The Netwide Assembler: NASM
===========================
Chapter 1: Introduction
-----------------------
1.1 What Is NASM?
The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler
designed for portability and modularity. It supports a range of
object file formats, including Linux and `*BSD' `a.out', `ELF',
`COFF', `Mach-O', Microsoft 16-bit `OBJ', `Win32' and `Win64'. 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 all currently known x86 architectural extensions, and
has strong support for macros.
1.1.1 Why Yet Another Assembler?
The Netwide Assembler grew out of an idea on `comp.lang.asm.x86' (or
possibly `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.
(*) `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.
(*) `gas' is free, and ports over to DOS and Unix, but it's not very
good, since it's designed to be a back end to `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 _write_ anything in it. Plus you can't write
16-bit code in it (properly.)
(*) `as86' is specific to Minix and Linux, and (my version at least)
doesn't seem to have much (or any) documentation.
(*) `MASM' isn't very good, and it's (was) expensive, and it runs
only under DOS.
(*) `TASM' is better, but still strives for MASM compatibility,
which means millions of directives and tons of red tape. And its
syntax is essentially 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, _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.
1.1.2 License Conditions
Please see the file `LICENSE', supplied as part of any NASM
distribution archive, for the license conditions under which you may
use NASM. NASM is now under the so-called 2-clause BSD license, also
known as the simplified BSD license.
Copyright 1996-2011 the NASM Authors - All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
(*) Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
(*) Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials provided
with the distribution.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
1.2 Contact Information
The current version of NASM (since about 0.98.08) is maintained by a
team of developers, accessible through the `nasm-devel' mailing list
(see below for the link). If you want to report a bug, please read
section 12.2 first.
NASM has a website at `http://www.nasm.us/'. If it's not there,
google for us!
New releases, release candidates, and daily development snapshots of
NASM are available from the official web site.
Announcements are posted to `comp.lang.asm.x86', and to the web site
`http://www.freshmeat.net/'.
If you want information about the current development status, please
subscribe to the `nasm-devel' email list; see link from the website.
1.3 Installation
1.3.1 Installing NASM under MS-DOS or Windows
Once you've obtained the appropriate archive for NASM,
`nasm-XXX-dos.zip' or `nasm-XXX-win32.zip' (where `XXX' denotes the
version number of NASM contained in the archive), unpack it into its
own directory (for example `c:\nasm').
The archive will contain a set of executable files: the NASM
executable file `nasm.exe', the NDISASM executable file
`ndisasm.exe', and possibly additional utilities to handle the RDOFF
file format.
The only file NASM needs to run is its own executable, so copy
`nasm.exe' to a directory on your PATH, or alternatively edit
`autoexec.bat' to add the `nasm' directory to your `PATH' (to do
that under Windows XP, go to Start > Control Panel > System >
Advanced > Environment Variables; these instructions may work under
other versions of Windows as well.)
That's it - NASM is installed. You don't need the nasm directory to
be present to run NASM (unless you've added it to your `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 DOS source archive, `nasm-XXX.zip', the
`nasm' directory will also contain the full NASM source code, and a
selection of Makefiles you can (hopefully) use to rebuild your copy
of NASM from scratch. See the file `INSTALL' in the source archive.
Note that a number of files are generated from other files by Perl
scripts. Although the NASM source distribution includes these
generated files, you will need to rebuild them (and hence, will need
a Perl interpreter) if you change insns.dat, standard.mac or the
documentation. It is possible future source distributions may not
include these files at all. Ports of Perl for a variety of
platforms, including DOS and Windows, are available from
www.cpan.org.
1.3.2 Installing NASM under Unix
Once you've obtained the Unix source archive for NASM,
`nasm-XXX.tar.gz' (where `XXX' denotes the version number of NASM
contained in the archive), unpack it into a directory such as
`/usr/local/src'. The archive, when unpacked, will create its own
subdirectory `nasm-XXX'.
NASM is an auto-configuring package: once you've unpacked it, `cd'
to the directory it's been unpacked into and type `./configure'.
This shell script will find the best C compiler to use for building
NASM and set up Makefiles accordingly.
Once NASM has auto-configured, you can type `make' to build the
`nasm' and `ndisasm' binaries, and then `make install' to install
them in `/usr/local/bin' and install the man pages `nasm.1' and
`ndisasm.1' in `/usr/local/man/man1'. Alternatively, you can give
options such as `--prefix' to the configure script (see the file
`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 `rdoff' subdirectory of
the NASM archive. You can build these with `make rdf' and install
them with `make rdf_install', if you want them.
Chapter 2: Running NASM
-----------------------
2.1 NASM Command-Line Syntax
To assemble a file, you issue a command of the form
nasm -f <format> <filename> [-o <output>]
For example,
nasm -f elf myfile.asm
will assemble `myfile.asm' into an `ELF' object file `myfile.o'. And
nasm -f bin myfile.asm -o myfile.com
will assemble `myfile.asm' into a raw binary file `myfile.com'.
To produce a listing file, with the hex codes output from NASM
displayed on the left of the original sources, use the `-l' option
to give a listing file name, for example:
nasm -f coff myfile.asm -l myfile.lst
To get further usage instructions from NASM, try typing
nasm -h
As `-hf', this will also list the available output file formats, and
what they are.
If you use Linux but aren't sure whether your system is `a.out' or
`ELF', type
file nasm
(in the directory in which you put the NASM binary when you
installed it). If it says something like
nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1
then your system is `ELF', and you should use the option `-f elf'
when you want NASM to produce Linux object files. If it says
nasm: Linux/i386 demand-paged executable (QMAGIC)
or something similar, your system is `a.out', and you should use
`-f aout' instead (Linux `a.out' systems have long been obsolete,
and are rare these days.)
Like Unix compilers and assemblers, NASM is silent unless it goes
wrong: you won't see any output at all, unless it gives error
messages.
2.1.1 The `-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 (`obj', `win32' and `win64'), it
will remove the `.asm' extension (or whatever extension you like to
use - NASM doesn't care) from your source file name and substitute
`.obj'. For Unix object file formats (`aout', `as86', `coff',
`elf32', `elf64', `elfx32', `ieee', `macho32' and `macho64') it will
substitute `.o'. For `dbg', `rdf', `ith' and `srec', it will use
`.dbg', `.rdf', `.ith' and `.srec', respectively, and for the `bin'
format it will simply remove the extension, so that `myfile.asm'
produces the output file `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 `nasm.out' as the output file name instead.
For situations in which this behaviour is unacceptable, NASM
provides the `-o' command-line option, which allows you to specify
your desired output file name. You invoke `-o' by following it with
the name you wish for the output file, either with or without an
intervening space. For example:
nasm -f bin program.asm -o program.com
nasm -f bin driver.asm -odriver.sys
Note that this is a small o, and is different from a capital O ,
which is used to specify the number of optimisation passes required.
See section 2.1.22.
2.1.2 The `-f' Option: Specifying the Output File Format
If you do not supply the `-f' option to NASM, it will choose an
output file format for you itself. In the distribution versions of
NASM, the default is always `bin'; if you've compiled your own copy
of NASM, you can redefine `OF_DEFAULT' at compile time and choose
what you want the default to be.
Like `-o', the intervening space between `-f' and the output file
format is optional; so `-f elf' and `-felf' are both valid.
A complete list of the available output file formats can be given by
issuing the command `nasm -hf'.
2.1.3 The `-l' Option: Generating a Listing File
If you supply the `-l' option to NASM, followed (with the usual
optional space) by a file name, NASM will generate a 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 section 4.3.11) on the right. For example:
nasm -f elf myfile.asm -l myfile.lst
If a list file is selected, you may turn off listing for a section
of your source with `[list -]', and turn it back on with `[list +]',
(the default, obviously). There is no "user form" (without the
brackets). This can be used to list only sections of interest,
avoiding excessively long listings.
2.1.4 The `-M' Option: Generate Makefile Dependencies
This option can be used to generate makefile dependencies on stdout.
This can be redirected to a file for further processing. For
example:
nasm -M myfile.asm > myfile.dep
2.1.5 The `-MG' Option: Generate Makefile Dependencies
This option can be used to generate makefile dependencies on stdout.
This differs from the `-M' option in that if a nonexisting file is
encountered, it is assumed to be a generated file and is added to
the dependency list without a prefix.
2.1.6 The `-MF' Option: Set Makefile Dependency File
This option can be used with the `-M' or `-MG' options to send the
output to a file, rather than to stdout. For example:
nasm -M -MF myfile.dep myfile.asm
2.1.7 The `-MD' Option: Assemble and Generate Dependencies
The `-MD' option acts as the combination of the `-M' and `-MF'
options (i.e. a filename has to be specified.) However, unlike the
`-M' or `-MG' options, `-MD' does _not_ inhibit the normal operation
of the assembler. Use this to automatically generate updated
dependencies with every assembly session. For example:
nasm -f elf -o myfile.o -MD myfile.dep myfile.asm
2.1.8 The `-MT' Option: Dependency Target Name
The `-MT' option can be used to override the default name of the
dependency target. This is normally the same as the output filename,
specified by the `-o' option.
2.1.9 The `-MQ' Option: Dependency Target Name (Quoted)
The `-MQ' option acts as the `-MT' option, except it tries to quote
characters that have special meaning in Makefile syntax. This is not
foolproof, as not all characters with special meaning are quotable
in Make. The default output (if no `-MT' or `-MQ' option is
specified) is automatically quoted.
2.1.10 The `-MP' Option: Emit phony targets
When used with any of the dependency generation options, the `-MP'
option causes NASM to emit a phony target without dependencies for
each header file. This prevents Make from complaining if a header
file has been removed.
2.1.11 The `-F' Option: Selecting a Debug Information Format
This option is used to select the format of the debug information
emitted into the output file, to be used by a debugger (or _will_
be). Prior to version 2.03.01, the use of this switch did _not_
enable output of the selected debug info format. Use `-g', see
section 2.1.12, to enable output. Versions 2.03.01 and later
automatically enable `-g' if `-F' is specified.
A complete list of the available debug file formats for an output
format can be seen by issuing the command `nasm -f <format> -y'. Not
all output formats currently support debugging output. See section
2.1.26.
This should not be confused with the `-f dbg' output format option
which is not built into NASM by default. For information on how to
enable it when building from the sources, see section 7.14.
2.1.12 The `-g' Option: Enabling Debug Information.
This option can be used to generate debugging information in the
specified format. See section 2.1.11. Using `-g' without `-F'
results in emitting debug info in the default format, if any, for
the selected output format. If no debug information is currently
implemented in the selected output format, `-g' is _silently
ignored_.
2.1.13 The `-X' Option: Selecting an Error Reporting Format
This option can be used to select an error reporting format for any
error messages that might be produced by NASM.
Currently, two error reporting formats may be selected. They are the
`-Xvc' option and the `-Xgnu' option. The GNU format is the default
and looks like this:
filename.asm:65: error: specific error message
where `filename.asm' is the name of the source file in which the
error was detected, `65' is the source file line number on which the
error was detected, `error' is the severity of the error (this could
be `warning'), and `specific error message' is a more detailed text
message which should help pinpoint the exact problem.
The other format, specified by `-Xvc' is the style used by Microsoft
Visual C++ and some other programs. It looks like this:
filename.asm(65) : error: specific error message
where the only difference is that the line number is in parentheses
instead of being delimited by colons.
See also the `Visual C++' output format, section 7.5.
2.1.14 The `-Z' Option: Send Errors to a File
Under `MS-DOS' it can be difficult (though there are ways) to
redirect the standard-error output of a program to a file. Since
NASM usually produces its warning and error messages on `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 `-Z' option, taking a filename argument
which causes errors to be sent to the specified files rather than
standard error. Therefore you can redirect the errors into a file by
typing
nasm -Z myfile.err -f obj myfile.asm
In earlier versions of NASM, this option was called `-E', but it was
changed since `-E' is an option conventionally used for
preprocessing only, with disastrous results. See section 2.1.20.
2.1.15 The `-s' Option: Send Errors to `stdout'
The `-s' option redirects error messages to `stdout' rather than
`stderr', so it can be redirected under `MS-DOS'. To assemble the
file `myfile.asm' and pipe its output to the `more' program, you can
type:
nasm -s -f obj myfile.asm | more
See also the `-Z' option, section 2.1.14.
2.1.16 The `-i' Option: Include File Search Directories
When NASM sees the `%include' or `%pathsearch' directive in a source
file (see section 4.6.1, section 4.6.2 or section 3.2.3), 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 `-i' option. Therefore you can include files from a macro
library, for example, by typing
nasm -ic:\macrolib\ -f obj myfile.asm
(As usual, a space between `-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 `-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 perverse, by
noting that the option `-ifoo' will cause `%include "bar.i"' to
search for the file `foobar.i'...)
If you want to define a _standard_ include search path, similar to
`/usr/include' on Unix systems, you should place one or more `-i'
directives in the `NASMENV' environment variable (see section
2.1.28).
For Makefile compatibility with many C compilers, this option can
also be specified as `-I'.
2.1.17 The `-p' Option: Pre-Include a File
NASM allows you to specify files to be _pre-included_ into your
source file, by the use of the `-p' option. So running
nasm myfile.asm -p myinc.inc
is equivalent to running `nasm myfile.asm' and placing the directive
`%include "myinc.inc"' at the start of the file.
For consistency with the `-I', `-D' and `-U' options, this option
can also be specified as `-P'.
2.1.18 The `-d' Option: Pre-Define a Macro
Just as the `-p' option gives an alternative to placing `%include'
directives at the start of a source file, the `-d' option gives an
alternative to placing a `%define' directive. You could code
nasm myfile.asm -dFOO=100
as an alternative to placing the directive
%define FOO 100
at the start of the file. You can miss off the macro value, as well:
the option `-dFOO' is equivalent to coding `%define FOO'. This form
of the directive may be useful for selecting assembly-time options
which are then tested using `%ifdef', for example `-dDEBUG'.
For Makefile compatibility with many C compilers, this option can
also be specified as `-D'.
2.1.19 The `-u' Option: Undefine a Macro
The `-u' option undefines a macro that would otherwise have been
pre-defined, either automatically or by a `-p' or `-d' option
specified earlier on the command lines.
For example, the following command line:
nasm myfile.asm -dFOO=100 -uFOO
would result in `FOO' _not_ being a predefined macro in the program.
This is useful to override options specified at a different point in
a Makefile.
For Makefile compatibility with many C compilers, this option can
also be specified as `-U'.
2.1.20 The `-E' Option: Preprocess Only
NASM allows the preprocessor to be run on its own, up to a point.
Using the `-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 `-o'
option is also used).
This option cannot be applied to programs which require the
preprocessor to evaluate expressions which depend on the values of
symbols: so code such as
%assign tablesize ($-tablestart)
will cause an error in preprocess-only mode.
For compatiblity with older version of NASM, this option can also be
written `-e'. `-E' in older versions of NASM was the equivalent of
the current `-Z' option, section 2.1.14.
2.1.21 The `-a' Option: Don't Preprocess At All
If NASM is being used as the back end to a compiler, it might be
desirable to suppress preprocessing completely and assume the
compiler has already done it, to save time and increase compilation
speeds. The `-a' option, requiring no argument, instructs NASM to
replace its powerful preprocessor with a stub preprocessor which
does nothing.
2.1.22 The `-O' Option: Specifying Multipass Optimization
Using the `-O' option, you can tell NASM to carry out different
levels of optimization. The syntax is:
(*) `-O0': No optimization. All operands take their long forms, if a
short form is not specified, except conditional jumps. This is
intended to match NASM 0.98 behavior.
(*) `-O1': Minimal optimization. As above, but immediate operands
which will fit in a signed byte are optimized, unless the long
form is specified. Conditional jumps default to the long form
unless otherwise specified.
(*) `-Ox' (where `x' is the actual letter `x'): Multipass
optimization. Minimize branch offsets and signed immediate
bytes, overriding size specification unless the `strict' keyword
has been used (see section 3.7). For compatibility with earlier
releases, the letter `x' may also be any number greater than
one. This number has no effect on the actual number of passes.
The `-Ox' mode is recommended for most uses, and is the default
since NASM 2.09.
Note that this is a capital `O', and is different from a small `o',
which is used to specify the output file name. See section 2.1.1.
2.1.23 The `-t' Option: Enable TASM Compatibility Mode
NASM includes a limited form of compatibility with Borland's `TASM'.
When NASM's `-t' option is used, the following changes are made:
(*) local labels may be prefixed with `@@' instead of `.'
(*) size override is supported within brackets. In TASM compatible
mode, a size override inside square brackets changes the size of
the operand, and not the address type of the operand as it does
in NASM syntax. E.g. `mov eax,[DWORD val]' is valid syntax in
TASM compatibility mode. Note that you lose the ability to
override the default address type for the instruction.
(*) unprefixed forms of some directives supported (`arg', `elif',
`else', `endif', `if', `ifdef', `ifdifi', `ifndef', `include',
`local')
2.1.24 The `-w' and `-W' Options: Enable or Disable Assembly 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
`-w' command-line option, which enables or disables certain classes
of assembly warning. Such warning classes are described by a name,
for example `orphan-labels'; you can enable warnings of this class
by the command-line option `-w+orphan-labels' and disable it by
`-w-orphan-labels'.
The suppressible warning classes are:
(*) `macro-params' covers warnings about multi-line macros being
invoked with the wrong number of parameters. This warning class
is enabled by default; see section 4.3.1 for an example of why
you might want to disable it.
(*) `macro-selfref' warns if a macro references itself. This warning
class is disabled by default.
(*) `macro-defaults' warns when a macro has more default parameters
than optional parameters. This warning class is enabled by
default; see section 4.3.5 for why you might want to disable it.
(*) `orphan-labels' covers warnings about source lines which contain
no instruction but define a label without a trailing colon. NASM
warns about this somewhat obscure condition by default; see
section 3.1 for more information.
(*) `number-overflow' covers warnings about numeric constants which
don't fit in 64 bits. This warning class is enabled by default.
(*) `gnu-elf-extensions' warns if 8-bit or 16-bit relocations are
used in `-f elf' format. The GNU extensions allow this. This
warning class is disabled by default.
(*) `float-overflow' warns about floating point overflow. Enabled by
default.
(*) `float-denorm' warns about floating point denormals. Disabled by
default.
(*) `float-underflow' warns about floating point underflow. Disabled
by default.
(*) `float-toolong' warns about too many digits in floating-point
numbers. Enabled by default.
(*) `user' controls `%warning' directives (see section 4.9). Enabled
by default.
(*) `lock' warns about `LOCK' prefixes on unlockable instructions.
Enabled by default.
(*) `hle' warns about invalid use of the HLE `XACQUIRE' or
`XRELEASE' prefixes. Enabled by default.
(*) `bnd' warns about ineffective use of the `BND' prefix when a
relaxed form of jmp instruction becomes jmp short form. Enabled
by default.
(*) `error' causes warnings to be treated as errors. Disabled by
default.
(*) `all' is an alias for _all_ suppressible warning classes (not
including `error'). Thus, `-w+all' enables all available
warnings.
In addition, you can set warning classes across sections. Warning
classes may be enabled with `[warning +warning-name]', disabled with
`[warning -warning-name]' or reset to their original value with
`[warning *warning-name]'. No "user form" (without the brackets)
exists.
Since version 2.00, NASM has also supported the gcc-like syntax
`-Wwarning' and `-Wno-warning' instead of `-w+warning' and
`-w-warning', respectively.
2.1.25 The `-v' Option: Display Version Info
Typing `NASM -v' will display the version of NASM which you are
using, and the date on which it was compiled.
You will need the version number if you report a bug.
2.1.26 The `-y' Option: Display Available Debug Info Formats
Typing `nasm -f <option> -y' will display a list of the available
debug info formats for the given output format. The default format
is indicated by an asterisk. For example:
nasm -f elf -y
valid debug formats for 'elf32' output format are
('*' denotes default):
* stabs ELF32 (i386) stabs debug format for Linux
dwarf elf32 (i386) dwarf debug format for Linux
2.1.27 The `--prefix' and `--postfix' Options.
The `--prefix' and `--postfix' options prepend or append
(respectively) the given argument to all `global' or `extern'
variables. E.g. `--prefix _' will prepend the underscore to all
global and external variables, as C sometimes (but not always) likes
it.
2.1.28 The `NASMENV' Environment Variable
If you define an environment variable called `NASMENV', the program
will interpret it as a list of extra command-line options, which are
processed before the real command line. You can use this to define
standard search directories for include files, by putting `-i'
options in the `NASMENV' variable.
The value of the variable is split up at white space, so that the
value `-s -ic:\nasmlib\' will be treated as two separate options.
However, that means that the value `-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 `-dNAME="my' and `name"'.
To get round this, NASM provides a feature whereby, if you begin the
`NASMENV' environment variable with some character that isn't a
minus sign, then NASM will treat this character as the separator
character for options. So setting the `NASMENV' variable to the
value `!-s!-ic:\nasmlib\' is equivalent to setting it to
`-s -ic:\nasmlib\', but `!-dNAME="my name"' will work.
This environment variable was previously called `NASM'. This was
changed with version 0.98.31.
2.2 Quick Start for MASM Users
If you're used to writing programs with MASM, or with TASM in MASM-
compatible (non-Ideal) mode, or with `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.
2.2.1 NASM Is Case-Sensitive
One simple difference is that NASM is case-sensitive. It makes a
difference whether you call your label `foo', `Foo' or `FOO'. If
you're assembling to `DOS' or `OS/2' `.OBJ' files, you can invoke
the `UPPERCASE' directive (documented in section 7.4) to ensure that
all symbols exported to other code modules are forced to be upper
case; but even then, _within_ a single module, NASM will distinguish
between labels differing only in case.
2.2.2 NASM Requires Square Brackets For Memory References
NASM was designed with simplicity of syntax in mind. One of the
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,
foo equ 1
bar dw 2
then the two lines of code
mov ax,foo
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 _contents_ of a memory location requires square brackets around
the address, and any access to the _address_ of a variable doesn't.
So an instruction of the form `mov ax,foo' will _always_ refer to a
compile-time constant, whether it's an `EQU' or the address of a
variable; and to access the _contents_ of the variable `bar', you
must code `mov ax,[bar]'.
This also means that NASM has no need for MASM's `OFFSET' keyword,
since the MASM code `mov ax,offset bar' means exactly the same thing
as NASM's `mov ax,bar'. If you're trying to get large amounts of
MASM code to assemble sensibly under NASM, you can always code
`%idefine offset' to make the preprocessor treat the `OFFSET'
keyword as a no-op.
This issue is even more confusing in `a86', where declaring a label
with a trailing colon defines it to be a `label' as opposed to a
`variable' and causes `a86' to adopt NASM-style semantics; so in
`a86', `mov ax,var' has different behaviour depending on whether
`var' was declared as `var: dw 0' (a label) or `var dw 0' (a word-
size variable). NASM is very simple by comparison: _everything_ is a
label.
NASM, in the interests of simplicity, also does not support the
hybrid syntaxes supported by MASM and its clones, such as
`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 `mov ax,[table+bx]'. Likewise,
`mov ax,es:[di]' is wrong and `mov ax,[es:di]' is right.
2.2.3 NASM Doesn't Store Variable Types
NASM, by design, chooses not to remember the types of variables you
declare. Whereas MASM will remember, on seeing `var dw 0', that you
declared `var' as a word-size variable, and will then be able to
fill in the ambiguity in the size of the instruction `mov var,2',
NASM will deliberately remember nothing about the symbol `var'
except where it begins, and so you must explicitly code
`mov word [var],2'.
For this reason, NASM doesn't support the `LODS', `MOVS', `STOS',
`SCAS', `CMPS', `INS', or `OUTS' instructions, but only supports the
forms such as `LODSB', `MOVSW', and `SCASD', which explicitly
specify the size of the components of the strings being manipulated.
2.2.4 NASM Doesn't `ASSUME'
As part of NASM's drive for simplicity, it also does not support the
`ASSUME' directive. NASM will not keep track of what values you
choose to put in your segment registers, and will never
_automatically_ generate a segment override prefix.
2.2.5 NASM Doesn't Support 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 far call and which with a near
call, and is responsible for putting the correct form of `RET'
instruction (`RETN' or `RETF'; NASM accepts `RET' itself as an
alternate form for `RETN'); in addition, the programmer is
responsible for coding CALL FAR instructions where necessary when
calling _external_ functions, and must also keep track of which
external variable definitions are far and which are near.
2.2.6 Floating-Point Differences
NASM uses different names to refer to floating-point registers from
MASM: where MASM would call them `ST(0)', `ST(1)' and so on, and
`a86' would call them simply `0', `1' and so on, NASM chooses to
call them `st0', `st1' etc.
As of version 0.96, NASM now treats the instructions with `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.
2.2.7 Other Differences
For historical reasons, NASM uses the keyword `TWORD' where MASM and
compatible assemblers use `TBYTE'.
NASM does not declare uninitialized storage in the same way as MASM:
where a MASM programmer might use `stack db 64 dup (?)', NASM
requires `stack resb 64', intended to be read as `reserve 64 bytes'.
For a limited amount of compatibility, since NASM treats `?' as a
valid character in symbol names, you can code `? equ 0' and then
writing `dw ?' will at least do something vaguely useful. `DUP' is
still not a supported syntax, however.
In addition to all of this, macros and directives work completely
differently to MASM. See chapter 4 and chapter 6 for further
details.
Chapter 3: The NASM Language
----------------------------
3.1 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
chapter 4 and chapter 6) some combination of the four fields
label: instruction operands ; comment
As usual, most of these fields are optional; the presence or absence
of any combination of a label, an instruction and a comment is
allowed. Of course, the operand field is either required or
forbidden by the presence and nature of the instruction field.
NASM uses backslash (\) as the line continuation character; if a
line ends with backslash, the next line is considered to be a part
of the backslash-ended line.
NASM places no restrictions on white space within a line: labels may
have white space before them, or instructions may have no space
before them, or anything. The colon after a label is also optional.
(Note that this means that if you intend to code `lodsb' alone on a
line, and type `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 `-w+orphan-labels' will cause it to warn you if
you define a label alone on a line without a trailing colon.)
Valid characters in labels are letters, numbers, `_', `$', `#', `@',
`~', `.', and `?'. The only characters which may be used as the
_first_ character of an identifier are letters, `.' (with special
meaning: see section 3.9), `_' and `?'. An identifier may also be
prefixed with a `$' 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 `eax', you can refer to
`$eax' in NASM code to distinguish the symbol from the register.
Maximum length of an identifier is 4095 characters.
The instruction field may contain any machine instruction: Pentium
and P6 instructions, FPU instructions, MMX instructions and even
undocumented instructions are all supported. The instruction may be
prefixed by `LOCK', `REP', `REPE'/`REPZ', `REPNE'/`REPNZ',
`XACQUIRE'/`XRELEASE' or `BND'/`NOBND', in the usual way. Explicit
address-size and operand-size prefixes `A16', `A32', `A64', `O16'
and `O32', `O64' are provided - one example of their use is given in
chapter 10. You can also use the name of a segment register as an
instruction prefix: coding `es mov [bx],ax' is equivalent to coding
`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 `LODSB', which has no operands and yet can
require a segment override, there is no clean syntactic way to
proceed apart from `es lodsb'.
An instruction is not required to use a prefix: prefixes such as
`CS', `A32', `LOCK' or `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 section 3.2.
Instruction operands may take a number of forms: they can be
registers, described simply by the register name (e.g. `ax', `bp',
`ebx', `cr0': NASM does not use the `gas'-style syntax in which
register names must be prefixed by a `%' sign), or they can be
effective addresses (see section 3.3), constants (section 3.4) or
expressions (section 3.5).
For x87 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. For
example, you can code:
fadd st1 ; this sets st0 := st0 + st1
fadd st0,st1 ; so does this
fadd st1,st0 ; this sets st1 := st1 + st0
fadd to st1 ; so does this
Almost any x87 floating-point instruction that references memory
must use one of the prefixes `DWORD', `QWORD' or `TWORD' to indicate
what size of memory operand it refers to.
3.2 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 `DB', `DW', `DD', `DQ', `DT', `DO', `DY' and `DZ';
their uninitialized counterparts `RESB', `RESW', `RESD', `RESQ',
`REST', `RESO', `RESY' and `RESZ'; the `INCBIN' command, the `EQU'
command, and the `TIMES' prefix.
3.2.1 `DB' and Friends: Declaring Initialized Data
`DB', `DW', `DD', `DQ', `DT', `DO', `DY' and `DZ' are used, much as
in MASM, to declare initialized data in the output file. They can be
invoked in a wide range of ways:
db 0x55 ; just the byte 0x55
db 0x55,0x56,0x57 ; three bytes in succession
db 'a',0x55 ; character constants are OK
db 'hello',13,10,'$' ; so are string constants
dw 0x1234 ; 0x34 0x12
dw 'a' ; 0x61 0x00 (it's just a number)
dw 'ab' ; 0x61 0x62 (character constant)
dw 'abc' ; 0x61 0x62 0x63 0x00 (string)
dd 0x12345678 ; 0x78 0x56 0x34 0x12
dd 1.234567e20 ; floating-point constant
dq 0x123456789abcdef0 ; eight byte constant
dq 1.234567e20 ; double-precision float
dt 1.234567e20 ; extended-precision float
`DT', `DO', `DY' and `DZ' do not accept numeric constants as
operands.
3.2.2 `RESB' and Friends: Declaring Uninitialized Data
`RESB', `RESW', `RESD', `RESQ', `REST', `RESO', `RESY' and `RESZ'
are designed to be used in the BSS section of a module: they declare
_uninitialized_ storage space. Each takes a single operand, which is
the number of bytes, words, doublewords or whatever to reserve. As
stated in section 2.2.7, NASM does not support the MASM/TASM syntax
of reserving uninitialized space by writing `DW ?' or similar
things: this is what it does instead. The operand to a `RESB'-type
pseudo-instruction is a _critical expression_: see section 3.8.
For example:
buffer: resb 64 ; reserve 64 bytes
wordvar: resw 1 ; reserve a word
realarray resq 10 ; array of ten reals
ymmval: resy 1 ; one YMM register
zmmvals: resz 32 ; 32 ZMM registers
3.2.3 `INCBIN': Including External Binary Files
`INCBIN' is borrowed from the old Amiga assembler DevPac: it
includes a binary file verbatim into the output file. This can be
handy for (for example) including graphics and sound data directly
into a game executable file. It can be called in one of these three
ways:
incbin "file.dat" ; include the whole file
incbin "file.dat",1024 ; skip the first 1024 bytes
incbin "file.dat",1024,512 ; skip the first 1024, and
; actually include at most 512
`INCBIN' is both a directive and a standard macro; the standard
macro version searches for the file in the include file search path
and adds the file to the dependency lists. This macro can be
overridden if desired.
3.2.4 `EQU': Defining Constants
`EQU' defines a symbol to a given constant value: when `EQU' is
used, the source line must contain a label. The action of `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,
message db 'hello, world'
msglen equ $-message
defines `msglen' to be the constant 12. `msglen' may not then be
redefined later. This is not a preprocessor definition either: the
value of `msglen' is evaluated _once_, using the value of `$' (see
section 3.5 for an explanation of `$') at the point of definition,
rather than being evaluated wherever it is referenced and using the
value of `$' at the point of reference.
3.2.5 `TIMES': Repeating Instructions or Data
The `TIMES' prefix causes the instruction to be assembled multiple
times. This is partly present as NASM's equivalent of the `DUP'
syntax supported by MASM-compatible assemblers, in that you can code
zerobuf: times 64 db 0
or similar things; but `TIMES' is more versatile than that. The
argument to `TIMES' is not just a numeric constant, but a numeric
_expression_, so you can do things like
buffer: db 'hello, world'
times 64-$+buffer db ' '
which will store exactly enough spaces to make the total length of
`buffer' up to 64. Finally, `TIMES' can be applied to ordinary
instructions, so you can code trivial unrolled loops in it:
times 100 movsb
Note that there is no effective difference between
`times 100 resb 1' and `resb 100', except that the latter will be
assembled about 100 times faster due to the internal structure of
the assembler.
The operand to `TIMES' is a critical expression (section 3.8).
Note also that `TIMES' can't be applied to macros: the reason for
this is that `TIMES' is processed after the macro phase, which
allows the argument to `TIMES' to contain expressions such as
`64-$+buffer' as above. To repeat more than one line of code, or a
complex macro, use the preprocessor `%rep' directive.
3.3 Effective Addresses
An effective address is any operand to an instruction which
references memory. Effective addresses, in NASM, have a very simple
syntax: they consist of an expression evaluating to the desired
address, enclosed in square brackets. For example:
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 `[eax*2+0]' and `[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 `[eax+ebx]' and
`[ebx+eax]' to generate different opcodes; this is occasionally
useful because `[esi+ebp]' and `[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 `BYTE', `WORD', `DWORD'
and `NOSPLIT'. If you need `[eax+3]' to be assembled using a double-
word offset field instead of the one byte NASM will normally
generate, you can code `[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 section 3.8 for an example of such a code
fragment) by using `[byte eax+offset]'. As special cases,
`[byte eax]' will code `[eax+0]' with a byte offset of zero, and
`[dword eax]' will code it with a double-word offset of zero. The
normal form, `[eax]', will be coded with no offset field.
The form described in the previous paragraph is also useful if you
are trying to access data in a 32-bit segment from within 16 bit
code. For more information on this see the section on mixed-size
addressing (section 10.2). In particular, if you need to access data
with a known offset that is larger than will fit in a 16-bit value,
if you don't specify that it is a dword offset, nasm will cause the
high word of the offset to be lost.
Similarly, NASM will split `[eax*2]' into `[eax+eax]' because that
allows the offset field to be absent and space to be saved; in fact,
it will also split `[eax*2+offset]' into `[eax+eax+offset]'. You can
combat this behaviour by the use of the `NOSPLIT' keyword:
`[nosplit eax*2]' will force `[eax*2+0]' to be generated literally.
`[nosplit eax*1]' also has the same effect. In another way, a split
EA form `[0, eax*2]' can be used, too. However, `NOSPLIT' in
`[nosplit eax+eax]' will be ignored because user's intention here is
considered as `[eax+eax]'.
In 64-bit mode, NASM will by default generate absolute addresses.
The `REL' keyword makes it produce `RIP'-relative addresses. Since
this is frequently the normally desired behaviour, see the `DEFAULT'
directive (section 6.2). The keyword `ABS' overrides `REL'.
A new form of split effective addres syntax is also supported. This
is mainly intended for mib operands as used by MPX instructions, but
can be used for any memory reference. The basic concept of this form
is splitting base and index.
NASM understands four different types of constant: numeric,
character, string and floating-point.
3.4.1 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 `H' or `X', `D' or `T', `Q' or `O', and `B' or `Y' for
hexadecimal, decimal, octal and binary respectively, or you can
prefix `0x', for hexadecimal in the style of C, or you can prefix
`$' for hexadecimal in the style of Borland Pascal or Motorola
Assemblers. Note, though, that the `$' prefix does double duty as a
prefix on identifiers (see section 3.1), so a hex number prefixed
with a `$' sign must have a digit after the `$' rather than a
letter. In addition, current versions of NASM accept the prefix `0h'
for hexadecimal, `0d' or `0t' for decimal, `0o' or `0q' for octal,
and `0b' or `0y' for binary. Please note that unlike C, a `0' prefix
by itself does _not_ imply an octal constant!
Numeric constants can have underscores (`_') interspersed to break
up long strings.
Some examples (all producing exactly the same code):
mov ax,200 ; decimal
mov ax,0200 ; still decimal
mov ax,0200d ; explicitly decimal
mov ax,0d200 ; also decimal
mov ax,0c8h ; hex
mov ax,$0c8 ; hex again: the 0 is required
mov ax,0xc8 ; hex yet again
mov ax,0hc8 ; still hex
mov ax,310q ; octal
mov ax,310o ; octal again
mov ax,0o310 ; octal yet again
mov ax,0q310 ; octal yet again
mov ax,11001000b ; binary
mov ax,1100_1000b ; same binary constant
mov ax,1100_1000y ; same binary constant once more
mov ax,0b1100_1000 ; same binary constant yet again
mov ax,0y1100_1000 ; same binary constant yet again
3.4.2 Character Strings
A character string consists of up to eight characters enclosed in
either single quotes (`'...''), double quotes (`"..."') or
backquotes (``...`'). Single or double quotes are equivalent to NASM
(except of course that surrounding the constant with single quotes
allows double quotes to appear within it and vice versa); the
contents of those are represented verbatim. Strings enclosed in
backquotes support C-style `\'-escapes for special characters.
The following escape sequences are recognized by backquoted strings:
All other escape sequences are reserved. Note that `\0', meaning a
`NUL' character (ASCII 0), is a special case of the octal escape
sequence.
Unicode characters specified with `\u' or `\U' are converted to
UTF-8. For example, the following lines are all equivalent:
db `\u263a` ; UTF-8 smiley face
db `\xe2\x98\xba` ; UTF-8 smiley face
db 0E2h, 098h, 0BAh ; UTF-8 smiley face
3.4.3 Character Constants
A character constant consists of a string up to eight bytes long,
used in an expression context. It is treated as if it was an
integer.
A character constant with more than one byte will be arranged with
little-endian order in mind: if you code
mov eax,'abcd'
then the constant generated is not `0x61626364', but `0x64636261',
so that if you were then to store the value into memory, it would
read `abcd' rather than `dcba'. This is also the sense of character
constants understood by the Pentium's `CPUID' instruction.
3.4.4 String Constants
String constants are character strings used in the context of some
pseudo-instructions, namely the `DB' family and `INCBIN' (where it
represents a filename.) They are also used in certain preprocessor
directives.
A string constant looks like a character constant, only longer. It
is treated as a concatenation of maximum-size character constants
for the conditions. So the following are equivalent:
db 'hello' ; string constant
db 'h','e','l','l','o' ; equivalent character constants
And the following are also equivalent:
dd 'ninechars' ; doubleword string constant
dd 'nine','char','s' ; becomes three doublewords
db 'ninechars',0,0,0 ; and really looks like this
Note that when used in a string-supporting context, quoted strings
are treated as a string constants even if they are short enough to
be a character constant, because otherwise `db 'ab'' would have the
same effect as `db 'a'', which would be silly. Similarly, three-
character or four-character constants are treated as strings when
they are operands to `DW', and so forth.
3.4.5 Unicode Strings
The special operators `__utf16__', `__utf16le__', `__utf16be__',
`__utf32__', `__utf32le__' and `__utf32be__' allows definition of
Unicode strings. They take a string in UTF-8 format and converts it
to UTF-16 or UTF-32, respectively. Unless the `be' forms are
specified, the output is littleendian.
dw u('C:\WINDOWS'), 0 ; Pathname in UTF-16
dd w(`A + B = \u206a`), 0 ; String in UTF-32
The UTF operators can be applied either to strings passed to the
`DB' family instructions, or to character constants in an expression
context.
3.4.6 Floating-Point Constants
Floating-point constants are acceptable only as arguments to `DB',
`DW', `DD', `DQ', `DT', and `DO', or as arguments to the special
operators `__float8__', `__float16__', `__float32__', `__float64__',
`__float80m__', `__float80e__', `__float128l__', and
`__float128h__'.
Floating-point constants are expressed in the traditional form:
digits, then a period, then optionally more digits, then optionally
an `E' followed by an exponent. The period is mandatory, so that
NASM can distinguish between `dd 1', which declares an integer
constant, and `dd 1.0' which declares a floating-point constant.
NASM also support C99-style hexadecimal floating-point: `0x',
hexadecimal digits, period, optionally more hexadeximal digits, then
optionally a `P' followed by a _binary_ (not hexadecimal) exponent
in decimal notation. As an extension, NASM additionally supports the
`0h' and `$' prefixes for hexadecimal, as well binary and octal
floating-point, using the `0b' or `0y' and `0o' or `0q' prefixes,
respectively.
Underscores to break up groups of digits are permitted in floating-
point constants as well.
Some examples:
db -0.2 ; "Quarter precision"
dw -0.5 ; IEEE 754r/SSE5 half precision
dd 1.2 ; an easy one
dd 1.222_222_222 ; underscores are permitted
dd 0x1p+2 ; 1.0x2^2 = 4.0
dq 0x1p+32 ; 1.0x2^32 = 4 294 967 296.0
dq 1.e10 ; 10 000 000 000.0
dq 1.e+10 ; synonymous with 1.e10
dq 1.e-10 ; 0.000 000 000 1
dt 3.141592653589793238462 ; pi
do 1.e+4000 ; IEEE 754r quad precision
The 8-bit "quarter-precision" floating-point format is
sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This
appears to be the most frequently used 8-bit floating-point format,
although it is not covered by any formal standard. This is sometimes
called a "minifloat."
The special operators are used to produce floating-point numbers in
other contexts. They produce the binary representation of a specific
floating-point number as an integer, and can use anywhere integer
constants are used in an expression. `__float80m__' and
`__float80e__' produce the 64-bit mantissa and 16-bit exponent of an
80-bit floating-point number, and `__float128l__' and
`__float128h__' produce the lower and upper 64-bit halves of a 128-
bit floating-point number, respectively.
For example:
mov rax,__float64__(3.141592653589793238462)
... would assign the binary representation of pi as a 64-bit
floating point number into `RAX'. This is exactly equivalent to:
mov rax,0x400921fb54442d18
NASM cannot do compile-time arithmetic on floating-point constants.
This is because NASM is designed to be portable - although it always
generates code to run on x86 processors, the assembler itself can
run on any system with an ANSI C compiler. Therefore, the assembler
cannot guarantee the presence of a floating-point unit capable of
handling the Intel number formats, and so for NASM to be able to do
floating arithmetic it would have to include its own complete set of
floating-point routines, which would significantly increase the size
of the assembler for very little benefit.
The special tokens `__Infinity__', `__QNaN__' (or `__NaN__') and
`__SNaN__' can be used to generate infinities, quiet NaNs, and
signalling NaNs, respectively. These are normally used as macros:
%define Inf __Infinity__
%define NaN __QNaN__
dq +1.5, -Inf, NaN ; Double-precision constants
The `%use fp' standard macro package contains a set of convenience
macros. See section 5.3.
3.4.7 Packed BCD Constants
x87-style packed BCD constants can be used in the same contexts as
80-bit floating-point numbers. They are suffixed with `p' or
prefixed with `0p', and can include up to 18 decimal digits.
As with other numeric constants, underscores can be used to separate
digits.
Expressions in NASM are similar in syntax to those in C. Expressions
are evaluated as 64-bit integers which are then adjusted to the
appropriate size.
NASM supports two special tokens in expressions, allowing
calculations to involve the current assembly position: the `$' and
`$$' tokens. `$' evaluates to the assembly position at the beginning
of the line containing the expression; so you can code an infinite
loop using `JMP $'. `$$' evaluates to the beginning of the current
section; so you can tell how far into the section you are by using
`($-$$)'.
The arithmetic operators provided by NASM are listed here, in
increasing order of precedence.
3.5.1 `|': Bitwise OR Operator
The `|' operator gives a bitwise OR, exactly as performed by the
`OR' machine instruction. Bitwise OR is the lowest-priority
arithmetic operator supported by NASM.
3.5.2 `^': Bitwise XOR Operator
`^' provides the bitwise XOR operation.
3.5.3 `&': Bitwise AND Operator
`&' provides the bitwise AND operation.
3.5.4 `<<' and `>>': Bit Shift Operators
`<<' gives a bit-shift to the left, just as it does in C. So `5<<3'
evaluates to 5 times 8, or 40. `>>' gives a bit-shift to the right;
in NASM, such a shift is _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.
3.5.5 `+' and `-': Addition and Subtraction Operators
The `+' and `-' operators do perfectly ordinary addition and
subtraction.
3.5.6 `*', `/', `//', `%' and `%%': Multiplication and Division
`*' is the multiplication operator. `/' and `//' are both division
operators: `/' is unsigned division and `//' is signed division.
Similarly, `%' and `%%' provide unsigned and signed modulo operators
respectively.
NASM, like ANSI C, provides no guarantees about the sensible
operation of the signed modulo operator.
Since the `%' character is used extensively by the macro
preprocessor, you should ensure that both the signed and unsigned
modulo operators are followed by white space wherever they appear.
3.5.7 Unary Operators
The highest-priority operators in NASM's expression grammar are
those which only apply to one argument. These are `+', `-', `~',
`!', `SEG', and the integer functions operators.
`-' negates its operand, `+' does nothing (it's provided for
symmetry with `-'), `~' computes the one's complement of its
operand, `!' is the logical negation operator.
`SEG' provides the segment address of its operand (explained in more
detail in section 3.6).
A set of additional operators with leading and trailing double
underscores are used to implement the integer functions of the
`ifunc' macro package, see section 5.4.
3.6 `SEG' and `WRT'
When writing large 16-bit programs, which must be split into
multiple segments, it is often necessary to be able to refer to the
segment part of the address of a symbol. NASM supports the `SEG'
operator to perform this function.
The `SEG' operator returns the _preferred_ segment base of a symbol,
defined as the segment base relative to which the offset of the
symbol makes sense. So the code
mov ax,seg symbol
mov es,ax
mov bx,symbol
will load `ES:BX' with a valid pointer to the symbol `symbol'.
Things can be more complex than this: since 16-bit segments and
groups may 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 `WRT' (With Reference To)
keyword. So you can do things like
mov ax,weird_seg ; weird_seg is a segment base
mov es,ax
mov bx,symbol wrt weird_seg
to load `ES:BX' with a different, but functionally equivalent,
pointer to the symbol `symbol'.
NASM supports far (inter-segment) calls and jumps by means of the
syntax `call segment:offset', where `segment' and `offset' both
represent immediate values. So to call a far procedure, you could
code either of
(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 `call far procedure' as a synonym for the
first of the above usages. `JMP' works identically to `CALL' in
these examples.
To declare a far pointer to a data item in a data segment, you must
code
dw symbol, seg symbol
NASM supports no convenient synonym for this, though you can always
invent one using the macro processor.
3.7 `STRICT': Inhibiting Optimization
When assembling with the optimizer set to level 2 or higher (see
section 2.1.22), NASM will use size specifiers (`BYTE', `WORD',
`DWORD', `QWORD', `TWORD', `OWORD', `YWORD' or `ZWORD'), but will
give them the smallest possible size. The keyword `STRICT' can be
used to inhibit optimization and force a particular operand to be
emitted in the specified size. For example, with the optimizer on,
and in `BITS 16' mode,
push dword 33
is encoded in three bytes `66 6A 21', whereas
push strict dword 33
is encoded in six bytes, with a full dword immediate operand
`66 68 21 00 00 00'.
With the optimizer off, the same code (six bytes) is generated
whether the `STRICT' keyword was used or not.
3.8 Critical Expressions
Although NASM has an optional multi-pass optimizer, there are some
expressions which must be resolvable on the first pass. These are
called _Critical Expressions_.
The first pass is used to determine the size of all the assembled
code and data, so that the second pass, when generating all the
code, knows all the symbol addresses the code refers to. So one
thing NASM can't handle is code whose size depends on the value of a
symbol declared after the code in question. For example,
times (label-$) db 0
label: db 'Where am I?'
The argument to `TIMES' in this case could equally legally evaluate
to anything at all; NASM will reject this example because it cannot
tell the size of the `TIMES' line when it first sees it. It will
just as firmly reject the slightly paradoxical code
times (label-$+1) db 0
label: db 'NOW where am I?'
in which _any_ value for the `TIMES' argument is by definition
wrong!
NASM rejects these examples by means of a concept called a _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 `TIMES' prefix is a critical expression.
3.9 Local Labels
NASM gives special treatment to symbols beginning with a period. A
label beginning with a single period is treated as a _local_ label,
which means that it is associated with the previous non-local label.
So, for example:
label1 ; some code
.loop
; some more code
jne .loop
ret
label2 ; some code
.loop
; some more code
jne .loop
ret
In the above code fragment, each `JNE' instruction jumps to the line
immediately before it, because the two definitions of `.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 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 _defining_ a local label in terms of the
previous non-local label: the first definition of `.loop' above is
really defining a symbol called `label1.loop', and the second
defines a symbol called `label2.loop'. So, if you really needed to,
you could write
label3 ; some more code
; and some more
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 special prefix `..@', then it does nothing to the local label
mechanism. So you could code
label1: ; a non-local label
.local: ; this is really label1.local
..@foo: ; this is a special symbol
label2: ; another non-local label
.local: ; this is really label2.local
jmp ..@foo ; this will jump three lines up
NASM has the capacity to define other special symbols beginning with
a double period: for example, `..start' is used to specify the entry
point in the `obj' output format (see section 7.4.6), `..imagebase'
is used to find out the offset from a base address of the current
image in the `win64' output format (see section 7.6.1). So just keep
in mind that symbols beginning with a double period are special.
Chapter 4: The NASM Preprocessor
--------------------------------
NASM contains a powerful 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 `%' sign.
The preprocessor collapses all lines which end with a backslash (\)
character into a single line. Thus:
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
%define a(x) 1+b(x)
%define b(x) 2*x
mov ax,a(8)
will evaluate in the expected way to `mov ax,1+2*8', even though the
macro `b' wasn't defined at the time of definition of `a'.
Macros defined with `%define' are case sensitive: after
`%define foo bar', only `foo' will expand to `bar': `Foo' or `FOO'
will not. By using `%idefine' instead of `%define' (the `i' stands
for `insensitive') you can define all the case variants of a macro
at once, so that `%idefine foo bar' would cause `foo', `Foo', `FOO',
`fOO' and so on all to expand to `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
circular references and infinite loops. If this happens, the
preprocessor will only expand the first occurrence of the macro.
Hence, if you code
%define a(x) 1+a(x)
mov ax,a(3)
the macro `a(3)' will expand once, becoming `1+a(3)', and will then
expand no further. This behaviour can be useful: see section 9.1 for
an example of its use.
You can overload single-line macros: if you write
%define foo(x) 1+x
%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 `foo(3)' will become `1+3'
whereas `foo(ebx,2)' will become `1+ebx*2'. However, if you define
%define foo bar
then no other definition of `foo' will be accepted: a macro with no
parameters prohibits the definition of the same name as a macro
_with_ parameters, and vice versa.
This doesn't prevent single-line macros being _redefined_: you can
perfectly well define a macro with
%define foo bar
and then re-define it later in the same source file with
%define foo baz
Then everywhere the macro `foo' is invoked, it will be expanded
according to the most recent definition. This is particularly useful
when defining single-line macros with `%assign' (see section 4.1.7).
You can pre-define single-line macros using the `-d' option on the
NASM command line: see section 2.1.18.
4.1.2 Resolving `%define': `%xdefine'
To have a reference to an embedded single-line macro resolved at the
time that the embedding macro is _defined_, as opposed to when the
embedding macro is _expanded_, you need a different mechanism to the
one offered by `%define'. The solution is to use `%xdefine', or it's
case-insensitive counterpart `%ixdefine'.
In this case, `val1' is equal to 0, and `val2' is equal to 1. This
is because, when a single-line macro is defined using `%define', it
is expanded only when it is called. As `isFalse' expands to
`isTrue', the expansion will be the current value of `isTrue'. The
first time it is called that is 0, and the second time it is 1.
If you wanted `isFalse' to expand to the value assigned to the
embedded macro `isTrue' at the time that `isFalse' was defined, you
need to change the above code to use `%xdefine'.
Now, each time that `isFalse' is called, it expands to 1, as that is
what the embedded macro `isTrue' expanded to at the time that
`isFalse' was defined.
4.1.3 Macro Indirection: `%[...]'
The `%[...]' construct can be used to expand macros in contexts
where macro expansion would otherwise not occur, including in the
names other macros. For example, if you have a set of macros named
`Foo16', `Foo32' and `Foo64', you could write:
mov ax,Foo%[__BITS__] ; The Foo value
to use the builtin macro `__BITS__' (see section 4.12.5) to
automatically select between them. Similarly, the two statements:
%xdefine Bar Quux ; Expands due to %xdefine
%define Bar %[Quux] ; Expands due to %[...]
have, in fact, exactly the same effect.
`%[...]' concatenates to adjacent tokens in the same way that multi-
line macro parameters do, see section 4.3.9 for details.
4.1.4 Concatenating Single Line Macro Tokens: `%+'
Individual tokens in single line macros can be concatenated, to
produce longer tokens for later processing. This can be useful if
there are several similar macros that perform similar functions.
Please note that a space is required after `%+', in order to
disambiguate it from the syntax `%+1' used in multiline macros.
As an example, consider the following:
%define BDASTART 400h ; Start of BIOS data area
struc tBIOSDA ; its structure
.COM1addr RESW 1
.COM2addr RESW 1
; ..and so on
endstruc
Now, if we need to access the elements of tBIOSDA in different
places, we can end up with:
This will become pretty ugly (and tedious) if used in many places,
and can be reduced in size significantly by using the following
macro:
; Macro to access BIOS variables by their names (from tBDA):
%define BDA(x) BDASTART + tBIOSDA. %+ x
Now the above code can be written as:
mov ax,BDA(COM1addr)
mov bx,BDA(COM2addr)
Using this feature, we can simplify references to a lot of macros
(and, in turn, reduce typing errors).
4.1.5 The Macro Name Itself: `%?' and `%??'
The special symbols `%?' and `%??' can be used to reference the
macro name itself inside a macro expansion, this is supported for
both single-and multi-line macros. `%?' refers to the macro name as
_invoked_, whereas `%??' refers to the macro name as _declared_. The
two are always the same for case-sensitive macros, but for case-
insensitive macros, they can differ.
For example:
%idefine Foo mov %?,%??
foo
FOO
will expand to:
mov foo,Foo
mov FOO,Foo
The sequence:
%idefine keyword $%?
can be used to make a keyword "disappear", for example in case a new
instruction has been used as a label in older code. For example:
%idefine pause $%? ; Hide the PAUSE instruction
4.1.6 Undefining Single-Line Macros: `%undef'
Single-line macros can be removed with the `%undef' directive. For
example, the following sequence:
%define foo bar
%undef foo
mov eax, foo
will expand to the instruction `mov eax, foo', since after `%undef'
the macro `foo' is no longer defined.
Macros that would otherwise be pre-defined can be undefined on the
command-line using the `-u' option on the NASM command line: see
section 2.1.19.
4.1.7 Preprocessor Variables: `%assign'
An alternative way to define single-line macros is by means of the
`%assign' command (and its case-insensitive counterpart `%iassign',
which differs from `%assign' in exactly the same way that `%idefine'
differs from `%define').
`%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
`%assign' directive is processed.
Like `%define', macros defined using `%assign' can be re-defined
later, so you can do things like
%assign i i+1
to increment the numeric value of a macro.
`%assign' is useful for controlling the termination of `%rep'
preprocessor loops: see section 4.5 for an example of this. Another
use for `%assign' is given in section 8.4 and section 9.1.
The expression passed to `%assign' is a critical expression (see
section 3.8), 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).
4.1.8 Defining Strings: `%defstr'
`%defstr', and its case-insensitive counterpart `%idefstr', define
or redefine a single-line macro without parameters but converts the
entire right-hand side, after macro expansion, to a quoted string
before definition.
For example:
%defstr test TEST
is equivalent to
%define test 'TEST'
This can be used, for example, with the `%!' construct (see section
4.10.2):
%defstr PATH %!PATH ; The operating system PATH variable
4.1.9 Defining Tokens: `%deftok'
`%deftok', and its case-insensitive counterpart `%ideftok', define
or redefine a single-line macro without parameters but converts the
second parameter, after string conversion, to a sequence of tokens.
For example:
%deftok test 'TEST'
is equivalent to
%define test TEST
4.2 String Manipulation in Macros
It's often useful to be able to handle strings in macros. NASM
supports a few simple string handling macro operators from which
more complex operations can be constructed.
All the string operators define or redefine a value (either a string
or a numeric value) to a single-line macro. When producing a string
value, it may change the style of quoting of the input string or
strings, and possibly use `\'-escapes inside ``'-quoted strings.
4.2.1 Concatenating Strings: `%strcat'
The `%strcat' operator concatenates quoted strings and assign them
to a single-line macro.
For example:
%strcat alpha "Alpha: ", '12" screen'
... would assign the value `'Alpha: 12" screen'' to `alpha'.
Similarly:
%strcat beta '"foo"\', "'bar'"
... would assign the value ``"foo"\\'bar'`' to `beta'.
The use of commas to separate strings is permitted but optional.
4.2.2 String Length: `%strlen'
The `%strlen' operator assigns the length of a string to a macro.
For example:
%strlen charcnt 'my string'
In this example, `charcnt' would receive the value 9, just as if an
`%assign' had been used. In this example, `'my string'' was a
literal string but it could also have been a single-line macro that
expands to a string, as in the following example:
As in the first case, this would result in `charcnt' being assigned
the value of 9.
4.2.3 Extracting Substrings: `%substr'
Individual letters or substrings in strings can be extracted using
the `%substr' operator. An example of its use is probably more
useful than the description:
%substr mychar 'xyzw' 1 ; equivalent to %define mychar 'x'
%substr mychar 'xyzw' 2 ; equivalent to %define mychar 'y'
%substr mychar 'xyzw' 3 ; equivalent to %define mychar 'z'
%substr mychar 'xyzw' 2,2 ; equivalent to %define mychar 'yz'
%substr mychar 'xyzw' 2,-1 ; equivalent to %define mychar 'yzw'
%substr mychar 'xyzw' 2,-2 ; equivalent to %define mychar 'yz'
As with `%strlen' (see section 4.2.2), the first parameter is the
single-line macro to be created and the second is the string. The
third parameter specifies the first character to be selected, and
the optional fourth parameter preceeded by comma) is the length.
Note that the first index is 1, not 0 and the last index is equal to
the value that `%strlen' would assign given the same string. Index
values out of range result in an empty string. A negative length
means "until N-1 characters before the end of string", i.e. `-1'
means until end of string, `-2' until one character before, etc.
4.3 Multi-Line Macros: `%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.
%macro prologue 1
push ebp
mov ebp,esp
sub esp,%1
%endmacro
This defines a C-like function prologue as a macro: so you would
invoke the macro with a call such as
myfunc: prologue 12
which would expand to the three lines of code
myfunc: push ebp
mov ebp,esp
sub esp,12
The number `1' after the macro name in the `%macro' line defines the
number of parameters the macro `prologue' expects to receive. The
use of `%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 `%2', `%3'
and so on.
Multi-line macros, like single-line macros, are case-sensitive,
unless you define them using the alternative directive `%imacro'.
If you need to pass a comma as _part_ of a parameter to a multi-line
macro, you can do that by enclosing the entire parameter in braces.
So you could code things like
%macro silly 2
%2: db %1
%endmacro
silly 'a', letter_a ; letter_a: db 'a'
silly 'ab', string_ab ; string_ab: db 'ab'
silly {13,10}, crlf ; crlf: db 13,10
4.3.1 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
%macro prologue 0
push ebp
mov ebp,esp
%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
%macro push 2
push %1
push %2
%endmacro
so that you could code
push ebx ; this line is not a macro call
push eax,ecx ; but this one is
Ordinarily, NASM will give a warning for the first of the above two
lines, since `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
`-w-macro-params' command-line option (see section 2.1.24).
4.3.2 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 `%%' to the label name. So you
can invent an instruction which executes a `RET' if the `Z' flag is
set by doing this:
%macro retz 0
jnz %%skip
ret
%%skip:
%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 `%%skip'. The names NASM invents are of the form
`..@2345.skip', where the number 2345 changes with every macro call.
The `..@' prefix prevents macro-local labels from interfering with
the local label mechanism, as described in section 3.9. You should
avoid defining your own labels in this form (the `..@' prefix, then
a number, then another period) in case they interfere with macro-
local labels.
4.3.3 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
writefile [filehandle],"hello, world",13,10
NASM allows you to define the last parameter of a macro to be
_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:
%macro writefile 2+
jmp %%endstr
%%str: db %2
%%endstr:
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
then the example call to `writefile' above will work as expected:
the text before the first comma, `[filehandle]', is used as the
first macro parameter and expanded when `%1' is referred to, and all
the subsequent text is lumped into `%2' and placed after the `db'.
The greedy nature of the macro is indicated to NASM by the use of
the `+' sign after the parameter count on the `%macro' line.
If you define a greedy macro, you are effectively telling NASM how
it should expand the macro given _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 `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
`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
writefile [filehandle], {"hello, world",13,10}
NASM provides both mechanisms for putting commas in macro
parameters, and you choose which one you prefer for each macro
definition.
See section 6.3.1 for a better way to write the above macro.
4.3.4 Macro Parameters Range
NASM allows you to expand parameters via special construction
`%{x:y}' where `x' is the first parameter index and `y' is the last.
Any index can be either negative or positive but must never be zero.
For example
%macro mpar 1-*
db %{3:5}
%endmacro
mpar 1,2,3,4,5,6
expands to `3,4,5' range.
Even more, the parameters can be reversed so that
%macro mpar 1-*
db %{5:3}
%endmacro
mpar 1,2,3,4,5,6
expands to `5,4,3' range.
But even this is not the last. The parameters can be addressed via
negative indices so NASM will count them reversed. The ones who know
Python may see the analogue here.
%macro mpar 1-*
db %{-1:-3}
%endmacro
mpar 1,2,3,4,5,6
expands to `6,5,4' range.
Note that NASM uses comma to separate parameters being expanded.
By the way, here is a trick - you might use the index `%{-1:-1'}
which gives you the last argument passed to a macro.
4.3.5 Default Macro Parameters
NASM also allows you to define a multi-line macro with a _range_ of
allowable parameter counts. If you do this, you can specify defaults
for omitted parameters. So, for example:
%macro die 0-1 "Painful program death has occurred."
writefile 2,%1
mov ax,0x4c01
int 0x21
%endmacro
This macro (which makes use of the `writefile' macro defined in
section 4.3.3) 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
%macro foobar 1-3 eax,[ebx+2]
then it could be called with between one and three parameters, and
`%1' would always be taken from the macro call. `%2', if not
specified by the macro call, would default to `eax', and `%3' if not
specified would default to `[ebx+2]'.
You can provide extra information to a macro by providing too many
default parameters:
%macro quux 1 something
This will trigger a warning by default; see section 2.1.24 for more
information. When `quux' is invoked, it receives not one but two
parameters. `something' can be referred to as `%2'. The difference
between passing `something' this way and writing `something' in the
macro body is that with this way `something' is evaluated when the
macro is defined, not when it is expanded.
You may omit parameter defaults from the macro definition, in which
case the parameter default is taken to be blank. This can be useful
for macros which can take a variable number of parameters, since the
`%0' token (see section 4.3.6) 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 `die' macro above could be made more powerful, and
more useful, by changing the first line of the definition to
%macro die 0-1+ "Painful program death has occurred.",13,10
The maximum parameter count can be infinite, denoted by `*'. In this
case, of course, it is impossible to provide a _full_ set of default
parameters. Examples of this usage are shown in section 4.3.8.
4.3.6 `%0': Macro Parameter Counter
The parameter reference `%0' will return a numeric constant giving
the number of parameters received, that is, if `%0' is n then `%'n
is the last parameter. `%0' is mostly useful for macros that can
take a variable number of parameters. It can be used as an argument
to `%rep' (see section 4.5) in order to iterate through all the
parameters of a macro. Examples are given in section 4.3.8.
4.3.7 `%00': Label Preceeding Macro
`%00' will return the label preceeding the macro invocation, if any.
The label must be on the same line as the macro invocation, may be a
local label (see section 3.9), and need not end in a colon.
4.3.8 `%rotate': Rotating Macro Parameters
Unix shell programmers will be familiar with the `shift' shell
command, which allows the arguments passed to a shell script
(referenced as `$1', `$2' and so on) to be moved left by one place,
so that the argument previously referenced as `$2' becomes available
as `$1', and the argument previously referenced as `$1' is no longer
available at all.
NASM provides a similar mechanism, in the form of `%rotate'. As its
name suggests, it differs from the Unix `shift' in that no
parameters are lost: parameters rotated off the left end of the
argument list reappear on the right, and vice versa.
`%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 `%rotate' is negative, the macro
parameters are rotated to the right.
So a pair of macros to save and restore a set of registers might
work as follows:
%macro multipush 1-*
%rep %0
push %1
%rotate 1
%endrep
%endmacro
This macro invokes the `PUSH' instruction on each of its arguments
in turn, from left to right. It begins by pushing its first
argument, `%1', then invokes `%rotate' to move all the arguments one
place to the left, so that the original second argument is now
available as `%1'. Repeating this procedure as many times as there
were arguments (achieved by supplying `%0' as the argument to
`%rep') causes each argument in turn to be pushed.
Note also the use of `*' as the maximum parameter count, indicating
that there is no upper limit on the number of parameters you may
supply to the `multipush' macro.
It would be convenient, when using this macro, to have a `POP'
equivalent, which _didn't_ require the arguments to be given in
reverse order. Ideally, you would write the `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 `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:
%macro multipop 1-*
%rep %0
%rotate -1
pop %1
%endrep
%endmacro
This macro begins by rotating its arguments one place to the
_right_, so that the original _last_ argument appears as `%1'. This
is then popped, and the arguments are rotated right again, so the
second-to-last argument becomes `%1'. Thus the arguments are
iterated through in reverse order.
4.3.9 Concatenating Macro Parameters
NASM can concatenate macro parameters and macro indirection
constructs on to other text surrounding them. This allows you to
declare a family of symbols, for example, in a macro definition. If,
for example, you wanted to generate a table of key codes along with
offsets into the table, you could code something like
keytab:
keyposF1 equ $-keytab
db 128+1
keyposF2 equ $-keytab
db 128+2
keyposReturn equ $-keytab
db 13
You can just as easily concatenate text on to the other end of a
macro parameter, by writing `%1foo'.
If you need to append a _digit_ to a macro parameter, for example
defining labels `foo1' and `foo2' when passed the parameter `foo',
you can't code `%11' because that would be taken as the eleventh
macro parameter. Instead, you must code `%{1}1', which will separate
the first `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 (section 4.3.2) and context-
local labels (section 4.7.2). In all cases, ambiguities in syntax
can be resolved by enclosing everything after the `%' sign and
before the literal text in braces: so `%{%foo}bar' concatenates the
text `bar' to the end of the real name of the macro-local label
`%%foo'. (This is unnecessary, since the form NASM uses for the real
names of macro-local labels means that the two usages `%{%foo}bar'
and `%%foobar' would both expand to the same thing anyway;
nevertheless, the capability is there.)
The single-line macro indirection construct, `%[...]' (section
4.1.3), behaves the same way as macro parameters for the purpose of
concatenation.
See also the `%+' operator, section 4.1.4.
4.3.10 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
`%1' by means of the alternative syntax `%+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 _not_ a valid condition
code.
Far more usefully, though, you can refer to the macro parameter by
means of `%-1', which NASM will expand as the _inverse_ condition
code. So the `retz' macro defined in section 4.3.2 can be replaced
by a general conditional-return macro like this:
%macro retc 1
j%-1 %%skip
ret
%%skip:
%endmacro
This macro can now be invoked using calls like `retc ne', which will
cause the conditional-jump instruction in the macro expansion to
come out as `JE', or `retc po' which will make the jump a `JPE'.
The `%+1' macro-parameter reference is quite happy to interpret the
arguments `CXZ' and `ECXZ' as valid condition codes; however, `%-1'
will report an error if passed either of these, because no inverse
condition code exists.
4.3.11 Disabling Listing Expansion
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 `.nolist' qualifier, which you can
include in a macro definition to inhibit the expansion of the macro
in the listing file. The `.nolist' qualifier comes directly after
the number of parameters, like this:
%macro foo 1.nolist
Or like this:
%macro bar 1-5+.nolist a,b,c,d,e,f,g,h
4.3.12 Undefining Multi-Line Macros: `%unmacro'
Multi-line macros can be removed with the `%unmacro' directive.
Unlike the `%undef' directive, however, `%unmacro' takes an argument
specification, and will only remove exact matches with that argument
specification.
For example:
%macro foo 1-3
; Do something
%endmacro
%unmacro foo 1-3
removes the previously defined macro `foo', but
%macro bar 1-3
; Do something
%endmacro
%unmacro bar 1
does _not_ remove the macro `bar', since the argument specification
does not match exactly.
4.4 Conditional Assembly
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:
%if<condition>
; some code which only appears if <condition> is met
%elif<condition2>
; only appears if <condition> is not met but <condition2> is
%else
; this appears if neither <condition> nor <condition2> was met
%endif
The inverse forms `%ifn' and `%elifn' are also supported.
The `%else' clause is optional, as is the `%elif' clause. You can
have more than one `%elif' clause as well.
There are a number of variants of the `%if' directive. Each has its
corresponding `%elif', `%ifn', and `%elifn' directives; for example,
the equivalents to the `%ifdef' directive are `%elifdef', `%ifndef',
and `%elifndef'.
4.4.1 `%ifdef': Testing Single-Line Macro Existence
Beginning a conditional-assembly block with the line `%ifdef MACRO'
will assemble the subsequent code if, and only if, a single-line
macro called `MACRO' is defined. If not, then the `%elif' and
`%else' blocks (if any) will be processed instead.
For example, when debugging a program, you might want to write code
such as
; perform some function
%ifdef DEBUG
writefile 2,"Function performed successfully",13,10
%endif
; go and do something else
Then you could use the command-line option `-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 _not_ being defined by using `%ifndef'
instead of `%ifdef'. You can also test for macro definitions in
`%elif' blocks by using `%elifdef' and `%elifndef'.
4.4.2 `%ifmacro': Testing Multi-Line Macro Existence
The `%ifmacro' directive operates in the same way as the `%ifdef'
directive, except that it checks for the existence of a multi-line
macro.
For example, you may be working with a large project and not have
control over the macros in a library. You may want to create a macro
with one name if it doesn't already exist, and another name if one
with that name does exist.
The `%ifmacro' is considered true if defining a macro with the given
name and number of arguments would cause a definitions conflict. For
example:
%ifmacro MyMacro 1-3
%error "MyMacro 1-3" causes a conflict with an existing macro.
%else
%macro MyMacro 1-3
; insert code to define the macro
%endmacro
%endif
This will create the macro "MyMacro 1-3" if no macro already exists
which would conflict with it, and emits a warning if there would be
a definition conflict.
You can test for the macro not existing by using the `%ifnmacro'
instead of `%ifmacro'. Additional tests can be performed in `%elif'
blocks by using `%elifmacro' and `%elifnmacro'.
4.4.3 `%ifctx': Testing the Context Stack
The conditional-assembly construct `%ifctx' will cause the
subsequent code to be assembled if and only if the top context on
the preprocessor's context stack has the same name as one of the
arguments. As with `%ifdef', the inverse and `%elif' forms
`%ifnctx', `%elifctx' and `%elifnctx' are also supported.
For more details of the context stack, see section 4.7. For a sample
use of `%ifctx', see section 4.7.6.
The conditional-assembly construct `%if expr' will cause the
subsequent code to be assembled if and only if the value of the
numeric expression `expr' is non-zero. An example of the use of this
feature is in deciding when to break out of a `%rep' preprocessor
loop: see section 4.5 for a detailed example.
The expression given to `%if', and its counterpart `%elif', is a
critical expression (see section 3.8).
`%if' extends the normal NASM expression syntax, by providing a set
of relational operators which are not normally available in
expressions. The operators `=', `<', `>', `<=', `>=' and `<>' test
equality, less-than, greater-than, less-or-equal, greater-or-equal
and not-equal respectively. The C-like forms `==' and `!=' are
supported as alternative forms of `=' and `<>'. In addition, low-
priority logical operators `&&', `^^' and `||' are provided,
supplying logical AND, logical XOR and 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 `^^', for example, returns 1 if exactly one of its inputs
is zero, and 0 otherwise). The relational operators also return 1
for true and 0 for false.
Like other `%if' constructs, `%if' has a counterpart `%elif', and
negative forms `%ifn' and `%elifn'.
4.4.5 `%ifidn' and `%ifidni': Testing Exact Text Identity
The construct `%ifidn text1,text2' will cause the subsequent code to
be assembled if and only if `text1' and `text2', after expanding
single-line macros, are identical pieces of text. Differences in
white space are not counted.
`%ifidni' is similar to `%ifidn', but is case-insensitive.
For example, the following macro pushes a register or number on the
stack, and allows you to treat `IP' as a real register:
Like other `%if' constructs, `%ifidn' has a counterpart `%elifidn',
and negative forms `%ifnidn' and `%elifnidn'. Similarly, `%ifidni'
has counterparts `%elifidni', `%ifnidni' and `%elifnidni'.
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 `%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.
`%ifnum' works similarly, but tests for the token being a numeric
constant; `%ifstr' tests for it being a string.
For example, the `writefile' macro defined in section 4.3.3 can be
extended to take advantage of `%ifstr' in the following fashion:
In the first, `strpointer' is used as the address of an already-
declared string, and `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 `%if' inside the `%ifstr': this is to detect whether
the macro was passed two arguments (so the string would be a single
string constant, and `db %2' would be adequate) or more (in which
case, all but the first two would be lumped together into `%3', and
`db %2,%3' would be required).
The usual `%elif'..., `%ifn'..., and `%elifn'... versions exist for
each of `%ifid', `%ifnum' and `%ifstr'.
4.4.7 `%iftoken': Test for a Single Token
Some macros will want to do different things depending on if it is
passed a single token (e.g. paste it to something else using `%+')
versus a multi-token sequence.
The conditional assembly construct `%iftoken' assembles the
subsequent code if and only if the expanded parameters consist of
exactly one token, possibly surrounded by whitespace.
For example:
%iftoken 1
will assemble the subsequent code, but
%iftoken -1
will not, since `-1' contains two tokens: the unary minus operator
`-', and the number `1'.
The usual `%eliftoken', `%ifntoken', and `%elifntoken' variants are
also provided.
4.4.8 `%ifempty': Test for Empty Expansion
The conditional assembly construct `%ifempty' assembles the
subsequent code if and only if the expanded parameters do not
contain any tokens at all, whitespace excepted.
The usual `%elifempty', `%ifnempty', and `%elifnempty' variants are
also provided.
4.4.9 `%ifenv': Test If Environment Variable Exists
The conditional assembly construct `%ifenv' assembles the subsequent
code if and only if the environment variable referenced by the
`%!<env>' directive exists.
The usual `%elifenv', `%ifnenv', and `%elifnenv' variants are also
provided.
Just as for `%!<env>' the argument should be written as a string if
it contains characters that would not be legal in an identifier. See
section 4.10.2.
4.5 Preprocessor Loops: `%rep'
NASM's `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: `%rep'.
The directives `%rep' and `%endrep' (`%rep' takes a numeric
argument, which can be an expression; `%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:
%assign i 0
%rep 64
inc word [table+2*i]
%assign i i+1
%endrep
This will generate a sequence of 64 `INC' instructions, incrementing
every word of memory from `[table]' to `[table+126]'.
For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the `%exitrep' directive to
terminate the loop, like this:
fibonacci:
%assign i 0
%assign j 1
%rep 100
%if j > 65535
%exitrep
%endif
dw j
%assign k j+i
%assign i j
%assign j k
%endrep
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
`%rep'. This is to prevent the possibility of NASM getting into an
infinite loop in the preprocessor, which (on multitasking or multi-
user systems) would typically cause all the system memory to be
gradually used up and other applications to start crashing.
Note a maximum repeat count is limited by 62 bit number, though it
is hardly possible that you ever need anything bigger.
4.6 Source Files and Dependencies
These commands allow you to split your sources into multiple files.
4.6.1 `%include': 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 `%include' directive:
%include "macros.mac"
will include the contents of the file `macros.mac' into the source
file containing the `%include' directive.
Include files are 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 `-i'
option.
The standard C idiom for preventing a file being included more than
once is just as applicable in NASM: if the file `macros.mac' has the
form
%ifndef MACROS_MAC
%define MACROS_MAC
; now define some macros
%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 `MACROS_MAC' will already be defined.
You can force a file to be included even if there is no `%include'
directive that explicitly includes it, by using the `-p' option on
the NASM command line (see section 2.1.17).
4.6.2 `%pathsearch': Search the Include Path
The `%pathsearch' directive takes a single-line macro name and a
filename, and declare or redefines the specified single-line macro
to be the include-path-resolved version of the filename, if the file
exists (otherwise, it is passed unchanged.)
For example,
%pathsearch MyFoo "foo.bin"
... with `-Ibins/' in the include path may end up defining the macro
`MyFoo' to be `"bins/foo.bin"'.
4.6.3 `%depend': Add Dependent Files
The `%depend' directive takes a filename and adds it to the list of
files to be emitted as dependency generation when the `-M' options
and its relatives (see section 2.1.4) are used. It produces no
output.
This is generally used in conjunction with `%pathsearch'. For
example, a simplified version of the standard macro wrapper for the
`INCBIN' directive looks like:
%imacro incbin 1-2+ 0
%pathsearch dep %1
%depend dep
incbin dep,%2
%endmacro
This first resolves the location of the file into the macro `dep',
then adds it to the dependency lists, and finally issues the
assembler-level `INCBIN' directive.
4.6.4 `%use': Include Standard Macro Package
The `%use' directive is similar to `%include', but rather than
including the contents of a file, it includes a named standard macro
package. The standard macro packages are part of NASM, and are
described in chapter 5.
Unlike the `%include' directive, package names for the `%use'
directive do not require quotes, but quotes are permitted. In NASM
2.04 and 2.05 the unquoted form would be macro-expanded; this is no
longer true. Thus, the following lines are equivalent:
%use altreg
%use 'altreg'
Standard macro packages are protected from multiple inclusion. When
a standard macro package is used, a testable single-line macro of
the form `__USE_'_package_`__' is also defined, see section 4.12.8.
4.7 The 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 `REPEAT' ...
`UNTIL' loop, in which the expansion of the `REPEAT' macro would
need to be able to refer to a label which the `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 _context stack_. The
preprocessor maintains a stack of _contexts_, each of which is
characterized by a name. You add a new context to the stack using
the `%push' directive, and remove one using `%pop'. You can define
labels that are local to a particular context on the stack.
4.7.1 `%push' and `%pop': Creating and Removing Contexts
The `%push' directive is used to create a new context and place it
on the top of the context stack. `%push' takes an optional argument,
which is the name of the context. For example:
%push foobar
This pushes a new context called `foobar' on the stack. You can have
several contexts on the stack with the same name: they can still be
distinguished. If no name is given, the context is unnamed (this is
normally used when both the `%push' and the `%pop' are inside a
single macro definition.)
The directive `%pop', taking one optional argument, removes the top
context from the context stack and destroys it, along with any
labels associated with it. If an argument is given, it must match
the name of the current context, otherwise it will issue an error.
4.7.2 Context-Local Labels
Just as the usage `%%foo' defines a label which is local to the
particular macro call in which it is used, the usage `%$foo' is used
to define a label which is local to the context on the top of the
context stack. So the `REPEAT' and `UNTIL' example given above could
be implemented by means of:
%macro repeat 0
%push repeat
%$begin:
%endmacro
%macro until 1
j%-1 %$begin
%pop
%endmacro
and invoked by means of, for example,
mov cx,string
repeat
add cx,3
scasb
until e
which would scan every fourth byte of a string in search of the byte
in `AL'.
If you need to define, or access, labels local to the context
_below_ the top one on the stack, you can use `%$$foo', or `%$$$foo'
for the context below that, and so on.
4.7.3 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:
%define %$localmac 3
will define the single-line macro `%$localmac' to be local to the
top context on the stack. Of course, after a subsequent `%push', it
can then still be accessed by the name `%$$localmac'.
4.7.4 Context Fall-Through Lookup
Context fall-through lookup (automatic searching of outer contexts)
is a feature that was added in NASM version 0.98.03. Unfortunately,
this feature is unintuitive and can result in buggy code that would
have otherwise been prevented by NASM's error reporting. As a
result, this feature has been _deprecated_. NASM version 2.09 will
issue a warning when usage of this _deprecated_ feature is detected.
Starting with NASM version 2.10, usage of this _deprecated_ feature
will simply result in an _expression syntax error_.
An example usage of this _deprecated_ feature follows:
As demonstrated, `%$external' is being defined in the `ctx1' context
and referenced within the `ctx2' context. With context fall-through
lookup, referencing an undefined context-local macro like this
implicitly searches through all outer contexts until a match is made
or isn't found in any context. As a result, `%$external' referenced
within the `ctx2' context would implicitly use `%$external' as
defined in `ctx1'. Most people would expect NASM to issue an error
in this situation because `%$external' was never defined within
`ctx2' and also isn't qualified with the proper context depth,
`%$$external'.
Here is a revision of the above example with proper context depth:
As demonstrated, `%$external' is still being defined in the `ctx1'
context and referenced within the `ctx2' context. However, the
reference to `%$external' within `ctx2' has been fully qualified
with the proper context depth, `%$$external', and thus is no longer
ambiguous, unintuitive or erroneous.
4.7.5 `%repl': 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 `%ifctx'), you
can execute a `%pop' followed by a `%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 `%repl', which _replaces_ a context with
a different name, without touching the associated macros and labels.
So you could replace the destructive code
%pop
%push newname
with the non-destructive version `%repl newname'.
4.7.6 Example Use of the Context Stack: Block IFs
This example makes use of almost all the context-stack features,
including the conditional-assembly construct `%ifctx', to implement
a block IF statement as a set of macros.
%macro if 1
%push if
j%-1 %$ifnot
%endmacro
%macro else 0
%ifctx if
%repl else
jmp %$ifend
%$ifnot:
%else
%error "expected `if' before `else'"
%endif
%endmacro
%macro endif 0
%ifctx if
%$ifnot:
%pop
%elifctx else
%$ifend:
%pop
%else
%error "expected `if' or `else' before `endif'"
%endif
%endmacro
This code is more robust than the `REPEAT' and `UNTIL' macros given
in section 4.7.2, because it uses conditional assembly to check that
the macros are issued in the right order (for example, not calling
`endif' before `if') and issues a `%error' if they're not.
In addition, the `endif' macro has to be able to cope with the two
distinct cases of either directly following an `if', or following an
`else'. It achieves this, again, by using conditional assembly to do
different things depending on whether the context on top of the
stack is `if' or `else'.
The `else' macro has to preserve the context on the stack, in order
to have the `%$ifnot' referred to by the `if' macro be the same as
the one defined by the `endif' macro, but has to change the
context's name so that `endif' will know there was an intervening
`else'. It does this by the use of `%repl'.
A sample usage of these macros might look like:
cmp ax,bx
if ae
cmp bx,cx
if ae
mov ax,cx
else
mov ax,bx
endif
else
cmp ax,cx
if ae
mov ax,cx
endif
endif
The block-`IF' macros handle nesting quite happily, by means of
pushing another context, describing the inner `if', on top of the
one describing the outer `if'; thus `else' and `endif' always refer
to the last unmatched `if' or `else'.
4.8 Stack Relative Preprocessor Directives
The following preprocessor directives provide a way to use labels to
refer to local variables allocated on the stack.
(*) `%arg' (see section 4.8.1)
(*) `%stacksize' (see section 4.8.2)
(*) `%local' (see section 4.8.3)
4.8.1 `%arg' Directive
The `%arg' directive is used to simplify the handling of parameters
passed on the stack. Stack based parameter passing is used by many
high level languages, including C, C++ and Pascal.
While NASM has macros which attempt to duplicate this functionality
(see section 8.4.5), the syntax is not particularly convenient to
use and is not TASM compatible. Here is an example which shows the
use of `%arg' without any external macros:
some_function:
%push mycontext ; save the current context
%stacksize large ; tell NASM to use bp
%arg i:word, j_ptr:word
mov ax,[i]
mov bx,[j_ptr]
add ax,[bx]
ret
%pop ; restore original context
This is similar to the procedure defined in section 8.4.5 and adds
the value in i to the value pointed to by j_ptr and returns the sum
in the ax register. See section 4.7.1 for an explanation of `push'
and `pop' and the use of context stacks.
4.8.2 `%stacksize' Directive
The `%stacksize' directive is used in conjunction with the `%arg'
(see section 4.8.1) and the `%local' (see section 4.8.3) directives.
It tells NASM the default size to use for subsequent `%arg' and
`%local' directives. The `%stacksize' directive takes one required
argument which is one of `flat', `flat64', `large' or `small'.
%stacksize flat
This form causes NASM to use stack-based parameter addressing
relative to `ebp' and it assumes that a near form of call was used
to get to this label (i.e. that `eip' is on the stack).
%stacksize flat64
This form causes NASM to use stack-based parameter addressing
relative to `rbp' and it assumes that a near form of call was used
to get to this label (i.e. that `rip' is on the stack).
%stacksize large
This form uses `bp' to do stack-based parameter addressing and
assumes that a far form of call was used to get to this address
(i.e. that `ip' and `cs' are on the stack).
%stacksize small
This form also uses `bp' to address stack parameters, but it is
different from `large' because it also assumes that the old value of
bp is pushed onto the stack (i.e. it expects an `ENTER'
instruction). In other words, it expects that `bp', `ip' and `cs'
are on the top of the stack, underneath any local space which may
have been allocated by `ENTER'. This form is probably most useful
when used in combination with the `%local' directive (see section
4.8.3).
4.8.3 `%local' Directive
The `%local' directive is used to simplify the use of local
temporary stack variables allocated in a stack frame. Automatic
local variables in C are an example of this kind of variable. The
`%local' directive is most useful when used with the `%stacksize'
(see section 4.8.2 and is also compatible with the `%arg' directive
(see section 4.8.1). It allows simplified reference to variables on
the stack which have been allocated typically by using the `ENTER'
instruction. An example of its use is the following:
silly_swap:
%push mycontext ; save the current context
%stacksize small ; tell NASM to use bp
%assign %$localsize 0 ; see text for explanation
%local old_ax:word, old_dx:word
enter %$localsize,0 ; see text for explanation
mov [old_ax],ax ; swap ax & bx
mov [old_dx],dx ; and swap dx & cx
mov ax,bx
mov dx,cx
mov bx,[old_ax]
mov cx,[old_dx]
leave ; restore old bp
ret ;
%pop ; restore original context
The `%$localsize' variable is used internally by the `%local'
directive and _must_ be defined within the current context before
the `%local' directive may be used. Failure to do so will result in
one expression syntax error for each `%local' variable declared. It
then may be used in the construction of an appropriately sized ENTER
instruction as shown in the example.
The preprocessor directive `%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:
%ifdef F1
; do some setup
%elifdef F2
; do some different setup
%else
%error "Neither F1 nor F2 was defined."
%endif
Then any user who fails to understand the way your code is supposed
to be assembled will be quickly warned of their mistake, rather than
having to wait until the program crashes on being run and then not
knowing what went wrong.
Similarly, `%warning' issues a warning, but allows assembly to
continue:
%ifdef F1
; do some setup
%elifdef F2
; do some different setup
%else
%warning "Neither F1 nor F2 was defined, assuming F1."
%define F1
%endif
`%error' and `%warning' are issued only on the final assembly pass.
This makes them safe to use in conjunction with tests that depend on
symbol values.
`%fatal' terminates assembly immediately, regardless of pass. This
is useful when there is no point in continuing the assembly further,
and doing so is likely just going to cause a spew of confusing error
messages.
It is optional for the message string after `%error', `%warning' or
`%fatal' to be quoted. If it is _not_, then single-line macros are
expanded in it, which can be used to display more information to the
user. For example:
%if foo > 64
%assign foo_over foo-64
%error foo is foo_over bytes too large
%endif
4.10 Other Preprocessor Directives
NASM also has preprocessor directives which allow access to
information from external sources. Currently they include:
(*) `%line' enables NASM to correctly handle the output of another
preprocessor (see section 4.10.1).
(*) `%!' enables NASM to read in the value of an environment
variable, which can then be used in your program (see section
4.10.2).
4.10.1 `%line' Directive
The `%line' directive is used to notify NASM that the input line
corresponds to a specific line number in another file. Typically
this other file would be an original source file, with the current
NASM input being the output of a pre-processor. The `%line'
directive allows NASM to output messages which indicate the line
number of the original source file, instead of the file that is
being read by NASM.
This preprocessor directive is not generally of use to programmers,
by may be of interest to preprocessor authors. The usage of the
`%line' preprocessor directive is as follows:
%line nnn[+mmm] [filename]
In this directive, `nnn' identifies the line of the original source
file which this line corresponds to. `mmm' is an optional parameter
which specifies a line increment value; each line of the input file
read in is considered to correspond to `mmm' lines of the original
source file. Finally, `filename' is an optional parameter which
specifies the file name of the original source file.
After reading a `%line' preprocessor directive, NASM will report all
file name and line numbers relative to the values specified therein.
4.10.2 `%!'`<env>': Read an environment variable.
The `%!<env>' directive makes it possible to read the value of an
environment variable at assembly time. This could, for example, be
used to store the contents of an environment variable into a string,
which could be used at some other point in your code.
For example, suppose that you have an environment variable `FOO',
and you want the contents of `FOO' to be embedded in your program.
You could do that as follows:
%defstr FOO %!FOO
See section 4.1.8 for notes on the `%defstr' directive.
If the name of the environment variable contains non-identifier
characters, you can use string quotes to surround the name of the
variable, for example:
%defstr C_colon %!'C:'
4.11 Comment Blocks: `%comment'
The `%comment' and `%endcomment' directives are used to specify a
block of commented (i.e. unprocessed) code/text. Everything between
`%comment' and `%endcomment' will be ignored by the preprocessor.
%comment
; some code, text or data to be ignored
%endcomment
4.12 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
`%clear' directive to empty the preprocessor of everything but
context-local preprocessor variables and single-line macros.
Most user-level assembler directives (see chapter 6) are implemented
as macros which invoke primitive directives; these are described in
chapter 6. The rest of the standard macro set is described here.
4.12.1 NASM Version Macros
The single-line macros `__NASM_MAJOR__', `__NASM_MINOR__',
`__NASM_SUBMINOR__' and `___NASM_PATCHLEVEL__' expand to the major,
minor, subminor and patch level parts of the version number of NASM
being used. So, under NASM 0.98.32p1 for example, `__NASM_MAJOR__'
would be defined to be 0, `__NASM_MINOR__' would be defined as 98,
`__NASM_SUBMINOR__' would be defined to 32, and
`___NASM_PATCHLEVEL__' would be defined as 1.
Additionally, the macro `__NASM_SNAPSHOT__' is defined for
automatically generated snapshot releases _only_.
4.12.2 `__NASM_VERSION_ID__': NASM Version ID
The single-line macro `__NASM_VERSION_ID__' expands to a dword
integer representing the full version number of the version of nasm
being used. The value is the equivalent to `__NASM_MAJOR__',
`__NASM_MINOR__', `__NASM_SUBMINOR__' and `___NASM_PATCHLEVEL__'
concatenated to produce a single doubleword. Hence, for 0.98.32p1,
the returned number would be equivalent to:
dd 0x00622001
or
db 1,32,98,0
Note that the above lines are generate exactly the same code, the
second line is used just to give an indication of the order that the
separate values will be present in memory.
4.12.3 `__NASM_VER__': NASM Version string
The single-line macro `__NASM_VER__' expands to a string which
defines the version number of nasm being used. So, under NASM
0.98.32 for example,
db __NASM_VER__
would expand to
db "0.98.32"
4.12.4 `__FILE__' and `__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
`__FILE__' expands to a string constant giving the name of the
current input file (which may change through the course of assembly
if `%include' directives are used), and `__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 `__LINE__' inside a macro
definition (either single-line or multi-line) will return the line
number of the macro _call_, rather than _definition_. So to
determine where in a piece of code a crash is occurring, for
example, one could write a routine `stillhere', which is passed a
line number in `EAX' and outputs something like `line 155: still
here'. You could then write a macro
%macro notdeadyet 0
push eax
mov eax,__LINE__
call stillhere
pop eax
%endmacro
and then pepper your code with calls to `notdeadyet' until you find
the crash point.
4.12.5 `__BITS__': Current BITS Mode
The `__BITS__' standard macro is updated every time that the BITS
mode is set using the `BITS XX' or `[BITS XX]' directive, where XX
is a valid mode number of 16, 32 or 64. `__BITS__' receives the
specified mode number and makes it globally available. This can be
very useful for those who utilize mode-dependent macros.
4.12.6 `__OUTPUT_FORMAT__': Current Output Format
The `__OUTPUT_FORMAT__' standard macro holds the current Output
Format, as given by the `-f' option or NASM's default. Type
`nasm -hf' for a list.
NASM provides a variety of macros that represent the timestamp of
the assembly session.
(*) The `__DATE__' and `__TIME__' macros give the assembly date and
time as strings, in ISO 8601 format (`"YYYY-MM-DD"' and
`"HH:MM:SS"', respectively.)
(*) The `__DATE_NUM__' and `__TIME_NUM__' macros give the assembly
date and time in numeric form; in the format `YYYYMMDD' and
`HHMMSS' respectively.
(*) The `__UTC_DATE__' and `__UTC_TIME__' macros give the assembly
date and time in universal time (UTC) as strings, in ISO 8601
format (`"YYYY-MM-DD"' and `"HH:MM:SS"', respectively.) If the
host platform doesn't provide UTC time, these macros are
undefined.
(*) The `__UTC_DATE_NUM__' and `__UTC_TIME_NUM__' macros give the
assembly date and time universal time (UTC) in numeric form; in
the format `YYYYMMDD' and `HHMMSS' respectively. If the host
platform doesn't provide UTC time, these macros are undefined.
(*) The `__POSIX_TIME__' macro is defined as a number containing the
number of seconds since the POSIX epoch, 1 January 1970 00:00:00
UTC; excluding any leap seconds. This is computed using UTC time
if available on the host platform, otherwise it is computed
using the local time as if it was UTC.
All instances of time and date macros in the same assembly session
produce consistent output. For example, in an assembly session
started at 42 seconds after midnight on January 1, 2010 in Moscow
(timezone UTC+3) these macros would have the following values,
assuming, of course, a properly configured environment with a
correct clock:
4.12.8 `__USE_'_package_`__': Package Include Test
When a standard macro package (see chapter 5) is included with the
`%use' directive (see section 4.6.4), a single-line macro of the
form `__USE_'_package_`__' is automatically defined. This allows
testing if a particular package is invoked or not.
For example, if the `altreg' package is included (see section 5.1),
then the macro `__USE_ALTREG__' is defined.
4.12.9 `__PASS__': Assembly Pass
The macro `__PASS__' is defined to be `1' on preparatory passes, and
`2' on the final pass. In preprocess-only mode, it is set to `3',
and when running only to generate dependencies (due to the `-M' or
`-MG' option, see section 2.1.4) it is set to `0'.
_Avoid using this macro if at all possible. It is tremendously easy
to generate very strange errors by misusing it, and the semantics
may change in future versions of NASM._
4.12.10 `STRUC' and `ENDSTRUC': 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
`STRUC' and `ENDSTRUC' are used to define a structure data type.
`STRUC' takes one or two parameters. The first parameter is the name
of the data type. The second, optional parameter is the base offset
of the structure. The name of the data type is defined as a symbol
with the value of the base offset, and the name of the data type
with the suffix `_size' appended to it is defined as an `EQU' giving
the size of the structure. Once `STRUC' has been issued, you are
defining the structure, and should define fields using the `RESB'
family of pseudo-instructions, and then invoke `ENDSTRUC' to finish
the definition.
For example, to define a structure called `mytype' containing a
longword, a word, a byte and a string of bytes, you might code
The above code defines six symbols: `mt_long' as 0 (the offset from
the beginning of a `mytype' structure to the longword field),
`mt_word' as 4, `mt_byte' as 6, `mt_str' as 7, `mytype_size' as 39,
and `mytype' itself as zero.
The reason why the structure type name is defined at zero by default
is a side effect of allowing structures to work with the local label
mechanism: if your structure members tend to have the same names in
more than one structure, you can define the above structure like
this:
This defines the offsets to the structure fields as `mytype.long',
`mytype.word', `mytype.byte' and `mytype.str'.
NASM, since it has no _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 `mov ax,[mystruc.mt_word]' is not valid. `mt_word'
is a constant just like any other constant, so the correct syntax is
`mov ax,[mystruc+mt_word]' or `mov ax,[mystruc+mytype.word]'.
Sometimes you only have the address of the structure displaced by an
offset. For example, consider this standard stack frame setup:
push ebp
mov ebp, esp
sub esp, 40
In this case, you could access an element by subtracting the offset:
mov [ebp - 40 + mytype.word], ax
However, if you do not want to repeat this offset, you can use -40
as a base offset:
struc mytype, -40
And access an element this way:
mov [ebp + mytype.word], ax
4.12.11 `ISTRUC', `AT' and `IEND': Declaring 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 `ISTRUC'
mechanism. To declare a structure of type `mytype' in a program, you
code something like this:
mystruc:
istruc mytype
at mt_long, dd 123456
at mt_word, dw 1024
at mt_byte, db 'x'
at mt_str, db 'hello, world', 13, 10, 0
iend
The function of the `AT' macro is to make use of the `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 `AT' line. For example:
at mt_str, db 123,134,145,156,167,178,189
db 190,100,0
Depending on personal taste, you can also omit the code part of the
`AT' line completely, and start the structure field on the next
line:
at mt_str
db 'hello, world'
db 13,10,0
4.12.12 `ALIGN' and `ALIGNB': Data Alignment
The `ALIGN' and `ALIGNB' macros provides a convenient way to align
code or data on a word, longword, paragraph or other boundary. (Some
assemblers call this directive `EVEN'.) The syntax of the `ALIGN'
and `ALIGNB' macros is
align 4 ; align on 4-byte boundary
align 16 ; align on 16-byte boundary
align 8,db 0 ; pad with 0s rather than NOPs
align 4,resb 1 ; align to 4 in the BSS
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 `TIMES' prefix to their second argument to
perform the alignment.
If the second argument is not specified, the default for `ALIGN' is
`NOP', and the default for `ALIGNB' is `RESB 1'. So if the second
argument is specified, the two macros are equivalent. Normally, you
can just use `ALIGN' in code and data sections and `ALIGNB' in BSS
sections, and never need the second argument except for special
purposes.
`ALIGN' and `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.
`ALIGNB' (or `ALIGN' with a second argument of `RESB 1') can be used
within structure definitions:
This will ensure that the structure members are sensibly aligned
relative to the base of the structure.
A final caveat: `ALIGN' and `ALIGNB' work relative to the beginning
of the _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
`ALIGN' or `ALIGNB'.
Both `ALIGN' and `ALIGNB' do call `SECTALIGN' macro implicitly. See
section 4.12.13 for details.
See also the `smartalign' standard macro package, section 5.2.
4.12.13 `SECTALIGN': Section Alignment
The `SECTALIGN' macros provides a way to modify alignment attribute
of output file section. Unlike the `align=' attribute (which is
allowed at section definition only) the `SECTALIGN' macro may be
used at any time.
For example the directive
SECTALIGN 16
sets the section alignment requirements to 16 bytes. Once increased
it can not be decreased, the magnitude may grow only.
Note that `ALIGN' (see section 4.12.12) calls the `SECTALIGN' macro
implicitly so the active section alignment requirements may be
updated. This is by default behaviour, if for some reason you want
the `ALIGN' do not call `SECTALIGN' at all use the directive
SECTALIGN OFF
It is still possible to turn in on again by
SECTALIGN ON
Chapter 5: Standard Macro Packages
----------------------------------
The `%use' directive (see section 4.6.4) includes one of the
standard macro packages included with the NASM distribution and
compiled into the NASM binary. It operates like the `%include'
directive (see section 4.6.1), but the included contents is provided
by NASM itself.
The names of standard macro packages are case insensitive, and can
be quoted or not.
5.1 `altreg': Alternate Register Names
The `altreg' standard macro package provides alternate register
names. It provides numeric register names for all registers (not
just `R8'-`R15'), the Intel-defined aliases `R8L'-`R15L' for the low
bytes of register (as opposed to the NASM/AMD standard names `R8B'-
`R15B'), and the names `R0H'-`R3H' (by analogy with `R0L'-`R3L') for
`AH', `CH', `DH', and `BH'.
Example use:
%use altreg
proc:
mov r0l,r3h ; mov al,bh
ret
See also section 11.1.
5.2 `smartalign': Smart `ALIGN' Macro
The `smartalign' standard macro package provides for an `ALIGN'
macro which is more powerful than the default (and backwards-
compatible) one (see section 4.12.12). When the `smartalign' package
is enabled, when `ALIGN' is used without a second argument, NASM
will generate a sequence of instructions more efficient than a
series of `NOP'. Furthermore, if the padding exceeds a specific
threshold, then NASM will generate a jump over the entire padding
sequence.
The specific instructions generated can be controlled with the new
`ALIGNMODE' macro. This macro takes two parameters: one mode, and an
optional jump threshold override. If (for any reason) you need to
turn off the jump completely just set jump threshold value to -1 (or
set it to `nojmp'). The following modes are possible:
(*) `generic': Works on all x86 CPUs and should have reasonable
performance. The default jump threshold is 8. This is the
default.
(*) `nop': Pad out with `NOP' instructions. The only difference
compared to the standard `ALIGN' macro is that NASM can still
jump over a large padding area. The default jump threshold is
16.
(*) `k7': Optimize for the AMD K7 (Athlon/Althon XP). These
instructions should still work on all x86 CPUs. The default jump
threshold is 16.
(*) `k8': Optimize for the AMD K8 (Opteron/Althon 64). These
instructions should still work on all x86 CPUs. The default jump
threshold is 16.
(*) `p6': Optimize for Intel CPUs. This uses the long `NOP'
instructions first introduced in Pentium Pro. This is
incompatible with all CPUs of family 5 or lower, as well as some
VIA CPUs and several virtualization solutions. The default jump
threshold is 16.
The macro `__ALIGNMODE__' is defined to contain the current
alignment mode. A number of other macros beginning with `__ALIGN_'
are used internally by this macro package.
5.3 `fp': Floating-point macros
This packages contains the following floating-point convenience
macros:
This package contains a set of macros which implement integer
functions. These are actually implemented as special operators, but
are most conveniently accessed via this macro package.
The macros provided are:
5.4.1 Integer logarithms
These functions calculate the integer logarithm base 2 of their
argument, considered as an unsigned integer. The only differences
between the functions is their behavior if the argument provided is
not a power of two.
The function `ilog2e()' (alias `ilog2()') generate an error if the
argument is not a power of two.
The function `ilog2w()' generate a warning if the argument is not a
power of two.
The function `ilog2f()' rounds the argument down to the nearest
power of two; if the argument is zero it returns zero.
The function `ilog2c()' rounds the argument up to the nearest power
of two.
NASM, though it attempts to avoid the bureaucracy of assemblers like
MASM and TASM, is nevertheless forced to support a _few_ directives.
These are described in this chapter.
NASM's directives come in two types: _user-level_ directives and
_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
_format-specific_ directives are documented along with the formats
that implement them, in chapter 7.
6.1 `BITS': Specifying Target Processor Mode
The `BITS' directive specifies whether NASM should generate code
designed to run on a processor operating in 16-bit mode, 32-bit mode
or 64-bit mode. The syntax is `BITS XX', where XX is 16, 32 or 64.
In most cases, you should not need to use `BITS' explicitly. The
`aout', `coff', `elf', `macho', `win32' and `win64' object formats,
which are designed for use in 32-bit or 64-bit operating systems,
all cause NASM to select 32-bit or 64-bit mode, respectively, by
default. The `obj' object format allows you to specify each segment
you define as either `USE16' or `USE32', and NASM will set its
operating mode accordingly, so the use of the `BITS' directive is
once again unnecessary.
The most likely reason for using the `BITS' directive is to write
32-bit or 64-bit code in a flat binary file; this is because the
`bin' output format defaults to 16-bit mode in anticipation of it
being used most frequently to write DOS `.COM' programs, DOS `.SYS'
device drivers and boot loader software.
You do _not_ need to specify `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 `BITS 16' mode, instructions which use 32-bit data
are prefixed with an 0x66 byte, and those referring to 32-bit
addresses have an 0x67 prefix. In `BITS 32' mode, the reverse is
true: 32-bit instructions require no prefixes, whereas instructions
using 16-bit data need an 0x66 and those working on 16-bit addresses
need an 0x67.
When NASM is in `BITS 64' mode, most instructions operate the same
as they do for `BITS 32' mode. However, there are 8 more general and
SSE registers, and 16-bit addressing is no longer supported.
The default address size is 64 bits; 32-bit addressing can be
selected with the 0x67 prefix. The default operand size is still 32
bits, however, and the 0x66 prefix selects 16-bit operand size. The
`REX' prefix is used both to select 64-bit operand size, and to
access the new registers. NASM automatically inserts REX prefixes
when necessary.
When the `REX' prefix is used, the processor does not know how to
address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead,
it is possible to access the the low 8-bits of the SP, BP SI and DI
registers as SPL, BPL, SIL and DIL, respectively; but only when the
REX prefix is used.
The `BITS' directive has an exactly equivalent primitive form,
`[BITS 16]', `[BITS 32]' and `[BITS 64]'. The user-level form is a
macro which has no function other than to call the primitive form.
Note that the space is neccessary, e.g. `BITS32' will _not_ work!
6.1.1 `USE16' & `USE32': Aliases for BITS
The ``USE16'' and ``USE32'' directives can be used in place of
``BITS 16'' and ``BITS 32'', for compatibility with other
assemblers.
6.2 `DEFAULT': Change the assembler defaults
The `DEFAULT' directive changes the assembler defaults. Normally,
NASM defaults to a mode where the programmer is expected to
explicitly specify most features directly. However, this is
occationally obnoxious, as the explicit form is pretty much the only
one one wishes to use.
Currently, `DEFAULT' can set `REL' & `ABS' and `BND' & `NOBND'.
6.2.1 `REL' & `ABS': RIP-relative addressing
This sets whether registerless instructions in 64-bit mode are
`RIP'-relative or not. By default, they are absolute unless
overridden with the `REL' specifier (see section 3.3). However, if
`DEFAULT REL' is specified, `REL' is default, unless overridden with
the `ABS' specifier, _except when used with an FS or GS segment
override_.
The special handling of `FS' and `GS' overrides are due to the fact
that these registers are generally used as thread pointers or other
special functions in 64-bit mode, and generating `RIP'-relative
addresses would be extremely confusing.
`DEFAULT REL' is disabled with `DEFAULT ABS'.
6.2.2 `BND' & `NOBND': `BND' prefix
If `DEFAULT BND' is set, all bnd-prefix available instructions
following this directive are prefixed with bnd. To override it,
`NOBND' prefix can be used.
DEFAULT BND
call foo ; BND will be prefixed
nobnd call foo ; BND will NOT be prefixed
`DEFAULT NOBND' can disable `DEFAULT BND' and then `BND' prefix will
be added only when explicitly specified in code.
`DEFAULT BND' is expected to be the normal configuration for writing
MPX-enabled code.
6.3 `SECTION' or `SEGMENT': Changing and Defining Sections
The `SECTION' directive (`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 `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 `bin' object format (but see
section 7.1.3, all support the standardized section names `.text',
`.data' and `.bss' for the code, data and uninitialized-data
sections. The `obj' format, by contrast, does not recognize these
section names as being special, and indeed will strip off the
leading period of any section name that has one.
6.3.1 The `__SECT__' Macro
The `SECTION' directive is unusual in that its user-level form
functions differently from its primitive form. The primitive form,
`[SECTION xyz]', simply switches the current target section to the
one given. The user-level form, `SECTION xyz', however, first
defines the single-line macro `__SECT__' to be the primitive
`[SECTION]' directive which it is about to issue, and then issues
it. So the user-level directive
SECTION .text
expands to the two lines
%define __SECT__ [SECTION .text]
[SECTION .text]
Users may find it useful to make use of this in their own macros.
For example, the `writefile' macro defined in section 4.3.3 can be
usefully rewritten in the following more sophisticated form:
%macro writefile 2+
[section .data]
%%str: db %2
%%endstr:
__SECT__
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%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 `SECTION' directive so as not to modify
`__SECT__'. It then declares its string in the data section, and
then invokes `__SECT__' to switch back to _whichever_ section the
user was previously working in. It thus avoids the need, in the
previous version of the macro, to include a `JMP' instruction to
jump over the data, and also does not fail if, in a complicated
`OBJ' format module, the user could potentially be assembling the
code in any of several separate code sections.
6.4 `ABSOLUTE': Defining Absolute Labels
The `ABSOLUTE' directive can be thought of as an alternative form of
`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 `RESB' family.
`ABSOLUTE' is used as follows:
absolute 0x1A
kbuf_chr resw 1
kbuf_free resw 1
kbuf resw 16
This example describes a section of the PC BIOS data area, at
segment address 0x40: the above code defines `kbuf_chr' to be 0x1A,
`kbuf_free' to be 0x1C, and `kbuf' to be 0x1E.
The user-level form of `ABSOLUTE', like that of `SECTION', redefines
the `__SECT__' macro when it is invoked.
`STRUC' and `ENDSTRUC' are defined as macros which use `ABSOLUTE'
(and also `__SECT__').
`ABSOLUTE' doesn't have to take an absolute constant as an argument:
it can take an expression (actually, a critical expression: see
section 3.8) 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:
org 100h ; it's a .COM program
jmp setup ; setup code comes last
; the resident part of the TSR goes here
setup:
; now write the code that installs the TSR here
absolute setup
runtimevar1 resw 1
runtimevar2 resd 20
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.
6.5 `EXTERN': Importing Symbols from Other Modules
`EXTERN' is similar to the MASM directive `EXTRN' and the C keyword
`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 `bin'
format cannot.
The `EXTERN' directive takes as many arguments as you like. Each
argument is the name of a symbol:
extern _printf
extern _sscanf,_fscanf
Some object-file formats provide extra features to the `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 `obj' format allows you to declare that the default
segment base of an external should be the group `dgroup' by means of
the directive
extern _variable:wrt dgroup
The primitive form of `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 `EXTERN' more than once: NASM
will quietly ignore the second and later redeclarations. You can't
declare a variable as `EXTERN' as well as something else, though.
6.6 `GLOBAL': Exporting Symbols to Other Modules
`GLOBAL' is the other end of `EXTERN': if one module declares a
symbol as `EXTERN' and refers to it, then in order to prevent linker
errors, some other module must actually _define_ the symbol and
declare it as `GLOBAL'. Some assemblers use the name `PUBLIC' for
this purpose.
The `GLOBAL' directive applying to a symbol must appear _before_ the
definition of the symbol.
`GLOBAL' uses the same syntax as `EXTERN', except that it must refer
to symbols which _are_ defined in the same module as the `GLOBAL'
directive. For example:
global _main
_main:
; some code
`GLOBAL', like `EXTERN', allows object formats to define private
extensions by means of a colon. The `elf' object format, for
example, lets you specify whether global data items are functions or
data:
global hashlookup:function, hashtable:data
Like `EXTERN', the primitive form of `GLOBAL' differs from the user-
level form only in that it can take only one argument at a time.
6.7 `COMMON': Defining Common Data Areas
The `COMMON' directive is used to declare _common variables_. A
common variable is much like a global variable declared in the
uninitialized data section, so that
common intvar 4
is similar in function to
global intvar
section .bss
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 _merged_,
and references to `intvar' in all modules will point at the same
piece of memory.
Like `GLOBAL' and `EXTERN', `COMMON' supports object-format specific
extensions. For example, the `obj' format allows common variables to
be NEAR or FAR, and the `elf' format allows you to specify the
alignment requirements of a common variable:
common commvar 4:near ; works in OBJ
common intarray 100:4 ; works in ELF: 4 byte aligned
Once again, like `EXTERN' and `GLOBAL', the primitive form of
`COMMON' differs from the user-level form only in that it can take
only one argument at a time.
6.8 `CPU': Defining CPU Dependencies
The `CPU' directive restricts assembly to those instructions which
are available on the specified CPU.
Options are:
(*) `CPU 8086' Assemble only 8086 instruction set
(*) `CPU 186' Assemble instructions up to the 80186 instruction set
(*) `CPU 286' Assemble instructions up to the 286 instruction set
(*) `CPU 386' Assemble instructions up to the 386 instruction set
(*) `CPU 486' 486 instruction set
(*) `CPU 586' Pentium instruction set
(*) `CPU PENTIUM' Same as 586
(*) `CPU 686' P6 instruction set
(*) `CPU PPRO' Same as 686
(*) `CPU P2' Same as 686
(*) `CPU P3' Pentium III (Katmai) instruction sets
(*) `CPU KATMAI' Same as P3
(*) `CPU P4' Pentium 4 (Willamette) instruction set
(*) `CPU WILLAMETTE' Same as P4
(*) `CPU PRESCOTT' Prescott instruction set
(*) `CPU X64' x86-64 (x64/AMD64/Intel 64) instruction set
(*) `CPU IA64' IA64 CPU (in x86 mode) instruction set
All options are case insensitive. All instructions will be selected
only if they apply to the selected CPU or lower. By default, all
instructions are available.
6.9 `FLOAT': Handling of floating-point constants
By default, floating-point constants are rounded to nearest, and
IEEE denormals are supported. The following options can be set to
alter this behaviour:
(*) `FLOAT DAZ' Flush denormals to zero
(*) `FLOAT NODAZ' Do not flush denormals to zero (default)
(*) `FLOAT NEAR' Round to nearest (default)
(*) `FLOAT UP' Round up (toward +Infinity)
(*) `FLOAT DOWN' Round down (toward -Infinity)
(*) `FLOAT ZERO' Round toward zero
(*) `FLOAT DEFAULT' Restore default settings
The standard macros `__FLOAT_DAZ__', `__FLOAT_ROUND__', and
`__FLOAT__' contain the current state, as long as the programmer has
avoided the use of the brackeded primitive form, (`[FLOAT]').
`__FLOAT__' contains the full set of floating-point settings; this
value can be saved away and invoked later to restore the setting.
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 `-f' option on the
NASM command line. Each of these formats, along with its extensions
to the base NASM syntax, is detailed in this chapter.
As stated in section 2.1.1, NASM chooses a default name for your
output file based on the input file name and the chosen output
format. This will be generated by removing the extension (`.asm',
`.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.
7.1 `bin': Flat-Form Binary Output
The `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 MS-DOS: `.COM' executables and `.SYS' device
drivers are pure binary files. Pure binary output is also useful for
operating system and boot loader development.
The `bin' format supports multiple section names. For details of how
NASM handles sections in the `bin' format, see section 7.1.3.
Using the `bin' format puts NASM by default into 16-bit mode (see
section 6.1). In order to use `bin' to write 32-bit or 64-bit code,
such as an OS kernel, you need to explicitly issue the `BITS 32' or
`BITS 64' directive.
`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 `binprog.asm'
into a binary file called `binprog'.
7.1.1 `ORG': Binary File Program Origin
The `bin' format provides an additional directive to the list given
in chapter 6: `ORG'. The function of the `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
`0x00000104':
org 0x100
dd label
label:
Unlike the `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 `ORG' does exactly what the
directive says: _origin_. Its sole function is to specify one offset
which is added to all internal address references within the
section; it does not permit any of the trickery that MASM's version
does. See section 12.1.3 for further comments.
7.1.2 `bin' Extensions to the `SECTION' Directive
The `bin' output format extends the `SECTION' (or `SEGMENT')
directive to allow you to specify the alignment requirements of
segments. This is done by appending the `ALIGN' qualifier to the end
of the section-definition line. For example,
section .data align=16
switches to the section `.data' and also specifies that it must be
aligned on a 16-byte boundary.
The parameter to `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.
7.1.3 Multisection Support for the `bin' Format
The `bin' format allows the use of multiple sections, of arbitrary
names, besides the "known" `.text', `.data', and `.bss' names.
(*) Sections may be designated `progbits' or `nobits'. Default is
`progbits' (except `.bss', which defaults to `nobits', of
course).
(*) Sections can be aligned at a specified boundary following the
previous section with `align=', or at an arbitrary byte-granular
position with `start='.
(*) Sections can be given a virtual start address, which will be
used for the calculation of all memory references within that
section with `vstart='.
(*) Sections can be ordered using `follows='`<section>' or
`vfollows='`<section>' as an alternative to specifying an
explicit start address.
(*) Arguments to `org', `start', `vstart', and `align=' are critical
expressions. See section 3.8. E.g. `align=(1 << ALIGN_SHIFT)' -
`ALIGN_SHIFT' must be defined before it is used here.
(*) Any code which comes before an explicit `SECTION' directive is
directed by default into the `.text' section.
(*) If an `ORG' statement is not given, `ORG 0' is used by default.
(*) The `.bss' section will be placed after the last `progbits'
section, unless `start=', `vstart=', `follows=', or `vfollows='
has been specified.
(*) All sections are aligned on dword boundaries, unless a different
alignment has been specified.
(*) Sections may not overlap.
(*) NASM creates the `section.<secname>.start' for each section,
which may be used in your code.
7.1.4 Map Files
Map files can be generated in `-f bin' format by means of the
`[map]' option. Map types of `all' (default), `brief', `sections',
`segments', or `symbols' may be specified. Output may be directed to
`stdout' (default), `stderr', or a specified file. E.g.
`[map symbols myfile.map]'. No "user form" exists, the square
brackets must be used.
7.2 `ith': Intel Hex Output
The `ith' file format produces Intel hex-format files. Just as the
`bin' format, this is a flat memory image format with no support for
relocation or linking. It is usually used with ROM programmers and
similar utilities.
All extensions supported by the `bin' file format is also supported
by the `ith' file format.
`ith' provides a default output file-name extension of `.ith'.
7.3 `srec': Motorola S-Records Output
The `srec' file format produces Motorola S-records files. Just as
the `bin' format, this is a flat memory image format with no support
for relocation or linking. It is usually used with ROM programmers
and similar utilities.
All extensions supported by the `bin' file format is also supported
by the `srec' file format.
`srec' provides a default output file-name extension of `.srec'.
7.4 `obj': Microsoft OMF Object Files
The `obj' file format (NASM calls it `obj' rather than `omf' for
historical reasons) is the one produced by MASM and TASM, which is
typically fed to 16-bit DOS linkers to produce `.EXE' files. It is
also the format used by OS/2.
`obj' provides a default output file-name extension of `.obj'.
`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 `obj' format files are used by Borland's Win32 compilers,
instead of using Microsoft's newer `win32' object file format.
The `obj' format does not define any special segment names: you can
call your segments anything you like. Typical names for segments in
`obj' format files are `CODE', `DATA' and `BSS'.
If your source file contains code before specifying an explicit
`SEGMENT' directive, then NASM will invent its own segment called
`__NASMDEFSEG' for you.
When you define a segment in an `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:
segment data
dvar: dw 1234
segment code
function:
mov ax,data ; get segment address of data
mov ds,ax ; and move it into DS
inc word [dvar] ; now this reference will work
ret
The `obj' format also enables the use of the `SEG' and `WRT'
operators, so that you can write code which does things like
extern foo
mov ax,seg foo ; get preferred segment of foo
mov ds,ax
mov ax,data ; a different segment
mov es,ax
mov ax,[ds:foo] ; this accesses `foo'
mov [es:foo wrt data],bx ; so does this
7.4.1 `obj' Extensions to the `SEGMENT' Directive
The `obj' output format extends the `SEGMENT' (or `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,
segment code private align=16
defines the segment `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:
(*) `PRIVATE', `PUBLIC', `COMMON' and `STACK' specify the
combination characteristics of the segment. `PRIVATE' segments
do not get combined with any others by the linker; `PUBLIC' and
`STACK' segments get concatenated together at link time; and
`COMMON' segments all get overlaid on top of each other rather
than stuck end-to-end.
(*) `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 PharLap extension to the format and
may not be supported by all linkers.
(*) `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. `CLASS=CODE'.
(*) `OVERLAY', like `CLASS', is specified with an arbitrary word as
an argument, and provides overlay information to an overlay-
capable linker.
(*) Segments can be declared as `USE16' or `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.
(*) When writing OS/2 object files, you should declare 32-bit
segments as `FLAT', which causes the default segment base for
anything in the segment to be the special group `FLAT', and also
defines the group if it is not already defined.
(*) The `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 `SEGMENT SCREEN ABSOLUTE=0xB800' if you need to. The
`ABSOLUTE' and `ALIGN' keywords are mutually exclusive.
NASM's default segment attributes are `PUBLIC', `ALIGN=1', no class,
no overlay, and `USE16'.
7.4.2 `GROUP': Defining Groups of Segments
The `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 `GROUP' directive, whereby you
can code
segment data
; some data
segment bss
; some uninitialized data
group dgroup data bss
which will define a group called `dgroup' to contain the segments
`data' and `bss'. Like `SEGMENT', `GROUP' causes the group name to
be defined as a symbol, so that you can refer to a variable `var' in
the `data' segment as `var wrt data' or as `var wrt dgroup',
depending on which segment value is currently in your segment
register.
If you just refer to `var', however, and `var' is declared in a
segment which is part of a group, then NASM will default to giving
you the offset of `var' from the beginning of the _group_, not the
_segment_. Therefore `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
`WRT' references to a group which does not contain the variable you
are referring to. OS/2, for example, defines the special group
`FLAT' with no segments in it.
7.4.3 `UPPERCASE': Disabling Case Sensitivity in Output
Although NASM itself is case sensitive, some OMF linkers are not;
therefore it can be useful for NASM to output single-case object
files. The `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.
`UPPERCASE' is used alone on a line; it requires no parameters.
7.4.4 `IMPORT': Importing DLL Symbols
The `IMPORT' format-specific directive defines a symbol to be
imported from a DLL, for use if you are writing a DLL's import
library in NASM. You still need to declare the symbol as `EXTERN' as
well as using the `IMPORT' directive.
The `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:
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:
import asyncsel wsock32.dll WSAAsyncSelect
7.4.5 `EXPORT': Exporting DLL Symbols
The `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 `GLOBAL' as well as using
the `EXPORT' directive.
`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 _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:
(*) `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.
(*) `nodata' indicates that the exported symbol is a function which
does not make use of any initialized data.
(*) `parm=NNN', where `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.
(*) An attribute which is just a number indicates that the symbol
should be exported with an identifying number (ordinal), and
gives the desired number.
`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 `..start' at the point where you wish execution
to begin.
7.4.7 `obj' Extensions to the `EXTERN' Directive
If you declare an external symbol with the directive
extern foo
then references such as `mov ax,foo' will give you the offset of
`foo' from its preferred segment base (as specified in whichever
module `foo' is actually defined in). So to access the contents of
`foo' you will usually need to do something like
mov ax,seg foo ; get preferred segment base
mov es,ax ; move it into ES
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
`dgroup'. So if `DS' already contained `dgroup', you could simply
code
mov ax,[foo wrt dgroup]
However, having to type this every time you want to access `foo' can
be a pain; so NASM allows you to declare `foo' in the alternative
form
extern foo:wrt dgroup
This form causes NASM to pretend that the preferred segment base of
`foo' is in fact `dgroup'; so the expression `seg foo' will now
return `dgroup', and the expression `foo' is equivalent to
`foo wrt dgroup'.
This default-`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 section 7.4.8.
7.4.8 `obj' Extensions to the `COMMON' Directive
The `obj' format allows common variables to be either near or far;
NASM allows you to specify which your variables should be by the use
of the syntax
common nearvar 2:near ; `nearvar' is a near common
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
_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 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:
common c_5by2 10:far 5 ; two five-byte elements
common c_2by5 10:far 2 ; five two-byte elements
If no element size is specified, the default is 1. Also, the `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
common c_5by2 10:5 ; two five-byte elements
common c_2by5 10:2 ; five two-byte elements
In addition to these extensions, the `COMMON' directive in `obj'
also supports default-`WRT' specification like `EXTERN' does
(explained in section 7.4.7). So you can also declare things like
common foo 10:wrt dgroup
common bar 16:far 2:wrt data
common baz 24:wrt data:6
7.5 `win32': Microsoft Win32 Object Files
The `win32' output format generates Microsoft Win32 object files,
suitable for passing to Microsoft linkers such as Visual C++. Note
that Borland Win32 compilers do not use this format, but use `obj'
instead (see section 7.4).
`win32' provides a default output file-name extension of `.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 `coff' output
format; conversely, the `coff' format does not produce object files
that Win32 linkers can generate correct output from.
7.5.1 `win32' Extensions to the `SECTION' Directive
Like the `obj' format, `win32' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names
`.text', `.data' and `.bss', but may still be overridden by these
qualifiers.
The available qualifiers are:
(*) `code', or equivalently `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.
(*) `data' and `bss' define the section to be a data section,
analogously to `code'. Data sections are marked as readable and
writable, but not executable. `data' declares an initialized
data section, whereas `bss' declares an uninitialized data
section.
(*) `rdata' declares an initialized data section that is readable
but not writable. Microsoft compilers use this section to place
constants in it.
(*) `info' defines the section to be an informational section, which
is not included in the executable file by the linker, but may
(for example) pass information _to_ the linker. For example,
declaring an `info'-type section called `.drectve' causes the
linker to interpret the contents of the section as command-line
options.
(*) `align=', used with a trailing number as in `obj', gives the
alignment requirements of the section. The maximum you may
specify is 64: the Win32 object file format contains no means to
request a greater section alignment than this. If alignment is
not explicitly specified, the defaults are 16-byte alignment for
code sections, 8-byte alignment for rdata sections and 4-byte
alignment for data (and BSS) sections. Informational sections
get a default alignment of 1 byte (no alignment), though the
value does not matter.
The defaults assumed by NASM if you do not specify the above
qualifiers are:
Any other section name is treated by default like `.text'.
7.5.2 `win32': Safe Structured Exception Handling
Among other improvements in Windows XP SP2 and Windows Server 2003
Microsoft has introduced concept of "safe structured exception
handling." General idea is to collect handlers' entry points in
designated read-only table and have alleged entry point verified
against this table prior exception control is passed to the handler.
In order for an executable module to be equipped with such "safe
exception handler table," all object modules on linker command line
has to comply with certain criteria. If one single module among them
does not, then the table in question is omitted and above mentioned
run-time checks will not be performed for application in question.
Table omission is by default silent and therefore can be easily
overlooked. One can instruct linker to refuse to produce binary
without such table by passing `/safeseh' command line option.
Without regard to this run-time check merits it's natural to expect
NASM to be capable of generating modules suitable for `/safeseh'
linking. From developer's viewpoint the problem is two-fold:
(*) how to adapt modules not deploying exception handlers of their
own;
(*) how to adapt/develop modules utilizing custom exception
handling;
Former can be easily achieved with any NASM version by adding
following line to source code:
$@feat.00 equ 1
As of version 2.03 NASM adds this absolute symbol automatically. If
it's not already present to be precise. I.e. if for whatever reason
developer would choose to assign another value in source file, it
would still be perfectly possible.
Registering custom exception handler on the other hand requires
certain "magic." As of version 2.03 additional directive is
implemented, `safeseh', which instructs the assembler to produce
appropriately formatted input data for above mentioned "safe
exception handler table." Its typical use would be:
section .text
extern _MessageBoxA@16
%if __NASM_VERSION_ID__ >= 0x02030000
safeseh handler ; register handler as "safe handler"
%endif
handler:
push DWORD 1 ; MB_OKCANCEL
push DWORD caption
push DWORD text
push DWORD 0
call _MessageBoxA@16
sub eax,1 ; incidentally suits as return value
; for exception handler
ret
global _main
_main:
push DWORD handler
push DWORD [fs:0]
mov DWORD [fs:0],esp ; engage exception handler
xor eax,eax
mov eax,DWORD[eax] ; cause exception
pop DWORD [fs:0] ; disengage exception handler
add esp,4
ret
text: db 'OK to rethrow, CANCEL to generate core dump',0
caption:db 'SEGV',0
section .drectve info
db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
As you might imagine, it's perfectly possible to produce .exe binary
with "safe exception handler table" and yet engage unregistered
exception handler. Indeed, handler is engaged by simply manipulating
`[fs:0]' location at run-time, something linker has no power over,
run-time that is. It should be explicitly mentioned that such
failure to register handler's entry point with `safeseh' directive
has undesired side effect at run-time. If exception is raised and
unregistered handler is to be executed, the application is abruptly
terminated without any notification whatsoever. One can argue that
system could at least have logged some kind "non-safe exception
handler in x.exe at address n" message in event log, but no,
literally no notification is provided and user is left with no clue
on what caused application failure.
Finally, all mentions of linker in this paragraph refer to Microsoft
linker version 7.x and later. Presence of `@feat.00' symbol and
input data for "safe exception handler table" causes no backward
incompatibilities and "safeseh" modules generated by NASM 2.03 and
later can still be linked by earlier versions or non-Microsoft
linkers.
7.6 `win64': Microsoft Win64 Object Files
The `win64' output format generates Microsoft Win64 object files,
which is nearly 100% identical to the `win32' object format (section
7.5) with the exception that it is meant to target 64-bit code and
the x86-64 platform altogether. This object file is used exactly the
same as the `win32' object format (section 7.5), in NASM, with
regard to this exception.
7.6.1 `win64': Writing Position-Independent Code
While `REL' takes good care of RIP-relative addressing, there is one
aspect that is easy to overlook for a Win64 programmer: indirect
references. Consider a switch dispatch table:
Even a novice Win64 assembler programmer will soon realize that the
code is not 64-bit savvy. Most notably linker will refuse to link it
with
'ADDR32' relocation to '.text' invalid without /LARGEADDRESSAWARE:NO
So [s]he will have to split jmp instruction as following:
lea rbx,[rel dsptch]
jmp qword [rbx+rax*8]
What happens behind the scene is that effective address in `lea' is
encoded relative to instruction pointer, or in perfectly position-
independent manner. But this is only part of the problem! Trouble is
that in .dll context `caseN' relocations will make their way to the
final module and might have to be adjusted at .dll load time. To be
specific when it can't be loaded at preferred address. And when this
occurs, pages with such relocations will be rendered private to
current process, which kind of undermines the idea of sharing .dll.
But no worry, it's trivial to fix:
NASM version 2.03 and later provides another alternative,
`wrt ..imagebase' operator, which returns offset from base address
of the current image, be it .exe or .dll module, therefore the name.
For those acquainted with PE-COFF format base address denotes start
of `IMAGE_DOS_HEADER' structure. Here is how to implement switch
with these image-relative references:
One can argue that the operator is redundant. Indeed, snippet before
last works just fine with any NASM version and is not even Windows
specific... The real reason for implementing `wrt ..imagebase' will
become apparent in next paragraph.
It should be noted that `wrt ..imagebase' is defined as 32-bit
operand only:
dd label wrt ..imagebase ; ok
dq label wrt ..imagebase ; bad
mov eax,label wrt ..imagebase ; ok
mov rax,label wrt ..imagebase ; bad
7.6.2 `win64': Structured Exception Handling
Structured exception handing in Win64 is completely different matter
from Win32. Upon exception program counter value is noted, and
linker-generated table comprising start and end addresses of all the
functions [in given executable module] is traversed and compared to
the saved program counter. Thus so called `UNWIND_INFO' structure is
identified. If it's not found, then offending subroutine is assumed
to be "leaf" and just mentioned lookup procedure is attempted for
its caller. In Win64 leaf function is such function that does not
call any other function _nor_ modifies any Win64 non-volatile
registers, including stack pointer. The latter ensures that it's
possible to identify leaf function's caller by simply pulling the
value from the top of the stack.
While majority of subroutines written in assembler are not calling
any other function, requirement for non-volatile registers'
immutability leaves developer with not more than 7 registers and no
stack frame, which is not necessarily what [s]he counted with.
Customarily one would meet the requirement by saving non-volatile
registers on stack and restoring them upon return, so what can go
wrong? If [and only if] an exception is raised at run-time and no
`UNWIND_INFO' structure is associated with such "leaf" function, the
stack unwind procedure will expect to find caller's return address
on the top of stack immediately followed by its frame. Given that
developer pushed caller's non-volatile registers on stack, would the
value on top point at some code segment or even addressable space?
Well, developer can attempt copying caller's return address to the
top of stack and this would actually work in some very specific
circumstances. But unless developer can guarantee that these
circumstances are always met, it's more appropriate to assume worst
case scenario, i.e. stack unwind procedure going berserk. Relevant
question is what happens then? Application is abruptly terminated
without any notification whatsoever. Just like in Win32 case, one
can argue that system could at least have logged "unwind procedure
went berserk in x.exe at address n" in event log, but no, no trace
of failure is left.
Now, when we understand significance of the `UNWIND_INFO' structure,
let's discuss what's in it and/or how it's processed. First of all
it is checked for presence of reference to custom language-specific
exception handler. If there is one, then it's invoked. Depending on
the return value, execution flow is resumed (exception is said to be
"handled"), _or_ rest of `UNWIND_INFO' structure is processed as
following. Beside optional reference to custom handler, it carries
information about current callee's stack frame and where non-
volatile registers are saved. Information is detailed enough to be
able to reconstruct contents of caller's non-volatile registers upon
call to current callee. And so caller's context is reconstructed,
and then unwind procedure is repeated, i.e. another `UNWIND_INFO'
structure is associated, this time, with caller's instruction
pointer, which is then checked for presence of reference to
language-specific handler, etc. The procedure is recursively
repeated till exception is handled. As last resort system "handles"
it by generating memory core dump and terminating the application.
As for the moment of this writing NASM unfortunately does not
facilitate generation of above mentioned detailed information about
stack frame layout. But as of version 2.03 it implements building
blocks for generating structures involved in stack unwinding. As
simplest example, here is how to deploy custom exception handler for
leaf function:
default rel
section .text
extern MessageBoxA
handler:
sub rsp,40
mov rcx,0
lea rdx,[text]
lea r8,[caption]
mov r9,1 ; MB_OKCANCEL
call MessageBoxA
sub eax,1 ; incidentally suits as return value
; for exception handler
add rsp,40
ret
global main
main:
xor rax,rax
mov rax,QWORD[rax] ; cause exception
ret
main_end:
text: db 'OK to rethrow, CANCEL to generate core dump',0
caption:db 'SEGV',0
section .pdata rdata align=4
dd main wrt ..imagebase
dd main_end wrt ..imagebase
dd xmain wrt ..imagebase
section .xdata rdata align=8
xmain: db 9,0,0,0
dd handler wrt ..imagebase
section .drectve info
db '/defaultlib:user32.lib /defaultlib:msvcrt.lib '
What you see in `.pdata' section is element of the "table comprising
start and end addresses of function" along with reference to
associated `UNWIND_INFO' structure. And what you see in `.xdata'
section is `UNWIND_INFO' structure describing function with no
frame, but with designated exception handler. References are
_required_ to be image-relative (which is the real reason for
implementing `wrt ..imagebase' operator). It should be noted that
`rdata align=n', as well as `wrt ..imagebase', are optional in these
two segments' contexts, i.e. can be omitted. Latter means that _all_
32-bit references, not only above listed required ones, placed into
these two segments turn out image-relative. Why is it important to
understand? Developer is allowed to append handler-specific data to
`UNWIND_INFO' structure, and if [s]he adds a 32-bit reference, then
[s]he will have to remember to adjust its value to obtain the real
pointer.
As already mentioned, in Win64 terms leaf function is one that does
not call any other function _nor_ modifies any non-volatile
register, including stack pointer. But it's not uncommon that
assembler programmer plans to utilize every single register and
sometimes even have variable stack frame. Is there anything one can
do with bare building blocks? I.e. besides manually composing fully-
fledged `UNWIND_INFO' structure, which would surely be considered
error-prone? Yes, there is. Recall that exception handler is called
first, before stack layout is analyzed. As it turned out, it's
perfectly possible to manipulate current callee's context in custom
handler in manner that permits further stack unwinding. General idea
is that handler would not actually "handle" the exception, but
instead restore callee's context, as it was at its entry point and
thus mimic leaf function. In other words, handler would simply
undertake part of unwinding procedure. Consider following example:
function:
mov rax,rsp ; copy rsp to volatile register
push r15 ; save non-volatile registers
push rbx
push rbp
mov r11,rsp ; prepare variable stack frame
sub r11,rcx
and r11,-64
mov QWORD[r11],rax ; check for exceptions
mov rsp,r11 ; allocate stack frame
mov QWORD[rsp],rax ; save original rsp value
magic_point:
...
mov r11,QWORD[rsp] ; pull original rsp value
mov rbp,QWORD[r11-24]
mov rbx,QWORD[r11-16]
mov r15,QWORD[r11-8]
mov rsp,r11 ; destroy frame
ret
The keyword is that up to `magic_point' original `rsp' value remains
in chosen volatile register and no non-volatile register, except for
`rsp', is modified. While past `magic_point' `rsp' remains constant
till the very end of the `function'. In this case custom language-
specific exception handler would look like this:
As custom handler mimics leaf function, corresponding `UNWIND_INFO'
structure does not have to contain any information about stack frame
and its layout.
7.7 `coff': Common Object File Format
The `coff' output type produces `COFF' object files suitable for
linking with the DJGPP linker.
`coff' provides a default output file-name extension of `.o'.
The `coff' format supports the same extensions to the `SECTION'
directive as `win32' does, except that the `align' qualifier and the
`info' section type are not supported.
7.8 `macho32' and `macho64': Mach Object File Format
The `macho32' and `macho64' output formts produces `Mach-O' object
files suitable for linking with the MacOS X linker. `macho' is a
synonym for `macho32'.
`macho' provides a default output file-name extension of `.o'.
7.9 `elf32', `elf64', `elfx32': Executable and Linkable Format Object Files
The `elf32', `elf64' and `elfx32' output formats generate
`ELF32 and ELF64' (Executable and Linkable Format) object files, as
used by Linux as well as Unix System V, including Solaris x86,
UnixWare and SCO Unix. `elf' provides a default output file-name
extension of `.o'. `elf' is a synonym for `elf32'.
The `elfx32' format is used for the x32 ABI, which is a 32-bit ABI
with the CPU in 64-bit mode.
7.9.1 ELF specific directive `osabi'
The ELF header specifies the application binary interface for the
target operating system (OSABI). This field can be set by using the
`osabi' directive with the numeric value (0-255) of the target
system. If this directive is not used, the default value will be
"UNIX System V ABI" (0) which will work on most systems which
support ELF.
7.9.2 `elf' Extensions to the `SECTION' Directive
Like the `obj' format, `elf' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names, but
may still be overridden by these qualifiers.
The available qualifiers are:
(*) `alloc' defines the section to be one which is loaded into
memory when the program is run. `noalloc' defines it to be one
which is not, such as an informational or comment section.
(*) `exec' defines the section to be one which should have execute
permission when the program is run. `noexec' defines it as one
which should not.
(*) `write' defines the section to be one which should be writable
when the program is run. `nowrite' defines it as one which
should not.
(*) `progbits' defines the section to be one with explicit contents
stored in the object file: an ordinary code or data section, for
example, `nobits' defines the section to be one with no explicit
contents given, such as a BSS section.
(*) `align=', used with a trailing number as in `obj', gives the
alignment requirements of the section.
(*) `tls' defines the section to be one which contains thread local
variables.
The defaults assumed by NASM if you do not specify the above
qualifiers are:
(Any section name other than those in the above table is treated by
default like `other' in the above table. Please note that section
names are case sensitive.)
7.9.3 Position-Independent Code: `elf' Special Symbols and `WRT'
The `ELF' specification contains enough features to allow position-
independent code (PIC) to be written, which makes ELF shared
libraries very flexible. However, it also means NASM has to be able
to generate a variety of ELF specific 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 `WRT'
operator is not used for its normal purpose; therefore NASM's `elf'
output format makes use of `WRT' for a different purpose, namely the
PIC-specific relocation types.
`elf' defines five special symbols which you can use as the right-
hand side of the `WRT' operator to obtain PIC relocation types. They
are `..gotpc', `..gotoff', `..got', `..plt' and `..sym'. Their
functions are summarized here:
(*) Referring to the symbol marking the global offset table base
using `wrt ..gotpc' will end up giving the distance from the
beginning of the current section to the global offset table.
(`_GLOBAL_OFFSET_TABLE_' is the standard symbol name used to
refer to the GOT.) So you would then need to add `$$' to the
result to get the real address of the GOT.
(*) Referring to a location in one of your own sections using
`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.
(*) Referring to an external or global symbol using `wrt ..got'
causes the linker to build an entry _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.
(*) Referring to a procedure name using `wrt ..plt' causes the
linker to build a procedure linkage table entry for the symbol,
and the reference gives the address of the PLT entry. You can
only use this in contexts which would generate a PC-relative
relocation normally (i.e. as the destination for `CALL' or
`JMP'), since ELF contains no relocation type to refer to PLT
entries absolutely.
(*) Referring to a symbol name using `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 section 9.2.
7.9.4 Thread Local Storage: `elf' Special Symbols and `WRT'
(*) In ELF32 mode, referring to an external or global symbol using
`wrt ..tlsie' causes the linker to build an entry _in_ the GOT
containing the offset of the symbol within the TLS block, so you
can access the value of the symbol with code such as:
mov eax,[tid wrt ..tlsie]
mov [gs:eax],ebx
(*) In ELF64 or ELFx32 mode, referring to an external or global
symbol using `wrt ..gottpoff' causes the linker to build an
entry _in_ the GOT containing the offset of the symbol within
the TLS block, so you can access the value of the symbol with
code such as:
mov rax,[rel tid wrt ..gottpoff]
mov rcx,[fs:rax]
7.9.5 `elf' Extensions to the `GLOBAL' Directive
`ELF' object files can contain more information about a global
symbol than just its address: they can contain the size of the
symbol and its type as well. These are not merely debugger
conveniences, but are actually necessary when the program being
written is a shared library. NASM therefore supports some extensions
to the `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 `function' or
`data'. (`object' is a synonym for `data'.) For example:
global hashlookup:function, hashtable:data
exports the global symbol `hashlookup' as a function and `hashtable'
as a data object.
Optionally, you can control the ELF visibility of the symbol. Just
add one of the visibility keywords: `default', `internal', `hidden',
or `protected'. The default is `default' of course. For example, to
make `hashlookup' hidden:
global hashlookup:function hidden
You can also specify the size of the data associated with the
symbol, as a numeric expression (which may involve labels, and even
forward references) after the type specifier. Like this:
global hashtable:data (hashtable.end - hashtable)
hashtable:
db this,that,theother ; some data here
.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 section
9.2.4.
7.9.6 `elf' Extensions to the `COMMON' Directive
`ELF' also allows you to specify alignment requirements 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:
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.
7.9.7 16-bit code and ELF
The `ELF32' specification doesn't provide relocations for 8- and 16-
bit values, but the GNU `ld' linker adds these as an extension. NASM
can generate GNU-compatible relocations, to allow 16-bit code to be
linked as ELF using GNU `ld'. If NASM is used with the
`-w+gnu-elf-extensions' option, a warning is issued when one of
these relocations is generated.
7.9.8 Debug formats and ELF
ELF provides debug information in `STABS' and `DWARF' formats. Line
number information is generated for all executable sections, but
please note that only the ".text" section is executable by default.
7.10 `aout': Linux `a.out' Object Files
The `aout' format generates `a.out' object files, in the form used
by early Linux systems (current Linux systems use ELF, see section
7.9.) These differ from other `a.out' object files in that the magic
number in the first four bytes of the file is different; also, some
implementations of `a.out', for example NetBSD's, support position-
independent code, which Linux's implementation does not.
`a.out' provides a default output file-name extension of `.o'.
`a.out' is a very simple object format. It supports no special
directives, no special symbols, no use of `SEG' or `WRT', and no
extensions to any standard directives. It supports only the three
standard section names `.text', `.data' and `.bss'.
The `aoutb' format generates `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 `aout' except for the magic number in the first four
bytes of the file. However, the `aoutb' format supports
position-independent code in the same way as the `elf' format, so
you can use it to write `BSD' shared libraries.
`aoutb' provides a default output file-name extension of `.o'.
`aoutb' supports no special directives, no special symbols, and only
the three standard section names `.text', `.data' and `.bss'.
However, it also supports the same use of `WRT' as `elf' does, to
provide position-independent code relocation types. See section
7.9.3 for full documentation of this feature.
`aoutb' also supports the same extensions to the `GLOBAL' directive
as `elf' does: see section 7.9.5 for documentation of this.
7.12 `as86': Minix/Linux `as86' Object Files
The Minix/Linux 16-bit assembler `as86' has its own non-standard
object file format. Although its companion linker `ld86' produces
something close to ordinary `a.out' binaries as output, the object
file format used to communicate between `as86' and `ld86' is not
itself `a.out'.
NASM supports this format, just in case it is useful, as `as86'.
`as86' provides a default output file-name extension of `.o'.
`as86' is a very simple object format (from the NASM user's point of
view). It supports no special directives, no use of `SEG' or `WRT',
and no extensions to any standard directives. It supports only the
three standard section names `.text', `.data' and `.bss'. The only
special symbol supported is `..start'.
7.13 `rdf': Relocatable Dynamic Object File Format
The `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 `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.
`rdf' supports only the standard section names `.text', `.data' and
`.bss'.
7.13.1 Requiring a Library: The `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 `LIBRARY' directive, which takes one argument
which is the name of the module:
library mylib.rdl
7.13.2 Specifying a Module Name: The `MODULE' Directive
Special `RDOFF' header record is used to store the name of the
module. It can be used, for example, by run-time loader to perform
dynamic linking. `MODULE' directive takes one argument which is the
name of current module:
module mymodname
Note that when you statically link modules and tell linker to strip
the symbols from output file, all module names will be stripped too.
To avoid it, you should start module names with `$', like:
module $kernel.core
7.13.3 `rdf' Extensions to the `GLOBAL' Directive
`RDOFF' global symbols can contain additional information needed by
the static linker. You can mark a global symbol as exported, thus
telling the linker do not strip it from target executable or library
file. Like in `ELF', you can also specify whether an exported symbol
is a procedure (function) or data object.
Suffixing the name with a colon and the word `export' you make the
symbol exported:
global sys_open:export
To specify that exported symbol is a procedure (function), you add
the word `proc' or `function' after declaration:
global sys_open:export proc
Similarly, to specify exported data object, add the word `data' or
`object' to the directive:
global kernel_ticks:export data
7.13.4 `rdf' Extensions to the `EXTERN' Directive
By default the `EXTERN' directive in `RDOFF' declares a "pure
external" symbol (i.e. the static linker will complain if such a
symbol is not resolved). To declare an "imported" symbol, which must
be resolved later during a dynamic linking phase, `RDOFF' offers an
additional `import' modifier. As in `GLOBAL', you can also specify
whether an imported symbol is a procedure (function) or data object.
For example:
library $libc
extern _open:import
extern _printf:import proc
extern _errno:import data
Here the directive `LIBRARY' is also included, which gives the
dynamic linker a hint as to where to find requested symbols.
7.14 `dbg': Debugging Format
The `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 `OF_DBG' in `output/outform.h' or on the
compiler command line, and obtain the `dbg' output format.
The `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 `dbg' format like this:
nasm -f dbg filename.asm
which will generate a diagnostic file called `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 `dbg' format. Therefore it can be useful
to run NASM twice, in order to do the preprocessing with the native
object format selected:
This preprocesses `rdfprog.asm' into `rdfprog.i', keeping the `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 `dbg' format to generate the final
diagnostic output.
This workaround will still typically not work for programs intended
for `obj' format, because the `obj' `SEGMENT' and `GROUP' directives
have side effects of defining the segment and group names as
symbols; `dbg' will not do this, so the program will not assemble.
You will have to work around that by defining the symbols yourself
(using `EXTERN', for example) if you really need to get a `dbg'
trace of an `obj'-specific source file.
`dbg' accepts any section name and any directives at all, and logs
them all to its output file.
Chapter 8: 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 `.EXE' or `.COM' files, how
to write `.SYS' device drivers, and how to interface assembly
language code with 16-bit C compilers and with Borland Pascal.
8.1 Producing `.EXE' Files
Any large program written under DOS needs to be built as a `.EXE'
file: only `.EXE' files have the necessary internal structure
required to span more than one 64K segment. Windows programs, also,
have to be built as `.EXE' files, since Windows does not support the
`.COM' format.
In general, you generate `.EXE' files by using the `obj' output
format to produce one or more `.OBJ' files, and then linking them
together using a linker. However, NASM also supports the direct
generation of simple DOS `.EXE' files using the `bin' output format
(by using `DB' and `DW' to construct the `.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 `.EXE' natively as another output format in
future releases.
8.1.1 Using the `obj' Format To Generate `.EXE' Files
This section describes the usual method of generating `.EXE' files
by linking `.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
VAL, available in `LZH' archive format from `x2ftp.oulu.fi'. An LZH
archiver can be found at `ftp.simtel.net'. There is another `free'
linker (though this one doesn't come with sources) called FREELINK,
available from `www.pcorner.com'. A third, `djlink', written by DJ
Delorie, is available at `www.delorie.com'. A fourth linker,
`ALINK', written by Anthony A.J. Williams, is available at
`alink.sourceforge.net'.
When linking several `.OBJ' files into a `.EXE' file, you should
ensure that exactly one of them has a start point defined (using the
`..start' special symbol defined by the `obj' format: see section
7.4.6). 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
_which_ value to use.
An example of a NASM source file which can be assembled to a `.OBJ'
file and linked on its own to a `.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 `test' subdirectory of the NASM archives, under
the name `objexe.asm'.
This initial piece of code sets up `DS' to point to the data
segment, and initializes `SS' and `SP' to point to the top of the
provided stack. Notice that interrupts are implicitly disabled for
one instruction after a move into `SS', precisely for this
situation, so that there's no chance of an interrupt occurring
between the loads of `SS' and `SP' and not having a stack to execute
on.
Note also that the special symbol `..start' is defined at the
beginning of this code, which means that will be the entry point
into the resulting executable file.
mov dx,hello
mov ah,9
int 0x21
The above is the main program: load `DS:DX' with a pointer to the
greeting message (`hello' is implicitly relative to the segment
`data', which was loaded into `DS' in the setup code, so the full
pointer is valid), and call the DOS print-string function.
mov ax,0x4c00
int 0x21
This terminates the program using another DOS system call.
segment data
hello: db 'hello, world', 13, 10, '$'
The data segment contains the string we want to display.
segment stack stack
resb 64
stacktop:
The above code declares a stack segment containing 64 bytes of
uninitialized stack space, and points `stacktop' at the top of it.
The directive `segment stack stack' defines a segment _called_
`stack', and also of _type_ `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 `STACK'.
The above file, when assembled into a `.OBJ' file, will link on its
own to a valid `.EXE' file, which when run will print `hello, world'
and then exit.
8.1.2 Using the `bin' Format To Generate `.EXE' Files
The `.EXE' file format is simple enough that it's possible to build
a `.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 `DB' and `DW' commands by NASM itself, so that
you can use the `bin' output format to directly generate `.EXE'
files.
Included in the NASM archives, in the `misc' subdirectory, is a file
`exebin.mac' of macros. It defines three macros: `EXE_begin',
`EXE_stack' and `EXE_end'.
To produce a `.EXE' file using this method, you should start by
using `%include' to load the `exebin.mac' macro package into your
source file. You should then issue the `EXE_begin' macro call (which
takes no arguments) to generate the file header data. Then write
code as normal for the `bin' format - you can use all three standard
sections `.text', `.data' and `.bss'. At the end of the file you
should call the `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 `EXE_begin'.
In this model, the code you end up writing starts at `0x100', just
like a `.COM' file - in fact, if you strip off the 32-byte header
from the resulting `.EXE' file, you will have a valid `.COM'
program. All the segment bases are the same, so you are limited to a
64K program, again just like a `.COM' file. Note that an `ORG'
directive is issued by the `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 `CS' instead.
On entry to your `.EXE' file, `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 `EXE_stack' macro. For example, to change the stack
size of your program to 64 bytes, you would call `EXE_stack 64'.
A sample program which generates a `.EXE' file in this way is given
in the `test' subdirectory of the NASM archive, as `binexe.asm'.
8.2 Producing `.COM' Files
While large DOS programs must be written as `.EXE' files, small ones
are often better written as `.COM' files. `.COM' files are pure
binary, and therefore most easily produced using the `bin' output
format.
8.2.1 Using the `bin' Format To Generate `.COM' Files
`.COM' files expect to be loaded at offset `100h' into their segment
(though the segment may change). Execution then begins at `100h',
i.e. right at the start of the program. So to write a `.COM'
program, you would create a source file looking like
org 100h
section .text
start:
; put your code here
section .data
; put data items here
section .bss
; put uninitialized data here
The `bin' format puts the `.text' section first in the file, so you
can declare data or BSS items before beginning to write code if you
want to and the code will still end up at the front of the file
where it belongs.
The BSS (uninitialized data) section does not take up space in the
`.COM' file itself: instead, addresses of BSS items are resolved to
point at space beyond the end of the file, on the grounds that this
will be free memory when the program is run. Therefore you should
not rely on your BSS being initialized to all zeros when you run.
To assemble the above program, you should use a command line like
nasm myprog.asm -fbin -o myprog.com
The `bin' format would produce a file called `myprog' if no explicit
output file name were specified, so you have to override it and give
the desired file name.
8.2.2 Using the `obj' Format To Generate `.COM' Files
If you are writing a `.COM' program as more than one module, you may
wish to assemble several `.OBJ' files and link them together into a
`.COM' program. You can do this, provided you have a linker capable
of outputting `.COM' files directly (TLINK does this), or
alternatively a converter program such as `EXE2BIN' to transform the
`.EXE' file output from the linker into a `.COM' file.
If you do this, you need to take care of several things:
(*) The first object file containing code should start its code
segment with a line like `RESB 100h'. This is to ensure that the
code begins at offset `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 `.COM' file. Other assemblers use an `ORG'
directive for this purpose, but `ORG' in NASM is a format-
specific directive to the `bin' output format, and does not mean
the same thing as it does in MASM-compatible assemblers.
(*) You don't need to define a stack segment.
(*) 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
`.COM' file is loaded, all the segment registers contain the
same value.
8.3 Producing `.SYS' Files
MS-DOS device drivers - `.SYS' files - are pure binary files,
similar to `.COM' files, except that they start at origin zero
rather than `100h'. Therefore, if you are writing a device driver
using the `bin' format, you do not need the `ORG' directive, since
the default origin for `bin' is zero. Similarly, if you are using
`obj', you do not need the `RESB 100h' at the start of your code
segment.
`.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 `.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
`comp.os.msdos.programmer'.
8.4 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 `.OBJ' file, and link it
with your C modules to produce a mixed-language program.
8.4.1 External Symbol Names
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 `printf' appears
to an assembly language programmer as `_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 `GLOBAL' and `EXTERN' directives as follows:
%macro cglobal 1
global _%1
%define %1 _%1
%endmacro
%macro cextern 1
extern _%1
%define %1 _%1
%endmacro
(These forms of the macros only take one argument at a time; a
`%rep' construct could solve this.)
If you then declare an external like this:
cextern printf
then the macro will expand it as
extern _printf
%define printf _printf
Thereafter, you can reference `printf' as if it was a symbol, and
the preprocessor will put the leading underscore on where necessary.
The `cglobal' macro works similarly. You must use `cglobal' before
defining the symbol in question, but you would have had to do that
anyway if you used `GLOBAL'.
Also see section 2.1.27.
8.4.2 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:
(*) 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 `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 `CALL' instructions and return
using `RETN' (which, in NASM, is synonymous with `RET' anyway).
This means both that you should write your own routines to
return with `RETN', and that you should call external C routines
with near `CALL' instructions.
(*) 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 `CALL FAR' (or
`CALL seg:offset') and return using `RETF'. Again, you should
therefore write your own routines to return with `RETF' and use
`CALL FAR' to call external routines.
(*) In models using a single data segment (tiny, small and medium),
data pointers are 16 bits long, containing only an offset field
(the `DS' register doesn't change its value, and always gives
the segment part of the full data item address).
(*) 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 `DS' in your routines without restoring it
afterwards, but `ES' is free for you to use to access the
contents of 32-bit data pointers you are passed.
(*) 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.
(*) In most memory models, there is a _default_ data segment, whose
segment address is kept in `DS' throughout the program. This
data segment is typically the same segment as the stack, kept in
`SS', so that functions' local variables (which are stored on
the stack) and global data items can both be accessed easily
without changing `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 `SS' and `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 `_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
`_DATA'.
8.4.3 Function Definitions and Function Calls
The C calling convention in 16-bit programs is as follows. In the
following description, the words _caller_ and _callee_ are used to
denote the function doing the calling and the function which gets
called.
(*) 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).
(*) The caller then executes a `CALL' instruction to pass control to
the callee. This `CALL' is either near or far depending on the
memory model.
(*) 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 `SP' in `BP' so
as to be able to use `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 `BP'
must be preserved by any C function. Hence the callee, if it is
going to set up `BP' as a _frame pointer_, must push the
previous value first.
(*) The callee may then access its parameters relative to `BP'. The
word at `[BP]' holds the previous value of `BP' as it was
pushed; the next word, at `[BP+2]', holds the offset part of the
return address, pushed implicitly by `CALL'. In a small-model
(near) function, the parameters start after that, at `[BP+4]';
in a large-model (far) function, the segment part of the return
address lives at `[BP+4]', and the parameters begin at `[BP+6]'.
The leftmost parameter of the function, since it was pushed
last, is accessible at this offset from `BP'; the others follow,
at successively greater offsets. Thus, in a function such as
`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.
(*) The callee may also wish to decrease `SP' further, so as to
allocate space on the stack for local variables, which will then
be accessible at negative offsets from `BP'.
(*) The callee, if it wishes to return a value to the caller, should
leave the value in `AL', `AX' or `DX:AX' depending on the size
of the value. Floating-point results are sometimes (depending on
the compiler) returned in `ST0'.
(*) Once the callee has finished processing, it restores `SP' from
`BP' if it had allocated local stack space, then pops the
previous value of `BP', and returns via `RETN' or `RETF'
depending on memory model.
(*) When the caller regains control from the callee, the function
parameters are still on the stack, so it typically adds an
immediate constant to `SP' to remove them (instead of executing
a number of slow `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 _knows_ how many
parameters it pushed, does the removing.
It is instructive to compare this calling convention with that for
Pascal programs (described in section 8.5.1). 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 `RET' or `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:
global _myfunc
_myfunc:
push bp
mov bp,sp
sub sp,0x40 ; 64 bytes of local stack space
mov bx,[bp+4] ; first parameter to function
; some more code
mov sp,bp ; undo "sub sp,0x40" above
pop bp
ret
For a large-model function, you would replace `RET' by `RETF', and
look for the first parameter at `[BP+6]' instead of `[BP+4]'. Of
course, if one of the parameters is a pointer, then the offsets of
_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:
extern _printf
; and then, further down...
push word [myint] ; one of my integer variables
push word mystring ; pointer into my data segment
call _printf
add sp,byte 4 ; `byte' saves space
; then those data items...
segment _DATA
myint dw 1234
mystring db 'This number -> %d <- should be 1234',10,0
This piece of code is the small-model assembly equivalent of the C
code
int myint = 1234;
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 `DS' already holds the segment base
of the segment `_DATA'. If not, you would have to initialize it
first.
push word [myint]
push word seg mystring ; Now push the segment, and...
push word mystring ; ... offset of "mystring"
call far _printf
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 `int' data type. The first
argument (pushed last) to `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, `PUSH DS' would have been a shorter instruction
than `PUSH WORD SEG mystring', if `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 `SP' has to be
increased by 6 rather than 4 afterwards to make up for the extra
word of parameters.
8.4.4 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 `GLOBAL' or
`EXTERN'. (Again, the names require leading underscores, as stated
in section 8.4.1.) Thus, a C variable declared as `int i' can be
accessed from assembler as
extern _i
mov ax,[_i]
And to declare your own integer variable which C programs can access
as `extern int j', you do this (making sure you are assembling in
the `_DATA' segment, if necessary):
global _j
_j dw 0
To access a C array, you need to know the size of the components of
the array. For example, `int' variables are two bytes long, so if a
C program declares an array as `int a[10]', you can access `a[3]' by
coding `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 `char', 2 for `short' and `int', 4 for `long' and `float', and
8 for `double'.
To access a C 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 `STRUC'), or by calculating the one
offset and using just that.
To do either of these, you should read your C compiler's manual to
find out how it organizes data structures. NASM gives no special
alignment to structure members in its own `STRUC' macro, so you have
to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like
struct {
char c;
int i;
} foo;
might be four bytes long rather than three, since the `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 `#pragma' lines, so you have to find
out how your own compiler does it.
8.4.5 `c16.mac': Helper Macros for the 16-bit C Interface
Included in the NASM archives, in the `misc' directory, is a file
`c16.mac' of macros. It defines three macros: `proc', `arg' and
`endproc'. These are intended to be used for C-style procedure
definitions, and they automate a lot of the work involved in keeping
track of the calling convention.
(An alternative, TASM compatible form of `arg' is also now built
into NASM's preprocessor. See section 4.8 for details.)
An example of an assembly function using the macro set is given
here:
This defines `_nearproc' to be a procedure taking two arguments, the
first (`i') an integer and the second (`j') a pointer to an integer.
It returns `i + *j'.
Note that the `arg' macro has an `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 `EQU' works, defining
`%$i' to be an offset from `BP'. A context-local variable is used,
local to the context pushed by the `proc' macro and popped by the
`endproc' macro, so that the same argument name can be used in later
procedures. Of course, you don't _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
`%define FARCODE'. This changes the kind of return instruction
generated by `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.
`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 `int'.
The large-model equivalent of the above function would look like
this:
This makes use of the argument to the `arg' macro to define a
parameter of size 4, because `j' is now a far pointer. When we load
from `j', we must load a segment and an offset.
8.5 Interfacing to Borland Pascal Programs
Interfacing to Borland Pascal programs is similar in concept to
interfacing to 16-bit C programs. The differences are:
(*) The leading underscore required for interfacing to C programs is
not required for Pascal.
(*) 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 `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 _pointers_, however, are far.
(*) The function calling convention is different - described below.
(*) Some data types, such as strings, are stored differently.
(*) 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.
8.5.1 The Pascal Calling Convention
The 16-bit Pascal calling convention is as follows. In the following
description, the words _caller_ and _callee_ are used to denote the
function doing the calling and the function which gets called.
(*) 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).
(*) The caller then executes a far `CALL' instruction to pass
control to the callee.
(*) 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 `SP' in `BP' so
as to be able to use `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 `BP'
must be preserved by any function. Hence the callee, if it is
going to set up `BP' as a frame pointer, must push the previous
value first.
(*) The callee may then access its parameters relative to `BP'. The
word at `[BP]' holds the previous value of `BP' as it was
pushed. The next word, at `[BP+2]', holds the offset part of the
return address, and the next one at `[BP+4]' the segment part.
The parameters begin at `[BP+6]'. The rightmost parameter of the
function, since it was pushed last, is accessible at this offset
from `BP'; the others follow, at successively greater offsets.
(*) The callee may also wish to decrease `SP' further, so as to
allocate space on the stack for local variables, which will then
be accessible at negative offsets from `BP'.
(*) The callee, if it wishes to return a value to the caller, should
leave the value in `AL', `AX' or `DX:AX' depending on the size
of the value. Floating-point results are returned in `ST0'.
Results of type `Real' (Borland's own custom floating-point data
type, not handled directly by the FPU) are returned in
`DX:BX:AX'. To return a result of type `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 `RETF' instruction.
(*) Once the callee has finished processing, it restores `SP' from
`BP' if it had allocated local stack space, then pops the
previous value of `BP', and returns via `RETF'. It uses the form
of `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.
(*) 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
`Integer'-type parameters, in the following way:
global myfunc
myfunc: push bp
mov bp,sp
sub sp,0x40 ; 64 bytes of local stack space
mov bx,[bp+8] ; first parameter to function
mov bx,[bp+6] ; second parameter to function
; some more code
mov sp,bp ; undo "sub sp,0x40" above
pop bp
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:
extern SomeFunc
; and then, further down...
push word seg mystring ; Now push the segment, and...
push word mystring ; ... offset of "mystring"
push word [myint] ; one of my variables
call far SomeFunc
Since Borland Pascal's internal unit file format is completely
different from `OBJ', it only makes a very sketchy job of actually
reading and understanding the various information contained in a
real `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:
(*) Procedures and functions must be in a segment whose name is
either `CODE', `CSEG', or something ending in `_TEXT'.
(*) initialized data must be in a segment whose name is either
`CONST' or something ending in `_DATA'.
(*) Uninitialized data must be in a segment whose name is either
`DATA', `DSEG', or something ending in `_BSS'.
(*) Any other segments in the object file are completely ignored.
`GROUP' directives and segment attributes are also ignored.
8.5.3 Using `c16.mac' With Pascal Programs
The `c16.mac' macro package, described in section 8.4.5, can also be
used to simplify writing functions to be called from Pascal
programs, if you code `%define PASCAL'. This definition ensures that
functions are far (it implies `FARCODE'), and also causes procedure
return instructions to be generated with an operand.
Defining `PASCAL' does not change the code which calculates the
argument offsets; you must declare your function's arguments in
reverse order. For example:
This defines the same routine, conceptually, as the example in
section 8.4.5: 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 `PASCAL' is defined
instead of `FARCODE', and that the arguments are declared in reverse
order.
This chapter attempts to cover some of the common issues involved
when writing 32-bit code, to run under Win32 or Unix, or to be
linked with C code generated by a Unix-style C compiler such as
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 _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
`CALL' and `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.
9.1 Interfacing to 32-bit C Programs
A lot of the discussion in section 8.4, 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.
9.1.1 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 _not_ have a leading
underscore on their assembly-language names.
The older Linux `a.out' C compiler, all `Win32' compilers, `DJGPP',
and `NetBSD' and `FreeBSD', all use the leading underscore; for
these compilers, the macros `cextern' and `cglobal', as given in
section 8.4.1, will still work. For `ELF', though, the leading
underscore should not be used.
See also section 2.1.27.
9.1.2 Function Definitions and Function Calls
The C calling convention in 32-bit programs is as follows. In the
following description, the words _caller_ and _callee_ are used to
denote the function doing the calling and the function which gets
called.
(*) 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).
(*) The caller then executes a near `CALL' instruction to pass
control to the callee.
(*) 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 `ESP' in `EBP'
so as to be able to use `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 `EBP'
must be preserved by any C function. Hence the callee, if it is
going to set up `EBP' as a frame pointer, must push the previous
value first.
(*) The callee may then access its parameters relative to `EBP'. The
doubleword at `[EBP]' holds the previous value of `EBP' as it
was pushed; the next doubleword, at `[EBP+4]', holds the return
address, pushed implicitly by `CALL'. The parameters start after
that, at `[EBP+8]'. The leftmost parameter of the function,
since it was pushed last, is accessible at this offset from
`EBP'; the others follow, at successively greater offsets. Thus,
in a function such as `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.
(*) The callee may also wish to decrease `ESP' further, so as to
allocate space on the stack for local variables, which will then
be accessible at negative offsets from `EBP'.
(*) The callee, if it wishes to return a value to the caller, should
leave the value in `AL', `AX' or `EAX' depending on the size of
the value. Floating-point results are typically returned in
`ST0'.
(*) Once the callee has finished processing, it restores `ESP' from
`EBP' if it had allocated local stack space, then pops the
previous value of `EBP', and returns via `RET' (equivalently,
`RETN').
(*) When the caller regains control from the callee, the function
parameters are still on the stack, so it typically adds an
immediate constant to `ESP' to remove them (instead of executing
a number of slow `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 _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 _by_ the
Windows API such as window procedures: they follow what Microsoft
calls the `__stdcall' convention. This is slightly closer to the
Pascal convention, in that the callee clears the stack by passing a
parameter to the `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:
global _myfunc
_myfunc:
push ebp
mov ebp,esp
sub esp,0x40 ; 64 bytes of local stack space
mov ebx,[ebp+8] ; first parameter to function
; some more code
leave ; mov esp,ebp / pop ebp
ret
At the other end of the process, to call a C function from your
assembly code, you would do something like this:
extern _printf
; and then, further down...
push dword [myint] ; one of my integer variables
push dword mystring ; pointer into my data segment
call _printf
add esp,byte 8 ; `byte' saves space
; then those data items...
segment _DATA
myint dd 1234
mystring db 'This number -> %d <- should be 1234',10,0
This piece of code is the assembly equivalent of the C code
int myint = 1234;
printf("This number -> %d <- should be 1234\n", myint);
9.1.3 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 `GLOBAL' or
`EXTERN'. (Again, the names require leading underscores, as stated
in section 9.1.1.) Thus, a C variable declared as `int i' can be
accessed from assembler as
extern _i
mov eax,[_i]
And to declare your own integer variable which C programs can access
as `extern int j', you do this (making sure you are assembling in
the `_DATA' segment, if necessary):
global _j
_j dd 0
To access a C array, you need to know the size of the components of
the array. For example, `int' variables are four bytes long, so if a
C program declares an array as `int a[10]', you can access `a[3]' by
coding `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 `char', 2 for `short', 4 for `int', `long' and `float', and 8
for `double'. Pointers, being 32-bit addresses, are also 4 bytes
long.
To access a C 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 `STRUC'), or by calculating the one
offset and using just that.
To do either of these, you should read your C compiler's manual to
find out how it organizes data structures. NASM gives no special
alignment to structure members in its own `STRUC' macro, so you have
to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like
struct {
char c;
int i;
} foo;
might be eight bytes long rather than five, since the `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 `#pragma' lines, so you have to find
out how your own compiler does it.
9.1.4 `c32.mac': Helper Macros for the 32-bit C Interface
Included in the NASM archives, in the `misc' directory, is a file
`c32.mac' of macros. It defines three macros: `proc', `arg' and
`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:
This defines `_proc32' to be a procedure taking two arguments, the
first (`i') an integer and the second (`j') a pointer to an integer.
It returns `i + *j'.
Note that the `arg' macro has an `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 `EQU' works, defining
`%$i' to be an offset from `BP'. A context-local variable is used,
local to the context pushed by the `proc' macro and popped by the
`endproc' macro, so that the same argument name can be used in later
procedures. Of course, you don't _have_ to do that.
`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 `int' or pointers.
9.2 Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries
`ELF' replaced the older `a.out' object file format under Linux
because it contains support for position-independent code (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.
NetBSD, and its close cousins FreeBSD and OpenBSD, take a different
approach by hacking PIC support into the `a.out' format. NASM
supports this as the `aoutb' output format, so you can write 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:
mov eax,[myvar] ; WRONG
Instead, the linker provides an area of memory called the _global
offset table_, or 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 `CALL' and `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 _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 section 9.2.4 for a caveat).
9.2.1 Obtaining the Address of the GOT
Each code module in your shared library should define the GOT as an
external symbol:
extern _GLOBAL_OFFSET_TABLE_ ; in ELF
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:
(For BSD, again, the symbol `_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 `EBX' register, because PIC shared
libraries use this register to store the address of the GOT.
The interesting bit is the `CALL' instruction and the following two
lines. The `CALL' and `POP' combination obtains the address of the
label `.get_GOT', without having to know in advance where the
program was loaded (since the `CALL' instruction is encoded relative
to the current position). The `ADD' instruction makes use of one of
the special PIC relocation types: GOTPC relocation. With the
`WRT ..gotpc' qualifier specified, the symbol referenced (here
`_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 `ADD'
instruction, but NASM simplifies this deliberately, so you do things
the same way for both `ELF' and `BSD'.) So the instruction then
_adds_ the beginning of the section, to get the real address of the
GOT, and subtracts the value of `.get_GOT' which it knows is in
`EBX'. Therefore, by the time that instruction has finished, `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:
%macro get_GOT 0
call %%getgot
%%getgot:
pop ebx
add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
%endmacro
9.2.2 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 `..gotoff' special `WRT'
type. The way this works is like this:
lea eax,[ebx+myvar wrt ..gotoff]
The expression `myvar wrt ..gotoff' is calculated, when the shared
library is linked, to be the offset to the local variable `myvar'
from the beginning of the GOT. Therefore, adding it to `EBX' as
above will place the real address of `myvar' in `EAX'.
If you declare variables as `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
`..gotoff' mechanism.
Note that due to a peculiarity of the way BSD `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.
9.2.3 Finding External and Common Data Items
If your library needs to get at an external variable (external to
the _library_, not just to one of the modules within it), you must
use the `..got' type to get at it. The `..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 _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 `extvar' in `EAX', you would code
mov eax,[ebx+extvar wrt ..got]
This loads the address of `extvar' out of an entry in the GOT. The
linker, when it builds the shared library, collects together every
relocation of type `..got', and builds the GOT so as to ensure it
has every necessary entry present.
Common variables must also be accessed in this way.
9.2.4 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 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
global func:function ; declare it as a function
func: push ebp
; etc.
And to export a data item such as an array, you would have to code
global array:data array.end-array ; give the size too
array: resd 128
.end:
Be careful: If you export a variable to the library user, by
declaring it as `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 `..got' mechanism rather
than `..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:
dataptr: dd global_data_item ; WRONG
NASM will interpret this code as an ordinary relocation, in which
`global_data_item' is merely an offset from the beginning of the
`.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
dataptr: dd global_data_item wrt ..sym
which makes use of the special `WRT' type `..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
funcptr: dd my_function
will give the user the address of the code you wrote, whereas
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 _believe_ the
function lives. Either address is a valid way to call the function.
9.2.5 Calling Procedures Outside the Library
Calling procedures outside your shared library has to be done by
means of a _procedure linkage table_, or 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, `WRT ..plt'. This is much easier than the GOT-based
ones: you simply replace calls such as `CALL printf' with the PLT-
relative version `CALL printf WRT ..plt'.
9.2.6 Generating the Library File
Having written some code modules and assembled them to `.o' files,
you then generate your shared library with a command such as
ld -shared -o library.so module1.o module2.o # for ELF
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 `/usr/lib' or `/lib', it is usually worth using
the `-soname' flag to the linker, to store the final library file
name, with a version number, into the library:
You would then copy `library.so.1.2' into the library directory, and
create `library.so.1' as a symbolic link to it.
Chapter 10: 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.
10.1 Mixed-Size Jumps
The most common form of 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 _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,
jmp 0x1234:0x56789ABC ; wrong!
will not work, since the offset part of the address will be
truncated to `0x9ABC' and the jump will be an ordinary 16-bit far
one.
The Linux kernel setup code gets round the inability of `as86' to
generate the required instruction by coding it manually, using `DB'
instructions. NASM can go one better than that, by actually
generating the right instruction itself. Here's how to do it right:
jmp dword 0x1234:0x56789ABC ; right
The `DWORD' prefix (strictly speaking, it should come _after_ the
colon, since it is declaring the _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 `WORD' prefix:
jmp word 0x8765:0x4321 ; 32 to 16 bit
If the `WORD' prefix is specified in 16-bit mode, or the `DWORD'
prefix in 32-bit mode, they will be ignored, since each is
explicitly forcing NASM into a mode it was in anyway.
10.2 Addressing Between Different-Size Segments
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
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 section 10.1, you need only prefix the address with
the `DWORD' keyword, and it will be forced to be a 32-bit address:
mov dword [fs:dword my_offset],0x11223344
Also as in section 10.1, NASM is not fussy about whether the `DWORD'
prefix comes before or after the segment override, so arguably a
nicer-looking way to code the above instruction is
mov dword [dword fs:my_offset],0x11223344
Don't confuse the `DWORD' prefix _outside_ the square brackets,
which controls the size of the data stored at the address, with the
one `inside' the square brackets which controls the length of the
address itself. The two can quite easily be different:
mov word [dword 0x12345678],0x9ABC
This moves 16 bits of data to an address specified by a 32-bit
offset.
You can also specify `WORD' or `DWORD' prefixes along with the `FAR'
prefix to indirect far jumps or calls. For example:
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.
10.3 Other Mixed-Size Instructions
The other way you might want to access data might be using the
string instructions (`LODSx', `STOSx' and so on) or the `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 `a16', `a32' and `a64' prefixes. If
you are coding `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 `ESI' and then code
a32 lodsb
The prefix forces the addressing size to 32 bits, meaning that
`LODSB' loads from `[DS:ESI]' instead of `[DS:SI]'. To access a
string in a 16-bit segment when coding in a 32-bit one, the
corresponding `a16' prefix can be used.
The `a16', `a32' and `a64' prefixes can be applied to any
instruction in NASM's instruction table, but most of them can
generate all the useful forms without them. The prefixes are
necessary only for instructions with implicit addressing: `CMPSx',
`SCASx', `LODSx', `STOSx', `MOVSx', `INSx', `OUTSx', and `XLATB'.
Also, the various push and pop instructions (`PUSHA' and `POPF' as
well as the more usual `PUSH' and `POP') can accept `a16', `a32' or
`a64' prefixes to force a particular one of `SP', `ESP' or `RSP' to
be used as a stack pointer, in case the stack segment in use is a
different size from the code segment.
`PUSH' and `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 `o16':
o16 push ss
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 `o32' prefix to force the 32-bit behaviour
when in 16-bit mode, but this seems less useful.)
This chapter attempts to cover some of the common issues involved
when writing 64-bit code, to run under Win64 or Unix. It covers how
to write assembly code to interface with 64-bit C routines, and how
to write position-independent code for shared libraries.
All 64-bit code uses a flat memory model, since segmentation is not
available in 64-bit mode. The one exception is the `FS' and `GS'
registers, which still add their bases.
Position independence in 64-bit mode is significantly simpler, since
the processor supports `RIP'-relative addressing directly; see the
`REL' keyword (section 3.3). On most 64-bit platforms, it is
probably desirable to make that the default, using the directive
`DEFAULT REL' (section 6.2).
64-bit programming is relatively similar to 32-bit programming, but
of course pointers are 64 bits long; additionally, all existing
platforms pass arguments in registers rather than on the stack.
Furthermore, 64-bit platforms use SSE2 by default for floating
point. Please see the ABI documentation for your platform.
64-bit platforms differ in the sizes of the fundamental datatypes,
not just from 32-bit platforms but from each other. If a specific
size data type is desired, it is probably best to use the types
defined in the Standard C header `<inttypes.h>'.
In 64-bit mode, the default instruction size is still 32 bits. When
loading a value into a 32-bit register (but not an 8- or 16-bit
register), the upper 32 bits of the corresponding 64-bit register
are set to zero.
11.1 Register Names in 64-bit Mode
NASM uses the following names for general-purpose registers in 64-
bit mode, for 8-, 16-, 32- and 64-bit references, respectively:
This is consistent with the AMD documentation and most other
assemblers. The Intel documentation, however, uses the names
`R8L-R15L' for 8-bit references to the higher registers. It is
possible to use those names by definiting them as macros; similarly,
if one wants to use numeric names for the low 8 registers, define
them as macros. The standard macro package `altreg' (see section
5.1) can be used for this purpose.
11.2 Immediates and Displacements in 64-bit Mode
In 64-bit mode, immediates and displacements are generally only 32
bits wide. NASM will therefore truncate most displacements and
immediates to 32 bits.
The only instruction which takes a full 64-bit immediate is:
MOV reg64,imm64
NASM will produce this instruction whenever the programmer uses
`MOV' with an immediate into a 64-bit register. If this is not
desirable, simply specify the equivalent 32-bit register, which will
be automatically zero-extended by the processor, or specify the
immediate as `DWORD':
The length of these instructions are 10, 5 and 7 bytes,
respectively.
The only instructions which take a full 64-bit _displacement_ is
loading or storing, using `MOV', `AL', `AX', `EAX' or `RAX' (but no
other registers) to an absolute 64-bit address. Since this is a
relatively rarely used instruction (64-bit code generally uses
relative addressing), the programmer has to explicitly declare the
displacement size as `QWORD':
A sign-extended absolute displacement can access from -2 GB to +2
GB; a zero-extended absolute displacement can access from 0 to 4 GB.
11.3 Interfacing to 64-bit C Programs (Unix)
On Unix, the 64-bit ABI is defined by the document:
`http://www.nasm.us/links/unix64abi'
Although written for AT&T-syntax assembly, the concepts apply
equally well for NASM-style assembly. What follows is a simplified
summary.
The first six integer arguments (from the left) are passed in `RDI',
`RSI', `RDX', `RCX', `R8', and `R9', in that order. Additional
integer arguments are passed on the stack. These registers, plus
`RAX', `R10' and `R11' are destroyed by function calls, and thus are
available for use by the function without saving.
Integer return values are passed in `RAX' and `RDX', in that order.
Floating point is done using SSE registers, except for
`long double'. Floating-point arguments are passed in `XMM0' to
`XMM7'; return is `XMM0' and `XMM1'. `long double' are passed on the
stack, and returned in `ST0' and `ST1'.
All SSE and x87 registers are destroyed by function calls.
On 64-bit Unix, `long' is 64 bits.
Integer and SSE register arguments are counted separately, so for
the case of
void foo(long a, double b, int c)
`a' is passed in `RDI', `b' in `XMM0', and `c' in `ESI'.
11.4 Interfacing to 64-bit C Programs (Win64)
The Win64 ABI is described at:
`http://www.nasm.us/links/win64abi'
What follows is a simplified summary.
The first four integer arguments are passed in `RCX', `RDX', `R8'
and `R9', in that order. Additional integer arguments are passed on
the stack. These registers, plus `RAX', `R10' and `R11' are
destroyed by function calls, and thus are available for use by the
function without saving.
Integer return values are passed in `RAX' only.
Floating point is done using SSE registers, except for
`long double'. Floating-point arguments are passed in `XMM0' to
`XMM3'; return is `XMM0' only.
On Win64, `long' is 32 bits; `long long' or `_int64' is 64 bits.
Integer and SSE register arguments are counted together, so for the
case of
void foo(long long a, double b, int c)
`a' is passed in `RCX', `b' in `XMM1', and `c' in `R8D'.
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.
12.1 Common Problems
12.1.1 NASM Generates Inefficient Code
We sometimes get `bug' reports about NASM generating inefficient, or
even `wrong', code on instructions such as `ADD ESP,8'. This is a
deliberate design feature, connected to predictability of output:
NASM, on seeing `ADD ESP,8', will generate the form of the
instruction which leaves room for a 32-bit offset. You need to code
`ADD ESP,BYTE 8' if you want the space-efficient form of the
instruction. This isn't a bug, it's user error: if you prefer to
have NASM produce the more efficient code automatically enable
optimization with the `-O' option (see section 2.1.22).
12.1.2 My Jumps are Out of Range
Similarly, people complain that when they issue conditional jumps
(which are `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 `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 `JNE'
instruction with a very short `JE' instruction that jumps over a
`JMP NEAR'; this is a sensible solution for processors below a 386,
but hardly efficient on processors which have good branch prediction
_and_ could have used `JNE NEAR' instead. So, once again, it's up to
the user, not the assembler, to decide what instructions should be
generated. See section 2.1.22.
12.1.3 `ORG' Doesn't Work
People writing boot sector programs in the `bin' format often
complain that `ORG' doesn't work the way they'd like: in order to
place the `0xAA55' signature word at the end of a 512-byte boot
sector, people who are used to MASM tend to code
ORG 0
; some boot sector code
ORG 510
DW 0xAA55
This is not the intended use of the `ORG' directive in NASM, and
will not work. The correct way to solve this problem in NASM is to
use the `TIMES' directive, like this:
ORG 0
; some boot sector code
TIMES 510-($-$$) DB 0
DW 0xAA55
The `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.
12.1.4 `TIMES' Doesn't Work
The other common problem with the above code is people who write the
`TIMES' line as
TIMES 510-$ DB 0
by reasoning that `$' should be a pure number, just like 510, so the
difference between them is also a pure number and can happily be fed
to `TIMES'.
NASM is a _modular_ assembler: the various component parts are
designed to be easily separable for re-use, so they don't exchange
information unnecessarily. In consequence, the `bin' output format,
even though it has been told by the `ORG' directive that the `.text'
section should start at 0, does not pass that information back to
the expression evaluator. So from the evaluator's point of view, `$'
isn't a pure number: it's an offset from a section base. Therefore
the difference between `$' and 510 is also not a pure number, but
involves a section base. Values involving section bases cannot be
passed as arguments to `TIMES'.
The solution, as in the previous section, is to code the `TIMES'
line in the form
TIMES 510-($-$$) DB 0
in which `$' and `$$' are offsets from the same section base, and so
their difference is a pure number. This will solve the problem and
generate sensible code.
12.2 Bugs
We have never yet released a version of NASM with any _known_ bugs.
That doesn't usually stop there being plenty we didn't know about,
though. Any that you find should be reported firstly via the
`bugtracker' at `http://www.nasm.us/' (click on "Bug Tracker"), or
if that fails then through one of the contacts in section 1.2.
Please read section 2.2 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
section 12.1, and don't bother reporting the bug if it's listed
there.
If you do report a bug, _please_ give us all of the following
information:
(*) What operating system you're running NASM under. DOS, Linux,
NetBSD, Win16, Win32, VMS (I'd be impressed), whatever.
(*) 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.
(*) Which version of NASM you're using, and exactly how you invoked
it. Give us the precise command line, and the contents of the
`NASMENV' environment variable if any.
(*) 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.)
(*) 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 _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 _not_ to be mailed large chunks of code anyway. The
smaller the file, the better. A three-line sample file that does
nothing useful _except_ demonstrate the problem is much easier
to work with than a fully fledged ten-thousand-line program. (Of
course, some errors _do_ only crop up in large files, so this
may not be possible.)
(*) A description of what the problem actually _is_. `It doesn't
work' is _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 _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'.
(*) 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.
(*) 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 _both_ object files, so we can see what
TASM is doing differently from us.
Appendix A: Ndisasm
-------------------
The Netwide Disassembler, NDISASM
A.1 Introduction
The Netwide Disassembler is a small companion program to the Netwide
Assembler, NASM. It seemed a shame to have an x86 assembler,
complete with a full instruction table, and not make as much use of
it as possible, so here's a disassembler which shares the
instruction table (and some other bits of code) with NASM.
The Netwide Disassembler does nothing except to produce
disassemblies of _binary_ source files. NDISASM does not have any
understanding of object file formats, like `objdump', and it will
not understand `DOS .EXE' files like `debug' will. It just
disassembles.
A.2 Getting Started: Installation
See section 1.3 for installation instructions. NDISASM, like NASM,
has a `man page' which you may want to put somewhere useful, if you
are on a Unix system.
A.3 Running NDISASM
To disassemble a file, you will typically use a command of the form
ndisasm -b {16|32|64} filename
NDISASM can disassemble 16-, 32- or 64-bit code equally easily,
provided of course that you remember to specify which it is to work
with. If no `-b' switch is present, NDISASM works in 16-bit mode by
default. The `-u' switch (for USE32) also invokes 32-bit mode.
Two more command line options are `-r' which reports the version
number of NDISASM you are running, and `-h' which gives a short
summary of command line options.
A.3.1 COM Files: Specifying an Origin
To disassemble a `DOS .COM' file correctly, a disassembler must
assume that the first instruction in the file is loaded at address
`0x100', rather than at zero. NDISASM, which assumes by default that
any file you give it is loaded at zero, will therefore need to be
informed of this.
The `-o' option allows you to declare a different origin for the
file you are disassembling. Its argument may be expressed in any of
the NASM numeric formats: decimal by default, if it begins with
``$'' or ``0x'' or ends in ``H'' it's `hex', if it ends in ``Q''
it's `octal', and if it ends in ``B'' it's `binary'.
Hence, to disassemble a `.COM' file:
ndisasm -o100h filename.com
will do the trick.
A.3.2 Code Following Data: Synchronisation
Suppose you are disassembling a file which contains some data which
isn't machine code, and _then_ contains some machine code. NDISASM
will faithfully plough through the data section, producing machine
instructions wherever it can (although most of them will look
bizarre, and some may have unusual prefixes, e.g.
``FS OR AX,0x240A''), and generating `DB' instructions ever so often
if it's totally stumped. Then it will reach the code section.
Supposing NDISASM has just finished generating a strange machine
instruction from part of the data section, and its file position is
now one byte _before_ the beginning of the code section. It's
entirely possible that another spurious instruction will get
generated, starting with the final byte of the data section, and
then the correct first instruction in the code section will not be
seen because the starting point skipped over it. This isn't really
ideal.
To avoid this, you can specify a ``synchronisation'' point, or
indeed as many synchronisation points as you like (although NDISASM
can only handle 2147483647 sync points internally). The definition
of a sync point is this: NDISASM guarantees to hit sync points
exactly during disassembly. If it is thinking about generating an
instruction which would cause it to jump over a sync point, it will
discard that instruction and output a ``db'' instead. So it _will_
start disassembly exactly from the sync point, and so you _will_ see
all the instructions in your code section.
Sync points are specified using the `-s' option: they are measured
in terms of the program origin, not the file position. So if you
want to synchronize after 32 bytes of a `.COM' file, you would have
to do
ndisasm -o100h -s120h file.com
rather than
ndisasm -o100h -s20h file.com
As stated above, you can specify multiple sync markers if you need
to, just by repeating the `-s' option.
A.3.3 Mixed Code and Data: Automatic (Intelligent) Synchronisation
Suppose you are disassembling the boot sector of a `DOS' floppy
(maybe it has a virus, and you need to understand the virus so that
you know what kinds of damage it might have done you). Typically,
this will contain a `JMP' instruction, then some data, then the rest
of the code. So there is a very good chance of NDISASM being
_misaligned_ when the data ends and the code begins. Hence a sync
point is needed.
On the other hand, why should you have to specify the sync point
manually? What you'd do in order to find where the sync point would
be, surely, would be to read the `JMP' instruction, and then to use
its target address as a sync point. So can NDISASM do that for you?
The answer, of course, is yes: using either of the synonymous
switches `-a' (for automatic sync) or `-i' (for intelligent sync)
will enable `auto-sync' mode. Auto-sync mode automatically generates
a sync point for any forward-referring PC-relative jump or call
instruction that NDISASM encounters. (Since NDISASM is one-pass, if
it encounters a PC-relative jump whose target has already been
processed, there isn't much it can do about it...)
Only PC-relative jumps are processed, since an absolute jump is
either through a register (in which case NDISASM doesn't know what
the register contains) or involves a segment address (in which case
the target code isn't in the same segment that NDISASM is working
in, and so the sync point can't be placed anywhere useful).
For some kinds of file, this mechanism will automatically put sync
points in all the right places, and save you from having to place
any sync points manually. However, it should be stressed that auto-
sync mode is _not_ guaranteed to catch all the sync points, and you
may still have to place some manually.
Auto-sync mode doesn't prevent you from declaring manual sync
points: it just adds automatically generated ones to the ones you
provide. It's perfectly feasible to specify `-i' _and_ some `-s'
options.
Another caveat with auto-sync mode is that if, by some unpleasant
fluke, something in your data section should disassemble to a PC-
relative call or jump instruction, NDISASM may obediently place a
sync point in a totally random place, for example in the middle of
one of the instructions in your code section. So you may end up with
a wrong disassembly even if you use auto-sync. Again, there isn't
much I can do about this. If you have problems, you'll have to use
manual sync points, or use the `-k' option (documented below) to
suppress disassembly of the data area.
A.3.4 Other Options
The `-e' option skips a header on the file, by ignoring the first N
bytes. This means that the header is _not_ counted towards the
disassembly offset: if you give `-e10 -o10', disassembly will start
at byte 10 in the file, and this will be given offset 10, not 20.
The `-k' option is provided with two comma-separated numeric
arguments, the first of which is an assembly offset and the second
is a number of bytes to skip. This _will_ count the skipped bytes
towards the assembly offset: its use is to suppress disassembly of a
data section which wouldn't contain anything you wanted to see
anyway.
A.4 Bugs and Improvements
There are no known bugs. However, any you find, with patches if
possible, should be sent to `nasm-bugs@lists.sourceforge.net', or to
the developer's site at `http://www.nasm.us/' and we'll try to fix
them. Feel free to send contributions and new features as well.
Appendix B: Instruction List
----------------------------
B.1 Introduction
The following sections show the instructions which NASM currently
supports. For each instruction, there is a separate entry for each
supported addressing mode. The third column shows the processor type
in which the instruction was introduced and, when appropriate, one
or more usage flags.
B.1.1 Special instructions...
DB
DW
DD
DQ
DT
DO
DY
DZ
RESB imm 8086
RESW
RESD
RESQ
REST
RESO
RESY
RESZ
B.1.4 Introduced in Deschutes but necessary for SSE support
FXRSTOR mem P6,SSE,FPU
FXRSTOR64 mem X64,SSE,FPU
FXSAVE mem P6,SSE,FPU
FXSAVE64 mem X64,SSE,FPU
B.1.5 XSAVE group (AVX and extended state)
XGETBV NEHALEM
XSETBV NEHALEM,PRIV
XSAVE mem NEHALEM
XSAVE64 mem LONG,NEHALEM
XSAVEOPT mem FUTURE
XSAVEOPT64 mem LONG,FUTURE
XRSTOR mem NEHALEM
XRSTOR64 mem LONG,NEHALEM
(*) 8 new opmask registers `(K0 ~ K7)'. One of 7 registers
`(K1 ~ K7)' can be used as an opmask for conditional execution.
(*) A new EVEX encoding prefix. EVEX is based on VEX and provides
more capabilities: opmasks, broadcasting, embedded rounding and
compressed displacements.
- opmask
VDIVPD zmm0{k1}{z}, zmm1, zmm3 ; conditional vector operation
; using opmask k1.
; {z} is for zero-masking
- broadcasting
VDIVPS zmm4, zmm5, [rbx]{1to16} ; load single-precision float and
; replicate it 16 times. 32 * 16 = 512
- embedded rounding
VCVTSI2SD xmm6, xmm7, {rz-sae}, rax ; round toward zero. note that it
; is used as if a separate operand.
; it comes after the last SIMD operand
(*) Add support for `ZWORD' (512 bits), `DZ' and `RESZ'.
(*) Add support for the MPX and SHA instruction sets.
(*) Better handling of section redefinition.
(*) Generate manpages when running `'make dist''.
(*) Handle all token chains in mmacro params range.
This is expected to be most useful for the MPX instructions.
(*) Support `BND' prefix for branch instructions (for MPX).
(*) The `DEFAULT' directive can now take `BND' and `NOBND' options
to indicate whether all relevant branches should be getting
`BND' prefixes. This is expected to be the normal for use in MPX
code.
(*) Add `{evex'}, `{vex3'} and `{vex2'} instruction prefixes to have
NASM encode the corresponding instruction, if possible, with an
EVEX, 3-byte VEX, or 2-byte VEX prefix, respectively.
(*) Support for section names longer than 8 bytes in Win32/Win64
COFF.
(*) The `NOSPLIT' directive by itself no longer forces a single
register to become an index register, unless it has an explicit
multiplier.
mov eax,[nosplit eax] ; eax as base register
mov eax,[nosplit eax*1] ; eax as index register
C.1.2 Version 2.10.09
(*) Pregenerate man pages.
C.1.3 Version 2.10.08
(*) Fix `VMOVNTDQA', `MOVNTDQA' and `MOVLPD' instructions.
(*) Fix collision for `VGATHERQPS', `VPGATHERQD' instructions.
To force a specific form, use the `STRICT' keyword, see section 3.7.
(*) Add support for the Intel AVX2 instruction set.
(*) Add support for Bit Manipulation Instructions 1 and 2.
(*) Add support for Intel Transactional Synchronization Extensions
(TSX).
(*) Add support for x32 ELF (32-bit ELF with the CPU in 64-bit
mode.) See section 7.9.
(*) Add support for bigendian UTF-16 and UTF-32. See section 3.4.5.
C.1.12 Version 2.09.10
(*) Fix up NSIS script to protect uninstaller against registry keys
absence or corruption. It brings in a few additional questions
to a user during deinstallation procedure but still it is better
than unpredictable file removal.
C.1.13 Version 2.09.09
(*) Fix initialization of section attributes of `bin' output format.
(*) Fix `mach64' output format bug that crashes NASM due to NULL
symbols.
C.1.14 Version 2.09.08
(*) Fix `__OUTPUT_FORMAT__' assignment when output driver alias is
used. For example when `-f elf' is used `__OUTPUT_FORMAT__' must
be set to `elf', if `-f elf32' is used `__OUTPUT_FORMAT__' must
be assigned accordingly, i.e. to `elf32'. The rule applies to
all output driver aliases. See section 4.12.6.
C.1.15 Version 2.09.07
(*) Fix attempts to close same file several times when `-a' option
is used.
(*) Fixes for VEXTRACTF128, VMASKMOVPS encoding.
C.1.16 Version 2.09.06
(*) Fix missed section attribute initialization in `bin' output
target.
C.1.17 Version 2.09.05
(*) Fix arguments encoding for VPEXTRW instruction.
(*) Remove invalid form of VPEXTRW instruction.
(*) Add `VLDDQU' as alias for `VLDQQU' to match specification.
C.1.18 Version 2.09.04
(*) Fix incorrect labels offset for VEX intructions.
(*) Eliminate bogus warning on implicit operand size override.
(*) `%if' term could not handle 64 bit numbers.
(*) The COFF backend was limiting relocations number to 16 bits even
if in real there were a way more relocations.
C.1.19 Version 2.09.03
(*) Print `%macro' name inside `%rep' blocks on error.
(*) Fix preprocessor expansion behaviour. It happened sometime too
early and sometime simply wrong. Move behaviour back to the
origins (down to NASM 2.05.01).
(*) Fix unitialized data dereference on OMF output format.
(*) Issue warning on unterminated `%{' construct.
(*) Fix for documentation typo.
C.1.20 Version 2.09.02
(*) Fix reversed tokens when `%deftok' produces more than one output
token.
(*) Fix segmentation fault on disassembling some VEX instructions.
(*) Missing `%endif' did not always cause error.
(*) Fix typo in documentation.
(*) Compound context local preprocessor single line macro
identifiers were not expanded early enough and as result lead to
unresolved symbols.
C.1.21 Version 2.09.01
(*) Fix NULL dereference on missed %deftok second parameter.
(*) Fix NULL dereference on invalid %substr parameters.
C.1.22 Version 2.09
(*) Fixed assignment the magnitude of `%rep' counter. It is limited
to 62 bits now.
(*) Fixed NULL dereference if argument of `%strlen' resolves to
whitespace. For example if nonexistent macro parameter is used.
(*) `%ifenv', `%elifenv', `%ifnenv', and `%elifnenv' directives
introduced. See section 4.4.9.
(*) Fixed NULL dereference if environment variable is missed.
(*) Updates of new AVX v7 Intel instructions.
(*) `PUSH imm32' is now officially documented.
(*) Fix for encoding the LFS, LGS and LSS in 64-bit mode.
(*) Fixes for compatibility with OpenWatcom compiler and DOS 8.3
file format limitation.
(*) Macros parameters range expansion introduced. See section 4.3.4.
(*) Backward compatibility on expanging of local sigle macros
restored.
(*) 8 bit relocations for `elf' and `bin' output formats are
introduced.
(*) Short intersegment jumps are permitted now.
(*) An alignment more than 64 bytes are allowed for `win32', `win64'
output formats.
(*) `SECTALIGN' directive introduced. See section 4.12.13.
(*) `nojmp' option introduced in `smartalign' package. See section
5.2.
(*) Short aliases `win', `elf' and `macho' for output formats are
introduced. Each stands for `win32', `elf32' and `macho32'
accordingly.
(*) Faster handling of missing directives implemented.
(*) Various small improvements in documentation.
(*) No hang anymore if unable to open malloc.log file.
(*) The environments without vsnprintf function are able to build
nasm again.
(*) AMD LWP instructions updated.
(*) Tighten EA checks. We warn a user if there overflow in EA
addressing.
(*) Make `-Ox' the default optimization level. For the legacy
behavior, specify `-O0' explicitly. See section 2.1.22.
(*) Environment variables read with `%!' or tested with `%ifenv' can
now contain non-identifier characters if surrounded by quotes.
See section 4.10.2.
(*) Add a new standard macro package `%use fp' for floating-point
convenience macros. See section 5.3.
C.1.23 Version 2.08.02
(*) Fix crash under certain circumstances when using the `%+'
operator.
C.1.24 Version 2.08.01
(*) Fix the `%use' statement, which was broken in 2.08.
C.1.25 Version 2.08
(*) A number of enhancements/fixes in macros area.
(*) Support for converting strings to tokens. See section 4.1.9.
(*) Fuzzy operand size logic introduced.
(*) Fix COFF stack overrun on too long export identifiers.
(*) Fix Macho-O alignment bug.
(*) Fix crashes with -fwin32 on file with many exports.
(*) Fix stack overrun for too long [DEBUG id].
(*) Fix incorrect sbyte usage in IMUL (hit only if optimization flag
passed).
(*) Append ending token for `.stabs' records in the ELF output
format.
(*) New NSIS script which uses ModernUI and MultiUser approach.
(*) Visual Studio 2008 NASM integration (rules file).
(*) Warn a user if a constant is too long (and as result will be
stripped).
(*) The obsoleted pre-XOP AMD SSE5 instruction set which was never
actualized was removed.
(*) Fix stack overrun on too long error file name passed from the
command line.
(*) Bind symbols to the .text section by default (ie in case if
SECTION directive was omitted) in the ELF output format.
(*) Fix sync points array index wrapping.
(*) A few fixes for FMA4 and XOP instruction templates.
(*) An undefined local macro (`%$') no longer matches a global macro
with the same name.
(*) Fix NULL dereference on too long local labels.
C.1.26 Version 2.07
(*) NASM is now under the 2-clause BSD license. See section 1.1.2.
(*) Fix the section type for the `.strtab' section in the `elf64'
output format.
(*) Fix the handling of `COMMON' directives in the `obj' output
format.
(*) New `ith' and `srec' output formats; these are variants of the
`bin' output format which output Intel hex and Motorola S-
records, respectively. See section 7.2 and section 7.3.
(*) `rdf2ihx' replaced with an enhanced `rdf2bin', which can output
binary, COM, Intel hex or Motorola S-records.
(*) The Windows installer now puts the NASM directory first in the
`PATH' of the "NASM Shell".
(*) Revert the early expansion behavior of `%+' to pre-2.06
behavior: `%+' is only expanded late.
(*) Yet another Mach-O alignment fix.
(*) Don't delete the list file on errors. Also, include error and
warning information in the list file.
(*) Support for 64-bit Mach-O output, see section 7.8.
(*) Fix assert failure on certain operations that involve strings
with high-bit bytes.
C.1.27 Version 2.06
(*) This release is dedicated to the memory of Charles A. Crayne,
long time NASM developer as well as moderator of
`comp.lang.asm.x86' and author of the book _Serious Assembler_.
We miss you, Chuck.
(*) Support for indirect macro expansion (`%[...]'). See section
4.1.3.
(*) `%pop' can now take an argument, see section 4.7.1.
(*) The argument to `%use' is no longer macro-expanded. Use `%[...]'
if macro expansion is desired.
(*) Support for thread-local storage in ELF32 and ELF64. See section
7.9.4.
(*) Fix crash on `%ifmacro' without an argument.
(*) Correct the arguments to the `POPCNT' instruction.
(*) Fix section alignment in the Mach-O format.
(*) Update AVX support to version 5 of the Intel specification.
(*) Fix the handling of accesses to context-local macros from higher
levels in the context stack.
(*) Treat `WAIT' as a prefix rather than as an instruction, thereby
allowing constructs like `O16 FSAVE' to work correctly.
(*) Support for structures with a non-zero base offset. See section
4.12.10.
(*) The `PINSR' series of instructions have been corrected and
rationalized.
(*) Removed AMD SSE5, replaced with the new XOP/FMA4/CVT16 (rev
3.03) spec.
(*) The ELF backends no longer automatically generate a `.comment'
section.
(*) Add additional "well-known" ELF sections with default
attributes. See section 7.9.2.
C.1.28 Version 2.05.01
(*) Fix the `-w'/`-W' option parsing, which was broken in NASM 2.05.
C.1.29 Version 2.05
(*) Fix redundant REX.W prefix on `JMP reg64'.
(*) Make the behaviour of `-O0' match NASM 0.98 legacy behavior. See
section 2.1.22.
(*) `-w-user' can be used to suppress the output of `%warning'
directives. See section 2.1.24.
(*) Fix bug where `ALIGN' would issue a full alignment datum instead
of zero bytes.
(*) Fix offsets in list files.
(*) Fix `%include' inside multi-line macros or loops.
(*) Fix error where NASM would generate a spurious warning on valid
optimizations of immediate values.
(*) Fix arguments to a number of the `CVT' SSE instructions.
(*) Fix RIP-relative offsets when the instruction carries an
immediate.
(*) Massive overhaul of the ELF64 backend for spec compliance.
(*) Fix the Geode `PFRCPV' and `PFRSQRTV' instruction.
(*) Fix the SSE 4.2 `CRC32' instruction.
C.1.30 Version 2.04
(*) Sanitize macro handing in the `%error' directive.
(*) New `%warning' directive to issue user-controlled warnings.
(*) `%error' directives are now deferred to the final assembly
phase.
(*) New `%fatal' directive to immediately terminate assembly.
(*) New `%strcat' directive to join quoted strings together.
(*) New `%use' macro directive to support standard macro directives.
See section 4.6.4.
(*) Excess default parameters to `%macro' now issues a warning by
default. See section 4.3.
(*) Fix `%ifn' and `%elifn'.
(*) Fix nested `%else' clauses.
(*) Correct the handling of nested `%rep's.
(*) New `%unmacro' directive to undeclare a multi-line macro. See
section 4.3.12.
(*) Builtin macro `__PASS__' which expands to the current assembly
pass. See section 4.12.9.
(*) `__utf16__' and `__utf32__' operators to generate UTF-16 and
UTF-32 strings. See section 3.4.5.
(*) Fix bug in case-insensitive matching when compiled on platforms
that don't use the `configure' script. Of the official release
binaries, that only affected the OS/2 binary.
(*) Support for x87 packed BCD constants. See section 3.4.7.
(*) Correct the `LTR' and `SLDT' instructions in 64-bit mode.
(*) Fix unnecessary REX.W prefix on indirect jumps in 64-bit mode.
(*) Add AVX versions of the AES instructions (`VAES'...).
(*) Fix the 256-bit FMA instructions.
(*) Add 256-bit AVX stores per the latest AVX spec.
(*) VIA XCRYPT instructions can now be written either with or
without `REP', apparently different versions of the VIA spec
wrote them differently.
(*) Add missing 64-bit `MOVNTI' instruction.
(*) Fix the operand size of `VMREAD' and `VMWRITE'.
(*) Numerous bug fixes, especially to the AES, AVX and VTX
instructions.
(*) The optimizer now always runs until it converges. It also runs
even when disabled, but doesn't optimize. This allows most
forward references to be resolved properly.
(*) `%push' no longer needs a context identifier; omitting the
context identifier results in an anonymous context.
C.1.31 Version 2.03.01
(*) Fix buffer overflow in the listing module.
(*) Fix the handling of hexadecimal escape codes in `...` strings.
(*) The Postscript/PDF documentation has been reformatted.
(*) The `-F' option now implies `-g'.
C.1.32 Version 2.03
(*) Add support for Intel AVX, CLMUL and FMA instructions, including
YMM registers.
(*) `dy', `resy' and `yword' for 32-byte operands.
(*) Fix some SSE5 instructions.
(*) Intel `INVEPT', `INVVPID' and `MOVBE' instructions.
(*) Fix checking for critical expressions when the optimizer is
enabled.
(*) Support the DWARF debugging format for ELF targets.
(*) Fix optimizations of signed bytes.
(*) Fix operation on bigendian machines.
(*) Fix buffer overflow in the preprocessor.
(*) `SAFESEH' support for Win32, `IMAGEREL' for Win64 (SEH).
(*) `%?' and `%??' to refer to the name of a macro itself. In
particular, `%idefine keyword $%?' can be used to make a keyword
"disappear".
(*) New options for dependency generation: `-MD', `-MF', `-MP',
`-MT', `-MQ'.
(*) New preprocessor directives `%pathsearch' and `%depend'; INCBIN
reimplemented as a macro.
(*) `%include' now resolves macros in a sane manner.
(*) `%substr' can now be used to get other than one-character
substrings.
(*) New type of character/string constants, using backquotes
(``...`'), which support C-style escape sequences.
(*) `%defstr' and `%idefstr' to stringize macro definitions before
creation.
(*) Fix forward references used in `EQU' statements.
C.1.33 Version 2.02
(*) Additional fixes for MMX operands with explicit `qword', as well
as (hopefully) SSE operands with `oword'.
(*) Fix handling of truncated strings with `DO'.
(*) Fix segfaults due to memory overwrites when floating-point
constants were used.
(*) Fix segfaults due to missing include files.
(*) Fix OpenWatcom Makefiles for DOS and OS/2.
(*) Add autogenerated instruction list back into the documentation.
(*) ELF: Fix segfault when generating stabs, and no symbols have
been defined.
(*) ELF: Experimental support for DWARF debugging information.
(*) New compile date and time standard macros.
(*) `%ifnum' now returns true for negative numbers.
(*) New `%iftoken' test for a single token.
(*) New `%ifempty' test for empty expansion.
(*) Add support for the `XSAVE' instruction group.
(*) Makefile for Netware/gcc.
(*) Fix issue with some warnings getting emitted way too many times.
(*) Autogenerated instruction list added to the documentation.
C.1.34 Version 2.01
(*) Fix the handling of MMX registers with explicit `qword' tags on
memory (broken in 2.00 due to 64-bit changes.)
(*) Fix the PREFETCH instructions.
(*) Fix the documentation.
(*) Fix debugging info when using `-f elf' (backwards compatibility
alias for `-f elf32').
(*) Man pages for rdoff tools (from the Debian project.)
(*) ELF: handle large numbers of sections.
(*) Fix corrupt output when the optimizer runs out of passes.
C.1.35 Version 2.00
(*) Added c99 data-type compliance.
(*) Added general x86-64 support.
(*) Added win64 (x86-64 COFF) output format.
(*) Added `__BITS__' standard macro.
(*) Renamed the `elf' output format to `elf32' for clarity.
(*) Added `elf64' and `macho' (MacOS X) output formats.
(*) Added Numeric constants in `dq' directive.
(*) Added `oword', `do' and `reso' pseudo operands.
(*) Allow underscores in numbers.
(*) Added 8-, 16- and 128-bit floating-point formats.
(*) Added binary, octal and hexadecimal floating-point.
(*) Correct the generation of floating-point constants.
(*) Added floating-point option control.
(*) Added Infinity and NaN floating point support.
(*) Added ELF Symbol Visibility support.
(*) Added setting OSABI value in ELF header directive.
(*) Added a large number of additional instructions.
(*) Significant performance improvements.
(*) `-w+warning' and `-w-warning' can now be written as -Wwarning
and -Wno-warning, respectively. See section 2.1.24.
(*) Add `-w+error' to treat warnings as errors. See section 2.1.24.
(*) Add `-w+all' and `-w-all' to enable or disable all suppressible
warnings. See section 2.1.24.
C.2 NASM 0.98 Series
The 0.98 series was the production versions of NASM from 1999 to
2007.
C.2.1 Version 0.98.39
(*) fix buffer overflow
(*) fix outas86's `.bss' handling
(*) "make spotless" no longer deletes config.h.in.
(*) `%(el)if(n)idn' insensitivity to string quotes difference
(#809300).
(*) (nasm.c)`__OUTPUT_FORMAT__' changed to string value instead of
symbol.
C.2.2 Version 0.98.38
(*) Add Makefile for 16-bit DOS binaries under OpenWatcom, and
modify `mkdep.pl' to be able to generate completely pathless
dependencies, as required by OpenWatcom wmake (it supports path
searches, but not explicit paths.)
(*) Fix the `STR' instruction.
(*) Fix the ELF output format, which was broken under certain
circumstances due to the addition of stabs support.
(*) Quick-fix Borland format debug-info for `-f obj'
(*) Fix for `%rep' with no arguments (#560568)
(*) Fix concatenation of preprocessor function call (#794686)
(*) Fix long label causes coredump (#677841)
(*) Use autoheader as well as autoconf to keep configure from
generating ridiculously long command lines.
(*) Make sure that all of the formats which support debugging output
actually will suppress debugging output when `-g' not specified.
C.2.3 Version 0.98.37
(*) Paths given in `-I' switch searched for `incbin'-ed as well as
`%include'-ed files.
(*) Added stabs debugging for the ELF output format, patch from
Martin Wawro.
(*) Fix `output/outbin.c' to allow origin > 80000000h.
(*) Make `-U' switch work.
(*) Fix the use of relative offsets with explicit prefixes, e.g.
`a32 loop foo'.
(*) Remove `backslash()'.
(*) Fix the `SMSW' and `SLDT' instructions.
(*) `-O2' and `-O3' are no longer aliases for `-O10' and `-O15'. If
you mean the latter, please say so! :)
C.2.4 Version 0.98.36
(*) Update rdoff - librarian/archiver - common rec - docs!
(*) Fix signed/unsigned problems.
(*) Fix `JMP FAR label' and `CALL FAR label'.
(*) Add new multisection support - map files - fix align bug
(*) Fix sysexit, movhps/movlps reg,reg bugs in insns.dat
(*) `Q' or `O' suffixes indicate octal
(*) Support Prescott new instructions (PNI).
(*) Cyrix `XSTORE' instruction.
C.2.5 Version 0.98.35
(*) Fix build failure on 16-bit DOS (Makefile.bc3 workaround for
compiler bug.)
(*) Fix dependencies and compiler warnings.
(*) Add "const" in a number of places.
(*) Add -X option to specify error reporting format (use -Xvc to
integrate with Microsoft Visual Studio.)
(*) Minor changes for code legibility.
(*) Drop use of tmpnam() in rdoff (security fix.)
C.2.6 Version 0.98.34
(*) Correct additional address-size vs. operand-size confusions.
(*) Generate dependencies for all Makefiles automatically.
(*) Add support for unimplemented (but theoretically available)
registers such as tr0 and cr5. Segment registers 6 and 7 are
called segr6 and segr7 for the operations which they can be
represented.
(*) Correct some disassembler bugs related to redundant address-size
prefixes. Some work still remains in this area.
(*) Correctly generate an error for things like "SEG eax".
(*) Add the JMPE instruction, enabled by "CPU IA64".
(*) Correct compilation on newer gcc/glibc platforms.
(*) Issue an error on things like "jmp far eax".
C.2.7 Version 0.98.33
(*) New __NASM_PATCHLEVEL__ and __NASM_VERSION_ID__ standard macros
to round out the version-query macros. version.pl now
understands X.YYplWW or X.YY.ZZplWW as a version number,
equivalent to X.YY.ZZ.WW (or X.YY.0.WW, as appropriate).
(*) New keyword "strict" to disable the optimization of specific
operands.
(*) Fix the handing of size overrides with JMP instructions
(instructions such as "jmp dword foo".)
(*) Fix the handling of "ABSOLUTE label", where "label" points into
a relocatable segment.
(*) Fix OBJ output format with lots of externs.
(*) More documentation updates.
(*) Add -Ov option to get verbose information about optimizations.
(*) Undo a braindead change which broke `%elif' directives.
(*) Makefile updates.
C.2.8 Version 0.98.32
(*) Fix NASM crashing when `%macro' directives were left
unterminated.
(*) Lots of documentation updates.
(*) Complete rewrite of the PostScript/PDF documentation generator.
(*) The MS Visual C++ Makefile was updated and corrected.
(*) Recognize .rodata as a standard section name in ELF.
(*) Fix some obsolete Perl4-isms in Perl scripts.
(*) Fix configure.in to work with autoconf 2.5x.
(*) Fix a couple of "make cleaner" misses.
(*) Make the normal "./configure && make" work with Cygwin.
C.2.9 Version 0.98.31
(*) Correctly build in a separate object directory again.
(*) Derive all references to the version number from the version
file.
(*) New standard macros __NASM_SUBMINOR__ and __NASM_VER__ macros.
(*) Lots of Makefile updates and bug fixes.
(*) New `%ifmacro' directive to test for multiline macros.
(*) Documentation updates.
(*) Fixes for 16-bit OBJ format output.
(*) Changed the NASM environment variable to NASMENV.
C.2.10 Version 0.98.30
(*) Changed doc files a lot: completely removed old READMExx and
Wishlist files, incorporating all information in CHANGES and
TODO.
(*) I waited a long time to rename zoutieee.c to (original)
outieee.c
(*) moved all output modules to output/ subdirectory.
(*) Added 'make strip' target to strip debug info from nasm &
ndisasm.
(*) Added INSTALL file with installation instructions.
(*) Added -v option description to nasm man.
(*) Added dist makefile target to produce source distributions.
(*) 16-bit support for ELF output format (GNU extension, but
useful.)
C.2.11 Version 0.98.28
(*) Fastcooked this for Debian's Woody release: Frank applied the
INCBIN bug patch to 0.98.25alt and called it 0.98.28 to not
confuse poor little apt-get.
C.2.12 Version 0.98.26
(*) Reorganised files even better from 0.98.25alt
C.2.13 Version 0.98.25alt
(*) Prettified the source tree. Moved files to more reasonable
places.
(*) Documentation - Ndisasm doc added to Nasm.doc.
C.2.17 Version 0.98.23
(*) Attempted to remove rdoff version1
(*) Lino Mastrodomenico's patches to preproc.c (%$$ bug?).
C.2.18 Version 0.98.22
(*) Update rdoff2 - attempt to remove v1.
C.2.19 Version 0.98.21
(*) Optimization fixes.
C.2.20 Version 0.98.20
(*) Optimization fixes.
C.2.21 Version 0.98.19
(*) H. J. Lu's patch back out.
C.2.22 Version 0.98.18
(*) Added ".rdata" to "-f win32".
C.2.23 Version 0.98.17
(*) H. J. Lu's "bogus elf" patch. (Red Hat problem?)
C.2.24 Version 0.98.16
(*) Fix whitespace before "[section ..." bug.
C.2.25 Version 0.98.15
(*) Rdoff changes (?).
(*) Fix fixes to memory leaks.
C.2.26 Version 0.98.14
(*) Fix memory leaks.
C.2.27 Version 0.98.13
(*) There was no 0.98.13
C.2.28 Version 0.98.12
(*) Update optimization (new function of "-O1")
(*) Changes to test/bintest.asm (?).
C.2.29 Version 0.98.11
(*) Optimization changes.
(*) Ndisasm fixed.
C.2.30 Version 0.98.10
(*) There was no 0.98.10
C.2.31 Version 0.98.09
(*) Add multiple sections support to "-f bin".
(*) Changed GLOBAL_TEMP_BASE in outelf.c from 6 to 15.
(*) Add "-v" as an alias to the "-r" switch.
(*) Remove "#ifdef" from Tasm compatibility options.
(*) Remove redundant size-overrides on "mov ds, ex", etc.
(*) Fixes to SSE2, other insns.dat (?).
(*) Enable uppercase "I" and "P" switches.
(*) Case insinsitive "seg" and "wrt".
(*) Update install.sh (?).
(*) Allocate tokens in blocks.
(*) Improve "invalid effective address" messages.
C.2.32 Version 0.98.08
(*) Add "`%strlen'" and "`%substr'" macro operators
(*) Fixed broken c16.mac.
(*) Unterminated string error reported.
(*) Fixed bugs as per 0.98bf
C.2.33 Version 0.98.09b with John Coffman patches released 28-Oct-2001
Changes from 0.98.07 release to 98.09b as of 28-Oct-2001
(*) More closely compatible with 0.98 when -O0 is implied or
specified. Not strictly identical, since backward branches in
range of short offsets are recognized, and signed byte values
with no explicit size specification will be assembled as a
single byte.
(*) More forgiving with the PUSH instruction. 0.98 requires a size
to be specified always. 0.98.09b will imply the size from the
current BITS setting (16 or 32).
(*) Changed definition of the optimization flag:
-O0 strict two-pass assembly, JMP and Jcc are handled more like
0.98, except that back- ward JMPs are short, if possible.
-O1 strict two-pass assembly, but forward branches are assembled
with code guaranteed to reach; may produce larger code than -O0, but
will produce successful assembly more often if branch offset sizes
are not specified.
-O2 multi-pass optimization, minimize branch offsets; also will
minimize signed immed- iate bytes, overriding size specification.
-O3 like -O2, but more passes taken, if needed
C.2.34 Version 0.98.07 released 01/28/01
(*) Added Stepane Denis' SSE2 instructions to a *working* version of
the code - some earlier versions were based on broken code -
sorry 'bout that. version "0.98.07"
01/28/01
(*) Cosmetic modifications to nasm.c, nasm.h, AUTHORS, MODIFIED
C.2.35 Version 0.98.06f released 01/18/01
(*) - Add "metalbrain"s jecxz bug fix in insns.dat - alter
nasmdoc.src to match - version "0.98.06f"
C.2.36 Version 0.98.06e released 01/09/01
(*) Removed the "outforms.h" file - it appears to be someone's old
backup of "outform.h". version "0.98.06e"
01/09/01
(*) fbk - finally added the fix for the "multiple %includes bug",
known since 7/27/99 - reported originally (?) and sent to us by
Austin Lunnen - he reports that John Fine had a fix within the
day. Here it is...
(*) Nelson Rush resigns from the group. Big thanks to Nelson for his
leadership and enthusiasm in getting these changes incorporated
into Nasm!
(*) fbk - [list +], [list -] directives - ineptly implemented,
should be re-written or removed, perhaps.
(*) Brian Raiter / fbk - "elfso bug" fix - applied to aoutb format
as well - testing might be desirable...
08/07/00
(*) James Seter - -postfix, -prefix command line switches.
(*) Yuri Zaporogets - rdoff utility changes.
C.2.37 Version 0.98p1
(*) GAS-like palign (Panos Minos)
(*) FIXME: Someone, fill this in with details
C.2.38 Version 0.98bf (bug-fixed)
(*) Fixed - elf and aoutb bug - shared libraries - multiple
"%include" bug in "-f obj" - jcxz, jecxz bug - unrecognized
option bug in ndisasm
C.2.39 Version 0.98.03 with John Coffman's changes released 27-Jul-2000
(*) Added signed byte optimizations for the 0x81/0x83 class of
instructions: ADC, ADD, AND, CMP, OR, SBB, SUB, XOR: when used
as 'ADD reg16,imm' or 'ADD reg32,imm.' Also optimization of
signed byte form of 'PUSH imm' and 'IMUL reg,imm'/'IMUL
reg,reg,imm.' No size specification is needed.
(*) Added multi-pass JMP and Jcc offset optimization. Offsets on
forward references will preferentially use the short form,
without the need to code a specific size (short or near) for the
branch. Added instructions for 'Jcc label' to use the form
'Jnotcc $+3/JMP label', in cases where a short offset is out of
bounds. If compiling for a 386 or higher CPU, then the 386 form
of Jcc will be used instead.
This feature is controlled by a new command-line switch: "O", (upper
case letter O). "-O0" reverts the assembler to no extra optimization
passes, "-O1" allows up to 5 extra passes, and "-O2"(default),
allows up to 10 extra optimization passes.
(*) Added a new directive: 'cpu XXX', where XXX is any of: 8086,
186, 286, 386, 486, 586, pentium, 686, PPro, P2, P3 or Katmai.
All are case insensitive. All instructions will be selected only
if they apply to the selected cpu or lower. Corrected a couple
of bugs in cpu-dependence in 'insns.dat'.
(*) Added to 'standard.mac', the "use16" and "use32" forms of the
"bits 16/32" directive. This is nothing new, just conforms to a
lot of other assemblers. (minor)
(*) Changed label allocation from 320/32 (10000 labels @ 200K+) to
32/37 (1000 labels); makes running under DOS much easier. Since
additional label space is allocated dynamically, this should
have no effect on large programs with lots of labels. The 37 is
a prime, believed to be better for hashing. (minor)
C.2.40 Version 0.98.03
"Integrated patchfile 0.98-0.98.01. I call this version 0.98.03 for
historical reasons: 0.98.02 was trashed." --John Coffman
<johninsd@san.rr.com>, 27-Jul-2000
(*) Kendall Bennett's SciTech MGL changes
(*) Note that you must define "TASM_COMPAT" at compile-time to get
the Tasm Ideal Mode compatibility.
(*) All changes can be compiled in and out using the TASM_COMPAT
macros, and when compiled without TASM_COMPAT defined we get the
exact same binary as the unmodified 0.98 sources.
(*) standard.mac, macros.c: Added macros to ignore TASM directives
before first include
(*) nasm.h: Added extern declaration for tasm_compatible_mode
(*) nasm.c: Added global variable tasm_compatible_mode
(*) Added command line switch for TASM compatible mode (-t)
(*) Changed version command line to reflect when compiled with TASM
additions
(*) Added response file processing to allow all arguments on a
single line (response file is @resp rather than -@resp for NASM
format).
(*) labels.c: Changes islocal() macro to support TASM style @@local
labels.
(*) Added islocalchar() macro to support TASM style @@local labels.
(*) parser.c: Added support for TASM style memory references (ie:
mov [DWORD eax],10 rather than the NASM style mov DWORD
[eax],10).
(*) preproc.c: Added new directives, `%arg', `%local', `%stacksize'
to directives table
(*) Added support for TASM style directives without a leading %
symbol.
(*) Integrated a block of changes from Andrew Zabolotny
<bit@eltech.ru>:
(*) A new keyword `%xdefine' and its case-insensitive counterpart
`%ixdefine'. They work almost the same way as `%define' and
`%idefine' but expand the definition immediately, not on the
invocation. Something like a cross between `%define' and
`%assign'. The "x" suffix stands for "eXpand", so "xdefine" can
be deciphered as "expand-and-define". Thus you can do things
like this:
(*) Changed the place where the expansion of %$name macros are
expanded. Now they are converted into ..@ctxnum.name form when
detokenizing, so there are no quirks as before when using %$name
arguments to macros, in macros etc. For example:
%macro abc 1
%define %1 hello
%endm
abc %$here
%$here
Now last line will be expanded into "hello" as expected. This also
allows for lots of goodies, a good example are extended "proc"
macros included in this archive.
(*) Added a check for "cstk" in smacro_defined() before calling
get_ctx() - this allows for things like:
%ifdef %$abc
%endif
to work without warnings even in no context.
(*) Added a check for "cstk" in %if*ctx and %elif*ctx directives -
this allows to use `%ifctx' without excessive warnings. If there
is no active context, `%ifctx' goes through "false" branch.
(*) Removed "user error: " prefix with `%error' directive: it just
clobbers the output and has absolutely no functionality.
Besides, this allows to write macros that does not differ from
built-in functions in any way.
(*) Added expansion of string that is output by `%error' directive.
Now you can do things like:
%define hello(x) Hello, x!
%define %$name andy
%error "hello(%$name)"
Same happened with `%include' directive.
(*) Now all directives that expect an identifier will try to expand
and concatenate everything without whitespaces in between before
usage. For example, with "unfixed" nasm the commands
would produce "incorrect" output: last line will expand to
hello goodbyehello
Not quite what you expected, eh? :-) The answer is that preprocessor
treats the `%define' construct as if it would be
%define __ %$abc goodbye
(note the white space between __ and %$abc). After my "fix" it will
"correctly" expand into
goodbye
as expected. Note that I use quotes around words "correct",
"incorrect" etc because this is rather a feature not a bug; however
current behaviour is more logical (and allows more advanced macro
usage :-).
Same change was applied to:
`%push',`%macro',`%imacro',`%define',`%idefine',`%xdefine',`%ixdefine',
`%assign',`%iassign',`%undef'
(*) A new directive [WARNING {+|-}warning-id] have been added. It
works only if the assembly phase is enabled (i.e. it doesn't
work with nasm -e).
(*) A new warning type: macro-selfref. By default this warning is
disabled; when enabled NASM warns when a macro self-references
itself; for example the following source:
will produce a warning, but if we remove the first line we won't see
it anymore (which is The Right Thing To Do {tm} IMHO since C
preprocessor eats such constructs without warnings at all).
(*) Added a "error" routine to preprocessor which always will set
ERR_PASS1 bit in severity_code. This removes annoying repeated
errors on first and second passes from preprocessor.
(*) Added the %+ operator in single-line macros for concatenating
two identifiers. Usage example:
%define _myfunc _otherfunc
%define cextern(x) _ %+ x
cextern (myfunc)
After first expansion, third line will become "_myfunc". After this
expansion is performed again so it becomes "_otherunc".
(*) Now if preprocessor is in a non-emitting state, no warning or
error will be emitted. Example:
%if 1
mov eax,ebx
%else
put anything you want between these two brackets,
even macro-parameter references %1 or local
labels %$zz or macro-local labels %%zz - no
warning will be emitted.
%endif
(*) Context-local variables on expansion as a last resort are looked
up in outer contexts. For example, the following piece:
%push outer
%define %$a [esp]
%push inner
%$a
%pop
%pop
will expand correctly the fourth line to [esp]; if we'll define
another %$a inside the "inner" context, it will take precedence over
outer definition. However, this modification has been applied only
to expand_smacro and not to smacro_define: as a consequence
expansion looks in outer contexts, but `%ifdef' won't look in outer
contexts.
This behaviour is needed because we don't want nested contexts to
act on already defined local macros. Example:
%define %$arg1 [esp+4]
test eax,eax
if nz
mov eax,%$arg1
endif
In this example the "if" mmacro enters into the "if" context, so
%$arg1 is not valid anymore inside "if". Of course it could be
worked around by using explicitely %$$arg1 but this is ugly IMHO.
(*) Fixed memory leak in `%undef'. The origline wasn't freed before
exiting on success.
(*) Fixed trap in preprocessor when line expanded to empty set of
tokens. This happens, for example, in the following case:
#define SOMETHING
SOMETHING
C.2.41 Version 0.98
All changes since NASM 0.98p3 have been produced by H. Peter Anvin
<hpa@zytor.com>.
(*) The documentation comment delimiter is
(*) Allow EQU definitions to refer to external labels; reported by
Pedro Gimeno.
(*) Re-enable support for RDOFF v1; reported by Pedro Gimeno.
(*) Updated License file per OK from Simon and Julian.
C.2.42 Version 0.98p9
(*) Update documentation (although the instruction set reference
will have to wait; I don't want to hold up the 0.98 release for
it.)
(*) Verified that the NASM implementation of the PEXTRW and PMOVMSKB
instructions is correct. The encoding differs from what the
Intel manuals document, but the Pentium III behaviour matches
NASM, not the Intel manuals.
(*) Fix handling of implicit sizes in PSHUFW and PINSRW, reported by
Stefan Hoffmeister.
(*) Resurrect the -s option, which was removed when changing the
diagnostic output to stdout.
C.2.43 Version 0.98p8
(*) Fix for "DB" when NASM is running on a bigendian machine.
(*) Invoke insns.pl once for each output script, making Makefile.in
legal for "make -j".
(*) Improve the Unix configure-based makefiles to make package
creation easier.
(*) Included an RPM .spec file for building RPM (RedHat Package
Manager) packages on Linux or Unix systems.
(*) Fix Makefile dependency problems.
(*) Change src/rdsrc.pl to include sectioning information in info
output; required for install-info to work.
(*) Updated the RDOFF distribution to version 2 from Jules; minor
massaging to make it compile in my environment.
(*) Split doc files that can be built by anyone with a Perl
interpreter off into a separate archive.
(*) "Dress rehearsal" release!
C.2.44 Version 0.98p7
(*) Fixed opcodes with a third byte-sized immediate argument to not
complain if given "byte" on the immediate.
(*) Allow `%undef' to remove single-line macros with arguments. This
matches the behaviour of #undef in the C preprocessor.
(*) Allow -d, -u, -i and -p to be specified as -D, -U, -I and -P for
compatibility with most C compilers and preprocessors. This
allows Makefile options to be shared between cc and nasm, for
example.
(*) Minor cleanups.
(*) Went through the list of Katmai instructions and hopefully fixed
the (rather few) mistakes in it.
(*) (Hopefully) fixed a number of disassembler bugs related to
ambiguous instructions (disambiguated by -p) and SSE
instructions with REP.
(*) Fix for bug reported by Mark Junger: "call dword 0x12345678"
should work and may add an OSP (affected CALL, JMP, Jcc).
(*) Fix for environments when "stderr" isn't a compile-time
constant.
C.2.45 Version 0.98p6
(*) Took officially over coordination of the 0.98 release; so drop
the p3.x notation. Skipped p4 and p5 to avoid confusion with
John Fine's J4 and J5 releases.
(*) Update the documentation; however, it still doesn't include
documentation for the various new instructions. I somehow wonder
if it makes sense to have an instruction set reference in the
assembler manual when Intel et al have PDF versions of their
manuals online.
(*) Recognize "idt" or "centaur" for the -p option to ndisasm.
(*) Changed error messages back to stderr where they belong, but add
an -E option to redirect them elsewhere (the DOS shell cannot
redirect stderr.)
(*) -M option to generate Makefile dependencies (based on code from
Alex Verstak.)
(*) `%undef' preprocessor directive, and -u option, that undefines a
single-line macro.
(*) OS/2 Makefile (Mkfiles/Makefile.os2) for Borland under OS/2;
from Chuck Crayne.
(*) Various minor bugfixes (reported by): - Dangling `%s' in
preproc.c (Martin Junker)
(*) THERE ARE KNOWN BUGS IN SSE AND THE OTHER KATMAI INSTRUCTIONS. I
am on a trip and didn't bring the Katmai instruction reference,
so I can't work on them right now.
(*) Updated the License file per agreement with Simon and Jules to
include a GPL distribution clause.
C.2.46 Version 0.98p3.7
(*) (Hopefully) fixed the canned Makefiles to include the outrdf2
and zoutieee modules.
(*) Renamed changes.asm to changed.asm.
C.2.47 Version 0.98p3.6
(*) Fixed a bunch of instructions that were added in 0.98p3.5 which
had memory operands, and the address-size prefix was missing
from the instruction pattern.
C.2.48 Version 0.98p3.5
(*) Merged in changes from John S. Fine's 0.98-J5 release. John's
based 0.98-J5 on my 0.98p3.3 release; this merges the changes.
(*) Expanded the instructions flag field to a long so we can fit
more flags; mark SSE (KNI) and AMD or Katmai-specific
instructions as such.
(*) Fix the "PRIV" flag on a bunch of instructions, and create new
"PROT" flag for protected-mode-only instructions (orthogonal to
if the instruction is privileged!) and new "SMM" flag for SMM-
only instructions.
(*) Added AMD-only SYSCALL and SYSRET instructions.
(*) Make SSE actually work, and add new Katmai MMX instructions.
(*) Added a -p (preferred vendor) option to ndisasm so that it can
distinguish e.g. Cyrix opcodes also used in SSE. For example:
(*) Made at least an attempt to modify all the additional Makefiles
(in the Mkfiles directory). I can't test it, but this was the
best I could do.
(*) DOS DJGPP+"Opus Make" Makefile from John S. Fine.
(*) changes.asm changes from John S. Fine.
C.2.50 Version 0.98p3.3
(*) Patch from Conan Brink to allow nesting of `%rep' directives.
(*) If we're going to allow INT01 as an alias for INT1/ICEBP (one of
Jules 0.98p3 changes), then we should allow INT03 as an alias
for INT3 as well.
(*) Updated changes.asm to include the latest changes.
(*) Tried to clean up the <CR>s that had snuck in from a DOS/Windows
environment into my Unix environment, and try to make sure than
DOS/Windows users get them back.
(*) We would silently generate broken tools if insns.dat wasn't
sorted properly. Change insns.pl so that the order doesn't
matter.
(*) Fix bug in insns.pl (introduced by me) which would cause
conditional instructions to have an extra "cc" in disassembly,
e.g. "jnz" disassembled as "jccnz".
C.2.51 Version 0.98p3.2
(*) Merged in John S. Fine's changes from his 0.98-J4 prerelease;
see http://www.csoft.net/cz/johnfine/
(*) Changed previous "spotless" Makefile target (appropriate for
distribution) to "distclean", and added "cleaner" target which
is same as "clean" except deletes files generated by Perl
scripts; "spotless" is union.
(*) Removed BASIC programs from distribution. Get a Perl interpreter
instead (see below.)
(*) Calling this "pre-release 3.2" rather than "p3-hpa2" because of
John's contributions.
(*) Actually link in the IEEE output format (zoutieee.c); fix a
bunch of compiler warnings in that file. Note I don't know what
IEEE output is supposed to look like, so these changes were made
"blind".
C.2.52 Version 0.98p3-hpa
(*) Merged nasm098p3.zip with nasm-0.97.tar.gz to create a fully
buildable version for Unix systems (Makefile.in updates, etc.)
(*) Changed insns.pl to create the instruction tables in nasm.h and
names.c, so that a new instruction can be added by adding it
*only* to insns.dat.
(*) Added the following new instructions: SYSENTER, SYSEXIT, FXSAVE,
FXRSTOR, UD1, UD2 (the latter two are two opcodes that Intel
guarantee will never be used; one of them is documented as UD2
in Intel documentation, the other one just as "Undefined Opcode"
-- calling it UD1 seemed to make sense.)
(*) MAX_SYMBOL was defined to be 9, but LOADALL286 and LOADALL386
are 10 characters long. Now MAX_SYMBOL is derived from
insns.dat.
(*) A note on the BASIC programs included: forget them. insns.bas is
already out of date. Get yourself a Perl interpreter for your
platform of choice at http://www.cpan.org/ports/index.html.
C.2.53 Version 0.98 pre-release 3
(*) added response file support, improved command line handling, new
layout help screen
(*) fixed limit checking bug, 'OUT byte nn, reg' bug, and a couple
of rdoff related bugs, updated Wishlist; 0.98 Prerelease 3.
C.2.54 Version 0.98 pre-release 2
(*) fixed bug in outcoff.c to do with truncating section names
longer than 8 characters, referencing beyond end of string; 0.98
pre-release 2
C.2.55 Version 0.98 pre-release 1
(*) Fixed a bug whereby STRUC didn't work at all in RDF.
(*) Fixed a problem with group specification in PUBDEFs in OBJ.
(*) Improved ease of adding new output formats. Contribution due to
Fox Cutter.
(*) Fixed a bug in relocations in the `bin' format: was showing up
when a relocatable reference crossed an 8192-byte boundary in
any output section.
(*) Fixed a bug in local labels: local-label lookups were
inconsistent between passes one and two if an EQU occurred
between the definition of a global label and the subsequent use
of a local label local to that global.
(*) Fixed a seg-fault in the preprocessor (again) which happened
when you use a blank line as the first line of a multi-line
macro definition and then defined a label on the same line as a
call to that macro.
(*) Fixed a stale-pointer bug in the handling of the NASM
environment variable. Thanks to Thomas McWilliams.
(*) ELF had a hard limit on the number of sections which caused
segfaults when transgressed. Fixed.
(*) Added ability for ndisasm to read from stdin by using `-' as the
filename.
(*) ndisasm wasn't outputting the TO keyword. Fixed.
(*) Fixed error cascade on bogus expression in `%if' - an error in
evaluation was causing the entire `%if' to be discarded, thus
creating trouble later when the `%else' or `%endif' was
encountered.
(*) Forward reference tracking was instruction-granular not operand-
granular, which was causing 286-specific code to be generated
needlessly on code of the form `shr word [forwardref],1'. Thanks
to Jim Hague for sending a patch.
(*) All messages now appear on stdout, as sending them to stderr
serves no useful purpose other than to make redirection
difficult.
(*) Fixed the problem with EQUs pointing to an external symbol -
this now generates an error message.
(*) Allowed multiple size prefixes to an operand, of which only the
first is taken into account.
(*) Incorporated John Fine's changes, including fixes of a large
number of preprocessor bugs, some small problems in OBJ, and a
reworking of label handling to define labels before their line
is assembled, rather than after.
(*) Reformatted a lot of the source code to be more readable.
Included 'coding.txt' as a guideline for how to format code for
contributors.
(*) Stopped nested `%reps' causing a panic - they now cause a
slightly more friendly error message instead.
(*) Fixed floating point constant problems (patch by Pedro Gimeno)
(*) Fixed the return value of insn_size() not being checked for -1,
indicating an error.
(*) Incorporated 3Dnow! instructions.
(*) Fixed the 'mov eax, eax + ebx' bug.
(*) Fixed the GLOBAL EQU bug in ELF. Released developers release 3.
(*) Incorporated John Fine's command line parsing changes
(*) Incorporated David Lindauer's OMF debug support
(*) Made changes for LCC 4.0 support (`__NASM_CDecl__', removed
register size specification warning when sizes agree).
C.3 NASM 0.9 Series
Revisions before 0.98.
C.3.1 Version 0.97 released December 1997
(*) This was entirely a bug-fix release to 0.96, which seems to have
got cursed. Silly me.
(*) Fixed stupid mistake in OBJ which caused `MOV EAX,<constant>' to
fail. Caused by an error in the `MOV EAX,<segment>' support.
(*) ndisasm hung at EOF when compiled with lcc on Linux because lcc
on Linux somehow breaks feof(). ndisasm now does not rely on
feof().
(*) A heading in the documentation was missing due to a markup error
in the indexing. Fixed.
(*) Fixed failure to update all pointers on realloc() within
extended- operand code in parser.c. Was causing wrong behaviour
and seg faults on lines such as `dd 0.0,0.0,0.0,0.0,...'
(*) Fixed a subtle preprocessor bug whereby invoking one multi-line
macro on the first line of the expansion of another, when the
second had been invoked with a label defined before it, didn't
expand the inner macro.
(*) Added internal.doc back in to the distribution archives - it was
missing in 0.96 *blush*
(*) Fixed bug causing 0.96 to be unable to assemble its own test
files, specifically objtest.asm. *blush again*
(*) Fixed seg-faults and bogus error messages caused by mismatching
`%rep' and `%endrep' within multi-line macro definitions.
(*) Fixed a problem with buffer overrun in OBJ, which was causing
corruption at ends of long PUBDEF records.
(*) Separated DOS archives into main-program and documentation to
reduce download size.
C.3.2 Version 0.96 released November 1997
(*) Fixed a bug whereby, if `nasm sourcefile' would cause a filename
collision warning and put output into `nasm.out', then `nasm
sourcefile -o outputfile' still gave the warning even though the
`-o' was honoured. Fixed name pollution under Digital UNIX: one
of its header files defined R_SP, which broke the enum in
nasm.h.
(*) Fixed minor instruction table problems: FUCOM and FUCOMP didn't
have two-operand forms; NDISASM didn't recognise the longer
register forms of PUSH and POP (eg FF F3 for PUSH BX); TEST
mem,imm32 was flagged as undocumented; the 32-bit forms of CMOV
had 16-bit operand size prefixes; `AAD imm' and `AAM imm' are no
longer flagged as undocumented because the Intel Architecture
reference documents them.
(*) Fixed a problem with the local-label mechanism, whereby strange
types of symbol (EQUs, auto-defined OBJ segment base symbols)
interfered with the `previous global label' value and screwed up
local labels.
(*) Fixed a bug whereby the stub preprocessor didn't communicate
with the listing file generator, so that the -a and -l options
in conjunction would produce a useless listing file.
(*) Merged `os2' object file format back into `obj', after
discovering that `obj' _also_ shouldn't have a link pass
separator in a module containing a non-trivial MODEND. Flat
segments are now declared using the FLAT attribute. `os2' is no
longer a valid object format name: use `obj'.
(*) Removed the fixed-size temporary storage in the evaluator. Very
very long expressions (like `mov ax,1+1+1+1+...' for two hundred
1s or so) should now no longer crash NASM.
(*) Fixed a bug involving segfaults on disassembly of MMX
instructions, by changing the meaning of one of the operand-type
flags in nasm.h. This may cause other apparently unrelated MMX
problems; it needs to be tested thoroughly.
(*) Fixed some buffer overrun problems with large OBJ output files.
Thanks to DJ Delorie for the bug report and fix.
(*) Made preprocess-only mode actually listen to the `%line' markers
as it prints them, so that it can report errors more sanely.
(*) Re-designed the evaluator to keep more sensible track of
expressions involving forward references: can now cope with
previously-nightmare situations such as:
mov ax,foo | bar
foo equ 1
bar equ 2
(*) Added the ALIGN and ALIGNB standard macros.
(*) Added PIC support in ELF: use of WRT to obtain the four extra
relocation types needed.
(*) Added the ability for output file formats to define their own
extensions to the GLOBAL, COMMON and EXTERN directives.
(*) Implemented common-variable alignment, and global-symbol type
and size declarations, in ELF.
(*) Implemented NEAR and FAR keywords for common variables, plus
far-common element size specification, in OBJ.
(*) Added a feature whereby EXTERNs and COMMONs in OBJ can be given
a default WRT specification (either a segment or a group).
(*) Transformed the Unix NASM archive into an auto-configuring
package.
(*) Added a sanity-check for people applying SEG to things which are
already segment bases: this previously went unnoticed by the SEG
processing and caused OBJ-driver panics later.
(*) Added the ability, in OBJ format, to deal with `MOV
EAX,<segment>' type references: OBJ doesn't directly support
dword-size segment base fixups, but as long as the low two bytes
of the constant term are zero, a word-size fixup can be
generated instead and it will work.
(*) Added the ability to specify sections' alignment requirements in
Win32 object files and pure binary files.
(*) Added preprocess-time expression evaluation: the `%assign' (and
`%iassign') directive and the bare `%if' (and `%elif')
conditional. Added relational operators to the evaluator, for
use only in `%if' constructs: the standard relationals = < > <=
>= <> (and C-like synonyms == and !=) plus low-precedence
logical operators &&, ^^ and ||.
(*) Added the __FILE__ and __LINE__ standard macros.
(*) Added a sanity check for number constants being greater than
0xFFFFFFFF. The warning can be disabled.
(*) Added the %0 token whereby a variadic multi-line macro can tell
how many parameters it's been given in a specific invocation.
(*) Added `%rotate', allowing multi-line macro parameters to be
cycled.
(*) Added the `*' option for the maximum parameter count on multi-
line macros, allowing them to take arbitrarily many parameters.
(*) Added the ability for the user-level forms of EXTERN, GLOBAL and
COMMON to take more than one argument.
(*) Added the IMPORT and EXPORT directives in OBJ format, to deal
with Windows DLLs.
(*) Added some more preprocessor `%if' constructs: `%ifidn' /
`%ifidni' (exact textual identity), and `%ifid' / `%ifnum' /
`%ifstr' (token type testing).
(*) Added the ability to distinguish SHL AX,1 (the 8086 version)
from SHL AX,BYTE 1 (the 286-and-upwards version whose constant
happens to be 1).
(*) Added NetBSD/FreeBSD/OpenBSD's variant of a.out format, complete
with PIC shared library features.
(*) Changed NASM's idiosyncratic handling of FCLEX, FDISI, FENI,
FINIT, FSAVE, FSTCW, FSTENV, and FSTSW to bring it into line
with the otherwise accepted standard. The previous behaviour,
though it was a deliberate feature, was a deliberate feature
based on a misunderstanding. Apologies for the inconvenience.
(*) Improved the flexibility of ABSOLUTE: you can now give it an
expression rather than being restricted to a constant, and it
can take relocatable arguments as well.
(*) Added the ability for a variable to be declared as EXTERN
multiple times, and the subsequent definitions are just ignored.
(*) We now allow instruction prefixes (CS, DS, LOCK, REPZ etc) to be
alone on a line (without a following instruction).
(*) Improved sanity checks on whether the arguments to EXTERN,
GLOBAL and COMMON are valid identifiers.
(*) Added misc/exebin.mac to allow direct generation of .EXE files
by hacking up an EXE header using DB and DW; also added
test/binexe.asm to demonstrate the use of this. Thanks to Yann
Guidon for contributing the EXE header code.
(*) ndisasm forgot to check whether the input file had been
successfully opened. Now it does. Doh!
(*) Added the Cyrix extensions to the MMX instruction set.
(*) Added a hinting mechanism to allow [EAX+EBX] and [EBX+EAX] to be
assembled differently. This is important since [ESI+EBP] and
[EBP+ESI] have different default base segment registers.
(*) Added support for the PharLap OMF extension for 4096-byte
segment alignment.
C.3.3 Version 0.95 released July 1997
(*) Fixed yet another ELF bug. This one manifested if the user
relied on the default segment, and attempted to define global
symbols without first explicitly declaring the target segment.
(*) Added makefiles (for NASM and the RDF tools) to build Win32
console apps under Symantec C++. Donated by Mark Junker.
(*) Added `macros.bas' and `insns.bas', QBasic versions of the Perl
scripts that convert `standard.mac' to `macros.c' and convert
`insns.dat' to `insnsa.c' and `insnsd.c'. Also thanks to Mark
Junker.
(*) Changed the diassembled forms of the conditional instructions so
that JB is now emitted as JC, and other similar changes.
Suggested list by Ulrich Doewich.
(*) Added `@' to the list of valid characters to begin an identifier
with.
(*) Documentary changes, notably the addition of the `Common
Problems' section in nasm.doc.
(*) Fixed a bug relating to 32-bit PC-relative fixups in OBJ.
(*) Fixed a bug in perm_copy() in labels.c which was causing
exceptions in cleanup_labels() on some systems.
(*) Positivity sanity check in TIMES argument changed from a warning
to an error following a further complaint.
(*) Changed the acceptable limits on byte and word operands to allow
things like `~10111001b' to work.
(*) Fixed a major problem in the preprocessor which caused seg-
faults if macro definitions contained blank lines or comment-
only lines.
(*) Fixed inadequate error checking on the commas separating the
arguments to `db', `dw' etc.
(*) Fixed a crippling bug in the handling of macros with operand
counts defined with a `+' modifier.
(*) Fixed a bug whereby object file formats which stored the input
file name in the output file (such as OBJ and COFF) weren't
doing so correctly when the output file name was specified on
the command line.
(*) Removed [INC] and [INCLUDE] support for good, since they were
obsolete anyway.
(*) Fixed a bug in OBJ which caused all fixups to be output in 16-
bit (old-format) FIXUPP records, rather than putting the 32-bit
ones in FIXUPP32 (new-format) records.
(*) Added, tentatively, OS/2 object file support (as a minor variant
on OBJ).
(*) Updates to Fox Cutter's Borland C makefile, Makefile.bc2.
(*) Removed a spurious second fclose() on the output file.
(*) Added the `-s' command line option to redirect all messages
which would go to stderr (errors, help text) to stdout instead.
(*) Added the `-w' command line option to selectively suppress some
classes of assembly warning messages.
(*) Added the `-p' pre-include and `-d' pre-define command-line
options.
(*) Added an include file search path: the `-i' command line option.
(*) Fixed a silly little preprocessor bug whereby starting a line
with a `%!' environment-variable reference caused an `unknown
directive' error.
(*) Added the long-awaited listing file support: the `-l' command
line option.
(*) Fixed a problem with OBJ format whereby, in the absence of any
explicit segment definition, non-global symbols declared in the
implicit default segment generated spurious EXTDEF records in
the output.
(*) Added the NASM environment variable.
(*) From this version forward, Win32 console-mode binaries will be
included in the DOS distribution in addition to the 16-bit
binaries. Added Makefile.vc for this purpose.
(*) Added `return 0;' to test/objlink.c to prevent compiler
warnings.
(*) Added the __NASM_MAJOR__ and __NASM_MINOR__ standard defines.
(*) Added an alternative memory-reference syntax in which prefixing
an operand with `&' is equivalent to enclosing it in square
brackets, at the request of Fox Cutter.
(*) Errors in pass two now cause the program to return a non-zero
error code, which they didn't before.
(*) Fixed the single-line macro cycle detection, which didn't work
at all on macros with no parameters (caused an infinite loop).
Also changed the behaviour of single-line macro cycle detection
to work like cpp, so that macros like `extrn' as given in the
documentation can be implemented.
(*) Fixed the implementation of WRT, which was too restrictive in
that you couldn't do `mov ax,[di+abc wrt dgroup]' because
(di+abc) wasn't a relocatable reference.
C.3.4 Version 0.94 released April 1997
(*) Major item: added the macro processor.
(*) Added undocumented instructions SMI, IBTS, XBTS and LOADALL286.
Also reorganised CMPXCHG instruction into early-486 and Pentium
forms. Thanks to Thobias Jones for the information.
(*) Fixed two more stupid bugs in ELF, which were causing `ld' to
continue to seg-fault in a lot of non-trivial cases.
(*) Fixed a seg-fault in the label manager.
(*) Stopped FBLD and FBSTP from _requiring_ the TWORD keyword, which
is the only option for BCD loads/stores in any case.
(*) Ensured FLDCW, FSTCW and FSTSW can cope with the WORD keyword,
if anyone bothers to provide it. Previously they complained
unless no keyword at all was present.
(*) Some forms of FDIV/FDIVR and FSUB/FSUBR were still inverted: a
vestige of a bug that I thought had been fixed in 0.92. This was
fixed, hopefully for good this time...
(*) Another minor phase error (insofar as a phase error can _ever_
be minor) fixed, this one occurring in code of the form
rol ax,forward_reference
forward_reference equ 1
(*) The number supplied to TIMES is now sanity-checked for
positivity, and also may be greater than 64K (which previously
didn't work on 16-bit systems).
(*) Added Watcom C makefiles, and misc/pmw.bat, donated by Dominik
Behr.
(*) Added the INCBIN pseudo-opcode.
(*) Due to the advent of the preprocessor, the [INCLUDE] and [INC]
directives have become obsolete. They are still supported in
this version, with a warning, but won't be in the next.
(*) Fixed a bug in OBJ format, which caused incorrect object records
to be output when absolute labels were made global.
(*) Updates to RDOFF subdirectory, and changes to outrdf.c.
C.3.5 Version 0.93 released January 1997
This release went out in a great hurry after semi-crippling bugs
were found in 0.92.
(*) Really _did_ fix the stack overflows this time. *blush*
(*) Had problems with EA instruction sizes changing between passes,
when an offset contained a forward reference and so 4 bytes were
allocated for the offset in pass one; by pass two the symbol had
been defined and happened to be a small absolute value, so only
1 byte got allocated, causing instruction size mismatch between
passes and hence incorrect address calculations. Fixed.
(*) Stupid bug in the revised ELF section generation fixed
(associated string-table section for .symtab was hard-coded as
7, even when this didn't fit with the real section table). Was
causing `ld' to seg-fault under Linux.
(*) Included a new Borland C makefile, Makefile.bc2, donated by Fox
Cutter <lmb@comtch.iea.com>.
C.3.6 Version 0.92 released January 1997
(*) The FDIVP/FDIVRP and FSUBP/FSUBRP pairs had been inverted: this
was fixed. This also affected the LCC driver.
(*) Fixed a bug regarding 32-bit effective addresses of the form
`[other_register+ESP]'.
(*) Documentary changes, notably documentation of the fact that
Borland Win32 compilers use `obj' rather than `win32' object
format.
(*) Fixed the COMENT record in OBJ files, which was formatted
incorrectly.
(*) Fixed a bug causing segfaults in large RDF files.
(*) OBJ format now strips initial periods from segment and group
definitions, in order to avoid complications with the local
label syntax.
(*) Fixed a bug in disassembling far calls and jumps in NDISASM.
(*) Added support for user-defined sections in COFF and ELF files.
(*) Compiled the DOS binaries with a sensible amount of stack, to
prevent stack overflows on any arithmetic expression containing
parentheses.
(*) Fixed a bug in handling of files that do not terminate in a
newline.
C.3.7 Version 0.91 released November 1996
(*) Loads of bug fixes.
(*) Support for RDF added.
(*) Support for DBG debugging format added.
(*) Support for 32-bit extensions to Microsoft OBJ format added.
(*) Revised for Borland C: some variable names changed, makefile
added.
(*) LCC support revised to actually work.
(*) JMP/CALL NEAR/FAR notation added.
(*) `a16', `o16', `a32' and `o32' prefixes added.
(*) Range checking on short jumps implemented.
(*) MMX instruction support added.
(*) Negative floating point constant support added.
(*) Memory handling improved to bypass 64K barrier under DOS.
(*) `$' prefix to force treatment of reserved words as identifiers
added.
(*) Default-size mechanism for object formats added.
(*) Compile-time configurability added.
(*) `#', `@', `~' and c{?} are now valid characters in labels.
(*) `-e' and `-k' options in NDISASM added.
C.3.8 Version 0.90 released October 1996
First release version. First support for object file output. Other
changes from previous version (0.3x) too numerous to document.
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