage that if you accidentally fill your boot sector too
7876 full, NASM will catch the problem at assembly time and report it, so
7877 you won't end up with a boot sector that you have to disassemble to
7878 find out what's wrong with it.
7881 \S{probtimes} \i\c{TIMES} Doesn't Work
7883 The other common problem with the above code is people who write the
7888 by reasoning that \c{$} should be a pure number, just like 510, so
7889 the difference between them is also a pure number and can happily be
7892 NASM is a \e{modular} assembler: the various component parts are
7893 designed to be easily separable for re-use, so they don't exchange
7894 information unnecessarily. In consequence, the \c{bin} output
7895 format, even though it has been told by the \c{ORG} directive that
7896 the \c{.text} section should start at 0, does not pass that
7897 information back to the expression evaluator. So from the
7898 evaluator's point of view, \c{$} isn't a pure number: it's an offset
7899 from a section base. Therefore the difference between \c{$} and 510
7900 is also not a pure number, but involves a section base. Values
7901 involving section bases cannot be passed as arguments to \c{TIMES}.
7903 The solution, as in the previous section, is to code the \c{TIMES}
7906 \c TIMES 510-($-$$) DB 0
7908 in which \c{$} and \c{$$} are offsets from the same section base,
7909 and so their difference is a pure number. This will solve the
7910 problem and generate sensible code.
7913 \H{bugs} \i{Bugs}\I{reporting bugs}
7915 We have never yet released a version of NASM with any \e{known}
7916 bugs. That doesn't usually stop there being plenty we didn't know
7917 about, though. Any that you find should be reported firstly via the
7919 \W{http://www.nasm.us/}\c{http://www.nasm.us/}
7920 (click on "Bug Tracker"), or if that fails then through one of the
7921 contacts in \k{contact}.
7923 Please read \k{qstart} first, and don't report the bug if it's
7924 listed in there as a deliberate feature. (If you think the feature
7925 is badly thought out, feel free to send us reasons why you think it
7926 should be changed, but don't just send us mail saying `This is a
7927 bug' if the documentation says we did it on purpose.) Then read
7928 \k{problems}, and don't bother reporting the bug if it's listed
7931 If you do report a bug, \e{please} give us all of the following
7934 \b What operating system you're running NASM under. DOS, Linux,
7935 NetBSD, Win16, Win32, VMS (I'd be impressed), whatever.
7937 \b If you're running NASM under DOS or Win32, tell us whether you've
7938 compiled your own executable from the DOS source archive, or whether
7939 you were using the standard distribution binaries out of the
7940 archive. If you were using a locally built executable, try to
7941 reproduce the problem using one of the standard binaries, as this
7942 will make it easier for us to reproduce your problem prior to fixing
7945 \b Which version of NASM you're using, and exactly how you invoked
7946 it. Give us the precise command line, and the contents of the
7947 \c{NASMENV} environment variable if any.
7949 \b Which versions of any supplementary programs you're using, and
7950 how you invoked them. If the problem only becomes visible at link
7951 time, tell us what linker you're using, what version of it you've
7952 got, and the exact linker command line. If the problem involves
7953 linking against object files generated by a compiler, tell us what
7954 compiler, what version, and what command line or options you used.
7955 (If you're compiling in an IDE, please try to reproduce the problem
7956 with the command-line version of the compiler.)
7958 \b If at all possible, send us a NASM source file which exhibits the
7959 problem. If this causes copyright problems (e.g. you can only
7960 reproduce the bug in restricted-distribution code) then bear in mind
7961 the following two points: firstly, we guarantee that any source code
7962 sent to us for the purposes of debugging NASM will be used \e{only}
7963 for the purposes of debugging NASM, and that we will delete all our
7964 copies of it as soon as we have found and fixed the bug or bugs in
7965 question; and secondly, we would prefer \e{not} to be mailed large
7966 chunks of code anyway. The smaller the file, the better. A
7967 three-line sample file that does nothing useful \e{except}
7968 demonstrate the problem is much easier to work with than a
7969 fully fledged ten-thousand-line program. (Of course, some errors
7970 \e{do} only crop up in large files, so this may not be possible.)
7972 \b A description of what the problem actually \e{is}. `It doesn't
7973 work' is \e{not} a helpful description! Please describe exactly what
7974 is happening that shouldn't be, or what isn't happening that should.
7975 Examples might be: `NASM generates an error message saying Line 3
7976 for an error that's actually on Line 5'; `NASM generates an error
7977 message that I believe it shouldn't be generating at all'; `NASM
7978 fails to generate an error message that I believe it \e{should} be
7979 generating'; `the object file produced from this source code crashes
7980 my linker'; `the ninth byte of the output file is 66 and I think it
7981 should be 77 instead'.
7983 \b If you believe the output file from NASM to be faulty, send it to
7984 us. That allows us to determine whether our own copy of NASM
7985 generates the same file, or whether the problem is related to
7986 portability issues between our development platforms and yours. We
7987 can handle binary files mailed to us as MIME attachments, uuencoded,
7988 and even BinHex. Alternatively, we may be able to provide an FTP
7989 site you can upload the suspect files to; but mailing them is easier
7992 \b Any other information or data files that might be helpful. If,
7993 for example, the problem involves NASM failing to generate an object
7994 file while TASM can generate an equivalent file without trouble,
7995 then send us \e{both} object files, so we can see what TASM is doing
7996 differently from us.
7999 \A{ndisasm} \i{Ndisasm}
8001 The Netwide Disassembler, NDISASM
8003 \H{ndisintro} Introduction
8006 The Netwide Disassembler is a small companion program to the Netwide
8007 Assembler, NASM. It seemed a shame to have an x86 assembler,
8008 complete with a full instruction table, and not make as much use of
8009 it as possible, so here's a disassembler which shares the
8010 instruction table (and some other bits of code) with NASM.
8012 The Netwide Disassembler does nothing except to produce
8013 disassemblies of \e{binary} source files. NDISASM does not have any
8014 understanding of object file formats, like \c{objdump}, and it will
8015 not understand \c{DOS .EXE} files like \c{debug} will. It just
8019 \H{ndisstart} Getting Started: Installation
8021 See \k{install} for installation instructions. NDISASM, like NASM,
8022 has a \c{man page} which you may want to put somewhere useful, if you
8023 are on a Unix system.
8026 \H{ndisrun} Running NDISASM
8028 To disassemble a file, you will typically use a command of the form
8030 \c ndisasm -b {16|32|64} filename
8032 NDISASM can disassemble 16-, 32- or 64-bit code equally easily,
8033 provided of course that you remember to specify which it is to work
8034 with. If no \i\c{-b} switch is present, NDISASM works in 16-bit mode
8035 by default. The \i\c{-u} switch (for USE32) also invokes 32-bit mode.
8037 Two more command line options are \i\c{-r} which reports the version
8038 number of NDISASM you are running, and \i\c{-h} which gives a short
8039 summary of command line options.
8042 \S{ndiscom} COM Files: Specifying an Origin
8044 To disassemble a \c{DOS .COM} file correctly, a disassembler must assume
8045 that the first instruction in the file is loaded at address \c{0x100},
8046 rather than at zero. NDISASM, which assumes by default that any file
8047 you give it is loaded at zero, will therefore need to be informed of
8050 The \i\c{-o} option allows you to declare a different origin for the
8051 file you are disassembling. Its argument may be expressed in any of
8052 the NASM numeric formats: decimal by default, if it begins with `\c{$}'
8053 or `\c{0x}' or ends in `\c{H}' it's \c{hex}, if it ends in `\c{Q}' it's
8054 \c{octal}, and if it ends in `\c{B}' it's \c{binary}.
8056 Hence, to disassemble a \c{.COM} file:
8058 \c ndisasm -o100h filename.com
8063 \S{ndissync} Code Following Data: Synchronisation
8065 Suppose you are disassembling a file which contains some data which
8066 isn't machine code, and \e{then} contains some machine code. NDISASM
8067 will faithfully plough through the data section, producing machine
8068 instructions wherever it can (although most of them will look
8069 bizarre, and some may have unusual prefixes, e.g. `\c{FS OR AX,0x240A}'),
8070 and generating `DB' instructions ever so often if it's totally stumped.
8071 Then it will reach the code section.
8073 Supposing NDISASM has just finished generating a strange machine
8074 instruction from part of the data section, and its file position is
8075 now one byte \e{before} the beginning of the code section. It's
8076 entirely possible that another spurious instruction will get
8077 generated, starting with the final byte of the data section, and
8078 then the correct first instruction in the code section will not be
8079 seen because the starting point skipped over it. This isn't really
8082 To avoid this, you can specify a `\i\c{synchronisation}' point, or indeed
8083 as many synchronisation points as you like (although NDISASM can
8084 only handle 2147483647 sync points internally). The definition of a sync
8085 point is this: NDISASM guarantees to hit sync points exactly during
8086 disassembly. If it is thinking about generating an instruction which
8087 would cause it to jump over a sync point, it will discard that
8088 instruction and output a `\c{db}' instead. So it \e{will} start
8089 disassembly exactly from the sync point, and so you \e{will} see all
8090 the instructions in your code section.
8092 Sync points are specified using the \i\c{-s} option: they are measured
8093 in terms of the program origin, not the file position. So if you
8094 want to synchronize after 32 bytes of a \c{.COM} file, you would have to
8097 \c ndisasm -o100h -s120h file.com
8101 \c ndisasm -o100h -s20h file.com
8103 As stated above, you can specify multiple sync markers if you need
8104 to, just by repeating the \c{-s} option.
8107 \S{ndisisync} Mixed Code and Data: Automatic (Intelligent) Synchronisation
8110 Suppose you are disassembling the boot sector of a \c{DOS} floppy (maybe
8111 it has a virus, and you need to understand the virus so that you
8112 know what kinds of damage it might have done you). Typically, this
8113 will contain a \c{JMP} instruction, then some data, then the rest of the
8114 code. So there is a very good chance of NDISASM being \e{misaligned}
8115 when the data ends and the code begins. Hence a sync point is
8118 On the other hand, why should you have to specify the sync point
8119 manually? What you'd do in order to find where the sync point would
8120 be, surely, would be to read the \c{JMP} instruction, and then to use
8121 its target address as a sync point. So can NDISASM do that for you?
8123 The answer, of course, is yes: using either of the synonymous
8124 switches \i\c{-a} (for automatic sync) or \i\c{-i} (for intelligent
8125 sync) will enable \c{auto-sync} mode. Auto-sync mode automatically
8126 generates a sync point for any forward-referring PC-relative jump or
8127 call instruction that NDISASM encounters. (Since NDISASM is one-pass,
8128 if it encounters a PC-relative jump whose target has already been
8129 processed, there isn't much it can do about it...)
8131 Only PC-relative jumps are processed, since an absolute jump is
8132 either through a register (in which case NDISASM doesn't know what
8133 the register contains) or involves a segment address (in which case
8134 the target code isn't in the same segment that NDISASM is working
8135 in, and so the sync point can't be placed anywhere useful).
8137 For some kinds of file, this mechanism will automatically put sync
8138 points in all the right places, and save you from having to place
8139 any sync points manually. However, it should be stressed that
8140 auto-sync mode is \e{not} guaranteed to catch all the sync points, and
8141 you may still have to place some manually.
8143 Auto-sync mode doesn't prevent you from declaring manual sync
8144 points: it just adds automatically generated ones to the ones you
8145 provide. It's perfectly feasible to specify \c{-i} \e{and} some \c{-s}
8148 Another caveat with auto-sync mode is that if, by some unpleasant
8149 fluke, something in your data section should disassemble to a
8150 PC-relative call or jump instruction, NDISASM may obediently place a
8151 sync point in a totally random place, for example in the middle of
8152 one of the instructions in your code section. So you may end up with
8153 a wrong disassembly even if you use auto-sync. Again, there isn't
8154 much I can do about this. If you have problems, you'll have to use
8155 manual sync points, or use the \c{-k} option (documented below) to
8156 suppress disassembly of the data area.
8159 \S{ndisother} Other Options
8161 The \i\c{-e} option skips a header on the file, by ignoring the first N
8162 bytes. This means that the header is \e{not} counted towards the
8163 disassembly offset: if you give \c{-e10 -o10}, disassembly will start
8164 at byte 10 in the file, and this will be given offset 10, not 20.
8166 The \i\c{-k} option is provided with two comma-separated numeric
8167 arguments, the first of which is an assembly offset and the second
8168 is a number of bytes to skip. This \e{will} count the skipped bytes
8169 towards the assembly offset: its use is to suppress disassembly of a
8170 data section which wouldn't contain anything you wanted to see
8174 \H{ndisbugs} Bugs and Improvements
8176 There are no known bugs. However, any you find, with patches if
8177 possible, should be sent to
8178 \W{mailto:nasm-bugs@lists.sourceforge.net}\c{nasm-bugs@lists.sourceforge.net}, or to the
8180 \W{http://www.nasm.us/}\c{http://www.nasm.us/}
8181 and we'll try to fix them. Feel free to send contributions and
8182 new features as well.
8184 \A{inslist} \i{Instruction List}
8186 \H{inslistintro} Introduction
8188 The following sections show the instructions which NASM currently supports. For each
8189 instruction, there is a separate entry for each supported addressing mode. The third
8190 column shows the processor type in which the instruction was introduced and,
8191 when appropriate, one or more usage flags.
8195 \A{changelog} \i{NASM Version History}