\IR{-o} \c{-o} option
\IR{-f} \c{-f} option
\IR{-l} \c{-l} option
\IR{-s} \c{-s} option
\IR{-i} \c{-i} option
\IR{-p} \c{-p} option
\IR{-d} \c{-d} option
\IR{-e} \c{-e} option
\IR{-a} \c{-a} option
\IR{-w} \c{-w} option
\IR{!=} \c{!=} operator
\IR{$ here} \c{$} Here token
\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{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{bitwise and} bitwise AND
\IR{bitwise or} bitwise OR
\IR{bitwise xor} bitwise XOR
\IR{block ifs} block IFs
\IR{borland pascal} Borland, Pascal
\IR{borland's win32 compilers} Borland, Win32 compilers
\IR{braces, after % sign} braces, after \c{%} sign
\IR{bsd} BSD
\IR{c calling convention} C calling convention
\IR{c symbol names} C symbol names
\IA{critical expressions}{critical expression}
\IA{command line}{command-line}
\IA{case sensitivity}{case sensitive}
\IA{case-sensitive}{case sensitive}
\IA{case-insensitive}{case sensitive}
\IA{character constants}{character constant}
\IR{common object file format} Common Object File Format
\IR{common variables, alignment in elf} common variables, alignment
in \c{elf}
\IR{common, elf extensions to} \c{COMMON}, \c{elf} extensions to
\IR{common, obj extensions to} \c{COMMON}, \c{obj} extensions to
\IR{declaring structure} declaring structures
\IR{default-wrt mechanism} default-\c{WRT} mechanism
\IR{devpac} DevPac
\IR{djgpp} DJGPP
\IR{dll symbols, exporting} DLL symbols, exporting
\IR{dll symbols, importing} DLL symbols, importing
\IR{dos} DOS
\IR{dos archive} DOS archive
\IR{dos source archive} DOS source archive
\IA{effective address}{effective addresses}
\IA{effective-address}{effective addresses}
\IR{elf shared libraries} \c{elf} shared libraries
\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{got} GOT
\IR{got relocations} \c{GOT} relocations
\IR{gotoff relocation} \c{GOTOFF} relocations
\IR{gotpc relocation} \c{GOTPC} relocations
\IR{linux elf} Linux ELF
\IR{logical and} logical AND
\IR{logical or} logical OR
\IR{logical xor} logical XOR
\IR{masm} MASM
\IA{memory reference}{memory references}
\IA{misc directory}{misc subdirectory}
\IR{misc subdirectory} \c{misc} subdirectory
\IR{microsoft omf} Microsoft OMF
\IR{mmx registers} MMX registers
\IA{modr/m}{modr/m byte}
\IR{modr/m byte} ModR/M byte
\IR{ms-dos} MS-DOS
\IR{ms-dos device drivers} MS-DOS device drivers
\IR{multipush} \c{multipush} macro
\IR{nasm version} NASM version
\IR{netbsd} NetBSD
\IR{omf} OMF
\IR{openbsd} OpenBSD
\IR{operating-system} operating system
\IR{os/2} OS/2
\IR{pascal calling convention}Pascal calling convention
\IR{passes} passes, assembly
\IR{perl} Perl
\IR{pic} PIC
\IR{pharlap} PharLap
\IR{plt} PLT
\IR{plt} \c{PLT} relocations
\IA{pre-defining macros}{pre-define}
\IR{qbasic} QBasic
\IA{rdoff subdirectory}{rdoff}
\IR{rdoff} \c{rdoff} subdirectory
\IR{relocatable dynamic object file format} Relocatable Dynamic
Object File Format
\IR{relocations, pic-specific} relocations, PIC-specific
\IA{repeating}{repeating code}
\IR{section alignment, in elf} section alignment, in \c{elf}
\IR{section alignment, in bin} section alignment, in \c{bin}
\IR{section alignment, in obj} section alignment, in \c{obj}
\IR{section alignment, in win32} section alignment, in \c{win32}
\IR{section, elf extensions to} \c{SECTION}, \c{elf} extensions to
\IR{section, win32 extensions to} \c{SECTION}, \c{win32} extensions to
\IR{segment alignment, in bin} segment alignment, in \c{bin}
\IR{segment alignment, in obj} segment alignment, in \c{obj}
\IR{segment, obj extensions to} \c{SEGMENT}, \c{elf} extensions to
\IR{segment names, borland pascal} segment names, Borland Pascal
\IR{shift commane} \c{shift} command
\IA{sib}{sib byte}
\IR{sib byte} SIB byte
\IA{standard section names}{standardised section names}
\IR{symbols, exporting from dlls} symbols, exporting from DLLs
\IR{symbols, importing from dlls} symbols, importing from DLLs
\IR{tasm} TASM
\IR{test subdirectory} \c{test} subdirectory
\IR{tlink} TLINK
\IR{underscore, in c symbols} underscore, in C symbols
\IR{unix} Unix
\IR{unix source archive} Unix source archive
\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{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 assembler designed for
portability and modularity. It supports a range of object file
formats, including Linux \c{a.out} and ELF, NetBSD/FreeBSD, COFF,
Microsoft 16-bit OBJ and Win32. It will also output plain binary
files. Its syntax is designed to be simple and easy to understand,
similar to Intel's but less complex. It supports Pentium, P6 and MMX
opcodes, and has macro capability.

\S{yaasm} Why Yet Another Assembler?

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

\b \i\c{a86} is good, but not free, and in particular you don't get any
32-bit capability until you pay. It's DOS only, too.

\b \i\c{gas} is free, and ports over DOS and Unix, but it's not very good,
since it's designed to be a back end to \i\c{gcc}, which always feeds
it correct code. So its error checking is minimal. Also, its syntax
is horrible, from the point of view of anyone trying to actually
\e{write} anything in it. Plus you can't write 16-bit code in it
(properly).

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

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

\b \i{TASM} is better, but still strives for \i{MASM} compatibility, which
means millions of directives and tons of red tape. And its syntax is
essentially \i{MASM}'s, with the contradictions and quirks that entails
(although it sorts out some of those by means of Ideal mode). It's
expensive too. And it's DOS-only.

So here, for your coding pleasure, is NASM. At present it's
still in prototype stage - we don't promise that it can outperform
any of these assemblers. But please, \e{please} send us bug reports,
fixes, helpful information, and anything else you can get your hands
on (and thanks to the many people who've done this already! You all
know who you are), and we'll improve it out of all recognition.
Again.

\S{legal} Licence Conditions

Please see the file \c{Licence}, supplied as part of any NASM
distribution archive, for the \i{licence} conditions under which you
may use NASM.

\H{contact} Contact Information

NASM has a \i{WWW page} at
\W{http://www.cryogen.com/Nasm}\c{http://www.cryogen.com/Nasm}. The
authors are \i{e\-mail}able as
\W{mailto:jules@earthcorp.com}\c{jules@earthcorp.com} and
\W{mailto:anakin@pobox.com}\c{anakin@pobox.com}. If you want to
report a bug to us, please read \k{bugs} first.

\i{New releases} of NASM are uploaded to
\W{ftp://sunsite.unc.edu/pub/Linux/devel/lang/assemblers/}\i\c{sunsite.unc.edu},
\W{ftp://ftp.simtel.net/pub/simtelnet/msdos/asmutl/}\i\c{ftp.simtel.net}
and
\W{ftp://ftp.coast.net/coast/msdos/asmutil/}\i\c{ftp.coast.net}.
Announcements are posted to
\W{news:comp.lang.asm.x86}\i\c{comp.lang.asm.x86},
\W{news:alt.lang.asm}\i\c{alt.lang.asm},
\W{news:comp.os.linux.announce}\i\c{comp.os.linux.announce} and
\W{news:comp.archives.msdos.announce}\i\c{comp.archives.msdos.announce}
(the last one is done automagically by uploading to
\W{ftp://ftp.simtel.net/pub/simtelnet/msdos/asmutl/}\c{ftp.simtel.net}).

If you don't have Usenet access, or would rather be informed by
\i{e\-mail} when new releases come out, e\-mail
\W{mailto:anakin@pobox.com}\c{anakin@pobox.com}
and ask.

\H{install} Installation

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

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

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

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

That's it - NASM is installed. You don't need the \c{nasm} directory
to be present to run NASM (unless you've added it to your \c{PATH}),
so you can delete it if you need to save space; however, you may
want to keep the documentation or test programs.

If you've downloaded the \i{DOS source archive}, \i\c{nasmXXXs.zip},
the \c{nasm} directory will also contain the full NASM \i{source
code}, and a selection of \i{Makefiles} you can (hopefully) use to
rebuild your copy of NASM from scratch. The file \c{Readme} lists
the various Makefiles and which compilers they work with. Note that
the source files \c{insnsa.c} and \c{insnsd.c} are automatically
generated from the master instruction table \c{insns.dat} by a Perl
script; a \i{QBasic} version of the program is provided, but it is
recommended that you use the Perl version. A DOS port of \i{Perl} is
available from
\W{http://www.perl.org/CPAN/ports/msdos/}\i{www.perl.org}.

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

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

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

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

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

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

\C{running} Running NASM

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

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

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

For example,

\c nasm -f elf myfile.asm

will assemble \c{myfile.asm} into an 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

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
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 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.

Like Unix compilers and assemblers, NASM is silent unless it
goes wrong: you won't see any output at all, unless it gives error
messages.

\S{opt-o} The \i\c{-o} Option: Specifying the Output File Name

NASM will normally choose the name of your output file for you;
precisely how it does this is dependent on the object file format.
For Microsoft object file formats (\i\c{obj} and \i\c{win32}), it
will remove the \c{.asm} \i{extension} (or whatever extension you
like to use - NASM doesn't care) from your source file name and
substitute \c{.obj}. For Unix object file formats (\i\c{aout},
\i\c{coff}, \i\c{elf} and \i\c{as86}) it will substitute \c{.o}. For
\i\c{rdf}, it will use \c{.rdf}, and for the \i\c{bin} format it
will simply remove the extension, so that \c{myfile.asm} produces
the output file \c{myfile}.

If the output file already exists, NASM will overwrite it, unless it
has the same name as the input file, in which case it will give a
warning and use \i\c{nasm.out} as the output file name instead.

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

\c nasm -f bin program.asm -o program.com
\c nasm -f bin driver.asm -odriver.sys

\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 -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

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

Under MS-\i{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{-s} option, requiring no argument,
which causes errors to be sent to standard output rather than
standard error. Therefore you can \I{redirecting errors}redirect
the errors into a file by typing

\c nasm -s -f obj myfile.asm > myfile.err

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

When NASM sees the \i\c{%include} directive in a source file (see
\k{include}), 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{NASM} environment variable (see
\k{nasmenv}).

\S{opt-p} The \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.

\S{opt-d} The \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}.

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

NASM allows the \i{preprocessor} to be run on its own, up to a
point. Using the \c{-e} option (which requires no arguments) will
cause NASM to preprocess its input file, expand all the macro
references, remove all the comments and preprocessor directives, and
print the resulting file on standard output (or save it to a file,
if the \c{-o} option is also used).

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

\c %assign tablesize ($-tablestart)

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

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

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

\S{opt-w} The \i\c{-w} Option: Enable or Disable Assembly \i{Warnings}

NASM can observe many conditions during the course of assembly which
are worth mentioning to the user, but not a sufficiently severe
error to justify NASM refusing to generate an output file. These
conditions are reported like errors, but come up with the word
`warning' before the message. Warnings do not prevent NASM from
generating an output file and returning a success status to the
operating system.

Some conditions are even less severe than that: they are only
sometimes worth mentioning to the user. Therefore NASM supports the
\c{-w} command-line option, which enables or disables certain
classes of assembly warning. Such warning classes are described by a
name, for example \c{orphan-labels}; you can enable warnings of
this class by the command-line option \c{-w+orphan-labels} and
disable it by \c{-w-orphan-labels}.

The \i{suppressible warning} classes are:

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

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

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

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

If you define an environment variable called \c{NASM}, 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{NASM} 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{NASM} 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{NASM} 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.

\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 DOS or 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{uninitialised 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 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.

The instruction field may contain any machine instruction: Pentium
and P6 instructions, FPU instructions, MMX instructions and even
undocumented instructions are all supported. The instruction may be
prefixed by \c{LOCK}, \c{REP}, \c{REPE}/\c{REPZ} or
\c{REPNE}/\c{REPNZ}, in the usual way. Explicit \I{address-size
prefixes}address-size and \i{operand-size prefixes} \c{A16},
\c{A32}, \c{O16} and \c{O32} are provided - one example of their use
is given in \k{mixsize}. You can also use the name of a \I{segment
override}segment register as an instruction prefix: coding
\c{es mov [bx],ax} is equivalent to coding \c{mov [es:bx],ax}. We
recommend the latter syntax, since it is consistent with other
syntactic features of the language, but for instructions such as
\c{LODSB}, which has no operands and yet can require a segment
override, there is no clean syntactic way to proceed apart from
\c{es lodsb}.

An instruction is not required to use a prefix: prefixes such as
\c{CS}, \c{A32}, \c{LOCK} or \c{REPE} can appear on a line by
themselves, and NASM will just generate the prefix bytes.

In addition to actual machine instructions, NASM also supports a
number of pseudo-instructions, described in \k{pseudop}.

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

For \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 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} and
\i\c{DT}, their \i{uninitialised} counterparts \i\c{RESB},
\i\c{RESW}, \i\c{RESD}, \i\c{RESQ} and \i\c{REST}, the \i\c{INCBIN}
command, the \i\c{EQU} command, and the \i\c{TIMES} prefix.

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

\i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ} and \i\c{DT} are used, much
as in MASM, to declare initialised 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'                 ; 0x41 0x00 (it's just a number)
\c           dw 'ab'                ; 0x41 0x42 (character constant)
\c           dw 'abc'               ; 0x41 0x42 0x43 0x00 (string)
\c           dd 0x12345678          ; 0x78 0x56 0x34 0x12
\c           dd 1.234567e20         ; floating-point constant
\c           dq 1.234567e20         ; double-precision float
\c           dt 1.234567e20         ; extended-precision float

\c{DQ} and \c{DT} do not accept \i{numeric constants} or string
constants as operands.

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

\i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ} and \i\c{REST} are
designed to be used in the BSS section of a module: they declare
\e{uninitialised} 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 uninitialised space by writing \I\c{?}\c{DW ?} or
similar things: this is what it does instead. The operand to a
\c{RESB}-type pseudo-instruction is a \i\e{critical expression}: see
\k{crit}.

For example:

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

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

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

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

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

\c{EQU} defines a symbol to a given constant value: when \c{EQU} is
used, the source line must contain a label. The action of \c{EQU} is
to define the given label name to the value of its (only) operand.
This definition is absolute, and cannot change later. So, for
example,

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

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

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

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

\c zerobuf:  times 64 db 0

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

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

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

\c           times 100 movsb

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

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

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

\H{effaddr} Effective Addresses

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

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

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

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

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

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

\c           mov eax,[ebx*5]        ; assembles as [ebx*4+ebx]
\c           mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]

Some forms of effective address have more than one assembled form;
in most such cases NASM will generate the smallest form it can. For
example, there are distinct assembled forms for the 32-bit effective
addresses \c{[eax*2+0]} and \c{[eax+eax]}, and NASM will generally
generate the latter on the grounds that the former requires four
bytes to store a zero offset.

NASM has a hinting mechanism which will cause \c{[eax+ebx]} and
\c{[ebx+eax]} to generate different opcodes; this is occasionally
useful because \c{[esi+ebp]} and \c{[ebp+esi]} have different
default segment registers.

However, you can force NASM to generate an effective address in a
particular form by the use of the keywords \c{BYTE}, \c{WORD},
\c{DWORD} and \c{NOSPLIT}. If you need \c{[eax+3]} to be assembled
using a double-word offset field instead of the one byte NASM will
normally generate, you can code \c{[dword eax+3]}. Similarly, you
can force NASM to use a byte offset for a small value which it
hasn't seen on the first pass (see \k{crit} for an example of such a
code fragment) by using \c{[byte eax+offset]}. As special cases,
\c{[byte eax]} will code \c{[eax+0]} with a byte offset of zero, and
\c{[dword eax]} will code it with a double-word offset of zero. The
normal form, \c{[eax]}, will be coded with no offset field.

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.

\H{const} \i{Constants}

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

\S{numconst} \i{Numeric Constants}

A numeric constant is simply a number. NASM allows you to specify
numbers in a variety of number bases, in a variety of ways: you can
suffix \c{H}, \c{Q} and \c{B} for \i{hex}, \i{octal} and \i{binary},
or you can prefix \c{0x} for hex in the style of C, or you can
prefix \c{$} for hex in the style of Borland Pascal. Note, though,
that the \I{$prefix}\c{$} prefix does double duty as a prefix on
identifiers (see \k{syntax}), so a hex number prefixed with a \c{$}
sign must have a digit after the \c{$} rather than a letter.

Some examples:

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

\S{chrconst} \i{Character Constants}

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

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

\c           mov eax,'abcd'

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

\S{strconst} String Constants

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

A string constant looks like a character constant, only longer. It
is treated as a concatenation of maximum-size character constants
for the conditions. So the following are equivalent:

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

And the following are also equivalent:

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

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

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

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

Some examples:

\c           dd 1.2                 ; an easy one
\c           dq 1.e10               ; 10,000,000,000
\c           dq 1.e+10              ; synonymous with 1.e10
\c           dq 1.e-10              ; 0.000 000 000 1
\c           dt 3.141592653589793238462 ; pi

NASM cannot do compile-time arithmetic on floating-point constants.
This is because NASM is designed to be portable - although it always
generates code to run on x86 processors, the assembler itself can
run on any system with an ANSI C compiler. Therefore, the assembler
cannot guarantee the presence of a floating-point unit capable of
handling the \i{Intel number formats}, and so for NASM to be able to
do floating arithmetic it would have to include its own complete set
of floating-point routines, which would significantly increase the
size of the assembler for very little benefit.

\H{expr} \i{Expressions}

Expressions in NASM are similar in syntax to those in C.

NASM does not guarantee the size of the integers used to evaluate
expressions at compile time: since NASM can compile and run on
64-bit systems quite happily, don't assume that expressions are
evaluated in 32-bit registers and so try to make deliberate use of
\i{integer overflow}. It might not always work. The only thing NASM
will guarantee is what's guaranteed by ANSI C: you always have \e{at
least} 32 bits to work in.

NASM supports two special tokens in expressions, allowing
calculations to involve the current assembly position: the
\I{$ here}\c{$} and \i\c{$$} tokens. \c{$} evaluates to the assembly
position at the beginning of the line containing the expression; so
you can code an \i{infinite loop} using \c{JMP $}. \c{$$} evaluates
to the beginning of the current section; so you can tell how far
into the section you are by using \c{($-$$)}.

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

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

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

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

\c{^} provides the bitwise XOR operation.

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

\c{&} provides the bitwise AND operation.

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

\c{<<} gives a bit-shift to the left, just as it does in C. So \c{5<<3}
evaluates to 5 times 8, or 40. \c{>>} gives a bit-shift to the
right; in NASM, such a shift is \e{always} unsigned, so that
the bits shifted in from the left-hand end are filled with zero
rather than a sign-extension of the previous highest bit.

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

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

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

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

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

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

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

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

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

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

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

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

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

Things can be more complex than this: since 16-bit segments and
\i{groups} may \I{overlapping segments}overlap, you might occasionally
want to refer to some symbol using a different segment base from the
preferred one. NASM lets you do this, by the use of the \c{WRT}
(With Reference To) keyword. So you can do things like

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

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

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

\c           call (seg procedure):procedure
\c           call weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended
parsing of the above instructions. They are not necessary in
practice.)

NASM supports the syntax \I\c{CALL FAR}\c{call far procedure} as a
synonym for the first of the above usages. \c{JMP} works identically
to \c{CALL} in these examples.

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

\c           dw symbol, seg symbol

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

\H{crit} \i{Critical Expressions}

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

The first pass is used to determine the size of all the assembled
code and data, so that the second pass, when generating all the
code, knows all the symbol addresses the code refers to. So one
thing NASM can't handle is code whose size depends on the value of a
symbol declared after the code in question. For example,

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

The argument to \i\c{TIMES} in this case could equally legally
evaluate to anything at all; NASM will reject this example because
it cannot tell the size of the \c{TIMES} line when it first sees it.
It will just as firmly reject the slightly \I{paradox}paradoxical
code

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

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

NASM rejects these examples by means of a concept called a
\e{critical expression}, which is defined to be an expression whose
value is required to be computable in the first pass, and which must
therefore depend only on symbols defined before it. The argument to
the \c{TIMES} prefix is a critical expression; for the same reason,
the arguments to the \i\c{RESB} family of pseudo-instructions are
also critical expressions.

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

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

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

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

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

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

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

\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 .loop     ; some more code
\c           jne .loop
\c           ret
\c label2    ; some code
\c .loop     ; some more code
\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           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           jmp ..@foo             ; this will jump three lines up

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

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

NASM contains a powerful \i{macro processor}, which supports
conditional assembly, multi-level file inclusion, two forms of macro
(single-line and multi-line), and a `context stack' mechanism for
extra macro power. Preprocessor directives all begin with a \c{%}
sign.

\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           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           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           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.

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

\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.

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

The expression passed to \c{%assign} is a \i{critical expression}
(see \k{crit}), and must also evaluate to a pure number (rather than
a relocatable reference such as a code or data address, or anything
involving a register).

\H{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           push ebp
\c           mov ebp,esp
\c           sub esp,%1
\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 %2:       db %1
\c %endmacro
\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} \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           push ebp
\c           mov ebp,esp
\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           push %1
\c           push %2
\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           jnz %%skip
\c           ret
\c %%skip:
\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           jmp %%endstr
\c %%str:    db %2
\c %%endstr: mov dx,%%str
\c           mov cx,%%endstr-%%str
\c           mov bx,%1
\c           mov ah,0x40
\c           int 0x21
\c %endmacro

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

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

If you define a greedy macro, you are effectively telling NASM how
it should expand the macro given \e{any} number of parameters from
the actual number specified up to infinity; in this case, for
example, NASM now knows what to do when it sees a call to
\c{writefile} with 2, 3, 4 or more parameters. NASM will take this
into account when overloading macros, and will not allow you to
define another form of \c{writefile} taking 4 parameters (for
example).

Of course, the above macro could have been implemented as a
non-greedy macro, in which case the call to it would have had to
look like

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

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

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

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

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

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

This macro (which makes use of the \c{writefile} macro defined in
\k{mlmacgre}) can be called with an explicit error message, which it
will display on the error output stream before exiting, or it can be
called with no parameters, in which case it will use the default
error message supplied in the macro definition.

In general, you supply a minimum and maximum number of parameters
for a macro of this type; the minimum number of parameters are then
required in the macro call, and then you provide defaults for the
optional ones. So if a macro definition began with the line

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

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

You may omit parameter defaults from the macro definition, in which
case the parameter default is taken to be blank. This can be useful
for macros which can take a variable number of parameters, since the
\i\c{%0} token (see \k{percent0}) allows you to determine how many
parameters were really passed to the macro call.

This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the \c{die} macro above could be made more powerful,
and more useful, by changing the first line of the definition to

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

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

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

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

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

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

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

\c{%rotate} is invoked with a single numeric argument (which may be
an expression). The macro parameters are rotated to the left by that
many places. If the argument to \c{%rotate} is negative, the macro
parameters are rotated to the right.

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

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

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

\S{concat} \i{Concatenating Macro Parameters}

NASM can concatenate macro parameters on to other text surrounding
them. This allows you to declare a family of symbols, for example,
in a macro definition. If, for example, you wanted to generate a
table of key codes along with offsets into the table, you could code
something like

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

which would expand to

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

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

If you need to append a \e{digit} to a macro parameter, for example
defining labels \c{foo1} and \c{foo2} when passed the parameter
\c{foo}, you can't code \c{%11} because that would be taken as the
eleventh macro parameter. Instead, you must code
\I{braces, after % sign}\c{%\{1\}1}, which will separate the first
\c{1} (giving the number of the macro parameter) from the second
(literal text to be concatenated to the parameter).

This concatenation can also be applied to other preprocessor in-line
objects, such as macro-local labels (\k{maclocal}) and context-local
labels (\k{ctxlocal}). In all cases, ambiguities in syntax can be
resolved by enclosing everything after the \c{%} sign and before the
literal text in braces: so \c{%\{%foo\}bar} concatenates the text
\c{bar} to the end of the real name of the macro-local label
\c{%%foo}. (This is unnecessary, since the form NASM uses for the
real names of macro-local labels means that the two usages
\c{%\{%foo\}bar} and \c{%%foobar} would both expand to the same
thing anyway; nevertheless, the capability is there.)

\S{mlmaccc} \i{Condition Codes as Macro Parameters}

NASM can give special treatment to a macro parameter which contains
a condition code. For a start, you can refer to the macro parameter
\c{%1} by means of the alternative syntax \i\c{%+1}, which informs
NASM that this macro parameter is supposed to contain a condition
code, and will cause the preprocessor to report an error message if
the macro is called with a parameter which is \e{not} a valid
condition code.

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

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

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

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

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

When NASM is generating a listing file from your program, it will
generally expand multi-line macros by means of writing the macro
call and then listing each line of the expansion. This allows you to
see which instructions in the macro expansion are generating what
code; however, for some macros this clutters the listing up
unnecessarily.

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

\c %macro foo 1.nolist

Or like this:

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

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

Similarly to the C preprocessor, NASM allows sections of a source
file to be assembled only if certain conditions are met. The general
syntax of this feature looks like this:

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

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

\S{ifdef} \i\c{%ifdef}: \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{ifctx} \i\c{%ifctx}: \i{Testing the Context Stack}

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

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

\S{if} \i\c{%if}: \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.

\S{ifidn} \i\c{%ifidn} and \i\c{%ifidni}: \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 %ifidni %1,ip
\c           call %%label
\c %%label:
\c %else
\c           push %1
\c %endif
\c %endmacro

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

\S{iftyp} \i\c{%ifid}, \i\c{%ifnum}, \i\c{%ifstr}: \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 %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 %endmacro

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

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

In the first, \c{strpointer} is used as the address of an
already-declared string, and \c{length} is used as its length; in
the second, a string is given to the macro, which therefore declares
it itself and works out the address and length for itself.

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

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

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

The preprocessor directive \c{%error} will cause NASM to report an
error if it occurs in assembled code. So if other users are going to
try to assemble your source files, you can ensure that they define
the right macros by means of code like this:

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

Then any user who fails to understand the way your code is supposed
to be assembled will be quickly warned of their mistake, rather than
having to wait until the program crashes on being run and then not
knowing what went wrong.

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

NASM's \c{TIMES} prefix, though useful, cannot be used to invoke a
multi-line macro multiple times, because it is processed by NASM
after macros have already been expanded. Therefore NASM provides
another form of loop, this time at the preprocessor level: \c{%rep}.

The directives \c{%rep} and \i\c{%endrep} (\c{%rep} takes a numeric
argument, which can be an expression; \c{%endrep} takes no
arguments) can be used to enclose a chunk of code, which is then
replicated as many times as specified by the preprocessor:

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

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

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

\c fibonacci:
\c %assign i 0
\c %assign j 1
\c %rep 100
\c %if j > 65535
\c %exitrep
\c %endif
\c           dw j
\c %assign k j+i
\c %assign i j
\c %assign j k
\c %endrep
\c fib_number equ ($-fibonacci)/2

This produces a list of all the Fibonacci numbers that will fit in
16 bits. Note that a maximum repeat count must still be given to
\c{%rep}. This is to prevent the possibility of NASM getting into an
infinite loop in the preprocessor, which (on multitasking or
multi-user systems) would typically cause all the system memory to
be gradually used up and other applications to start crashing.

\H{include} \i{Including Other Files}

Using, once again, a very similar syntax to the C preprocessor,
NASM's preprocessor lets you include other source files into your
code. This is done by the use of the \i\c{%include} directive:

\c %include "macros.mac"

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

Include files are \I{searching for include files}searched for in the
current directory (the directory you're in when you run NASM, as
opposed to the location of the NASM executable or the location of
the source file), plus any directories specified on the NASM command
line using the \c{-i} option.

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

\c %ifndef MACROS_MAC
\c %define MACROS_MAC
\c ; now define some macros
\c %endif

then including the file more than once will not cause errors,
because the second time the file is included nothing will happen
because the macro \c{MACROS_MAC} will already be defined.

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

\H{ctxstack} The \i{Context Stack}

Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a \c{REPEAT} ...
\c{UNTIL} loop, in which the expansion of the \c{REPEAT} macro
would need to be able to refer to a label which the \c{UNTIL} macro
had defined. However, for such a macro you would also want to be
able to nest these loops.

NASM provides this level of power by means of a \e{context stack}.
The preprocessor maintains a stack of \e{contexts}, each of which is
characterised by a name. You add a new context to the stack using
the \i\c{%push} directive, and remove one using \i\c{%pop}. You can
define labels that are local to a particular context on the stack.

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

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

\c %push foobar

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

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

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

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

\c %macro repeat 0
\c %push repeat
\c %$begin:
\c %endmacro

\c %macro until 1
\c           j%-1 %$begin
\c %pop
\c %endmacro

and invoked by means of, for example,

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

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

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

\S{ctxdefine} \i{Context-Local Single-Line Macros}

NASM also allows you to define single-line macros which are local to
a particular context, in just the same way:

\c %define %$localmac 3

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

\S{ctxrepl} \i\c{%repl}: \I{renaming contexts}Renaming a Context

If you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to \c{%ifctx}),
you can execute a \c{%pop} followed by a \c{%push}; but this will
have the side effect of destroying all context-local labels and
macros associated with the context that was just popped.

NASM provides the directive \c{%repl}, which \e{replaces} a context
with a different name, without touching the associated macros and
labels. So you could replace the destructive code

\c %pop
\c %push newname

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

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

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

\c %macro if 1
\c     %push if
\c     j%-1 %$ifnot
\c %endmacro

\c %macro else 0
\c     %ifctx if
\c         %repl else
\c         jmp %$ifend
\c         %$ifnot:
\c     %else
\c         %error "expected `if' before `else'"
\c     %endif
\c %endmacro

\c %macro endif 0
\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 %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           if ae
\c             cmp bx,cx
\c             if ae
\c               mov ax,cx
\c             else
\c               mov ax,bx
\c             endif
\c           else
\c             cmp ax,cx
\c             if ae
\c               mov ax,cx
\c             endif
\c           endif

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

\H{stdmac} \i{Standard Macros}

NASM defines a set of standard macros, which are already defined
when it starts to process any source file. If you really need a
program to be assembled with no pre-defined macros, you can use the
\i\c{%clear} directive to empty the preprocessor of everything.

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

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

The single-line macros \c{__NASM_MAJOR__} and \c{__NASM_MINOR__}
expand to the major and minor parts of the \i{version number of
NASM} being used. So, under NASM 0.96 for example,
\c{__NASM_MAJOR__} would be defined to be 0 and \c{__NASM_MINOR__}
would be defined as 96.

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

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

\S{struc} \i\c{STRUC} and \i\c{ENDSTRUC}: \i{Declaring Structure} Data Types

The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that
data structures can be implemented as a set of macros. The macros
\c{STRUC} and \c{ENDSTRUC} are used to define a structure data type.

\c{STRUC} takes one parameter, which is the name of the data type.
This name is defined as a symbol with the value zero, and also has
the suffix \c{_size} appended to it and is then defined as an
\c{EQU} giving the size of the structure. Once \c{STRUC} has been
issued, you are defining the structure, and should define fields
using the \c{RESB} family of pseudo-instructions, and then invoke
\c{ENDSTRUC} to finish the definition.

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

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

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

The reason why the structure type name is defined at zero is a side
effect of allowing structures to work with the local label
mechanism: if your structure members tend to have the same names in
more than one structure, you can define the above structure like this:

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

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

NASM, since it has no \e{intrinsic} structure support, does not
support any form of period notation to refer to the elements of a
structure once you have one (except the above local-label notation),
so code such as \c{mov ax,[mystruc.mt_word]} is not valid.
\c{mt_word} is a constant just like any other constant, so the
correct syntax is \c{mov ax,[mystruc+mt_word]} or \c{mov
ax,[mystruc+mytype.word]}.

\S{istruc} \i\c{ISTRUC}, \i\c{AT} and \i\c{IEND}: Declaring
\i{Instances of Structures}

Having defined a structure type, the next thing you typically want
to do is to declare instances of that structure in your data
segment. NASM provides an easy way to do this in the \c{ISTRUC}
mechanism. To declare a structure of type \c{mytype} in a program,
you code something like this:

\c mystruc:  istruc mytype
\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           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 mt_byte:  resb 1
\c           alignb 2
\c mt_word:  resw 1
\c           alignb 4
\c mt_long:  resd 1
\c mt_str:   resb 32
\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}.

\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, or code designed to run on a processor
operating in 32-bit mode. The syntax is \c{BITS 16} or \c{BITS 32}.

In most cases, you should not need to use \c{BITS} explicitly. The
\c{aout}, \c{coff}, \c{elf} and \c{win32} object formats, which are
designed for use in 32-bit operating systems, all cause NASM to
select 32-bit mode 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 code in a flat binary file; this is because the \c{bin}
output format defaults to 16-bit mode in anticipation of it being
used most frequently to write DOS \c{.COM} programs, DOS \c{.SYS}
device drivers and boot loader software.

You do \e{not} need to specify \c{BITS 32} merely in order to use
32-bit instructions in a 16-bit DOS program; if you do, the
assembler will generate incorrect code because it will be writing
code targeted at a 32-bit platform, to be run on a 16-bit one.

When NASM is in \c{BITS 16} state, 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} state, the reverse is
true: 32-bit instructions require no prefixes, whereas instructions
using 16-bit data need an 0x66 and those working in 16-bit addresses
need an 0x67.

The \c{BITS} directive has an exactly equivalent primitive form,
\c{[BITS 16]} and \c{[BITS 32]}. The user-level form is a macro
which has no function other than to call the primitive form.

\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, all support
the \i{standardised section names} \c{.text}, \c{.data} and \c{.bss}
for the code, data and uninitialised-data sections. The \c{obj}
format, by contrast, does not recognise 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           [section .data]
\c %%str:    db %2
\c %%endstr:
\c           __SECT__
\c           mov dx,%%str
\c           mov cx,%%endstr-%%str
\c           mov bx,%1
\c           mov ah,0x40
\c           int 0x21
\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 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           jmp setup              ; setup code comes last
\c           ; the resident part of the TSR goes here
\c setup:    ; now write the code that installs the TSR here
\c           absolute setup
\c runtimevar1 resw 1
\c runtimevar2 resd 20
\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:    ; 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
uninitialised data section, so that

\c           common intvar 4

is similar in function to

\c           global intvar
\c           section .bss
\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.

\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.

\c{bin} supports the three \i{standardised section names} \i\c{.text},
\i\c{.data} and \i\c{.bss} only. The file NASM outputs will contain the
contents of the \c{.text} section first, followed by the contents of
the \c{.data} section, aligned on a four-byte boundary. The \c{.bss}
section is not stored in the output file at all, but is assumed to
appear directly after the end of the \c{.data} section, again
aligned on a four-byte boundary.

If you specify no explicit \c{SECTION} directive, the code you write
will be directed by default into the \c{.text} section.

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 code such as an
OS kernel, you need to explicitly issue the \I\c{BITS}\c{BITS 32}
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
file; 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}

\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 dvar:     dw 1234
\c           segment code
\c function: 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           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           ; some data
\c           segment bss
\c           ; some uninitialised data
\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 initialised 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}

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 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
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 initialised data
section, whereas \c{bss} declares an uninitialised data section.

\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, 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 .bss bss align=4

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

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

The \c{coff} output type produces 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{elffmt} \i\c{elf}: \i{Linux ELF}\I{Executable and Linkable
Format}Object Files

The \c{elf} output format generates ELF32 (Executable and Linkable
Format) object files, as used by Linux. \c{elf} provides a default
output file-name extension of \c{.o}.

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

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

The available qualifiers are:

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

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

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

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

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

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

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

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

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

The ELF specification contains enough features to allow
position-independent code (PIC) to be written, which makes \i{ELF
shared libraries} very flexible. However, it also means NASM has to
be able to generate a variety of strange relocation types in ELF
object files, if it is to be an assembler which can write PIC.

Since 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 summarised here:

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

\b Referring to a location in one of your own sections using \c{wrt
..gotoff} will give the distance from the beginning of the GOT to
the specified location, so that adding on the address of the GOT
would give the real address of the location you wanted.

\b Referring to an external or global symbol using \c{wrt ..got}
causes the linker to build an entry \e{in} the GOT containing the
address of the symbol, and the reference gives the distance from the
beginning of the GOT to the entry; so you can add on the address of
the GOT, load from the resulting address, and end up with the
address of the symbol.

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

\b Referring to a symbol name using \c{wrt ..sym} causes NASM to
write an ordinary relocation, but instead of making the relocation
relative to the start of the section and then adding on the offset
to the symbol, it will write a relocation record aimed directly at
the symbol in question. The distinction is a necessary one due to a
peculiarity of the dynamic linker.

A fuller explanation of how to use these relocation types to write
shared libraries entirely in NASM is given in \k{picdll}.

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

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.

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 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 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}

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.

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

The \c{aout} format generates \c{a.out} object files, in the form
used by early Linux systems. (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 doesn't.)

\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 BSD Unix clones, NetBSD, FreeBSD and
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 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}: Linux \i\c{as86} Object Files

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

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

\c{as86} is a very simple object format (from the NASM user's point
of view). It supports no special directives, no special symbols, no
use of \c{SEG} or \c{WRT}, and no extensions to any standard
directives. It supports only the three \i{standard section names}
\i\c{.text}, \i\c{.data} and \i\c{.bss}.

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

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

RDOFF is not used by any well-known operating systems. Those writing
their own systems, however, may well wish to use 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 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

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

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

The \c{dbg} output format is not built into NASM in the default
configuration. If you are building your own NASM executable from the
sources, you can define \i\c{OF_DBG} in \c{outform.h} or on the
compiler command line, and obtain the \c{dbg} output format.

The \c{dbg} format does not output an object file as such; instead,
it outputs a text file which contains a complete list of all the
transactions between the main body of NASM and the output-format
back end module. It is primarily intended to aid people who want to
write their own output drivers, so that they can get a clearer idea
of the various requests the main program makes of the output driver,
and in what order they happen.

For simple files, one can easily use the \c{dbg} format like this:

\c nasm -f dbg filename.asm

which will generate a diagnostic file called \c{filename.dbg}.
However, this will not work well on files which were designed for a
different object format, because each object format defines its own
macros (usually user-level forms of directives), and those macros
will not be defined in the \c{dbg} format. Therefore it can be
useful to run NASM twice, in order to do the preprocessing with the
native object format selected:

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

This preprocesses \c{rdfprog.asm} into \c{rdfprog.i}, keeping the
\c{rdf} object format selected in order to make sure RDF special
directives are converted into primitive form correctly. Then the
preprocessed source is fed through the \c{dbg} format to generate
the final diagnostic output.

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

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

\C{16bit} Writing 16-bit Code (DOS, Windows 3/3.1)

This chapter attempts to cover some of the common issues encountered
when writing 16-bit code to run under MS-DOS or 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}.

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:  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 initialises \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 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
uninitialised 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           section .text
\c start:    ; put your code here
\c           section .data
\c           ; put data items here
\c           section .bss
\c           ; put uninitialised 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 (uninitialised 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 initialised 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           global _%1
\c %define %1 _%1
\c %endmacro

\c %macro cextern 1
\c           extern _%1
\c %define %1 _%1
\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}.

\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 _myfunc:  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           ; some more code
\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           ; and then, further down...
\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           ; then those data items...
\c           segment _DATA
\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 initialise
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           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 _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 organises 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 example of an assembly function using the macro set is given
here:

\c           proc _nearproc
\c %$i       arg
\c %$j       arg
\c           mov ax,[bp + %$i]
\c           mov bx,[bp + %$j]
\c           add ax,[bx]
\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           proc _farproc
\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           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 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           ; some more code
\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           ; and then, further down...
\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 Initialised data must be in a segment whose name is either
\c{CONST} or something ending in \c{_DATA}.

\b Uninitialised 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           proc _pascalproc
\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           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
Win32, 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 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 Win32 compilers, DJGPP,
and NetBSD and 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 ELF, though, the leading
underscore should not be used.

\S{32cfunc} Function Definitions and Function Calls

\I{functions, C calling convention}The \i{C calling convention}The C
calling convention in 32-bit programs is as follows. In the
following description, the words \e{caller} and \e{callee} are used
to denote the function doing the calling and the function which gets
called.

\b The caller pushes the function's parameters on the stack, one
after another, in reverse order (right to left, so that the first
argument specified to the function is pushed last).

\b The caller then executes a near \c{CALL} instruction to pass
control to the callee.

\b The callee receives control, and typically (although this is not
actually necessary, in functions which do not need to access their
parameters) starts by saving the value of \c{ESP} in \c{EBP} so as
to be able to use \c{EBP} as a base pointer to find its parameters
on the stack. However, the caller was probably doing this too, so
part of the calling convention states that \c{EBP} must be preserved
by any C function. Hence the callee, if it is going to set up
\c{EBP} as a \i{frame pointer}, must push the previous value first.

\b The callee may then access its parameters relative to \c{EBP}.
The doubleword at \c{[EBP]} holds the previous value of \c{EBP} as
it was pushed; the next doubleword, at \c{[EBP+4]}, holds the return
address, pushed implicitly by \c{CALL}. The parameters start after
that, at \c{[EBP+8]}. The leftmost parameter of the function, since
it was pushed last, is accessible at this offset from \c{EBP}; the
others follow, at successively greater offsets. Thus, in a function
such as \c{printf} which takes a variable number of parameters, the
pushing of the parameters in reverse order means that the function
knows where to find its first parameter, which tells it the number
and type of the remaining ones.

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

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

\b Once the callee has finished processing, it restores \c{ESP} from
\c{EBP} if it had allocated local stack space, then pops the previous
value of \c{EBP}, and returns via \c{RET} (equivalently, \c{RETN}).

\b When the caller regains control from the callee, the function
parameters are still on the stack, so it typically adds an immediate
constant to \c{ESP} to remove them (instead of executing a number of
slow \c{POP} instructions). Thus, if a function is accidentally
called with the wrong number of parameters due to a prototype
mismatch, the stack will still be returned to a sensible state since
the caller, which \e{knows} how many parameters it pushed, does the
removing.

There is an alternative calling convention used by Win32 programs
for Windows API calls, and also for functions called \e{by} the
Windows API such as window procedures: they follow what Microsoft
calls the \c{__stdcall} convention. This is slightly closer to the
Pascal convention, in that the callee clears the stack by passing a
parameter to the \c{RET} instruction. However, the parameters are
still pushed in right-to-left order.

Thus, you would define a function in C style in the following way:

\c           global _myfunc
\c _myfunc:  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           ; some more code
\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           ; and then, further down...
\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           ; then those data items...
\c           segment _DATA
\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 organises 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 %$i       arg
\c %$j       arg
\c           mov eax,[ebp + %$i]
\c           mov ebx,[ebp + %$j]
\c           add eax,[ebx]
\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}

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 ELF position-independent code features, so you can
write Linux 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: pop ebx
\c           add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
\c           ; the function body comes here
\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, 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 ELF and 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           call %%getgot
\c %%getgot: pop ebx
\c           add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
\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 func:     push ebp
\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 array:    resd 128
\c .end:

Be careful: If you export a variable to the library user, by
declaring it as \c{GLOBAL} and supplying a size, the variable will
end up living in the data section of the main program, rather than
in your library's data section, where you declared it. So you will
have to access your own global variable with the \c{..got} mechanism
rather than \c{..gotoff}, as if it were external (which,
effectively, it has become).

Equally, if you need to store the address of an exported global in
one of your data sections, you can't do it by means of the standard
sort of code:

\c dataptr:  dd global_data_item    ; WRONG

NASM will interpret this code as an ordinary relocation, in which
\c{global_data_item} is merely an offset from the beginning of the
\c{.data} section (or whatever); so this reference will end up
pointing at your data section instead of at the exported global
which resides elsewhere.

Instead of the above code, then, you must write

\c dataptr:  dd global_data_item wrt ..sym

which makes use of the special \c{WRT} type \I\c{WRT ..sym}\c{..sym}
to instruct NASM to search the symbol table for a particular symbol
at that address, rather than just relocating by section base.

Either method will work for functions: referring to one of your
functions by means of

\c funcptr:  dd my_function

will give the user the address of the code you wrote, whereas

\c funcptr:  dd my_function wrt ..sym

will give the address of the procedure linkage table for the
function, which is where the calling program will \e{believe} the
function lives. Either address is a valid way to call the function.

\S{picproc} Calling Procedures Outside the Library

Calling procedures outside your shared library has to be done by
means of a \i\e{procedure linkage table}, or \i{PLT}. The PLT is
placed at a known offset from where the library is loaded, so the
library code can make calls to the PLT in a position-independent
way. Within the PLT there is code to jump to offsets contained in
the GOT, so function calls to other shared libraries or to routines
in the main program can be transparently passed off to their real
destinations.

To call an external routine, you must use another special PIC
relocation type, \I{PLT relocations}\i\c{WRT ..plt}. This is much
easier than the GOT-based ones: you simply replace calls such as
\c{CALL printf} with the PLT-relative version \c{CALL printf WRT
..plt}.

\S{link} Generating the Library File

Having written some code modules and assembled them to \c{.o} files,
you then generate your shared library with a command such as

\c ld -shared -o library.so module1.o module2.o       # for ELF
\c ld -Bshareable -o library.so module1.o module2.o   # for BSD

For ELF, if your shared library is going to reside in system
directories such as \c{/usr/lib} or \c{/lib}, it is usually worth
using the \i\c{-soname} flag to the linker, to store the final
library file name, with a version number, into the library:

\c ld -shared -soname library.so.1 -o library.so.1.2 *.o

You would then copy \c{library.so.1.2} into the library directory,
and create \c{library.so.1} as a symbolic link to it.

\C{mixsize} Mixing 16 and 32 Bit Code

This chapter tries to cover some of the issues, largely related to
unusual forms of addressing and jump instructions, encountered when
writing operating system code such as protected-mode initialisation
routines, which require code that operates in mixed segment sizes,
such as code in a 16-bit segment trying to modify data in a 32-bit
one, or jumps between different-size segments.

\H{mixjump} Mixed-Size Jumps\I{jumps, mixed-size}

\I{operating system, writing}\I{writing operating systems}The most
common form of \i{mixed-size instruction} is the one used when
writing a 32-bit OS: having done your setup in 16-bit mode, such as
loading the kernel, you then have to boot it by switching into
protected mode and jumping to the 32-bit kernel start address. In a
fully 32-bit OS, this tends to be the \e{only} mixed-size
instruction you need, since everything before it can be done in pure
16-bit code, and everything after it can be pure 32-bit.

This jump must specify a 48-bit far address, since the target
segment is a 32-bit one. However, it must be assembled in a 16-bit
segment, so just coding, for example,

\c           jmp 0x1234:0x56789ABC  ; wrong!

will not work, since the offset part of the address will be
truncated to \c{0x9ABC} and the jump will be an ordinary 16-bit far
one.

The Linux kernel setup code gets round the inability of \c{as86} to
generate the required instruction by coding it manually, using
\c{DB} instructions. NASM can go one better than that, by actually
generating the right instruction itself. Here's how to do it right:

\c           jmp dword 0x1234:0x56789ABC  ; right

\I\c{JMP DWORD}The \c{DWORD} prefix (strictly speaking, it should
come \e{after} the colon, since it is declaring the \e{offset} field
to be a doubleword; but NASM will accept either form, since both are
unambiguous) forces the offset part to be treated as far, in the
assumption that you are deliberately writing a jump from a 16-bit
segment to a 32-bit one.

You can do the reverse operation, jumping from a 32-bit segment to a
16-bit one, by means of the \c{WORD} prefix:

\c           jmp word 0x8765:0x4321 ; 32 to 16 bit

If the \c{WORD} prefix is specified in 16-bit mode, or the \c{DWORD}
prefix in 32-bit mode, they will be ignored, since each is
explicitly forcing NASM into a mode it was in anyway.

\H{mixaddr} Addressing Between Different-Size Segments\I{addressing,
mixed-size}\I{mixed-size addressing}

If your OS is mixed 16 and 32-bit, or if you are writing a DOS
extender, you are likely to have to deal with some 16-bit segments
and some 32-bit ones. At some point, you will probably end up
writing code in a 16-bit segment which has to access data in a
32-bit segment, or vice versa.

If the data you are trying to access in a 32-bit segment lies within
the first 64K of the segment, you may be able to get away with using
an ordinary 16-bit addressing operation for the purpose; but sooner
or later, you will want to do 32-bit addressing from 16-bit mode.

The easiest way to do this is to make sure you use a register for
the address, since any effective address containing a 32-bit
register is forced to be a 32-bit address. So you can do

\c           mov eax,offset_into_32_bit_segment_specified_by_fs
\c           mov dword [fs:eax],0x11223344

This is fine, but slightly cumbersome (since it wastes an
instruction and a register) if you already know the precise offset
you are aiming at. The x86 architecture does allow 32-bit effective
addresses to specify nothing but a 4-byte offset, so why shouldn't
NASM be able to generate the best instruction for the purpose?

It can. As in \k{mixjump}, you need only prefix the address with the
\c{DWORD} keyword, and it will be forced to be a 32-bit address:

\c           mov dword [fs:dword my_offset],0x11223344

Also as in \k{mixjump}, NASM is not fussy about whether the
\c{DWORD} prefix comes before or after the segment override, so
arguably a nicer-looking way to code the above instruction is

\c           mov dword [dword fs:my_offset],0x11223344

Don't confuse the \c{DWORD} prefix \e{outside} the square brackets,
which controls the size of the data stored at the address, with the
one \c{inside} the square brackets which controls the length of the
address itself. The two can quite easily be different:

\c           mov word [dword 0x12345678],0x9ABC

This moves 16 bits of data to an address specified by a 32-bit
offset.

You can also specify \c{WORD} or \c{DWORD} prefixes along with the
\c{FAR} prefix to indirect far jumps or calls. For example:

\c           call dword far [fs:word 0x4321]

This instruction contains an address specified by a 16-bit offset;
it loads a 48-bit far pointer from that (16-bit segment and 32-bit
offset), and calls that address.

\H{mixother} Other Mixed-Size Instructions

The other way you might want to access data might be using the
string instructions (\c{LODSx}, \c{STOSx} and so on) or the
\c{XLATB} instruction. These instructions, since they take no
parameters, might seem to have no easy way to make them perform
32-bit addressing when assembled in a 16-bit segment.

This is the purpose of NASM's \i\c{a16} and \i\c{a32} prefixes. If
you are coding \c{LODSB} in a 16-bit segment but it is supposed to
be accessing a string in a 32-bit segment, you should load the
desired address into \c{ESI} and then code

\c           a32 lodsb

The prefix forces the addressing size to 32 bits, meaning that
\c{LODSB} loads from \c{[DS:ESI]} instead of \c{[DS:SI]}. To access
a string in a 16-bit segment when coding in a 32-bit one, the
corresponding \c{a16} prefix can be used.

The \c{a16} and \c{a32} prefixes can be applied to any instruction
in NASM's instruction table, but most of them can generate all the
useful forms without them. The prefixes are necessary only for
instructions with implicit addressing: \c{CMPSx} (\k{insCMPSB}),
\c{SCASx} (\k{insSCASB}), \c{LODSx} (\k{insLODSB}), \c{STOSx}
(\k{insSTOSB}), \c{MOVSx} (\k{insMOVSB}), \c{INSx} (\k{insINSB}),
\c{OUTSx} (\k{insOUTSB}), and \c{XLATB} (\k{insXLATB}). Also, the
various push and pop instructions (\c{PUSHA} and \c{POPF} as well as
the more usual \c{PUSH} and \c{POP}) can accept \c{a16} or \c{a32}
prefixes to force a particular one of \c{SP} or \c{ESP} to be used
as a stack pointer, in case the stack segment in use is a different
size from the code segment.

\c{PUSH} and \c{POP}, when applied to segment registers in 32-bit
mode, also have the slightly odd behaviour that they push and pop 4
bytes at a time, of which the top two are ignored and the bottom two
give the value of the segment register being manipulated. To force
the 16-bit behaviour of segment-register push and pop instructions,
you can use the operand-size prefix \i\c{o16}:

\c           o16 push ss
\c           o16 push ds

This code saves a doubleword of stack space by fitting two segment
registers into the space which would normally be consumed by pushing
one.

(You can also use the \i\c{o32} prefix to force the 32-bit behaviour
when in 16-bit mode, but this seems less useful.)

\C{trouble} Troubleshooting

This chapter describes some of the common problems that users have
been known to encounter with NASM, and answers them. It also gives
instructions for reporting bugs in NASM if you find a difficulty
that isn't listed here.

\H{problems} Common Problems

\S{inefficient} NASM Generates \i{Inefficient Code}

I get a lot of `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: at worst it's a
misfeature, and that's a matter of opinion only.

\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.

\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           ; some boot sector code
\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           ; some boot sector code
\c           TIMES 510-($-$$) DB 0
\c           DW 0xAA55

The \c{TIMES} directive will insert exactly enough zero bytes into
the output to move the assembly point up to 510. This method also
has the advantage that if you accidentally fill your boot sector too
full, NASM will catch the problem at assembly time and report it, so
you won't end up with a boot sector that you have to disassemble to
find out what's wrong with it.

\S{probtimes} \i\c{TIMES} Doesn't Work

The other common problem with the above code is people who write the
\c{TIMES} line as

\c           TIMES 510-$ DB 0

by reasoning that \c{$} should be a pure number, just like 510, so
the difference between them is also a pure number and can happily be
fed to \c{TIMES}.

NASM is a \e{modular} assembler: the various component parts are
designed to be easily separable for re-use, so they don't exchange
information unnecessarily. In consequence, the \c{bin} output
format, even though it has been told by the \c{ORG} directive that
the \c{.text} section should start at 0, does not pass that
information back to the expression evaluator. So from the
evaluator's point of view, \c{$} isn't a pure number: it's an offset
from a section base. Therefore the difference between \c{$} and 510
is also not a pure number, but involves a section base. Values
involving section bases cannot be passed as arguments to \c{TIMES}.

The solution, as in the previous section, is to code the \c{TIMES}
line in the form

\c           TIMES 510-($-$$) DB 0

in which \c{$} and \c{$$} are offsets from the same section base,
and so their difference is a pure number. This will solve the
problem and generate sensible code.

\H{bugs} \i{Bugs}\I{reporting bugs}

We have never yet released a version of NASM with any \e{known}
bugs. That doesn't usually stop there being plenty we didn't know
about, though. Any that you find should be reported to
\W{mailto:anakin@pobox.com}\c{anakin@pobox.com}.

Please read \k{qstart} first, and don't report the bug if it's
listed in there as a deliberate feature. (If you think the feature
is badly thought out, feel free to send us reasons why you think it
should be changed, but don't just send us mail saying `This is a
bug' if the documentation says we did it on purpose.) Then read
\k{problems}, and don't bother reporting the bug if it's listed
there.

If you do report a bug, \e{please} give us all of the following
information:

\b What operating system you're running NASM under. DOS, Linux,
NetBSD, Win16, Win32, VMS (I'd be impressed), whatever.

\b If you're running NASM under DOS or Win32, tell us whether you've
compiled your own executable from the DOS source archive, or whether
you were using the standard distribution binaries out of the
archive. If you were using a locally built executable, try to
reproduce the problem using one of the standard binaries, as this
will make it easier for us to reproduce your problem prior to fixing
it.

\b Which version of NASM you're using, and exactly how you invoked
it. Give us the precise command line, and the contents of the
\c{NASM} environment variable if any.

\b Which versions of any supplementary programs you're using, and
how you invoked them. If the problem only becomes visible at link
time, tell us what linker you're using, what version of it you've
got, and the exact linker command line. If the problem involves
linking against object files generated by a compiler, tell us what
compiler, what version, and what command line or options you used.
(If you're compiling in an IDE, please try to reproduce the problem
with the command-line version of the compiler.)

\b If at all possible, send us a NASM source file which exhibits the
problem. If this causes copyright problems (e.g. you can only
reproduce the bug in restricted-distribution code) then bear in mind
the following two points: firstly, we guarantee that any source code
sent to us for the purposes of debugging NASM will be used \e{only}
for the purposes of debugging NASM, and that we will delete all our
copies of it as soon as we have found and fixed the bug or bugs in
question; and secondly, we would prefer \e{not} to be mailed large
chunks of code anyway. The smaller the file, the better. A
three-line sample file that does nothing useful \e{except}
demonstrate the problem is much easier to work with than a
fully fledged ten-thousand-line program. (Of course, some errors
\e{do} only crop up in large files, so this may not be possible.)

\b A description of what the problem actually \e{is}. `It doesn't
work' is \e{not} a helpful description! Please describe exactly what
is happening that shouldn't be, or what isn't happening that should.
Examples might be: `NASM generates an error message saying Line 3
for an error that's actually on Line 5'; `NASM generates an error
message that I believe it shouldn't be generating at all'; `NASM
fails to generate an error message that I believe it \e{should} be
generating'; `the object file produced from this source code crashes
my linker'; `the ninth byte of the output file is 66 and I think it
should be 77 instead'.

\b If you believe the output file from NASM to be faulty, send it to
us. That allows us to determine whether our own copy of NASM
generates the same file, or whether the problem is related to
portability issues between our development platforms and yours. We
can handle binary files mailed to us as MIME attachments, uuencoded,
and even BinHex. Alternatively, we may be able to provide an FTP
site you can upload the suspect files to; but mailing them is easier
for us.

\b Any other information or data files that might be helpful. If,
for example, the problem involves NASM failing to generate an object
file while TASM can generate an equivalent file without trouble,
then send us \e{both} object files, so we can see what TASM is doing
differently from us.

\A{iref} Intel x86 Instruction Reference

This appendix provides a complete list of the machine instructions
which NASM will assemble, and a short description of the function of
each one.

It is not intended to be exhaustive documentation on the fine
details of the instructions' function, such as which exceptions they
can trigger: for such documentation, you should go to Intel's Web
site, \W{http://www.intel.com}\c{http://www.intel.com}.

Instead, this appendix is intended primarily to provide
documentation on the way the instructions may be used within NASM.
For example, looking up \c{LOOP} will tell you that NASM allows
\c{CX} or \c{ECX} to be specified as an optional second argument to
the \c{LOOP} instruction, to enforce which of the two possible
counter registers should be used if the default is not the one
desired.

The instructions are not quite listed in alphabetical order, since
groups of instructions with similar functions are lumped together in
the same entry. Most of them don't move very far from their
alphabetic position because of this.

\H{iref-opr} Key to Operand Specifications

The instruction descriptions in this appendix specify their operands
using the following notation:

\b Registers: \c{reg8} denotes an 8-bit \i{general purpose
register}, \c{reg16} denotes a 16-bit general purpose register, and
\c{reg32} a 32-bit one. \c{fpureg} denotes one of the eight FPU
stack registers, \c{mmxreg} denotes one of the eight 64-bit MMX
registers, and \c{segreg} denotes a segment register. In addition,
some registers (such as \c{AL}, \c{DX} or
\c{ECX}) may be specified explicitly.

\b Immediate operands: \c{imm} denotes a generic \i{immediate operand}.
\c{imm8}, \c{imm16} and \c{imm32} are used when the operand is
intended to be a specific size. For some of these instructions, NASM
needs an explicit specifier: for example, \c{ADD ESP,16} could be
interpreted as either \c{ADD r/m32,imm32} or \c{ADD r/m32,imm8}.
NASM chooses the former by default, and so you must specify \c{ADD
ESP,BYTE 16} for the latter.

\b Memory references: \c{mem} denotes a generic \i{memory reference};
\c{mem8}, \c{mem16}, \c{mem32}, \c{mem64} and \c{mem80} are used
when the operand needs to be a specific size. Again, a specifier is
needed in some cases: \c{DEC [address]} is ambiguous and will be
rejected by NASM. You must specify \c{DEC BYTE [address]}, \c{DEC
WORD [address]} or \c{DEC DWORD [address]} instead.

\b \i{Restricted memory references}: one form of the \c{MOV}
instruction allows a memory address to be specified \e{without}
allowing the normal range of register combinations and effective
address processing. This is denoted by \c{memoffs8}, \c{memoffs16}
and \c{memoffs32}.

\b Register or memory choices: many instructions can accept either a
register \e{or} a memory reference as an operand. \c{r/m8} is a
shorthand for \c{reg8/mem8}; similarly \c{r/m16} and \c{r/m32}.
\c{r/m64} is MMX-related, and is a shorthand for \c{mmxreg/mem64}.

\H{iref-opc} Key to Opcode Descriptions

This appendix also provides the opcodes which NASM will generate for
each form of each instruction. The opcodes are listed in the
following way:

\b A hex number, such as \c{3F}, indicates a fixed byte containing
that number.

\b A hex number followed by \c{+r}, such as \c{C8+r}, indicates that
one of the operands to the instruction is a register, and the
`register value' of that register should be added to the hex number
to produce the generated byte. For example, EDX has register value
2, so the code \c{C8+r}, when the register operand is EDX, generates
the hex byte \c{CA}. Register values for specific registers are
given in \k{iref-rv}.

\b A hex number followed by \c{+cc}, such as \c{40+cc}, indicates
that the instruction name has a condition code suffix, and the
numeric representation of the condition code should be added to the
hex number to produce the generated byte. For example, the code
\c{40+cc}, when the instruction contains the \c{NE} condition,
generates the hex byte \c{45}. Condition codes and their numeric
representations are given in \k{iref-cc}.

\b A slash followed by a digit, such as \c{/2}, indicates that one
of the operands to the instruction is a memory address or register
(denoted \c{mem} or \c{r/m}, with an optional size). This is to be
encoded as an effective address, with a \i{ModR/M byte}, an optional
\i{SIB byte}, and an optional displacement, and the spare (register)
field of the ModR/M byte should be the digit given (which will be
from 0 to 7, so it fits in three bits). The encoding of effective
addresses is given in \k{iref-ea}.

\b The code \c{/r} combines the above two: it indicates that one of
the operands is a memory address or \c{r/m}, and another is a
register, and that an effective address should be generated with the
spare (register) field in the ModR/M byte being equal to the
`register value' of the register operand. The encoding of effective
addresses is given in \k{iref-ea}; register values are given in
\k{iref-rv}.

\b The codes \c{ib}, \c{iw} and \c{id} indicate that one of the
operands to the instruction is an immediate value, and that this is
to be encoded as a byte, little-endian word or little-endian
doubleword respectively.

\b The codes \c{rb}, \c{rw} and \c{rd} indicate that one of the
operands to the instruction is an immediate value, and that the
\e{difference} between this value and the address of the end of the
instruction is to be encoded as a byte, word or doubleword
respectively. Where the form \c{rw/rd} appears, it indicates that
either \c{rw} or \c{rd} should be used according to whether assembly
is being performed in \c{BITS 16} or \c{BITS 32} state respectively.

\b The codes \c{ow} and \c{od} indicate that one of the operands to
the instruction is a reference to the contents of a memory address
specified as an immediate value: this encoding is used in some forms
of the \c{MOV} instruction in place of the standard
effective-address mechanism. The displacement is encoded as a word
or doubleword. Again, \c{ow/od} denotes that \c{ow} or \c{od} should
be chosen according to the \c{BITS} setting.

\b The codes \c{o16} and \c{o32} indicate that the given form of the
instruction should be assembled with operand size 16 or 32 bits. In
other words, \c{o16} indicates a \c{66} prefix in \c{BITS 32} state,
but generates no code in \c{BITS 16} state; and \c{o32} indicates a
\c{66} prefix in \c{BITS 16} state but generates nothing in \c{BITS
32}.

\b The codes \c{a16} and \c{a32}, similarly to \c{o16} and \c{o32},
indicate the address size of the given form of the instruction.
Where this does not match the \c{BITS} setting, a \c{67} prefix is
required.

\S{iref-rv} Register Values

Where an instruction requires a register value, it is already
implicit in the encoding of the rest of the instruction what type of
register is intended: an 8-bit general-purpose register, a segment
register, a debug register, an MMX register, or whatever. Therefore
there is no problem with registers of different types sharing an
encoding value.

The encodings for the various classes of register are:

\b 8-bit general registers: \c{AL} is 0, \c{CL} is 1, \c{DL} is 2,
\c{BL} is 3, \c{AH} is 4, \c{CH} is 5, \c{DH} is 6, and \c{BH} is
7.

\b 16-bit general registers: \c{AX} is 0, \c{CX} is 1, \c{DX} is 2,
\c{BX} is 3, \c{SP} is 4, \c{BP} is 5, \c{SI} is 6, and \c{DI} is 7.

\b 32-bit general registers: \c{EAX} is 0, \c{ECX} is 1, \c{EDX} is
2, \c{EBX} is 3, \c{ESP} is 4, \c{EBP} is 5, \c{ESI} is 6, and
\c{EDI} is 7.

\b \i{Segment registers}: \c{ES} is 0, \c{CS} is 1, \c{SS} is 2, \c{DS}
is 3, \c{FS} is 4, and \c{GS} is 5.

\b \I{floating-point, registers}{Floating-point registers}: \c{ST0}
is 0, \c{ST1} is 1, \c{ST2} is 2, \c{ST3} is 3, \c{ST4} is 4,
\c{ST5} is 5, \c{ST6} is 6, and \c{ST7} is 7.

\b 64-bit \i{MMX registers}: \c{MM0} is 0, \c{MM1} is 1, \c{MM2} is 2,
\c{MM3} is 3, \c{MM4} is 4, \c{MM5} is 5, \c{MM6} is 6, and \c{MM7}
is 7.

\b \i{Control registers}: \c{CR0} is 0, \c{CR2} is 2, \c{CR3} is 3,
and \c{CR4} is 4.

\b \i{Debug registers}: \c{DR0} is 0, \c{DR1} is 1, \c{DR2} is 2,
\c{DR3} is 3, \c{DR6} is 6, and \c{DR7} is 7.

\b \i{Test registers}: \c{TR3} is 3, \c{TR4} is 4, \c{TR5} is 5,
\c{TR6} is 6, and \c{TR7} is 7.

(Note that wherever a register name contains a number, that number
is also the register value for that register.)

\S{iref-cc} \i{Condition Codes}

The available condition codes are given here, along with their
numeric representations as part of opcodes. Many of these condition
codes have synonyms, so several will be listed at a time.

In the following descriptions, the word `either', when applied to two
possible trigger conditions, is used to mean `either or both'. If
`either but not both' is meant, the phrase `exactly one of' is used.

\b \c{O} is 0 (trigger if the overflow flag is set); \c{NO} is 1.

\b \c{B}, \c{C} and \c{NAE} are 2 (trigger if the carry flag is
set); \c{AE}, \c{NB} and \c{NC} are 3.

\b \c{E} and \c{Z} are 4 (trigger if the zero flag is set); \c{NE}
and \c{NZ} are 5.

\b \c{BE} and \c{NA} are 6 (trigger if either of the carry or zero
flags is set); \c{A} and \c{NBE} are 7.

\b \c{S} is 8 (trigger if the sign flag is set); \c{NS} is 9.

\b \c{P} and \c{PE} are 10 (trigger if the parity flag is set);
\c{NP} and \c{PO} are 11.

\b \c{L} and \c{NGE} are 12 (trigger if exactly one of the sign and
overflow flags is set); \c{GE} and \c{NL} are 13.

\b \c{LE} and \c{NG} are 14 (trigger if either the zero flag is set,
or exactly one of the sign and overflow flags is set); \c{G} and
\c{NLE} are 15.

Note that in all cases, the sense of a condition code may be
reversed by changing the low bit of the numeric representation.

\S{iref-ea} Effective Address Encoding: \i{ModR/M} and \i{SIB}

An \i{effective address} is encoded in up to three parts: a ModR/M
byte, an optional SIB byte, and an optional byte, word or doubleword
displacement field.

The ModR/M byte consists of three fields: the \c{mod} field, ranging
from 0 to 3, in the upper two bits of the byte, the \c{r/m} field,
ranging from 0 to 7, in the lower three bits, and the spare
(register) field in the middle (bit 3 to bit 5). The spare field is
not relevant to the effective address being encoded, and either
contains an extension to the instruction opcode or the register
value of another operand.

The ModR/M system can be used to encode a direct register reference
rather than a memory access. This is always done by setting the
\c{mod} field to 3 and the \c{r/m} field to the register value of
the register in question (it must be a general-purpose register, and
the size of the register must already be implicit in the encoding of
the rest of the instruction). In this case, the SIB byte and
displacement field are both absent.

In 16-bit addressing mode (either \c{BITS 16} with no \c{67} prefix,
or \c{BITS 32} with a \c{67} prefix), the SIB byte is never used.
The general rules for \c{mod} and \c{r/m} (there is an exception,
given below) are:

\b The \c{mod} field gives the length of the displacement field: 0
means no displacement, 1 means one byte, and 2 means two bytes.

\b The \c{r/m} field encodes the combination of registers to be
added to the displacement to give the accessed address: 0 means
\c{BX+SI}, 1 means \c{BX+DI}, 2 means \c{BP+SI}, 3 means \c{BP+DI},
4 means \c{SI} only, 5 means \c{DI} only, 6 means \c{BP} only, and 7
means \c{BX} only.

However, there is a special case:

\b If \c{mod} is 0 and \c{r/m} is 6, the effective address encoded
is not \c{[BP]} as the above rules would suggest, but instead
\c{[disp16]}: the displacement field is present and is two bytes
long, and no registers are added to the displacement.

Therefore the effective address \c{[BP]} cannot be encoded as
efficiently as \c{[BX]}; so if you code \c{[BP]} in a program, NASM
adds a notional 8-bit zero displacement, and sets \c{mod} to 1,
\c{r/m} to 6, and the one-byte displacement field to 0.

In 32-bit addressing mode (either \c{BITS 16} with a \c{67} prefix,
or \c{BITS 32} with no \c{67} prefix) the general rules (again,
there are exceptions) for \c{mod} and \c{r/m} are:

\b The \c{mod} field gives the length of the displacement field: 0
means no displacement, 1 means one byte, and 2 means four bytes.

\b If only one register is to be added to the displacement, and it
is not \c{ESP}, the \c{r/m} field gives its register value, and the
SIB byte is absent. If the \c{r/m} field is 4 (which would encode
\c{ESP}), the SIB byte is present and gives the combination and
scaling of registers to be added to the displacement.

If the SIB byte is present, it describes the combination of
registers (an optional base register, and an optional index register
scaled by multiplication by 1, 2, 4 or 8) to be added to the
displacement. The SIB byte is divided into the \c{scale} field, in
the top two bits, the \c{index} field in the next three, and the
\c{base} field in the bottom three. The general rules are:

\b The \c{base} field encodes the register value of the base
register.

\b The \c{index} field encodes the register value of the index
register, unless it is 4, in which case no index register is used
(so \c{ESP} cannot be used as an index register).

\b The \c{scale} field encodes the multiplier by which the index
register is scaled before adding it to the base and displacement: 0
encodes a multiplier of 1, 1 encodes 2, 2 encodes 4 and 3 encodes 8.

The exceptions to the 32-bit encoding rules are:

\b If \c{mod} is 0 and \c{r/m} is 5, the effective address encoded
is not \c{[EBP]} as the above rules would suggest, but instead
\c{[disp32]}: the displacement field is present and is four bytes
long, and no registers are added to the displacement.

\b If \c{mod} is 0, \c{r/m} is 4 (meaning the SIB byte is present)
and \c{base} is 4, the effective address encoded is not
\c{[EBP+index]} as the above rules would suggest, but instead
\c{[disp32+index]}: the displacement field is present and is four
bytes long, and there is no base register (but the index register is
still processed in the normal way).

\H{iref-flg} Key to Instruction Flags

Given along with each instruction in this appendix is a set of
flags, denoting the type of the instruction. The types are as follows:

\b \c{8086}, \c{186}, \c{286}, \c{386}, \c{486}, \c{PENT} and \c{P6}
denote the lowest processor type that supports the instruction. Most
instructions run on all processors above the given type; those that
do not are documented. The Pentium II contains no additional
instructions beyond the P6 (Pentium Pro); from the point of view of
its instruction set, it can be thought of as a P6 with MMX
capability.

\b \c{CYRIX} indicates that the instruction is specific to Cyrix
processors, for example the extra MMX instructions in the Cyrix
extended MMX instruction set.

\b \c{FPU} indicates that the instruction is a floating-point one,
and will only run on machines with a coprocessor (automatically
including 486DX, Pentium and above).

\b \c{MMX} indicates that the instruction is an MMX one, and will
run on MMX-capable Pentium processors and the Pentium II.

\b \c{PRIV} indicates that the instruction is a protected-mode
management instruction. Many of these may only be used in protected
mode, or only at privilege level zero.

\b \c{UNDOC} indicates that the instruction is an undocumented one,
and not part of the official Intel Architecture; it may or may not
be supported on any given machine.

\H{insAAA} \i\c{AAA}, \i\c{AAS}, \i\c{AAM}, \i\c{AAD}: ASCII
Adjustments

\c AAA                           ; 37                   [8086]

\c AAS                           ; 3F                   [8086]

\c AAD                           ; D5 0A                [8086]
\c AAD imm                       ; D5 ib                [8086]

\c AAM                           ; D4 0A                [8086]
\c AAM imm                       ; D4 ib                [8086]

These instructions are used in conjunction with the add, subtract,
multiply and divide instructions to perform binary-coded decimal
arithmetic in \e{unpacked} (one BCD digit per byte - easy to
translate to and from ASCII, hence the instruction names) form.
There are also packed BCD instructions \c{DAA} and \c{DAS}: see
\k{insDAA}.

\c{AAA} should be used after a one-byte \c{ADD} instruction whose
destination was the \c{AL} register: by means of examining the value
in the low nibble of \c{AL} and also the auxiliary carry flag
\c{AF}, it determines whether the addition has overflowed, and
adjusts it (and sets the carry flag) if so. You can add long BCD
strings together by doing \c{ADD}/\c{AAA} on the low digits, then
doing \c{ADC}/\c{AAA} on each subsequent digit.

\c{AAS} works similarly to \c{AAA}, but is for use after \c{SUB}
instructions rather than \c{ADD}.

\c{AAM} is for use after you have multiplied two decimal digits
together and left the result in \c{AL}: it divides \c{AL} by ten and
stores the quotient in \c{AH}, leaving the remainder in \c{AL}. The
divisor 10 can be changed by specifying an operand to the
instruction: a particularly handy use of this is \c{AAM 16}, causing
the two nibbles in \c{AL} to be separated into \c{AH} and \c{AL}.

\c{AAD} performs the inverse operation to \c{AAM}: it multiplies
\c{AH} by ten, adds it to \c{AL}, and sets \c{AH} to zero. Again,
the multiplier 10 can be changed.

\H{insADC} \i\c{ADC}: Add with Carry

\c ADC r/m8,reg8                 ; 10 /r                [8086]
\c ADC r/m16,reg16               ; o16 11 /r            [8086]
\c ADC r/m32,reg32               ; o32 11 /r            [386]

\c ADC reg8,r/m8                 ; 12 /r                [8086]
\c ADC reg16,r/m16               ; o16 13 /r            [8086]
\c ADC reg32,r/m32               ; o32 13 /r            [386]

\c ADC r/m8,imm8                 ; 80 /2 ib             [8086]
\c ADC r/m16,imm16               ; o16 81 /2 iw         [8086]
\c ADC r/m32,imm32               ; o32 81 /2 id         [386]

\c ADC r/m16,imm8                ; o16 83 /2 ib         [8086]
\c ADC r/m32,imm8                ; o32 83 /2 ib         [386]

\c ADC AL,imm8                   ; 14 ib                [8086]
\c ADC AX,imm16                  ; o16 15 iw            [8086]
\c ADC EAX,imm32                 ; o32 15 id            [386]

\c{ADC} performs integer addition: it adds its two operands
together, plus the value of the carry flag, and leaves the result in
its destination (first) operand. The flags are set according to the
result of the operation: in particular, the carry flag is affected
and can be used by a subsequent \c{ADC} instruction.

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

To add two numbers without also adding the contents of the carry
flag, use \c{ADD} (\k{insADD}).

\H{insADD} \i\c{ADD}: Add Integers

\c ADD r/m8,reg8                 ; 00 /r                [8086]
\c ADD r/m16,reg16               ; o16 01 /r            [8086]
\c ADD r/m32,reg32               ; o32 01 /r            [386]

\c ADD reg8,r/m8                 ; 02 /r                [8086]
\c ADD reg16,r/m16               ; o16 03 /r            [8086]
\c ADD reg32,r/m32               ; o32 03 /r            [386]

\c ADD r/m8,imm8                 ; 80 /0 ib             [8086]
\c ADD r/m16,imm16               ; o16 81 /0 iw         [8086]
\c ADD r/m32,imm32               ; o32 81 /0 id         [386]

\c ADD r/m16,imm8                ; o16 83 /0 ib         [8086]
\c ADD r/m32,imm8                ; o32 83 /0 ib         [386]

\c ADD AL,imm8                   ; 04 ib                [8086]
\c ADD AX,imm16                  ; o16 05 iw            [8086]
\c ADD EAX,imm32                 ; o32 05 id            [386]

\c{ADD} performs integer addition: it adds its two operands
together, and leaves the result in its destination (first) operand.
The flags are set according to the result of the operation: in
particular, the carry flag is affected and can be used by a
subsequent \c{ADC} instruction (\k{insADC}).

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

\H{insAND} \i\c{AND}: Bitwise AND

\c AND r/m8,reg8                 ; 20 /r                [8086]
\c AND r/m16,reg16               ; o16 21 /r            [8086]
\c AND r/m32,reg32               ; o32 21 /r            [386]

\c AND reg8,r/m8                 ; 22 /r                [8086]
\c AND reg16,r/m16               ; o16 23 /r            [8086]
\c AND reg32,r/m32               ; o32 23 /r            [386]

\c AND r/m8,imm8                 ; 80 /4 ib             [8086]
\c AND r/m16,imm16               ; o16 81 /4 iw         [8086]
\c AND r/m32,imm32               ; o32 81 /4 id         [386]

\c AND r/m16,imm8                ; o16 83 /4 ib         [8086]
\c AND r/m32,imm8                ; o32 83 /4 ib         [386]

\c AND AL,imm8                   ; 24 ib                [8086]
\c AND AX,imm16                  ; o16 25 iw            [8086]
\c AND EAX,imm32                 ; o32 25 id            [386]

\c{AND} performs a bitwise AND operation between its two operands
(i.e. each bit of the result is 1 if and only if the corresponding
bits of the two inputs were both 1), and stores the result in the
destination (first) operand.

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

The MMX instruction \c{PAND} (see \k{insPAND}) performs the same
operation on the 64-bit MMX registers.

\H{insARPL} \i\c{ARPL}: Adjust RPL Field of Selector

\c ARPL r/m16,reg16              ; 63 /r                [286,PRIV]

\c{ARPL} expects its two word operands to be segment selectors. It
adjusts the RPL (requested privilege level - stored in the bottom
two bits of the selector) field of the destination (first) operand
to ensure that it is no less (i.e. no more privileged than) the RPL
field of the source operand. The zero flag is set if and only if a
change had to be made.

\H{insBOUND} \i\c{BOUND}: Check Array Index against Bounds

\c BOUND reg16,mem               ; o16 62 /r            [186]
\c BOUND reg32,mem               ; o32 62 /r            [386]

\c{BOUND} expects its second operand to point to an area of memory
containing two signed values of the same size as its first operand
(i.e. two words for the 16-bit form; two doublewords for the 32-bit
form). It performs two signed comparisons: if the value in the
register passed as its first operand is less than the first of the
in-memory values, or is greater than or equal to the second, it
throws a BR exception. Otherwise, it does nothing.

\H{insBSF} \i\c{BSF}, \i\c{BSR}: Bit Scan

\c BSF reg16,r/m16               ; o16 0F BC /r         [386]
\c BSF reg32,r/m32               ; o32 0F BC /r         [386]

\c BSR reg16,r/m16               ; o16 0F BD /r         [386]
\c BSR reg32,r/m32               ; o32 0F BD /r         [386]

\c{BSF} searches for a set bit in its source (second) operand,
starting from the bottom, and if it finds one, stores the index in
its destination (first) operand. If no set bit is found, the
contents of the destination operand are undefined.

\c{BSR} performs the same function, but searches from the top
instead, so it finds the most significant set bit.

Bit indices are from 0 (least significant) to 15 or 31 (most
significant).

\H{insBSWAP} \i\c{BSWAP}: Byte Swap

\c BSWAP reg32                   ; o32 0F C8+r          [486]

\c{BSWAP} swaps the order of the four bytes of a 32-bit register:
bits 0-7 exchange places with bits 24-31, and bits 8-15 swap with
bits 16-23. There is no explicit 16-bit equivalent: to byte-swap
\c{AX}, \c{BX}, \c{CX} or \c{DX}, \c{XCHG} can be used.

\H{insBT} \i\c{BT}, \i\c{BTC}, \i\c{BTR}, \i\c{BTS}: Bit Test

\c BT r/m16,reg16                ; o16 0F A3 /r         [386]
\c BT r/m32,reg32                ; o32 0F A3 /r         [386]
\c BT r/m16,imm8                 ; o16 0F BA /4 ib      [386]
\c BT r/m32,imm8                 ; o32 0F BA /4 ib      [386]

\c BTC r/m16,reg16               ; o16 0F BB /r         [386]
\c BTC r/m32,reg32               ; o32 0F BB /r         [386]
\c BTC r/m16,imm8                ; o16 0F BA /7 ib      [386]
\c BTC r/m32,imm8                ; o32 0F BA /7 ib      [386]

\c BTR r/m16,reg16               ; o16 0F B3 /r         [386]
\c BTR r/m32,reg32               ; o32 0F B3 /r         [386]
\c BTR r/m16,imm8                ; o16 0F BA /6 ib      [386]
\c BTR r/m32,imm8                ; o32 0F BA /6 ib      [386]

\c BTS r/m16,reg16               ; o16 0F AB /r         [386]
\c BTS r/m32,reg32               ; o32 0F AB /r         [386]
\c BTS r/m16,imm                 ; o16 0F BA /5 ib      [386]
\c BTS r/m32,imm                 ; o32 0F BA /5 ib      [386]

These instructions all test one bit of their first operand, whose
index is given by the second operand, and store the value of that
bit into the carry flag. Bit indices are from 0 (least significant)
to 15 or 31 (most significant).

In addition to storing the original value of the bit into the carry
flag, \c{BTR} also resets (clears) the bit in the operand itself.
\c{BTS} sets the bit, and \c{BTC} complements the bit. \c{BT} does
not modify its operands.

The bit offset should be no greater than the size of the operand.

\H{insCALL} \i\c{CALL}: Call Subroutine

\c CALL imm                      ; E8 rw/rd             [8086]
\c CALL imm:imm16                ; o16 9A iw iw         [8086]
\c CALL imm:imm32                ; o32 9A id iw         [386]
\c CALL FAR mem16                ; o16 FF /3            [8086]
\c CALL FAR mem32                ; o32 FF /3            [386]
\c CALL r/m16                    ; o16 FF /2            [8086]
\c CALL r/m32                    ; o32 FF /2            [386]

\c{CALL} calls a subroutine, by means of pushing the current
instruction pointer (\c{IP}) and optionally \c{CS} as well on the
stack, and then jumping to a given address.

\c{CS} is pushed as well as \c{IP} if and only if the call is a far
call, i.e. a destination segment address is specified in the
instruction. The forms involving two colon-separated arguments are
far calls; so are the \c{CALL FAR mem} forms.

You can choose between the two immediate \i{far call} forms (\c{CALL
imm:imm}) by the use of the \c{WORD} and \c{DWORD} keywords: \c{CALL
WORD 0x1234:0x5678}) or \c{CALL DWORD 0x1234:0x56789abc}.

The \c{CALL FAR mem} forms execute a far call by loading the
destination address out of memory. The address loaded consists of 16
or 32 bits of offset (depending on the operand size), and 16 bits of
segment. The operand size may be overridden using \c{CALL WORD FAR
mem} or \c{CALL DWORD FAR mem}.

The \c{CALL r/m} forms execute a \i{near call} (within the same
segment), loading the destination address out of memory or out of a
register. The keyword \c{NEAR} may be specified, for clarity, in
these forms, but is not necessary. Again, operand size can be
overridden using \c{CALL WORD mem} or \c{CALL DWORD mem}.

As a convenience, NASM does not require you to call a far procedure
symbol by coding the cumbersome \c{CALL SEG routine:routine}, but
instead allows the easier synonym \c{CALL FAR routine}.

The \c{CALL r/m} forms given above are near calls; NASM will accept
the \c{NEAR} keyword (e.g. \c{CALL NEAR [address]}), even though it
is not strictly necessary.

\H{insCBW} \i\c{CBW}, \i\c{CWD}, \i\c{CDQ}, \i\c{CWDE}: Sign Extensions

\c CBW                           ; o16 98               [8086]
\c CWD                           ; o16 99               [8086]
\c CDQ                           ; o32 99               [386]
\c CWDE                          ; o32 98               [386]

All these instructions sign-extend a short value into a longer one,
by replicating the top bit of the original value to fill the
extended one.

\c{CBW} extends \c{AL} into \c{AX} by repeating the top bit of
\c{AL} in every bit of \c{AH}. \c{CWD} extends \c{AX} into \c{DX:AX}
by repeating the top bit of \c{AX} throughout \c{DX}. \c{CWDE}
extends \c{AX} into \c{EAX}, and \c{CDQ} extends \c{EAX} into
\c{EDX:EAX}.

\H{insCLC} \i\c{CLC}, \i\c{CLD}, \i\c{CLI}, \i\c{CLTS}: Clear Flags

\c CLC                           ; F8                   [8086]
\c CLD                           ; FC                   [8086]
\c CLI                           ; FA                   [8086]
\c CLTS                          ; 0F 06                [286,PRIV]

These instructions clear various flags. \c{CLC} clears the carry
flag; \c{CLD} clears the direction flag; \c{CLI} clears the
interrupt flag (thus disabling interrupts); and \c{CLTS} clears the
task-switched (\c{TS}) flag in \c{CR0}.

To set the carry, direction, or interrupt flags, use the \c{STC},
\c{STD} and \c{STI} instructions (\k{insSTC}). To invert the carry
flag, use \c{CMC} (\k{insCMC}).

\H{insCMC} \i\c{CMC}: Complement Carry Flag

\c CMC                           ; F5                   [8086]

\c{CMC} changes the value of the carry flag: if it was 0, it sets it
to 1, and vice versa.

\H{insCMOVcc} \i\c{CMOVcc}: Conditional Move

\c CMOVcc reg16,r/m16            ; o16 0F 40+cc /r      [P6]
\c CMOVcc reg32,r/m32            ; o32 0F 40+cc /r      [P6]

\c{CMOV} moves its source (second) operand into its destination
(first) operand if the given condition code is satisfied; otherwise
it does nothing.

For a list of condition codes, see \k{iref-cc}.

Although the \c{CMOV} instructions are flagged \c{P6} above, they
may not be supported by all Pentium Pro processors; the \c{CPUID}
instruction (\k{insCPUID}) will return a bit which indicates whether
conditional moves are supported.

\H{insCMP} \i\c{CMP}: Compare Integers

\c CMP r/m8,reg8                 ; 38 /r                [8086]
\c CMP r/m16,reg16               ; o16 39 /r            [8086]
\c CMP r/m32,reg32               ; o32 39 /r            [386]

\c CMP reg8,r/m8                 ; 3A /r                [8086]
\c CMP reg16,r/m16               ; o16 3B /r            [8086]
\c CMP reg32,r/m32               ; o32 3B /r            [386]

\c CMP r/m8,imm8                 ; 80 /0 ib             [8086]
\c CMP r/m16,imm16               ; o16 81 /0 iw         [8086]
\c CMP r/m32,imm32               ; o32 81 /0 id         [386]

\c CMP r/m16,imm8                ; o16 83 /0 ib         [8086]
\c CMP r/m32,imm8                ; o32 83 /0 ib         [386]

\c CMP AL,imm8                   ; 3C ib                [8086]
\c CMP AX,imm16                  ; o16 3D iw            [8086]
\c CMP EAX,imm32                 ; o32 3D id            [386]

\c{CMP} performs a `mental' subtraction of its second operand from
its first operand, and affects the flags as if the subtraction had
taken place, but does not store the result of the subtraction
anywhere.

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

\H{insCMPSB} \i\c{CMPSB}, \i\c{CMPSW}, \i\c{CMPSD}: Compare Strings

\c CMPSB                         ; A6                   [8086]
\c CMPSW                         ; o16 A7               [8086]
\c CMPSD                         ; o32 A7               [386]

\c{CMPSB} compares the byte at \c{[DS:SI]} or \c{[DS:ESI]} with the
byte at \c{[ES:DI]} or \c{[ES:EDI]}, and sets the flags accordingly.
It then increments or decrements (depending on the direction flag:
increments if the flag is clear, decrements if it is set) \c{SI} and
\c{DI} (or \c{ESI} and \c{EDI}).

The registers used are \c{SI} and \c{DI} if the address size is 16
bits, and \c{ESI} and \c{EDI} if it is 32 bits. If you need to use
an address size not equal to the current \c{BITS} setting, you can
use an explicit \i\c{a16} or \i\c{a32} prefix.

The segment register used to load from \c{[SI]} or \c{[ESI]} can be
overridden by using a segment register name as a prefix (for
example, \c{es cmpsb}). The use of \c{ES} for the load from \c{[DI]}
or \c{[EDI]} cannot be overridden.

\c{CMPSW} and \c{CMPSD} work in the same way, but they compare a
word or a doubleword instead of a byte, and increment or decrement
the addressing registers by 2 or 4 instead of 1.

The \c{REPE} and \c{REPNE} prefixes (equivalently, \c{REPZ} and
\c{REPNZ}) may be used to repeat the instruction up to \c{CX} (or
\c{ECX} - again, the address size chooses which) times until the
first unequal or equal byte is found.

\H{insCMPXCHG} \i\c{CMPXCHG}, \i\c{CMPXCHG486}: Compare and Exchange

\c CMPXCHG r/m8,reg8             ; 0F B0 /r             [PENT]
\c CMPXCHG r/m16,reg16           ; o16 0F B1 /r         [PENT]
\c CMPXCHG r/m32,reg32           ; o32 0F B1 /r         [PENT]

\c CMPXCHG486 r/m8,reg8          ; 0F A6 /r             [486,UNDOC]
\c CMPXCHG486 r/m16,reg16        ; o16 0F A7 /r         [486,UNDOC]
\c CMPXCHG486 r/m32,reg32        ; o32 0F A7 /r         [486,UNDOC]

These two instructions perform exactly the same operation; however,
apparently some (not all) 486 processors support it under a
non-standard opcode, so NASM provides the undocumented
\c{CMPXCHG486} form to generate the non-standard opcode.

\c{CMPXCHG} compares its destination (first) operand to the value in
\c{AL}, \c{AX} or \c{EAX} (depending on the size of the
instruction). If they are equal, it copies its source (second)
operand into the destination and sets the zero flag. Otherwise, it
clears the zero flag and leaves the destination alone.

\c{CMPXCHG} is intended to be used for atomic operations in
multitasking or multiprocessor environments. To safely update a
value in shared memory, for example, you might load the value into
\c{EAX}, load the updated value into \c{EBX}, and then execute the
instruction \c{lock cmpxchg [value],ebx}. If \c{value} has not
changed since being loaded, it is updated with your desired new
value, and the zero flag is set to let you know it has worked. (The
\c{LOCK} prefix prevents another processor doing anything in the
middle of this operation: it guarantees atomicity.) However, if
another processor has modified the value in between your load and
your attempted store, the store does not happen, and you are
notified of the failure by a cleared zero flag, so you can go round
and try again.

\H{insCMPXCHG8B} \i\c{CMPXCHG8B}: Compare and Exchange Eight Bytes

\c CMPXCHG8B mem                 ; 0F C7 /1             [PENT]

This is a larger and more unwieldy version of \c{CMPXCHG}: it
compares the 64-bit (eight-byte) value stored at \c{[mem]} with the
value in \c{EDX:EAX}. If they are equal, it sets the zero flag and
stores \c{ECX:EBX} into the memory area. If they are unequal, it
clears the zero flag and leaves the memory area untouched.

\H{insCPUID} \i\c{CPUID}: Get CPU Identification Code

\c CPUID                         ; 0F A2                [PENT]

\c{CPUID} returns various information about the processor it is
being executed on. It fills the four registers \c{EAX}, \c{EBX},
\c{ECX} and \c{EDX} with information, which varies depending on the
input contents of \c{EAX}.

\c{CPUID} also acts as a barrier to serialise instruction execution:
executing the \c{CPUID} instruction guarantees that all the effects
(memory modification, flag modification, register modification) of
previous instructions have been completed before the next
instruction gets fetched.

The information returned is as follows:

\b If \c{EAX} is zero on input, \c{EAX} on output holds the maximum
acceptable input value of \c{EAX}, and \c{EBX:EDX:ECX} contain the
string \c{"GenuineIntel"} (or not, if you have a clone processor).
That is to say, \c{EBX} contains \c{"Genu"} (in NASM's own sense of
character constants, described in \k{chrconst}), \c{EDX} contains
\c{"ineI"} and \c{ECX} contains \c{"ntel"}.

\b If \c{EAX} is one on input, \c{EAX} on output contains version
information about the processor, and \c{EDX} contains a set of
feature flags, showing the presence and absence of various features.
For example, bit 8 is set if the \c{CMPXCHG8B} instruction
(\k{insCMPXCHG8B}) is supported, bit 15 is set if the conditional
move instructions (\k{insCMOVcc} and \k{insFCMOVB}) are supported,
and bit 23 is set if MMX instructions are supported.

\b If \c{EAX} is two on input, \c{EAX}, \c{EBX}, \c{ECX} and \c{EDX}
all contain information about caches and TLBs (Translation Lookahead
Buffers).

For more information on the data returned from \c{CPUID}, see the
documentation on Intel's web site.

\H{insDAA} \i\c{DAA}, \i\c{DAS}: Decimal Adjustments

\c DAA                           ; 27                   [8086]
\c DAS                           ; 2F                   [8086]

These instructions are used in conjunction with the add and subtract
instructions to perform binary-coded decimal arithmetic in
\e{packed} (one BCD digit per nibble) form. For the unpacked
equivalents, see \k{insAAA}.

\c{DAA} should be used after a one-byte \c{ADD} instruction whose
destination was the \c{AL} register: by means of examining the value
in the \c{AL} and also the auxiliary carry flag \c{AF}, it
determines whether either digit of the addition has overflowed, and
adjusts it (and sets the carry and auxiliary-carry flags) if so. You
can add long BCD strings together by doing \c{ADD}/\c{DAA} on the
low two digits, then doing \c{ADC}/\c{DAA} on each subsequent pair
of digits.

\c{DAS} works similarly to \c{DAA}, but is for use after \c{SUB}
instructions rather than \c{ADD}.

\H{insDEC} \i\c{DEC}: Decrement Integer

\c DEC reg16                     ; o16 48+r             [8086]
\c DEC reg32                     ; o32 48+r             [386]
\c DEC r/m8                      ; FE /1                [8086]
\c DEC r/m16                     ; o16 FF /1            [8086]
\c DEC r/m32                     ; o32 FF /1            [386]

\c{DEC} subtracts 1 from its operand. It does \e{not} affect the
carry flag: to affect the carry flag, use \c{SUB something,1} (see
\k{insSUB}). See also \c{INC} (\k{insINC}).

\H{insDIV} \i\c{DIV}: Unsigned Integer Divide

\c DIV r/m8                      ; F6 /6                [8086]
\c DIV r/m16                     ; o16 F7 /6            [8086]
\c DIV r/m32                     ; o32 F7 /6            [386]

\c{DIV} performs unsigned integer division. The explicit operand
provided is the divisor; the dividend and destination operands are
implicit, in the following way:

\b For \c{DIV r/m8}, \c{AX} is divided by the given operand; the
quotient is stored in \c{AL} and the remainder in \c{AH}.

\b For \c{DIV r/m16}, \c{DX:AX} is divided by the given operand; the
quotient is stored in \c{AX} and the remainder in \c{DX}.

\b For \c{DIV r/m32}, \c{EDX:EAX} is divided by the given operand;
the quotient is stored in \c{EAX} and the remainder in \c{EDX}.

Signed integer division is performed by the \c{IDIV} instruction:
see \k{insIDIV}.

\H{insEMMS} \i\c{EMMS}: Empty MMX State

\c EMMS                          ; 0F 77                [PENT,MMX]

\c{EMMS} sets the FPU tag word (marking which floating-point
registers are available) to all ones, meaning all registers are
available for the FPU to use. It should be used after executing MMX
instructions and before executing any subsequent floating-point
operations.

\H{insENTER} \i\c{ENTER}: Create Stack Frame

\c ENTER imm,imm                 ; C8 iw ib             [186]

\c{ENTER} constructs a stack frame for a high-level language
procedure call. The first operand (the \c{iw} in the opcode
definition above refers to the first operand) gives the amount of
stack space to allocate for local variables; the second (the \c{ib}
above) gives the nesting level of the procedure (for languages like
Pascal, with nested procedures).

The function of \c{ENTER}, with a nesting level of zero, is
equivalent to

\c           PUSH EBP            ; or PUSH BP         in 16 bits
\c           MOV EBP,ESP         ; or MOV BP,SP       in 16 bits
\c           SUB ESP,operand1    ; or SUB SP,operand1 in 16 bits

This creates a stack frame with the procedure parameters accessible
upwards from \c{EBP}, and local variables accessible downwards from
\c{EBP}.

With a nesting level of one, the stack frame created is 4 (or 2)
bytes bigger, and the value of the final frame pointer \c{EBP} is
accessible in memory at \c{[EBP-4]}.

This allows \c{ENTER}, when called with a nesting level of two, to
look at the stack frame described by the \e{previous} value of
\c{EBP}, find the frame pointer at offset -4 from that, and push it
along with its new frame pointer, so that when a level-two procedure
is called from within a level-one procedure, \c{[EBP-4]} holds the
frame pointer of the most recent level-one procedure call and
\c{[EBP-8]} holds that of the most recent level-two call. And so on,
for nesting levels up to 31.

Stack frames created by \c{ENTER} can be destroyed by the \c{LEAVE}
instruction: see \k{insLEAVE}.

\H{insF2XM1} \i\c{F2XM1}: Calculate 2**X-1

\c F2XM1                         ; D9 F0                [8086,FPU]

\c{F2XM1} raises 2 to the power of \c{ST0}, subtracts one, and
stores the result back into \c{ST0}. The initial contents of \c{ST0}
must be a number in the range -1 to +1.

\H{insFABS} \i\c{FABS}: Floating-Point Absolute Value

\c FABS                          ; D9 E1                [8086,FPU]

\c{FABS} computes the absolute value of \c{ST0}, storing the result
back in \c{ST0}.

\H{insFADD} \i\c{FADD}, \i\c{FADDP}: Floating-Point Addition

\c FADD mem32                    ; D8 /0                [8086,FPU]
\c FADD mem64                    ; DC /0                [8086,FPU]

\c FADD fpureg                   ; D8 C0+r              [8086,FPU]
\c FADD ST0,fpureg               ; D8 C0+r              [8086,FPU]

\c FADD TO fpureg                ; DC C0+r              [8086,FPU]
\c FADD fpureg,ST0               ; DC C0+r              [8086,FPU]

\c FADDP fpureg                  ; DE C0+r              [8086,FPU]
\c FADDP fpureg,ST0              ; DE C0+r              [8086,FPU]

\c{FADD}, given one operand, adds the operand to \c{ST0} and stores
the result back in \c{ST0}. If the operand has the \c{TO} modifier,
the result is stored in the register given rather than in \c{ST0}.

\c{FADDP} performs the same function as \c{FADD TO}, but pops the
register stack after storing the result.

The given two-operand forms are synonyms for the one-operand forms.

\H{insFBLD} \i\c{FBLD}, \i\c{FBSTP}: BCD Floating-Point Load and Store

\c FBLD mem80                    ; DF /4                [8086,FPU]
\c FBSTP mem80                   ; DF /6                [8086,FPU]

\c{FBLD} loads an 80-bit (ten-byte) packed binary-coded decimal
number from the given memory address, converts it to a real, and
pushes it on the register stack. \c{FBSTP} stores the value of
\c{ST0}, in packed BCD, at the given address and then pops the
register stack.

\H{insFCHS} \i\c{FCHS}: Floating-Point Change Sign

\c FCHS                          ; D9 E0                [8086,FPU]

\c{FCHS} negates the number in \c{ST0}: negative numbers become
positive, and vice versa.

\H{insFCLEX} \i\c{FCLEX}, \{FNCLEX}: Clear Floating-Point Exceptions

\c FCLEX                         ; 9B DB E2             [8086,FPU]
\c FNCLEX                        ; DB E2                [8086,FPU]

\c{FCLEX} clears any floating-point exceptions which may be pending.
\c{FNCLEX} does the same thing but doesn't wait for previous
floating-point operations (including the \e{handling} of pending
exceptions) to finish first.

\H{insFCMOVB} \i\c{FCMOVcc}: Floating-Point Conditional Move

\c FCMOVB fpureg                 ; DA C0+r              [P6,FPU]
\c FCMOVB ST0,fpureg             ; DA C0+r              [P6,FPU]

\c FCMOVBE fpureg                ; DA D0+r              [P6,FPU]
\c FCMOVBE ST0,fpureg            ; DA D0+r              [P6,FPU]

\c FCMOVE fpureg                 ; DA C8+r              [P6,FPU]
\c FCMOVE ST0,fpureg             ; DA C8+r              [P6,FPU]

\c FCMOVNB fpureg                ; DB C0+r              [P6,FPU]
\c FCMOVNB ST0,fpureg            ; DB C0+r              [P6,FPU]

\c FCMOVNBE fpureg               ; DB D0+r              [P6,FPU]
\c FCMOVNBE ST0,fpureg           ; DB D0+r              [P6,FPU]

\c FCMOVNE fpureg                ; DB C8+r              [P6,FPU]
\c FCMOVNE ST0,fpureg            ; DB C8+r              [P6,FPU]

\c FCMOVNU fpureg                ; DB D8+r              [P6,FPU]
\c FCMOVNU ST0,fpureg            ; DB D8+r              [P6,FPU]

\c FCMOVU fpureg                 ; DA D8+r              [P6,FPU]
\c FCMOVU ST0,fpureg             ; DA D8+r              [P6,FPU]

The \c{FCMOV} instructions perform conditional move operations: each
of them moves the contents of the given register into \c{ST0} if its
condition is satisfied, and does nothing if not.

The conditions are not the same as the standard condition codes used
with conditional jump instructions. The conditions \c{B}, \c{BE},
\c{NB}, \c{NBE}, \c{E} and \c{NE} are exactly as normal, but none of
the other standard ones are supported. Instead, the condition \c{U}
and its counterpart \c{NU} are provided; the \c{U} condition is
satisfied if the last two floating-point numbers compared were
\e{unordered}, i.e. they were not equal but neither one could be
said to be greater than the other, for example if they were NaNs.
(The flag state which signals this is the setting of the parity
flag: so the \c{U} condition is notionally equivalent to \c{PE}, and
\c{NU} is equivalent to \c{PO}.)

The \c{FCMOV} conditions test the main processor's status flags, not
the FPU status flags, so using \c{FCMOV} directly after \c{FCOM}
will not work. Instead, you should either use \c{FCOMI} which writes
directly to the main CPU flags word, or use \c{FSTSW} to extract the
FPU flags.

Although the \c{FCMOV} instructions are flagged \c{P6} above, they
may not be supported by all Pentium Pro processors; the \c{CPUID}
instruction (\k{insCPUID}) will return a bit which indicates whether
conditional moves are supported.

\H{insFCOM} \i\c{FCOM}, \i\c{FCOMP}, \i\c{FCOMPP}, \i\c{FCOMI}, \i\c{FCOMIP}: Floating-Point Compare

\c FCOM mem32                    ; D8 /2                [8086,FPU]
\c FCOM mem64                    ; DC /2                [8086,FPU]
\c FCOM fpureg                   ; D8 D0+r              [8086,FPU]
\c FCOM ST0,fpureg               ; D8 D0+r              [8086,FPU]

\c FCOMP mem32                   ; D8 /3                [8086,FPU]
\c FCOMP mem64                   ; DC /3                [8086,FPU]
\c FCOMP fpureg                  ; D8 D8+r              [8086,FPU]
\c FCOMP ST0,fpureg              ; D8 D8+r              [8086,FPU]

\c FCOMPP                        ; DE D9                [8086,FPU]

\c FCOMI fpureg                  ; DB F0+r              [P6,FPU]
\c FCOMI ST0,fpureg              ; DB F0+r              [P6,FPU]

\c FCOMIP fpureg                 ; DF F0+r              [P6,FPU]
\c FCOMIP ST0,fpureg             ; DF F0+r              [P6,FPU]

\c{FCOM} compares \c{ST0} with the given operand, and sets the FPU
flags accordingly. \c{ST0} is treated as the left-hand side of the
comparison, so that the carry flag is set (for a `less-than' result)
if \c{ST0} is less than the given operand.

\c{FCOMP} does the same as \c{FCOM}, but pops the register stack
afterwards. \c{FCOMPP} compares \c{ST0} with \c{ST1} and then pops
the register stack twice.

\c{FCOMI} and \c{FCOMIP} work like the corresponding forms of
\c{FCOM} and \c{FCOMP}, but write their results directly to the CPU
flags register rather than the FPU status word, so they can be
immediately followed by conditional jump or conditional move
instructions.

The \c{FCOM} instructions differ from the \c{FUCOM} instructions
(\k{insFUCOM}) only in the way they handle quiet NaNs: \c{FUCOM}
will handle them silently and set the condition code flags to an
`unordered' result, whereas \c{FCOM} will generate an exception.

\H{insFCOS} \i\c{FCOS}: Cosine

\c FCOS                          ; D9 FF                [386,FPU]

\c{FCOS} computes the cosine of \c{ST0} (in radians), and stores the
result in \c{ST0}. See also \c{FSINCOS} (\k{insFSIN}).

\H{insFDECSTP} \i\c{FDECSTP}: Decrement Floating-Point Stack Pointer

\c FDECSTP                       ; D9 F6                [8086,FPU]

\c{FDECSTP} decrements the `top' field in the floating-point status
word. This has the effect of rotating the FPU register stack by one,
as if the contents of \c{ST7} had been pushed on the stack. See also
\c{FINCSTP} (\k{insFINCSTP}).

\H{insFDISI} \i\c{FxDISI}, \i\c{FxENI}: Disable and Enable Floating-Point Interrupts

\c FDISI                         ; 9B DB E1             [8086,FPU]
\c FNDISI                        ; DB E1                [8086,FPU]

\c FENI                          ; 9B DB E0             [8086,FPU]
\c FNENI                         ; DB E0                [8086,FPU]

\c{FDISI} and \c{FENI} disable and enable floating-point interrupts.
These instructions are only meaningful on original 8087 processors:
the 287 and above treat them as no-operation instructions.

\c{FNDISI} and \c{FNENI} do the same thing as \c{FDISI} and \c{FENI}
respectively, but without waiting for the floating-point processor
to finish what it was doing first.

\H{insFDIV} \i\c{FDIV}, \i\c{FDIVP}, \i\c{FDIVR}, \i\c{FDIVRP}: Floating-Point Division

\c FDIV mem32                    ; D8 /6                [8086,FPU]
\c FDIV mem64                    ; DC /6                [8086,FPU]

\c FDIV fpureg                   ; D8 F0+r              [8086,FPU]
\c FDIV ST0,fpureg               ; D8 F0+r              [8086,FPU]

\c FDIV TO fpureg                ; DC F8+r              [8086,FPU]
\c FDIV fpureg,ST0               ; DC F8+r              [8086,FPU]

\c FDIVR mem32                   ; D8 /0                [8086,FPU]
\c FDIVR mem64                   ; DC /0                [8086,FPU]

\c FDIVR fpureg                  ; D8 F8+r              [8086,FPU]
\c FDIVR ST0,fpureg              ; D8 F8+r              [8086,FPU]

\c FDIVR TO fpureg               ; DC F0+r              [8086,FPU]
\c FDIVR fpureg,ST0              ; DC F0+r              [8086,FPU]

\c FDIVP fpureg                  ; DE F8+r              [8086,FPU]
\c FDIVP fpureg,ST0              ; DE F8+r              [8086,FPU]

\c FDIVRP fpureg                 ; DE F0+r              [8086,FPU]
\c FDIVRP fpureg,ST0             ; DE F0+r              [8086,FPU]

\c{FDIV} divides \c{ST0} by the given operand and stores the result
back in \c{ST0}, unless the \c{TO} qualifier is given, in which case
it divides the given operand by \c{ST0} and stores the result in the
operand.

\c{FDIVR} does the same thing, but does the division the other way
up: so if \c{TO} is not given, it divides the given operand by
\c{ST0} and stores the result in \c{ST0}, whereas if \c{TO} is given
it divides \c{ST0} by its operand and stores the result in the
operand.

\c{FDIVP} operates like \c{FDIV TO}, but pops the register stack
once it has finished. \c{FDIVRP} operates like \c{FDIVR TO}, but
pops the register stack once it has finished.

\H{insFFREE} \i\c{FFREE}: Flag Floating-Point Register as Unused

\c FFREE fpureg                  ; DD C0+r              [8086,FPU]

\c{FFREE} marks the given register as being empty.

\H{insFIADD} \i\c{FIADD}: Floating-Point/Integer Addition

\c FIADD mem16                   ; DE /0                [8086,FPU]
\c FIADD mem32                   ; DA /0                [8086,FPU]

\c{FIADD} adds the 16-bit or 32-bit integer stored in the given
memory location to \c{ST0}, storing the result in \c{ST0}.

\H{insFICOM} \i\c{FICOM}, \i\c{FICOMP}: Floating-Point/Integer Compare

\c FICOM mem16                   ; DE /2                [8086,FPU]
\c FICOM mem32                   ; DA /2                [8086,FPU]

\c FICOMP mem16                  ; DE /3                [8086,FPU]
\c FICOMP mem32                  ; DA /3                [8086,FPU]

\c{FICOM} compares \c{ST0} with the 16-bit or 32-bit integer stored
in the given memory location, and sets the FPU flags accordingly.
\c{FICOMP} does the same, but pops the register stack afterwards.

\H{insFIDIV} \i\c{FIDIV}, \i\c{FIDIVR}: Floating-Point/Integer Division

\c FIDIV mem16                   ; DE /6                [8086,FPU]
\c FIDIV mem32                   ; DA /6                [8086,FPU]

\c FIDIVR mem16                  ; DE /0                [8086,FPU]
\c FIDIVR mem32                  ; DA /0                [8086,FPU]

\c{FIDIV} divides \c{ST0} by the 16-bit or 32-bit integer stored in
the given memory location, and stores the result in \c{ST0}.
\c{FIDIVR} does the division the other way up: it divides the
integer by \c{ST0}, but still stores the result in \c{ST0}.

\H{insFILD} \i\c{FILD}, \i\c{FIST}, \i\c{FISTP}: Floating-Point/Integer Conversion

\c FILD mem16                    ; DF /0                [8086,FPU]
\c FILD mem32                    ; DB /0                [8086,FPU]
\c FILD mem64                    ; DF /5                [8086,FPU]

\c FIST mem16                    ; DF /2                [8086,FPU]
\c FIST mem32                    ; DB /2                [8086,FPU]

\c FISTP mem16                   ; DF /3                [8086,FPU]
\c FISTP mem32                   ; DB /3                [8086,FPU]
\c FISTP mem64                   ; DF /0                [8086,FPU]

\c{FILD} loads an integer out of a memory location, converts it to a
real, and pushes it on the FPU register stack. \c{FIST} converts
\c{ST0} to an integer and stores that in memory; \c{FISTP} does the
same as \c{FIST}, but pops the register stack afterwards.

\H{insFIMUL} \i\c{FIMUL}: Floating-Point/Integer Multiplication

\c FIMUL mem16                   ; DE /1                [8086,FPU]
\c FIMUL mem32                   ; DA /1                [8086,FPU]

\c{FIMUL} multiplies \c{ST0} by the 16-bit or 32-bit integer stored
in the given memory location, and stores the result in \c{ST0}.

\H{insFINCSTP} \i\c{FINCSTP}: Increment Floating-Point Stack Pointer

\c FINCSTP                       ; D9 F7                [8086,FPU]

\c{FINCSTP} increments the `top' field in the floating-point status
word. This has the effect of rotating the FPU register stack by one,
as if the register stack had been popped; however, unlike the
popping of the stack performed by many FPU instructions, it does not
flag the new \c{ST7} (previously \c{ST0}) as empty. See also
\c{FDECSTP} (\k{insFDECSTP}).

\H{insFINIT} \i\c{FINIT}, \i\c{FNINIT}: Initialise Floating-Point Unit

\c FINIT                         ; 9B DB E3             [8086,FPU]
\c FNINIT                        ; DB E3                [8086,FPU]

\c{FINIT} initialises the FPU to its default state. It flags all
registers as empty, though it does not actually change their values.
\c{FNINIT} does the same, without first waiting for pending
exceptions to clear.

\H{insFISUB} \i\c{FISUB}: Floating-Point/Integer Subtraction

\c FISUB mem16                   ; DE /4                [8086,FPU]
\c FISUB mem32                   ; DA /4                [8086,FPU]

\c FISUBR mem16                  ; DE /5                [8086,FPU]
\c FISUBR mem32                  ; DA /5                [8086,FPU]

\c{FISUB} subtracts the 16-bit or 32-bit integer stored in the given
memory location from \c{ST0}, and stores the result in \c{ST0}.
\c{FISUBR} does the subtraction the other way round, i.e. it
subtracts \c{ST0} from the given integer, but still stores the
result in \c{ST0}.

\H{insFLD} \i\c{FLD}: Floating-Point Load

\c FLD mem32                     ; D9 /0                [8086,FPU]
\c FLD mem64                     ; DD /0                [8086,FPU]
\c FLD mem80                     ; DB /5                [8086,FPU]
\c FLD fpureg                    ; D9 C0+r              [8086,FPU]

\c{FLD} loads a floating-point value out of the given register or
memory location, and pushes it on the FPU register stack.

\H{insFLD1} \i\c{FLDxx}: Floating-Point Load Constants

\c FLD1                          ; D9 E8                [8086,FPU]
\c FLDL2E                        ; D9 EA                [8086,FPU]
\c FLDL2T                        ; D9 E9                [8086,FPU]
\c FLDLG2                        ; D9 EC                [8086,FPU]
\c FLDLN2                        ; D9 ED                [8086,FPU]
\c FLDPI                         ; D9 EB                [8086,FPU]
\c FLDZ                          ; D9 EE                [8086,FPU]

These instructions push specific standard constants on the FPU
register stack. \c{FLD1} pushes the value 1; \c{FLDL2E} pushes the
base-2 logarithm of e; \c{FLDL2T} pushes the base-2 log of 10;
\c{FLDLG2} pushes the base-10 log of 2; \c{FLDLN2} pushes the base-e
log of 2; \c{FLDPI} pushes pi; and \c{FLDZ} pushes zero.

\H{insFLDCW} \i\c{FLDCW}: Load Floating-Point Control Word

\c FLDCW mem16                   ; D9 /5                [8086,FPU]

\c{FLDCW} loads a 16-bit value out of memory and stores it into the
FPU control word (governing things like the rounding mode, the
precision, and the exception masks). See also \c{FSTCW}
(\k{insFSTCW}).

\H{insFLDENV} \i\c{FLDENV}: Load Floating-Point Environment

\c FLDENV mem                    ; D9 /4                [8086,FPU]

\c{FLDENV} loads the FPU operating environment (control word, status
word, tag word, instruction pointer, data pointer and last opcode)
from memory. The memory area is 14 or 28 bytes long, depending on
the CPU mode at the time. See also \c{FSTENV} (\k{insFSTENV}).

\H{insFMUL} \i\c{FMUL}, \i\c{FMULP}: Floating-Point Multiply

\c FMUL mem32                    ; D8 /1                [8086,FPU]
\c FMUL mem64                    ; DC /1                [8086,FPU]

\c FMUL fpureg                   ; D8 C8+r              [8086,FPU]
\c FMUL ST0,fpureg               ; D8 C8+r              [8086,FPU]

\c FMUL TO fpureg                ; DC C8+r              [8086,FPU]
\c FMUL fpureg,ST0               ; DC C8+r              [8086,FPU]

\c FMULP fpureg                  ; DE C8+r              [8086,FPU]
\c FMULP fpureg,ST0              ; DE C8+r              [8086,FPU]

\c{FMUL} multiplies \c{ST0} by the given operand, and stores the
result in \c{ST0}, unless the \c{TO} qualifier is used in which case
it stores the result in the operand. \c{FMULP} performs the same
operation as \c{FMUL TO}, and then pops the register stack.

\H{insFNOP} \i\c{FNOP}: Floating-Point No Operation

\c FNOP                          ; D9 D0                [8086,FPU]

\c{FNOP} does nothing.

\H{insFPATAN} \i\c{FPATAN}, \i\c{FPTAN}: Arctangent and Tangent

\c FPATAN                        ; D9 F3                [8086,FPU]
\c FPTAN                         ; D9 F2                [8086,FPU]

\c{FPATAN} computes the arctangent, in radians, of the result of
dividing \c{ST1} by \c{ST0}, stores the result in \c{ST1}, and pops
the register stack. It works like the C \c{atan2} function, in that
changing the sign of both \c{ST0} and \c{ST1} changes the output
value by pi (so it performs true rectangular-to-polar coordinate
conversion, with \c{ST1} being the Y coordinate and \c{ST0} being
the X coordinate, not merely an arctangent).

\c{FPTAN} computes the tangent of the value in \c{ST0} (in radians),
and stores the result back into \c{ST0}.

\H{insFPREM} \i\c{FPREM}, \i\c{FPREM1}: Floating-Point Partial Remainder

\c FPREM                         ; D9 F8                [8086,FPU]
\c FPREM1                        ; D9 F5                [386,FPU]

These instructions both produce the remainder obtained by dividing
\c{ST0} by \c{ST1}. This is calculated, notionally, by dividing
\c{ST0} by \c{ST1}, rounding the result to an integer, multiplying
by \c{ST1} again, and computing the value which would need to be
added back on to the result to get back to the original value in
\c{ST0}.

The two instructions differ in the way the notional round-to-integer
operation is performed. \c{FPREM} does it by rounding towards zero,
so that the remainder it returns always has the same sign as the
original value in \c{ST0}; \c{FPREM1} does it by rounding to the
nearest integer, so that the remainder always has at most half the
magnitude of \c{ST1}.

Both instructions calculate \e{partial} remainders, meaning that
they may not manage to provide the final result, but might leave
intermediate results in \c{ST0} instead. If this happens, they will
set the C2 flag in the FPU status word; therefore, to calculate a
remainder, you should repeatedly execute \c{FPREM} or \c{FPREM1}
until C2 becomes clear.

\H{insFRNDINT} \i\c{FRNDINT}: Floating-Point Round to Integer

\c FRNDINT                       ; D9 FC                [8086,FPU]

\c{FRNDINT} rounds the contents of \c{ST0} to an integer, according
to the current rounding mode set in the FPU control word, and stores
the result back in \c{ST0}.

\H{insFRSTOR} \i\c{FSAVE}, \i\c{FRSTOR}: Save/Restore Floating-Point State

\c FSAVE mem                     ; 9B DD /6             [8086,FPU]
\c FNSAVE mem                    ; DD /6                [8086,FPU]

\c FRSTOR mem                    ; DD /4                [8086,FPU]

\c{FSAVE} saves the entire floating-point unit state, including all
the information saved by \c{FSTENV} (\k{insFSTENV}) plus the
contents of all the registers, to a 94 or 108 byte area of memory
(depending on the CPU mode). \c{FRSTOR} restores the floating-point
state from the same area of memory.

\c{FNSAVE} does the same as \c{FSAVE}, without first waiting for
pending floating-point exceptions to clear.

\H{insFSCALE} \i\c{FSCALE}: Scale Floating-Point Value by Power of Two

\c FSCALE                        ; D9 FD                [8086,FPU]

\c{FSCALE} scales a number by a power of two: it rounds \c{ST1}
towards zero to obtain an integer, then multiplies \c{ST0} by two to
the power of that integer, and stores the result in \c{ST0}.

\H{insFSETPM} \i\c{FSETPM}: Set Protected Mode

\c FSETPM                        ; DB E4                [286,FPU]

This instruction initalises protected mode on the 287 floating-point
coprocessor. It is only meaningful on that processor: the 387 and
above treat the instruction as a no-operation.

\H{insFSIN} \i\c{FSIN}, \i\c{FSINCOS}: Sine and Cosine

\c FSIN                          ; D9 FE                [386,FPU]
\c FSINCOS                       ; D9 FB                [386,FPU]

\c{FSIN} calculates the sine of \c{ST0} (in radians) and stores the
result in \c{ST0}. \c{FSINCOS} does the same, but then pushes the
cosine of the same value on the register stack, so that the sine
ends up in \c{ST1} and the cosine in \c{ST0}. \c{FSINCOS} is faster
than executing \c{FSIN} and \c{FCOS} (see \k{insFCOS}) in
succession.

\H{insFSQRT} \i\c{FSQRT}: Floating-Point Square Root

\c FSQRT                         ; D9 FA                [8086,FPU]

\c{FSQRT} calculates the square root of \c{ST0} and stores the
result in \c{ST0}.

\H{insFST} \i\c{FST}, \i\c{FSTP}: Floating-Point Store

\c FST mem32                     ; D9 /2                [8086,FPU]
\c FST mem64                     ; DD /2                [8086,FPU]
\c FST fpureg                    ; DD D0+r              [8086,FPU]

\c FSTP mem32                    ; D9 /3                [8086,FPU]
\c FSTP mem64                    ; DD /3                [8086,FPU]
\c FSTP mem80                    ; DB /0                [8086,FPU]
\c FSTP fpureg                   ; DD D8+r              [8086,FPU]

\c{FST} stores the value in \c{ST0} into the given memory location
or other FPU register. \c{FSTP} does the same, but then pops the
register stack.

\H{insFSTCW} \i\c{FSTCW}: Store Floating-Point Control Word

\c FSTCW mem16                   ; 9B D9 /0             [8086,FPU]
\c FNSTCW mem16                  ; D9 /0                [8086,FPU]

\c{FSTCW} stores the FPU control word (governing things like the
rounding mode, the precision, and the exception masks) into a 2-byte
memory area. See also \c{FLDCW} (\k{insFLDCW}).

\c{FNSTCW} does the same thing as \c{FSTCW}, without first waiting
for pending floating-point exceptions to clear.

\H{insFSTENV} \i\c{FSTENV}: Store Floating-Point Environment

\c FSTENV mem                    ; 9B D9 /6             [8086,FPU]
\c FNSTENV mem                   ; D9 /6                [8086,FPU]

\c{FSTENV} stores the FPU operating environment (control word,
status word, tag word, instruction pointer, data pointer and last
opcode) into memory. The memory area is 14 or 28 bytes long,
depending on the CPU mode at the time. See also \c{FLDENV}
(\k{insFLDENV}).

\c{FNSTENV} does the same thing as \c{FSTENV}, without first waiting
for pending floating-point exceptions to clear.

\H{insFSTSW} \i\c{FSTSW}: Store Floating-Point Status Word

\c FSTSW mem16                   ; 9B DD /0             [8086,FPU]
\c FSTSW AX                      ; 9B DF E0             [286,FPU]

\c FNSTSW mem16                  ; DD /0                [8086,FPU]
\c FNSTSW AX                     ; DF E0                [286,FPU]

\c{FSTSW} stores the FPU status word into \c{AX} or into a 2-byte
memory area.

\c{FNSTSW} does the same thing as \c{FSTSW}, without first waiting
for pending floating-point exceptions to clear.

\H{insFSUB} \i\c{FSUB}, \i\c{FSUBP}, \i\c{FSUBR}, \i\c{FSUBRP}: Floating-Point Subtract

\c FSUB mem32                    ; D8 /4                [8086,FPU]
\c FSUB mem64                    ; DC /4                [8086,FPU]

\c FSUB fpureg                   ; D8 E0+r              [8086,FPU]
\c FSUB ST0,fpureg               ; D8 E0+r              [8086,FPU]

\c FSUB TO fpureg                ; DC E8+r              [8086,FPU]
\c FSUB fpureg,ST0               ; DC E8+r              [8086,FPU]

\c FSUBR mem32                   ; D8 /5                [8086,FPU]
\c FSUBR mem64                   ; DC /5                [8086,FPU]

\c FSUBR fpureg                  ; D8 E8+r              [8086,FPU]
\c FSUBR ST0,fpureg              ; D8 E8+r              [8086,FPU]

\c FSUBR TO fpureg               ; DC E0+r              [8086,FPU]
\c FSUBR fpureg,ST0              ; DC E0+r              [8086,FPU]

\c FSUBP fpureg                  ; DE E8+r              [8086,FPU]
\c FSUBP fpureg,ST0              ; DE E8+r              [8086,FPU]

\c FSUBRP fpureg                 ; DE E0+r              [8086,FPU]
\c FSUBRP fpureg,ST0             ; DE E0+r              [8086,FPU]

\c{FSUB} subtracts the given operand from \c{ST0} and stores the
result back in \c{ST0}, unless the \c{TO} qualifier is given, in
which case it subtracts \c{ST0} from the given operand and stores
the result in the operand.

\c{FSUBR} does the same thing, but does the subtraction the other way
up: so if \c{TO} is not given, it subtracts \c{ST0} from the given
operand and stores the result in \c{ST0}, whereas if \c{TO} is given
it subtracts its operand from \c{ST0} and stores the result in the
operand.

\c{FSUBP} operates like \c{FSUB TO}, but pops the register stack
once it has finished. \c{FSUBRP} operates like \c{FSUBR TO}, but
pops the register stack once it has finished.

\H{insFTST} \i\c{FTST}: Test \c{ST0} Against Zero

\c FTST                          ; D9 E4                [8086,FPU]

\c{FTST} compares \c{ST0} with zero and sets the FPU flags
accordingly. \c{ST0} is treated as the left-hand side of the
comparison, so that a `less-than' result is generated if \c{ST0} is
negative.

\H{insFUCOM} \i\c{FUCOMxx}: Floating-Point Unordered Compare

\c FUCOM fpureg                  ; DD E0+r              [386,FPU]
\c FUCOM ST0,fpureg              ; DD E0+r              [386,FPU]

\c FUCOMP fpureg                 ; DD E8+r              [386,FPU]
\c FUCOMP ST0,fpureg             ; DD E8+r              [386,FPU]

\c FUCOMPP                       ; DA E9                [386,FPU]

\c FUCOMI fpureg                 ; DB E8+r              [P6,FPU]
\c FUCOMI ST0,fpureg             ; DB E8+r              [P6,FPU]

\c FUCOMIP fpureg                ; DF E8+r              [P6,FPU]
\c FUCOMIP ST0,fpureg            ; DF E8+r              [P6,FPU]

\c{FUCOM} compares \c{ST0} with the given operand, and sets the FPU
flags accordingly. \c{ST0} is treated as the left-hand side of the
comparison, so that the carry flag is set (for a `less-than' result)
if \c{ST0} is less than the given operand.

\c{FUCOMP} does the same as \c{FUCOM}, but pops the register stack
afterwards. \c{FUCOMPP} compares \c{ST0} with \c{ST1} and then pops
the register stack twice.

\c{FUCOMI} and \c{FUCOMIP} work like the corresponding forms of
\c{FUCOM} and \c{FUCOMP}, but write their results directly to the CPU
flags register rather than the FPU status word, so they can be
immediately followed by conditional jump or conditional move
instructions.

The \c{FUCOM} instructions differ from the \c{FCOM} instructions
(\k{insFCOM}) only in the way they handle quiet NaNs: \c{FUCOM} will
handle them silently and set the condition code flags to an
`unordered' result, whereas \c{FCOM} will generate an exception.

\H{insFXAM} \i\c{FXAM}: Examine Class of Value in \c{ST0}

\c FXAM                          ; D9 E5                [8086,FPU]

\c{FXAM} sets the FPU flags C3, C2 and C0 depending on the type of
value stored in \c{ST0}: 000 (respectively) for an unsupported
format, 001 for a NaN, 010 for a normal finite number, 011 for an
infinity, 100 for a zero, 101 for an empty register, and 110 for a
denormal. It also sets the C1 flag to the sign of the number.

\H{insFXCH} \i\c{FXCH}: Floating-Point Exchange

\c FXCH                          ; D9 C9                [8086,FPU]
\c FXCH fpureg                   ; D9 C8+r              [8086,FPU]
\c FXCH fpureg,ST0               ; D9 C8+r              [8086,FPU]
\c FXCH ST0,fpureg               ; D9 C8+r              [8086,FPU]

\c{FXCH} exchanges \c{ST0} with a given FPU register. The no-operand
form exchanges \c{ST0} with \c{ST1}.

\H{insFXTRACT} \i\c{FXTRACT}: Extract Exponent and Significand

\c FXTRACT                       ; D9 F4                [8086,FPU]

\c{FXTRACT} separates the number in \c{ST0} into its exponent and
significand (mantissa), stores the exponent back into \c{ST0}, and
then pushes the significand on the register stack (so that the
significand ends up in \c{ST0}, and the exponent in \c{ST1}).

\H{insFYL2X} \i\c{FYL2X}, \i\c{FYL2XP1}: Compute Y times Log2(X) or Log2(X+1)

\c FYL2X                         ; D9 F1                [8086,FPU]
\c FYL2XP1                       ; D9 F9                [8086,FPU]

\c{FYL2X} multiplies \c{ST1} by the base-2 logarithm of \c{ST0},
stores the result in \c{ST1}, and pops the register stack (so that
the result ends up in \c{ST0}). \c{ST0} must be non-zero and
positive.

\c{FYL2XP1} works the same way, but replacing the base-2 log of
\c{ST0} with that of \c{ST0} plus one. This time, \c{ST0} must have
magnitude no greater than 1 minus half the square root of two.

\H{insHLT} \i\c{HLT}: Halt Processor

\c HLT                           ; F4                   [8086]

\c{HLT} puts the processor into a halted state, where it will
perform no more operations until restarted by an interrupt or a
reset.

\H{insIBTS} \i\c{IBTS}: Insert Bit String

\c IBTS r/m16,reg16              ; o16 0F A7 /r         [386,UNDOC]
\c IBTS r/m32,reg32              ; o32 0F A7 /r         [386,UNDOC]

No clear documentation seems to be available for this instruction:
the best I've been able to find reads `Takes a string of bits from
the second operand and puts them in the first operand'. It is
present only in early 386 processors, and conflicts with the opcodes
for \c{CMPXCHG486}. NASM supports it only for completeness. Its
counterpart is \c{XBTS} (see \k{insXBTS}).

\H{insIDIV} \i\c{IDIV}: Signed Integer Divide

\c IDIV r/m8                     ; F6 /7                [8086]
\c IDIV r/m16                    ; o16 F7 /7            [8086]
\c IDIV r/m32                    ; o32 F7 /7            [386]

\c{IDIV} performs signed integer division. The explicit operand
provided is the divisor; the dividend and destination operands are
implicit, in the following way:

\b For \c{IDIV r/m8}, \c{AX} is divided by the given operand; the
quotient is stored in \c{AL} and the remainder in \c{AH}.

\b For \c{IDIV r/m16}, \c{DX:AX} is divided by the given operand; the
quotient is stored in \c{AX} and the remainder in \c{DX}.

\b For \c{IDIV r/m32}, \c{EDX:EAX} is divided by the given operand;
the quotient is stored in \c{EAX} and the remainder in \c{EDX}.

Unsigned integer division is performed by the \c{DIV} instruction:
see \k{insDIV}.

\H{insIMUL} \i\c{IMUL}: Signed Integer Multiply

\c IMUL r/m8                     ; F6 /5                [8086]
\c IMUL r/m16                    ; o16 F7 /5            [8086]
\c IMUL r/m32                    ; o32 F7 /5            [386]

\c IMUL reg16,r/m16              ; o16 0F AF /r         [386]
\c IMUL reg32,r/m32              ; o32 0F AF /r         [386]

\c IMUL reg16,imm8               ; o16 6B /r ib         [286]
\c IMUL reg16,imm16              ; o16 69 /r iw         [286]
\c IMUL reg32,imm8               ; o32 6B /r ib         [386]
\c IMUL reg32,imm32              ; o32 69 /r id         [386]

\c IMUL reg16,r/m16,imm8         ; o16 6B /r ib         [286]
\c IMUL reg16,r/m16,imm16        ; o16 69 /r iw         [286]
\c IMUL reg32,r/m32,imm8         ; o32 6B /r ib         [386]
\c IMUL reg32,r/m32,imm32        ; o32 69 /r id         [386]

\c{IMUL} performs signed integer multiplication. For the
single-operand form, the other operand and destination are implicit,
in the following way:

\b For \c{IMUL r/m8}, \c{AL} is multiplied by the given operand; the
product is stored in \c{AX}.

\b For \c{IMUL r/m16}, \c{AX} is multiplied by the given operand;
the product is stored in \c{DX:AX}.

\b For \c{IMUL r/m32}, \c{EAX} is multiplied by the given operand;
the product is stored in \c{EDX:EAX}.

The two-operand form multiplies its two operands and stores the
result in the destination (first) operand. The three-operand form
multiplies its last two operands and stores the result in the first
operand.

The two-operand form is in fact a shorthand for the three-operand
form, as can be seen by examining the opcode descriptions: in the
two-operand form, the code \c{/r} takes both its register and
\c{r/m} parts from the same operand (the first one).

In the forms with an 8-bit immediate operand and another longer
source operand, the immediate operand is considered to be signed,
and is sign-extended to the length of the other source operand. In
these cases, the \c{BYTE} qualifier is necessary to force NASM to
generate this form of the instruction.

Unsigned integer multiplication is performed by the \c{MUL}
instruction: see \k{insMUL}.

\H{insIN} \i\c{IN}: Input from I/O Port

\c IN AL,imm8                    ; E4 ib                [8086]
\c IN AX,imm8                    ; o16 E5 ib            [8086]
\c IN EAX,imm8                   ; o32 E5 ib            [386]
\c IN AL,DX                      ; EC                   [8086]
\c IN AX,DX                      ; o16 ED               [8086]
\c IN EAX,DX                     ; o32 ED               [386]

\c{IN} reads a byte, word or doubleword from the specified I/O port,
and stores it in the given destination register. The port number may
be specified as an immediate value if it is between 0 and 255, and
otherwise must be stored in \c{DX}. See also \c{OUT} (\k{insOUT}).

\H{insINC} \i\c{INC}: Increment Integer

\c INC reg16                     ; o16 40+r             [8086]
\c INC reg32                     ; o32 40+r             [386]
\c INC r/m8                      ; FE /0                [8086]
\c INC r/m16                     ; o16 FF /0            [8086]
\c INC r/m32                     ; o32 FF /0            [386]

\c{INC} adds 1 to its operand. It does \e{not} affect the carry
flag: to affect the carry flag, use \c{ADD something,1} (see
\k{insADD}). See also \c{DEC} (\k{insDEC}).

\H{insINSB} \i\c{INSB}, \i\c{INSW}, \i\c{INSD}: Input String from I/O Port

\c INSB                          ; 6C                   [186]
\c INSW                          ; o16 6D               [186]
\c INSD                          ; o32 6D               [386]

\c{INSB} inputs a byte from the I/O port specified in \c{DX} and
stores it at \c{[ES:DI]} or \c{[ES:EDI]}. It then increments or
decrements (depending on the direction flag: increments if the flag
is clear, decrements if it is set) \c{DI} or \c{EDI}.

The register used is \c{DI} if the address size is 16 bits, and
\c{EDI} if it is 32 bits. If you need to use an address size not
equal to the current \c{BITS} setting, you can use an explicit
\i\c{a16} or \i\c{a32} prefix.

Segment override prefixes have no effect for this instruction: the
use of \c{ES} for the load from \c{[DI]} or \c{[EDI]} cannot be
overridden.

\c{INSW} and \c{INSD} work in the same way, but they input a word or
a doubleword instead of a byte, and increment or decrement the
addressing register by 2 or 4 instead of 1.

The \c{REP} prefix may be used to repeat the instruction \c{CX} (or
\c{ECX} - again, the address size chooses which) times.

See also \c{OUTSB}, \c{OUTSW} and \c{OUTSD} (\k{insOUTSB}).

\H{insINT} \i\c{INT}: Software Interrupt

\c INT imm8                      ; CD ib                [8086]

\c{INT} causes a software interrupt through a specified vector
number from 0 to 255.

The code generated by the \c{INT} instruction is always two bytes
long: although there are short forms for some \c{INT} instructions,
NASM does not generate them when it sees the \c{INT} mnemonic. In
order to generate single-byte breakpoint instructions, use the
\c{INT3} or \c{INT1} instructions (see \k{insINT1}) instead.

\H{insINT1} \i\c{INT3}, \i\c{INT1}, \i\c{ICEBP}, \i\c{INT01}: Breakpoints

\c INT1                          ; F1                   [P6]
\c ICEBP                         ; F1                   [P6]
\c INT01                         ; F1                   [P6]

\c INT3                          ; CC                   [8086]

\c{INT1} and \c{INT3} are short one-byte forms of the instructions
\c{INT 1} and \c{INT 3} (see \k{insINT}). They perform a similar
function to their longer counterparts, but take up less code space.
They are used as breakpoints by debuggers.

\c{INT1}, and its alternative synonyms \c{INT01} and \c{ICEBP}, is
an instruction used by in-circuit emulators (ICEs). It is present,
though not documented, on some processors down to the 286, but is
only documented for the Pentium Pro. \c{INT3} is the instruction
normally used as a breakpoint by debuggers.

\c{INT3} is not precisely equivalent to \c{INT 3}: the short form,
since it is designed to be used as a breakpoint, bypasses the normal
IOPL checks in virtual-8086 mode, and also does not go through
interrupt redirection.

\H{insINTO} \i\c{INTO}: Interrupt if Overflow

\c INTO                          ; CE                   [8086]

\c{INTO} performs an \c{INT 4} software interrupt (see \k{insINT})
if and only if the overflow flag is set.

\H{insINVD} \i\c{INVD}: Invalidate Internal Caches

\c INVD                          ; 0F 08                [486]

\c{INVD} invalidates and empties the processor's internal caches,
and causes the processor to instruct external caches to do the same.
It does not write the contents of the caches back to memory first:
any modified data held in the caches will be lost. To write the data
back first, use \c{WBINVD} (\k{insWBINVD}).

\H{insINVLPG} \i\c{INVLPG}: Invalidate TLB Entry

\c INVLPG mem                    ; 0F 01 /0             [486]

\c{INVLPG} invalidates the translation lookahead buffer (TLB) entry
associated with the supplied memory address.

\H{insIRET} \i\c{IRET}, \i\c{IRETW}, \i\c{IRETD}: Return from Interrupt

\c IRET                          ; CF                   [8086]
\c IRETW                         ; o16 CF               [8086]
\c IRETD                         ; o32 CF               [386]

\c{IRET} returns from an interrupt (hardware or software) by means
of popping \c{IP} (or \c{EIP}), \c{CS} and the flags off the stack
and then continuing execution from the new \c{CS:IP}.

\c{IRETW} pops \c{IP}, \c{CS} and the flags as 2 bytes each, taking
6 bytes off the stack in total. \c{IRETD} pops \c{EIP} as 4 bytes,
pops a further 4 bytes of which the top two are discarded and the
bottom two go into \c{CS}, and pops the flags as 4 bytes as well,
taking 12 bytes off the stack.

\c{IRET} is a shorthand for either \c{IRETW} or \c{IRETD}, depending
on the default \c{BITS} setting at the time.

\H{insJCXZ} \i\c{JCXZ}, \i\c{JECXZ}: Jump if CX/ECX Zero

\c JCXZ imm                      ; o16 E3 rb            [8086]
\c JECXZ imm                     ; o32 E3 rb            [386]

\c{JCXZ} performs a short jump (with maximum range 128 bytes) if and
only if the contents of the \c{CX} register is 0. \c{JECXZ} does the
same thing, but with \c{ECX}.

\H{insJMP} \i\c{JMP}: Jump

\c JMP imm                       ; E9 rw/rd             [8086]
\c JMP SHORT imm                 ; EB rb                [8086]
\c JMP imm:imm16                 ; o16 EA iw iw         [8086]
\c JMP imm:imm32                 ; o32 EA id iw         [386]
\c JMP FAR mem                   ; o16 FF /5            [8086]
\c JMP FAR mem                   ; o32 FF /5            [386]
\c JMP r/m16                     ; o16 FF /4            [8086]
\c JMP r/m32                     ; o32 FF /4            [386]

\c{JMP} jumps to a given address. The address may be specified as an
absolute segment and offset, or as a relative jump within the
current segment.

\c{JMP SHORT imm} has a maximum range of 128 bytes, since the
displacement is specified as only 8 bits, but takes up less code
space. NASM does not choose when to generate \c{JMP SHORT} for you:
you must explicitly code \c{SHORT} every time you want a short jump.

You can choose between the two immediate \i{far jump} forms (\c{JMP
imm:imm}) by the use of the \c{WORD} and \c{DWORD} keywords: \c{JMP
WORD 0x1234:0x5678}) or \c{JMP DWORD 0x1234:0x56789abc}.

The \c{JMP FAR mem} forms execute a far jump by loading the
destination address out of memory. The address loaded consists of 16
or 32 bits of offset (depending on the operand size), and 16 bits of
segment. The operand size may be overridden using \c{JMP WORD FAR
mem} or \c{JMP DWORD FAR mem}.

The \c{JMP r/m} forms execute a \i{near jump} (within the same
segment), loading the destination address out of memory or out of a
register. The keyword \c{NEAR} may be specified, for clarity, in
these forms, but is not necessary. Again, operand size can be
overridden using \c{JMP WORD mem} or \c{JMP DWORD mem}.

As a convenience, NASM does not require you to jump to a far symbol
by coding the cumbersome \c{JMP SEG routine:routine}, but instead
allows the easier synonym \c{JMP FAR routine}.

The \c{CALL r/m} forms given above are near calls; NASM will accept
the \c{NEAR} keyword (e.g. \c{CALL NEAR [address]}), even though it
is not strictly necessary.

\H{insJcc} \i\c{Jcc}: Conditional Branch

\c Jcc imm                       ; 70+cc rb             [8086]
\c Jcc NEAR imm                  ; 0F 80+cc rw/rd       [386]

The \i{conditional jump} instructions execute a near (same segment)
jump if and only if their conditions are satisfied. For example,
\c{JNZ} jumps only if the zero flag is not set.

The ordinary form of the instructions has only a 128-byte range; the
\c{NEAR} form is a 386 extension to the instruction set, and can
span the full size of a segment. NASM will not override your choice
of jump instruction: if you want \c{Jcc NEAR}, you have to use the
\c{NEAR} keyword.

The \c{SHORT} keyword is allowed on the first form of the
instruction, for clarity, but is not necessary.

\H{insLAHF} \i\c{LAHF}: Load AH from Flags

\c LAHF                          ; 9F                   [8086]

\c{LAHF} sets the \c{AH} register according to the contents of the
low byte of the flags word. See also \c{SAHF} (\k{insSAHF}).

\H{insLAR} \i\c{LAR}: Load Access Rights

\c LAR reg16,r/m16               ; o16 0F 02 /r         [286,PRIV]
\c LAR reg32,r/m32               ; o32 0F 02 /r         [286,PRIV]

\c{LAR} takes the segment selector specified by its source (second)
operand, finds the corresponding segment descriptor in the GDT or
LDT, and loads the access-rights byte of the descriptor into its
destination (first) operand.

\H{insLDS} \i\c{LDS}, \i\c{LES}, \i\c{LFS}, \i\c{LGS}, \i\c{LSS}: Load Far Pointer

\c LDS reg16,mem                 ; o16 C5 /r            [8086]
\c LDS reg32,mem                 ; o32 C5 /r            [8086]

\c LES reg16,mem                 ; o16 C4 /r            [8086]
\c LES reg32,mem                 ; o32 C4 /r            [8086]

\c LFS reg16,mem                 ; o16 0F B4 /r         [386]
\c LFS reg32,mem                 ; o32 0F B4 /r         [386]

\c LGS reg16,mem                 ; o16 0F B5 /r         [386]
\c LGS reg32,mem                 ; o32 0F B5 /r         [386]

\c LSS reg16,mem                 ; o16 0F B2 /r         [386]
\c LSS reg32,mem                 ; o32 0F B2 /r         [386]

These instructions load an entire far pointer (16 or 32 bits of
offset, plus 16 bits of segment) out of memory in one go. \c{LDS},
for example, loads 16 or 32 bits from the given memory address into
the given register (depending on the size of the register), then
loads the \e{next} 16 bits from memory into \c{DS}. \c{LES},
\c{LFS}, \c{LGS} and \c{LSS} work in the same way but use the other
segment registers.

\H{insLEA} \i\c{LEA}: Load Effective Address

\c LEA reg16,mem                 ; o16 8D /r            [8086]
\c LEA reg32,mem                 ; o32 8D /r            [8086]

\c{LEA}, despite its syntax, does not access memory. It calculates
the effective address specified by its second operand as if it were
going to load or store data from it, but instead it stores the
calculated address into the register specified by its first operand.
This can be used to perform quite complex calculations (e.g. \c{LEA
EAX,[EBX+ECX*4+100]}) in one instruction.

\c{LEA}, despite being a purely arithmetic instruction which
accesses no memory, still requires square brackets around its second
operand, as if it were a memory reference.

\H{insLEAVE} \i\c{LEAVE}: Destroy Stack Frame

\c LEAVE                         ; C9                   [186]

\c{LEAVE} destroys a stack frame of the form created by the
\c{ENTER} instruction (see \k{insENTER}). It is functionally
equivalent to \c{MOV ESP,EBP} followed by \c{POP EBP} (or \c{MOV
SP,BP} followed by \c{POP BP} in 16-bit mode).

\H{insLGDT} \i\c{LGDT}, \i\c{LIDT}, \i\c{LLDT}: Load Descriptor Tables

\c LGDT mem                      ; 0F 01 /2             [286,PRIV]
\c LIDT mem                      ; 0F 01 /3             [286,PRIV]
\c LLDT r/m16                    ; 0F 00 /2             [286,PRIV]

\c{LGDT} and \c{LIDT} both take a 6-byte memory area as an operand:
they load a 32-bit linear address and a 16-bit size limit from that
area (in the opposite order) into the GDTR (global descriptor table
register) or IDTR (interrupt descriptor table register). These are
the only instructions which directly use \e{linear} addresses,
rather than segment/offset pairs.

\c{LLDT} takes a segment selector as an operand. The processor looks
up that selector in the GDT and stores the limit and base address
given there into the LDTR (local descriptor table register).

See also \c{SGDT}, \c{SIDT} and \c{SLDT} (\k{insSGDT}).

\H{insLMSW} \i\c{LMSW}: Load/Store Machine Status Word

\c LMSW r/m16                    ; 0F 01 /6             [286,PRIV]

\c{LMSW} loads the bottom four bits of the source operand into the
bottom four bits of the \c{CR0} control register (or the Machine
Status Word, on 286 processors). See also \c{SMSW} (\k{insSMSW}).

\H{insLOADALL} \i\c{LOADALL}, \i\c{LOADALL286}: Load Processor State

\c LOADALL                       ; 0F 07                [386,UNDOC]
\c LOADALL286                    ; 0F 05                [286,UNDOC]

This instruction, in its two different-opcode forms, is apparently
supported on most 286 processors, some 386 and possibly some 486.
The opcode differs between the 286 and the 386.

The function of the instruction is to load all information relating
to the state of the processor out of a block of memory: on the 286,
this block is located implicitly at absolute address \c{0x800}, and
on the 386 and 486 it is at \c{[ES:EDI]}.

\H{insLODSB} \i\c{LODSB}, \i\c{LODSW}, \i\c{LODSD}: Load from String

\c LODSB                         ; AC                   [8086]
\c LODSW                         ; o16 AD               [8086]
\c LODSD                         ; o32 AD               [386]

\c{LODSB} loads a byte from \c{[DS:SI]} or \c{[DS:ESI]} into \c{AL}.
It then increments or decrements (depending on the direction flag:
increments if the flag is clear, decrements if it is set) \c{SI} or
\c{ESI}.

The register used is \c{SI} if the address size is 16 bits, and
\c{ESI} if it is 32 bits. If you need to use an address size not
equal to the current \c{BITS} setting, you can use an explicit
\i\c{a16} or \i\c{a32} prefix.

The segment register used to load from \c{[SI]} or \c{[ESI]} can be
overridden by using a segment register name as a prefix (for
example, \c{es lodsb}).

\c{LODSW} and \c{LODSD} work in the same way, but they load a
word or a doubleword instead of a byte, and increment or decrement
the addressing registers by 2 or 4 instead of 1.

\H{insLOOP} \i\c{LOOP}, \i\c{LOOPE}, \i\c{LOOPZ}, \i\c{LOOPNE}, \i\c{LOOPNZ}: Loop with Counter

\c LOOP imm                      ; E2 rb                [8086]
\c LOOP imm,CX                   ; a16 E2 rb            [8086]
\c LOOP imm,ECX                  ; a32 E2 rb            [386]

\c LOOPE imm                     ; E1 rb                [8086]
\c LOOPE imm,CX                  ; a16 E1 rb            [8086]
\c LOOPE imm,ECX                 ; a32 E1 rb            [386]
\c LOOPZ imm                     ; E1 rb                [8086]
\c LOOPZ imm,CX                  ; a16 E1 rb            [8086]
\c LOOPZ imm,ECX                 ; a32 E1 rb            [386]

\c LOOPNE imm                    ; E0 rb                [8086]
\c LOOPNE imm,CX                 ; a16 E0 rb            [8086]
\c LOOPNE imm,ECX                ; a32 E0 rb            [386]
\c LOOPNZ imm                    ; E0 rb                [8086]
\c LOOPNZ imm,CX                 ; a16 E0 rb            [8086]
\c LOOPNZ imm,ECX                ; a32 E0 rb            [386]

\c{LOOP} decrements its counter register (either \c{CX} or \c{ECX} -
if one is not specified explicitly, the \c{BITS} setting dictates
which is used) by one, and if the counter does not become zero as a
result of this operation, it jumps to the given label. The jump has
a range of 128 bytes.

\c{LOOPE} (or its synonym \c{LOOPZ}) adds the additional condition
that it only jumps if the counter is nonzero \e{and} the zero flag
is set. Similarly, \c{LOOPNE} (and \c{LOOPNZ}) jumps only if the
counter is nonzero and the zero flag is clear.

\H{insLSL} \i\c{LSL}: Load Segment Limit

\c LSL reg16,r/m16               ; o16 0F 03 /r         [286,PRIV]
\c LSL reg32,r/m32               ; o32 0F 03 /r         [286,PRIV]

\c{LSL} is given a segment selector in its source (second) operand;
it computes the segment limit value by loading the segment limit
field from the associated segment descriptor in the GDT or LDT.
(This involves shifting left by 12 bits if the segment limit is
page-granular, and not if it is byte-granular; so you end up with a
byte limit in either case.) The segment limit obtained is then
loaded into the destination (first) operand.

\H{insLTR} \i\c{LTR}: Load Task Register

\c LTR r/m16                     ; 0F 00 /3             [286,PRIV]

\c{LTR} looks up the segment base and limit in the GDT or LDT
descriptor specified by the segment selector given as its operand,
and loads them into the Task Register.

\H{insMOV} \i\c{MOV}: Move Data

\c MOV r/m8,reg8                 ; 88 /r                [8086]
\c MOV r/m16,reg16               ; o16 89 /r            [8086]
\c MOV r/m32,reg32               ; o32 89 /r            [386]
\c MOV reg8,r/m8                 ; 8A /r                [8086]
\c MOV reg16,r/m16               ; o16 8B /r            [8086]
\c MOV reg32,r/m32               ; o32 8B /r            [386]

\c MOV reg8,imm8                 ; B0+r ib              [8086]
\c MOV reg16,imm16               ; o16 B8+r iw          [8086]
\c MOV reg32,imm32               ; o32 B8+r id          [386]
\c MOV r/m8,imm8                 ; C6 /0 ib             [8086]
\c MOV r/m16,imm16               ; o16 C7 /0 iw         [8086]
\c MOV r/m32,imm32               ; o32 C7 /0 id         [386]

\c MOV AL,memoffs8               ; A0 ow/od             [8086]
\c MOV AX,memoffs16              ; o16 A1 ow/od         [8086]
\c MOV EAX,memoffs32             ; o32 A1 ow/od         [386]
\c MOV memoffs8,AL               ; A2 ow/od             [8086]
\c MOV memoffs16,AX              ; o16 A3 ow/od         [8086]
\c MOV memoffs32,EAX             ; o32 A3 ow/od         [386]

\c MOV r/m16,segreg              ; o16 8C /r            [8086]
\c MOV r/m32,segreg              ; o32 8C /r            [386]
\c MOV segreg,r/m16              ; o16 8E /r            [8086]
\c MOV segreg,r/m32              ; o32 8E /r            [386]

\c MOV reg32,CR0/2/3/4           ; 0F 20 /r             [386]
\c MOV reg32,DR0/1/2/3/6/7       ; 0F 21 /r             [386]
\c MOV reg32,TR3/4/5/6/7         ; 0F 24 /r             [386]
\c MOV CR0/2/3/4,reg32           ; 0F 22 /r             [386]
\c MOV DR0/1/2/3/6/7,reg32       ; 0F 23 /r             [386]
\c MOV TR3/4/5/6/7,reg32         ; 0F 26 /r             [386]

\c{MOV} copies the contents of its source (second) operand into its
destination (first) operand.

In all forms of the \c{MOV} instruction, the two operands are the
same size, except for moving between a segment register and an
\c{r/m32} operand. These instructions are treated exactly like the
corresponding 16-bit equivalent (so that, for example, \c{MOV
DS,EAX} functions identically to \c{MOV DS,AX} but saves a prefix
when in 32-bit mode), except that when a segment register is moved
into a 32-bit destination, the top two bytes of the result are
undefined.

\c{MOV} may not use \c{CS} as a destination.

\c{CR4} is only a supported register on the Pentium and above.

\H{insMOVD} \i\c{MOVD}: Move Doubleword to/from MMX Register

\c MOVD mmxreg,r/m32             ; 0F 6E /r             [PENT,MMX]
\c MOVD r/m32,mmxreg             ; 0F 7E /r             [PENT,MMX]

\c{MOVD} copies 32 bits from its source (second) operand into its
destination (first) operand. When the destination is a 64-bit MMX
register, the top 32 bits are set to zero.

\H{insMOVQ} \i\c{MOVQ}: Move Quadword to/from MMX Register

\c MOVQ mmxreg,r/m64             ; 0F 6F /r             [PENT,MMX]
\c MOVQ r/m64,mmxreg             ; 0F 7F /r             [PENT,MMX]

\c{MOVQ} copies 64 bits from its source (second) operand into its
destination (first) operand.

\H{insMOVSB} \i\c{MOVSB}, \i\c{MOVSW}, \i\c{MOVSD}: Move String

\c MOVSB                         ; A4                   [8086]
\c MOVSW                         ; o16 A5               [8086]
\c MOVSD                         ; o32 A5               [386]

\c{MOVSB} copies the byte at \c{[ES:DI]} or \c{[ES:EDI]} to
\c{[DS:SI]} or \c{[DS:ESI]}. It then increments or decrements
(depending on the direction flag: increments if the flag is clear,
decrements if it is set) \c{SI} and \c{DI} (or \c{ESI} and \c{EDI}).

The registers used are \c{SI} and \c{DI} if the address size is 16
bits, and \c{ESI} and \c{EDI} if it is 32 bits. If you need to use
an address size not equal to the current \c{BITS} setting, you can
use an explicit \i\c{a16} or \i\c{a32} prefix.

The segment register used to load from \c{[SI]} or \c{[ESI]} can be
overridden by using a segment register name as a prefix (for
example, \c{es movsb}). The use of \c{ES} for the store to \c{[DI]}
or \c{[EDI]} cannot be overridden.

\c{MOVSW} and \c{MOVSD} work in the same way, but they copy a word
or a doubleword instead of a byte, and increment or decrement the
addressing registers by 2 or 4 instead of 1.

The \c{REP} prefix may be used to repeat the instruction \c{CX} (or
\c{ECX} - again, the address size chooses which) times.

\H{insMOVSX} \i\c{MOVSX}, \i\c{MOVZX}: Move Data with Sign or Zero Extend

\c MOVSX reg16,r/m8              ; o16 0F BE /r         [386]
\c MOVSX reg32,r/m8              ; o32 0F BE /r         [386]
\c MOVSX reg32,r/m16             ; o32 0F BF /r         [386]

\c MOVZX reg16,r/m8              ; o16 0F B6 /r         [386]
\c MOVZX reg32,r/m8              ; o32 0F B6 /r         [386]
\c MOVZX reg32,r/m16             ; o32 0F B7 /r         [386]

\c{MOVSX} sign-extends its source (second) operand to the length of
its destination (first) operand, and copies the result into the
destination operand. \c{MOVZX} does the same, but zero-extends
rather than sign-extending.

\H{insMUL} \i\c{MUL}: Unsigned Integer Multiply

\c MUL r/m8                      ; F6 /4                [8086]
\c MUL r/m16                     ; o16 F7 /4            [8086]
\c MUL r/m32                     ; o32 F7 /4            [386]

\c{MUL} performs unsigned integer multiplication. The other operand
to the multiplication, and the destination operand, are implicit, in
the following way:

\b For \c{MUL r/m8}, \c{AL} is multiplied by the given operand; the
product is stored in \c{AX}.

\b For \c{MUL r/m16}, \c{AX} is multiplied by the given operand;
the product is stored in \c{DX:AX}.

\b For \c{MUL r/m32}, \c{EAX} is multiplied by the given operand;
the product is stored in \c{EDX:EAX}.

Signed integer multiplication is performed by the \c{IMUL}
instruction: see \k{insIMUL}.

\H{insNEG} \i\c{NEG}, \i\c{NOT}: Two's and One's Complement

\c NEG r/m8                      ; F6 /3                [8086]
\c NEG r/m16                     ; o16 F7 /3            [8086]
\c NEG r/m32                     ; o32 F7 /3            [386]

\c NOT r/m8                      ; F6 /2                [8086]
\c NOT r/m16                     ; o16 F7 /2            [8086]
\c NOT r/m32                     ; o32 F7 /2            [386]

\c{NEG} replaces the contents of its operand by the two's complement
negation (invert all the bits and then add one) of the original
value. \c{NOT}, similarly, performs one's complement (inverts all
the bits).

\H{insNOP} \i\c{NOP}: No Operation

\c NOP                           ; 90                   [8086]

\c{NOP} performs no operation. Its opcode is the same as that
generated by \c{XCHG AX,AX} or \c{XCHG EAX,EAX} (depending on the
processor mode; see \k{insXCHG}).

\H{insOR} \i\c{OR}: Bitwise OR

\c OR r/m8,reg8                  ; 08 /r                [8086]
\c OR r/m16,reg16                ; o16 09 /r            [8086]
\c OR r/m32,reg32                ; o32 09 /r            [386]

\c OR reg8,r/m8                  ; 0A /r                [8086]
\c OR reg16,r/m16                ; o16 0B /r            [8086]
\c OR reg32,r/m32                ; o32 0B /r            [386]

\c OR r/m8,imm8                  ; 80 /1 ib             [8086]
\c OR r/m16,imm16                ; o16 81 /1 iw         [8086]
\c OR r/m32,imm32                ; o32 81 /1 id         [386]

\c OR r/m16,imm8                 ; o16 83 /1 ib         [8086]
\c OR r/m32,imm8                 ; o32 83 /1 ib         [386]

\c OR AL,imm8                    ; 0C ib                [8086]
\c OR AX,imm16                   ; o16 0D iw            [8086]
\c OR EAX,imm32                  ; o32 0D id            [386]

\c{OR} performs a bitwise OR operation between its two operands
(i.e. each bit of the result is 1 if and only if at least one of the
corresponding bits of the two inputs was 1), and stores the result
in the destination (first) operand.

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

The MMX instruction \c{POR} (see \k{insPOR}) performs the same
operation on the 64-bit MMX registers.

\H{insOUT} \i\c{OUT}: Output Data to I/O Port

\c OUT imm8,AL                   ; E6 ib                [8086]
\c OUT imm8,AX                   ; o16 E7 ib            [8086]
\c OUT imm8,EAX                  ; o32 E7 ib            [386]
\c OUT DX,AL                     ; EE                   [8086]
\c OUT DX,AX                     ; o16 EF               [8086]
\c OUT DX,EAX                    ; o32 EF               [386]

\c{IN} writes the contents of the given source register to the
specified I/O port. The port number may be specified as an immediate
value if it is between 0 and 255, and otherwise must be stored in
\c{DX}. See also \c{IN} (\k{insIN}).

\H{insOUTSB} \i\c{OUTSB}, \i\c{OUTSW}, \i\c{OUTSD}: Output String to I/O Port

\c OUTSB                         ; 6E                   [186]

\c OUTSW                         ; o16 6F               [186]

\c OUTSD                         ; o32 6F               [386]

\c{OUTSB} loads a byte from \c{[DS:SI]} or \c{[DS:ESI]} and writes
it to the I/O port specified in \c{DX}. It then increments or
decrements (depending on the direction flag: increments if the flag
is clear, decrements if it is set) \c{SI} or \c{ESI}.

The register used is \c{SI} if the address size is 16 bits, and
\c{ESI} if it is 32 bits. If you need to use an address size not
equal to the current \c{BITS} setting, you can use an explicit
\i\c{a16} or \i\c{a32} prefix.

The segment register used to load from \c{[SI]} or \c{[ESI]} can be
overridden by using a segment register name as a prefix (for
example, \c{es outsb}).

\c{OUTSW} and \c{OUTSD} work in the same way, but they output a
word or a doubleword instead of a byte, and increment or decrement
the addressing registers by 2 or 4 instead of 1.

The \c{REP} prefix may be used to repeat the instruction \c{CX} (or
\c{ECX} - again, the address size chooses which) times.

\H{insPACKSSDW} \i\c{PACKSSDW}, \i\c{PACKSSWB}, \i\c{PACKUSWB}: Pack Data

\c PACKSSDW mmxreg,r/m64         ; 0F 6B /r             [PENT,MMX]
\c PACKSSWB mmxreg,r/m64         ; 0F 63 /r             [PENT,MMX]
\c PACKUSWB mmxreg,r/m64         ; 0F 67 /r             [PENT,MMX]

All these instructions start by forming a notional 128-bit word by
placing the source (second) operand on the left of the destination
(first) operand. \c{PACKSSDW} then splits this 128-bit word into
four doublewords, converts each to a word, and loads them side by
side into the destination register; \c{PACKSSWB} and \c{PACKUSWB}
both split the 128-bit word into eight words, converts each to a
byte, and loads \e{those} side by side into the destination
register.

\c{PACKSSDW} and \c{PACKSSWB} perform signed saturation when
reducing the length of numbers: if the number is too large to fit
into the reduced space, they replace it by the largest signed number
(\c{7FFFh} or \c{7Fh}) that \e{will} fit, and if it is too small
then they replace it by the smallest signed number (\c{8000h} or
\c{80h}) that will fit. \c{PACKUSWB} performs unsigned saturation:
it treats its input as unsigned, and replaces it by the largest
unsigned number that will fit.

\H{insPADDB} \i\c{PADDxx}: MMX Packed Addition

\c PADDB mmxreg,r/m64            ; 0F FC /r             [PENT,MMX]
\c PADDW mmxreg,r/m64            ; 0F FD /r             [PENT,MMX]
\c PADDD mmxreg,r/m64            ; 0F FE /r             [PENT,MMX]

\c PADDSB mmxreg,r/m64           ; 0F EC /r             [PENT,MMX]
\c PADDSW mmxreg,r/m64           ; 0F ED /r             [PENT,MMX]

\c PADDUSB mmxreg,r/m64          ; 0F DC /r             [PENT,MMX]
\c PADDUSW mmxreg,r/m64          ; 0F DD /r             [PENT,MMX]

\c{PADDxx} all perform packed addition between their two 64-bit
operands, storing the result in the destination (first) operand. The
\c{PADDxB} forms treat the 64-bit operands as vectors of eight
bytes, and add each byte individually; \c{PADDxW} treat the operands
as vectors of four words; and \c{PADDD} treats its operands as
vectors of two doublewords.

\c{PADDSB} and \c{PADDSW} perform signed saturation on the sum of
each pair of bytes or words: if the result of an addition is too
large or too small to fit into a signed byte or word result, it is
clipped (saturated) to the largest or smallest value which \e{will}
fit. \c{PADDUSB} and \c{PADDUSW} similarly perform unsigned
saturation, clipping to \c{0FFh} or \c{0FFFFh} if the result is
larger than that.

\H{insPADDSIW} \i\c{PADDSIW}: MMX Packed Addition to Implicit
Destination

\c PADDSIW mmxreg,r/m64          ; 0F 51 /r             [CYRIX,MMX]

\c{PADDSIW}, specific to the Cyrix extensions to the MMX instruction
set, performs the same function as \c{PADDSW}, except that the
result is not placed in the register specified by the first operand,
but instead in the register whose number differs from the first
operand only in the last bit. So \c{PADDSIW MM0,MM2} would put the
result in \c{MM1}, but \c{PADDSIW MM1,MM2} would put the result in
\c{MM0}.

\H{insPAND} \i\c{PAND}, \i\c{PANDN}: MMX Bitwise AND and AND-NOT

\c PAND mmxreg,r/m64             ; 0F DB /r             [PENT,MMX]
\c PANDN mmxreg,r/m64            ; 0F DF /r             [PENT,MMX]

\c{PAND} performs a bitwise AND operation between its two operands
(i.e. each bit of the result is 1 if and only if the corresponding
bits of the two inputs were both 1), and stores the result in the
destination (first) operand.

\c{PANDN} performs the same operation, but performs a one's
complement operation on the destination (first) operand first.

\H{insPAVEB} \i\c{PAVEB}: MMX Packed Average

\c PAVEB mmxreg,r/m64            ; 0F 50 /r             [CYRIX,MMX]

\c{PAVEB}, specific to the Cyrix MMX extensions, treats its two
operands as vectors of eight unsigned bytes, and calculates the
average of the corresponding bytes in the operands. The resulting
vector of eight averages is stored in the first operand.

\H{insPCMPEQB} \i\c{PCMPxx}: MMX Packed Comparison

\c PCMPEQB mmxreg,r/m64          ; 0F 74 /r             [PENT,MMX]
\c PCMPEQW mmxreg,r/m64          ; 0F 75 /r             [PENT,MMX]
\c PCMPEQD mmxreg,r/m64          ; 0F 76 /r             [PENT,MMX]

\c PCMPGTB mmxreg,r/m64          ; 0F 64 /r             [PENT,MMX]
\c PCMPGTW mmxreg,r/m64          ; 0F 65 /r             [PENT,MMX]
\c PCMPGTD mmxreg,r/m64          ; 0F 66 /r             [PENT,MMX]

The \c{PCMPxx} instructions all treat their operands as vectors of
bytes, words, or doublewords; corresponding elements of the source
and destination are compared, and the corresponding element of the
destination (first) operand is set to all zeros or all ones
depending on the result of the comparison.

\c{PCMPxxB} treats the operands as vectors of eight bytes,
\c{PCMPxxW} treats them as vectors of four words, and \c{PCMPxxD} as
two doublewords.

\c{PCMPEQx} sets the corresponding element of the destination
operand to all ones if the two elements compared are equal;
\c{PCMPGTx} sets the destination element to all ones if the element
of the first (destination) operand is greater (treated as a signed
integer) than that of the second (source) operand.

\H{insPDISTIB} \i\c{PDISTIB}: MMX Packed Distance and Accumulate
with Implied Register

\c PDISTIB mmxreg,mem64          ; 0F 54 /r             [CYRIX,MMX]

\c{PDISTIB}, specific to the Cyrix MMX extensions, treats its two
input operands as vectors of eight unsigned bytes. For each byte
position, it finds the absolute difference between the bytes in that
position in the two input operands, and adds that value to the byte
in the same position in the implied output register. The addition is
saturated to an unsigned byte in the same way as \c{PADDUSB}.

The implied output register is found in the same way as \c{PADDSIW}
(\k{insPADDSIW}).

Note that \c{PDISTIB} cannot take a register as its second source
operand.

\H{insPMACHRIW} \i\c{PMACHRIW}: MMX Packed Multiply and Accumulate
with Rounding

\c PMACHRIW mmxreg,mem64         ; 0F 5E /r             [CYRIX,MMX]

\c{PMACHRIW} acts almost identically to \c{PMULHRIW}
(\k{insPMULHRW}), but instead of \e{storing} its result in the
implied destination register, it \e{adds} its result, as four packed
words, to the implied destination register. No saturation is done:
the addition can wrap around.

Note that \c{PMACHRIW} cannot take a register as its second source
operand.

\H{insPMADDWD} \i\c{PMADDWD}: MMX Packed Multiply and Add

\c PMADDWD mmxreg,r/m64          ; 0F F5 /r             [PENT,MMX]

\c{PMADDWD} treats its two inputs as vectors of four signed words.
It multiplies corresponding elements of the two operands, giving
four signed doubleword results. The top two of these are added and
placed in the top 32 bits of the destination (first) operand; the
bottom two are added and placed in the bottom 32 bits.

\H{insPMAGW} \i\c{PMAGW}: MMX Packed Magnitude

\c PMAGW mmxreg,r/m64            ; 0F 52 /r             [CYRIX,MMX]

\c{PMAGW}, specific to the Cyrix MMX extensions, treats both its
operands as vectors of four signed words. It compares the absolute
values of the words in corresponding positions, and sets each word
of the destination (first) operand to whichever of the two words in
that position had the larger absolute value.

\H{insPMULHRW} \i\c{PMULHRW}, \i\c{PMULHRIW}: MMX Packed Multiply
High with Rounding

\c PMULHRW mmxreg,r/m64          ; 0F 59 /r             [CYRIX,MMX]
\c PMULHRIW mmxreg,r/m64         ; 0F 5D /r             [CYRIX,MMX]

These instructions, specific to the Cyrix MMX extensions, treat
their operands as vectors of four signed words. Words in
corresponding positions are multiplied, to give a 32-bit value in
which bits 30 and 31 are guaranteed equal. Bits 30 to 15 of this
value (bit mask \c{0x7FFF8000}) are taken and stored in the
corresponding position of the destination operand, after first
rounding the low bit (equivalent to adding \c{0x4000} before
extracting bits 30 to 15).

For \c{PMULHRW}, the destination operand is the first operand; for
\c{PMULHRIW} the destination operand is implied by the first operand
in the manner of \c{PADDSIW} (\k{insPADDSIW}).

\H{insPMULHW} \i\c{PMULHW}, \i\c{PMULLW}: MMX Packed Multiply

\c PMULHW mmxreg,r/m64           ; 0F E5 /r             [PENT,MMX]
\c PMULLW mmxreg,r/m64           ; 0F D5 /r             [PENT,MMX]

\c{PMULxW} treats its two inputs as vectors of four signed words. It
multiplies corresponding elements of the two operands, giving four
signed doubleword results.

\c{PMULHW} then stores the top 16 bits of each doubleword in the
destination (first) operand; \c{PMULLW} stores the bottom 16 bits of
each doubleword in the destination operand.

\H{insPMVccZB} \i\c{PMVccZB}: MMX Packed Conditional Move

\c PMVZB mmxreg,mem64            ; 0F 58 /r             [CYRIX,MMX]
\c PMVNZB mmxreg,mem64           ; 0F 5A /r             [CYRIX,MMX]
\c PMVLZB mmxreg,mem64           ; 0F 5B /r             [CYRIX,MMX]
\c PMVGEZB mmxreg,mem64          ; 0F 5C /r             [CYRIX,MMX]

These instructions, specific to the Cyrix MMX extensions, perform
parallel conditional moves. The two input operands are treated as
vectors of eight bytes. Each byte of the destination (first) operand
is either written from the corresponding byte of the source (second)
operand, or left alone, depending on the value of the byte in the
\e{implied} operand (specified in the same way as \c{PADDSIW}, in
\k{insPADDSIW}).

\c{PMVZB} performs each move if the corresponding byte in the
implied operand is zero. \c{PMVNZB} moves if the byte is non-zero.
\c{PMVLZB} moves if the byte is less than zero, and \c{PMVGEZB}
moves if the byte is greater than or equal to zero.

Note that these instructions cannot take a register as their second
source operand.

\H{insPOP} \i\c{POP}: Pop Data from Stack

\c POP reg16                     ; o16 58+r             [8086]
\c POP reg32                     ; o32 58+r             [386]

\c POP r/m16                     ; o16 8F /0            [8086]
\c POP r/m32                     ; o32 8F /0            [386]

\c POP CS                        ; 0F                   [8086,UNDOC]
\c POP DS                        ; 1F                   [8086]
\c POP ES                        ; 07                   [8086]
\c POP SS                        ; 17                   [8086]
\c POP FS                        ; 0F A1                [386]
\c POP GS                        ; 0F A9                [386]

\c{POP} loads a value from the stack (from \c{[SS:SP]} or
\c{[SS:ESP]}) and then increments the stack pointer.

The address-size attribute of the instruction determines whether
\c{SP} or \c{ESP} is used as the stack pointer: to deliberately
override the default given by the \c{BITS} setting, you can use an
\i\c{a16} or \i\c{a32} prefix.

The operand-size attribute of the instruction determines whether the
stack pointer is incremented by 2 or 4: this means that segment
register pops in \c{BITS 32} mode will pop 4 bytes off the stack and
discard the upper two of them. If you need to override that, you can
use an \i\c{o16} or \i\c{o32} prefix.

The above opcode listings give two forms for general-purpose
register pop instructions: for example, \c{POP BX} has the two forms
\c{5B} and \c{8F C3}. NASM will always generate the shorter form
when given \c{POP BX}. NDISASM will disassemble both.

\c{POP CS} is not a documented instruction, and is not supported on
any processor above the 8086 (since they use \c{0Fh} as an opcode
prefix for instruction set extensions). However, at least some 8086
processors do support it, and so NASM generates it for completeness.

\H{insPOPA} \i\c{POPAx}: Pop All General-Purpose Registers

\c POPA                          ; 61                   [186]
\c POPAW                         ; o16 61               [186]
\c POPAD                         ; o32 61               [386]

\c{POPAW} pops a word from the stack into each of, successively,
\c{DI}, \c{SI}, \c{BP}, nothing (it discards a word from the stack
which was a placeholder for \c{SP}), \c{BX}, \c{DX}, \c{CX} and
\c{AX}. It is intended to reverse the operation of \c{PUSHAW} (see
\k{insPUSHA}), but it ignores the value for \c{SP} that was pushed
on the stack by \c{PUSHAW}.

\c{POPAD} pops twice as much data, and places the results in
\c{EDI}, \c{ESI}, \c{EBP}, nothing (placeholder for \c{ESP}),
\c{EBX}, \c{EDX}, \c{ECX} and \c{EAX}. It reverses the operation of
\c{PUSHAD}.

\c{POPA} is an alias mnemonic for either \c{POPAW} or \c{POPAD},
depending on the current \c{BITS} setting.

Note that the registers are popped in reverse order of their numeric
values in opcodes (see \k{iref-rv}).

\H{insPOPF} \i\c{POPFx}: Pop Flags Register

\c POPF                          ; 9D                   [186]
\c POPFW                         ; o16 9D               [186]
\c POPFD                         ; o32 9D               [386]

\c{POPFW} pops a word from the stack and stores it in the bottom 16
bits of the flags register (or the whole flags register, on
processors below a 386). \c{POPFD} pops a doubleword and stores it
in the entire flags register.

\c{POPF} is an alias mnemonic for either \c{POPFW} or \c{POPFD},
depending on the current \c{BITS} setting.

See also \c{PUSHF} (\k{insPUSHF}).

\H{insPOR} \i\c{POR}: MMX Bitwise OR

\c POR mmxreg,r/m64              ; 0F EB /r             [PENT,MMX]

\c{POR} performs a bitwise OR operation between its two operands
(i.e. each bit of the result is 1 if and only if at least one of the
corresponding bits of the two inputs was 1), and stores the result
in the destination (first) operand.

\H{insPSLLD} \i\c{PSLLx}, \i\c{PSRLx}, \i\c{PSRAx}: MMX Bit Shifts

\c PSLLW mmxreg,r/m64            ; 0F F1 /r             [PENT,MMX]
\c PSLLW mmxreg,imm8             ; 0F 71 /6 ib          [PENT,MMX]

\c PSLLD mmxreg,r/m64            ; 0F F2 /r             [PENT,MMX]
\c PSLLD mmxreg,imm8             ; 0F 72 /6 ib          [PENT,MMX]

\c PSLLQ mmxreg,r/m64            ; 0F F3 /r             [PENT,MMX]
\c PSLLQ mmxreg,imm8             ; 0F 73 /6 ib          [PENT,MMX]

\c PSRAW mmxreg,r/m64            ; 0F E1 /r             [PENT,MMX]
\c PSRAW mmxreg,imm8             ; 0F 71 /4 ib          [PENT,MMX]

\c PSRAD mmxreg,r/m64            ; 0F E2 /r             [PENT,MMX]
\c PSRAD mmxreg,imm8             ; 0F 72 /4 ib          [PENT,MMX]

\c PSRLW mmxreg,r/m64            ; 0F D1 /r             [PENT,MMX]
\c PSRLW mmxreg,imm8             ; 0F 71 /2 ib          [PENT,MMX]

\c PSRLD mmxreg,r/m64            ; 0F D2 /r             [PENT,MMX]
\c PSRLD mmxreg,imm8             ; 0F 72 /2 ib          [PENT,MMX]

\c PSRLQ mmxreg,r/m64            ; 0F D3 /r             [PENT,MMX]
\c PSRLQ mmxreg,imm8             ; 0F 73 /2 ib          [PENT,MMX]

\c{PSxxQ} perform simple bit shifts on the 64-bit MMX registers: the
destination (first) operand is shifted left or right by the number of
bits given in the source (second) operand, and the vacated bits are
filled in with zeros (for a logical shift) or copies of the original
sign bit (for an arithmetic right shift).

\c{PSxxW} and \c{PSxxD} perform packed bit shifts: the destination
operand is treated as a vector of four words or two doublewords, and
each element is shifted individually, so bits shifted out of one
element do not interfere with empty bits coming into the next.

\c{PSLLx} and \c{PSRLx} perform logical shifts: the vacated bits at
one end of the shifted number are filled with zeros. \c{PSRAx}
performs an arithmetic right shift: the vacated bits at the top of
the shifted number are filled with copies of the original top (sign)
bit.

\H{insPSUBB} \i\c{PSUBxx}: MMX Packed Subtraction

\c PSUBB mmxreg,r/m64            ; 0F F8 /r             [PENT,MMX]
\c PSUBW mmxreg,r/m64            ; 0F F9 /r             [PENT,MMX]
\c PSUBD mmxreg,r/m64            ; 0F FA /r             [PENT,MMX]

\c PSUBSB mmxreg,r/m64           ; 0F E8 /r             [PENT,MMX]
\c PSUBSW mmxreg,r/m64           ; 0F E9 /r             [PENT,MMX]

\c PSUBUSB mmxreg,r/m64          ; 0F D8 /r             [PENT,MMX]
\c PSUBUSW mmxreg,r/m64          ; 0F D9 /r             [PENT,MMX]

\c{PSUBxx} all perform packed subtraction between their two 64-bit
operands, storing the result in the destination (first) operand. The
\c{PSUBxB} forms treat the 64-bit operands as vectors of eight
bytes, and subtract each byte individually; \c{PSUBxW} treat the operands
as vectors of four words; and \c{PSUBD} treats its operands as
vectors of two doublewords.

In all cases, the elements of the operand on the right are
subtracted from the corresponding elements of the operand on the
left, not the other way round.

\c{PSUBSB} and \c{PSUBSW} perform signed saturation on the sum of
each pair of bytes or words: if the result of a subtraction is too
large or too small to fit into a signed byte or word result, it is
clipped (saturated) to the largest or smallest value which \e{will}
fit. \c{PSUBUSB} and \c{PSUBUSW} similarly perform unsigned
saturation, clipping to \c{0FFh} or \c{0FFFFh} if the result is
larger than that.

\H{insPSUBSIW} \i\c{PSUBSIW}: MMX Packed Subtract with Saturation to
Implied Destination

\c PSUBSIW mmxreg,r/m64          ; 0F 55 /r             [CYRIX,MMX]

\c{PSUBSIW}, specific to the Cyrix extensions to the MMX instruction
set, performs the same function as \c{PSUBSW}, except that the
result is not placed in the register specified by the first operand,
but instead in the implied destination register, specified as for
\c{PADDSIW} (\k{insPADDSIW}).

\H{insPUNPCKHBW} \i\c{PUNPCKxxx}: Unpack Data

\c PUNPCKHBW mmxreg,r/m64        ; 0F 68 /r             [PENT,MMX]
\c PUNPCKHWD mmxreg,r/m64        ; 0F 69 /r             [PENT,MMX]
\c PUNPCKHDQ mmxreg,r/m64        ; 0F 6A /r             [PENT,MMX]

\c PUNPCKLBW mmxreg,r/m64        ; 0F 60 /r             [PENT,MMX]
\c PUNPCKLWD mmxreg,r/m64        ; 0F 61 /r             [PENT,MMX]
\c PUNPCKLDQ mmxreg,r/m64        ; 0F 62 /r             [PENT,MMX]

\c{PUNPCKxx} all treat their operands as vectors, and produce a new
vector generated by interleaving elements from the two inputs. The
\c{PUNPCKHxx} instructions start by throwing away the bottom half of
each input operand, and the \c{PUNPCKLxx} instructions throw away
the top half.

The remaining elements, totalling 64 bits, are then interleaved into
the destination, alternating elements from the second (source)
operand and the first (destination) operand: so the leftmost element
in the result always comes from the second operand, and the
rightmost from the destination.

\c{PUNPCKxBW} works a byte at a time, \c{PUNPCKxWD} a word at a
time, and \c{PUNPCKxDQ} a doubleword at a time.

So, for example, if the first operand held \c{0x7A6A5A4A3A2A1A0A}
and the second held \c{0x7B6B5B4B3B2B1B0B}, then:

\b \c{PUNPCKHBW} would return \c{0x7B7A6B6A5B5A4B4A}.

\b \c{PUNPCKHWD} would return \c{0x7B6B7A6A5B4B5A4A}.

\b \c{PUNPCKHDQ} would return \c{0x7B6B5B4B7A6A5A4A}.

\b \c{PUNPCKLBW} would return \c{0x3B3A2B2A1B1A0B0A}.

\b \c{PUNPCKLWD} would return \c{0x3B2B3A2A1B0B1A0A}.

\b \c{PUNPCKLDQ} would return \c{0x3B2B1B0B3A2A1A0A}.

\H{insPUSH} \i\c{PUSH}: Push Data on Stack

\c PUSH reg16                    ; o16 50+r             [8086]
\c PUSH reg32                    ; o32 50+r             [386]

\c PUSH r/m16                    ; o16 FF /6            [8086]
\c PUSH r/m32                    ; o32 FF /6            [386]

\c PUSH CS                       ; 0E                   [8086]
\c PUSH DS                       ; 1E                   [8086]
\c PUSH ES                       ; 06                   [8086]
\c PUSH SS                       ; 16                   [8086]
\c PUSH FS                       ; 0F A0                [386]
\c PUSH GS                       ; 0F A8                [386]

\c PUSH imm8                     ; 6A ib                [286]
\c PUSH imm16                    ; o16 68 iw            [286]
\c PUSH imm32                    ; o32 68 id            [386]

\c{PUSH} decrements the stack pointer (\c{SP} or \c{ESP}) by 2 or 4,
and then stores the given value at \c{[SS:SP]} or \c{[SS:ESP]}.

The address-size attribute of the instruction determines whether
\c{SP} or \c{ESP} is used as the stack pointer: to deliberately
override the default given by the \c{BITS} setting, you can use an
\i\c{a16} or \i\c{a32} prefix.

The operand-size attribute of the instruction determines whether the
stack pointer is decremented by 2 or 4: this means that segment
register pushes in \c{BITS 32} mode will push 4 bytes on the stack,
of which the upper two are undefined. If you need to override that,
you can use an \i\c{o16} or \i\c{o32} prefix.

The above opcode listings give two forms for general-purpose
\i{register push} instructions: for example, \c{PUSH BX} has the two
forms \c{53} and \c{FF F3}. NASM will always generate the shorter
form when given \c{PUSH BX}. NDISASM will disassemble both.

Unlike the undocumented and barely supported \c{POP CS}, \c{PUSH CS}
is a perfectly valid and sensible instruction, supported on all
processors.

The instruction \c{PUSH SP} may be used to distinguish an 8086 from
later processors: on an 8086, the value of \c{SP} stored is the
value it has \e{after} the push instruction, whereas on later
processors it is the value \e{before} the push instruction.

\H{insPUSHA} \i\c{PUSHAx}: Push All General-Purpose Registers

\c PUSHA                         ; 60                   [186]
\c PUSHAD                        ; o32 60               [386]
\c PUSHAW                        ; o16 60               [186]

\c{PUSHAW} pushes, in succession, \c{AX}, \c{CX}, \c{DX}, \c{BX},
\c{SP}, \c{BP}, \c{SI} and \c{DI} on the stack, decrementing the
stack pointer by a total of 16.

\c{PUSHAD} pushes, in succession, \c{EAX}, \c{ECX}, \c{EDX},
\c{EBX}, \c{ESP}, \c{EBP}, \c{ESI} and \c{EDI} on the stack,
decrementing the stack pointer by a total of 32.

In both cases, the value of \c{SP} or \c{ESP} pushed is its
\e{original} value, as it had before the instruction was executed.

\c{PUSHA} is an alias mnemonic for either \c{PUSHAW} or \c{PUSHAD},
depending on the current \c{BITS} setting.

Note that the registers are pushed in order of their numeric values
in opcodes (see \k{iref-rv}).

See also \c{POPA} (\k{insPOPA}).

\H{insPUSHF} \i\c{PUSHFx}: Push Flags Register

\c PUSHF                         ; 9C                   [186]
\c PUSHFD                        ; o32 9C               [386]
\c PUSHFW                        ; o16 9C               [186]

\c{PUSHFW} pops a word from the stack and stores it in the bottom 16
bits of the flags register (or the whole flags register, on
processors below a 386). \c{PUSHFD} pops a doubleword and stores it
in the entire flags register.

\c{PUSHF} is an alias mnemonic for either \c{PUSHFW} or \c{PUSHFD},
depending on the current \c{BITS} setting.

See also \c{POPF} (\k{insPOPF}).

\H{insPXOR} \i\c{PXOR}: MMX Bitwise XOR

\c PXOR mmxreg,r/m64             ; 0F EF /r             [PENT,MMX]

\c{PXOR} performs a bitwise XOR operation between its two operands
(i.e. each bit of the result is 1 if and only if exactly one of the
corresponding bits of the two inputs was 1), and stores the result
in the destination (first) operand.

\H{insRCL} \i\c{RCL}, \i\c{RCR}: Bitwise Rotate through Carry Bit

\c RCL r/m8,1                    ; D0 /2                [8086]
\c RCL r/m8,CL                   ; D2 /2                [8086]
\c RCL r/m8,imm8                 ; C0 /2 ib             [286]
\c RCL r/m16,1                   ; o16 D1 /2            [8086]
\c RCL r/m16,CL                  ; o16 D3 /2            [8086]
\c RCL r/m16,imm8                ; o16 C1 /2 ib         [286]
\c RCL r/m32,1                   ; o32 D1 /2            [386]
\c RCL r/m32,CL                  ; o32 D3 /2            [386]
\c RCL r/m32,imm8                ; o32 C1 /2 ib         [386]

\c RCR r/m8,1                    ; D0 /3                [8086]
\c RCR r/m8,CL                   ; D2 /3                [8086]
\c RCR r/m8,imm8                 ; C0 /3 ib             [286]
\c RCR r/m16,1                   ; o16 D1 /3            [8086]
\c RCR r/m16,CL                  ; o16 D3 /3            [8086]
\c RCR r/m16,imm8                ; o16 C1 /3 ib         [286]
\c RCR r/m32,1                   ; o32 D1 /3            [386]
\c RCR r/m32,CL                  ; o32 D3 /3            [386]
\c RCR r/m32,imm8                ; o32 C1 /3 ib         [386]

\c{RCL} and \c{RCR} perform a 9-bit, 17-bit or 33-bit bitwise
rotation operation, involving the given source/destination (first)
operand and the carry bit. Thus, for example, in the operation
\c{RCR AL,1}, a 9-bit rotation is performed in which \c{AL} is
shifted left by 1, the top bit of \c{AL} moves into the carry flag,
and the original value of the carry flag is placed in the low bit of
\c{AL}.

The number of bits to rotate by is given by the second operand. Only
the bottom five bits of the rotation count are considered by
processors above the 8086.

You can force the longer (286 and upwards, beginning with a \c{C1}
byte) form of \c{RCL foo,1} by using a \c{BYTE} prefix: \c{RCL
foo,BYTE 1}. Similarly with \c{RCR}.

\H{insRDMSR} \i\c{RDMSR}: Read Model-Specific Registers

\c RDMSR                         ; 0F 32                [PENT]

\c{RDMSR} reads the processor Model-Specific Register (MSR) whose
index is stored in \c{ECX}, and stores the result in \c{EDX:EAX}.
See also \c{WRMSR} (\k{insWRMSR}).

\H{insRDPMC} \i\c{RDPMC}: Read Performance-Monitoring Counters

\c RDPMC                         ; 0F 33                [P6]

\c{RDPMC} reads the processor performance-monitoring counter whose
index is stored in \c{ECX}, and stores the result in \c{EDX:EAX}.

\H{insRDTSC} \i\c{RDTSC}: Read Time-Stamp Counter

\c RDTSC                         ; 0F 31                [PENT]

\c{RDTSC} reads the processor's time-stamp counter into \c{EDX:EAX}.

\H{insRET} \i\c{RET}, \i\c{RETF}, \i\c{RETN}: Return from Procedure Call

\c RET                           ; C3                   [8086]
\c RET imm16                     ; C2 iw                [8086]

\c RETF                          ; CB                   [8086]
\c RETF imm16                    ; CA iw                [8086]

\c RETN                          ; C3                   [8086]
\c RETN imm16                    ; C2 iw                [8086]

\c{RET}, and its exact synonym \c{RETN}, pop \c{IP} or \c{EIP} from
the stack and transfer control to the new address. Optionally, if a
numeric second operand is provided, they increment the stack pointer
by a further \c{imm16} bytes after popping the return address.

\c{RETF} executes a far return: after popping \c{IP}/\c{EIP}, it
then pops \c{CS}, and \e{then} increments the stack pointer by the
optional argument if present.

\H{insROL} \i\c{ROL}, \i\c{ROR}: Bitwise Rotate

\c ROL r/m8,1                    ; D0 /0                [8086]
\c ROL r/m8,CL                   ; D2 /0                [8086]
\c ROL r/m8,imm8                 ; C0 /0 ib             [286]
\c ROL r/m16,1                   ; o16 D1 /0            [8086]
\c ROL r/m16,CL                  ; o16 D3 /0            [8086]
\c ROL r/m16,imm8                ; o16 C1 /0 ib         [286]
\c ROL r/m32,1                   ; o32 D1 /0            [386]
\c ROL r/m32,CL                  ; o32 D3 /0            [386]
\c ROL r/m32,imm8                ; o32 C1 /0 ib         [386]

\c ROR r/m8,1                    ; D0 /1                [8086]
\c ROR r/m8,CL                   ; D2 /1                [8086]
\c ROR r/m8,imm8                 ; C0 /1 ib             [286]
\c ROR r/m16,1                   ; o16 D1 /1            [8086]
\c ROR r/m16,CL                  ; o16 D3 /1            [8086]
\c ROR r/m16,imm8                ; o16 C1 /1 ib         [286]
\c ROR r/m32,1                   ; o32 D1 /1            [386]
\c ROR r/m32,CL                  ; o32 D3 /1            [386]
\c ROR r/m32,imm8                ; o32 C1 /1 ib         [386]

\c{ROL} and \c{ROR} perform a bitwise rotation operation on the given
source/destination (first) operand. Thus, for example, in the
operation \c{ROR AL,1}, an 8-bit rotation is performed in which
\c{AL} is shifted left by 1 and the original top bit of \c{AL} moves
round into the low bit.

The number of bits to rotate by is given by the second operand. Only
the bottom 3, 4 or 5 bits (depending on the source operand size) of
the rotation count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a \c{C1}
byte) form of \c{ROL foo,1} by using a \c{BYTE} prefix: \c{ROL
foo,BYTE 1}. Similarly with \c{ROR}.

\H{insRSM} \i\c{RSM}: Resume from System-Management Mode

\c RSM                           ; 0F AA                [PENT]

\c{RSM} returns the processor to its normal operating mode when it
was in System-Management Mode.

\H{insSAHF} \i\c{SAHF}: Store AH to Flags

\c SAHF                          ; 9E                   [8086]

\c{SAHF} sets the low byte of the flags word according to the
contents of the \c{AH} register. See also \c{LAHF} (\k{insLAHF}).

\H{insSAL} \i\c{SAL}, \i\c{SAR}: Bitwise Arithmetic Shifts

\c SAL r/m8,1                    ; D0 /4                [8086]
\c SAL r/m8,CL                   ; D2 /4                [8086]
\c SAL r/m8,imm8                 ; C0 /4 ib             [286]
\c SAL r/m16,1                   ; o16 D1 /4            [8086]
\c SAL r/m16,CL                  ; o16 D3 /4            [8086]
\c SAL r/m16,imm8                ; o16 C1 /4 ib         [286]
\c SAL r/m32,1                   ; o32 D1 /4            [386]
\c SAL r/m32,CL                  ; o32 D3 /4            [386]
\c SAL r/m32,imm8                ; o32 C1 /4 ib         [386]

\c SAR r/m8,1                    ; D0 /0                [8086]
\c SAR r/m8,CL                   ; D2 /0                [8086]
\c SAR r/m8,imm8                 ; C0 /0 ib             [286]
\c SAR r/m16,1                   ; o16 D1 /0            [8086]
\c SAR r/m16,CL                  ; o16 D3 /0            [8086]
\c SAR r/m16,imm8                ; o16 C1 /0 ib         [286]
\c SAR r/m32,1                   ; o32 D1 /0            [386]
\c SAR r/m32,CL                  ; o32 D3 /0            [386]
\c SAR r/m32,imm8                ; o32 C1 /0 ib         [386]

\c{SAL} and \c{SAR} perform an arithmetic shift operation on the given
source/destination (first) operand. The vacated bits are filled with
zero for \c{SAL}, and with copies of the original high bit of the
source operand for \c{SAR}.

\c{SAL} is a synonym for \c{SHL} (see \k{insSHL}). NASM will
assemble either one to the same code, but NDISASM will always
disassemble that code as \c{SHL}.

The number of bits to shift by is given by the second operand. Only
the bottom 3, 4 or 5 bits (depending on the source operand size) of
the shift count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a \c{C1}
byte) form of \c{SAL foo,1} by using a \c{BYTE} prefix: \c{SAL
foo,BYTE 1}. Similarly with \c{SAR}.

\H{insSALC} \i\c{SALC}: Set AL from Carry Flag

\c SALC                          ; D6                   [8086,UNDOC]

\c{SALC} is an early undocumented instruction similar in concept to
\c{SETcc} (\k{insSETcc}). Its function is to set \c{AL} to zero if
the carry flag is clear, or to \c{0xFF} if it is set.

\H{insSBB} \i\c{SBB}: Subtract with Borrow

\c SBB r/m8,reg8                 ; 18 /r                [8086]
\c SBB r/m16,reg16               ; o16 19 /r            [8086]
\c SBB r/m32,reg32               ; o32 19 /r            [386]

\c SBB reg8,r/m8                 ; 1A /r                [8086]
\c SBB reg16,r/m16               ; o16 1B /r            [8086]
\c SBB reg32,r/m32               ; o32 1B /r            [386]

\c SBB r/m8,imm8                 ; 80 /3 ib             [8086]
\c SBB r/m16,imm16               ; o16 81 /3 iw         [8086]
\c SBB r/m32,imm32               ; o32 81 /3 id         [386]

\c SBB r/m16,imm8                ; o16 83 /3 ib         [8086]
\c SBB r/m32,imm8                ; o32 83 /3 ib         [8086]

\c SBB AL,imm8                   ; 1C ib                [8086]
\c SBB AX,imm16                  ; o16 1D iw            [8086]
\c SBB EAX,imm32                 ; o32 1D id            [386]

\c{SBB} performs integer subtraction: it subtracts its second
operand, plus the value of the carry flag, from its first, and
leaves the result in its destination (first) operand. The flags are
set according to the result of the operation: in particular, the
carry flag is affected and can be used by a subsequent \c{SBB}
instruction.

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

To subtract one number from another without also subtracting the
contents of the carry flag, use \c{SUB} (\k{insSUB}).

\H{insSCASB} \i\c{SCASB}, \i\c{SCASW}, \i\c{SCASD}: Scan String

\c SCASB                         ; AE                   [8086]
\c SCASW                         ; o16 AF               [8086]
\c SCASD                         ; o32 AF               [386]

\c{SCASB} compares the byte in \c{AL} with the byte at \c{[ES:DI]}
or \c{[ES:EDI]}, and sets the flags accordingly. It then increments
or decrements (depending on the direction flag: increments if the
flag is clear, decrements if it is set) \c{DI} (or \c{EDI}).

The register used is \c{DI} if the address size is 16 bits, and
\c{EDI} if it is 32 bits. If you need to use an address size not
equal to the current \c{BITS} setting, you can use an explicit
\i\c{a16} or \i\c{a32} prefix.

Segment override prefixes have no effect for this instruction: the
use of \c{ES} for the load from \c{[DI]} or \c{[EDI]} cannot be
overridden.

\c{SCASW} and \c{SCASD} work in the same way, but they compare a
word to \c{AX} or a doubleword to \c{EAX} instead of a byte to
\c{AL}, and increment or decrement the addressing registers by 2 or
4 instead of 1.

The \c{REPE} and \c{REPNE} prefixes (equivalently, \c{REPZ} and
\c{REPNZ}) may be used to repeat the instruction up to \c{CX} (or
\c{ECX} - again, the address size chooses which) times until the
first unequal or equal byte is found.

\H{insSETcc} \i\c{SETcc}: Set Register from Condition

\c SETcc r/m8                    ; 0F 90+cc /2          [386]

\c{SETcc} sets the given 8-bit operand to zero if its condition is
not satisfied, and to 1 if it is.

\H{insSGDT} \i\c{SGDT}, \i\c{SIDT}, \i\c{SLDT}: Store Descriptor Table Pointers

\c SGDT mem                      ; 0F 01 /0             [286,PRIV]
\c SIDT mem                      ; 0F 01 /1             [286,PRIV]
\c SLDT r/m16                    ; 0F 00 /0             [286,PRIV]

\c{SGDT} and \c{SIDT} both take a 6-byte memory area as an operand:
they store the contents of the GDTR (global descriptor table
register) or IDTR (interrupt descriptor table register) into that
area as a 32-bit linear address and a 16-bit size limit from that
area (in that order). These are the only instructions which directly
use \e{linear} addresses, rather than segment/offset pairs.

\c{SLDT} stores the segment selector corresponding to the LDT (local
descriptor table) into the given operand.

See also \c{LGDT}, \c{LIDT} and \c{LLDT} (\k{insLGDT}).

\H{insSHL} \i\c{SHL}, \i\c{SHR}: Bitwise Logical Shifts

\c SHL r/m8,1                    ; D0 /4                [8086]
\c SHL r/m8,CL                   ; D2 /4                [8086]
\c SHL r/m8,imm8                 ; C0 /4 ib             [286]
\c SHL r/m16,1                   ; o16 D1 /4            [8086]
\c SHL r/m16,CL                  ; o16 D3 /4            [8086]
\c SHL r/m16,imm8                ; o16 C1 /4 ib         [286]
\c SHL r/m32,1                   ; o32 D1 /4            [386]
\c SHL r/m32,CL                  ; o32 D3 /4            [386]
\c SHL r/m32,imm8                ; o32 C1 /4 ib         [386]

\c SHR r/m8,1                    ; D0 /5                [8086]
\c SHR r/m8,CL                   ; D2 /5                [8086]
\c SHR r/m8,imm8                 ; C0 /5 ib             [286]
\c SHR r/m16,1                   ; o16 D1 /5            [8086]
\c SHR r/m16,CL                  ; o16 D3 /5            [8086]
\c SHR r/m16,imm8                ; o16 C1 /5 ib         [286]
\c SHR r/m32,1                   ; o32 D1 /5            [386]
\c SHR r/m32,CL                  ; o32 D3 /5            [386]
\c SHR r/m32,imm8                ; o32 C1 /5 ib         [386]

\c{SHL} and \c{SHR} perform a logical shift operation on the given
source/destination (first) operand. The vacated bits are filled with
zero.

A synonym for \c{SHL} is \c{SAL} (see \k{insSAL}). NASM will
assemble either one to the same code, but NDISASM will always
disassemble that code as \c{SHL}.

The number of bits to shift by is given by the second operand. Only
the bottom 3, 4 or 5 bits (depending on the source operand size) of
the shift count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a \c{C1}
byte) form of \c{SHL foo,1} by using a \c{BYTE} prefix: \c{SHL
foo,BYTE 1}. Similarly with \c{SHR}.

\H{insSHLD} \i\c{SHLD}, \i\c{SHRD}: Bitwise Double-Precision Shifts

\c SHLD r/m16,reg16,imm8         ; o16 0F A4 /r ib      [386]
\c SHLD r/m16,reg32,imm8         ; o32 0F A4 /r ib      [386]
\c SHLD r/m16,reg16,CL           ; o16 0F A5 /r         [386]
\c SHLD r/m16,reg32,CL           ; o32 0F A5 /r         [386]

\c SHRD r/m16,reg16,imm8         ; o16 0F AC /r ib      [386]
\c SHRD r/m32,reg32,imm8         ; o32 0F AC /r ib      [386]
\c SHRD r/m16,reg16,CL           ; o16 0F AD /r         [386]
\c SHRD r/m32,reg32,CL           ; o32 0F AD /r         [386]

\c{SHLD} performs a double-precision left shift. It notionally places
its second operand to the right of its first, then shifts the entire
bit string thus generated to the left by a number of bits specified
in the third operand. It then updates only the \e{first} operand
according to the result of this. The second operand is not modified.

\c{SHRD} performs the corresponding right shift: it notionally
places the second operand to the \e{left} of the first, shifts the
whole bit string right, and updates only the first operand.

For example, if \c{EAX} holds \c{0x01234567} and \c{EBX} holds
\c{0x89ABCDEF}, then the instruction \c{SHLD EAX,EBX,4} would update
\c{EAX} to hold \c{0x12345678}. Under the same conditions, \c{SHRD
EAX,EBX,4} would update \c{EAX} to hold \c{0xF0123456}.

The number of bits to shift by is given by the third operand. Only
the bottom 5 bits of the shift count are considered.

\H{insSMI} \i\c{SMI}: System Management Interrupt

\c SMI                           ; F1                   [386,UNDOC]

This is an opcode apparently supported by some AMD processors (which
is why it can generate the same opcode as \c{INT1}), and places the
machine into system-management mode, a special debugging mode.

\H{insSMSW} \i\c{SMSW}: Store Machine Status Word

\c SMSW r/m16                    ; 0F 01 /4             [286,PRIV]

\c{SMSW} stores the bottom half of the \c{CR0} control register (or
the Machine Status Word, on 286 processors) into the destination
operand. See also \c{LMSW} (\k{insLMSW}).

\H{insSTC} \i\c{STC}, \i\c{STD}, \i\c{STI}: Set Flags

\c STC                           ; F9                   [8086]
\c STD                           ; FD                   [8086]
\c STI                           ; FB                   [8086]

These instructions set various flags. \c{STC} sets the carry flag;
\c{STD} sets the direction flag; and \c{STI} sets the interrupt flag
(thus enabling interrupts).

To clear the carry, direction, or interrupt flags, use the \c{CLC},
\c{CLD} and \c{CLI} instructions (\k{insCLC}). To invert the carry
flag, use \c{CMC} (\k{insCMC}).

\H{insSTOSB} \i\c{STOSB}, \i\c{STOSW}, \i\c{STOSD}: Store Byte to String

\c STOSB                         ; AA                   [8086]
\c STOSW                         ; o16 AB               [8086]
\c STOSD                         ; o32 AB               [386]

\c{STOSB} stores the byte in \c{AL} at \c{[ES:DI]} or \c{[ES:EDI]},
and sets the flags accordingly. It then increments or decrements
(depending on the direction flag: increments if the flag is clear,
decrements if it is set) \c{DI} (or \c{EDI}).

The register used is \c{DI} if the address size is 16 bits, and
\c{EDI} if it is 32 bits. If you need to use an address size not
equal to the current \c{BITS} setting, you can use an explicit
\i\c{a16} or \i\c{a32} prefix.

Segment override prefixes have no effect for this instruction: the
use of \c{ES} for the store to \c{[DI]} or \c{[EDI]} cannot be
overridden.

\c{STOSW} and \c{STOSD} work in the same way, but they store the
word in \c{AX} or the doubleword in \c{EAX} instead of the byte in
\c{AL}, and increment or decrement the addressing registers by 2 or
4 instead of 1.

The \c{REP} prefix may be used to repeat the instruction \c{CX} (or
\c{ECX} - again, the address size chooses which) times.

\H{insSTR} \i\c{STR}: Store Task Register

\c STR r/m16                     ; 0F 00 /1             [286,PRIV]

\c{STR} stores the segment selector corresponding to the contents of
the Task Register into its operand.

\H{insSUB} \i\c{SUB}: Subtract Integers

\c SUB r/m8,reg8                 ; 28 /r                [8086]
\c SUB r/m16,reg16               ; o16 29 /r            [8086]
\c SUB r/m32,reg32               ; o32 29 /r            [386]

\c SUB reg8,r/m8                 ; 2A /r                [8086]
\c SUB reg16,r/m16               ; o16 2B /r            [8086]
\c SUB reg32,r/m32               ; o32 2B /r            [386]

\c SUB r/m8,imm8                 ; 80 /5 ib             [8086]
\c SUB r/m16,imm16               ; o16 81 /5 iw         [8086]
\c SUB r/m32,imm32               ; o32 81 /5 id         [386]

\c SUB r/m16,imm8                ; o16 83 /5 ib         [8086]
\c SUB r/m32,imm8                ; o32 83 /5 ib         [386]

\c SUB AL,imm8                   ; 2C ib                [8086]
\c SUB AX,imm16                  ; o16 2D iw            [8086]
\c SUB EAX,imm32                 ; o32 2D id            [386]

\c{SUB} performs integer subtraction: it subtracts its second
operand from its first, and leaves the result in its destination
(first) operand. The flags are set according to the result of the
operation: in particular, the carry flag is affected and can be used
by a subsequent \c{SBB} instruction (\k{insSBB}).

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

\H{insTEST} \i\c{TEST}: Test Bits (notional bitwise AND)

\c TEST r/m8,reg8                ; 84 /r                [8086]
\c TEST r/m16,reg16              ; o16 85 /r            [8086]
\c TEST r/m32,reg32              ; o32 85 /r            [386]

\c TEST r/m8,imm8                ; F6 /7 ib             [8086]
\c TEST r/m16,imm16              ; o16 F7 /7 iw         [8086]
\c TEST r/m32,imm32              ; o32 F7 /7 id         [386]

\c TEST AL,imm8                  ; A8 ib                [8086]
\c TEST AX,imm16                 ; o16 A9 iw            [8086]
\c TEST EAX,imm32                ; o32 A9 id            [386]

\c{TEST} performs a `mental' bitwise AND of its two operands, and
affects the flags as if the operation had taken place, but does not
store the result of the operation anywhere.

\H{insUMOV} \i\c{UMOV}: User Move Data

\c UMOV r/m8,reg8                ; 0F 10 /r             [386,UNDOC]
\c UMOV r/m16,reg16              ; o16 0F 11 /r         [386,UNDOC]
\c UMOV r/m32,reg32              ; o32 0F 11 /r         [386,UNDOC]

\c UMOV reg8,r/m8                ; 0F 12 /r             [386,UNDOC]
\c UMOV reg16,r/m16              ; o16 0F 13 /r         [386,UNDOC]
\c UMOV reg32,r/m32              ; o32 0F 13 /r         [386,UNDOC]

This undocumented instruction is used by in-circuit emulators to
access user memory (as opposed to host memory). It is used just like
an ordinary memory/register or register/register \c{MOV}
instruction, but accesses user space.

\H{insVERR} \i\c{VERR}, \i\c{VERW}: Verify Segment Readability/Writability

\c VERR r/m16                    ; 0F 00 /4             [286,PRIV]

\c VERW r/m16                    ; 0F 00 /5             [286,PRIV]

\c{VERR} sets the zero flag if the segment specified by the selector
in its operand can be read from at the current privilege level.
\c{VERW} sets the zero flag if the segment can be written.

\H{insWAIT} \i\c{WAIT}: Wait for Floating-Point Processor

\c WAIT                          ; 9B                   [8086]

\c{WAIT}, on 8086 systems with a separate 8087 FPU, waits for the
FPU to have finished any operation it is engaged in before
continuing main processor operations, so that (for example) an FPU
store to main memory can be guaranteed to have completed before the
CPU tries to read the result back out.

On higher processors, \c{WAIT} is unnecessary for this purpose, and
it has the alternative purpose of ensuring that any pending unmasked
FPU exceptions have happened before execution continues.

\H{insWBINVD} \i\c{WBINVD}: Write Back and Invalidate Cache

\c WBINVD                        ; 0F 09                [486]

\c{WBINVD} invalidates and empties the processor's internal caches,
and causes the processor to instruct external caches to do the same.
It writes the contents of the caches back to memory first, so no
data is lost. To flush the caches quickly without bothering to write
the data back first, use \c{INVD} (\k{insINVD}).

\H{insWRMSR} \i\c{WRMSR}: Write Model-Specific Registers

\c WRMSR                         ; 0F 30                [PENT]

\c{WRMSR} writes the value in \c{EDX:EAX} to the processor
Model-Specific Register (MSR) whose index is stored in \c{ECX}. See
also \c{RDMSR} (\k{insRDMSR}).

\H{insXADD} \i\c{XADD}: Exchange and Add

\c XADD r/m8,reg8                ; 0F C0 /r             [486]
\c XADD r/m16,reg16              ; o16 0F C1 /r         [486]
\c XADD r/m32,reg32              ; o32 0F C1 /r         [486]

\c{XADD} exchanges the values in its two operands, and then adds
them together and writes the result into the destination (first)
operand. This instruction can be used with a \c{LOCK} prefix for
multi-processor synchronisation purposes.

\H{insXBTS} \i\c{XBTS}: Extract Bit String

\c XBTS reg16,r/m16              ; o16 0F A6 /r         [386,UNDOC]
\c XBTS reg32,r/m32              ; o32 0F A6 /r         [386,UNDOC]

No clear documentation seems to be available for this instruction:
the best I've been able to find reads `Takes a string of bits from
the first operand and puts them in the second operand'. It is
present only in early 386 processors, and conflicts with the opcodes
for \c{CMPXCHG486}. NASM supports it only for completeness. Its
counterpart is \c{IBTS} (see \k{insIBTS}).

\H{insXCHG} \i\c{XCHG}: Exchange

\c XCHG reg8,r/m8                ; 86 /r                [8086]
\c XCHG reg16,r/m8               ; o16 87 /r            [8086]
\c XCHG reg32,r/m32              ; o32 87 /r            [386]

\c XCHG r/m8,reg8                ; 86 /r                [8086]
\c XCHG r/m16,reg16              ; o16 87 /r            [8086]
\c XCHG r/m32,reg32              ; o32 87 /r            [386]

\c XCHG AX,reg16                 ; o16 90+r             [8086]
\c XCHG EAX,reg32                ; o32 90+r             [386]
\c XCHG reg16,AX                 ; o16 90+r             [8086]
\c XCHG reg32,EAX                ; o32 90+r             [386]

\c{XCHG} exchanges the values in its two operands. It can be used
with a \c{LOCK} prefix for purposes of multi-processor
synchronisation.

\c{XCHG AX,AX} or \c{XCHG EAX,EAX} (depending on the \c{BITS}
setting) generates the opcode \c{90h}, and so is a synonym for
\c{NOP} (\k{insNOP}).

\H{insXLATB} \i\c{XLATB}: Translate Byte in Lookup Table

\c XLATB                         ; D7                   [8086]

\c{XLATB} adds the value in \c{AL}, treated as an unsigned byte, to
\c{BX} or \c{EBX}, and loads the byte from the resulting address (in
the segment specified by \c{DS}) back into \c{AL}.

The base register used is \c{BX} if the address size is 16 bits, and
\c{EBX} if it is 32 bits. If you need to use an address size not
equal to the current \c{BITS} setting, you can use an explicit
\i\c{a16} or \i\c{a32} prefix.

The segment register used to load from \c{[BX+AL]} or \c{[EBX+AL]}
can be overridden by using a segment register name as a prefix (for
example, \c{es xlatb}).

\H{insXOR} \i\c{XOR}: Bitwise Exclusive OR

\c XOR r/m8,reg8                 ; 30 /r                [8086]
\c XOR r/m16,reg16               ; o16 31 /r            [8086]
\c XOR r/m32,reg32               ; o32 31 /r            [386]

\c XOR reg8,r/m8                 ; 32 /r                [8086]
\c XOR reg16,r/m16               ; o16 33 /r            [8086]
\c XOR reg32,r/m32               ; o32 33 /r            [386]

\c XOR r/m8,imm8                 ; 80 /6 ib             [8086]
\c XOR r/m16,imm16               ; o16 81 /6 iw         [8086]
\c XOR r/m32,imm32               ; o32 81 /6 id         [386]

\c XOR r/m16,imm8                ; o16 83 /6 ib         [8086]
\c XOR r/m32,imm8                ; o32 83 /6 ib         [386]

\c XOR AL,imm8                   ; 34 ib                [8086]
\c XOR AX,imm16                  ; o16 35 iw            [8086]
\c XOR EAX,imm32                 ; o32 35 id            [386]

\c{XOR} performs a bitwise XOR operation between its two operands
(i.e. each bit of the result is 1 if and only if exactly one of the
corresponding bits of the two inputs was 1), and stores the result
in the destination (first) operand.

In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign-extended to the length of the first operand. In these cases,
the \c{BYTE} qualifier is necessary to force NASM to generate this
form of the instruction.

The MMX instruction \c{PXOR} (see \k{insPXOR}) performs the same
operation on the 64-bit MMX registers.
