trAvis - MANAGER

Edit File: nasmdoc.src

\# -------------------------------------------------------------------------- \# \# Copyright 1996-2017 The NASM Authors - All Rights Reserved \# See the file AUTHORS included with the NASM distribution for \# the specific copyright holders. \# \# Redistribution and use in source and binary forms, with or without \# modification, are permitted provided that the following \# conditions are met: \# \# * Redistributions of source code must retain the above copyright \# notice, this list of conditions and the following disclaimer. \# * Redistributions in binary form must reproduce the above \# copyright notice, this list of conditions and the following \# disclaimer in the documentation and/or other materials provided \# with the distribution. \# \# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND \# CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, \# INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF \# MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE \# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR \# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, \# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT \# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; \# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) \# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN \# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR \# OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, \# EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. \# \# -------------------------------------------------------------------------- \# \# Source code to NASM documentation \# \M{category}{Programming} \M{title}{NASM - The Netwide Assembler} \M{year}{1996-2017} \M{author}{The NASM Development Team} \M{copyright_tail}{-- All Rights Reserved} \M{license}{This document is redistributable under the license given in the file "LICENSE" distributed in the NASM archive.} \M{summary}{This file documents NASM, the Netwide Assembler: an assembler targetting the Intel x86 series of processors, with portable source.} \M{infoname}{NASM} \M{infofile}{nasm} \M{infotitle}{The Netwide Assembler for x86} \M{epslogo}{nasmlogo.eps} \M{logoyadj}{-72} \& version.src \IR{-D} \c{-D} option \IR{-E} \c{-E} option \IR{-F} \c{-F} option \IR{-I} \c{-I} option \IR{-M} \c{-M} option \IR{-MD} \c{-MD} option \IR{-MF} \c{-MF} option \IR{-MG} \c{-MG} option \IR{-MP} \c{-MP} option \IR{-MQ} \c{-MQ} option \IR{-MT} \c{-MT} option \IR{-O} \c{-O} option \IR{-P} \c{-P} option \IR{-U} \c{-U} option \IR{-X} \c{-X} option \IR{-a} \c{-a} option \IR{-d} \c{-d} option \IR{-e} \c{-e} option \IR{-f} \c{-f} option \IR{-g} \c{-g} option \IR{-i} \c{-i} option \IR{-l} \c{-l} option \IR{-o} \c{-o} option \IR{-p} \c{-p} option \IR{-s} \c{-s} option \IR{-u} \c{-u} option \IR{-v} \c{-v} option \IR{-W} \c{-W} option \IR{-Werror} \c{-Werror} option \IR{-Wno-error} \c{-Wno-error} option \IR{-w} \c{-w} option \IR{-y} \c{-y} option \IR{-Z} \c{-Z} option \IR{!=} \c{!=} operator \IR{$, here} \c{$}, Here token \IR{$, prefix} \c{$}, prefix \IR{$$} \c{$$} token \IR{%} \c{%} operator \IR{%%} \c{%%} operator \IR{%+1} \c{%+1} and \c{%-1} syntax \IA{%-1}{%+1} \IR{%0} \c{%0} parameter count \IR{&} \c{&} operator \IR{&&} \c{&&} operator \IR{*} \c{*} operator \IR{..@} \c{..@} symbol prefix \IR{/} \c{/} operator \IR{//} \c{//} operator \IR{<} \c{<} operator \IR{<<} \c{<<} operator \IR{<=} \c{<=} operator \IR{<>} \c{<>} operator \IR{=} \c{=} operator \IR{==} \c{==} operator \IR{>} \c{>} operator \IR{>=} \c{>=} operator \IR{>>} \c{>>} operator \IR{?} \c{?} MASM syntax \IR{^} \c{^} operator \IR{^^} \c{^^} operator \IR{|} \c{|} operator \IR{||} \c{||} operator \IR{~} \c{~} operator \IR{%$} \c{%$} and \c{%$$} prefixes \IA{%$$}{%$} \IR{+ opaddition} \c{+} operator, binary \IR{+ opunary} \c{+} operator, unary \IR{+ modifier} \c{+} modifier \IR{- opsubtraction} \c{-} operator, binary \IR{- opunary} \c{-} operator, unary \IR{! opunary} \c{!} operator, unary \IR{alignment, in bin sections} alignment, in \c{bin} sections \IR{alignment, in elf sections} alignment, in \c{elf} sections \IR{alignment, in win32 sections} alignment, in \c{win32} sections \IR{alignment, of elf common variables} alignment, of \c{elf} common variables \IR{alignment, in obj sections} alignment, in \c{obj} sections \IR{a.out, bsd version} \c{a.out}, BSD version \IR{a.out, linux version} \c{a.out}, Linux version \IR{autoconf} Autoconf \IR{bin} bin \IR{bitwise and} bitwise AND \IR{bitwise or} bitwise OR \IR{bitwise xor} bitwise XOR \IR{block ifs} block IFs \IR{borland pascal} Borland, Pascal \IR{borland's win32 compilers} Borland, Win32 compilers \IR{braces, after % sign} braces, after \c{%} sign \IR{bsd} BSD \IR{c calling convention} C calling convention \IR{c symbol names} C symbol names \IA{critical expressions}{critical expression} \IA{command line}{command-line} \IA{case sensitivity}{case sensitive} \IA{case-sensitive}{case sensitive} \IA{case-insensitive}{case sensitive} \IA{character constants}{character constant} \IR{codeview} CodeView debugging format \IR{common object file format} Common Object File Format \IR{common variables, alignment in elf} common variables, alignment in \c{elf} \IR{common, elf extensions to} \c{COMMON}, \c{elf} extensions to \IR{common, obj extensions to} \c{COMMON}, \c{obj} extensions to \IR{declaring structure} declaring structures \IR{default-wrt mechanism} default-\c{WRT} mechanism \IR{devpac} DevPac \IR{djgpp} DJGPP \IR{dll symbols, exporting} DLL symbols, exporting \IR{dll symbols, importing} DLL symbols, importing \IR{dos} DOS \IR{dos archive} DOS archive \IR{dos source archive} DOS source archive \IA{effective address}{effective addresses} \IA{effective-address}{effective addresses} \IR{elf} ELF \IR{elf, 16-bit code and} ELF, 16-bit code and \IR{elf shared libraries} ELF, shared libraries \IR{elf32} \c{elf32} \IR{elf64} \c{elf64} \IR{elfx32} \c{elfx32} \IR{executable and linkable format} Executable and Linkable Format \IR{extern, obj extensions to} \c{EXTERN}, \c{obj} extensions to \IR{extern, rdf extensions to} \c{EXTERN}, \c{rdf} extensions to \IR{floating-point, constants} floating-point, constants \IR{floating-point, packed bcd constants} floating-point, packed BCD constants \IR{freebsd} FreeBSD \IR{freelink} FreeLink \IR{functions, c calling convention} functions, C calling convention \IR{functions, pascal calling convention} functions, Pascal calling convention \IR{global, aoutb extensions to} \c{GLOBAL}, \c{aoutb} extensions to \IR{global, elf extensions to} \c{GLOBAL}, \c{elf} extensions to \IR{global, rdf extensions to} \c{GLOBAL}, \c{rdf} extensions to \IR{got} GOT \IR{got relocations} \c{GOT} relocations \IR{gotoff relocation} \c{GOTOFF} relocations \IR{gotpc relocation} \c{GOTPC} relocations \IR{intel number formats} Intel number formats \IR{linux, elf} Linux, ELF \IR{linux, a.out} Linux, \c{a.out} \IR{linux, as86} Linux, \c{as86} \IR{logical and} logical AND \IR{logical or} logical OR \IR{logical xor} logical XOR \IR{mach object file format} Mach, object file format \IA{mach-o}{macho} \IR{mach-o} Mach-O, object file format \IR{macho32} \c{macho32} \IR{macho64} \c{macho64} \IR{macos x} MacOS X \IR{masm} MASM \IA{memory reference}{memory references} \IR{minix} Minix \IA{misc directory}{misc subdirectory} \IR{misc subdirectory} \c{misc} subdirectory \IR{microsoft omf} Microsoft OMF \IR{mmx registers} MMX registers \IA{modr/m}{modr/m byte} \IR{modr/m byte} ModR/M byte \IR{ms-dos} MS-DOS \IR{ms-dos device drivers} MS-DOS device drivers \IR{multipush} \c{multipush} macro \IR{nan} NaN \IR{nasm version} NASM version \IR{netbsd} NetBSD \IR{nsis} NSIS \IR{nullsoft scriptable installer} Nullsoft Scriptable Installer \IR{omf} OMF \IR{openbsd} OpenBSD \IR{operating system} operating system \IR{os/2} OS/2 \IR{pascal calling convention}Pascal calling convention \IR{passes} passes, assembly \IR{perl} Perl \IR{pic} PIC \IR{pharlap} PharLap \IR{plt} PLT \IR{plt} \c{PLT} relocations \IA{pre-defining macros}{pre-define} \IA{preprocessor expressions}{preprocessor, expressions} \IA{preprocessor loops}{preprocessor, loops} \IA{preprocessor variables}{preprocessor, variables} \IA{rdoff subdirectory}{rdoff} \IR{rdoff} \c{rdoff} subdirectory \IR{relocatable dynamic object file format} Relocatable Dynamic Object File Format \IR{relocations, pic-specific} relocations, PIC-specific \IA{repeating}{repeating code} \IR{section alignment, in elf} section alignment, in \c{elf} \IR{section alignment, in bin} section alignment, in \c{bin} \IR{section alignment, in obj} section alignment, in \c{obj} \IR{section alignment, in win32} section alignment, in \c{win32} \IR{section, elf extensions to} \c{SECTION}, \c{elf} extensions to \IR{section, macho extensions to} \c{SECTION}, \c{macho} extensions to \IR{section, win32 extensions to} \c{SECTION}, \c{win32} extensions to \IR{segment alignment, in bin} segment alignment, in \c{bin} \IR{segment alignment, in obj} segment alignment, in \c{obj} \IR{segment, obj extensions to} \c{SEGMENT}, \c{elf} extensions to \IR{segment names, borland pascal} segment names, Borland Pascal \IR{shift command} \c{shift} command \IA{sib}{sib byte} \IR{sib byte} SIB byte \IR{align, smart} \c{ALIGN}, smart \IA{sectalign}{sectalign} \IR{solaris x86} Solaris x86 \IA{standard section names}{standardized section names} \IR{symbols, exporting from dlls} symbols, exporting from DLLs \IR{symbols, importing from dlls} symbols, importing from DLLs \IR{test subdirectory} \c{test} subdirectory \IR{tlink} \c{TLINK} \IR{underscore, in c symbols} underscore, in C symbols \IR{unicode} Unicode \IR{unix} Unix \IR{utf-8} UTF-8 \IR{utf-16} UTF-16 \IR{utf-32} UTF-32 \IA{sco unix}{unix, sco} \IR{unix, sco} Unix, SCO \IA{unix source archive}{unix, source archive} \IR{unix, source archive} Unix, source archive \IA{unix system v}{unix, system v} \IR{unix, system v} Unix, System V \IR{unixware} UnixWare \IR{val} VAL \IR{version number of nasm} version number of NASM \IR{visual c++} Visual C++ \IR{www page} WWW page \IR{win32} Win32 \IR{win32} Win64 \IR{windows} Windows \IR{windows 95} Windows 95 \IR{windows nt} Windows NT \# \IC{program entry point}{entry point, program} \# \IC{program entry point}{start point, program} \# \IC{MS-DOS device drivers}{device drivers, MS-DOS} \# \IC{16-bit mode, versus 32-bit mode}{32-bit mode, versus 16-bit mode} \# \IC{c symbol names}{symbol names, in C} \C{intro} Introduction \H{whatsnasm} What Is NASM? The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and \c{*BSD} \c{a.out}, \c{ELF}, \c{COFF}, \c{Mach-O}, 16-bit and 32-bit \c{OBJ} (OMF) format, \c{Win32} and \c{Win64}. It will also output plain binary files, Intel hex and Motorola S-Record formats. Its syntax is designed to be simple and easy to understand, similar to the syntax in the Intel Software Developer Manual with minimal complexity. It supports all currently known x86 architectural extensions, and has strong support for macros. NASM also comes with a set of utilities for handling the \c{RDOFF} custom object-file format. \S{legal} \i{License} Conditions Please see the file \c{LICENSE}, supplied as part of any NASM distribution archive, for the license conditions under which you may use NASM. NASM is now under the so-called 2-clause BSD license, also known as the simplified BSD license. Copyright 1996-2017 the NASM Authors - All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: \b Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. \b Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. \C{running} Running NASM \H{syntax} NASM \i{Command-Line} Syntax To assemble a file, you issue a command of the form \c nasm -f <format> <filename> [-o <output>] For example, \c nasm -f elf myfile.asm will assemble \c{myfile.asm} into an \c{ELF} object file \c{myfile.o}. And \c nasm -f bin myfile.asm -o myfile.com will assemble \c{myfile.asm} into a raw binary file \c{myfile.com}. To produce a listing file, with the hex codes output from NASM displayed on the left of the original sources, use the \c{-l} option to give a listing file name, for example: \c nasm -f coff myfile.asm -l myfile.lst To get further usage instructions from NASM, try typing \c nasm -h As \c{-hf}, this will also list the available output file formats, and what they are. If you use Linux but aren't sure whether your system is \c{a.out} or \c{ELF}, type \c file nasm (in the directory in which you put the NASM binary when you installed it). If it says something like \c nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1 then your system is \c{ELF}, and you should use the option \c{-f elf} when you want NASM to produce Linux object files. If it says \c nasm: Linux/i386 demand-paged executable (QMAGIC) or something similar, your system is \c{a.out}, and you should use \c{-f aout} instead (Linux \c{a.out} systems have long been obsolete, and are rare these days.) Like Unix compilers and assemblers, NASM is silent unless it goes wrong: you won't see any output at all, unless it gives error messages. \S{opt-o} The \i\c{-o} Option: Specifying the Output File Name NASM will normally choose the name of your output file for you; precisely how it does this is dependent on the object file format. For Microsoft object file formats (\c{obj}, \c{win32} and \c{win64}), it will remove the \c{.asm} \i{extension} (or whatever extension you like to use - NASM doesn't care) from your source file name and substitute \c{.obj}. For Unix object file formats (\c{aout}, \c{as86}, \c{coff}, \c{elf32}, \c{elf64}, \c{elfx32}, \c{ieee}, \c{macho32} and \c{macho64}) it will substitute \c{.o}. For \c{dbg}, \c{rdf}, \c{ith} and \c{srec}, it will use \c{.dbg}, \c{.rdf}, \c{.ith} and \c{.srec}, respectively, and for the \c{bin} format it will simply remove the extension, so that \c{myfile.asm} produces the output file \c{myfile}. If the output file already exists, NASM will overwrite it, unless it has the same name as the input file, in which case it will give a warning and use \i\c{nasm.out} as the output file name instead. For situations in which this behaviour is unacceptable, NASM provides the \c{-o} command-line option, which allows you to specify your desired output file name. You invoke \c{-o} by following it with the name you wish for the output file, either with or without an intervening space. For example: \c nasm -f bin program.asm -o program.com \c nasm -f bin driver.asm -odriver.sys Note that this is a small o, and is different from a capital O , which is used to specify the number of optimisation passes required. See \k{opt-O}. \S{opt-f} The \i\c{-f} Option: Specifying the \i{Output File Format} If you do not supply the \c{-f} option to NASM, it will choose an output file format for you itself. In the distribution versions of NASM, the default is always \i\c{bin}; if you've compiled your own copy of NASM, you can redefine \i\c{OF_DEFAULT} at compile time and choose what you want the default to be. Like \c{-o}, the intervening space between \c{-f} and the output file format is optional; so \c{-f elf} and \c{-felf} are both valid. A complete list of the available output file formats can be given by issuing the command \i\c{nasm -hf}. \S{opt-l} The \i\c{-l} Option: Generating a \i{Listing File} If you supply the \c{-l} option to NASM, followed (with the usual optional space) by a file name, NASM will generate a \i{source-listing file} for you, in which addresses and generated code are listed on the left, and the actual source code, with expansions of multi-line macros (except those which specifically request no expansion in source listings: see \k{nolist}) on the right. For example: \c nasm -f elf myfile.asm -l myfile.lst If a list file is selected, you may turn off listing for a section of your source with \c{[list -]}, and turn it back on with \c{[list +]}, (the default, obviously). There is no "user form" (without the brackets). This can be used to list only sections of interest, avoiding excessively long listings. \S{opt-M} The \i\c{-M} Option: Generate \i{Makefile Dependencies} This option can be used to generate makefile dependencies on stdout. This can be redirected to a file for further processing. For example: \c nasm -M myfile.asm > myfile.dep \S{opt-MG} The \i\c{-MG} Option: Generate \i{Makefile Dependencies} This option can be used to generate makefile dependencies on stdout. This differs from the \c{-M} option in that if a nonexisting file is encountered, it is assumed to be a generated file and is added to the dependency list without a prefix. \S{opt-MF} The \i\c\{-MF} Option: Set Makefile Dependency File This option can be used with the \c{-M} or \c{-MG} options to send the output to a file, rather than to stdout. For example: \c nasm -M -MF myfile.dep myfile.asm \S{opt-MD} The \i\c{-MD} Option: Assemble and Generate Dependencies The \c{-MD} option acts as the combination of the \c{-M} and \c{-MF} options (i.e. a filename has to be specified.) However, unlike the \c{-M} or \c{-MG} options, \c{-MD} does \e{not} inhibit the normal operation of the assembler. Use this to automatically generate updated dependencies with every assembly session. For example: \c nasm -f elf -o myfile.o -MD myfile.dep myfile.asm \S{opt-MT} The \i\c{-MT} Option: Dependency Target Name The \c{-MT} option can be used to override the default name of the dependency target. This is normally the same as the output filename, specified by the \c{-o} option. \S{opt-MQ} The \i\c{-MQ} Option: Dependency Target Name (Quoted) The \c{-MQ} option acts as the \c{-MT} option, except it tries to quote characters that have special meaning in Makefile syntax. This is not foolproof, as not all characters with special meaning are quotable in Make. The default output (if no \c{-MT} or \c{-MQ} option is specified) is automatically quoted. \S{opt-MP} The \i\c{-MP} Option: Emit phony targets When used with any of the dependency generation options, the \c{-MP} option causes NASM to emit a phony target without dependencies for each header file. This prevents Make from complaining if a header file has been removed. \S{opt-F} The \i\c{-F} Option: Selecting a \i{Debug Information Format} This option is used to select the format of the debug information emitted into the output file, to be used by a debugger (or \e{will} be). Prior to version 2.03.01, the use of this switch did \e{not} enable output of the selected debug info format. Use \c{-g}, see \k{opt-g}, to enable output. Versions 2.03.01 and later automatically enable \c{-g} if \c{-F} is specified. A complete list of the available debug file formats for an output format can be seen by issuing the command \c{nasm -f <format> -y}. Not all output formats currently support debugging output. See \k{opt-y}. This should not be confused with the \c{-f dbg} output format option which is not built into NASM by default. For information on how to enable it when building from the sources, see \k{dbgfmt}. \S{opt-g} The \i\c{-g} Option: Enabling \i{Debug Information}. This option can be used to generate debugging information in the specified format. See \k{opt-F}. Using \c{-g} without \c{-F} results in emitting debug info in the default format, if any, for the selected output format. If no debug information is currently implemented in the selected output format, \c{-g} is \e{silently ignored}. \S{opt-X} The \i\c{-X} Option: Selecting an \i{Error Reporting Format} This option can be used to select an error reporting format for any error messages that might be produced by NASM. Currently, two error reporting formats may be selected. They are the \c{-Xvc} option and the \c{-Xgnu} option. The GNU format is the default and looks like this: \c filename.asm:65: error: specific error message where \c{filename.asm} is the name of the source file in which the error was detected, \c{65} is the source file line number on which the error was detected, \c{error} is the severity of the error (this could be \c{warning}), and \c{specific error message} is a more detailed text message which should help pinpoint the exact problem. The other format, specified by \c{-Xvc} is the style used by Microsoft Visual C++ and some other programs. It looks like this: \c filename.asm(65) : error: specific error message where the only difference is that the line number is in parentheses instead of being delimited by colons. See also the \c{Visual C++} output format, \k{win32fmt}. \S{opt-Z} The \i\c{-Z} Option: Send Errors to a File Under \I{DOS}\c{MS-DOS} it can be difficult (though there are ways) to redirect the standard-error output of a program to a file. Since NASM usually produces its warning and \i{error messages} on \i\c{stderr}, this can make it hard to capture the errors if (for example) you want to load them into an editor. NASM therefore provides the \c{-Z} option, taking a filename argument which causes errors to be sent to the specified files rather than standard error. Therefore you can \I{redirecting errors}redirect the errors into a file by typing \c nasm -Z myfile.err -f obj myfile.asm In earlier versions of NASM, this option was called \c{-E}, but it was changed since \c{-E} is an option conventionally used for preprocessing only, with disastrous results. See \k{opt-E}. \S{opt-s} The \i\c{-s} Option: Send Errors to \i\c{stdout} The \c{-s} option redirects \i{error messages} to \c{stdout} rather than \c{stderr}, so it can be redirected under \I{DOS}\c{MS-DOS}. To assemble the file \c{myfile.asm} and pipe its output to the \c{more} program, you can type: \c nasm -s -f obj myfile.asm | more See also the \c{-Z} option, \k{opt-Z}. \S{opt-i} The \i\c{-i}\I\c{-I} Option: Include File Search Directories When NASM sees the \i\c{%include} or \i\c{%pathsearch} directive in a source file (see \k{include}, \k{pathsearch} or \k{incbin}), it will search for the given file not only in the current directory, but also in any directories specified on the command line by the use of the \c{-i} option. Therefore you can include files from a \i{macro library}, for example, by typing \c nasm -ic:\macrolib\ -f obj myfile.asm (As usual, a space between \c{-i} and the path name is allowed, and optional). NASM, in the interests of complete source-code portability, does not understand the file naming conventions of the OS it is running on; the string you provide as an argument to the \c{-i} option will be prepended exactly as written to the name of the include file. Therefore the trailing backslash in the above example is necessary. Under Unix, a trailing forward slash is similarly necessary. (You can use this to your advantage, if you're really \i{perverse}, by noting that the option \c{-ifoo} will cause \c{%include "bar.i"} to search for the file \c{foobar.i}...) If you want to define a \e{standard} \i{include search path}, similar to \c{/usr/include} on Unix systems, you should place one or more \c{-i} directives in the \c{NASMENV} environment variable (see \k{nasmenv}). For Makefile compatibility with many C compilers, this option can also be specified as \c{-I}. \S{opt-p} The \i\c{-p}\I\c{-P} Option: \I{pre-including files}Pre-Include a File \I\c{%include}NASM allows you to specify files to be \e{pre-included} into your source file, by the use of the \c{-p} option. So running \c nasm myfile.asm -p myinc.inc is equivalent to running \c{nasm myfile.asm} and placing the directive \c{%include "myinc.inc"} at the start of the file. For consistency with the \c{-I}, \c{-D} and \c{-U} options, this option can also be specified as \c{-P}. \S{opt-d} The \i\c{-d}\I\c{-D} Option: \I{pre-defining macros}Pre-Define a Macro \I\c{%define}Just as the \c{-p} option gives an alternative to placing \c{%include} directives at the start of a source file, the \c{-d} option gives an alternative to placing a \c{%define} directive. You could code \c nasm myfile.asm -dFOO=100 as an alternative to placing the directive \c %define FOO 100 at the start of the file. You can miss off the macro value, as well: the option \c{-dFOO} is equivalent to coding \c{%define FOO}. This form of the directive may be useful for selecting \i{assembly-time options} which are then tested using \c{%ifdef}, for example \c{-dDEBUG}. For Makefile compatibility with many C compilers, this option can also be specified as \c{-D}. \S{opt-u} The \i\c{-u}\I\c{-U} Option: \I{Undefining macros}Undefine a Macro \I\c{%undef}The \c{-u} option undefines a macro that would otherwise have been pre-defined, either automatically or by a \c{-p} or \c{-d} option specified earlier on the command lines. For example, the following command line: \c nasm myfile.asm -dFOO=100 -uFOO would result in \c{FOO} \e{not} being a predefined macro in the program. This is useful to override options specified at a different point in a Makefile. For Makefile compatibility with many C compilers, this option can also be specified as \c{-U}. \S{opt-E} The \i\c{-E}\I{-e} Option: Preprocess Only NASM allows the \i{preprocessor} to be run on its own, up to a point. Using the \c{-E} option (which requires no arguments) will cause NASM to preprocess its input file, expand all the macro references, remove all the comments and preprocessor directives, and print the resulting file on standard output (or save it to a file, if the \c{-o} option is also used). This option cannot be applied to programs which require the preprocessor to evaluate \I{preprocessor expressions}\i{expressions} which depend on the values of symbols: so code such as \c %assign tablesize ($-tablestart) will cause an error in \i{preprocess-only mode}. For compatiblity with older version of NASM, this option can also be written \c{-e}. \c{-E} in older versions of NASM was the equivalent of the current \c{-Z} option, \k{opt-Z}. \S{opt-a} The \i\c{-a} Option: Don't Preprocess At All If NASM is being used as the back end to a compiler, it might be desirable to \I{suppressing preprocessing}suppress preprocessing completely and assume the compiler has already done it, to save time and increase compilation speeds. The \c{-a} option, requiring no argument, instructs NASM to replace its powerful \i{preprocessor} with a \i{stub preprocessor} which does nothing. \S{opt-O} The \i\c{-O} Option: Specifying \i{Multipass Optimization} Using the \c{-O} option, you can tell NASM to carry out different levels of optimization. The syntax is: \b \c{-O0}: No optimization. All operands take their long forms, if a short form is not specified, except conditional jumps. This is intended to match NASM 0.98 behavior. \b \c{-O1}: Minimal optimization. As above, but immediate operands which will fit in a signed byte are optimized, unless the long form is specified. Conditional jumps default to the long form unless otherwise specified. \b \c{-Ox} (where \c{x} is the actual letter \c{x}): Multipass optimization. Minimize branch offsets and signed immediate bytes, overriding size specification unless the \c{strict} keyword has been used (see \k{strict}). For compatibility with earlier releases, the letter \c{x} may also be any number greater than one. This number has no effect on the actual number of passes. The \c{-Ox} mode is recommended for most uses, and is the default since NASM 2.09. Note that this is a capital \c{O}, and is different from a small \c{o}, which is used to specify the output file name. See \k{opt-o}. \S{opt-t} The \i\c{-t} Option: Enable TASM Compatibility Mode NASM includes a limited form of compatibility with Borland's \i\c{TASM}. When NASM's \c{-t} option is used, the following changes are made: \b local labels may be prefixed with \c{@@} instead of \c{.} \b size override is supported within brackets. In TASM compatible mode, a size override inside square brackets changes the size of the operand, and not the address type of the operand as it does in NASM syntax. E.g. \c{mov eax,[DWORD val]} is valid syntax in TASM compatibility mode. Note that you lose the ability to override the default address type for the instruction. \b unprefixed forms of some directives supported (\c{arg}, \c{elif}, \c{else}, \c{endif}, \c{if}, \c{ifdef}, \c{ifdifi}, \c{ifndef}, \c{include}, \c{local}) \S{opt-w} The \i\c{-w} and \i\c{-W} Options: Enable or Disable Assembly \i{Warnings} NASM can observe many conditions during the course of assembly which are worth mentioning to the user, but not a sufficiently severe error to justify NASM refusing to generate an output file. These conditions are reported like errors, but come up with the word `warning' before the message. Warnings do not prevent NASM from generating an output file and returning a success status to the operating system. Some conditions are even less severe than that: they are only sometimes worth mentioning to the user. Therefore NASM supports the \c{-w} command-line option, which enables or disables certain classes of assembly warning. Such warning classes are described by a name, for example \c{orphan-labels}; you can enable warnings of this class by the command-line option \c{-w+orphan-labels} and disable it by \c{-w-orphan-labels}. The current \i{warning classes} are: \b \i\c{other} specifies any warning not otherwise specified in any class. Enabled by default. \b \i\c{macro-params} covers warnings about \i{multi-line macros} being invoked with the wrong number of parameters. Enabled by default; see \k{mlmacover} for an example of why you might want to disable it. \b \i\c{macro-selfref} warns if a macro references itself. Disabled by default. \b \i\c{macro-defaults} warns when a macro has more default parameters than optional parameters. Enabled by default; see \k{mlmacdef} for why you might want to disable it. \b \i\c{orphan-labels} covers warnings about source lines which contain no instruction but define a label without a trailing colon. NASM warns about this somewhat obscure condition by default; see \k{syntax} for more information. \b \i\c{number-overflow} covers warnings about numeric constants which don't fit in 64 bits. Enabled by default. \b \i\c{gnu-elf-extensions} warns if 8-bit or 16-bit relocations are used in \c{-f elf} format. The GNU extensions allow this. Disabled by default. \b \i\c{float-overflow} warns about floating point overflow. Enabled by default. \b \i\c{float-denorm} warns about floating point denormals. Disabled by default. \b \i\c{float-underflow} warns about floating point underflow. Disabled by default. \b \i\c{float-toolong} warns about too many digits in floating-point numbers. Enabled by default. \b \i\c{user} controls \c{%warning} directives (see \k{pperror}). Enabled by default. \b \i\c{lock} warns about \c{LOCK} prefixes on unlockable instructions. Enabled by default. \b \i\c{hle} warns about invalid use of the HLE \c{XACQUIRE} or \c{XRELEASE} prefixes. Enabled by default. \b \i\c{bnd} warns about ineffective use of the \c{BND} prefix when a relaxed form of jmp instruction becomes jmp short form. Enabled by default. \b \i\c{zext-reloc} warns that a relocation has been zero-extended due to limitations in the output format. Enabled by default. \b \i\c\{ptr} warns about keywords used in other assemblers that might indicate a mistake in the source code. Currently only the MASM \c{PTR} keyword is recognized. Enabled by default. \b \i\c{bad-pragma} warns about a malformed or otherwise unparsable \c{%pragma} directive. Disabled by default. \b \i\c{unknown-pragma} warns about an unknown \c{%pragma} directive. This is not yet implemented. Disabled by default. \b \i\c{not-my-pragma} warns about a \c{%pragma} directive which is not applicable to this particular assembly session. This is not yet implemented. Disabled by default. \b \i\c{unknown-warning} warns about a \c{-w} or \c{-W} option or a \c{[WARNING]} directive that contains an unknown warning name or is otherwise not possible to process. Disabled by default. \b \i\c{all} is an alias for \e{all} suppressible warning classes. Thus, \c{-w+all} enables all available warnings, and \c{-w-all} disables warnings entirely (since NASM 2.13). Since version 2.00, NASM has also supported the \c{gcc}-like syntax \c{-Wwarning-class} and \c{-Wno-warning-class} instead of \c{-w+warning-class} and \c{-w-warning-class}, respectively; both syntaxes work identically. The option \c{-w+error} or \i\c{-Werror} can be used to treat warnings as errors. This can be controlled on a per warning class basis (\c{-w+error=}\e{warning-class} or \c{-Werror=}\e{warning-class}); if no \e{warning-class} is specified NASM treats it as \c{-w+error=all}; the same applies to \c{-w-error} or \i\c{-Wno-error}, of course. In addition, you can control warnings in the source code itself, using the \i\c{[WARNING]} directive. See \k{asmdir-warning}. \S{opt-v} The \i\c{-v} Option: Display \i{Version} Info Typing \c{NASM -v} will display the version of NASM which you are using, and the date on which it was compiled. You will need the version number if you report a bug. For command-line compatibility with Yasm, the form \i\c{--v} is also accepted for this option starting in NASM version 2.11.05. \S{opt-y} The \i\c{-y} Option: Display Available Debug Info Formats Typing \c{nasm -f <option> -y} will display a list of the available debug info formats for the given output format. The default format is indicated by an asterisk. For example: \c nasm -f elf -y \c valid debug formats for 'elf32' output format are \c ('*' denotes default): \c * stabs ELF32 (i386) stabs debug format for Linux \c dwarf elf32 (i386) dwarf debug format for Linux \S{opt-pfix} The \i\c{--prefix} and \i\c{--postfix} Options. The \c{--prefix} and \c{--postfix} options prepend or append (respectively) the given argument to all \c{global} or \c{extern} variables. E.g. \c{--prefix _} will prepend the underscore to all global and external variables, as C requires it in some, but not all, system calling conventions. \S{nasmenv} The \i\c{NASMENV} \i{Environment} Variable If you define an environment variable called \c{NASMENV}, the program will interpret it as a list of extra command-line options, which are processed before the real command line. You can use this to define standard search directories for include files, by putting \c{-i} options in the \c{NASMENV} variable. The value of the variable is split up at white space, so that the value \c{-s -ic:\\nasmlib\\} will be treated as two separate options. However, that means that the value \c{-dNAME="my name"} won't do what you might want, because it will be split at the space and the NASM command-line processing will get confused by the two nonsensical words \c{-dNAME="my} and \c{name"}. To get round this, NASM provides a feature whereby, if you begin the \c{NASMENV} environment variable with some character that isn't a minus sign, then NASM will treat this character as the \i{separator character} for options. So setting the \c{NASMENV} variable to the value \c{!-s!-ic:\\nasmlib\\} is equivalent to setting it to \c{-s -ic:\\nasmlib\\}, but \c{!-dNAME="my name"} will work. This environment variable was previously called \c{NASM}. This was changed with version 0.98.31. \H{qstart} \i{Quick Start} for \i{MASM} Users If you're used to writing programs with MASM, or with \i{TASM} in MASM-compatible (non-Ideal) mode, or with \i\c{a86}, this section attempts to outline the major differences between MASM's syntax and NASM's. If you're not already used to MASM, it's probably worth skipping this section. \S{qscs} NASM Is \I{case sensitivity}Case-Sensitive One simple difference is that NASM is case-sensitive. It makes a difference whether you call your label \c{foo}, \c{Foo} or \c{FOO}. If you're assembling to \c{DOS} or \c{OS/2} \c{.OBJ} files, you can invoke the \i\c{UPPERCASE} directive (documented in \k{objfmt}) to ensure that all symbols exported to other code modules are forced to be upper case; but even then, \e{within} a single module, NASM will distinguish between labels differing only in case. \S{qsbrackets} NASM Requires \i{Square Brackets} For \i{Memory References} NASM was designed with simplicity of syntax in mind. One of the \i{design goals} of NASM is that it should be possible, as far as is practical, for the user to look at a single line of NASM code and tell what opcode is generated by it. You can't do this in MASM: if you declare, for example, \c foo equ 1 \c bar dw 2 then the two lines of code \c mov ax,foo \c mov ax,bar generate completely different opcodes, despite having identical-looking syntaxes. NASM avoids this undesirable situation by having a much simpler syntax for memory references. The rule is simply that any access to the \e{contents} of a memory location requires square brackets around the address, and any access to the \e{address} of a variable doesn't. So an instruction of the form \c{mov ax,foo} will \e{always} refer to a compile-time constant, whether it's an \c{EQU} or the address of a variable; and to access the \e{contents} of the variable \c{bar}, you must code \c{mov ax,[bar]}. This also means that NASM has no need for MASM's \i\c{OFFSET} keyword, since the MASM code \c{mov ax,offset bar} means exactly the same thing as NASM's \c{mov ax,bar}. If you're trying to get large amounts of MASM code to assemble sensibly under NASM, you can always code \c{%idefine offset} to make the preprocessor treat the \c{OFFSET} keyword as a no-op. This issue is even more confusing in \i\c{a86}, where declaring a label with a trailing colon defines it to be a `label' as opposed to a `variable' and causes \c{a86} to adopt NASM-style semantics; so in \c{a86}, \c{mov ax,var} has different behaviour depending on whether \c{var} was declared as \c{var: dw 0} (a label) or \c{var dw 0} (a word-size variable). NASM is very simple by comparison: \e{everything} is a label. NASM, in the interests of simplicity, also does not support the \i{hybrid syntaxes} supported by MASM and its clones, such as \c{mov ax,table[bx]}, where a memory reference is denoted by one portion outside square brackets and another portion inside. The correct syntax for the above is \c{mov ax,[table+bx]}. Likewise, \c{mov ax,es:[di]} is wrong and \c{mov ax,[es:di]} is right. \S{qstypes} NASM Doesn't Store \i{Variable Types} NASM, by design, chooses not to remember the types of variables you declare. Whereas MASM will remember, on seeing \c{var dw 0}, that you declared \c{var} as a word-size variable, and will then be able to fill in the \i{ambiguity} in the size of the instruction \c{mov var,2}, NASM will deliberately remember nothing about the symbol \c{var} except where it begins, and so you must explicitly code \c{mov word [var],2}. For this reason, NASM doesn't support the \c{LODS}, \c{MOVS}, \c{STOS}, \c{SCAS}, \c{CMPS}, \c{INS}, or \c{OUTS} instructions, but only supports the forms such as \c{LODSB}, \c{MOVSW}, and \c{SCASD}, which explicitly specify the size of the components of the strings being manipulated. \S{qsassume} NASM Doesn't \i\c{ASSUME} As part of NASM's drive for simplicity, it also does not support the \c{ASSUME} directive. NASM will not keep track of what values you choose to put in your segment registers, and will never \e{automatically} generate a \i{segment override} prefix. \S{qsmodel} NASM Doesn't Support \i{Memory Models} NASM also does not have any directives to support different 16-bit memory models. The programmer has to keep track of which functions are supposed to be called with a \i{far call} and which with a \i{near call}, and is responsible for putting the correct form of \c{RET} instruction (\c{RETN} or \c{RETF}; NASM accepts \c{RET} itself as an alternate form for \c{RETN}); in addition, the programmer is responsible for coding CALL FAR instructions where necessary when calling \e{external} functions, and must also keep track of which external variable definitions are far and which are near. \S{qsfpu} \i{Floating-Point} Differences NASM uses different names to refer to floating-point registers from MASM: where MASM would call them \c{ST(0)}, \c{ST(1)} and so on, and \i\c{a86} would call them simply \c{0}, \c{1} and so on, NASM chooses to call them \c{st0}, \c{st1} etc. As of version 0.96, NASM now treats the instructions with \i{`nowait'} forms in the same way as MASM-compatible assemblers. The idiosyncratic treatment employed by 0.95 and earlier was based on a misunderstanding by the authors. \S{qsother} Other Differences For historical reasons, NASM uses the keyword \i\c{TWORD} where MASM and compatible assemblers use \i\c{TBYTE}. NASM does not declare \i{uninitialized storage} in the same way as MASM: where a MASM programmer might use \c{stack db 64 dup (?)}, NASM requires \c{stack resb 64}, intended to be read as `reserve 64 bytes'. For a limited amount of compatibility, since NASM treats \c{?} as a valid character in symbol names, you can code \c{? equ 0} and then writing \c{dw ?} will at least do something vaguely useful. \I\c{RESB}\i\c{DUP} is still not a supported syntax, however. In addition to all of this, macros and directives work completely differently to MASM. See \k{preproc} and \k{directive} for further details. \C{lang} The NASM Language \H{syntax} Layout of a NASM Source Line Like most assemblers, each NASM source line contains (unless it is a macro, a preprocessor directive or an assembler directive: see \k{preproc} and \k{directive}) some combination of the four fields \c label: instruction operands ; comment As usual, most of these fields are optional; the presence or absence of any combination of a label, an instruction and a comment is allowed. Of course, the operand field is either required or forbidden by the presence and nature of the instruction field. NASM uses backslash (\\) as the line continuation character; if a line ends with backslash, the next line is considered to be a part of the backslash-ended line. NASM places no restrictions on white space within a line: labels may have white space before them, or instructions may have no space before them, or anything. The \i{colon} after a label is also optional. (Note that this means that if you intend to code \c{lodsb} alone on a line, and type \c{lodab} by accident, then that's still a valid source line which does nothing but define a label. Running NASM with the command-line option \I{orphan-labels}\c{-w+orphan-labels} will cause it to warn you if you define a label alone on a line without a \i{trailing colon}.) \i{Valid characters} in labels are letters, numbers, \c{_}, \c{$}, \c{#}, \c{@}, \c{~}, \c{.}, and \c{?}. The only characters which may be used as the \e{first} character of an identifier are letters, \c{.} (with special meaning: see \k{locallab}), \c{_} and \c{?}. An identifier may also be prefixed with a \I{$, prefix}\c{$} to indicate that it is intended to be read as an identifier and not a reserved word; thus, if some other module you are linking with defines a symbol called \c{eax}, you can refer to \c{$eax} in NASM code to distinguish the symbol from the register. Maximum length of an identifier is 4095 characters. The instruction field may contain any machine instruction: Pentium and P6 instructions, FPU instructions, MMX instructions and even undocumented instructions are all supported. The instruction may be prefixed by \c{LOCK}, \c{REP}, \c{REPE}/\c{REPZ}, \c{REPNE}/\c{REPNZ}, \c{XACQUIRE}/\c{XRELEASE} or \c{BND}/\c{NOBND}, in the usual way. Explicit \I{address-size prefixes}address-size and \i{operand-size prefixes} \i\c{A16}, \i\c{A32}, \i\c{A64}, \i\c{O16} and \i\c{O32}, \i\c{O64} are provided - one example of their use is given in \k{mixsize}. You can also use the name of a \I{segment override}segment register as an instruction prefix: coding \c{es mov [bx],ax} is equivalent to coding \c{mov [es:bx],ax}. We recommend the latter syntax, since it is consistent with other syntactic features of the language, but for instructions such as \c{LODSB}, which has no operands and yet can require a segment override, there is no clean syntactic way to proceed apart from \c{es lodsb}. An instruction is not required to use a prefix: prefixes such as \c{CS}, \c{A32}, \c{LOCK} or \c{REPE} can appear on a line by themselves, and NASM will just generate the prefix bytes. In addition to actual machine instructions, NASM also supports a number of pseudo-instructions, described in \k{pseudop}. Instruction \i{operands} may take a number of forms: they can be registers, described simply by the register name (e.g. \c{ax}, \c{bp}, \c{ebx}, \c{cr0}: NASM does not use the \c{gas}-style syntax in which register names must be prefixed by a \c{%} sign), or they can be \i{effective addresses} (see \k{effaddr}), constants (\k{const}) or expressions (\k{expr}). For x87 \i{floating-point} instructions, NASM accepts a wide range of syntaxes: you can use two-operand forms like MASM supports, or you can use NASM's native single-operand forms in most cases. \# Details of \# all forms of each supported instruction are given in \# \k{iref}. For example, you can code: \c fadd st1 ; this sets st0 := st0 + st1 \c fadd st0,st1 ; so does this \c \c fadd st1,st0 ; this sets st1 := st1 + st0 \c fadd to st1 ; so does this Almost any x87 floating-point instruction that references memory must use one of the prefixes \i\c{DWORD}, \i\c{QWORD} or \i\c{TWORD} to indicate what size of \i{memory operand} it refers to. \H{pseudop} \i{Pseudo-Instructions} Pseudo-instructions are things which, though not real x86 machine instructions, are used in the instruction field anyway because that's the most convenient place to put them. The current pseudo-instructions are \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, \i\c{DO}, \i\c{DY} and \i\c\{DZ}; their \i{uninitialized} counterparts \i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST}, \i\c{RESO}, \i\c{RESY} and \i\c\{RESZ}; the \i\c{INCBIN} command, the \i\c{EQU} command, and the \i\c{TIMES} prefix. \S{db} \c{DB} and Friends: Declaring Initialized Data \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, \i\c{DO}, \i\c{DY} and \i\c{DZ} are used, much as in MASM, to declare initialized data in the output file. They can be invoked in a wide range of ways: \I{floating-point}\I{character constant}\I{string constant} \c db 0x55 ; just the byte 0x55 \c db 0x55,0x56,0x57 ; three bytes in succession \c db 'a',0x55 ; character constants are OK \c db 'hello',13,10,'$' ; so are string constants \c dw 0x1234 ; 0x34 0x12 \c dw 'a' ; 0x61 0x00 (it's just a number) \c dw 'ab' ; 0x61 0x62 (character constant) \c dw 'abc' ; 0x61 0x62 0x63 0x00 (string) \c dd 0x12345678 ; 0x78 0x56 0x34 0x12 \c dd 1.234567e20 ; floating-point constant \c dq 0x123456789abcdef0 ; eight byte constant \c dq 1.234567e20 ; double-precision float \c dt 1.234567e20 ; extended-precision float \c{DT}, \c{DO}, \c{DY} and \c{DZ} do not accept \i{numeric constants} as operands. \S{resb} \c{RESB} and Friends: Declaring \i{Uninitialized} Data \i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST}, \i\c{RESO}, \i\c{RESY} and \i\c\{RESZ} are designed to be used in the BSS section of a module: they declare \e{uninitialized} storage space. Each takes a single operand, which is the number of bytes, words, doublewords or whatever to reserve. As stated in \k{qsother}, NASM does not support the MASM/TASM syntax of reserving uninitialized space by writing \I\c{?}\c{DW ?} or similar things: this is what it does instead. The operand to a \c{RESB}-type pseudo-instruction is a \i\e{critical expression}: see \k{crit}. For example: \c buffer: resb 64 ; reserve 64 bytes \c wordvar: resw 1 ; reserve a word \c realarray resq 10 ; array of ten reals \c ymmval: resy 1 ; one YMM register \c zmmvals: resz 32 ; 32 ZMM registers \S{incbin} \i\c{INCBIN}: Including External \i{Binary Files} \c{INCBIN} is borrowed from the old Amiga assembler \i{DevPac}: it includes a binary file verbatim into the output file. This can be handy for (for example) including \i{graphics} and \i{sound} data directly into a game executable file. It can be called in one of these three ways: \c incbin "file.dat" ; include the whole file \c incbin "file.dat",1024 ; skip the first 1024 bytes \c incbin "file.dat",1024,512 ; skip the first 1024, and \c ; actually include at most 512 \c{INCBIN} is both a directive and a standard macro; the standard macro version searches for the file in the include file search path and adds the file to the dependency lists. This macro can be overridden if desired. \S{equ} \i\c{EQU}: Defining Constants \c{EQU} defines a symbol to a given constant value: when \c{EQU} is used, the source line must contain a label. The action of \c{EQU} is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example, \c message db 'hello, world' \c msglen equ $-message defines \c{msglen} to be the constant 12. \c{msglen} may not then be redefined later. This is not a \i{preprocessor} definition either: the value of \c{msglen} is evaluated \e{once}, using the value of \c{$} (see \k{expr} for an explanation of \c{$}) at the point of definition, rather than being evaluated wherever it is referenced and using the value of \c{$} at the point of reference. \S{times} \i\c{TIMES}: \i{Repeating} Instructions or Data The \c{TIMES} prefix causes the instruction to be assembled multiple times. This is partly present as NASM's equivalent of the \i\c{DUP} syntax supported by \i{MASM}-compatible assemblers, in that you can code \c zerobuf: times 64 db 0 or similar things; but \c{TIMES} is more versatile than that. The argument to \c{TIMES} is not just a numeric constant, but a numeric \e{expression}, so you can do things like \c buffer: db 'hello, world' \c times 64-$+buffer db ' ' which will store exactly enough spaces to make the total length of \c{buffer} up to 64. Finally, \c{TIMES} can be applied to ordinary instructions, so you can code trivial \i{unrolled loops} in it: \c times 100 movsb Note that there is no effective difference between \c{times 100 resb 1} and \c{resb 100}, except that the latter will be assembled about 100 times faster due to the internal structure of the assembler. The operand to \c{TIMES} is a critical expression (\k{crit}). Note also that \c{TIMES} can't be applied to \i{macros}: the reason for this is that \c{TIMES} is processed after the macro phase, which allows the argument to \c{TIMES} to contain expressions such as \c{64-$+buffer} as above. To repeat more than one line of code, or a complex macro, use the preprocessor \i\c{%rep} directive. \H{effaddr} Effective Addresses An \i{effective address} is any operand to an instruction which \I{memory reference}references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in \i{square brackets}. For example: \c wordvar dw 123 \c mov ax,[wordvar] \c mov ax,[wordvar+1] \c mov ax,[es:wordvar+bx] Anything not conforming to this simple system is not a valid memory reference in NASM, for example \c{es:wordvar[bx]}. More complicated effective addresses, such as those involving more than one register, work in exactly the same way: \c mov eax,[ebx*2+ecx+offset] \c mov ax,[bp+di+8] NASM is capable of doing \i{algebra} on these effective addresses, so that things which don't necessarily \e{look} legal are perfectly all right: \c mov eax,[ebx*5] ; assembles as [ebx*4+ebx] \c mov eax,[label1*2-label2] ; ie [label1+(label1-label2)] Some forms of effective address have more than one assembled form; in most such cases NASM will generate the smallest form it can. For example, there are distinct assembled forms for the 32-bit effective addresses \c{[eax*2+0]} and \c{[eax+eax]}, and NASM will generally generate the latter on the grounds that the former requires four bytes to store a zero offset. NASM has a hinting mechanism which will cause \c{[eax+ebx]} and \c{[ebx+eax]} to generate different opcodes; this is occasionally useful because \c{[esi+ebp]} and \c{[ebp+esi]} have different default segment registers. However, you can force NASM to generate an effective address in a particular form by the use of the keywords \c{BYTE}, \c{WORD}, \c{DWORD} and \c{NOSPLIT}. If you need \c{[eax+3]} to be assembled using a double-word offset field instead of the one byte NASM will normally generate, you can code \c{[dword eax+3]}. Similarly, you can force NASM to use a byte offset for a small value which it hasn't seen on the first pass (see \k{crit} for an example of such a code fragment) by using \c{[byte eax+offset]}. As special cases, \c{[byte eax]} will code \c{[eax+0]} with a byte offset of zero, and \c{[dword eax]} will code it with a double-word offset of zero. The normal form, \c{[eax]}, will be coded with no offset field. The form described in the previous paragraph is also useful if you are trying to access data in a 32-bit segment from within 16 bit code. For more information on this see the section on mixed-size addressing (\k{mixaddr}). In particular, if you need to access data with a known offset that is larger than will fit in a 16-bit value, if you don't specify that it is a dword offset, nasm will cause the high word of the offset to be lost. Similarly, NASM will split \c{[eax*2]} into \c{[eax+eax]} because that allows the offset field to be absent and space to be saved; in fact, it will also split \c{[eax*2+offset]} into \c{[eax+eax+offset]}. You can combat this behaviour by the use of the \c{NOSPLIT} keyword: \c{[nosplit eax*2]} will force \c{[eax*2+0]} to be generated literally. \c{[nosplit eax*1]} also has the same effect. In another way, a split EA form \c{[0, eax*2]} can be used, too. However, \c{NOSPLIT} in \c{[nosplit eax+eax]} will be ignored because user's intention here is considered as \c{[eax+eax]}. In 64-bit mode, NASM will by default generate absolute addresses. The \i\c{REL} keyword makes it produce \c{RIP}-relative addresses. Since this is frequently the normally desired behaviour, see the \c{DEFAULT} directive (\k{default}). The keyword \i\c{ABS} overrides \i\c{REL}. A new form of split effective addres syntax is also supported. This is mainly intended for mib operands as used by MPX instructions, but can be used for any memory reference. The basic concept of this form is splitting base and index. \c mov eax,[ebx+8,ecx*4] ; ebx=base, ecx=index, 4=scale, 8=disp For mib operands, there are several ways of writing effective address depending on the tools. NASM supports all currently possible ways of mib syntax: \c ; bndstx \c ; next 5 lines are parsed same \c ; base=rax, index=rbx, scale=1, displacement=3 \c bndstx [rax+0x3,rbx], bnd0 ; NASM - split EA \c bndstx [rbx*1+rax+0x3], bnd0 ; GAS - '*1' indecates an index reg \c bndstx [rax+rbx+3], bnd0 ; GAS - without hints \c bndstx [rax+0x3], bnd0, rbx ; ICC-1 \c bndstx [rax+0x3], rbx, bnd0 ; ICC-2 When broadcasting decorator is used, the opsize keyword should match the size of each element. \c VDIVPS zmm4, zmm5, dword [rbx]{1to16} ; single-precision float \c VDIVPS zmm4, zmm5, zword [rbx] ; packed 512 bit memory \H{const} \i{Constants} NASM understands four different types of constant: numeric, character, string and floating-point. \S{numconst} \i{Numeric Constants} A numeric constant is simply a number. NASM allows you to specify numbers in a variety of number bases, in a variety of ways: you can suffix \c{H} or \c{X}, \c{D} or \c{T}, \c{Q} or \c{O}, and \c{B} or \c{Y} for \i{hexadecimal}, \i{decimal}, \i{octal} and \i{binary} respectively, or you can prefix \c{0x}, for hexadecimal in the style of C, or you can prefix \c{$} for hexadecimal in the style of Borland Pascal or Motorola Assemblers. Note, though, that the \I{$, prefix}\c{$} prefix does double duty as a prefix on identifiers (see \k{syntax}), so a hex number prefixed with a \c{$} sign must have a digit after the \c{$} rather than a letter. In addition, current versions of NASM accept the prefix \c{0h} for hexadecimal, \c{0d} or \c{0t} for decimal, \c{0o} or \c{0q} for octal, and \c{0b} or \c{0y} for binary. Please note that unlike C, a \c{0} prefix by itself does \e{not} imply an octal constant! Numeric constants can have underscores (\c{_}) interspersed to break up long strings. Some examples (all producing exactly the same code): \c mov ax,200 ; decimal \c mov ax,0200 ; still decimal \c mov ax,0200d ; explicitly decimal \c mov ax,0d200 ; also decimal \c mov ax,0c8h ; hex \c mov ax,$0c8 ; hex again: the 0 is required \c mov ax,0xc8 ; hex yet again \c mov ax,0hc8 ; still hex \c mov ax,310q ; octal \c mov ax,310o ; octal again \c mov ax,0o310 ; octal yet again \c mov ax,0q310 ; octal yet again \c mov ax,11001000b ; binary \c mov ax,1100_1000b ; same binary constant \c mov ax,1100_1000y ; same binary constant once more \c mov ax,0b1100_1000 ; same binary constant yet again \c mov ax,0y1100_1000 ; same binary constant yet again \S{strings} \I{Strings}\i{Character Strings} A character string consists of up to eight characters enclosed in either single quotes (\c{'...'}), double quotes (\c{"..."}) or backquotes (\c{`...`}). Single or double quotes are equivalent to NASM (except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa); the contents of those are represented verbatim. Strings enclosed in backquotes support C-style \c{\\}-escapes for special characters. The following \i{escape sequences} are recognized by backquoted strings: \c \' single quote (') \c \" double quote (") \c \` backquote (`) \c \\\ backslash (\) \c \? question mark (?) \c \a BEL (ASCII 7) \c \b BS (ASCII 8) \c \t TAB (ASCII 9) \c \n LF (ASCII 10) \c \v VT (ASCII 11) \c \f FF (ASCII 12) \c \r CR (ASCII 13) \c \e ESC (ASCII 27) \c \377 Up to 3 octal digits - literal byte \c \xFF Up to 2 hexadecimal digits - literal byte \c \u1234 4 hexadecimal digits - Unicode character \c \U12345678 8 hexadecimal digits - Unicode character All other escape sequences are reserved. Note that \c{\\0}, meaning a \c{NUL} character (ASCII 0), is a special case of the octal escape sequence. \i{Unicode} characters specified with \c{\\u} or \c{\\U} are converted to \i{UTF-8}. For example, the following lines are all equivalent: \c db `\u263a` ; UTF-8 smiley face \c db `\xe2\x98\xba` ; UTF-8 smiley face \c db 0E2h, 098h, 0BAh ; UTF-8 smiley face \S{chrconst} \i{Character Constants} A character constant consists of a string up to eight bytes long, used in an expression context. It is treated as if it was an integer. A character constant with more than one byte will be arranged with \i{little-endian} order in mind: if you code \c mov eax,'abcd' then the constant generated is not \c{0x61626364}, but \c{0x64636261}, so that if you were then to store the value into memory, it would read \c{abcd} rather than \c{dcba}. This is also the sense of character constants understood by the Pentium's \i\c{CPUID} instruction. \S{strconst} \i{String Constants} String constants are character strings used in the context of some pseudo-instructions, namely the \I\c{DW}\I\c{DD}\I\c{DQ}\I\c{DT}\I\c{DO}\I\c{DY}\i\c{DB} family and \i\c{INCBIN} (where it represents a filename.) They are also used in certain preprocessor directives. A string constant looks like a character constant, only longer. It is treated as a concatenation of maximum-size character constants for the conditions. So the following are equivalent: \c db 'hello' ; string constant \c db 'h','e','l','l','o' ; equivalent character constants And the following are also equivalent: \c dd 'ninechars' ; doubleword string constant \c dd 'nine','char','s' ; becomes three doublewords \c db 'ninechars',0,0,0 ; and really looks like this Note that when used in a string-supporting context, quoted strings are treated as a string constants even if they are short enough to be a character constant, because otherwise \c{db 'ab'} would have the same effect as \c{db 'a'}, which would be silly. Similarly, three-character or four-character constants are treated as strings when they are operands to \c{DW}, and so forth. \S{unicode} \I{UTF-16}\I{UTF-32}\i{Unicode} Strings The special operators \i\c{__utf16__}, \i\c{__utf16le__}, \i\c{__utf16be__}, \i\c{__utf32__}, \i\c{__utf32le__} and \i\c{__utf32be__} allows definition of Unicode strings. They take a string in UTF-8 format and converts it to UTF-16 or UTF-32, respectively. Unless the \c{be} forms are specified, the output is littleendian. For example: \c %define u(x) __utf16__(x) \c %define w(x) __utf32__(x) \c \c dw u('C:\WINDOWS'), 0 ; Pathname in UTF-16 \c dd w(`A + B = \u206a`), 0 ; String in UTF-32 The UTF operators can be applied either to strings passed to the \c{DB} family instructions, or to character constants in an expression context. \S{fltconst} \I{floating-point, constants}Floating-Point Constants \i{Floating-point} constants are acceptable only as arguments to \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT}, and \i\c{DO}, or as arguments to the special operators \i\c{__float8__}, \i\c{__float16__}, \i\c{__float32__}, \i\c{__float64__}, \i\c{__float80m__}, \i\c{__float80e__}, \i\c{__float128l__}, and \i\c{__float128h__}. Floating-point constants are expressed in the traditional form: digits, then a period, then optionally more digits, then optionally an \c{E} followed by an exponent. The period is mandatory, so that NASM can distinguish between \c{dd 1}, which declares an integer constant, and \c{dd 1.0} which declares a floating-point constant. NASM also support C99-style hexadecimal floating-point: \c{0x}, hexadecimal digits, period, optionally more hexadeximal digits, then optionally a \c{P} followed by a \e{binary} (not hexadecimal) exponent in decimal notation. As an extension, NASM additionally supports the \c{0h} and \c{$} prefixes for hexadecimal, as well binary and octal floating-point, using the \c{0b} or \c{0y} and \c{0o} or \c{0q} prefixes, respectively. Underscores to break up groups of digits are permitted in floating-point constants as well. Some examples: \c db -0.2 ; "Quarter precision" \c dw -0.5 ; IEEE 754r/SSE5 half precision \c dd 1.2 ; an easy one \c dd 1.222_222_222 ; underscores are permitted \c dd 0x1p+2 ; 1.0x2^2 = 4.0 \c dq 0x1p+32 ; 1.0x2^32 = 4 294 967 296.0 \c dq 1.e10 ; 10 000 000 000.0 \c dq 1.e+10 ; synonymous with 1.e10 \c dq 1.e-10 ; 0.000 000 000 1 \c dt 3.141592653589793238462 ; pi \c do 1.e+4000 ; IEEE 754r quad precision The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "\i{minifloat}." The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. \c{__float80m__} and \c{__float80e__} produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and \c{__float128l__} and \c{__float128h__} produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively. For example: \c mov rax,__float64__(3.141592653589793238462) ... would assign the binary representation of pi as a 64-bit floating point number into \c{RAX}. This is exactly equivalent to: \c mov rax,0x400921fb54442d18 NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the \i{Intel number formats}, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit. The special tokens \i\c{__Infinity__}, \i\c{__QNaN__} (or \i\c{__NaN__}) and \i\c{__SNaN__} can be used to generate \I{infinity}infinities, quiet \i{NaN}s, and signalling NaNs, respectively. These are normally used as macros: \c %define Inf __Infinity__ \c %define NaN __QNaN__ \c \c dq +1.5, -Inf, NaN ; Double-precision constants The \c{%use fp} standard macro package contains a set of convenience macros. See \k{pkg_fp}. \S{bcdconst} \I{floating-point, packed BCD constants}Packed BCD Constants x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with \c{p} or prefixed with \c{0p}, and can include up to 18 decimal digits. As with other numeric constants, underscores can be used to separate digits. For example: \c dt 12_345_678_901_245_678p \c dt -12_345_678_901_245_678p \c dt +0p33 \c dt 33p \H{expr} \i{Expressions} Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size. NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the \I{$, here}\c{$} and \i\c{$$} tokens. \c{$} evaluates to the assembly position at the beginning of the line containing the expression; so you can code an \i{infinite loop} using \c{JMP $}. \c{$$} evaluates to the beginning of the current section; so you can tell how far into the section you are by using \c{($-$$)}. The arithmetic \i{operators} provided by NASM are listed here, in increasing order of \i{precedence}. \S{expor} \i\c{|}: \i{Bitwise OR} Operator The \c{|} operator gives a bitwise OR, exactly as performed by the \c{OR} machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM. \S{expxor} \i\c{^}: \i{Bitwise XOR} Operator \c{^} provides the bitwise XOR operation. \S{expand} \i\c{&}: \i{Bitwise AND} Operator \c{&} provides the bitwise AND operation. \S{expshift} \i\c{<<} and \i\c{>>}: \i{Bit Shift} Operators \c{<<} gives a bit-shift to the left, just as it does in C. So \c{5<<3} evaluates to 5 times 8, or 40. \c{>>} gives a bit-shift to the right; in NASM, such a shift is \e{always} unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit. \S{expplmi} \I{+ opaddition}\c{+} and \I{- opsubtraction}\c{-}: \i{Addition} and \i{Subtraction} Operators The \c{+} and \c{-} operators do perfectly ordinary addition and subtraction. \S{expmul} \i\c{*}, \i\c{/}, \i\c{//}, \i\c{%} and \i\c{%%}: \i{Multiplication} and \i{Division} \c{*} is the multiplication operator. \c{/} and \c{//} are both division operators: \c{/} is \i{unsigned division} and \c{//} is \i{signed division}. Similarly, \c{%} and \c{%%} provide \I{unsigned modulo}\I{modulo operators}unsigned and \i{signed modulo} operators respectively. NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator. Since the \c{%} character is used extensively by the macro \i{preprocessor}, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear. \S{expmul} \i{Unary Operators} The highest-priority operators in NASM's expression grammar are those which only apply to one argument. These are \I{+ opunary}\c{+}, \I{- opunary}\c{-}, \i\c{~}, \I{! opunary}\c{!}, \i\c{SEG}, and the \i{integer functions} operators. \c{-} negates its operand, \c{+} does nothing (it's provided for symmetry with \c{-}), \c{~} computes the \i{one's complement} of its operand, \c{!} is the \i{logical negation} operator. \c{SEG} provides the \i{segment address} of its operand (explained in more detail in \k{segwrt}). A set of additional operators with leading and trailing double underscores are used to implement the integer functions of the \c{ifunc} macro package, see \k{pkg_ifunc}. \H{segwrt} \i\c{SEG} and \i\c{WRT} When writing large 16-bit programs, which must be split into multiple \i{segments}, it is often necessary to be able to refer to the \I{segment address}segment part of the address of a symbol. NASM supports the \c{SEG} operator to perform this function. The \c{SEG} operator returns the \i\e{preferred} segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code \c mov ax,seg symbol \c mov es,ax \c mov bx,symbol will load \c{ES:BX} with a valid pointer to the symbol \c{symbol}. Things can be more complex than this: since 16-bit segments and \i{groups} may \I{overlapping segments}overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the \c{WRT} (With Reference To) keyword. So you can do things like \c mov ax,weird_seg ; weird_seg is a segment base \c mov es,ax \c mov bx,symbol wrt weird_seg to load \c{ES:BX} with a different, but functionally equivalent, pointer to the symbol \c{symbol}. NASM supports far (inter-segment) calls and jumps by means of the syntax \c{call segment:offset}, where \c{segment} and \c{offset} both represent immediate values. So to call a far procedure, you could code either of \c call (seg procedure):procedure \c call weird_seg:(procedure wrt weird_seg) (The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.) NASM supports the syntax \I\c{CALL FAR}\c{call far procedure} as a synonym for the first of the above usages. \c{JMP} works identically to \c{CALL} in these examples. To declare a \i{far pointer} to a data item in a data segment, you must code \c dw symbol, seg symbol NASM supports no convenient synonym for this, though you can always invent one using the macro processor. \H{strict} \i\c{STRICT}: Inhibiting Optimization When assembling with the optimizer set to level 2 or higher (see \k{opt-O}), NASM will use size specifiers (\c{BYTE}, \c{WORD}, \c{DWORD}, \c{QWORD}, \c{TWORD}, \c{OWORD}, \c{YWORD} or \c{ZWORD}), but will give them the smallest possible size. The keyword \c{STRICT} can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in \c{BITS 16} mode, \c push dword 33 is encoded in three bytes \c{66 6A 21}, whereas \c push strict dword 33 is encoded in six bytes, with a full dword immediate operand \c{66 68 21 00 00 00}. With the optimizer off, the same code (six bytes) is generated whether the \c{STRICT} keyword was used or not. \H{crit} \i{Critical Expressions} Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called \e{Critical Expressions}. The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example, \c times (label-$) db 0 \c label: db 'Where am I?' The argument to \i\c{TIMES} in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the \c{TIMES} line when it first sees it. It will just as firmly reject the slightly \I{paradox}paradoxical code \c times (label-$+1) db 0 \c label: db 'NOW where am I?' in which \e{any} value for the \c{TIMES} argument is by definition wrong! NASM rejects these examples by means of a concept called a \e{critical expression}, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the \c{TIMES} prefix is a critical expression. \H{locallab} \i{Local Labels} NASM gives special treatment to symbols beginning with a \i{period}. A label beginning with a single period is treated as a \e{local} label, which means that it is associated with the previous non-local label. So, for example: \c label1 ; some code \c \c .loop \c ; some more code \c \c jne .loop \c ret \c \c label2 ; some code \c \c .loop \c ; some more code \c \c jne .loop \c ret In the above code fragment, each \c{JNE} instruction jumps to the line immediately before it, because the two definitions of \c{.loop} are kept separate by virtue of each being associated with the previous non-local label. This form of local label handling is borrowed from the old Amiga assembler \i{DevPac}; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of \e{defining} a local label in terms of the previous non-local label: the first definition of \c{.loop} above is really defining a symbol called \c{label1.loop}, and the second defines a symbol called \c{label2.loop}. So, if you really needed to, you could write \c label3 ; some more code \c ; and some more \c \c jmp label1.loop Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the \I{label prefix}special prefix \i\c{..@}, then it does nothing to the local label mechanism. So you could code \c label1: ; a non-local label \c .local: ; this is really label1.local \c ..@foo: ; this is a special symbol \c label2: ; another non-local label \c .local: ; this is really label2.local \c \c jmp ..@foo ; this will jump three lines up NASM has the capacity to define other special symbols beginning with a double period: for example, \c{..start} is used to specify the entry point in the \c{obj} output format (see \k{dotdotstart}), \c{..imagebase} is used to find out the offset from a base address of the current image in the \c{win64} output format (see \k{win64pic}). So just keep in mind that symbols beginning with a double period are special. \C{preproc} The NASM \i{Preprocessor} NASM contains a powerful \i{macro processor}, which supports conditional assembly, multi-level file inclusion, two forms of macro (single-line and multi-line), and a `context stack' mechanism for extra macro power. Preprocessor directives all begin with a \c{%} sign. The preprocessor collapses all lines which end with a backslash (\\) character into a single line. Thus: \c %define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \\ \c THIS_VALUE will work like a single-line macro without the backslash-newline sequence. \H{slmacro} \i{Single-Line Macros} \S{define} The Normal Way: \I\c{%idefine}\i\c{%define} Single-line macros are defined using the \c{%define} preprocessor directive. The definitions work in a similar way to C; so you can do things like \c %define ctrl 0x1F & \c %define param(a,b) ((a)+(a)*(b)) \c \c mov byte [param(2,ebx)], ctrl 'D' which will expand to \c mov byte [(2)+(2)*(ebx)], 0x1F & 'D' When the expansion of a single-line macro contains tokens which invoke another macro, the expansion is performed at invocation time, not at definition time. Thus the code \c %define a(x) 1+b(x) \c %define b(x) 2*x \c \c mov ax,a(8) will evaluate in the expected way to \c{mov ax,1+2*8}, even though the macro \c{b} wasn't defined at the time of definition of \c{a}. Macros defined with \c{%define} are \i{case sensitive}: after \c{%define foo bar}, only \c{foo} will expand to \c{bar}: \c{Foo} or \c{FOO} will not. By using \c{%idefine} instead of \c{%define} (the `i' stands for `insensitive') you can define all the case variants of a macro at once, so that \c{%idefine foo bar} would cause \c{foo}, \c{Foo}, \c{FOO}, \c{fOO} and so on all to expand to \c{bar}. There is a mechanism which detects when a macro call has occurred as a result of a previous expansion of the same macro, to guard against \i{circular references} and infinite loops. If this happens, the preprocessor will only expand the first occurrence of the macro. Hence, if you code \c %define a(x) 1+a(x) \c \c mov ax,a(3) the macro \c{a(3)} will expand once, becoming \c{1+a(3)}, and will then expand no further. This behaviour can be useful: see \k{32c} for an example of its use. You can \I{overloading, single-line macros}overload single-line macros: if you write \c %define foo(x) 1+x \c %define foo(x,y) 1+x*y the preprocessor will be able to handle both types of macro call, by counting the parameters you pass; so \c{foo(3)} will become \c{1+3} whereas \c{foo(ebx,2)} will become \c{1+ebx*2}. However, if you define \c %define foo bar then no other definition of \c{foo} will be accepted: a macro with no parameters prohibits the definition of the same name as a macro \e{with} parameters, and vice versa. This doesn't prevent single-line macros being \e{redefined}: you can perfectly well define a macro with \c %define foo bar and then re-define it later in the same source file with \c %define foo baz Then everywhere the macro \c{foo} is invoked, it will be expanded according to the most recent definition. This is particularly useful when defining single-line macros with \c{%assign} (see \k{assign}). You can \i{pre-define} single-line macros using the `-d' option on the NASM command line: see \k{opt-d}. \S{xdefine} Resolving \c{%define}: \I\c{%ixdefine}\i\c{%xdefine} To have a reference to an embedded single-line macro resolved at the time that the embedding macro is \e{defined}, as opposed to when the embedding macro is \e{expanded}, you need a different mechanism to the one offered by \c{%define}. The solution is to use \c{%xdefine}, or it's \I{case sensitive}case-insensitive counterpart \c{%ixdefine}. Suppose you have the following code: \c %define isTrue 1 \c %define isFalse isTrue \c %define isTrue 0 \c \c val1: db isFalse \c \c %define isTrue 1 \c \c val2: db isFalse In this case, \c{val1} is equal to 0, and \c{val2} is equal to 1. This is because, when a single-line macro is defined using \c{%define}, it is expanded only when it is called. As \c{isFalse} expands to \c{isTrue}, the expansion will be the current value of \c{isTrue}. The first time it is called that is 0, and the second time it is 1. If you wanted \c{isFalse} to expand to the value assigned to the embedded macro \c{isTrue} at the time that \c{isFalse} was defined, you need to change the above code to use \c{%xdefine}. \c %xdefine isTrue 1 \c %xdefine isFalse isTrue \c %xdefine isTrue 0 \c \c val1: db isFalse \c \c %xdefine isTrue 1 \c \c val2: db isFalse Now, each time that \c{isFalse} is called, it expands to 1, as that is what the embedded macro \c{isTrue} expanded to at the time that \c{isFalse} was defined. \S{indmacro} \i{Macro Indirection}: \I\c{%[}\c{%[...]} The \c{%[...]} construct can be used to expand macros in contexts where macro expansion would otherwise not occur, including in the names other macros. For example, if you have a set of macros named \c{Foo16}, \c{Foo32} and \c{Foo64}, you could write: \c mov ax,Foo%[__BITS__] ; The Foo value to use the builtin macro \c{__BITS__} (see \k{bitsm}) to automatically select between them. Similarly, the two statements: \c %xdefine Bar Quux ; Expands due to %xdefine \c %define Bar %[Quux] ; Expands due to %[...] have, in fact, exactly the same effect. \c{%[...]} concatenates to adjacent tokens in the same way that multi-line macro parameters do, see \k{concat} for details. \S{concat%+} Concatenating Single Line Macro Tokens: \i\c{%+} Individual tokens in single line macros can be concatenated, to produce longer tokens for later processing. This can be useful if there are several similar macros that perform similar functions. Please note that a space is required after \c{%+}, in order to disambiguate it from the syntax \c{%+1} used in multiline macros. As an example, consider the following: \c %define BDASTART 400h ; Start of BIOS data area \c struc tBIOSDA ; its structure \c .COM1addr RESW 1 \c .COM2addr RESW 1 \c ; ..and so on \c endstruc Now, if we need to access the elements of tBIOSDA in different places, we can end up with: \c mov ax,BDASTART + tBIOSDA.COM1addr \c mov bx,BDASTART + tBIOSDA.COM2addr This will become pretty ugly (and tedious) if used in many places, and can be reduced in size significantly by using the following macro: \c ; Macro to access BIOS variables by their names (from tBDA): \c %define BDA(x) BDASTART + tBIOSDA. %+ x Now the above code can be written as: \c mov ax,BDA(COM1addr) \c mov bx,BDA(COM2addr) Using this feature, we can simplify references to a lot of macros (and, in turn, reduce typing errors). \S{selfref%?} The Macro Name Itself: \i\c{%?} and \i\c{%??} The special symbols \c{%?} and \c{%??} can be used to reference the macro name itself inside a macro expansion, this is supported for both single-and multi-line macros. \c{%?} refers to the macro name as \e{invoked}, whereas \c{%??} refers to the macro name as \e{declared}. The two are always the same for case-sensitive macros, but for case-insensitive macros, they can differ. For example: \c %idefine Foo mov %?,%?? \c \c foo \c FOO will expand to: \c mov foo,Foo \c mov FOO,Foo The sequence: \c %idefine keyword $%? can be used to make a keyword "disappear", for example in case a new instruction has been used as a label in older code. For example: \c %idefine pause $%? ; Hide the PAUSE instruction \S{undef} Undefining Single-Line Macros: \i\c{%undef} Single-line macros can be removed with the \c{%undef} directive. For example, the following sequence: \c %define foo bar \c %undef foo \c \c mov eax, foo will expand to the instruction \c{mov eax, foo}, since after \c{%undef} the macro \c{foo} is no longer defined. Macros that would otherwise be pre-defined can be undefined on the command-line using the `-u' option on the NASM command line: see \k{opt-u}. \S{assign} \i{Preprocessor Variables}: \i\c{%assign} An alternative way to define single-line macros is by means of the \c{%assign} command (and its \I{case sensitive}case-insensitive counterpart \i\c{%iassign}, which differs from \c{%assign} in exactly the same way that \c{%idefine} differs from \c{%define}). \c{%assign} is used to define single-line macros which take no parameters and have a numeric value. This value can be specified in the form of an expression, and it will be evaluated once, when the \c{%assign} directive is processed. Like \c{%define}, macros defined using \c{%assign} can be re-defined later, so you can do things like \c %assign i i+1 to increment the numeric value of a macro. \c{%assign} is useful for controlling the termination of \c{%rep} preprocessor loops: see \k{rep} for an example of this. Another use for \c{%assign} is given in \k{16c} and \k{32c}. The expression passed to \c{%assign} is a \i{critical expression} (see \k{crit}), and must also evaluate to a pure number (rather than a relocatable reference such as a code or data address, or anything involving a register). \S{defstr} Defining Strings: \I\c{%idefstr}\i\c{%defstr} \c{%defstr}, and its case-insensitive counterpart \c{%idefstr}, define or redefine a single-line macro without parameters but converts the entire right-hand side, after macro expansion, to a quoted string before definition. For example: \c %defstr test TEST is equivalent to \c %define test 'TEST' This can be used, for example, with the \c{%!} construct (see \k{getenv}): \c %defstr PATH %!PATH ; The operating system PATH variable \S{deftok} Defining Tokens: \I\c{%ideftok}\i\c{%deftok} \c{%deftok}, and its case-insensitive counterpart \c{%ideftok}, define or redefine a single-line macro without parameters but converts the second parameter, after string conversion, to a sequence of tokens. For example: \c %deftok test 'TEST' is equivalent to \c %define test TEST \H{strlen} \i{String Manipulation in Macros} It's often useful to be able to handle strings in macros. NASM supports a few simple string handling macro operators from which more complex operations can be constructed. All the string operators define or redefine a value (either a string or a numeric value) to a single-line macro. When producing a string value, it may change the style of quoting of the input string or strings, and possibly use \c{\\}-escapes inside \c{`}-quoted strings. \S{strcat} \i{Concatenating Strings}: \i\c{%strcat} The \c{%strcat} operator concatenates quoted strings and assign them to a single-line macro. For example: \c %strcat alpha "Alpha: ", '12" screen' ... would assign the value \c{'Alpha: 12" screen'} to \c{alpha}. Similarly: \c %strcat beta '"foo"\', "'bar'" ... would assign the value \c{`"foo"\\\\'bar'`} to \c{beta}. The use of commas to separate strings is permitted but optional. \S{strlen} \i{String Length}: \i\c{%strlen} The \c{%strlen} operator assigns the length of a string to a macro. For example: \c %strlen charcnt 'my string' In this example, \c{charcnt} would receive the value 9, just as if an \c{%assign} had been used. In this example, \c{'my string'} was a literal string but it could also have been a single-line macro that expands to a string, as in the following example: \c %define sometext 'my string' \c %strlen charcnt sometext As in the first case, this would result in \c{charcnt} being assigned the value of 9. \S{substr} \i{Extracting Substrings}: \i\c{%substr} Individual letters or substrings in strings can be extracted using the \c{%substr} operator. An example of its use is probably more useful than the description: \c %substr mychar 'xyzw' 1 ; equivalent to %define mychar 'x' \c %substr mychar 'xyzw' 2 ; equivalent to %define mychar 'y' \c %substr mychar 'xyzw' 3 ; equivalent to %define mychar 'z' \c %substr mychar 'xyzw' 2,2 ; equivalent to %define mychar 'yz' \c %substr mychar 'xyzw' 2,-1 ; equivalent to %define mychar 'yzw' \c %substr mychar 'xyzw' 2,-2 ; equivalent to %define mychar 'yz' As with \c{%strlen} (see \k{strlen}), the first parameter is the single-line macro to be created and the second is the string. The third parameter specifies the first character to be selected, and the optional fourth parameter preceeded by comma) is the length. Note that the first index is 1, not 0 and the last index is equal to the value that \c{%strlen} would assign given the same string. Index values out of range result in an empty string. A negative length means "until N-1 characters before the end of string", i.e. \c{-1} means until end of string, \c{-2} until one character before, etc. \H{mlmacro} \i{Multi-Line Macros}: \I\c{%imacro}\i\c{%macro} Multi-line macros are much more like the type of macro seen in MASM and TASM: a multi-line macro definition in NASM looks something like this. \c %macro prologue 1 \c \c push ebp \c mov ebp,esp \c sub esp,%1 \c \c %endmacro This defines a C-like function prologue as a macro: so you would invoke the macro with a call such as \c myfunc: prologue 12 which would expand to the three lines of code \c myfunc: push ebp \c mov ebp,esp \c sub esp,12 The number \c{1} after the macro name in the \c{%macro} line defines the number of parameters the macro \c{prologue} expects to receive. The use of \c{%1} inside the macro definition refers to the first parameter to the macro call. With a macro taking more than one parameter, subsequent parameters would be referred to as \c{%2}, \c{%3} and so on. Multi-line macros, like single-line macros, are \i{case-sensitive}, unless you define them using the alternative directive \c{%imacro}. If you need to pass a comma as \e{part} of a parameter to a multi-line macro, you can do that by enclosing the entire parameter in \I{braces, around macro parameters}braces. So you could code things like \c %macro silly 2 \c \c %2: db %1 \c \c %endmacro \c \c silly 'a', letter_a ; letter_a: db 'a' \c silly 'ab', string_ab ; string_ab: db 'ab' \c silly {13,10}, crlf ; crlf: db 13,10 \S{mlmacover} Overloading Multi-Line Macros\I{overloading, multi-line macros} As with single-line macros, multi-line macros can be overloaded by defining the same macro name several times with different numbers of parameters. This time, no exception is made for macros with no parameters at all. So you could define \c %macro prologue 0 \c \c push ebp \c mov ebp,esp \c \c %endmacro to define an alternative form of the function prologue which allocates no local stack space. Sometimes, however, you might want to `overload' a machine instruction; for example, you might want to define \c %macro push 2 \c \c push %1 \c push %2 \c \c %endmacro so that you could code \c push ebx ; this line is not a macro call \c push eax,ecx ; but this one is Ordinarily, NASM will give a warning for the first of the above two lines, since \c{push} is now defined to be a macro, and is being invoked with a number of parameters for which no definition has been given. The correct code will still be generated, but the assembler will give a warning. This warning can be disabled by the use of the \c{-w-macro-params} command-line option (see \k{opt-w}). \S{maclocal} \i{Macro-Local Labels} NASM allows you to define labels within a multi-line macro definition in such a way as to make them local to the macro call: so calling the same macro multiple times will use a different label each time. You do this by prefixing \i\c{%%} to the label name. So you can invent an instruction which executes a \c{RET} if the \c{Z} flag is set by doing this: \c %macro retz 0 \c \c jnz %%skip \c ret \c %%skip: \c \c %endmacro You can call this macro as many times as you want, and every time you call it NASM will make up a different `real' name to substitute for the label \c{%%skip}. The names NASM invents are of the form \c{..@2345.skip}, where the number 2345 changes with every macro call. The \i\c{..@} prefix prevents macro-local labels from interfering with the local label mechanism, as described in \k{locallab}. You should avoid defining your own labels in this form (the \c{..@} prefix, then a number, then another period) in case they interfere with macro-local labels. \S{mlmacgre} \i{Greedy Macro Parameters} Occasionally it is useful to define a macro which lumps its entire command line into one parameter definition, possibly after extracting one or two smaller parameters from the front. An example might be a macro to write a text string to a file in MS-DOS, where you might want to be able to write \c writefile [filehandle],"hello, world",13,10 NASM allows you to define the last parameter of a macro to be \e{greedy}, meaning that if you invoke the macro with more parameters than it expects, all the spare parameters get lumped into the last defined one along with the separating commas. So if you code: \c %macro writefile 2+ \c \c jmp %%endstr \c %%str: db %2 \c %%endstr: \c mov dx,%%str \c mov cx,%%endstr-%%str \c mov bx,%1 \c mov ah,0x40 \c int 0x21 \c \c %endmacro then the example call to \c{writefile} above will work as expected: the text before the first comma, \c{[filehandle]}, is used as the first macro parameter and expanded when \c{%1} is referred to, and all the subsequent text is lumped into \c{%2} and placed after the \c{db}. The greedy nature of the macro is indicated to NASM by the use of the \I{+ modifier}\c{+} sign after the parameter count on the \c{%macro} line. If you define a greedy macro, you are effectively telling NASM how it should expand the macro given \e{any} number of parameters from the actual number specified up to infinity; in this case, for example, NASM now knows what to do when it sees a call to \c{writefile} with 2, 3, 4 or more parameters. NASM will take this into account when overloading macros, and will not allow you to define another form of \c{writefile} taking 4 parameters (for example). Of course, the above macro could have been implemented as a non-greedy macro, in which case the call to it would have had to look like \c writefile [filehandle], {"hello, world",13,10} NASM provides both mechanisms for putting \i{commas in macro parameters}, and you choose which one you prefer for each macro definition. See \k{sectmac} for a better way to write the above macro. \S{mlmacrange} \i{Macro Parameters Range} NASM allows you to expand parameters via special construction \c{%\{x:y\}} where \c{x} is the first parameter index and \c{y} is the last. Any index can be either negative or positive but must never be zero. For example \c %macro mpar 1-* \c db %{3:5} \c %endmacro \c \c mpar 1,2,3,4,5,6 expands to \c{3,4,5} range. Even more, the parameters can be reversed so that \c %macro mpar 1-* \c db %{5:3} \c %endmacro \c \c mpar 1,2,3,4,5,6 expands to \c{5,4,3} range. But even this is not the last. The parameters can be addressed via negative indices so NASM will count them reversed. The ones who know Python may see the analogue here. \c %macro mpar 1-* \c db %{-1:-3} \c %endmacro \c \c mpar 1,2,3,4,5,6 expands to \c{6,5,4} range. Note that NASM uses \i{comma} to separate parameters being expanded. By the way, here is a trick - you might use the index \c{%{-1:-1}} which gives you the \i{last} argument passed to a macro. \S{mlmacdef} \i{Default Macro Parameters} NASM also allows you to define a multi-line macro with a \e{range} of allowable parameter counts. If you do this, you can specify defaults for \i{omitted parameters}. So, for example: \c %macro die 0-1 "Painful program death has occurred." \c \c writefile 2,%1 \c mov ax,0x4c01 \c int 0x21 \c \c %endmacro This macro (which makes use of the \c{writefile} macro defined in \k{mlmacgre}) can be called with an explicit error message, which it will display on the error output stream before exiting, or it can be called with no parameters, in which case it will use the default error message supplied in the macro definition. In general, you supply a minimum and maximum number of parameters for a macro of this type; the minimum number of parameters are then required in the macro call, and then you provide defaults for the optional ones. So if a macro definition began with the line \c %macro foobar 1-3 eax,[ebx+2] then it could be called with between one and three parameters, and \c{%1} would always be taken from the macro call. \c{%2}, if not specified by the macro call, would default to \c{eax}, and \c{%3} if not specified would default to \c{[ebx+2]}. You can provide extra information to a macro by providing too many default parameters: \c %macro quux 1 something This will trigger a warning by default; see \k{opt-w} for more information. When \c{quux} is invoked, it receives not one but two parameters. \c{something} can be referred to as \c{%2}. The difference between passing \c{something} this way and writing \c{something} in the macro body is that with this way \c{something} is evaluated when the macro is defined, not when it is expanded. You may omit parameter defaults from the macro definition, in which case the parameter default is taken to be blank. This can be useful for macros which can take a variable number of parameters, since the \i\c{%0} token (see \k{percent0}) allows you to determine how many parameters were really passed to the macro call. This defaulting mechanism can be combined with the greedy-parameter mechanism; so the \c{die} macro above could be made more powerful, and more useful, by changing the first line of the definition to \c %macro die 0-1+ "Painful program death has occurred.",13,10 The maximum parameter count can be infinite, denoted by \c{*}. In this case, of course, it is impossible to provide a \e{full} set of default parameters. Examples of this usage are shown in \k{rotate}. \S{percent0} \i\c{%0}: \I{counting macro parameters}Macro Parameter Counter The parameter reference \c{%0} will return a numeric constant giving the number of parameters received, that is, if \c{%0} is n then \c{%}n is the last parameter. \c{%0} is mostly useful for macros that can take a variable number of parameters. It can be used as an argument to \c{%rep} (see \k{rep}) in order to iterate through all the parameters of a macro. Examples are given in \k{rotate}. \S{percent00} \i\c{%00}: \I{label preceeding macro}Label Preceeding Macro \c{%00} will return the label preceeding the macro invocation, if any. The label must be on the same line as the macro invocation, may be a local label (see \k{locallab}), and need not end in a colon. \S{rotate} \i\c{%rotate}: \i{Rotating Macro Parameters} Unix shell programmers will be familiar with the \I{shift command}\c{shift} shell command, which allows the arguments passed to a shell script (referenced as \c{$1}, \c{$2} and so on) to be moved left by one place, so that the argument previously referenced as \c{$2} becomes available as \c{$1}, and the argument previously referenced as \c{$1} is no longer available at all. NASM provides a similar mechanism, in the form of \c{%rotate}. As its name suggests, it differs from the Unix \c{shift} in that no parameters are lost: parameters rotated off the left end of the argument list reappear on the right, and vice versa. \c{%rotate} is invoked with a single numeric argument (which may be an expression). The macro parameters are rotated to the left by that many places. If the argument to \c{%rotate} is negative, the macro parameters are rotated to the right. \I{iterating over macro parameters}So a pair of macros to save and restore a set of registers might work as follows: \c %macro multipush 1-* \c \c %rep %0 \c push %1 \c %rotate 1 \c %endrep \c \c %endmacro This macro invokes the \c{PUSH} instruction on each of its arguments in turn, from left to right. It begins by pushing its first argument, \c{%1}, then invokes \c{%rotate} to move all the arguments one place to the left, so that the original second argument is now available as \c{%1}. Repeating this procedure as many times as there were arguments (achieved by supplying \c{%0} as the argument to \c{%rep}) causes each argument in turn to be pushed. Note also the use of \c{*} as the maximum parameter count, indicating that there is no upper limit on the number of parameters you may supply to the \i\c{multipush} macro. It would be convenient, when using this macro, to have a \c{POP} equivalent, which \e{didn't} require the arguments to be given in reverse order. Ideally, you would write the \c{multipush} macro call, then cut-and-paste the line to where the pop needed to be done, and change the name of the called macro to \c{multipop}, and the macro would take care of popping the registers in the opposite order from the one in which they were pushed. This can be done by the following definition: \c %macro multipop 1-* \c \c %rep %0 \c %rotate -1 \c pop %1 \c %endrep \c \c %endmacro This macro begins by rotating its arguments one place to the \e{right}, so that the original \e{last} argument appears as \c{%1}. This is then popped, and the arguments are rotated right again, so the second-to-last argument becomes \c{%1}. Thus the arguments are iterated through in reverse order. \S{concat} \i{Concatenating Macro Parameters} NASM can concatenate macro parameters and macro indirection constructs on to other text surrounding them. This allows you to declare a family of symbols, for example, in a macro definition. If, for example, you wanted to generate a table of key codes along with offsets into the table, you could code something like \c %macro keytab_entry 2 \c \c keypos%1 equ $-keytab \c db %2 \c \c %endmacro \c \c keytab: \c keytab_entry F1,128+1 \c keytab_entry F2,128+2 \c keytab_entry Return,13 which would expand to \c keytab: \c keyposF1 equ $-keytab \c db 128+1 \c keyposF2 equ $-keytab \c db 128+2 \c keyposReturn equ $-keytab \c db 13 You can just as easily concatenate text on to the other end of a macro parameter, by writing \c{%1foo}. If you need to append a \e{digit} to a macro parameter, for example defining labels \c{foo1} and \c{foo2} when passed the parameter \c{foo}, you can't code \c{%11} because that would be taken as the eleventh macro parameter. Instead, you must code \I{braces, after % sign}\c{%\{1\}1}, which will separate the first \c{1} (giving the number of the macro parameter) from the second (literal text to be concatenated to the parameter). This concatenation can also be applied to other preprocessor in-line objects, such as macro-local labels (\k{maclocal}) and context-local labels (\k{ctxlocal}). In all cases, ambiguities in syntax can be resolved by enclosing everything after the \c{%} sign and before the literal text in braces: so \c{%\{%foo\}bar} concatenates the text \c{bar} to the end of the real name of the macro-local label \c{%%foo}. (This is unnecessary, since the form NASM uses for the real names of macro-local labels means that the two usages \c{%\{%foo\}bar} and \c{%%foobar} would both expand to the same thing anyway; nevertheless, the capability is there.) The single-line macro indirection construct, \c{%[...]} (\k{indmacro}), behaves the same way as macro parameters for the purpose of concatenation. See also the \c{%+} operator, \k{concat%+}. \S{mlmaccc} \i{Condition Codes as Macro Parameters} NASM can give special treatment to a macro parameter which contains a condition code. For a start, you can refer to the macro parameter \c{%1} by means of the alternative syntax \i\c{%+1}, which informs NASM that this macro parameter is supposed to contain a condition code, and will cause the preprocessor to report an error message if the macro is called with a parameter which is \e{not} a valid condition code. Far more usefully, though, you can refer to the macro parameter by means of \i\c{%-1}, which NASM will expand as the \e{inverse} condition code. So the \c{retz} macro defined in \k{maclocal} can be replaced by a general \i{conditional-return macro} like this: \c %macro retc 1 \c \c j%-1 %%skip \c ret \c %%skip: \c \c %endmacro This macro can now be invoked using calls like \c{retc ne}, which will cause the conditional-jump instruction in the macro expansion to come out as \c{JE}, or \c{retc po} which will make the jump a \c{JPE}. The \c{%+1} macro-parameter reference is quite happy to interpret the arguments \c{CXZ} and \c{ECXZ} as valid condition codes; however, \c{%-1} will report an error if passed either of these, because no inverse condition code exists. \S{nolist} \i{Disabling Listing Expansion}\I\c{.nolist} When NASM is generating a listing file from your program, it will generally expand multi-line macros by means of writing the macro call and then listing each line of the expansion. This allows you to see which instructions in the macro expansion are generating what code; however, for some macros this clutters the listing up unnecessarily. NASM therefore provides the \c{.nolist} qualifier, which you can include in a macro definition to inhibit the expansion of the macro in the listing file. The \c{.nolist} qualifier comes directly after the number of parameters, like this: \c %macro foo 1.nolist Or like this: \c %macro bar 1-5+.nolist a,b,c,d,e,f,g,h \S{unmacro} Undefining Multi-Line Macros: \i\c{%unmacro} Multi-line macros can be removed with the \c{%unmacro} directive. Unlike the \c{%undef} directive, however, \c{%unmacro} takes an argument specification, and will only remove \i{exact matches} with that argument specification. For example: \c %macro foo 1-3 \c ; Do something \c %endmacro \c %unmacro foo 1-3 removes the previously defined macro \c{foo}, but \c %macro bar 1-3 \c ; Do something \c %endmacro \c %unmacro bar 1 does \e{not} remove the macro \c{bar}, since the argument specification does not match exactly. \H{condasm} \i{Conditional Assembly}\I\c{%if} Similarly to the C preprocessor, NASM allows sections of a source file to be assembled only if certain conditions are met. The general syntax of this feature looks like this: \c %if<condition> \c ; some code which only appears if <condition> is met \c %elif<condition2> \c ; only appears if <condition> is not met but <condition2> is \c %else \c ; this appears if neither <condition> nor <condition2> was met \c %endif The inverse forms \i\c{%ifn} and \i\c{%elifn} are also supported. The \i\c{%else} clause is optional, as is the \i\c{%elif} clause. You can have more than one \c{%elif} clause as well. There are a number of variants of the \c{%if} directive. Each has its corresponding \c{%elif}, \c{%ifn}, and \c{%elifn} directives; for example, the equivalents to the \c{%ifdef} directive are \c{%elifdef}, \c{%ifndef}, and \c{%elifndef}. \S{ifdef} \i\c{%ifdef}: Testing Single-Line Macro Existence\I{testing, single-line macro existence} Beginning a conditional-assembly block with the line \c{%ifdef MACRO} will assemble the subsequent code if, and only if, a single-line macro called \c{MACRO} is defined. If not, then the \c{%elif} and \c{%else} blocks (if any) will be processed instead. For example, when debugging a program, you might want to write code such as \c ; perform some function \c %ifdef DEBUG \c writefile 2,"Function performed successfully",13,10 \c %endif \c ; go and do something else Then you could use the command-line option \c{-dDEBUG} to create a version of the program which produced debugging messages, and remove the option to generate the final release version of the program. You can test for a macro \e{not} being defined by using \i\c{%ifndef} instead of \c{%ifdef}. You can also test for macro definitions in \c{%elif} blocks by using \i\c{%elifdef} and \i\c{%elifndef}. \S{ifmacro} \i\c{%ifmacro}: Testing Multi-Line Macro Existence\I{testing, multi-line macro existence} The \c{%ifmacro} directive operates in the same way as the \c{%ifdef} directive, except that it checks for the existence of a multi-line macro. For example, you may be working with a large project and not have control over the macros in a library. You may want to create a macro with one name if it doesn't already exist, and another name if one with that name does exist. The \c{%ifmacro} is considered true if defining a macro with the given name and number of arguments would cause a definitions conflict. For example: \c %ifmacro MyMacro 1-3 \c \c %error "MyMacro 1-3" causes a conflict with an existing macro. \c \c %else \c \c %macro MyMacro 1-3 \c \c ; insert code to define the macro \c \c %endmacro \c \c %endif This will create the macro "MyMacro 1-3" if no macro already exists which would conflict with it, and emits a warning if there would be a definition conflict. You can test for the macro not existing by using the \i\c{%ifnmacro} instead of \c{%ifmacro}. Additional tests can be performed in \c{%elif} blocks by using \i\c{%elifmacro} and \i\c{%elifnmacro}. \S{ifctx} \i\c{%ifctx}: Testing the Context Stack\I{testing, context stack} The conditional-assembly construct \c{%ifctx} will cause the subsequent code to be assembled if and only if the top context on the preprocessor's context stack has the same name as one of the arguments. As with \c{%ifdef}, the inverse and \c{%elif} forms \i\c{%ifnctx}, \i\c{%elifctx} and \i\c{%elifnctx} are also supported. For more details of the context stack, see \k{ctxstack}. For a sample use of \c{%ifctx}, see \k{blockif}. \S{if} \i\c{%if}: Testing Arbitrary Numeric Expressions\I{testing, arbitrary numeric expressions} The conditional-assembly construct \c{%if expr} will cause the subsequent code to be assembled if and only if the value of the numeric expression \c{expr} is non-zero. An example of the use of this feature is in deciding when to break out of a \c{%rep} preprocessor loop: see \k{rep} for a detailed example. The expression given to \c{%if}, and its counterpart \i\c{%elif}, is a critical expression (see \k{crit}). \c{%if} extends the normal NASM expression syntax, by providing a set of \i{relational operators} which are not normally available in expressions. The operators \i\c{=}, \i\c{<}, \i\c{>}, \i\c{<=}, \i\c{>=} and \i\c{<>} test equality, less-than, greater-than, less-or-equal, greater-or-equal and not-equal respectively. The C-like forms \i\c{==} and \i\c{!=} are supported as alternative forms of \c{=} and \c{<>}. In addition, low-priority logical operators \i\c{&&}, \i\c{^^} and \i\c{||} are provided, supplying \i{logical AND}, \i{logical XOR} and \i{logical OR}. These work like the C logical operators (although C has no logical XOR), in that they always return either 0 or 1, and treat any non-zero input as 1 (so that \c{^^}, for example, returns 1 if exactly one of its inputs is zero, and 0 otherwise). The relational operators also return 1 for true and 0 for false. Like other \c{%if} constructs, \c{%if} has a counterpart \i\c{%elif}, and negative forms \i\c{%ifn} and \i\c{%elifn}. \S{ifidn} \i\c{%ifidn} and \i\c{%ifidni}: Testing Exact Text Identity\I{testing, exact text identity} The construct \c{%ifidn text1,text2} will cause the subsequent code to be assembled if and only if \c{text1} and \c{text2}, after expanding single-line macros, are identical pieces of text. Differences in white space are not counted. \c{%ifidni} is similar to \c{%ifidn}, but is \i{case-insensitive}. For example, the following macro pushes a register or number on the stack, and allows you to treat \c{IP} as a real register: \c %macro pushparam 1 \c \c %ifidni %1,ip \c call %%label \c %%label: \c %else \c push %1 \c %endif \c \c %endmacro Like other \c{%if} constructs, \c{%ifidn} has a counterpart \i\c{%elifidn}, and negative forms \i\c{%ifnidn} and \i\c{%elifnidn}. Similarly, \c{%ifidni} has counterparts \i\c{%elifidni}, \i\c{%ifnidni} and \i\c{%elifnidni}. \S{iftyp} \i\c{%ifid}, \i\c{%ifnum}, \i\c{%ifstr}: Testing Token Types\I{testing, token types} Some macros will want to perform different tasks depending on whether they are passed a number, a string, or an identifier. For example, a string output macro might want to be able to cope with being passed either a string constant or a pointer to an existing string. The conditional assembly construct \c{%ifid}, taking one parameter (which may be blank), assembles the subsequent code if and only if the first token in the parameter exists and is an identifier. \c{%ifnum} works similarly, but tests for the token being a numeric constant; \c{%ifstr} tests for it being a string. For example, the \c{writefile} macro defined in \k{mlmacgre} can be extended to take advantage of \c{%ifstr} in the following fashion: \c %macro writefile 2-3+ \c \c %ifstr %2 \c jmp %%endstr \c %if %0 = 3 \c %%str: db %2,%3 \c %else \c %%str: db %2 \c %endif \c %%endstr: mov dx,%%str \c mov cx,%%endstr-%%str \c %else \c mov dx,%2 \c mov cx,%3 \c %endif \c mov bx,%1 \c mov ah,0x40 \c int 0x21 \c \c %endmacro Then the \c{writefile} macro can cope with being called in either of the following two ways: \c writefile [file], strpointer, length \c writefile [file], "hello", 13, 10 In the first, \c{strpointer} is used as the address of an already-declared string, and \c{length} is used as its length; in the second, a string is given to the macro, which therefore declares it itself and works out the address and length for itself. Note the use of \c{%if} inside the \c{%ifstr}: this is to detect whether the macro was passed two arguments (so the string would be a single string constant, and \c{db %2} would be adequate) or more (in which case, all but the first two would be lumped together into \c{%3}, and \c{db %2,%3} would be required). The usual \I\c{%elifid}\I\c{%elifnum}\I\c{%elifstr}\c{%elif}..., \I\c{%ifnid}\I\c{%ifnnum}\I\c{%ifnstr}\c{%ifn}..., and \I\c{%elifnid}\I\c{%elifnnum}\I\c{%elifnstr}\c{%elifn}... versions exist for each of \c{%ifid}, \c{%ifnum} and \c{%ifstr}. \S{iftoken} \i\c{%iftoken}: Test for a Single Token Some macros will want to do different things depending on if it is passed a single token (e.g. paste it to something else using \c{%+}) versus a multi-token sequence. The conditional assembly construct \c{%iftoken} assembles the subsequent code if and only if the expanded parameters consist of exactly one token, possibly surrounded by whitespace. For example: \c %iftoken 1 will assemble the subsequent code, but \c %iftoken -1 will not, since \c{-1} contains two tokens: the unary minus operator \c{-}, and the number \c{1}. The usual \i\c{%eliftoken}, \i\c\{%ifntoken}, and \i\c{%elifntoken} variants are also provided. \S{ifempty} \i\c{%ifempty}: Test for Empty Expansion The conditional assembly construct \c{%ifempty} assembles the subsequent code if and only if the expanded parameters do not contain any tokens at all, whitespace excepted. The usual \i\c{%elifempty}, \i\c\{%ifnempty}, and \i\c{%elifnempty} variants are also provided. \S{ifenv} \i\c{%ifenv}: Test If Environment Variable Exists The conditional assembly construct \c{%ifenv} assembles the subsequent code if and only if the environment variable referenced by the \c{%!}\e{variable} directive exists. The usual \i\c{%elifenv}, \i\c\{%ifnenv}, and \i\c{%elifnenv} variants are also provided. Just as for \c{%!}\e{variable} the argument should be written as a string if it contains characters that would not be legal in an identifier. See \k{getenv}. \H{rep} \i{Preprocessor Loops}\I{repeating code}: \i\c{%rep} NASM's \c{TIMES} prefix, though useful, cannot be used to invoke a multi-line macro multiple times, because it is processed by NASM after macros have already been expanded. Therefore NASM provides another form of loop, this time at the preprocessor level: \c{%rep}. The directives \c{%rep} and \i\c{%endrep} (\c{%rep} takes a numeric argument, which can be an expression; \c{%endrep} takes no arguments) can be used to enclose a chunk of code, which is then replicated as many times as specified by the preprocessor: \c %assign i 0 \c %rep 64 \c inc word [table+2*i] \c %assign i i+1 \c %endrep This will generate a sequence of 64 \c{INC} instructions, incrementing every word of memory from \c{[table]} to \c{[table+126]}. For more complex termination conditions, or to break out of a repeat loop part way along, you can use the \i\c{%exitrep} directive to terminate the loop, like this: \c fibonacci: \c %assign i 0 \c %assign j 1 \c %rep 100 \c %if j > 65535 \c %exitrep \c %endif \c dw j \c %assign k j+i \c %assign i j \c %assign j k \c %endrep \c \c fib_number equ ($-fibonacci)/2 This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeat count must still be given to \c{%rep}. This is to prevent the possibility of NASM getting into an infinite loop in the preprocessor, which (on multitasking or multi-user systems) would typically cause all the system memory to be gradually used up and other applications to start crashing. Note a maximum repeat count is limited by 62 bit number, though it is hardly possible that you ever need anything bigger. \H{files} Source Files and Dependencies These commands allow you to split your sources into multiple files. \S{include} \i\c{%include}: \i{Including Other Files} Using, once again, a very similar syntax to the C preprocessor, NASM's preprocessor lets you include other source files into your code. This is done by the use of the \i\c{%include} directive: \c %include "macros.mac" will include the contents of the file \c{macros.mac} into the source file containing the \c{%include} directive. Include files are \I{searching for include files}searched for in the current directory (the directory you're in when you run NASM, as opposed to the location of the NASM executable or the location of the source file), plus any directories specified on the NASM command line using the \c{-i} option. The standard C idiom for preventing a file being included more than once is just as applicable in NASM: if the file \c{macros.mac} has the form \c %ifndef MACROS_MAC \c %define MACROS_MAC \c ; now define some macros \c %endif then including the file more than once will not cause errors, because the second time the file is included nothing will happen because the macro \c{MACROS_MAC} will already be defined. You can force a file to be included even if there is no \c{%include} directive that explicitly includes it, by using the \i\c{-p} option on the NASM command line (see \k{opt-p}). \S{pathsearch} \i\c{%pathsearch}: Search the Include Path The \c{%pathsearch} directive takes a single-line macro name and a filename, and declare or redefines the specified single-line macro to be the include-path-resolved version of the filename, if the file exists (otherwise, it is passed unchanged.) For example, \c %pathsearch MyFoo "foo.bin" ... with \c{-Ibins/} in the include path may end up defining the macro \c{MyFoo} to be \c{"bins/foo.bin"}. \S{depend} \i\c{%depend}: Add Dependent Files The \c{%depend} directive takes a filename and adds it to the list of files to be emitted as dependency generation when the \c{-M} options and its relatives (see \k{opt-M}) are used. It produces no output. This is generally used in conjunction with \c{%pathsearch}. For example, a simplified version of the standard macro wrapper for the \c{INCBIN} directive looks like: \c %imacro incbin 1-2+ 0 \c %pathsearch dep %1 \c %depend dep \c incbin dep,%2 \c %endmacro This first resolves the location of the file into the macro \c{dep}, then adds it to the dependency lists, and finally issues the assembler-level \c{INCBIN} directive. \S{use} \i\c{%use}: Include Standard Macro Package The \c{%use} directive is similar to \c{%include}, but rather than including the contents of a file, it includes a named standard macro package. The standard macro packages are part of NASM, and are described in \k{macropkg}. Unlike the \c{%include} directive, package names for the \c{%use} directive do not require quotes, but quotes are permitted. In NASM 2.04 and 2.05 the unquoted form would be macro-expanded; this is no longer true. Thus, the following lines are equivalent: \c %use altreg \c %use 'altreg' Standard macro packages are protected from multiple inclusion. When a standard macro package is used, a testable single-line macro of the form \c{__USE_}\e{package}\c{__} is also defined, see \k{use_def}. \H{ctxstack} The \i{Context Stack} Having labels that are local to a macro definition is sometimes not quite powerful enough: sometimes you want to be able to share labels between several macro calls. An example might be a \c{REPEAT} ... \c{UNTIL} loop, in which the expansion of the \c{REPEAT} macro would need to be able to refer to a label which the \c{UNTIL} macro had defined. However, for such a macro you would also want to be able to nest these loops. NASM provides this level of power by means of a \e{context stack}. The preprocessor maintains a stack of \e{contexts}, each of which is characterized by a name. You add a new context to the stack using the \i\c{%push} directive, and remove one using \i\c{%pop}. You can define labels that are local to a particular context on the stack. \S{pushpop} \i\c{%push} and \i\c{%pop}: \I{creating contexts}\I{removing contexts}Creating and Removing Contexts The \c{%push} directive is used to create a new context and place it on the top of the context stack. \c{%push} takes an optional argument, which is the name of the context. For example: \c %push foobar This pushes a new context called \c{foobar} on the stack. You can have several contexts on the stack with the same name: they can still be distinguished. If no name is given, the context is unnamed (this is normally used when both the \c{%push} and the \c{%pop} are inside a single macro definition.) The directive \c{%pop}, taking one optional argument, removes the top context from the context stack and destroys it, along with any labels associated with it. If an argument is given, it must match the name of the current context, otherwise it will issue an error. \S{ctxlocal} \i{Context-Local Labels} Just as the usage \c{%%foo} defines a label which is local to the particular macro call in which it is used, the usage \I{%$}\c{%$foo} is used to define a label which is local to the context on the top of the context stack. So the \c{REPEAT} and \c{UNTIL} example given above could be implemented by means of: \c %macro repeat 0 \c \c %push repeat \c %$begin: \c \c %endmacro \c \c %macro until 1 \c \c j%-1 %$begin \c %pop \c \c %endmacro and invoked by means of, for example, \c mov cx,string \c repeat \c add cx,3 \c scasb \c until e which would scan every fourth byte of a string in search of the byte in \c{AL}. If you need to define, or access, labels local to the context \e{below} the top one on the stack, you can use \I{%$$}\c{%$$foo}, or \c{%$$$foo} for the context below that, and so on. \S{ctxdefine} \i{Context-Local Single-Line Macros} NASM also allows you to define single-line macros which are local to a particular context, in just the same way: \c %define %$localmac 3 will define the single-line macro \c{%$localmac} to be local to the top context on the stack. Of course, after a subsequent \c{%push}, it can then still be accessed by the name \c{%$$localmac}. \S{ctxfallthrough} \i{Context Fall-Through Lookup} \e{(deprecated)} Context fall-through lookup (automatic searching of outer contexts) is a feature that was added in NASM version 0.98.03. Unfortunately, this feature is unintuitive and can result in buggy code that would have otherwise been prevented by NASM's error reporting. As a result, this feature has been \e{deprecated}. NASM version 2.09 will issue a warning when usage of this \e{deprecated} feature is detected. Starting with NASM version 2.10, usage of this \e{deprecated} feature will simply result in an \e{expression syntax error}. An example usage of this \e{deprecated} feature follows: \c %macro ctxthru 0 \c %push ctx1 \c %assign %$external 1 \c %push ctx2 \c %assign %$internal 1 \c mov eax, %$external \c mov eax, %$internal \c %pop \c %pop \c %endmacro As demonstrated, \c{%$external} is being defined in the \c{ctx1} context and referenced within the \c{ctx2} context. With context fall-through lookup, referencing an undefined context-local macro like this implicitly searches through all outer contexts until a match is made or isn't found in any context. As a result, \c{%$external} referenced within the \c{ctx2} context would implicitly use \c{%$external} as defined in \c{ctx1}. Most people would expect NASM to issue an error in this situation because \c{%$external} was never defined within \c{ctx2} and also isn't qualified with the proper context depth, \c{%$$external}. Here is a revision of the above example with proper context depth: \c %macro ctxthru 0 \c %push ctx1 \c %assign %$external 1 \c %push ctx2 \c %assign %$internal 1 \c mov eax, %$$external \c mov eax, %$internal \c %pop \c %pop \c %endmacro As demonstrated, \c{%$external} is still being defined in the \c{ctx1} context and referenced within the \c{ctx2} context. However, the reference to \c{%$external} within \c{ctx2} has been fully qualified with the proper context depth, \c{%$$external}, and thus is no longer ambiguous, unintuitive or erroneous. \S{ctxrepl} \i\c{%repl}: \I{renaming contexts}Renaming a Context If you need to change the name of the top context on the stack (in order, for example, to have it respond differently to \c{%ifctx}), you can execute a \c{%pop} followed by a \c{%push}; but this will have the side effect of destroying all context-local labels and macros associated with the context that was just popped. NASM provides the directive \c{%repl}, which \e{replaces} a context with a different name, without touching the associated macros and labels. So you could replace the destructive code \c %pop \c %push newname with the non-destructive version \c{%repl newname}. \S{blockif} Example Use of the \i{Context Stack}: \i{Block IFs} This example makes use of almost all the context-stack features, including the conditional-assembly construct \i\c{%ifctx}, to implement a block IF statement as a set of macros. \c %macro if 1 \c \c %push if \c j%-1 %$ifnot \c \c %endmacro \c \c %macro else 0 \c \c %ifctx if \c %repl else \c jmp %$ifend \c %$ifnot: \c %else \c %error "expected `if' before `else'" \c %endif \c \c %endmacro \c \c %macro endif 0 \c \c %ifctx if \c %$ifnot: \c %pop \c %elifctx else \c %$ifend: \c %pop \c %else \c %error "expected `if' or `else' before `endif'" \c %endif \c \c %endmacro This code is more robust than the \c{REPEAT} and \c{UNTIL} macros given in \k{ctxlocal}, because it uses conditional assembly to check that the macros are issued in the right order (for example, not calling \c{endif} before \c{if}) and issues a \c{%error} if they're not. In addition, the \c{endif} macro has to be able to cope with the two distinct cases of either directly following an \c{if}, or following an \c{else}. It achieves this, again, by using conditional assembly to do different things depending on whether the context on top of the stack is \c{if} or \c{else}. The \c{else} macro has to preserve the context on the stack, in order to have the \c{%$ifnot} referred to by the \c{if} macro be the same as the one defined by the \c{endif} macro, but has to change the context's name so that \c{endif} will know there was an intervening \c{else}. It does this by the use of \c{%repl}. A sample usage of these macros might look like: \c cmp ax,bx \c \c if ae \c cmp bx,cx \c \c if ae \c mov ax,cx \c else \c mov ax,bx \c endif \c \c else \c cmp ax,cx \c \c if ae \c mov ax,cx \c endif \c \c endif The block-\c{IF} macros handle nesting quite happily, by means of pushing another context, describing the inner \c{if}, on top of the one describing the outer \c{if}; thus \c{else} and \c{endif} always refer to the last unmatched \c{if} or \c{else}. \H{stackrel} \i{Stack Relative Preprocessor Directives} The following preprocessor directives provide a way to use labels to refer to local variables allocated on the stack. \b\c{%arg} (see \k{arg}) \b\c{%stacksize} (see \k{stacksize}) \b\c{%local} (see \k{local}) \S{arg} \i\c{%arg} Directive The \c{%arg} directive is used to simplify the handling of parameters passed on the stack. Stack based parameter passing is used by many high level languages, including C, C++ and Pascal. While NASM has macros which attempt to duplicate this functionality (see \k{16cmacro}), the syntax is not particularly convenient to use and is not TASM compatible. Here is an example which shows the use of \c{%arg} without any external macros: \c some_function: \c \c %push mycontext ; save the current context \c %stacksize large ; tell NASM to use bp \c %arg i:word, j_ptr:word \c \c mov ax,[i] \c mov bx,[j_ptr] \c add ax,[bx] \c ret \c \c %pop ; restore original context This is similar to the procedure defined in \k{16cmacro} and adds the value in i to the value pointed to by j_ptr and returns the sum in the ax register. See \k{pushpop} for an explanation of \c{push} and \c{pop} and the use of context stacks. \S{stacksize} \i\c{%stacksize} Directive The \c{%stacksize} directive is used in conjunction with the \c{%arg} (see \k{arg}) and the \c{%local} (see \k{local}) directives. It tells NASM the default size to use for subsequent \c{%arg} and \c{%local} directives. The \c{%stacksize} directive takes one required argument which is one of \c{flat}, \c{flat64}, \c{large} or \c{small}. \c %stacksize flat This form causes NASM to use stack-based parameter addressing relative to \c{ebp} and it assumes that a near form of call was used to get to this label (i.e. that \c{eip} is on the stack). \c %stacksize flat64 This form causes NASM to use stack-based parameter addressing relative to \c{rbp} and it assumes that a near form of call was used to get to this label (i.e. that \c{rip} is on the stack). \c %stacksize large This form uses \c{bp} to do stack-based parameter addressing and assumes that a far form of call was used to get to this address (i.e. that \c{ip} and \c{cs} are on the stack). \c %stacksize small This form also uses \c{bp} to address stack parameters, but it is different from \c{large} because it also assumes that the old value of bp is pushed onto the stack (i.e. it expects an \c{ENTER} instruction). In other words, it expects that \c{bp}, \c{ip} and \c{cs} are on the top of the stack, underneath any local space which may have been allocated by \c{ENTER}. This form is probably most useful when used in combination with the \c{%local} directive (see \k{local}). \S{local} \i\c{%local} Directive The \c{%local} directive is used to simplify the use of local temporary stack variables allocated in a stack frame. Automatic local variables in C are an example of this kind of variable. The \c{%local} directive is most useful when used with the \c{%stacksize} (see \k{stacksize} and is also compatible with the \c{%arg} directive (see \k{arg}). It allows simplified reference to variables on the stack which have been allocated typically by using the \c{ENTER} instruction. \# (see \k{insENTER} for a description of that instruction). An example of its use is the following: \c silly_swap: \c \c %push mycontext ; save the current context \c %stacksize small ; tell NASM to use bp \c %assign %$localsize 0 ; see text for explanation \c %local old_ax:word, old_dx:word \c \c enter %$localsize,0 ; see text for explanation \c mov [old_ax],ax ; swap ax & bx \c mov [old_dx],dx ; and swap dx & cx \c mov ax,bx \c mov dx,cx \c mov bx,[old_ax] \c mov cx,[old_dx] \c leave ; restore old bp \c ret ; \c \c %pop ; restore original context The \c{%$localsize} variable is used internally by the \c{%local} directive and \e{must} be defined within the current context before the \c{%local} directive may be used. Failure to do so will result in one expression syntax error for each \c{%local} variable declared. It then may be used in the construction of an appropriately sized ENTER instruction as shown in the example. \H{pperror} Reporting \i{User-Defined Errors}: \i\c{%error}, \i\c{%warning}, \i\c{%fatal} The preprocessor directive \c{%error} will cause NASM to report an error if it occurs in assembled code. So if other users are going to try to assemble your source files, you can ensure that they define the right macros by means of code like this: \c %ifdef F1 \c ; do some setup \c %elifdef F2 \c ; do some different setup \c %else \c %error "Neither F1 nor F2 was defined." \c %endif Then any user who fails to understand the way your code is supposed to be assembled will be quickly warned of their mistake, rather than having to wait until the program crashes on being run and then not knowing what went wrong. Similarly, \c{%warning} issues a warning, but allows assembly to continue: \c %ifdef F1 \c ; do some setup \c %elifdef F2 \c ; do some different setup \c %else \c %warning "Neither F1 nor F2 was defined, assuming F1." \c %define F1 \c %endif \c{%error} and \c{%warning} are issued only on the final assembly pass. This makes them safe to use in conjunction with tests that depend on symbol values. \c{%fatal} terminates assembly immediately, regardless of pass. This is useful when there is no point in continuing the assembly further, and doing so is likely just going to cause a spew of confusing error messages. It is optional for the message string after \c{%error}, \c{%warning} or \c{%fatal} to be quoted. If it is \e{not}, then single-line macros are expanded in it, which can be used to display more information to the user. For example: \c %if foo > 64 \c %assign foo_over foo-64 \c %error foo is foo_over bytes too large \c %endif \H{otherpreproc} \i{Other Preprocessor Directives} NASM also has preprocessor directives which allow access to information from external sources. Currently they include: \b\c{%line} enables NASM to correctly handle the output of another preprocessor (see \k{line}). \b\c{%!} enables NASM to read in the value of an environment variable, which can then be used in your program (see \k{getenv}). \S{line} \i\c{%line} Directive The \c{%line} directive is used to notify NASM that the input line corresponds to a specific line number in another file. Typically this other file would be an original source file, with the current NASM input being the output of a pre-processor. The \c{%line} directive allows NASM to output messages which indicate the line number of the original source file, instead of the file that is being read by NASM. This preprocessor directive is not generally of use to programmers, by may be of interest to preprocessor authors. The usage of the \c{%line} preprocessor directive is as follows: \c %line nnn[+mmm] [filename] In this directive, \c{nnn} identifies the line of the original source file which this line corresponds to. \c{mmm} is an optional parameter which specifies a line increment value; each line of the input file read in is considered to correspond to \c{mmm} lines of the original source file. Finally, \c{filename} is an optional parameter which specifies the file name of the original source file. After reading a \c{%line} preprocessor directive, NASM will report all file name and line numbers relative to the values specified therein. \S{getenv} \i\c{%!}\e{variable}: Read an Environment Variable. The \c{%!}\e{variable} directive makes it possible to read the value of an environment variable at assembly time. This could, for example, be used to store the contents of an environment variable into a string, which could be used at some other point in your code. For example, suppose that you have an environment variable \c{FOO}, and you want the contents of \c{FOO} to be embedded in your program as a quoted string. You could do that as follows: \c %defstr FOO %!FOO See \k{defstr} for notes on the \c{%defstr} directive. If the name of the environment variable contains non-identifier characters, you can use string quotes to surround the name of the variable, for example: \c %defstr C_colon %!'C:' \H{stdmac} \i{Standard Macros} NASM defines a set of standard macros, which are already defined when it starts to process any source file. If you really need a program to be assembled with no pre-defined macros, you can use the \i\c{%clear} directive to empty the preprocessor of everything but context-local preprocessor variables and single-line macros. Most \i{user-level assembler directives} (see \k{directive}) are implemented as macros which invoke primitive directives; these are described in \k{directive}. The rest of the standard macro set is described here. \S{stdmacver} \i{NASM Version} Macros The single-line macros \i\c{__NASM_MAJOR__}, \i\c{__NASM_MINOR__}, \i\c{__NASM_SUBMINOR__} and \i\c{___NASM_PATCHLEVEL__} expand to the major, minor, subminor and patch level parts of the \i{version number of NASM} being used. So, under NASM 0.98.32p1 for example, \c{__NASM_MAJOR__} would be defined to be 0, \c{__NASM_MINOR__} would be defined as 98, \c{__NASM_SUBMINOR__} would be defined to 32, and \c{___NASM_PATCHLEVEL__} would be defined as 1. Additionally, the macro \i\c{__NASM_SNAPSHOT__} is defined for automatically generated snapshot releases \e{only}. \S{stdmacverid} \i\c{__NASM_VERSION_ID__}: \i{NASM Version ID} The single-line macro \c{__NASM_VERSION_ID__} expands to a dword integer representing the full version number of the version of nasm being used. The value is the equivalent to \c{__NASM_MAJOR__}, \c{__NASM_MINOR__}, \c{__NASM_SUBMINOR__} and \c{___NASM_PATCHLEVEL__} concatenated to produce a single doubleword. Hence, for 0.98.32p1, the returned number would be equivalent to: \c dd 0x00622001 or \c db 1,32,98,0 Note that the above lines are generate exactly the same code, the second line is used just to give an indication of the order that the separate values will be present in memory. \S{stdmacverstr} \i\c{__NASM_VER__}: \i{NASM Version string} The single-line macro \c{__NASM_VER__} expands to a string which defines the version number of nasm being used. So, under NASM 0.98.32 for example, \c db __NASM_VER__ would expand to \c db "0.98.32" \S{fileline} \i\c{__FILE__} and \i\c{__LINE__}: File Name and Line Number Like the C preprocessor, NASM allows the user to find out the file name and line number containing the current instruction. The macro \c{__FILE__} expands to a string constant giving the name of the current input file (which may change through the course of assembly if \c{%include} directives are used), and \c{__LINE__} expands to a numeric constant giving the current line number in the input file. These macros could be used, for example, to communicate debugging information to a macro, since invoking \c{__LINE__} inside a macro definition (either single-line or multi-line) will return the line number of the macro \e{call}, rather than \e{definition}. So to determine where in a piece of code a crash is occurring, for example, one could write a routine \c{stillhere}, which is passed a line number in \c{EAX} and outputs something like `line 155: still here'. You could then write a macro \c %macro notdeadyet 0 \c \c push eax \c mov eax,__LINE__ \c call stillhere \c pop eax \c \c %endmacro and then pepper your code with calls to \c{notdeadyet} until you find the crash point. \S{bitsm} \i\c{__BITS__}: Current BITS Mode The \c{__BITS__} standard macro is updated every time that the BITS mode is set using the \c{BITS XX} or \c{[BITS XX]} directive, where XX is a valid mode number of 16, 32 or 64. \c{__BITS__} receives the specified mode number and makes it globally available. This can be very useful for those who utilize mode-dependent macros. \S{ofmtm} \i\c{__OUTPUT_FORMAT__}: Current Output Format The \c{__OUTPUT_FORMAT__} standard macro holds the current Output Format, as given by the \c{-f} option or NASM's default. Type \c{nasm -hf} for a list. \c %ifidn __OUTPUT_FORMAT__, win32 \c %define NEWLINE 13, 10 \c %elifidn __OUTPUT_FORMAT__, elf32 \c %define NEWLINE 10 \c %endif \S{datetime} Assembly Date and Time Macros NASM provides a variety of macros that represent the timestamp of the assembly session. \b The \i\c{__DATE__} and \i\c{__TIME__} macros give the assembly date and time as strings, in ISO 8601 format (\c{"YYYY-MM-DD"} and \c{"HH:MM:SS"}, respectively.) \b The \i\c{__DATE_NUM__} and \i\c{__TIME_NUM__} macros give the assembly date and time in numeric form; in the format \c{YYYYMMDD} and \c{HHMMSS} respectively. \b The \i\c{__UTC_DATE__} and \i\c{__UTC_TIME__} macros give the assembly date and time in universal time (UTC) as strings, in ISO 8601 format (\c{"YYYY-MM-DD"} and \c{"HH:MM:SS"}, respectively.) If the host platform doesn't provide UTC time, these macros are undefined. \b The \i\c{__UTC_DATE_NUM__} and \i\c{__UTC_TIME_NUM__} macros give the assembly date and time universal time (UTC) in numeric form; in the format \c{YYYYMMDD} and \c{HHMMSS} respectively. If the host platform doesn't provide UTC time, these macros are undefined. \b The \c{__POSIX_TIME__} macro is defined as a number containing the number of seconds since the POSIX epoch, 1 January 1970 00:00:00 UTC; excluding any leap seconds. This is computed using UTC time if available on the host platform, otherwise it is computed using the local time as if it was UTC. All instances of time and date macros in the same assembly session produce consistent output. For example, in an assembly session started at 42 seconds after midnight on January 1, 2010 in Moscow (timezone UTC+3) these macros would have the following values, assuming, of course, a properly configured environment with a correct clock: \c __DATE__ "2010-01-01" \c __TIME__ "00:00:42" \c __DATE_NUM__ 20100101 \c __TIME_NUM__ 000042 \c __UTC_DATE__ "2009-12-31" \c __UTC_TIME__ "21:00:42" \c __UTC_DATE_NUM__ 20091231 \c __UTC_TIME_NUM__ 210042 \c __POSIX_TIME__ 1262293242 \S{use_def} \I\c{__USE_*__}\c{__USE_}\e{package}\c{__}: Package Include Test When a standard macro package (see \k{macropkg}) is included with the \c{%use} directive (see \k{use}), a single-line macro of the form \c{__USE_}\e{package}\c{__} is automatically defined. This allows testing if a particular package is invoked or not. For example, if the \c{altreg} package is included (see \k{pkg_altreg}), then the macro \c{__USE_ALTREG__} is defined. \S{pass_macro} \i\c{__PASS__}: Assembly Pass The macro \c{__PASS__} is defined to be \c{1} on preparatory passes, and \c{2} on the final pass. In preprocess-only mode, it is set to \c{3}, and when running only to generate dependencies (due to the \c{-M} or \c{-MG} option, see \k{opt-M}) it is set to \c{0}. \e{Avoid using this macro if at all possible. It is tremendously easy to generate very strange errors by misusing it, and the semantics may change in future versions of NASM.} \S{struc} \i\c{STRUC} and \i\c{ENDSTRUC}: \i{Declaring Structure} Data Types The core of NASM contains no intrinsic means of defining data structures; instead, the preprocessor is sufficiently powerful that data structures can be implemented as a set of macros. The macros \c{STRUC} and \c{ENDSTRUC} are used to define a structure data type. \c{STRUC} takes one or two parameters. The first parameter is the name of the data type. The second, optional parameter is the base offset of the structure. The name of the data type is defined as a symbol with the value of the base offset, and the name of the data type with the suffix \c{_size} appended to it is defined as an \c{EQU} giving the size of the structure. Once \c{STRUC} has been issued, you are defining the structure, and should define fields using the \c{RESB} family of pseudo-instructions, and then invoke \c{ENDSTRUC} to finish the definition. For example, to define a structure called \c{mytype} containing a longword, a word, a byte and a string of bytes, you might code \c struc mytype \c \c mt_long: resd 1 \c mt_word: resw 1 \c mt_byte: resb 1 \c mt_str: resb 32 \c \c endstruc The above code defines six symbols: \c{mt_long} as 0 (the offset from the beginning of a \c{mytype} structure to the longword field), \c{mt_word} as 4, \c{mt_byte} as 6, \c{mt_str} as 7, \c{mytype_size} as 39, and \c{mytype} itself as zero. The reason why the structure type name is defined at zero by default is a side effect of allowing structures to work with the local label mechanism: if your structure members tend to have the same names in more than one structure, you can define the above structure like this: \c struc mytype \c \c .long: resd 1 \c .word: resw 1 \c .byte: resb 1 \c .str: resb 32 \c \c endstruc This defines the offsets to the structure fields as \c{mytype.long}, \c{mytype.word}, \c{mytype.byte} and \c{mytype.str}. NASM, since it has no \e{intrinsic} structure support, does not support any form of period notation to refer to the elements of a structure once you have one (except the above local-label notation), so code such as \c{mov ax,[mystruc.mt_word]} is not valid. \c{mt_word} is a constant just like any other constant, so the correct syntax is \c{mov ax,[mystruc+mt_word]} or \c{mov ax,[mystruc+mytype.word]}. Sometimes you only have the address of the structure displaced by an offset. For example, consider this standard stack frame setup: \c push ebp \c mov ebp, esp \c sub esp, 40 In this case, you could access an element by subtracting the offset: \c mov [ebp - 40 + mytype.word], ax However, if you do not want to repeat this offset, you can use -40 as a base offset: \c struc mytype, -40 And access an element this way: \c mov [ebp + mytype.word], ax \S{istruc} \i\c{ISTRUC}, \i\c{AT} and \i\c{IEND}: Declaring \i{Instances of Structures} Having defined a structure type, the next thing you typically want to do is to declare instances of that structure in your data segment. NASM provides an easy way to do this in the \c{ISTRUC} mechanism. To declare a structure of type \c{mytype} in a program, you code something like this: \c mystruc: \c istruc mytype \c \c at mt_long, dd 123456 \c at mt_word, dw 1024 \c at mt_byte, db 'x' \c at mt_str, db 'hello, world', 13, 10, 0 \c \c iend The function of the \c{AT} macro is to make use of the \c{TIMES} prefix to advance the assembly position to the correct point for the specified structure field, and then to declare the specified data. Therefore the structure fields must be declared in the same order as they were specified in the structure definition. If the data to go in a structure field requires more than one source line to specify, the remaining source lines can easily come after the \c{AT} line. For example: \c at mt_str, db 123,134,145,156,167,178,189 \c db 190,100,0 Depending on personal taste, you can also omit the code part of the \c{AT} line completely, and start the structure field on the next line: \c at mt_str \c db 'hello, world' \c db 13,10,0 \S{align} \i\c{ALIGN} and \i\c{ALIGNB}: Data Alignment The \c{ALIGN} and \c{ALIGNB} macros provides a convenient way to align code or data on a word, longword, paragraph or other boundary. (Some assemblers call this directive \i\c{EVEN}.) The syntax of the \c{ALIGN} and \c{ALIGNB} macros is \c align 4 ; align on 4-byte boundary \c align 16 ; align on 16-byte boundary \c align 8,db 0 ; pad with 0s rather than NOPs \c align 4,resb 1 ; align to 4 in the BSS \c alignb 4 ; equivalent to previous line Both macros require their first argument to be a power of two; they both compute the number of additional bytes required to bring the length of the current section up to a multiple of that power of two, and then apply the \c{TIMES} prefix to their second argument to perform the alignment. If the second argument is not specified, the default for \c{ALIGN} is \c{NOP}, and the default for \c{ALIGNB} is \c{RESB 1}. So if the second argument is specified, the two macros are equivalent. Normally, you can just use \c{ALIGN} in code and data sections and \c{ALIGNB} in BSS sections, and never need the second argument except for special purposes. \c{ALIGN} and \c{ALIGNB}, being simple macros, perform no error checking: they cannot warn you if their first argument fails to be a power of two, or if their second argument generates more than one byte of code. In each of these cases they will silently do the wrong thing. \c{ALIGNB} (or \c{ALIGN} with a second argument of \c{RESB 1}) can be used within structure definitions: \c struc mytype2 \c \c mt_byte: \c resb 1 \c alignb 2 \c mt_word: \c resw 1 \c alignb 4 \c mt_long: \c resd 1 \c mt_str: \c resb 32 \c \c endstruc This will ensure that the structure members are sensibly aligned relative to the base of the structure. A final caveat: \c{ALIGN} and \c{ALIGNB} work relative to the beginning of the \e{section}, not the beginning of the address space in the final executable. Aligning to a 16-byte boundary when the section you're in is only guaranteed to be aligned to a 4-byte boundary, for example, is a waste of effort. Again, NASM does not check that the section's alignment characteristics are sensible for the use of \c{ALIGN} or \c{ALIGNB}. Both \c{ALIGN} and \c{ALIGNB} do call \c{SECTALIGN} macro implicitly. See \k{sectalign} for details. See also the \c{smartalign} standard macro package, \k{pkg_smartalign}. \S{sectalign} \i\c{SECTALIGN}: Section Alignment The \c{SECTALIGN} macros provides a way to modify alignment attribute of output file section. Unlike the \c{align=} attribute (which is allowed at section definition only) the \c{SECTALIGN} macro may be used at any time. For example the directive \c SECTALIGN 16 sets the section alignment requirements to 16 bytes. Once increased it can not be decreased, the magnitude may grow only. Note that \c{ALIGN} (see \k{align}) calls the \c{SECTALIGN} macro implicitly so the active section alignment requirements may be updated. This is by default behaviour, if for some reason you want the \c{ALIGN} do not call \c{SECTALIGN} at all use the directive \c SECTALIGN OFF It is still possible to turn in on again by \c SECTALIGN ON \C{macropkg} \i{Standard Macro Packages} The \i\c{%use} directive (see \k{use}) includes one of the standard macro packages included with the NASM distribution and compiled into the NASM binary. It operates like the \c{%include} directive (see \k{include}), but the included contents is provided by NASM itself. The names of standard macro packages are case insensitive, and can be quoted or not. \H{pkg_altreg} \i\c{altreg}: \i{Alternate Register Names} The \c{altreg} standard macro package provides alternate register names. It provides numeric register names for all registers (not just \c{R8}-\c{R15}), the Intel-defined aliases \c{R8L}-\c{R15L} for the low bytes of register (as opposed to the NASM/AMD standard names \c{R8B}-\c{R15B}), and the names \c{R0H}-\c{R3H} (by analogy with \c{R0L}-\c{R3L}) for \c{AH}, \c{CH}, \c{DH}, and \c{BH}. Example use: \c %use altreg \c \c proc: \c mov r0l,r3h ; mov al,bh \c ret See also \k{reg64}. \H{pkg_smartalign} \i\c{smartalign}\I{align, smart}: Smart \c{ALIGN} Macro The \c{smartalign} standard macro package provides for an \i\c{ALIGN} macro which is more powerful than the default (and backwards-compatible) one (see \k{align}). When the \c{smartalign} package is enabled, when \c{ALIGN} is used without a second argument, NASM will generate a sequence of instructions more efficient than a series of \c{NOP}. Furthermore, if the padding exceeds a specific threshold, then NASM will generate a jump over the entire padding sequence. The specific instructions generated can be controlled with the new \i\c{ALIGNMODE} macro. This macro takes two parameters: one mode, and an optional jump threshold override. If (for any reason) you need to turn off the jump completely just set jump threshold value to -1 (or set it to \c{nojmp}). The following modes are possible: \b \c{generic}: Works on all x86 CPUs and should have reasonable performance. The default jump threshold is 8. This is the default. \b \c{nop}: Pad out with \c{NOP} instructions. The only difference compared to the standard \c{ALIGN} macro is that NASM can still jump over a large padding area. The default jump threshold is 16. \b \c{k7}: Optimize for the AMD K7 (Athlon/Althon XP). These instructions should still work on all x86 CPUs. The default jump threshold is 16. \b \c{k8}: Optimize for the AMD K8 (Opteron/Althon 64). These instructions should still work on all x86 CPUs. The default jump threshold is 16. \b \c{p6}: Optimize for Intel CPUs. This uses the long \c{NOP} instructions first introduced in Pentium Pro. This is incompatible with all CPUs of family 5 or lower, as well as some VIA CPUs and several virtualization solutions. The default jump threshold is 16. The macro \i\c{__ALIGNMODE__} is defined to contain the current alignment mode. A number of other macros beginning with \c{__ALIGN_} are used internally by this macro package. \H{pkg_fp} \i\c\{fp}: Floating-point macros This packages contains the following floating-point convenience macros: \c %define Inf __Infinity__ \c %define NaN __QNaN__ \c %define QNaN __QNaN__ \c %define SNaN __SNaN__ \c \c %define float8(x) __float8__(x) \c %define float16(x) __float16__(x) \c %define float32(x) __float32__(x) \c %define float64(x) __float64__(x) \c %define float80m(x) __float80m__(x) \c %define float80e(x) __float80e__(x) \c %define float128l(x) __float128l__(x) \c %define float128h(x) __float128h__(x) \H{pkg_ifunc} \i\c{ifunc}: \i{Integer functions} This package contains a set of macros which implement integer functions. These are actually implemented as special operators, but are most conveniently accessed via this macro package. The macros provided are: \S{ilog2} \i{Integer logarithms} These functions calculate the integer logarithm base 2 of their argument, considered as an unsigned integer. The only differences between the functions is their respective behavior if the argument provided is not a power of two. The function \i\c{ilog2e()} (alias \i\c{ilog2()}) generates an error if the argument is not a power of two. The function \i\c{ilog2f()} rounds the argument down to the nearest power of two; if the argument is zero it returns zero. The function \i\c{ilog2c()} rounds the argument up to the nearest power of two. The functions \i\c{ilog2fw()} (alias \i\c{ilog2w()}) and \i\c{ilog2cw()} generate a warning if the argument is not a power of two, but otherwise behaves like \c{ilog2f()} and \c{ilog2c()}, respectively. \C{directive} \i{Assembler Directives} NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM, is nevertheless forced to support a \e{few} directives. These are described in this chapter. NASM's directives come in two types: \I{user-level directives}\e{user-level} directives and \I{primitive directives}\e{primitive} directives. Typically, each directive has a user-level form and a primitive form. In almost all cases, we recommend that users use the user-level forms of the directives, which are implemented as macros which call the primitive forms. Primitive directives are enclosed in square brackets; user-level directives are not. In addition to the universal directives described in this chapter, each object file format can optionally supply extra directives in order to control particular features of that file format. These \I{format-specific directives}\e{format-specific} directives are documented along with the formats that implement them, in \k{outfmt}. \H{bits} \i\c{BITS}: Specifying Target \i{Processor Mode} The \c{BITS} directive specifies whether NASM should generate code \I{16-bit mode, versus 32-bit mode}designed to run on a processor operating in 16-bit mode, 32-bit mode or 64-bit mode. The syntax is \c{BITS XX}, where XX is 16, 32 or 64. In most cases, you should not need to use \c{BITS} explicitly. The \c{aout}, \c{coff}, \c{elf}, \c{macho}, \c{win32} and \c{win64} object formats, which are designed for use in 32-bit or 64-bit operating systems, all cause NASM to select 32-bit or 64-bit mode, respectively, by default. The \c{obj} object format allows you to specify each segment you define as either \c{USE16} or \c{USE32}, and NASM will set its operating mode accordingly, so the use of the \c{BITS} directive is once again unnecessary. The most likely reason for using the \c{BITS} directive is to write 32-bit or 64-bit code in a flat binary file; this is because the \c{bin} output format defaults to 16-bit mode in anticipation of it being used most frequently to write DOS \c{.COM} programs, DOS \c{.SYS} device drivers and boot loader software. The \c{BITS} directive can also be used to generate code for a different mode than the standard one for the output format. You do \e{not} need to specify \c{BITS 32} merely in order to use 32-bit instructions in a 16-bit DOS program; if you do, the assembler will generate incorrect code because it will be writing code targeted at a 32-bit platform, to be run on a 16-bit one. When NASM is in \c{BITS 16} mode, instructions which use 32-bit data are prefixed with an 0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In \c{BITS 32} mode, the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working on 16-bit addresses need an 0x67. When NASM is in \c{BITS 64} mode, most instructions operate the same as they do for \c{BITS 32} mode. However, there are 8 more general and SSE registers, and 16-bit addressing is no longer supported. The default address size is 64 bits; 32-bit addressing can be selected with the 0x67 prefix. The default operand size is still 32 bits, however, and the 0x66 prefix selects 16-bit operand size. The \c{REX} prefix is used both to select 64-bit operand size, and to access the new registers. NASM automatically inserts REX prefixes when necessary. When the \c{REX} prefix is used, the processor does not know how to address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead, it is possible to access the the low 8-bits of the SP, BP SI and DI registers as SPL, BPL, SIL and DIL, respectively; but only when the REX prefix is used. The \c{BITS} directive has an exactly equivalent primitive form, \c{[BITS 16]}, \c{[BITS 32]} and \c{[BITS 64]}. The user-level form is a macro which has no function other than to call the primitive form. Note that the space is neccessary, e.g. \c{BITS32} will \e{not} work! \S{USE16 & USE32} \i\c{USE16} & \i\c{USE32}: Aliases for BITS The `\c{USE16}' and `\c{USE32}' directives can be used in place of `\c{BITS 16}' and `\c{BITS 32}', for compatibility with other assemblers. \H{default} \i\c{DEFAULT}: Change the assembler defaults The \c{DEFAULT} directive changes the assembler defaults. Normally, NASM defaults to a mode where the programmer is expected to explicitly specify most features directly. However, this is occasionally obnoxious, as the explicit form is pretty much the only one one wishes to use. Currently, \c{DEFAULT} can set \c{REL} & \c{ABS} and \c{BND} & \c{NOBND}. \S{REL & ABS} \i\c{REL} & \i\c{ABS}: RIP-relative addressing This sets whether registerless instructions in 64-bit mode are \c{RIP}-relative or not. By default, they are absolute unless overridden with the \i\c{REL} specifier (see \k{effaddr}). However, if \c{DEFAULT REL} is specified, \c{REL} is default, unless overridden with the \c{ABS} specifier, \e{except when used with an FS or GS segment override}. The special handling of \c{FS} and \c{GS} overrides are due to the fact that these registers are generally used as thread pointers or other special functions in 64-bit mode, and generating \c{RIP}-relative addresses would be extremely confusing. \c{DEFAULT REL} is disabled with \c{DEFAULT ABS}. \S{BND & NOBND} \i\c{BND} & \i\c{NOBND}: \c{BND} prefix If \c{DEFAULT BND} is set, all bnd-prefix available instructions following this directive are prefixed with bnd. To override it, \c{NOBND} prefix can be used. \c DEFAULT BND \c call foo ; BND will be prefixed \c nobnd call foo ; BND will NOT be prefixed \c{DEFAULT NOBND} can disable \c{DEFAULT BND} and then \c{BND} prefix will be added only when explicitly specified in code. \c{DEFAULT BND} is expected to be the normal configuration for writing MPX-enabled code. \H{section} \i\c{SECTION} or \i\c{SEGMENT}: Changing and \i{Defining Sections} \I{changing sections}\I{switching between sections}The \c{SECTION} directive (\c{SEGMENT} is an exactly equivalent synonym) changes which section of the output file the code you write will be assembled into. In some object file formats, the number and names of sections are fixed; in others, the user may make up as many as they wish. Hence \c{SECTION} may sometimes give an error message, or may define a new section, if you try to switch to a section that does not (yet) exist. The Unix object formats, and the \c{bin} object format (but see \k{multisec}), all support the \i{standardized section names} \c{.text}, \c{.data} and \c{.bss} for the code, data and uninitialized-data sections. The \c{obj} format, by contrast, does not recognize these section names as being special, and indeed will strip off the leading period of any section name that has one. \S{sectmac} The \i\c{__SECT__} Macro The \c{SECTION} directive is unusual in that its user-level form functions differently from its primitive form. The primitive form, \c{[SECTION xyz]}, simply switches the current target section to the one given. The user-level form, \c{SECTION xyz}, however, first defines the single-line macro \c{__SECT__} to be the primitive \c{[SECTION]} directive which it is about to issue, and then issues it. So the user-level directive \c SECTION .text expands to the two lines \c %define __SECT__ [SECTION .text] \c [SECTION .text] Users may find it useful to make use of this in their own macros. For example, the \c{writefile} macro defined in \k{mlmacgre} can be usefully rewritten in the following more sophisticated form: \c %macro writefile 2+ \c \c [section .data] \c \c %%str: db %2 \c %%endstr: \c \c __SECT__ \c \c mov dx,%%str \c mov cx,%%endstr-%%str \c mov bx,%1 \c mov ah,0x40 \c int 0x21 \c \c %endmacro This form of the macro, once passed a string to output, first switches temporarily to the data section of the file, using the primitive form of the \c{SECTION} directive so as not to modify \c{__SECT__}. It then declares its string in the data section, and then invokes \c{__SECT__} to switch back to \e{whichever} section the user was previously working in. It thus avoids the need, in the previous version of the macro, to include a \c{JMP} instruction to jump over the data, and also does not fail if, in a complicated \c{OBJ} format module, the user could potentially be assembling the code in any of several separate code sections. \H{absolute} \i\c{ABSOLUTE}: Defining Absolute Labels The \c{ABSOLUTE} directive can be thought of as an alternative form of \c{SECTION}: it causes the subsequent code to be directed at no physical section, but at the hypothetical section starting at the given absolute address. The only instructions you can use in this mode are the \c{RESB} family. \c{ABSOLUTE} is used as follows: \c absolute 0x1A \c \c kbuf_chr resw 1 \c kbuf_free resw 1 \c kbuf resw 16 This example describes a section of the PC BIOS data area, at segment address 0x40: the above code defines \c{kbuf_chr} to be 0x1A, \c{kbuf_free} to be 0x1C, and \c{kbuf} to be 0x1E. The user-level form of \c{ABSOLUTE}, like that of \c{SECTION}, redefines the \i\c{__SECT__} macro when it is invoked. \i\c{STRUC} and \i\c{ENDSTRUC} are defined as macros which use \c{ABSOLUTE} (and also \c{__SECT__}). \c{ABSOLUTE} doesn't have to take an absolute constant as an argument: it can take an expression (actually, a \i{critical expression}: see \k{crit}) and it can be a value in a segment. For example, a TSR can re-use its setup code as run-time BSS like this: \c org 100h ; it's a .COM program \c \c jmp setup ; setup code comes last \c \c ; the resident part of the TSR goes here \c setup: \c ; now write the code that installs the TSR here \c \c absolute setup \c \c runtimevar1 resw 1 \c runtimevar2 resd 20 \c \c tsr_end: This defines some variables `on top of' the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol `tsr_end' can be used to calculate the total size of the part of the TSR that needs to be made resident. \H{extern} \i\c{EXTERN}: \i{Importing Symbols} from Other Modules \c{EXTERN} is similar to the MASM directive \c{EXTRN} and the C keyword \c{extern}: it is used to declare a symbol which is not defined anywhere in the module being assembled, but is assumed to be defined in some other module and needs to be referred to by this one. Not every object-file format can support external variables: the \c{bin} format cannot. The \c{EXTERN} directive takes as many arguments as you like. Each argument is the name of a symbol: \c extern _printf \c extern _sscanf,_fscanf Some object-file formats provide extra features to the \c{EXTERN} directive. In all cases, the extra features are used by suffixing a colon to the symbol name followed by object-format specific text. For example, the \c{obj} format allows you to declare that the default segment base of an external should be the group \c{dgroup} by means of the directive \c extern _variable:wrt dgroup The primitive form of \c{EXTERN} differs from the user-level form only in that it can take only one argument at a time: the support for multiple arguments is implemented at the preprocessor level. You can declare the same variable as \c{EXTERN} more than once: NASM will quietly ignore the second and later redeclarations. You can't declare a variable as \c{EXTERN} as well as something else, though. \H{global} \i\c{GLOBAL}: \i{Exporting Symbols} to Other Modules \c{GLOBAL} is the other end of \c{EXTERN}: if one module declares a symbol as \c{EXTERN} and refers to it, then in order to prevent linker errors, some other module must actually \e{define} the symbol and declare it as \c{GLOBAL}. Some assemblers use the name \i\c{PUBLIC} for this purpose. The \c{GLOBAL} directive applying to a symbol must appear \e{before} the definition of the symbol. \c{GLOBAL} uses the same syntax as \c{EXTERN}, except that it must refer to symbols which \e{are} defined in the same module as the \c{GLOBAL} directive. For example: \c global _main \c _main: \c ; some code \c{GLOBAL}, like \c{EXTERN}, allows object formats to define private extensions by means of a colon. The \c{elf} object format, for example, lets you specify whether global data items are functions or data: \c global hashlookup:function, hashtable:data Like \c{EXTERN}, the primitive form of \c{GLOBAL} differs from the user-level form only in that it can take only one argument at a time. \H{common} \i\c{COMMON}: Defining Common Data Areas The \c{COMMON} directive is used to declare \i\e{common variables}. A common variable is much like a global variable declared in the uninitialized data section, so that \c common intvar 4 is similar in function to \c global intvar \c section .bss \c \c intvar resd 1 The difference is that if more than one module defines the same common variable, then at link time those variables will be \e{merged}, and references to \c{intvar} in all modules will point at the same piece of memory. Like \c{GLOBAL} and \c{EXTERN}, \c{COMMON} supports object-format specific extensions. For example, the \c{obj} format allows common variables to be NEAR or FAR, and the \c{elf} format allows you to specify the alignment requirements of a common variable: \c common commvar 4:near ; works in OBJ \c common intarray 100:4 ; works in ELF: 4 byte aligned Once again, like \c{EXTERN} and \c{GLOBAL}, the primitive form of \c{COMMON} differs from the user-level form only in that it can take only one argument at a time. \H{CPU} \i\c{CPU}: Defining CPU Dependencies The \i\c{CPU} directive restricts assembly to those instructions which are available on the specified CPU. Options are: \b\c{CPU 8086} Assemble only 8086 instruction set \b\c{CPU 186} Assemble instructions up to the 80186 instruction set \b\c{CPU 286} Assemble instructions up to the 286 instruction set \b\c{CPU 386} Assemble instructions up to the 386 instruction set \b\c{CPU 486} 486 instruction set \b\c{CPU 586} Pentium instruction set \b\c{CPU PENTIUM} Same as 586 \b\c{CPU 686} P6 instruction set \b\c{CPU PPRO} Same as 686 \b\c{CPU P2} Same as 686 \b\c{CPU P3} Pentium III (Katmai) instruction sets \b\c{CPU KATMAI} Same as P3 \b\c{CPU P4} Pentium 4 (Willamette) instruction set \b\c{CPU WILLAMETTE} Same as P4 \b\c{CPU PRESCOTT} Prescott instruction set \b\c{CPU X64} x86-64 (x64/AMD64/Intel 64) instruction set \b\c{CPU IA64} IA64 CPU (in x86 mode) instruction set All options are case insensitive. All instructions will be selected only if they apply to the selected CPU or lower. By default, all instructions are available. \H{FLOAT} \i\c{FLOAT}: Handling of \I{floating-point, constants}floating-point constants By default, floating-point constants are rounded to nearest, and IEEE denormals are supported. The following options can be set to alter this behaviour: \b\c{FLOAT DAZ} Flush denormals to zero \b\c{FLOAT NODAZ} Do not flush denormals to zero (default) \b\c{FLOAT NEAR} Round to nearest (default) \b\c{FLOAT UP} Round up (toward +Infinity) \b\c{FLOAT DOWN} Round down (toward -Infinity) \b\c{FLOAT ZERO} Round toward zero \b\c{FLOAT DEFAULT} Restore default settings The standard macros \i\c{__FLOAT_DAZ__}, \i\c{__FLOAT_ROUND__}, and \i\c{__FLOAT__} contain the current state, as long as the programmer has avoided the use of the brackeded primitive form, (\c{[FLOAT]}). \c{__FLOAT__} contains the full set of floating-point settings; this value can be saved away and invoked later to restore the setting. \H{asmdir-warning} \i\c{[WARNING]}: Enable or disable warnings The \c{[WARNING]} directive can be used to enable or disable classes of warnings in the same way as the \c{-w} option, see \k{opt-w} for more details about warning classes. \b \c{[warning +}\e{warning-class}\c{]} enables warnings for \e{warning-class}. \b \c{[warning -}\e{warning-class}\c{]} disables warnings for \e{warning-class}. \b \c{[warning *}\e{warning-class}\c{]} restores \e{warning-class} to the original value, either the default value or as specified on the command line. The \c{[WARNING]} directive also accepts the \c{all}, \c{error} and \c{error=}\e{warning-class} specifiers. No "user form" (without the brackets) currently exists. \C{outfmt} \i{Output Formats} NASM is a portable assembler, designed to be able to compile on any ANSI C-supporting platform and produce output to run on a variety of Intel x86 operating systems. For this reason, it has a large number of available output formats, selected using the \i\c{-f} option on the NASM \i{command line}. Each of these formats, along with its extensions to the base NASM syntax, is detailed in this chapter. As stated in \k{opt-o}, NASM chooses a \i{default name} for your output file based on the input file name and the chosen output format. This will be generated by removing the \i{extension} (\c{.asm}, \c{.s}, or whatever you like to use) from the input file name, and substituting an extension defined by the output format. The extensions are given with each format below. \H{binfmt} \i\c{bin}: \i{Flat-Form Binary}\I{pure binary} Output The \c{bin} format does not produce object files: it generates nothing in the output file except the code you wrote. Such `pure binary' files are used by \i{MS-DOS}: \i\c{.COM} executables and \i\c{.SYS} device drivers are pure binary files. Pure binary output is also useful for \i{operating system} and \i{boot loader} development. The \c{bin} format supports \i{multiple section names}. For details of how NASM handles sections in the \c{bin} format, see \k{multisec}. Using the \c{bin} format puts NASM by default into 16-bit mode (see \k{bits}). In order to use \c{bin} to write 32-bit or 64-bit code, such as an OS kernel, you need to explicitly issue the \I\c{BITS}\c{BITS 32} or \I\c{BITS}\c{BITS 64} directive. \c{bin} has no default output file name extension: instead, it leaves your file name as it is once the original extension has been removed. Thus, the default is for NASM to assemble \c{binprog.asm} into a binary file called \c{binprog}. \S{org} \i\c{ORG}: Binary File \i{Program Origin} The \c{bin} format provides an additional directive to the list given in \k{directive}: \c{ORG}. The function of the \c{ORG} directive is to specify the origin address which NASM will assume the program begins at when it is loaded into memory. For example, the following code will generate the longword \c{0x00000104}: \c org 0x100 \c dd label \c label: Unlike the \c{ORG} directive provided by MASM-compatible assemblers, which allows you to jump around in the object file and overwrite code you have already generated, NASM's \c{ORG} does exactly what the directive says: \e{origin}. Its sole function is to specify one offset which is added to all internal address references within the section; it does not permit any of the trickery that MASM's version does. See \k{proborg} for further comments. \S{binseg} \c{bin} Extensions to the \c{SECTION} Directive\I{SECTION, bin extensions to} The \c{bin} output format extends the \c{SECTION} (or \c{SEGMENT}) directive to allow you to specify the alignment requirements of segments. This is done by appending the \i\c{ALIGN} qualifier to the end of the section-definition line. For example, \c section .data align=16 switches to the section \c{.data} and also specifies that it must be aligned on a 16-byte boundary. The parameter to \c{ALIGN} specifies how many low bits of the section start address must be forced to zero. The alignment value given may be any power of two.\I{section alignment, in bin}\I{segment alignment, in bin}\I{alignment, in bin sections} \S{multisec} \i{Multisection}\I{bin, multisection} Support for the \c{bin} Format The \c{bin} format allows the use of multiple sections, of arbitrary names, besides the "known" \c{.text}, \c{.data}, and \c{.bss} names. \b Sections may be designated \i\c{progbits} or \i\c{nobits}. Default is \c{progbits} (except \c{.bss}, which defaults to \c{nobits}, of course). \b Sections can be aligned at a specified boundary following the previous section with \c{align=}, or at an arbitrary byte-granular position with \i\c{start=}. \b Sections can be given a virtual start address, which will be used for the calculation of all memory references within that section with \i\c{vstart=}. \b Sections can be ordered using \i\c{follows=}\c{<section>} or \i\c{vfollows=}\c{<section>} as an alternative to specifying an explicit start address. \b Arguments to \c{org}, \c{start}, \c{vstart}, and \c{align=} are critical expressions. See \k{crit}. E.g. \c{align=(1 << ALIGN_SHIFT)} - \c{ALIGN_SHIFT} must be defined before it is used here. \b Any code which comes before an explicit \c{SECTION} directive is directed by default into the \c{.text} section. \b If an \c{ORG} statement is not given, \c{ORG 0} is used by default. \b The \c{.bss} section will be placed after the last \c{progbits} section, unless \c{start=}, \c{vstart=}, \c{follows=}, or \c{vfollows=} has been specified. \b All sections are aligned on dword boundaries, unless a different alignment has been specified. \b Sections may not overlap. \b NASM creates the \c{section.<secname>.start} for each section, which may be used in your code. \S{map}\i{Map Files} Map files can be generated in \c{-f bin} format by means of the \c{[map]} option. Map types of \c{all} (default), \c{brief}, \c{sections}, \c{segments}, or \c{symbols} may be specified. Output may be directed to \c{stdout} (default), \c{stderr}, or a specified file. E.g. \c{[map symbols myfile.map]}. No "user form" exists, the square brackets must be used. \H{ithfmt} \i\c{ith}: \i{Intel Hex} Output The \c{ith} file format produces Intel hex-format files. Just as the \c{bin} format, this is a flat memory image format with no support for relocation or linking. It is usually used with ROM programmers and similar utilities. All extensions supported by the \c{bin} file format is also supported by the \c{ith} file format. \c{ith} provides a default output file-name extension of \c{.ith}. \H{srecfmt} \i\c{srec}: \i{Motorola S-Records} Output The \c{srec} file format produces Motorola S-records files. Just as the \c{bin} format, this is a flat memory image format with no support for relocation or linking. It is usually used with ROM programmers and similar utilities. All extensions supported by the \c{bin} file format is also supported by the \c{srec} file format. \c{srec} provides a default output file-name extension of \c{.srec}. \H{objfmt} \i\c{obj}: \i{Microsoft OMF}\I{OMF} Object Files The \c{obj} file format (NASM calls it \c{obj} rather than \c{omf} for historical reasons) is the one produced by \i{MASM} and \i{TASM}, which is typically fed to 16-bit DOS linkers to produce \i\c{.EXE} files. It is also the format used by \i{OS/2}. \c{obj} provides a default output file-name extension of \c{.obj}. \c{obj} is not exclusively a 16-bit format, though: NASM has full support for the 32-bit extensions to the format. In particular, 32-bit \c{obj} format files are used by \i{Borland's Win32 compilers}, instead of using Microsoft's newer \i\c{win32} object file format. The \c{obj} format does not define any special segment names: you can call your segments anything you like. Typical names for segments in \c{obj} format files are \c{CODE}, \c{DATA} and \c{BSS}. If your source file contains code before specifying an explicit \c{SEGMENT} directive, then NASM will invent its own segment called \i\c{__NASMDEFSEG} for you. When you define a segment in an \c{obj} file, NASM defines the segment name as a symbol as well, so that you can access the segment address of the segment. So, for example: \c segment data \c \c dvar: dw 1234 \c \c segment code \c \c function: \c mov ax,data ; get segment address of data \c mov ds,ax ; and move it into DS \c inc word [dvar] ; now this reference will work \c ret The \c{obj} format also enables the use of the \i\c{SEG} and \i\c{WRT} operators, so that you can write code which does things like \c extern foo \c \c mov ax,seg foo ; get preferred segment of foo \c mov ds,ax \c mov ax,data ; a different segment \c mov es,ax \c mov ax,[ds:foo] ; this accesses `foo' \c mov [es:foo wrt data],bx ; so does this \S{objseg} \c{obj} Extensions to the \c{SEGMENT} Directive\I{SEGMENT, obj extensions to} The \c{obj} output format extends the \c{SEGMENT} (or \c{SECTION}) directive to allow you to specify various properties of the segment you are defining. This is done by appending extra qualifiers to the end of the segment-definition line. For example, \c segment code private align=16 defines the segment \c{code}, but also declares it to be a private segment, and requires that the portion of it described in this code module must be aligned on a 16-byte boundary. The available qualifiers are: \b \i\c{PRIVATE}, \i\c{PUBLIC}, \i\c{COMMON} and \i\c{STACK} specify the combination characteristics of the segment. \c{PRIVATE} segments do not get combined with any others by the linker; \c{PUBLIC} and \c{STACK} segments get concatenated together at link time; and \c{COMMON} segments all get overlaid on top of each other rather than stuck end-to-end. \b \i\c{ALIGN} is used, as shown above, to specify how many low bits of the segment start address must be forced to zero. The alignment value given may be any power of two from 1 to 4096; in reality, the only values supported are 1, 2, 4, 16, 256 and 4096, so if 8 is specified it will be rounded up to 16, and 32, 64 and 128 will all be rounded up to 256, and so on. Note that alignment to 4096-byte boundaries is a \i{PharLap} extension to the format and may not be supported by all linkers.\I{section alignment, in OBJ}\I{segment alignment, in OBJ}\I{alignment, in OBJ sections} \b \i\c{CLASS} can be used to specify the segment class; this feature indicates to the linker that segments of the same class should be placed near each other in the output file. The class name can be any word, e.g. \c{CLASS=CODE}. \b \i\c{OVERLAY}, like \c{CLASS}, is specified with an arbitrary word as an argument, and provides overlay information to an overlay-capable linker. \b Segments can be declared as \i\c{USE16} or \i\c{USE32}, which has the effect of recording the choice in the object file and also ensuring that NASM's default assembly mode when assembling in that segment is 16-bit or 32-bit respectively. \b When writing \i{OS/2} object files, you should declare 32-bit segments as \i\c{FLAT}, which causes the default segment base for anything in the segment to be the special group \c{FLAT}, and also defines the group if it is not already defined. \b The \c{obj} file format also allows segments to be declared as having a pre-defined absolute segment address, although no linkers are currently known to make sensible use of this feature; nevertheless, NASM allows you to declare a segment such as \c{SEGMENT SCREEN ABSOLUTE=0xB800} if you need to. The \i\c{ABSOLUTE} and \c{ALIGN} keywords are mutually exclusive. NASM's default segment attributes are \c{PUBLIC}, \c{ALIGN=1}, no class, no overlay, and \c{USE16}. \S{group} \i\c{GROUP}: Defining Groups of Segments\I{segments, groups of} The \c{obj} format also allows segments to be grouped, so that a single segment register can be used to refer to all the segments in a group. NASM therefore supplies the \c{GROUP} directive, whereby you can code \c segment data \c \c ; some data \c \c segment bss \c \c ; some uninitialized data \c \c group dgroup data bss which will define a group called \c{dgroup} to contain the segments \c{data} and \c{bss}. Like \c{SEGMENT}, \c{GROUP} causes the group name to be defined as a symbol, so that you can refer to a variable \c{var} in the \c{data} segment as \c{var wrt data} or as \c{var wrt dgroup}, depending on which segment value is currently in your segment register. If you just refer to \c{var}, however, and \c{var} is declared in a segment which is part of a group, then NASM will default to giving you the offset of \c{var} from the beginning of the \e{group}, not the \e{segment}. Therefore \c{SEG var}, also, will return the group base rather than the segment base. NASM will allow a segment to be part of more than one group, but will generate a warning if you do this. Variables declared in a segment which is part of more than one group will default to being relative to the first group that was defined to contain the segment. A group does not have to contain any segments; you can still make \c{WRT} references to a group which does not contain the variable you are referring to. OS/2, for example, defines the special group \c{FLAT} with no segments in it. \S{uppercase} \i\c{UPPERCASE}: Disabling Case Sensitivity in Output Although NASM itself is \i{case sensitive}, some OMF linkers are not; therefore it can be useful for NASM to output single-case object files. The \c{UPPERCASE} format-specific directive causes all segment, group and symbol names that are written to the object file to be forced to upper case just before being written. Within a source file, NASM is still case-sensitive; but the object file can be written entirely in upper case if desired. \c{UPPERCASE} is used alone on a line; it requires no parameters. \S{import} \i\c{IMPORT}: Importing DLL Symbols\I{DLL symbols, importing}\I{symbols, importing from DLLs} The \c{IMPORT} format-specific directive defines a symbol to be imported from a DLL, for use if you are writing a DLL's \i{import library} in NASM. You still need to declare the symbol as \c{EXTERN} as well as using the \c{IMPORT} directive. The \c{IMPORT} directive takes two required parameters, separated by white space, which are (respectively) the name of the symbol you wish to import and the name of the library you wish to import it from. For example: \c import WSAStartup wsock32.dll A third optional parameter gives the name by which the symbol is known in the library you are importing it from, in case this is not the same as the name you wish the symbol to be known by to your code once you have imported it. For example: \c import asyncsel wsock32.dll WSAAsyncSelect \S{export} \i\c{EXPORT}: Exporting DLL Symbols\I{DLL symbols, exporting}\I{symbols, exporting from DLLs} The \c{EXPORT} format-specific directive defines a global symbol to be exported as a DLL symbol, for use if you are writing a DLL in NASM. You still need to declare the symbol as \c{GLOBAL} as well as using the \c{EXPORT} directive. \c{EXPORT} takes one required parameter, which is the name of the symbol you wish to export, as it was defined in your source file. An optional second parameter (separated by white space from the first) gives the \e{external} name of the symbol: the name by which you wish the symbol to be known to programs using the DLL. If this name is the same as the internal name, you may leave the second parameter off. Further parameters can be given to define attributes of the exported symbol. These parameters, like the second, are separated by white space. If further parameters are given, the external name must also be specified, even if it is the same as the internal name. The available attributes are: \b \c{resident} indicates that the exported name is to be kept resident by the system loader. This is an optimisation for frequently used symbols imported by name. \b \c{nodata} indicates that the exported symbol is a function which does not make use of any initialized data. \b \c{parm=NNN}, where \c{NNN} is an integer, sets the number of parameter words for the case in which the symbol is a call gate between 32-bit and 16-bit segments. \b An attribute which is just a number indicates that the symbol should be exported with an identifying number (ordinal), and gives the desired number. For example: \c export myfunc \c export myfunc TheRealMoreFormalLookingFunctionName \c export myfunc myfunc 1234 ; export by ordinal \c export myfunc myfunc resident parm=23 nodata \S{dotdotstart} \i\c{..start}: Defining the \i{Program Entry Point} \c{OMF} linkers require exactly one of the object files being linked to define the program entry point, where execution will begin when the program is run. If the object file that defines the entry point is assembled using NASM, you specify the entry point by declaring the special symbol \c{..start} at the point where you wish execution to begin. \S{objextern} \c{obj} Extensions to the \c{EXTERN} Directive\I{EXTERN, obj extensions to} If you declare an external symbol with the directive \c extern foo then references such as \c{mov ax,foo} will give you the offset of \c{foo} from its preferred segment base (as specified in whichever module \c{foo} is actually defined in). So to access the contents of \c{foo} you will usually need to do something like \c mov ax,seg foo ; get preferred segment base \c mov es,ax ; move it into ES \c mov ax,[es:foo] ; and use offset `foo' from it This is a little unwieldy, particularly if you know that an external is going to be accessible from a given segment or group, say \c{dgroup}. So if \c{DS} already contained \c{dgroup}, you could simply code \c mov ax,[foo wrt dgroup] However, having to type this every time you want to access \c{foo} can be a pain; so NASM allows you to declare \c{foo} in the alternative form \c extern foo:wrt dgroup This form causes NASM to pretend that the preferred segment base of \c{foo} is in fact \c{dgroup}; so the expression \c{seg foo} will now return \c{dgroup}, and the expression \c{foo} is equivalent to \c{foo wrt dgroup}. This \I{default-WRT mechanism}default-\c{WRT} mechanism can be used to make externals appear to be relative to any group or segment in your program. It can also be applied to common variables: see \k{objcommon}. \S{objcommon} \c{obj} Extensions to the \c{COMMON} Directive\I{COMMON, obj extensions to} The \c{obj} format allows common variables to be either near\I{near common variables} or far\I{far common variables}; NASM allows you to specify which your variables should be by the use of the syntax \c common nearvar 2:near ; `nearvar' is a near common \c common farvar 10:far ; and `farvar' is far Far common variables may be greater in size than 64Kb, and so the OMF specification says that they are declared as a number of \e{elements} of a given size. So a 10-byte far common variable could be declared as ten one-byte elements, five two-byte elements, two five-byte elements or one ten-byte element. Some \c{OMF} linkers require the \I{element size, in common variables}\I{common variables, element size}element size, as well as the variable size, to match when resolving common variables declared in more than one module. Therefore NASM must allow you to specify the element size on your far common variables. This is done by the following syntax: \c common c_5by2 10:far 5 ; two five-byte elements \c common c_2by5 10:far 2 ; five two-byte elements If no element size is specified, the default is 1. Also, the \c{FAR} keyword is not required when an element size is specified, since only far commons may have element sizes at all. So the above declarations could equivalently be \c common c_5by2 10:5 ; two five-byte elements \c common c_2by5 10:2 ; five two-byte elements In addition to these extensions, the \c{COMMON} directive in \c{obj} also supports default-\c{WRT} specification like \c{EXTERN} does (explained in \k{objextern}). So you can also declare things like \c common foo 10:wrt dgroup \c common bar 16:far 2:wrt data \c common baz 24:wrt data:6 \H{win32fmt} \i\c{win32}: Microsoft Win32 Object Files The \c{win32} output format generates Microsoft Win32 object files, suitable for passing to Microsoft linkers such as \i{Visual C++}. Note that Borland Win32 compilers do not use this format, but use \c{obj} instead (see \k{objfmt}). \c{win32} provides a default output file-name extension of \c{.obj}. Note that although Microsoft say that Win32 object files follow the \c{COFF} (Common Object File Format) standard, the object files produced by Microsoft Win32 compilers are not compatible with COFF linkers such as DJGPP's, and vice versa. This is due to a difference of opinion over the precise semantics of PC-relative relocations. To produce COFF files suitable for DJGPP, use NASM's \c{coff} output format; conversely, the \c{coff} format does not produce object files that Win32 linkers can generate correct output from. \S{win32sect} \c{win32} Extensions to the \c{SECTION} Directive\I{SECTION, win32 extensions to} Like the \c{obj} format, \c{win32} allows you to specify additional information on the \c{SECTION} directive line, to control the type and properties of sections you declare. Section types and properties are generated automatically by NASM for the \i{standard section names} \c{.text}, \c{.data} and \c{.bss}, but may still be overridden by these qualifiers. The available qualifiers are: \b \c{code}, or equivalently \c{text}, defines the section to be a code section. This marks the section as readable and executable, but not writable, and also indicates to the linker that the type of the section is code. \b \c{data} and \c{bss} define the section to be a data section, analogously to \c{code}. Data sections are marked as readable and writable, but not executable. \c{data} declares an initialized data section, whereas \c{bss} declares an uninitialized data section. \b \c{rdata} declares an initialized data section that is readable but not writable. Microsoft compilers use this section to place constants in it. \b \c{info} defines the section to be an \i{informational section}, which is not included in the executable file by the linker, but may (for example) pass information \e{to} the linker. For example, declaring an \c{info}-type section called \i\c{.drectve} causes the linker to interpret the contents of the section as command-line options. \b \c{align=}, used with a trailing number as in \c{obj}, gives the \I{section alignment, in win32}\I{alignment, in win32 sections}alignment requirements of the section. The maximum you may specify is 64: the Win32 object file format contains no means to request a greater section alignment than this. If alignment is not explicitly specified, the defaults are 16-byte alignment for code sections, 8-byte alignment for rdata sections and 4-byte alignment for data (and BSS) sections. Informational sections get a default alignment of 1 byte (no alignment), though the value does not matter. The defaults assumed by NASM if you do not specify the above qualifiers are: \c section .text code align=16 \c section .data data align=4 \c section .rdata rdata align=8 \c section .bss bss align=4 Any other section name is treated by default like \c{.text}. \S{win32safeseh} \c{win32}: Safe Structured Exception Handling Among other improvements in Windows XP SP2 and Windows Server 2003 Microsoft has introduced concept of "safe structured exception handling." General idea is to collect handlers' entry points in designated read-only table and have alleged entry point verified against this table prior exception control is passed to the handler. In order for an executable module to be equipped with such "safe exception handler table," all object modules on linker command line has to comply with certain criteria. If one single module among them does not, then the table in question is omitted and above mentioned run-time checks will not be performed for application in question. Table omission is by default silent and therefore can be easily overlooked. One can instruct linker to refuse to produce binary without such table by passing \c{/safeseh} command line option. Without regard to this run-time check merits it's natural to expect NASM to be capable of generating modules suitable for \c{/safeseh} linking. From developer's viewpoint the problem is two-fold: \b how to adapt modules not deploying exception handlers of their own; \b how to adapt/develop modules utilizing custom exception handling; Former can be easily achieved with any NASM version by adding following line to source code: \c $@feat.00 equ 1 As of version 2.03 NASM adds this absolute symbol automatically. If it's not already present to be precise. I.e. if for whatever reason developer would choose to assign another value in source file, it would still be perfectly possible. Registering custom exception handler on the other hand requires certain "magic." As of version 2.03 additional directive is implemented, \c{safeseh}, which instructs the assembler to produce appropriately formatted input data for above mentioned "safe exception handler table." Its typical use would be: \c section .text \c extern _MessageBoxA@16 \c %if __NASM_VERSION_ID__ >= 0x02030000 \c safeseh handler ; register handler as "safe handler" \c %endif \c handler: \c push DWORD 1 ; MB_OKCANCEL \c push DWORD caption \c push DWORD text \c push DWORD 0 \c call _MessageBoxA@16 \c sub eax,1 ; incidentally suits as return value \c ; for exception handler \c ret \c global _main \c _main: \c push DWORD handler \c push DWORD [fs:0] \c mov DWORD [fs:0],esp ; engage exception handler \c xor eax,eax \c mov eax,DWORD[eax] ; cause exception \c pop DWORD [fs:0] ; disengage exception handler \c add esp,4 \c ret \c text: db 'OK to rethrow, CANCEL to generate core dump',0 \c caption:db 'SEGV',0 \c \c section .drectve info \c db '/defaultlib:user32.lib /defaultlib:msvcrt.lib ' As you might imagine, it's perfectly possible to produce .exe binary with "safe exception handler table" and yet engage unregistered exception handler. Indeed, handler is engaged by simply manipulating \c{[fs:0]} location at run-time, something linker has no power over, run-time that is. It should be explicitly mentioned that such failure to register handler's entry point with \c{safeseh} directive has undesired side effect at run-time. If exception is raised and unregistered handler is to be executed, the application is abruptly terminated without any notification whatsoever. One can argue that system could at least have logged some kind "non-safe exception handler in x.exe at address n" message in event log, but no, literally no notification is provided and user is left with no clue on what caused application failure. Finally, all mentions of linker in this paragraph refer to Microsoft linker version 7.x and later. Presence of \c{@feat.00} symbol and input data for "safe exception handler table" causes no backward incompatibilities and "safeseh" modules generated by NASM 2.03 and later can still be linked by earlier versions or non-Microsoft linkers. \S{codeview} Debugging formats for Windows \I{Windows debugging formats} The \c{win32} and \c{win64} formats support the Microsoft CodeView debugging format. Currently CodeView version 8 format is supported (\i\c{cv8}), but newer versions of the CodeView debugger should be able to handle this format as well. \H{win64fmt} \i\c{win64}: Microsoft Win64 Object Files The \c{win64} output format generates Microsoft Win64 object files, which is nearly 100% identical to the \c{win32} object format (\k{win32fmt}) with the exception that it is meant to target 64-bit code and the x86-64 platform altogether. This object file is used exactly the same as the \c{win32} object format (\k{win32fmt}), in NASM, with regard to this exception. \S{win64pic} \c{win64}: Writing Position-Independent Code While \c{REL} takes good care of RIP-relative addressing, there is one aspect that is easy to overlook for a Win64 programmer: indirect references. Consider a switch dispatch table: \c jmp qword [dsptch+rax*8] \c ... \c dsptch: dq case0 \c dq case1 \c ... Even a novice Win64 assembler programmer will soon realize that the code is not 64-bit savvy. Most notably linker will refuse to link it with \c 'ADDR32' relocation to '.text' invalid without /LARGEADDRESSAWARE:NO So [s]he will have to split jmp instruction as following: \c lea rbx,[rel dsptch] \c jmp qword [rbx+rax*8] What happens behind the scene is that effective address in \c{lea} is encoded relative to instruction pointer, or in perfectly position-independent manner. But this is only part of the problem! Trouble is that in .dll context \c{caseN} relocations will make their way to the final module and might have to be adjusted at .dll load time. To be specific when it can't be loaded at preferred address. And when this occurs, pages with such relocations will be rendered private to current process, which kind of undermines the idea of sharing .dll. But no worry, it's trivial to fix: \c lea rbx,[rel dsptch] \c add rbx,[rbx+rax*8] \c jmp rbx \c ... \c dsptch: dq case0-dsptch \c dq case1-dsptch \c ... NASM version 2.03 and later provides another alternative, \c{wrt ..imagebase} operator, which returns offset from base address of the current image, be it .exe or .dll module, therefore the name. For those acquainted with PE-COFF format base address denotes start of \c{IMAGE_DOS_HEADER} structure. Here is how to implement switch with these image-relative references: \c lea rbx,[rel dsptch] \c mov eax,[rbx+rax*4] \c sub rbx,dsptch wrt ..imagebase \c add rbx,rax \c jmp rbx \c ... \c dsptch: dd case0 wrt ..imagebase \c dd case1 wrt ..imagebase One can argue that the operator is redundant. Indeed, snippet before last works just fine with any NASM version and is not even Windows specific... The real reason for implementing \c{wrt ..imagebase} will become apparent in next paragraph. It should be noted that \c{wrt ..imagebase} is defined as 32-bit operand only: \c dd label wrt ..imagebase ; ok \c dq label wrt ..imagebase ; bad \c mov eax,label wrt ..imagebase ; ok \c mov rax,label wrt ..imagebase ; bad \S{win64seh} \c{win64}: Structured Exception Handling Structured exception handing in Win64 is completely different matter from Win32. Upon exception program counter value is noted, and linker-generated table comprising start and end addresses of all the functions [in given executable module] is traversed and compared to the saved program counter. Thus so called \c{UNWIND_INFO} structure is identified. If it's not found, then offending subroutine is assumed to be "leaf" and just mentioned lookup procedure is attempted for its caller. In Win64 leaf function is such function that does not call any other function \e{nor} modifies any Win64 non-volatile registers, including stack pointer. The latter ensures that it's possible to identify leaf function's caller by simply pulling the value from the top of the stack. While majority of subroutines written in assembler are not calling any other function, requirement for non-volatile registers' immutability leaves developer with not more than 7 registers and no stack frame, which is not necessarily what [s]he counted with. Customarily one would meet the requirement by saving non-volatile registers on stack and restoring them upon return, so what can go wrong? If [and only if] an exception is raised at run-time and no \c{UNWIND_INFO} structure is associated with such "leaf" function, the stack unwind procedure will expect to find caller's return address on the top of stack immediately followed by its frame. Given that developer pushed caller's non-volatile registers on stack, would the value on top point at some code segment or even addressable space? Well, developer can attempt copying caller's return address to the top of stack and this would actually work in some very specific circumstances. But unless developer can guarantee that these circumstances are always met, it's more appropriate to assume worst case scenario, i.e. stack unwind procedure going berserk. Relevant question is what happens then? Application is abruptly terminated without any notification whatsoever. Just like in Win32 case, one can argue that system could at least have logged "unwind procedure went berserk in x.exe at address n" in event log, but no, no trace of failure is left. Now, when we understand significance of the \c{UNWIND_INFO} structure, let's discuss what's in it and/or how it's processed. First of all it is checked for presence of reference to custom language-specific exception handler. If there is one, then it's invoked. Depending on the return value, execution flow is resumed (exception is said to be "handled"), \e{or} rest of \c{UNWIND_INFO} structure is processed as following. Beside optional reference to custom handler, it carries information about current callee's stack frame and where non-volatile registers are saved. Information is detailed enough to be able to reconstruct contents of caller's non-volatile registers upon call to current callee. And so caller's context is reconstructed, and then unwind procedure is repeated, i.e. another \c{UNWIND_INFO} structure is associated, this time, with caller's instruction pointer, which is then checked for presence of reference to language-specific handler, etc. The procedure is recursively repeated till exception is handled. As last resort system "handles" it by generating memory core dump and terminating the application. As for the moment of this writing NASM unfortunately does not facilitate generation of above mentioned detailed information about stack frame layout. But as of version 2.03 it implements building blocks for generating structures involved in stack unwinding. As simplest example, here is how to deploy custom exception handler for leaf function: \c default rel \c section .text \c extern MessageBoxA \c handler: \c sub rsp,40 \c mov rcx,0 \c lea rdx,[text] \c lea r8,[caption] \c mov r9,1 ; MB_OKCANCEL \c call MessageBoxA \c sub eax,1 ; incidentally suits as return value \c ; for exception handler \c add rsp,40 \c ret \c global main \c main: \c xor rax,rax \c mov rax,QWORD[rax] ; cause exception \c ret \c main_end: \c text: db 'OK to rethrow, CANCEL to generate core dump',0 \c caption:db 'SEGV',0 \c \c section .pdata rdata align=4 \c dd main wrt ..imagebase \c dd main_end wrt ..imagebase \c dd xmain wrt ..imagebase \c section .xdata rdata align=8 \c xmain: db 9,0,0,0 \c dd handler wrt ..imagebase \c section .drectve info \c db '/defaultlib:user32.lib /defaultlib:msvcrt.lib ' What you see in \c{.pdata} section is element of the "table comprising start and end addresses of function" along with reference to associated \c{UNWIND_INFO} structure. And what you see in \c{.xdata} section is \c{UNWIND_INFO} structure describing function with no frame, but with designated exception handler. References are \e{required} to be image-relative (which is the real reason for implementing \c{wrt ..imagebase} operator). It should be noted that \c{rdata align=n}, as well as \c{wrt ..imagebase}, are optional in these two segments' contexts, i.e. can be omitted. Latter means that \e{all} 32-bit references, not only above listed required ones, placed into these two segments turn out image-relative. Why is it important to understand? Developer is allowed to append handler-specific data to \c{UNWIND_INFO} structure, and if [s]he adds a 32-bit reference, then [s]he will have to remember to adjust its value to obtain the real pointer. As already mentioned, in Win64 terms leaf function is one that does not call any other function \e{nor} modifies any non-volatile register, including stack pointer. But it's not uncommon that assembler programmer plans to utilize every single register and sometimes even have variable stack frame. Is there anything one can do with bare building blocks? I.e. besides manually composing fully-fledged \c{UNWIND_INFO} structure, which would surely be considered error-prone? Yes, there is. Recall that exception handler is called first, before stack layout is analyzed. As it turned out, it's perfectly possible to manipulate current callee's context in custom handler in manner that permits further stack unwinding. General idea is that handler would not actually "handle" the exception, but instead restore callee's context, as it was at its entry point and thus mimic leaf function. In other words, handler would simply undertake part of unwinding procedure. Consider following example: \c function: \c mov rax,rsp ; copy rsp to volatile register \c push r15 ; save non-volatile registers \c push rbx \c push rbp \c mov r11,rsp ; prepare variable stack frame \c sub r11,rcx \c and r11,-64 \c mov QWORD[r11],rax ; check for exceptions \c mov rsp,r11 ; allocate stack frame \c mov QWORD[rsp],rax ; save original rsp value \c magic_point: \c ... \c mov r11,QWORD[rsp] ; pull original rsp value \c mov rbp,QWORD[r11-24] \c mov rbx,QWORD[r11-16] \c mov r15,QWORD[r11-8] \c mov rsp,r11 ; destroy frame \c ret The keyword is that up to \c{magic_point} original \c{rsp} value remains in chosen volatile register and no non-volatile register, except for \c{rsp}, is modified. While past \c{magic_point} \c{rsp} remains constant till the very end of the \c{function}. In this case custom language-specific exception handler would look like this: \c EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame, \c CONTEXT *context,DISPATCHER_CONTEXT *disp) \c { ULONG64 *rsp; \c if (context->Rip<(ULONG64)magic_point) \c rsp = (ULONG64 *)context->Rax; \c else \c { rsp = ((ULONG64 **)context->Rsp)[0]; \c context->Rbp = rsp[-3]; \c context->Rbx = rsp[-2]; \c context->R15 = rsp[-1]; \c } \c context->Rsp = (ULONG64)rsp; \c \c memcpy (disp->ContextRecord,context,sizeof(CONTEXT)); \c RtlVirtualUnwind(UNW_FLAG_NHANDLER,disp->ImageBase, \c dips->ControlPc,disp->FunctionEntry,disp->ContextRecord, \c &disp->HandlerData,&disp->EstablisherFrame,NULL); \c return ExceptionContinueSearch; \c } As custom handler mimics leaf function, corresponding \c{UNWIND_INFO} structure does not have to contain any information about stack frame and its layout. \H{cofffmt} \i\c{coff}: \i{Common Object File Format} The \c{coff} output type produces \c{COFF} object files suitable for linking with the \i{DJGPP} linker. \c{coff} provides a default output file-name extension of \c{.o}. The \c{coff} format supports the same extensions to the \c{SECTION} directive as \c{win32} does, except that the \c{align} qualifier and the \c{info} section type are not supported. \H{machofmt} \I{Mach-O}\i\c{macho32} and \i\c{macho64}: \i{Mach Object File Format} The \c{macho32} and \c{macho64} output formts produces Mach-O object files suitable for linking with the \i{MacOS X} linker. \i\c{macho} is a synonym for \c{macho32}. \c{macho} provides a default output file-name extension of \c{.o}. \S{machosect} \c{macho} extensions to the \c{SECTION} Directive \I{SECTION, macho extensions to} The \c{macho} output format specifies section names in the format "\e{segment}\c{,}\e{section}". No spaces are allowed around the comma. The following flags can also be specified: \b \c{data} - this section contains initialized data items \b \c{text} - this section contains code exclusively \b \c{mixed} - this section contains both code and data \b \c{bss} - this section is uninitialized and filled with zero \b \c{zerofill} - same as \c{bss} \b \c{no_dead_strip} - inhibit dead code stripping for this section \b \c{live_support} - set the live support flag for this section \b \c{strip_static_syms} - strip static symbols for this section \b \c{align=}\e{alignment} - specify section alignment The default is \c{data}, unless the section name is \c{__text} or \c{__bss} in which case the default is \c{text} or \c{bss}, respectively. For compatibility with other Unix platforms, the following standard names are also supported: \c .text = __TEXT,__text text \c .rodata = __DATA,__const data \c .data = __DATA,__data data \c .bss = __DATA,__bss bss If the \c{.rodata} section contains no relocations, it is instead put into the \c{__TEXT,__const} section unless this section has already been specified explicitly. However, it is probably better to specify \c{__TEXT,__const} and \c{__DATA,__const} explicitly as appropriate. \S{machotls} \i{Thread Local Storage in Mach-O}\I{TLS}: \c{macho} special symbols and \i\c{WRT} Mach-O defines the following special symbols that can be used on the right-hand side of the \c{WRT} operator: \b \c{..tlvp} is used to specify access to thread-local storage. \b \c{..gotpcrel} is used to specify references to the Global Offset Table. The GOT is supported in the \c{macho64} format only. \S{macho-ssvs} \c{macho} specfic directive \i\c{subsections_via_symbols} The directive \c{subsections_via_symbols} sets the \c{MH_SUBSECTIONS_VIA_SYMBOLS} flag in the Mach-O header, which tells the linker that the symbols in the file matches the conventions required to allow for link-time dead code elimination. This directive takes no arguments. This is a macro implemented as a \c{%pragma}. It can also be specified in its \c{%pragma} form, in which case it will not affect non-Mach-O builds of the same source code: \c %pragma macho subsections_via_symbols \S{macho-ssvs} \c{macho} specfic directive \i\c{no_dead_strip} The directive \c{no_dead_strip} sets the Mach-O \c{SH_NO_DEAD_STRIP} section flag on the section containing a a specific symbol. This directive takes a list of symbols as its arguments. This is a macro implemented as a \c{%pragma}. It can also be specified in its \c{%pragma} form, in which case it will not affect non-Mach-O builds of the same source code: \c %pragma macho no_dead_strip symbol... \H{elffmt} \i\c{elf32}, \i\c{elf64}, \i\c{elfx32}: \I{ELF}\I{linux, elf}\i{Executable and Linkable Format} Object Files The \c{elf32}, \c{elf64} and \c{elfx32} output formats generate \c{ELF32 and ELF64} (Executable and Linkable Format) object files, as used by Linux as well as \i{Unix System V}, including \i{Solaris x86}, \i{UnixWare} and \i{SCO Unix}. \c{elf} provides a default output file-name extension of \c{.o}. \c{elf} is a synonym for \c{elf32}. The \c{elfx32} format is used for the \i{x32} ABI, which is a 32-bit ABI with the CPU in 64-bit mode. \S{abisect} ELF specific directive \i\c{osabi} The ELF header specifies the application binary interface for the target operating system (OSABI). This field can be set by using the \c{osabi} directive with the numeric value (0-255) of the target system. If this directive is not used, the default value will be "UNIX System V ABI" (0) which will work on most systems which support ELF. \S{elfsect} \c{elf} extensions to the \c{SECTION} Directive \I{SECTION, elf extensions to} Like the \c{obj} format, \c{elf} allows you to specify additional information on the \c{SECTION} directive line, to control the type and properties of sections you declare. Section types and properties are generated automatically by NASM for the \i{standard section names}, but may still be overridden by these qualifiers. The available qualifiers are: \b \i\c{alloc} defines the section to be one which is loaded into memory when the program is run. \i\c{noalloc} defines it to be one which is not, such as an informational or comment section. \b \i\c{exec} defines the section to be one which should have execute permission when the program is run. \i\c{noexec} defines it as one which should not. \b \i\c{write} defines the section to be one which should be writable when the program is run. \i\c{nowrite} defines it as one which should not. \b \i\c{progbits} defines the section to be one with explicit contents stored in the object file: an ordinary code or data section, for example, \i\c{nobits} defines the section to be one with no explicit contents given, such as a BSS section. \b \c{align=}, used with a trailing number as in \c{obj}, gives the \I{section alignment, in elf}\I{alignment, in elf sections}alignment requirements of the section. \b \i\c{tls} defines the section to be one which contains thread local variables. The defaults assumed by NASM if you do not specify the above qualifiers are: \I\c{.text} \I\c{.rodata} \I\c{.lrodata} \I\c{.data} \I\c{.ldata} \I\c{.bss} \I\c{.lbss} \I\c{.tdata} \I\c{.tbss} \I\c\{.comment} \c section .text progbits alloc exec nowrite align=16 \c section .rodata progbits alloc noexec nowrite align=4 \c section .lrodata progbits alloc noexec nowrite align=4 \c section .data progbits alloc noexec write align=4 \c section .ldata progbits alloc noexec write align=4 \c section .bss nobits alloc noexec write align=4 \c section .lbss nobits alloc noexec write align=4 \c section .tdata progbits alloc noexec write align=4 tls \c section .tbss nobits alloc noexec write align=4 tls \c section .comment progbits noalloc noexec nowrite align=1 \c section other progbits alloc noexec nowrite align=1 (Any section name other than those in the above table is treated by default like \c{other} in the above table. Please note that section names are case sensitive.) \S{elfwrt} \i{Position-Independent Code}\I{PIC}: \c{macho} Special Symbols and \i\c{WRT} Since \c{ELF} does not support segment-base references, the \c{WRT} operator is not used for its normal purpose; therefore NASM's \c{elf} output format makes use of \c{WRT} for a different purpose, namely the PIC-specific \I{relocations, PIC-specific}relocation types. \c{elf} defines five special symbols which you can use as the right-hand side of the \c{WRT} operator to obtain PIC relocation types. They are \i\c{..gotpc}, \i\c{..gotoff}, \i\c{..got}, \i\c{..plt} and \i\c{..sym}. Their functions are summarized here: \b Referring to the symbol marking the global offset table base using \c{wrt ..gotpc} will end up giving the distance from the beginning of the current section to the global offset table. (\i\c{_GLOBAL_OFFSET_TABLE_} is the standard symbol name used to refer to the \i{GOT}.) So you would then need to add \i\c{$$} to the result to get the real address of the GOT. \b Referring to a location in one of your own sections using \c{wrt ..gotoff} will give the distance from the beginning of the GOT to the specified location, so that adding on the address of the GOT would give the real address of the location you wanted. \b Referring to an external or global symbol using \c{wrt ..got} causes the linker to build an entry \e{in} the GOT containing the address of the symbol, and the reference gives the distance from the beginning of the GOT to the entry; so you can add on the address of the GOT, load from the resulting address, and end up with the address of the symbol. \b Referring to a procedure name using \c{wrt ..plt} causes the linker to build a \i{procedure linkage table} entry for the symbol, and the reference gives the address of the \i{PLT} entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for \c{CALL} or \c{JMP}), since ELF contains no relocation type to refer to PLT entries absolutely. \b Referring to a symbol name using \c{wrt ..sym} causes NASM to write an ordinary relocation, but instead of making the relocation relative to the start of the section and then adding on the offset to the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction is a necessary one due to a peculiarity of the dynamic linker. A fuller explanation of how to use these relocation types to write shared libraries entirely in NASM is given in \k{picdll}. \S{elftls} \i{Thread Local Storage in ELF}\I{TLS}: \c{elf} Special Symbols and \i\c{WRT} \b In ELF32 mode, referring to an external or global symbol using \c{wrt ..tlsie} \I\c{..tlsie} causes the linker to build an entry \e{in} the GOT containing the offset of the symbol within the TLS block, so you can access the value of the symbol with code such as: \c mov eax,[tid wrt ..tlsie] \c mov [gs:eax],ebx \b In ELF64 or ELFx32 mode, referring to an external or global symbol using \c{wrt ..gottpoff} \I\c{..gottpoff} causes the linker to build an entry \e{in} the GOT containing the offset of the symbol within the TLS block, so you can access the value of the symbol with code such as: \c mov rax,[rel tid wrt ..gottpoff] \c mov rcx,[fs:rax] \S{elfglob} \c{elf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL, elf extensions to}\I{GLOBAL, aoutb extensions to} \c{ELF} object files can contain more information about a global symbol than just its address: they can contain the \I{symbol sizes, specifying}\I{size, of symbols}size of the symbol and its \I{symbol types, specifying}\I{type, of symbols}type as well. These are not merely debugger conveniences, but are actually necessary when the program being written is a \i{shared library}. NASM therefore supports some extensions to the \c{GLOBAL} directive, allowing you to specify these features. You can specify whether a global variable is a function or a data object by suffixing the name with a colon and the word \i\c{function} or \i\c{data}. (\i\c{object} is a synonym for \c{data}.) For example: \c global hashlookup:function, hashtable:data exports the global symbol \c{hashlookup} as a function and \c{hashtable} as a data object. Optionally, you can control the ELF visibility of the symbol. Just add one of the visibility keywords: \i\c{default}, \i\c{internal}, \i\c{hidden}, or \i\c{protected}. The default is \i\c{default} of course. For example, to make \c{hashlookup} hidden: \c global hashlookup:function hidden You can also specify the size of the data associated with the symbol, as a numeric expression (which may involve labels, and even forward references) after the type specifier. Like this: \c global hashtable:data (hashtable.end - hashtable) \c \c hashtable: \c db this,that,theother ; some data here \c .end: This makes NASM automatically calculate the length of the table and place that information into the \c{ELF} symbol table. Declaring the type and size of global symbols is necessary when writing shared library code. For more information, see \k{picglobal}. \S{elfcomm} \c{elf} Extensions to the \c{COMMON} Directive \I{COMMON, elf extensions to} \c{ELF} also allows you to specify alignment requirements \I{common variables, alignment in elf}\I{alignment, of elf common variables}on common variables. This is done by putting a number (which must be a power of two) after the name and size of the common variable, separated (as usual) by a colon. For example, an array of doublewords would benefit from 4-byte alignment: \c common dwordarray 128:4 This declares the total size of the array to be 128 bytes, and requires that it be aligned on a 4-byte boundary. \S{elf16} 16-bit code and ELF \I{ELF, 16-bit code and} The \c{ELF32} specification doesn't provide relocations for 8- and 16-bit values, but the GNU \c{ld} linker adds these as an extension. NASM can generate GNU-compatible relocations, to allow 16-bit code to be linked as ELF using GNU \c{ld}. If NASM is used with the \c{-w+gnu-elf-extensions} option, a warning is issued when one of these relocations is generated. \S{elfdbg} Debug formats and ELF \I{ELF, Debug formats and} ELF provides debug information in \c{STABS} and \c{DWARF} formats. Line number information is generated for all executable sections, but please note that only the ".text" section is executable by default. \H{aoutfmt} \i\c{aout}: Linux \I{a.out, Linux version}\I{linux, a.out}\c{a.out} Object Files The \c{aout} format generates \c{a.out} object files, in the form used by early Linux systems (current Linux systems use ELF, see \k{elffmt}.) These differ from other \c{a.out} object files in that the magic number in the first four bytes of the file is different; also, some implementations of \c{a.out}, for example NetBSD's, support position-independent code, which Linux's implementation does not. \c{a.out} provides a default output file-name extension of \c{.o}. \c{a.out} is a very simple object format. It supports no special directives, no special symbols, no use of \c{SEG} or \c{WRT}, and no extensions to any standard directives. It supports only the three \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}. \H{aoutfmt} \i\c{aoutb}: \i{NetBSD}/\i{FreeBSD}/\i{OpenBSD} \I{a.out, BSD version}\c{a.out} Object Files The \c{aoutb} format generates \c{a.out} object files, in the form used by the various free \c{BSD Unix} clones, \c{NetBSD}, \c{FreeBSD} and \c{OpenBSD}. For simple object files, this object format is exactly the same as \c{aout} except for the magic number in the first four bytes of the file. However, the \c{aoutb} format supports \I{PIC}\i{position-independent code} in the same way as the \c{elf} format, so you can use it to write \c{BSD} \i{shared libraries}. \c{aoutb} provides a default output file-name extension of \c{.o}. \c{aoutb} supports no special directives, no special symbols, and only the three \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}. However, it also supports the same use of \i\c{WRT} as \c{elf} does, to provide position-independent code relocation types. See \k{elfwrt} for full documentation of this feature. \c{aoutb} also supports the same extensions to the \c{GLOBAL} directive as \c{elf} does: see \k{elfglob} for documentation of this. \H{as86fmt} \c{as86}: \i{Minix}/Linux\I{linux, as86} \i\c{as86} Object Files The Minix/Linux 16-bit assembler \c{as86} has its own non-standard object file format. Although its companion linker \i\c{ld86} produces something close to ordinary \c{a.out} binaries as output, the object file format used to communicate between \c{as86} and \c{ld86} is not itself \c{a.out}. NASM supports this format, just in case it is useful, as \c{as86}. \c{as86} provides a default output file-name extension of \c{.o}. \c{as86} is a very simple object format (from the NASM user's point of view). It supports no special directives, no use of \c{SEG} or \c{WRT}, and no extensions to any standard directives. It supports only the three \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}. The only special symbol supported is \c{..start}. \H{rdffmt} \I{RDOFF}\i\c{rdf}: \i{Relocatable Dynamic Object File Format} The \c{rdf} output format produces \c{RDOFF} object files. \c{RDOFF} (Relocatable Dynamic Object File Format) is a home-grown object-file format, designed alongside NASM itself and reflecting in its file format the internal structure of the assembler. \c{RDOFF} is not used by any well-known operating systems. Those writing their own systems, however, may well wish to use \c{RDOFF} as their object format, on the grounds that it is designed primarily for simplicity and contains very little file-header bureaucracy. The Unix NASM archive, and the DOS archive which includes sources, both contain an \I{rdoff subdirectory}\c{rdoff} subdirectory holding a set of RDOFF utilities: an RDF linker, an \c{RDF} static-library manager, an RDF file dump utility, and a program which will load and execute an RDF executable under Linux. \c{rdf} supports only the \i{standard section names} \i\c{.text}, \i\c{.data} and \i\c{.bss}. \S{rdflib} Requiring a Library: The \i\c{LIBRARY} Directive \c{RDOFF} contains a mechanism for an object file to demand a given library to be linked to the module, either at load time or run time. This is done by the \c{LIBRARY} directive, which takes one argument which is the name of the module: \c library mylib.rdl \S{rdfmod} Specifying a Module Name: The \i\c{MODULE} Directive Special \c{RDOFF} header record is used to store the name of the module. It can be used, for example, by run-time loader to perform dynamic linking. \c{MODULE} directive takes one argument which is the name of current module: \c module mymodname Note that when you statically link modules and tell linker to strip the symbols from output file, all module names will be stripped too. To avoid it, you should start module names with \I{$, prefix}\c{$}, like: \c module $kernel.core \S{rdfglob} \c{rdf} Extensions to the \c{GLOBAL} Directive\I{GLOBAL, rdf extensions to} \c{RDOFF} global symbols can contain additional information needed by the static linker. You can mark a global symbol as exported, thus telling the linker do not strip it from target executable or library file. Like in \c{ELF}, you can also specify whether an exported symbol is a procedure (function) or data object. Suffixing the name with a colon and the word \i\c{export} you make the symbol exported: \c global sys_open:export To specify that exported symbol is a procedure (function), you add the word \i\c{proc} or \i\c{function} after declaration: \c global sys_open:export proc Similarly, to specify exported data object, add the word \i\c{data} or \i\c{object} to the directive: \c global kernel_ticks:export data \S{rdfimpt} \c{rdf} Extensions to the \c{EXTERN} Directive\I{EXTERN, rdf extensions to} By default the \c{EXTERN} directive in \c{RDOFF} declares a "pure external" symbol (i.e. the static linker will complain if such a symbol is not resolved). To declare an "imported" symbol, which must be resolved later during a dynamic linking phase, \c{RDOFF} offers an additional \c{import} modifier. As in \c{GLOBAL}, you can also specify whether an imported symbol is a procedure (function) or data object. For example: \c library $libc \c extern _open:import \c extern _printf:import proc \c extern _errno:import data Here the directive \c{LIBRARY} is also included, which gives the dynamic linker a hint as to where to find requested symbols. \H{dbgfmt} \i\c{dbg}: Debugging Format The \c{dbg} format does not output an object file as such; instead, it outputs a text file which contains a complete list of all the transactions between the main body of NASM and the output-format back end module. It is primarily intended to aid people who want to write their own output drivers, so that they can get a clearer idea of the various requests the main program makes of the output driver, and in what order they happen. For simple files, one can easily use the \c{dbg} format like this: \c nasm -f dbg filename.asm which will generate a diagnostic file called \c{filename.dbg}. However, this will not work well on files which were designed for a different object format, because each object format defines its own macros (usually user-level forms of directives), and those macros will not be defined in the \c{dbg} format. Therefore it can be useful to run NASM twice, in order to do the preprocessing with the native object format selected: \c nasm -e -f rdf -o rdfprog.i rdfprog.asm \c nasm -a -f dbg rdfprog.i This preprocesses \c{rdfprog.asm} into \c{rdfprog.i}, keeping the \c{rdf} object format selected in order to make sure RDF special directives are converted into primitive form correctly. Then the preprocessed source is fed through the \c{dbg} format to generate the final diagnostic output. This workaround will still typically not work for programs intended for \c{obj} format, because the \c{obj} \c{SEGMENT} and \c{GROUP} directives have side effects of defining the segment and group names as symbols; \c{dbg} will not do this, so the program will not assemble. You will have to work around that by defining the symbols yourself (using \c{EXTERN}, for example) if you really need to get a \c{dbg} trace of an \c{obj}-specific source file. \c{dbg} accepts any section name and any directives at all, and logs them all to its output file. \c{dbg} accepts and logs any \c{%pragma}, but the specific \c{%pragma}: \c %pragma dbg maxdump <size> where \c{<size>} is either a number or \c{unlimited}, can be used to control the maximum size for dumping the full contents of a \c{rawdata} output object. \C{16bit} Writing 16-bit Code (DOS, Windows 3/3.1) This chapter attempts to cover some of the common issues encountered when writing 16-bit code to run under \c{MS-DOS} or \c{Windows 3.x}. It covers how to link programs to produce \c{.EXE} or \c{.COM} files, how to write \c{.SYS} device drivers, and how to interface assembly language code with 16-bit C compilers and with Borland Pascal. \H{exefiles} Producing \i\c{.EXE} Files Any large program written under DOS needs to be built as a \c{.EXE} file: only \c{.EXE} files have the necessary internal structure required to span more than one 64K segment. \i{Windows} programs, also, have to be built as \c{.EXE} files, since Windows does not support the \c{.COM} format. In general, you generate \c{.EXE} files by using the \c{obj} output format to produce one or more \i\c{.OBJ} files, and then linking them together using a linker. However, NASM also supports the direct generation of simple DOS \c{.EXE} files using the \c{bin} output format (by using \c{DB} and \c{DW} to construct the \c{.EXE} file header), and a macro package is supplied to do this. Thanks to Yann Guidon for contributing the code for this. NASM may also support \c{.EXE} natively as another output format in future releases. \S{objexe} Using the \c{obj} Format To Generate \c{.EXE} Files This section describes the usual method of generating \c{.EXE} files by linking \c{.OBJ} files together. Most 16-bit programming language packages come with a suitable linker; if you have none of these, there is a free linker called \i{VAL}\I{linker, free}, available in \c{LZH} archive format from \W{ftp://x2ftp.oulu.fi/pub/msdos/programming/lang/}\i\c{x2ftp.oulu.fi}. An LZH archiver can be found at \W{ftp://ftp.simtel.net/pub/simtelnet/msdos/arcers}\i\c{ftp.simtel.net}. There is another `free' linker (though this one doesn't come with sources) called \i{FREELINK}, available from \W{http://www.pcorner.com/tpc/old/3-101.html}\i\c{www.pcorner.com}. A third, \i\c{djlink}, written by DJ Delorie, is available at \W{http://www.delorie.com/djgpp/16bit/djlink/}\i\c{www.delorie.com}. A fourth linker, \i\c{ALINK}, written by Anthony A.J. Williams, is available at \W{http://alink.sourceforge.net}\i\c{alink.sourceforge.net}. When linking several \c{.OBJ} files into a \c{.EXE} file, you should ensure that exactly one of them has a start point defined (using the \I{program entry point}\i\c{..start} special symbol defined by the \c{obj} format: see \k{dotdotstart}). If no module defines a start point, the linker will not know what value to give the entry-point field in the output file header; if more than one defines a start point, the linker will not know \e{which} value to use. An example of a NASM source file which can be assembled to a \c{.OBJ} file and linked on its own to a \c{.EXE} is given here. It demonstrates the basic principles of defining a stack, initialising the segment registers, and declaring a start point. This file is also provided in the \I{test subdirectory}\c{test} subdirectory of the NASM archives, under the name \c{objexe.asm}. \c segment code \c \c ..start: \c mov ax,data \c mov ds,ax \c mov ax,stack \c mov ss,ax \c mov sp,stacktop This initial piece of code sets up \c{DS} to point to the data segment, and initializes \c{SS} and \c{SP} to point to the top of the provided stack. Notice that interrupts are implicitly disabled for one instruction after a move into \c{SS}, precisely for this situation, so that there's no chance of an interrupt occurring between the loads of \c{SS} and \c{SP} and not having a stack to execute on. Note also that the special symbol \c{..start} is defined at the beginning of this code, which means that will be the entry point into the resulting executable file. \c mov dx,hello \c mov ah,9 \c int 0x21 The above is the main program: load \c{DS:DX} with a pointer to the greeting message (\c{hello} is implicitly relative to the segment \c{data}, which was loaded into \c{DS} in the setup code, so the full pointer is valid), and call the DOS print-string function. \c mov ax,0x4c00 \c int 0x21 This terminates the program using another DOS system call. \c segment data \c \c hello: db 'hello, world', 13, 10, '$' The data segment contains the string we want to display. \c segment stack stack \c resb 64 \c stacktop: The above code declares a stack segment containing 64 bytes of uninitialized stack space, and points \c{stacktop} at the top of it. The directive \c{segment stack stack} defines a segment \e{called} \c{stack}, and also of \e{type} \c{STACK}. The latter is not necessary to the correct running of the program, but linkers are likely to issue warnings or errors if your program has no segment of type \c{STACK}. The above file, when assembled into a \c{.OBJ} file, will link on its own to a valid \c{.EXE} file, which when run will print `hello, world' and then exit. \S{binexe} Using the \c{bin} Format To Generate \c{.EXE} Files The \c{.EXE} file format is simple enough that it's possible to build a \c{.EXE} file by writing a pure-binary program and sticking a 32-byte header on the front. This header is simple enough that it can be generated using \c{DB} and \c{DW} commands by NASM itself, so that you can use the \c{bin} output format to directly generate \c{.EXE} files. Included in the NASM archives, in the \I{misc subdirectory}\c{misc} subdirectory, is a file \i\c{exebin.mac} of macros. It defines three macros: \i\c{EXE_begin}, \i\c{EXE_stack} and \i\c{EXE_end}. To produce a \c{.EXE} file using this method, you should start by using \c{%include} to load the \c{exebin.mac} macro package into your source file. You should then issue the \c{EXE_begin} macro call (which takes no arguments) to generate the file header data. Then write code as normal for the \c{bin} format - you can use all three standard sections \c{.text}, \c{.data} and \c{.bss}. At the end of the file you should call the \c{EXE_end} macro (again, no arguments), which defines some symbols to mark section sizes, and these symbols are referred to in the header code generated by \c{EXE_begin}. In this model, the code you end up writing starts at \c{0x100}, just like a \c{.COM} file - in fact, if you strip off the 32-byte header from the resulting \c{.EXE} file, you will have a valid \c{.COM} program. All the segment bases are the same, so you are limited to a 64K program, again just like a \c{.COM} file. Note that an \c{ORG} directive is issued by the \c{EXE_begin} macro, so you should not explicitly issue one of your own. You can't directly refer to your segment base value, unfortunately, since this would require a relocation in the header, and things would get a lot more complicated. So you should get your segment base by copying it out of \c{CS} instead. On entry to your \c{.EXE} file, \c{SS:SP} are already set up to point to the top of a 2Kb stack. You can adjust the default stack size of 2Kb by calling the \c{EXE_stack} macro. For example, to change the stack size of your program to 64 bytes, you would call \c{EXE_stack 64}. A sample program which generates a \c{.EXE} file in this way is given in the \c{test} subdirectory of the NASM archive, as \c{binexe.asm}. \H{comfiles} Producing \i\c{.COM} Files While large DOS programs must be written as \c{.EXE} files, small ones are often better written as \c{.COM} files. \c{.COM} files are pure binary, and therefore most easily produced using the \c{bin} output format. \S{combinfmt} Using the \c{bin} Format To Generate \c{.COM} Files \c{.COM} files expect to be loaded at offset \c{100h} into their segment (though the segment may change). Execution then begins at \I\c{ORG}\c{100h}, i.e. right at the start of the program. So to write a \c{.COM} program, you would create a source file looking like \c org 100h \c \c section .text \c \c start: \c ; put your code here \c \c section .data \c \c ; put data items here \c \c section .bss \c \c ; put uninitialized data here The \c{bin} format puts the \c{.text} section first in the file, so you can declare data or BSS items before beginning to write code if you want to and the code will still end up at the front of the file where it belongs. The BSS (uninitialized data) section does not take up space in the \c{.COM} file itself: instead, addresses of BSS items are resolved to point at space beyond the end of the file, on the grounds that this will be free memory when the program is run. Therefore you should not rely on your BSS being initialized to all zeros when you run. To assemble the above program, you should use a command line like \c nasm myprog.asm -fbin -o myprog.com The \c{bin} format would produce a file called \c{myprog} if no explicit output file name were specified, so you have to override it and give the desired file name. \S{comobjfmt} Using the \c{obj} Format To Generate \c{.COM} Files If you are writing a \c{.COM} program as more than one module, you may wish to assemble several \c{.OBJ} files and link them together into a \c{.COM} program. You can do this, provided you have a linker capable of outputting \c{.COM} files directly (\i{TLINK} does this), or alternatively a converter program such as \i\c{EXE2BIN} to transform the \c{.EXE} file output from the linker into a \c{.COM} file. If you do this, you need to take care of several things: \b The first object file containing code should start its code segment with a line like \c{RESB 100h}. This is to ensure that the code begins at offset \c{100h} relative to the beginning of the code segment, so that the linker or converter program does not have to adjust address references within the file when generating the \c{.COM} file. Other assemblers use an \i\c{ORG} directive for this purpose, but \c{ORG} in NASM is a format-specific directive to the \c{bin} output format, and does not mean the same thing as it does in MASM-compatible assemblers. \b You don't need to define a stack segment. \b All your segments should be in the same group, so that every time your code or data references a symbol offset, all offsets are relative to the same segment base. This is because, when a \c{.COM} file is loaded, all the segment registers contain the same value. \H{sysfiles} Producing \i\c{.SYS} Files \i{MS-DOS device drivers} - \c{.SYS} files - are pure binary files, similar to \c{.COM} files, except that they start at origin zero rather than \c{100h}. Therefore, if you are writing a device driver using the \c{bin} format, you do not need the \c{ORG} directive, since the default origin for \c{bin} is zero. Similarly, if you are using \c{obj}, you do not need the \c{RESB 100h} at the start of your code segment. \c{.SYS} files start with a header structure, containing pointers to the various routines inside the driver which do the work. This structure should be defined at the start of the code segment, even though it is not actually code. For more information on the format of \c{.SYS} files, and the data which has to go in the header structure, a list of books is given in the Frequently Asked Questions list for the newsgroup \W{news:comp.os.msdos.programmer}\i\c{comp.os.msdos.programmer}. \H{16c} Interfacing to 16-bit C Programs This section covers the basics of writing assembly routines that call, or are called from, C programs. To do this, you would typically write an assembly module as a \c{.OBJ} file, and link it with your C modules to produce a \i{mixed-language program}. \S{16cunder} External Symbol Names \I{C symbol names}\I{underscore, in C symbols}C compilers have the convention that the names of all global symbols (functions or data) they define are formed by prefixing an underscore to the name as it appears in the C program. So, for example, the function a C programmer thinks of as \c{printf} appears to an assembly language programmer as \c{_printf}. This means that in your assembly programs, you can define symbols without a leading underscore, and not have to worry about name clashes with C symbols. If you find the underscores inconvenient, you can define macros to replace the \c{GLOBAL} and \c{EXTERN} directives as follows: \c %macro cglobal 1 \c \c global _%1 \c %define %1 _%1 \c \c %endmacro \c \c %macro cextern 1 \c \c extern _%1 \c %define %1 _%1 \c \c %endmacro (These forms of the macros only take one argument at a time; a \c{%rep} construct could solve this.) If you then declare an external like this: \c cextern printf then the macro will expand it as \c extern _printf \c %define printf _printf Thereafter, you can reference \c{printf} as if it was a symbol, and the preprocessor will put the leading underscore on where necessary. The \c{cglobal} macro works similarly. You must use \c{cglobal} before defining the symbol in question, but you would have had to do that anyway if you used \c{GLOBAL}. Also see \k{opt-pfix}. \S{16cmodels} \i{Memory Models} NASM contains no mechanism to support the various C memory models directly; you have to keep track yourself of which one you are writing for. This means you have to keep track of the following things: \b In models using a single code segment (tiny, small and compact), functions are near. This means that function pointers, when stored in data segments or pushed on the stack as function arguments, are 16 bits long and contain only an offset field (the \c{CS} register never changes its value, and always gives the segment part of the full function address), and that functions are called using ordinary near \c{CALL} instructions and return using \c{RETN} (which, in NASM, is synonymous with \c{RET} anyway). This means both that you should write your own routines to return with \c{RETN}, and that you should call external C routines with near \c{CALL} instructions. \b In models using more than one code segment (medium, large and huge), functions are far. This means that function pointers are 32 bits long (consisting of a 16-bit offset followed by a 16-bit segment), and that functions are called using \c{CALL FAR} (or \c{CALL seg:offset}) and return using \c{RETF}. Again, you should therefore write your own routines to return with \c{RETF} and use \c{CALL FAR} to call external routines. \b In models using a single data segment (tiny, small and medium), data pointers are 16 bits long, containing only an offset field (the \c{DS} register doesn't change its value, and always gives the segment part of the full data item address). \b In models using more than one data segment (compact, large and huge), data pointers are 32 bits long, consisting of a 16-bit offset followed by a 16-bit segment. You should still be careful not to modify \c{DS} in your routines without restoring it afterwards, but \c{ES} is free for you to use to access the contents of 32-bit data pointers you are passed. \b The huge memory model allows single data items to exceed 64K in size. In all other memory models, you can access the whole of a data item just by doing arithmetic on the offset field of the pointer you are given, whether a segment field is present or not; in huge model, you have to be more careful of your pointer arithmetic. \b In most memory models, there is a \e{default} data segment, whose segment address is kept in \c{DS} throughout the program. This data segment is typically the same segment as the stack, kept in \c{SS}, so that functions' local variables (which are stored on the stack) and global data items can both be accessed easily without changing \c{DS}. Particularly large data items are typically stored in other segments. However, some memory models (though not the standard ones, usually) allow the assumption that \c{SS} and \c{DS} hold the same value to be removed. Be careful about functions' local variables in this latter case. In models with a single code segment, the segment is called \i\c{_TEXT}, so your code segment must also go by this name in order to be linked into the same place as the main code segment. In models with a single data segment, or with a default data segment, it is called \i\c{_DATA}. \S{16cfunc} Function Definitions and Function Calls \I{functions, C calling convention}The \i{C calling convention} in 16-bit programs is as follows. In the following description, the words \e{caller} and \e{callee} are used to denote the function doing the calling and the function which gets called. \b The caller pushes the function's parameters on the stack, one after another, in reverse order (right to left, so that the first argument specified to the function is pushed last). \b The caller then executes a \c{CALL} instruction to pass control to the callee. This \c{CALL} is either near or far depending on the memory model. \b The callee receives control, and typically (although this is not actually necessary, in functions which do not need to access their parameters) starts by saving the value of \c{SP} in \c{BP} so as to be able to use \c{BP} as a base pointer to find its parameters on the stack. However, the caller was probably doing this too, so part of the calling convention states that \c{BP} must be preserved by any C function. Hence the callee, if it is going to set up \c{BP} as a \i\e{frame pointer}, must push the previous value first. \b The callee may then access its parameters relative to \c{BP}. The word at \c{[BP]} holds the previous value of \c{BP} as it was pushed; the next word, at \c{[BP+2]}, holds the offset part of the return address, pushed implicitly by \c{CALL}. In a small-model (near) function, the parameters start after that, at \c{[BP+4]}; in a large-model (far) function, the segment part of the return address lives at \c{[BP+4]}, and the parameters begin at \c{[BP+6]}. The leftmost parameter of the function, since it was pushed last, is accessible at this offset from \c{BP}; the others follow, at successively greater offsets. Thus, in a function such as \c{printf} which takes a variable number of parameters, the pushing of the parameters in reverse order means that the function knows where to find its first parameter, which tells it the number and type of the remaining ones. \b The callee may also wish to decrease \c{SP} further, so as to allocate space on the stack for local variables, which will then be accessible at negative offsets from \c{BP}. \b The callee, if it wishes to return a value to the caller, should leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size of the value. Floating-point results are sometimes (depending on the compiler) returned in \c{ST0}. \b Once the callee has finished processing, it restores \c{SP} from \c{BP} if it had allocated local stack space, then pops the previous value of \c{BP}, and returns via \c{RETN} or \c{RETF} depending on memory model. \b When the caller regains control from the callee, the function parameters are still on the stack, so it typically adds an immediate constant to \c{SP} to remove them (instead of executing a number of slow \c{POP} instructions). Thus, if a function is accidentally called with the wrong number of parameters due to a prototype mismatch, the stack will still be returned to a sensible state since the caller, which \e{knows} how many parameters it pushed, does the removing. It is instructive to compare this calling convention with that for Pascal programs (described in \k{16bpfunc}). Pascal has a simpler convention, since no functions have variable numbers of parameters. Therefore the callee knows how many parameters it should have been passed, and is able to deallocate them from the stack itself by passing an immediate argument to the \c{RET} or \c{RETF} instruction, so the caller does not have to do it. Also, the parameters are pushed in left-to-right order, not right-to-left, which means that a compiler can give better guarantees about sequence points without performance suffering. Thus, you would define a function in C style in the following way. The following example is for small model: \c global _myfunc \c \c _myfunc: \c push bp \c mov bp,sp \c sub sp,0x40 ; 64 bytes of local stack space \c mov bx,[bp+4] ; first parameter to function \c \c ; some more code \c \c mov sp,bp ; undo "sub sp,0x40" above \c pop bp \c ret For a large-model function, you would replace \c{RET} by \c{RETF}, and look for the first parameter at \c{[BP+6]} instead of \c{[BP+4]}. Of course, if one of the parameters is a pointer, then the offsets of \e{subsequent} parameters will change depending on the memory model as well: far pointers take up four bytes on the stack when passed as a parameter, whereas near pointers take up two. At the other end of the process, to call a C function from your assembly code, you would do something like this: \c extern _printf \c \c ; and then, further down... \c \c push word [myint] ; one of my integer variables \c push word mystring ; pointer into my data segment \c call _printf \c add sp,byte 4 ; `byte' saves space \c \c ; then those data items... \c \c segment _DATA \c \c myint dw 1234 \c mystring db 'This number -> %d <- should be 1234',10,0 This piece of code is the small-model assembly equivalent of the C code \c int myint = 1234; \c printf("This number -> %d <- should be 1234\n", myint); In large model, the function-call code might look more like this. In this example, it is assumed that \c{DS} already holds the segment base of the segment \c{_DATA}. If not, you would have to initialize it first. \c push word [myint] \c push word seg mystring ; Now push the segment, and... \c push word mystring ; ... offset of "mystring" \c call far _printf \c add sp,byte 6 The integer value still takes up one word on the stack, since large model does not affect the size of the \c{int} data type. The first argument (pushed last) to \c{printf}, however, is a data pointer, and therefore has to contain a segment and offset part. The segment should be stored second in memory, and therefore must be pushed first. (Of course, \c{PUSH DS} would have been a shorter instruction than \c{PUSH WORD SEG mystring}, if \c{DS} was set up as the above example assumed.) Then the actual call becomes a far call, since functions expect far calls in large model; and \c{SP} has to be increased by 6 rather than 4 afterwards to make up for the extra word of parameters. \S{16cdata} Accessing Data Items To get at the contents of C variables, or to declare variables which C can access, you need only declare the names as \c{GLOBAL} or \c{EXTERN}. (Again, the names require leading underscores, as stated in \k{16cunder}.) Thus, a C variable declared as \c{int i} can be accessed from assembler as \c extern _i \c \c mov ax,[_i] And to declare your own integer variable which C programs can access as \c{extern int j}, you do this (making sure you are assembling in the \c{_DATA} segment, if necessary): \c global _j \c \c _j dw 0 To access a C array, you need to know the size of the components of the array. For example, \c{int} variables are two bytes long, so if a C program declares an array as \c{int a[10]}, you can access \c{a[3]} by coding \c{mov ax,[_a+6]}. (The byte offset 6 is obtained by multiplying the desired array index, 3, by the size of the array element, 2.) The sizes of the C base types in 16-bit compilers are: 1 for \c{char}, 2 for \c{short} and \c{int}, 4 for \c{long} and \c{float}, and 8 for \c{double}. To access a C \i{data structure}, you need to know the offset from the base of the structure to the field you are interested in. You can either do this by converting the C structure definition into a NASM structure definition (using \i\c{STRUC}), or by calculating the one offset and using just that. To do either of these, you should read your C compiler's manual to find out how it organizes data structures. NASM gives no special alignment to structure members in its own \c{STRUC} macro, so you have to specify alignment yourself if the C compiler generates it. Typically, you might find that a structure like \c struct { \c char c; \c int i; \c } foo; might be four bytes long rather than three, since the \c{int} field would be aligned to a two-byte boundary. However, this sort of feature tends to be a configurable option in the C compiler, either using command-line options or \c{#pragma} lines, so you have to find out how your own compiler does it. \S{16cmacro} \i\c{c16.mac}: Helper Macros for the 16-bit C Interface Included in the NASM archives, in the \I{misc subdirectory}\c{misc} directory, is a file \c{c16.mac} of macros. It defines three macros: \i\c{proc}, \i\c{arg} and \i\c{endproc}. These are intended to be used for C-style procedure definitions, and they automate a lot of the work involved in keeping track of the calling convention. (An alternative, TASM compatible form of \c{arg} is also now built into NASM's preprocessor. See \k{stackrel} for details.) An example of an assembly function using the macro set is given here: \c proc _nearproc \c \c %$i arg \c %$j arg \c mov ax,[bp + %$i] \c mov bx,[bp + %$j] \c add ax,[bx] \c \c endproc This defines \c{_nearproc} to be a procedure taking two arguments, the first (\c{i}) an integer and the second (\c{j}) a pointer to an integer. It returns \c{i + *j}. Note that the \c{arg} macro has an \c{EQU} as the first line of its expansion, and since the label before the macro call gets prepended to the first line of the expanded macro, the \c{EQU} works, defining \c{%$i} to be an offset from \c{BP}. A context-local variable is used, local to the context pushed by the \c{proc} macro and popped by the \c{endproc} macro, so that the same argument name can be used in later procedures. Of course, you don't \e{have} to do that. The macro set produces code for near functions (tiny, small and compact-model code) by default. You can have it generate far functions (medium, large and huge-model code) by means of coding \I\c{FARCODE}\c{%define FARCODE}. This changes the kind of return instruction generated by \c{endproc}, and also changes the starting point for the argument offsets. The macro set contains no intrinsic dependency on whether data pointers are far or not. \c{arg} can take an optional parameter, giving the size of the argument. If no size is given, 2 is assumed, since it is likely that many function parameters will be of type \c{int}. The large-model equivalent of the above function would look like this: \c %define FARCODE \c \c proc _farproc \c \c %$i arg \c %$j arg 4 \c mov ax,[bp + %$i] \c mov bx,[bp + %$j] \c mov es,[bp + %$j + 2] \c add ax,[bx] \c \c endproc This makes use of the argument to the \c{arg} macro to define a parameter of size 4, because \c{j} is now a far pointer. When we load from \c{j}, we must load a segment and an offset. \H{16bp} Interfacing to \i{Borland Pascal} Programs Interfacing to Borland Pascal programs is similar in concept to interfacing to 16-bit C programs. The differences are: \b The leading underscore required for interfacing to C programs is not required for Pascal. \b The memory model is always large: functions are far, data pointers are far, and no data item can be more than 64K long. (Actually, some functions are near, but only those functions that are local to a Pascal unit and never called from outside it. All assembly functions that Pascal calls, and all Pascal functions that assembly routines are able to call, are far.) However, all static data declared in a Pascal program goes into the default data segment, which is the one whose segment address will be in \c{DS} when control is passed to your assembly code. The only things that do not live in the default data segment are local variables (they live in the stack segment) and dynamically allocated variables. All data \e{pointers}, however, are far. \b The function calling convention is different - described below. \b Some data types, such as strings, are stored differently. \b There are restrictions on the segment names you are allowed to use - Borland Pascal will ignore code or data declared in a segment it doesn't like the name of. The restrictions are described below. \S{16bpfunc} The Pascal Calling Convention \I{functions, Pascal calling convention}\I{Pascal calling convention}The 16-bit Pascal calling convention is as follows. In the following description, the words \e{caller} and \e{callee} are used to denote the function doing the calling and the function which gets called. \b The caller pushes the function's parameters on the stack, one after another, in normal order (left to right, so that the first argument specified to the function is pushed first). \b The caller then executes a far \c{CALL} instruction to pass control to the callee. \b The callee receives control, and typically (although this is not actually necessary, in functions which do not need to access their parameters) starts by saving the value of \c{SP} in \c{BP} so as to be able to use \c{BP} as a base pointer to find its parameters on the stack. However, the caller was probably doing this too, so part of the calling convention states that \c{BP} must be preserved by any function. Hence the callee, if it is going to set up \c{BP} as a \i{frame pointer}, must push the previous value first. \b The callee may then access its parameters relative to \c{BP}. The word at \c{[BP]} holds the previous value of \c{BP} as it was pushed. The next word, at \c{[BP+2]}, holds the offset part of the return address, and the next one at \c{[BP+4]} the segment part. The parameters begin at \c{[BP+6]}. The rightmost parameter of the function, since it was pushed last, is accessible at this offset from \c{BP}; the others follow, at successively greater offsets. \b The callee may also wish to decrease \c{SP} further, so as to allocate space on the stack for local variables, which will then be accessible at negative offsets from \c{BP}. \b The callee, if it wishes to return a value to the caller, should leave the value in \c{AL}, \c{AX} or \c{DX:AX} depending on the size of the value. Floating-point results are returned in \c{ST0}. Results of type \c{Real} (Borland's own custom floating-point data type, not handled directly by the FPU) are returned in \c{DX:BX:AX}. To return a result of type \c{String}, the caller pushes a pointer to a temporary string before pushing the parameters, and the callee places the returned string value at that location. The pointer is not a parameter, and should not be removed from the stack by the \c{RETF} instruction. \b Once the callee has finished processing, it restores \c{SP} from \c{BP} if it had allocated local stack space, then pops the previous value of \c{BP}, and returns via \c{RETF}. It uses the form of \c{RETF} with an immediate parameter, giving the number of bytes taken up by the parameters on the stack. This causes the parameters to be removed from the stack as a side effect of the return instruction. \b When the caller regains control from the callee, the function parameters have already been removed from the stack, so it needs to do nothing further. Thus, you would define a function in Pascal style, taking two \c{Integer}-type parameters, in the following way: \c global myfunc \c \c myfunc: push bp \c mov bp,sp \c sub sp,0x40 ; 64 bytes of local stack space \c mov bx,[bp+8] ; first parameter to function \c mov bx,[bp+6] ; second parameter to function \c \c ; some more code \c \c mov sp,bp ; undo "sub sp,0x40" above \c pop bp \c retf 4 ; total size of params is 4 At the other end of the process, to call a Pascal function from your assembly code, you would do something like this: \c extern SomeFunc \c \c ; and then, further down... \c \c push word seg mystring ; Now push the segment, and... \c push word mystring ; ... offset of "mystring" \c push word [myint] ; one of my variables \c call far SomeFunc This is equivalent to the Pascal code \c procedure SomeFunc(String: PChar; Int: Integer); \c SomeFunc(@mystring, myint); \S{16bpseg} Borland Pascal \I{segment names, Borland Pascal}Segment Name Restrictions Since Borland Pascal's internal unit file format is completely different from \c{OBJ}, it only makes a very sketchy job of actually reading and understanding the various information contained in a real \c{OBJ} file when it links that in. Therefore an object file intended to be linked to a Pascal program must obey a number of restrictions: \b Procedures and functions must be in a segment whose name is either \c{CODE}, \c{CSEG}, or something ending in \c{_TEXT}. \b initialized data must be in a segment whose name is either \c{CONST} or something ending in \c{_DATA}. \b Uninitialized data must be in a segment whose name is either \c{DATA}, \c{DSEG}, or something ending in \c{_BSS}. \b Any other segments in the object file are completely ignored. \c{GROUP} directives and segment attributes are also ignored. \S{16bpmacro} Using \i\c{c16.mac} With Pascal Programs The \c{c16.mac} macro package, described in \k{16cmacro}, can also be used to simplify writing functions to be called from Pascal programs, if you code \I\c{PASCAL}\c{%define PASCAL}. This definition ensures that functions are far (it implies \i\c{FARCODE}), and also causes procedure return instructions to be generated with an operand. Defining \c{PASCAL} does not change the code which calculates the argument offsets; you must declare your function's arguments in reverse order. For example: \c %define PASCAL \c \c proc _pascalproc \c \c %$j arg 4 \c %$i arg \c mov ax,[bp + %$i] \c mov bx,[bp + %$j] \c mov es,[bp + %$j + 2] \c add ax,[bx] \c \c endproc This defines the same routine, conceptually, as the example in \k{16cmacro}: it defines a function taking two arguments, an integer and a pointer to an integer, which returns the sum of the integer and the contents of the pointer. The only difference between this code and the large-model C version is that \c{PASCAL} is defined instead of \c{FARCODE}, and that the arguments are declared in reverse order. \C{32bit} Writing 32-bit Code (Unix, Win32, DJGPP) This chapter attempts to cover some of the common issues involved when writing 32-bit code, to run under \i{Win32} or Unix, or to be linked with C code generated by a Unix-style C compiler such as \i{DJGPP}. It covers how to write assembly code to interface with 32-bit C routines, and how to write position-independent code for shared libraries. Almost all 32-bit code, and in particular all code running under \c{Win32}, \c{DJGPP} or any of the PC Unix variants, runs in \I{flat memory model}\e{flat} memory model. This means that the segment registers and paging have already been set up to give you the same 32-bit 4Gb address space no matter what segment you work relative to, and that you should ignore all segment registers completely. When writing flat-model application code, you never need to use a segment override or modify any segment register, and the code-section addresses you pass to \c{CALL} and \c{JMP} live in the same address space as the data-section addresses you access your variables by and the stack-section addresses you access local variables and procedure parameters by. Every address is 32 bits long and contains only an offset part. \H{32c} Interfacing to 32-bit C Programs A lot of the discussion in \k{16c}, about interfacing to 16-bit C programs, still applies when working in 32 bits. The absence of memory models or segmentation worries simplifies things a lot. \S{32cunder} External Symbol Names Most 32-bit C compilers share the convention used by 16-bit compilers, that the names of all global symbols (functions or data) they define are formed by prefixing an underscore to the name as it appears in the C program. However, not all of them do: the \c{ELF} specification states that C symbols do \e{not} have a leading underscore on their assembly-language names. The older Linux \c{a.out} C compiler, all \c{Win32} compilers, \c{DJGPP}, and \c{NetBSD} and \c{FreeBSD}, all use the leading underscore; for these compilers, the macros \c{cextern} and \c{cglobal}, as given in \k{16cunder}, will still work. For \c{ELF}, though, the leading underscore should not be used. See also \k{opt-pfix}. \S{32cfunc} Function Definitions and Function Calls \I{functions, C calling convention}The \i{C calling convention} in 32-bit programs is as follows. In the following description, the words \e{caller} and \e{callee} are used to denote the function doing the calling and the function which gets called. \b The caller pushes the function's parameters on the stack, one after another, in reverse order (right to left, so that the first argument specified to the function is pushed last). \b The caller then executes a near \c{CALL} instruction to pass control to the callee. \b The callee receives control, and typically (although this is not actually necessary, in functions which do not need to access their parameters) starts by saving the value of \c{ESP} in \c{EBP} so as to be able to use \c{EBP} as a base pointer to find its parameters on the stack. However, the caller was probably doing this too, so part of the calling convention states that \c{EBP} must be preserved by any C function. Hence the callee, if it is going to set up \c{EBP} as a \i{frame pointer}, must push the previous value first. \b The callee may then access its parameters relative to \c{EBP}. The doubleword at \c{[EBP]} holds the previous value of \c{EBP} as it was pushed; the next doubleword, at \c{[EBP+4]}, holds the return address, pushed implicitly by \c{CALL}. The parameters start after that, at \c{[EBP+8]}. The leftmost parameter of the function, since it was pushed last, is accessible at this offset from \c{EBP}; the others follow, at successively greater offsets. Thus, in a function such as \c{printf} which takes a variable number of parameters, the pushing of the parameters in reverse order means that the function knows where to find its first parameter, which tells it the number and type of the remaining ones. \b The callee may also wish to decrease \c{ESP} further, so as to allocate space on the stack for local variables, which will then be accessible at negative offsets from \c{EBP}. \b The callee, if it wishes to return a value to the caller, should leave the value in \c{AL}, \c{AX} or \c{EAX} depending on the size of the value. Floating-point results are typically returned in \c{ST0}. \b Once the callee has finished processing, it restores \c{ESP} from \c{EBP} if it had allocated local stack space, then pops the previous value of \c{EBP}, and returns via \c{RET} (equivalently, \c{RETN}). \b When the caller regains control from the callee, the function parameters are still on the stack, so it typically adds an immediate constant to \c{ESP} to remove them (instead of executing a number of slow \c{POP} instructions). Thus, if a function is accidentally called with the wrong number of parameters due to a prototype mismatch, the stack will still be returned to a sensible state since the caller, which \e{knows} how many parameters it pushed, does the removing. There is an alternative calling convention used by Win32 programs for Windows API calls, and also for functions called \e{by} the Windows API such as window procedures: they follow what Microsoft calls the \c{__stdcall} convention. This is slightly closer to the Pascal convention, in that the callee clears the stack by passing a parameter to the \c{RET} instruction. However, the parameters are still pushed in right-to-left order. Thus, you would define a function in C style in the following way: \c global _myfunc \c \c _myfunc: \c push ebp \c mov ebp,esp \c sub esp,0x40 ; 64 bytes of local stack space \c mov ebx,[ebp+8] ; first parameter to function \c \c ; some more code \c \c leave ; mov esp,ebp / pop ebp \c ret At the other end of the process, to call a C function from your assembly code, you would do something like this: \c extern _printf \c \c ; and then, further down... \c \c push dword [myint] ; one of my integer variables \c push dword mystring ; pointer into my data segment \c call _printf \c add esp,byte 8 ; `byte' saves space \c \c ; then those data items... \c \c segment _DATA \c \c myint dd 1234 \c mystring db 'This number -> %d <- should be 1234',10,0 This piece of code is the assembly equivalent of the C code \c int myint = 1234; \c printf("This number -> %d <- should be 1234\n", myint); \S{32cdata} Accessing Data Items To get at the contents of C variables, or to declare variables which C can access, you need only declare the names as \c{GLOBAL} or \c{EXTERN}. (Again, the names require leading underscores, as stated in \k{32cunder}.) Thus, a C variable declared as \c{int i} can be accessed from assembler as \c extern _i \c mov eax,[_i] And to declare your own integer variable which C programs can access as \c{extern int j}, you do this (making sure you are assembling in the \c{_DATA} segment, if necessary): \c global _j \c _j dd 0 To access a C array, you need to know the size of the components of the array. For example, \c{int} variables are four bytes long, so if a C program declares an array as \c{int a[10]}, you can access \c{a[3]} by coding \c{mov ax,[_a+12]}. (The byte offset 12 is obtained by multiplying the desired array index, 3, by the size of the array element, 4.) The sizes of the C base types in 32-bit compilers are: 1 for \c{char}, 2 for \c{short}, 4 for \c{int}, \c{long} and \c{float}, and 8 for \c{double}. Pointers, being 32-bit addresses, are also 4 bytes long. To access a C \i{data structure}, you need to know the offset from the base of the structure to the field you are interested in. You can either do this by converting the C structure definition into a NASM structure definition (using \c{STRUC}), or by calculating the one offset and using just that. To do either of these, you should read your C compiler's manual to find out how it organizes data structures. NASM gives no special alignment to structure members in its own \i\c{STRUC} macro, so you have to specify alignment yourself if the C compiler generates it. Typically, you might find that a structure like \c struct { \c char c; \c int i; \c } foo; might be eight bytes long rather than five, since the \c{int} field would be aligned to a four-byte boundary. However, this sort of feature is sometimes a configurable option in the C compiler, either using command-line options or \c{#pragma} lines, so you have to find out how your own compiler does it. \S{32cmacro} \i\c{c32.mac}: Helper Macros for the 32-bit C Interface Included in the NASM archives, in the \I{misc directory}\c{misc} directory, is a file \c{c32.mac} of macros. It defines three macros: \i\c{proc}, \i\c{arg} and \i\c{endproc}. These are intended to be used for C-style procedure definitions, and they automate a lot of the work involved in keeping track of the calling convention. An example of an assembly function using the macro set is given here: \c proc _proc32 \c \c %$i arg \c %$j arg \c mov eax,[ebp + %$i] \c mov ebx,[ebp + %$j] \c add eax,[ebx] \c \c endproc This defines \c{_proc32} to be a procedure taking two arguments, the first (\c{i}) an integer and the second (\c{j}) a pointer to an integer. It returns \c{i + *j}. Note that the \c{arg} macro has an \c{EQU} as the first line of its expansion, and since the label before the macro call gets prepended to the first line of the expanded macro, the \c{EQU} works, defining \c{%$i} to be an offset from \c{BP}. A context-local variable is used, local to the context pushed by the \c{proc} macro and popped by the \c{endproc} macro, so that the same argument name can be used in later procedures. Of course, you don't \e{have} to do that. \c{arg} can take an optional parameter, giving the size of the argument. If no size is given, 4 is assumed, since it is likely that many function parameters will be of type \c{int} or pointers. \H{picdll} Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF \i{Shared Libraries} \c{ELF} replaced the older \c{a.out} object file format under Linux because it contains support for \i{position-independent code} (\i{PIC}), which makes writing shared libraries much easier. NASM supports the \c{ELF} position-independent code features, so you can write Linux \c{ELF} shared libraries in NASM. \i{NetBSD}, and its close cousins \i{FreeBSD} and \i{OpenBSD}, take a different approach by hacking PIC support into the \c{a.out} format. NASM supports this as the \i\c{aoutb} output format, so you can write \i{BSD} shared libraries in NASM too. The operating system loads a PIC shared library by memory-mapping the library file at an arbitrarily chosen point in the address space of the running process. The contents of the library's code section must therefore not depend on where it is loaded in memory. Therefore, you cannot get at your variables by writing code like this: \c mov eax,[myvar] ; WRONG Instead, the linker provides an area of memory called the \i\e{global offset table}, or \i{GOT}; the GOT is situated at a constant distance from your library's code, so if you can find out where your library is loaded (which is typically done using a \c{CALL} and \c{POP} combination), you can obtain the address of the GOT, and you can then load the addresses of your variables out of linker-generated entries in the GOT. The \e{data} section of a PIC shared library does not have these restrictions: since the data section is writable, it has to be copied into memory anyway rather than just paged in from the library file, so as long as it's being copied it can be relocated too. So you can put ordinary types of relocation in the data section without too much worry (but see \k{picglobal} for a caveat). \S{picgot} Obtaining the Address of the GOT Each code module in your shared library should define the GOT as an external symbol: \c extern _GLOBAL_OFFSET_TABLE_ ; in ELF \c extern __GLOBAL_OFFSET_TABLE_ ; in BSD a.out At the beginning of any function in your shared library which plans to access your data or BSS sections, you must first calculate the address of the GOT. This is typically done by writing the function in this form: \c func: push ebp \c mov ebp,esp \c push ebx \c call .get_GOT \c .get_GOT: \c pop ebx \c add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc \c \c ; the function body comes here \c \c mov ebx,[ebp-4] \c mov esp,ebp \c pop ebp \c ret (For BSD, again, the symbol \c{_GLOBAL_OFFSET_TABLE} requires a second leading underscore.) The first two lines of this function are simply the standard C prologue to set up a stack frame, and the last three lines are standard C function epilogue. The third line, and the fourth to last line, save and restore the \c{EBX} register, because PIC shared libraries use this register to store the address of the GOT. The interesting bit is the \c{CALL} instruction and the following two lines. The \c{CALL} and \c{POP} combination obtains the address of the label \c{.get_GOT}, without having to know in advance where the program was loaded (since the \c{CALL} instruction is encoded relative to the current position). The \c{ADD} instruction makes use of one of the special PIC relocation types: \i{GOTPC relocation}. With the \i\c{WRT ..gotpc} qualifier specified, the symbol referenced (here \c{_GLOBAL_OFFSET_TABLE_}, the special symbol assigned to the GOT) is given as an offset from the beginning of the section. (Actually, \c{ELF} encodes it as the offset from the operand field of the \c{ADD} instruction, but NASM simplifies this deliberately, so you do things the same way for both \c{ELF} and \c{BSD}.) So the instruction then \e{adds} the beginning of the section, to get the real address of the GOT, and subtracts the value of \c{.get_GOT} which it knows is in \c{EBX}. Therefore, by the time that instruction has finished, \c{EBX} contains the address of the GOT. If you didn't follow that, don't worry: it's never necessary to obtain the address of the GOT by any other means, so you can put those three instructions into a macro and safely ignore them: \c %macro get_GOT 0 \c \c call %%getgot \c %%getgot: \c pop ebx \c add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc \c \c %endmacro \S{piclocal} Finding Your Local Data Items Having got the GOT, you can then use it to obtain the addresses of your data items. Most variables will reside in the sections you have declared; they can be accessed using the \I{GOTOFF relocation}\c{..gotoff} special \I\c{WRT ..gotoff}\c{WRT} type. The way this works is like this: \c lea eax,[ebx+myvar wrt ..gotoff] The expression \c{myvar wrt ..gotoff} is calculated, when the shared library is linked, to be the offset to the local variable \c{myvar} from the beginning of the GOT. Therefore, adding it to \c{EBX} as above will place the real address of \c{myvar} in \c{EAX}. If you declare variables as \c{GLOBAL} without specifying a size for them, they are shared between code modules in the library, but do not get exported from the library to the program that loaded it. They will still be in your ordinary data and BSS sections, so you can access them in the same way as local variables, using the above \c{..gotoff} mechanism. Note that due to a peculiarity of the way BSD \c{a.out} format handles this relocation type, there must be at least one non-local symbol in the same section as the address you're trying to access. \S{picextern} Finding External and Common Data Items If your library needs to get at an external variable (external to the \e{library}, not just to one of the modules within it), you must use the \I{GOT relocations}\I\c{WRT ..got}\c{..got} type to get at it. The \c{..got} type, instead of giving you the offset from the GOT base to the variable, gives you the offset from the GOT base to a GOT \e{entry} containing the address of the variable. The linker will set up this GOT entry when it builds the library, and the dynamic linker will place the correct address in it at load time. So to obtain the address of an external variable \c{extvar} in \c{EAX}, you would code \c mov eax,[ebx+extvar wrt ..got] This loads the address of \c{extvar} out of an entry in the GOT. The linker, when it builds the shared library, collects together every relocation of type \c{..got}, and builds the GOT so as to ensure it has every necessary entry present. Common variables must also be accessed in this way. \S{picglobal} Exporting Symbols to the Library User If you want to export symbols to the user of the library, you have to declare whether they are functions or data, and if they are data, you have to give the size of the data item. This is because the dynamic linker has to build \I{PLT}\i{procedure linkage table} entries for any exported functions, and also moves exported data items away from the library's data section in which they were declared. So to export a function to users of the library, you must use \c global func:function ; declare it as a function \c \c func: push ebp \c \c ; etc. And to export a data item such as an array, you would have to code \c global array:data array.end-array ; give the size too \c \c array: resd 128 \c .end: Be careful: If you export a variable to the library user, by declaring it as \c{GLOBAL} and supplying a size, the variable will end up living in the data section of the main program, rather than in your library's data section, where you declared it. So you will have to access your own global variable with the \c{..got} mechanism rather than \c{..gotoff}, as if it were external (which, effectively, it has become). Equally, if you need to store the address of an exported global in one of your data sections, you can't do it by means of the standard sort of code: \c dataptr: dd global_data_item ; WRONG NASM will interpret this code as an ordinary relocation, in which \c{global_data_item} is merely an offset from the beginning of the \c{.data} section (or whatever); so this reference will end up pointing at your data section instead of at the exported global which resides elsewhere. Instead of the above code, then, you must write \c dataptr: dd global_data_item wrt ..sym which makes use of the special \c{WRT} type \I\c{WRT ..sym}\c{..sym} to instruct NASM to search the symbol table for a particular symbol at that address, rather than just relocating by section base. Either method will work for functions: referring to one of your functions by means of \c funcptr: dd my_function will give the user the address of the code you wrote, whereas \c funcptr: dd my_function wrt ..sym will give the address of the procedure linkage table for the function, which is where the calling program will \e{believe} the function lives. Either address is a valid way to call the function. \S{picproc} Calling Procedures Outside the Library Calling procedures outside your shared library has to be done by means of a \i\e{procedure linkage table}, or \i{PLT}. The PLT is placed at a known offset from where the library is loaded, so the library code can make calls to the PLT in a position-independent way. Within the PLT there is code to jump to offsets contained in the GOT, so function calls to other shared libraries or to routines in the main program can be transparently passed off to their real destinations. To call an external routine, you must use another special PIC relocation type, \I{PLT relocations}\i\c{WRT ..plt}. This is much easier than the GOT-based ones: you simply replace calls such as \c{CALL printf} with the PLT-relative version \c{CALL printf WRT ..plt}. \S{link} Generating the Library File Having written some code modules and assembled them to \c{.o} files, you then generate your shared library with a command such as \c ld -shared -o library.so module1.o module2.o # for ELF \c ld -Bshareable -o library.so module1.o module2.o # for BSD For ELF, if your shared library is going to reside in system directories such as \c{/usr/lib} or \c{/lib}, it is usually worth using the \i\c{-soname} flag to the linker, to store the final library file name, with a version number, into the library: \c ld -shared -soname library.so.1 -o library.so.1.2 *.o You would then copy \c{library.so.1.2} into the library directory, and create \c{library.so.1} as a symbolic link to it. \C{mixsize} Mixing 16 and 32 Bit Code This chapter tries to cover some of the issues, largely related to unusual forms of addressing and jump instructions, encountered when writing operating system code such as protected-mode initialisation routines, which require code that operates in mixed segment sizes, such as code in a 16-bit segment trying to modify data in a 32-bit one, or jumps between different-size segments. \H{mixjump} Mixed-Size Jumps\I{jumps, mixed-size} \I{operating system, writing}\I{writing operating systems}The most common form of \i{mixed-size instruction} is the one used when writing a 32-bit OS: having done your setup in 16-bit mode, such as loading the kernel, you then have to boot it by switching into protected mode and jumping to the 32-bit kernel start address. In a fully 32-bit OS, this tends to be the \e{only} mixed-size instruction you need, since everything before it can be done in pure 16-bit code, and everything after it can be pure 32-bit. This jump must specify a 48-bit far address, since the target segment is a 32-bit one. However, it must be assembled in a 16-bit segment, so just coding, for example, \c jmp 0x1234:0x56789ABC ; wrong! will not work, since the offset part of the address will be truncated to \c{0x9ABC} and the jump will be an ordinary 16-bit far one. The Linux kernel setup code gets round the inability of \c{as86} to generate the required instruction by coding it manually, using \c{DB} instructions. NASM can go one better than that, by actually generating the right instruction itself. Here's how to do it right: \c jmp dword 0x1234:0x56789ABC ; right \I\c{JMP DWORD}The \c{DWORD} prefix (strictly speaking, it should come \e{after} the colon, since it is declaring the \e{offset} field to be a doubleword; but NASM will accept either form, since both are unambiguous) forces the offset part to be treated as far, in the assumption that you are deliberately writing a jump from a 16-bit segment to a 32-bit one. You can do the reverse operation, jumping from a 32-bit segment to a 16-bit one, by means of the \c{WORD} prefix: \c jmp word 0x8765:0x4321 ; 32 to 16 bit If the \c{WORD} prefix is specified in 16-bit mode, or the \c{DWORD} prefix in 32-bit mode, they will be ignored, since each is explicitly forcing NASM into a mode it was in anyway. \H{mixaddr} Addressing Between Different-Size Segments\I{addressing, mixed-size}\I{mixed-size addressing} If your OS is mixed 16 and 32-bit, or if you are writing a DOS extender, you are likely to have to deal with some 16-bit segments and some 32-bit ones. At some point, you will probably end up writing code in a 16-bit segment which has to access data in a 32-bit segment, or vice versa. If the data you are trying to access in a 32-bit segment lies within the first 64K of the segment, you may be able to get away with using an ordinary 16-bit addressing operation for the purpose; but sooner or later, you will want to do 32-bit addressing from 16-bit mode. The easiest way to do this is to make sure you use a register for the address, since any effective address containing a 32-bit register is forced to be a 32-bit address. So you can do \c mov eax,offset_into_32_bit_segment_specified_by_fs \c mov dword [fs:eax],0x11223344 This is fine, but slightly cumbersome (since it wastes an instruction and a register) if you already know the precise offset you are aiming at. The x86 architecture does allow 32-bit effective addresses to specify nothing but a 4-byte offset, so why shouldn't NASM be able to generate the best instruction for the purpose? It can. As in \k{mixjump}, you need only prefix the address with the \c{DWORD} keyword, and it will be forced to be a 32-bit address: \c mov dword [fs:dword my_offset],0x11223344 Also as in \k{mixjump}, NASM is not fussy about whether the \c{DWORD} prefix comes before or after the segment override, so arguably a nicer-looking way to code the above instruction is \c mov dword [dword fs:my_offset],0x11223344 Don't confuse the \c{DWORD} prefix \e{outside} the square brackets, which controls the size of the data stored at the address, with the one \c{inside} the square brackets which controls the length of the address itself. The two can quite easily be different: \c mov word [dword 0x12345678],0x9ABC This moves 16 bits of data to an address specified by a 32-bit offset. You can also specify \c{WORD} or \c{DWORD} prefixes along with the \c{FAR} prefix to indirect far jumps or calls. For example: \c call dword far [fs:word 0x4321] This instruction contains an address specified by a 16-bit offset; it loads a 48-bit far pointer from that (16-bit segment and 32-bit offset), and calls that address. \H{mixother} Other Mixed-Size Instructions The other way you might want to access data might be using the string instructions (\c{LODSx}, \c{STOSx} and so on) or the \c{XLATB} instruction. These instructions, since they take no parameters, might seem to have no easy way to make them perform 32-bit addressing when assembled in a 16-bit segment. This is the purpose of NASM's \i\c{a16}, \i\c{a32} and \i\c{a64} prefixes. If you are coding \c{LODSB} in a 16-bit segment but it is supposed to be accessing a string in a 32-bit segment, you should load the desired address into \c{ESI} and then code \c a32 lodsb The prefix forces the addressing size to 32 bits, meaning that \c{LODSB} loads from \c{[DS:ESI]} instead of \c{[DS:SI]}. To access a string in a 16-bit segment when coding in a 32-bit one, the corresponding \c{a16} prefix can be used. The \c{a16}, \c{a32} and \c{a64} prefixes can be applied to any instruction in NASM's instruction table, but most of them can generate all the useful forms without them. The prefixes are necessary only for instructions with implicit addressing: \# \c{CMPSx} (\k{insCMPSB}), \# \c{SCASx} (\k{insSCASB}), \c{LODSx} (\k{insLODSB}), \c{STOSx} \# (\k{insSTOSB}), \c{MOVSx} (\k{insMOVSB}), \c{INSx} (\k{insINSB}), \# \c{OUTSx} (\k{insOUTSB}), and \c{XLATB} (\k{insXLATB}). \c{CMPSx}, \c{SCASx}, \c{LODSx}, \c{STOSx}, \c{MOVSx}, \c{INSx}, \c{OUTSx}, and \c{XLATB}. Also, the various push and pop instructions (\c{PUSHA} and \c{POPF} as well as the more usual \c{PUSH} and \c{POP}) can accept \c{a16}, \c{a32} or \c{a64} prefixes to force a particular one of \c{SP}, \c{ESP} or \c{RSP} to be used as a stack pointer, in case the stack segment in use is a different size from the code segment. \c{PUSH} and \c{POP}, when applied to segment registers in 32-bit mode, also have the slightly odd behaviour that they push and pop 4 bytes at a time, of which the top two are ignored and the bottom two give the value of the segment register being manipulated. To force the 16-bit behaviour of segment-register push and pop instructions, you can use the operand-size prefix \i\c{o16}: \c o16 push ss \c o16 push ds This code saves a doubleword of stack space by fitting two segment registers into the space which would normally be consumed by pushing one. (You can also use the \i\c{o32} prefix to force the 32-bit behaviour when in 16-bit mode, but this seems less useful.) \C{64bit} Writing 64-bit Code (Unix, Win64) This chapter attempts to cover some of the common issues involved when writing 64-bit code, to run under \i{Win64} or Unix. It covers how to write assembly code to interface with 64-bit C routines, and how to write position-independent code for shared libraries. All 64-bit code uses a flat memory model, since segmentation is not available in 64-bit mode. The one exception is the \c{FS} and \c{GS} registers, which still add their bases. Position independence in 64-bit mode is significantly simpler, since the processor supports \c{RIP}-relative addressing directly; see the \c{REL} keyword (\k{effaddr}). On most 64-bit platforms, it is probably desirable to make that the default, using the directive \c{DEFAULT REL} (\k{default}). 64-bit programming is relatively similar to 32-bit programming, but of course pointers are 64 bits long; additionally, all existing platforms pass arguments in registers rather than on the stack. Furthermore, 64-bit platforms use SSE2 by default for floating point. Please see the ABI documentation for your platform. 64-bit platforms differ in the sizes of the fundamental datatypes, not just from 32-bit platforms but from each other. If a specific size data type is desired, it is probably best to use the types defined in the Standard C header \c{<inttypes.h>}. In 64-bit mode, the default instruction size is still 32 bits. When loading a value into a 32-bit register (but not an 8- or 16-bit register), the upper 32 bits of the corresponding 64-bit register are set to zero. \H{reg64} Register Names in 64-bit Mode NASM uses the following names for general-purpose registers in 64-bit mode, for 8-, 16-, 32- and 64-bit references, respectively: \c AL/AH, CL/CH, DL/DH, BL/BH, SPL, BPL, SIL, DIL, R8B-R15B \c AX, CX, DX, BX, SP, BP, SI, DI, R8W-R15W \c EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI, R8D-R15D \c RAX, RCX, RDX, RBX, RSP, RBP, RSI, RDI, R8-R15 This is consistent with the AMD documentation and most other assemblers. The Intel documentation, however, uses the names \c{R8L-R15L} for 8-bit references to the higher registers. It is possible to use those names by definiting them as macros; similarly, if one wants to use numeric names for the low 8 registers, define them as macros. The standard macro package \c{altreg} (see \k{pkg_altreg}) can be used for this purpose. \H{id64} Immediates and Displacements in 64-bit Mode In 64-bit mode, immediates and displacements are generally only 32 bits wide. NASM will therefore truncate most displacements and immediates to 32 bits. The only instruction which takes a full \i{64-bit immediate} is: \c MOV reg64,imm64 NASM will produce this instruction whenever the programmer uses \c{MOV} with an immediate into a 64-bit register. If this is not desirable, simply specify the equivalent 32-bit register, which will be automatically zero-extended by the processor, or specify the immediate as \c{DWORD}: \c mov rax,foo ; 64-bit immediate \c mov rax,qword foo ; (identical) \c mov eax,foo ; 32-bit immediate, zero-extended \c mov rax,dword foo ; 32-bit immediate, sign-extended The length of these instructions are 10, 5 and 7 bytes, respectively. The only instructions which take a full \I{64-bit displacement}64-bit \e{displacement} is loading or storing, using \c{MOV}, \c{AL}, \c{AX}, \c{EAX} or \c{RAX} (but no other registers) to an absolute 64-bit address. Since this is a relatively rarely used instruction (64-bit code generally uses relative addressing), the programmer has to explicitly declare the displacement size as \c{QWORD}: \c default abs \c \c mov eax,[foo] ; 32-bit absolute disp, sign-extended \c mov eax,[a32 foo] ; 32-bit absolute disp, zero-extended \c mov eax,[qword foo] ; 64-bit absolute disp \c \c default rel \c \c mov eax,[foo] ; 32-bit relative disp \c mov eax,[a32 foo] ; d:o, address truncated to 32 bits(!) \c mov eax,[qword foo] ; error \c mov eax,[abs qword foo] ; 64-bit absolute disp A sign-extended absolute displacement can access from -2 GB to +2 GB; a zero-extended absolute displacement can access from 0 to 4 GB. \H{unix64} Interfacing to 64-bit C Programs (Unix) On Unix, the 64-bit ABI as well as the x32 ABI (32-bit ABI with the CPU in 64-bit mode) is defined by the documents at: \W{http://www.nasm.us/abi/unix64}\c{http://www.nasm.us/abi/unix64} Although written for AT&T-syntax assembly, the concepts apply equally well for NASM-style assembly. What follows is a simplified summary. The first six integer arguments (from the left) are passed in \c{RDI}, \c{RSI}, \c{RDX}, \c{RCX}, \c{R8}, and \c{R9}, in that order. Additional integer arguments are passed on the stack. These registers, plus \c{RAX}, \c{R10} and \c{R11} are destroyed by function calls, and thus are available for use by the function without saving. Integer return values are passed in \c{RAX} and \c{RDX}, in that order. Floating point is done using SSE registers, except for \c{long double}. Floating-point arguments are passed in \c{XMM0} to \c{XMM7}; return is \c{XMM0} and \c{XMM1}. \c{long double} are passed on the stack, and returned in \c{ST0} and \c{ST1}. All SSE and x87 registers are destroyed by function calls. On 64-bit Unix, \c{long} is 64 bits. Integer and SSE register arguments are counted separately, so for the case of \c void foo(long a, double b, int c) \c{a} is passed in \c{RDI}, \c{b} in \c{XMM0}, and \c{c} in \c{ESI}. \H{win64} Interfacing to 64-bit C Programs (Win64) The Win64 ABI is described by the document at: \W{http://www.nasm.us/abi/win64}\c{http://www.nasm.us/abi/win64} What follows is a simplified summary. The first four integer arguments are passed in \c{RCX}, \c{RDX}, \c{R8} and \c{R9}, in that order. Additional integer arguments are passed on the stack. These registers, plus \c{RAX}, \c{R10} and \c{R11} are destroyed by function calls, and thus are available for use by the function without saving. Integer return values are passed in \c{RAX} only. Floating point is done using SSE registers, except for \c{long double}. Floating-point arguments are passed in \c{XMM0} to \c{XMM3}; return is \c{XMM0} only. On Win64, \c{long} is 32 bits; \c{long long} or \c{_int64} is 64 bits. Integer and SSE register arguments are counted together, so for the case of \c void foo(long long a, double b, int c) \c{a} is passed in \c{RCX}, \c{b} in \c{XMM1}, and \c{c} in \c{R8D}. \C{trouble} Troubleshooting This chapter describes some of the common problems that users have been known to encounter with NASM, and answers them. If you think you have found a bug in NASM, please see \k{bugs}. \H{problems} Common Problems \S{inefficient} NASM Generates \i{Inefficient Code} We sometimes get `bug' reports about NASM generating inefficient, or even `wrong', code on instructions such as \c{ADD ESP,8}. This is a deliberate design feature, connected to predictability of output: NASM, on seeing \c{ADD ESP,8}, will generate the form of the instruction which leaves room for a 32-bit offset. You need to code \I\c{BYTE}\c{ADD ESP,BYTE 8} if you want the space-efficient form of the instruction. This isn't a bug, it's user error: if you prefer to have NASM produce the more efficient code automatically enable optimization with the \c{-O} option (see \k{opt-O}). \S{jmprange} My Jumps are Out of Range\I{out of range, jumps} Similarly, people complain that when they issue \i{conditional jumps} (which are \c{SHORT} by default) that try to jump too far, NASM reports `short jump out of range' instead of making the jumps longer. This, again, is partly a predictability issue, but in fact has a more practical reason as well. NASM has no means of being told what type of processor the code it is generating will be run on; so it cannot decide for itself that it should generate \i\c{Jcc NEAR} type instructions, because it doesn't know that it's working for a 386 or above. Alternatively, it could replace the out-of-range short \c{JNE} instruction with a very short \c{JE} instruction that jumps over a \c{JMP NEAR}; this is a sensible solution for processors below a 386, but hardly efficient on processors which have good branch prediction \e{and} could have used \c{JNE NEAR} instead. So, once again, it's up to the user, not the assembler, to decide what instructions should be generated. See \k{opt-O}. \S{proborg} \i\c{ORG} Doesn't Work People writing \i{boot sector} programs in the \c{bin} format often complain that \c{ORG} doesn't work the way they'd like: in order to place the \c{0xAA55} signature word at the end of a 512-byte boot sector, people who are used to MASM tend to code \c ORG 0 \c \c ; some boot sector code \c \c ORG 510 \c DW 0xAA55 This is not the intended use of the \c{ORG} directive in NASM, and will not work. The correct way to solve this problem in NASM is to use the \i\c{TIMES} directive, like this: \c ORG 0 \c \c ; some boot sector code \c \c TIMES 510-($-$$) DB 0 \c DW 0xAA55 The \c{TIMES} directive will insert exactly enough zero bytes into the output to move the assembly point up to 510. This method also has the advantage that if you accidentally fill your boot sector too full, NASM will catch the problem at assembly time and report it, so you won't end up with a boot sector that you have to disassemble to find out what's wrong with it. \S{probtimes} \i\c{TIMES} Doesn't Work The other common problem with the above code is people who write the \c{TIMES} line as \c TIMES 510-$ DB 0 by reasoning that \c{$} should be a pure number, just like 510, so the difference between them is also a pure number and can happily be fed to \c{TIMES}. NASM is a \e{modular} assembler: the various component parts are designed to be easily separable for re-use, so they don't exchange information unnecessarily. In consequence, the \c{bin} output format, even though it has been told by the \c{ORG} directive that the \c{.text} section should start at 0, does not pass that information back to the expression evaluator. So from the evaluator's point of view, \c{$} isn't a pure number: it's an offset from a section base. Therefore the difference between \c{$} and 510 is also not a pure number, but involves a section base. Values involving section bases cannot be passed as arguments to \c{TIMES}. The solution, as in the previous section, is to code the \c{TIMES} line in the form \c TIMES 510-($-$$) DB 0 in which \c{$} and \c{$$} are offsets from the same section base, and so their difference is a pure number. This will solve the problem and generate sensible code. \A{ndisasm} \i{Ndisasm} The Netwide Disassembler, NDISASM \H{ndisintro} Introduction The Netwide Disassembler is a small companion program to the Netwide Assembler, NASM. It seemed a shame to have an x86 assembler, complete with a full instruction table, and not make as much use of it as possible, so here's a disassembler which shares the instruction table (and some other bits of code) with NASM. The Netwide Disassembler does nothing except to produce disassemblies of \e{binary} source files. NDISASM does not have any understanding of object file formats, like \c{objdump}, and it will not understand \c{DOS .EXE} files like \c{debug} will. It just disassembles. \H{ndisrun} Running NDISASM To disassemble a file, you will typically use a command of the form \c ndisasm -b {16|32|64} filename NDISASM can disassemble 16-, 32- or 64-bit code equally easily, provided of course that you remember to specify which it is to work with. If no \i\c{-b} switch is present, NDISASM works in 16-bit mode by default. The \i\c{-u} switch (for USE32) also invokes 32-bit mode. Two more command line options are \i\c{-r} which reports the version number of NDISASM you are running, and \i\c{-h} which gives a short summary of command line options. \S{ndiscom} COM Files: Specifying an Origin To disassemble a \c{DOS .COM} file correctly, a disassembler must assume that the first instruction in the file is loaded at address \c{0x100}, rather than at zero. NDISASM, which assumes by default that any file you give it is loaded at zero, will therefore need to be informed of this. The \i\c{-o} option allows you to declare a different origin for the file you are disassembling. Its argument may be expressed in any of the NASM numeric formats: decimal by default, if it begins with `\c{$}' or `\c{0x}' or ends in `\c{H}' it's \c{hex}, if it ends in `\c{Q}' it's \c{octal}, and if it ends in `\c{B}' it's \c{binary}. Hence, to disassemble a \c{.COM} file: \c ndisasm -o100h filename.com will do the trick. \S{ndissync} Code Following Data: Synchronisation Suppose you are disassembling a file which contains some data which isn't machine code, and \e{then} contains some machine code. NDISASM will faithfully plough through the data section, producing machine instructions wherever it can (although most of them will look bizarre, and some may have unusual prefixes, e.g. `\c{FS OR AX,0x240A}'), and generating `DB' instructions ever so often if it's totally stumped. Then it will reach the code section. Supposing NDISASM has just finished generating a strange machine instruction from part of the data section, and its file position is now one byte \e{before} the beginning of the code section. It's entirely possible that another spurious instruction will get generated, starting with the final byte of the data section, and then the correct first instruction in the code section will not be seen because the starting point skipped over it. This isn't really ideal. To avoid this, you can specify a `\i\c{synchronisation}' point, or indeed as many synchronisation points as you like (although NDISASM can only handle 2147483647 sync points internally). The definition of a sync point is this: NDISASM guarantees to hit sync points exactly during disassembly. If it is thinking about generating an instruction which would cause it to jump over a sync point, it will discard that instruction and output a `\c{db}' instead. So it \e{will} start disassembly exactly from the sync point, and so you \e{will} see all the instructions in your code section. Sync points are specified using the \i\c{-s} option: they are measured in terms of the program origin, not the file position. So if you want to synchronize after 32 bytes of a \c{.COM} file, you would have to do \c ndisasm -o100h -s120h file.com rather than \c ndisasm -o100h -s20h file.com As stated above, you can specify multiple sync markers if you need to, just by repeating the \c{-s} option. \S{ndisisync} Mixed Code and Data: Automatic (Intelligent) Synchronisation \I\c{auto-sync} Suppose you are disassembling the boot sector of a \c{DOS} floppy (maybe it has a virus, and you need to understand the virus so that you know what kinds of damage it might have done you). Typically, this will contain a \c{JMP} instruction, then some data, then the rest of the code. So there is a very good chance of NDISASM being \e{misaligned} when the data ends and the code begins. Hence a sync point is needed. On the other hand, why should you have to specify the sync point manually? What you'd do in order to find where the sync point would be, surely, would be to read the \c{JMP} instruction, and then to use its target address as a sync point. So can NDISASM do that for you? The answer, of course, is yes: using either of the synonymous switches \i\c{-a} (for automatic sync) or \i\c{-i} (for intelligent sync) will enable \c{auto-sync} mode. Auto-sync mode automatically generates a sync point for any forward-referring PC-relative jump or call instruction that NDISASM encounters. (Since NDISASM is one-pass, if it encounters a PC-relative jump whose target has already been processed, there isn't much it can do about it...) Only PC-relative jumps are processed, since an absolute jump is either through a register (in which case NDISASM doesn't know what the register contains) or involves a segment address (in which case the target code isn't in the same segment that NDISASM is working in, and so the sync point can't be placed anywhere useful). For some kinds of file, this mechanism will automatically put sync points in all the right places, and save you from having to place any sync points manually. However, it should be stressed that auto-sync mode is \e{not} guaranteed to catch all the sync points, and you may still have to place some manually. Auto-sync mode doesn't prevent you from declaring manual sync points: it just adds automatically generated ones to the ones you provide. It's perfectly feasible to specify \c{-i} \e{and} some \c{-s} options. Another caveat with auto-sync mode is that if, by some unpleasant fluke, something in your data section should disassemble to a PC-relative call or jump instruction, NDISASM may obediently place a sync point in a totally random place, for example in the middle of one of the instructions in your code section. So you may end up with a wrong disassembly even if you use auto-sync. Again, there isn't much I can do about this. If you have problems, you'll have to use manual sync points, or use the \c{-k} option (documented below) to suppress disassembly of the data area. \S{ndisother} Other Options The \i\c{-e} option skips a header on the file, by ignoring the first N bytes. This means that the header is \e{not} counted towards the disassembly offset: if you give \c{-e10 -o10}, disassembly will start at byte 10 in the file, and this will be given offset 10, not 20. The \i\c{-k} option is provided with two comma-separated numeric arguments, the first of which is an assembly offset and the second is a number of bytes to skip. This \e{will} count the skipped bytes towards the assembly offset: its use is to suppress disassembly of a data section which wouldn't contain anything you wanted to see anyway. \A{inslist} \i{Instruction List} \H{inslistintro} Introduction The following sections show the instructions which NASM currently supports. For each instruction, there is a separate entry for each supported addressing mode. The third column shows the processor type in which the instruction was introduced and, when appropriate, one or more usage flags. \& inslist.src \A{changelog} \i{NASM Version History} \& changes.src \A{source} Building NASM from Source The source code for NASM is available from our website, \W{http://www.nasm.us/}{http://wwww.nasm.us/}, see \k{website}. \H{tarball} Building from a Source Archive The source archives available on the web site should be capable of building on a number of platforms. This is the recommended method for building NASM to support platforms for which executables are not available. On a system which has Unix shell (\c{sh}), run: \c sh configure \c make everything A number of options can be passed to \c{configure}; see \c{sh configure --help}. A set of Makefiles for some other environments are also available; please see the file \c{Mkfiles/README}. To build the installer for the Windows platform, you will need the \i\e{Nullsoft Scriptable Installer}, \i{NSIS}, installed. To build the documentation, you will need a set of additional tools. The documentation is not likely to be able to build on non-Unix systems. \H{git} Building from the \i\c{git} Repository The NASM development tree is kept in a source code repository using the \c{git} distributed source control system. The link is available on the website. This is recommended only to participate in the development of NASM or to assist with testing the development code. To build NASM from the \c{git} repository you will need a Perl and, if building on a Unix system, GNU autoconf. To build on a Unix system, run: \c sh autogen.sh to create the \c{configure} script and then build as listed above. \A{contact} Contact Information \H{website} Website NASM has a \i{website} at \W{http://www.nasm.us/}\c{http://www.nasm.us/}. \i{New releases}, \i{release candidates}, and \I{snapshots, daily development}\i{daily development snapshots} of NASM are available from the official web site in source form as well as binaries for a number of common platforms. \S{forums} User Forums Users of NASM may find the Forums on the website useful. These are, however, not frequented much by the developers of NASM, so they are not suitable for reporting bugs. \S{develcom} Development Community The development of NASM is coordinated primarily though the \i\c{nasm-devel} mailing list. If you wish to participate in development of NASM, please join this mailing list. Subscription links and archives of past posts are available on the website. \H{bugs} \i{Reporting Bugs}\I{bugs} To report bugs in NASM, please use the \i{bug tracker} at \W{http://www.nasm.us/}\c{http://www.nasm.us/} (click on "Bug Tracker"), or if that fails then through one of the contacts in \k{website}. Please read \k{qstart} first, and don't report the bug if it's listed in there as a deliberate feature. (If you think the feature is badly thought out, feel free to send us reasons why you think it should be changed, but don't just send us mail saying `This is a bug' if the documentation says we did it on purpose.) Then read \k{problems}, and don't bother reporting the bug if it's listed there. If you do report a bug, \e{please} make sure your bug report includes the following information: \b What operating system you're running NASM under. Linux, FreeBSD, NetBSD, MacOS X, Win16, Win32, Win64, MS-DOS, OS/2, VMS, whatever. \b If you compiled your own executable from a source archive, compiled your own executable from \c{git}, used the standard distribution binaries from the website, or got an executable from somewhere else (e.g. a Linux distribution.) If you were using a locally built executable, try to reproduce the problem using one of the standard binaries, as this will make it easier for us to reproduce your problem prior to fixing it. \b Which version of NASM you're using, and exactly how you invoked it. Give us the precise command line, and the contents of the \c{NASMENV} environment variable if any. \b Which versions of any supplementary programs you're using, and how you invoked them. If the problem only becomes visible at link time, tell us what linker you're using, what version of it you've got, and the exact linker command line. If the problem involves linking against object files generated by a compiler, tell us what compiler, what version, and what command line or options you used. (If you're compiling in an IDE, please try to reproduce the problem with the command-line version of the compiler.) \b If at all possible, send us a NASM source file which exhibits the problem. If this causes copyright problems (e.g. you can only reproduce the bug in restricted-distribution code) then bear in mind the following two points: firstly, we guarantee that any source code sent to us for the purposes of debugging NASM will be used \e{only} for the purposes of debugging NASM, and that we will delete all our copies of it as soon as we have found and fixed the bug or bugs in question; and secondly, we would prefer \e{not} to be mailed large chunks of code anyway. The smaller the file, the better. A three-line sample file that does nothing useful \e{except} demonstrate the problem is much easier to work with than a fully fledged ten-thousand-line program. (Of course, some errors \e{do} only crop up in large files, so this may not be possible.) \b A description of what the problem actually \e{is}. `It doesn't work' is \e{not} a helpful description! Please describe exactly what is happening that shouldn't be, or what isn't happening that should. Examples might be: `NASM generates an error message saying Line 3 for an error that's actually on Line 5'; `NASM generates an error message that I believe it shouldn't be generating at all'; `NASM fails to generate an error message that I believe it \e{should} be generating'; `the object file produced from this source code crashes my linker'; `the ninth byte of the output file is 66 and I think it should be 77 instead'. \b If you believe the output file from NASM to be faulty, send it to us. That allows us to determine whether our own copy of NASM generates the same file, or whether the problem is related to portability issues between our development platforms and yours. We can handle binary files mailed to us as MIME attachments, uuencoded, and even BinHex. Alternatively, we may be able to provide an FTP site you can upload the suspect files to; but mailing them is easier for us. \b Any other information or data files that might be helpful. If, for example, the problem involves NASM failing to generate an object file while TASM can generate an equivalent file without trouble, then send us \e{both} object files, so we can see what TASM is doing differently from us.