nasm/doc/running.src

\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 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

The option \c{--help} is an alias for the \c{-h} option.

If you use Linux but aren't sure whether your system is \c{a.out}
or 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: 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{ith}
and \c{srec}, it will use \c{.dbg}, \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 optimization passes required. See \k{opt-O}.


\S{opt-f} The \i\c{-f} Option: \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 -h}.


\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-L} The \i\c{-L} Option: Additional or Modified Listing Info

Use this option to specify listing output details.

Supported options are:

\b \c{-Lb} show builtin macro packages (standard and \c{%use})

\b \c{-Ld} show byte and repeat counts in decimal, not hex

\b \c{-Le} show the preprocessed input

\b \c{-Lf} ignore \c{.nolist} and force listing output

\b \c{-Lm} show multi-line macro calls with expanded parameters

\b \c{-Lp} output a list file in every pass, in case of errors

\b \c{-Ls} show all single-line macro definitions

\b \c{-Lw} flush the output after every line (very slow, mainly useful
to debug NASM crashes)

\b \c{-L+} enable \e{all} listing options except \c{-Lw} (very verbose)

These options can be enabled or disabled at runtime using the
\c{%pragma list options} directive:

\c %pragma list options [+|-]flags...

For example, to turn on the \c{d} and \c{m} flags but disable the
\c{s} flag:

\c %pragma list options +dm -s

For forward compatility reasons, an undefined flag will be
ignored. Thus, a new flag introduced in a newer version of NASM can be
specified without breaking older versions. Listing flags will always
be a single alphanumeric character and are case sensitive.

\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

If the argument after \c{-MD} is an option rather than a filename,
then the output filename is the first applicable one of:

\b the filename set in the \c{-MF} option;

\b the output filename from the \c{-o} option with \c{.d} appended;

\b the input filename with the extension set to \c{.d}.


\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 \c{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 Makefile 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 \c{make} from complaining if a header
file has been removed.


\S{opt-MW} The \i\c{-MW} Option: Watcom \c{make} quoting style

This option causes NASM to attempt to quote dependencies according to
Watcom \c{make} conventions rather than POSIX \c{make} conventions (also used
by most other \c{make} variants.)  This quotes \c{#} as \c{$#} rather than
\c{\\#}, uses \c{&} rather than \c{\\} for continuation lines, and
encloses filenames containing whitespace in double quotes.


\S{opt-F} The \i\c{-F} Option: \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 -h}.  Not
all output formats currently support debugging output.

This should not be confused with the \c{-f dbg} output format option,
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).

Prior NASM 2.14 a path provided in the option has been considered as
a verbatim copy and providing a path separator been up to a caller.
One could implicitly concatenate a search path together with a filename.
Still this was rather a trick than something useful. Now the trailing
path separator is made to always present, thus \c{-ifoo} will be
considered as the \c{-ifoo/} directory.

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.

\c{--include} option is also accepted.

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 compatibility 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: Suppress Preprocessing

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: \i{Multipass Optimization}

Using the \c{-O} option, you can tell NASM to carry out different
levels of optimization. Multiple flags can be specified after the
\c{-O} options, some of which can be combined in a single option,
e.g. \c{-Oxv}.

\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.

\b \c{-Ov}: At the end of assembly, print the number of passes
        actually executed.

The \c{-Ox} mode is recommended for most uses, and is the default
since NASM 2.09. \e{Any other mode will generate worse quality
output.} Use \c{-O0} or \c{-O1} only if you need the finer
programmer-level control of output and \c{strict} is not suitable for
your use case.

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: \i{TASM} Compatibility Mode

NASM includes a limited form of compatibility with Borland's 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{label-orphan}; you can enable warnings of
this class by the command-line option \c{-w+label-orphan} and
disable it by \c{-w-label-orphan}.

Since version 2.15, NASM has group aliases for all prefixed warnings,
so they can be used to enable or disable all warnings in the group.
For example, -w+float enables all warnings with names starting with float-*.

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}.

See \k{warnings} for the complete list of warning classes.

\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-pfix} The \i\c{--(g|l)prefix}, \i\c{--(g|l)postfix} Options.

The \c{--(g)prefix} options prepend the given argument
to all \c{extern}, \c{common}, \c{static}, and \c{global} symbols, and the
\c{--lprefix} option prepends to all other symbols. Similarly,
\c{--(g)postfix} and \c{--lpostfix} options append
the argument in the exactly same way as the \c{--xxprefix} options does.

Running this:

\c nasm -f macho --gprefix _

is equivalent to place the directive with \c{%pragma macho gprefix _}
at the start of the file (\k{mangling}). It will prepend the underscore
to all global and external variables, as C requires it in some, but not all,
system calling conventions.

\S{opt-pragma} The \i\c{--pragma} Option

NASM accepts an argument as \c{%pragma} option, which is like placing
a \c{%pragma} preprocess statement at the beginning of the source.
Running this:

\c nasm -f macho --pragma "macho gprefix _"

is equivalent to the example in \k{opt-pfix}. See \k{pragma}.


\S{opt-before} The \i\c{--before} Option

A preprocess statement can be accepted with this option. The example
shown in \k{opt-pragma} is the same as running this:

\c nasm -f macho --before "%pragma macho gprefix _"


\S{opt-limit} The \i\c{--limit-X} Option

This option allows user to setup various maximum values after which
NASM will terminate with a fatal error rather than consume arbitrary
amount of compute time. Each limit can be set to a positive number or
\c{unlimited}.

\b\c{--limit-passes}: Number of maximum allowed passes. Default is
\c{unlimited}.

\b\c{--limit-stalled-passes}: Maximum number of allowed unfinished
passes. Default is 1000.

\b\c{--limit-macro-levels}: Define maximum depth of macro expansion
(in preprocess). Default is 10000

\b\c{--limit-macro-tokens}: Maximum number of tokens processed during
single-line macro expansion. Default is 10000000.

\b\c{--limit-mmacros}: Maximum number of multi-line macros processed
before returning to the top-level input. Default is 100000.

\b\c{--limit-rep}: Maximum number of allowed preprocessor loop, defined
under \c{%rep}. Default is 1000000.

\b\c{--limit-eval}: This number sets the boundary condition of allowed
expression length. Default is 8192 on most systems.

\b\c{--limit-lines}: Total number of source lines allowed to be
processed. Default is 2000000000.

For example, set the maximum line count to 1000:

\c nasm --limit-lines 1000

Limits can also be set via the directive \c{%pragma limit}, for
example:

\c %pragma limit lines 1000


\S{opt-keep-all} The \i\c{--keep-all} Option

This option prevents NASM from deleting any output files even if an
error happens.

\S{opt-no-line} The \i\c{--no-line} Option

If this option is given, all \i\c{%line} directives in the source code
are ignored. This can be useful for debugging already preprocessed
code. See \k{line}.

\S{opt-reproducible} The \i\c{--reproducible} Option

If this option is given, NASM will not emit information that is
inherently dependent on the NASM version or different from run to run
(such as timestamps) into the output file.


\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}.

Historically, 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.

As of NASM 2.15, the MASM syntax is also supported.

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

\S{masm-compat} MASM compatibility package

See \k{pkg_masm}.