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| HowTo Linux | Linux Zone Home | E-Mail Me | Assembly HOWTO François-René Rideau fare@tunes.org
v0.4q, 22 June 1999
This is the Linux Assembly HOWTO. This document describes how to pro­
gram in assembly using FREE programming tools, focusing on development
for or from the Linux Operating System on i386 platforms. Included
material may or may not be applicable to other hardware and/or soft­
ware platforms. Contributions about these would be gladly accepted.
keywords: assembly, assembler, free, macroprocessor, preprocessor,
asm, inline asm, 32-bit, x86, i386, gas, as86, nasm
______________________________________________________________________
Table of Contents
1. INTRODUCTION
1.1 Legal Blurp
1.2 Important Note
1.3 Foreword
1.3.1 How to use this document
1.3.2 Other related documents
1.4 History
1.5 Credits
2. DO YOU NEED ASSEMBLY?
2.1 Pros and Cons
2.1.1 The advantages of Assembly
2.1.2 The disadvantages of Assembly
2.1.3 Assessment
2.2 How to NOT use Assembly
2.2.1 General procedure to achieve efficient code
2.2.2 Languages with optimizing compilers
2.2.3 General procedure to speed your code up
2.2.4 Inspecting compiler-generated code
3. ASSEMBLERS
3.1 GCC Inline Assembly
3.1.1 Where to find GCC
3.1.2 Where to find docs for GCC Inline Asm
3.1.3 Invoking GCC to have it properly inline assembly code ?
3.2 GAS
3.2.1 Where to find it
3.2.2 What is this AT&T syntax
3.2.3 Limited 16-bit mode
3.3 GASP
3.3.1 Where to find GASP
3.3.2 How it works
3.4 NASM
3.4.1 Where to find NASM
3.4.2 What it does
3.5 AS86
3.5.1 Where to get AS86
3.5.2 How to invoke the assembler?
3.5.3 Where to find docs
3.5.4 What if I can't compile Linux anymore with this new version ?
3.6 OTHER ASSEMBLERS
3.6.1 Win32Forth assembler
3.6.2 Terse
3.6.3 Non-free and/or Non-32bit x86 assemblers.
4. METAPROGRAMMING/MACROPROCESSING
4.1 What's integrated into the above
4.1.1 GCC
4.1.2 GAS
4.1.3 GASP
4.1.4 NASM
4.1.5 AS86
4.1.6 OTHER ASSEMBLERS
4.2 External Filters
4.2.1 CPP
4.2.2 M4
4.2.3 Macroprocessing with yer own filter
4.2.4 Metaprogramming
4.2.4.1 Backends from compilers
4.2.4.2 The New-Jersey Machine-Code Toolkit
4.2.4.3 TUNES
5. CALLING CONVENTIONS
5.1 Linux
5.1.1 Linking to GCC
5.1.2 ELF vs a.out problems
5.1.3 Direct Linux syscalls
5.1.4 Hardware I/O under Linux
5.1.5 Accessing 16-bit drivers from Linux/i386
5.2 DOS
5.3 Winblows and suches
5.4 Yer very own OS
6. TODO & POINTERS
______________________________________________________________________
1. INTRODUCTION
1.1. Legal Blurp
Copyright © 1996-1999 by François-René Rideau.
This document is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.
1.2. Important Note
This is an interactively evolving document: you are especially invited
to ask questions, to answer to questions, to correct given answers, to
add new FAQ answers, to give pointers to other software, to point the
current maintainer to bugs or deficiencies in the pages. If you're
motivated, you could even take over the maintenance of the HOWTO. In
one word, contribute!
To contribute, please contact whoever appears to maintain the
Assembly-HOWTO. At the time of this writing, it's me, i.e. François-
René Rideau <mailto:fare@tunes.org>.
However, it's been some time since I've been looking for a serious
hacker to replace me as maintainer of this document. Disadvantages are
you must spend some time updating and correcting the document, and
learning the LDP publication tools. Advantages are you get some fame
and you can receive complimentary copies of HOWTO compendiums.
1.3. Foreword
This document aims at answering frequently asked questions of people
who program or want to program 32-bit x86 assembly using free
software, particularly under the Linux operating system. It may also
point to other documents about non-free, non-x86, or non-32-bit
assemblers, though such is not its primary goal.
Because the main interest of assembly programming is to build to write
the guts of operating systems, interpreters, compilers, and games,
where a C compiler fails to provide the needed expressiveness
(performance is more and more seldom an issue), we stress on
development of such software.
1.3.1. How to use this document
This document contains answers to some frequently asked questions. At
many places, Universal Resource Locators (URL) are given for some
software or documentation repository. Please see that the most useful
repositories are mirrored, and that by accessing a nearer mirror site,
you relieve the whole Internet from unneeded network traffic, while
saving your own precious time. Particularly, there are large
repositories all over the world, that mirror other popular
repositories. You should learn and note what are those places near
you (networkwise). Sometimes, the list of mirrors is listed in a
file, or in a login message. Please heed the advice. Else, you should
ask archie about the software you're looking for...
The most recent version for this documents sits in
<http://www.tunes.org/~fare/files/asm/Assembly-HOWTO.en.sgml> but
what's in Linux HOWTO repositories should be fairly up to date, too (I
can't know): <http://metalab.unc.edu/LDP/HOWTO/>. A french
translation of this HOWTO can be found around
<ftp://ftp.lip6.fr/pub/linux/french/HOWTO/>.
1.3.2. Other related documents
· If you don't know what free software is, please do read carefully
the GNU General Public License, which is used in a lot of free
software, and is a model for most of their licenses. It generally
comes in a file named COPYING, with a library version in a file
named COPYING.LIB. Literature from the FSF <http://www.fsf.org>
(free software foundation) might help you, too.
· Particularly, the interesting kind of free software comes with
sources that you can consult and correct, or sometimes even borrow
from. Read your particular license carefully, and do comply to it.
· There is a FAQ for comp.lang.asm.x86 that answers generic questions
about x86 assembly programming, and questions about some commercial
assemblers in a 16-bit DOS environment. Some of it apply to free
32-bit asm programming, so you may want to read this FAQ...
<http://www2.dgsys.com/~raymoon/faq/asmfaq.zip>
· FAQs and docs exist about programming on your favorite platform,
whichever it is, that you should consult for platform-specific
issues not directly related to programming in assembler.
1.4. History
Each version includes a few fixes and minor corrections, which needs
not be repeatedly mentionned every time.
Version 0.1 23 Apr 1996
Francois-Rene "Faré" Rideau <fare@tunes.org> creates and
publishes the first mini-HOWTO, because ``I'm sick of answering
ever the same questions on comp.lang.asm.x86''
Version 0.2 4 May 1996
*
Version 0.3c 15 Jun 1996
*
Version 0.3f 17 Oct 1996
*
Version 0.3g 2 Nov 1996
Created the History. Added pointers in cross-compiling section.
Added section about I/O programming under Linux (particularly
video).
Version 0.3h 6 Nov 1996
more about cross-compiling -- See on sunsite: devel/msdos/
Version 0.3i 16 Nov 1996
NASM is getting pretty slick
Version 0.3j 24 Nov 1996
point to french translated version
Version 0.3k 19 Dec 1996
What? I had forgotten to point to terse???
Version 0.3l 11 Jan 1997
*
Version 0.4pre1 13 Jan 1997
text mini-HOWTO transformed into a full linuxdoc-sgml HOWTO, to
see what the SGML tools are like.
Version 0.4 20 Jan 1997
first release of the HOWTO as such.
Version 0.4a 20 Jan 1997
CREDITS section added
Version 0.4b 3 Feb 1997
NASM moved: now is before AS86
Version 0.4c 9 Feb 1997
Added section "DO YOU NEED ASSEMBLY?"
Version 0.4d 28 Feb 1997
Vapor announce of a new Assembly-HOWTO maintainer.
Version 0.4e 13 Mar 1997
Release for DrLinux
Version 0.4f 20 Mar 1997
*
Version 0.4g 30 Mar 1997
*
Version 0.4h 19 Jun 1997
still more on "how not to use assembly"; updates on NASM, GAS.
Version 0.4i 17 July 1997
info on 16-bit mode access from Linux.
Version 0.4j 7 September 1997
*
Version 0.4k 19 October 1997
*
Version 0.4l 16 November 1997
release for LSL 6th edition.
Version 0.4m 23 March 1998
corrections about gcc invocation
Version 0.4o 1 December 1998
*
Version 0.4p 6 June 1999
clean up and updates.
Version 0.4q 22 June 1999
process argument passing (argc,argv,environ) in assembly.
This is yet another ``last release by Faré before new maintainer
takes over''. Only nobody knows who the new maintainer might
be.
1.5. Credits
I would like to thanks the following persons, by order of appearance:
· Linus Torvalds <mailto:buried.alive@in.mail> for Linux
· Bruce Evans <mailto:bde@zeta.org.au> for bcc from which as86 is
extracted
· Simon Tatham <mailto:anakin@pobox.com> and Julian Hall
<mailto:jules@earthcorp.com> for NASM
· Greg Hankins <mailto:gregh@metalab.unc.edu> and now Tim Bynum
<mailto:linux-howto@metalab.unc.edu> for maintaining HOWTOs
· Raymond Moon <mailto:raymoon@moonware.dgsys.com> for his FAQ
· Eric Dumas <mailto:dumas@linux.eu.org> for his translation of the
mini-HOWTO into french (sad thing for the original author to be
french and write in english)
· Paul Anderson <mailto:paul@geeky1.ebtech.net> and Rahim Azizarab
<mailto:rahim@megsinet.net> for helping me, if not for taking over
the HOWTO.
· Marc Lehman <mailto:pcg@goof.com> for his insight on GCC
invocation.
· Abhijit Menon-Sen <mailto:ams@wiw.org> for helping me figure out
the process argument passing convention
· All the people who have contributed ideas, remarks, and moral
support.
2. DO YOU NEED ASSEMBLY?
Well, I wouldn't want to interfere with what you're doing, but here
are a few advice from hard-earned experience.
2.1. Pros and Cons
2.1.1. The advantages of Assembly
Assembly can express very low-level things:
· you can access machine-dependent registers and I/O.
· you can control the exact behavior of code in critical sections
that might otherwise involve deadlock between multiple software
threads or hardware devices.
· you can break the conventions of your usual compiler, which might
allow some optimizations (like temporarily breaking rules about
memory allocation, threading, calling conventions, etc).
· you can build interfaces between code fragments using incompatible
such conventions (e.g. produced by different compilers, or
separated by a low-level interface).
· you can get access to unusual programming modes of your processor
(e.g. 16 bit mode to interface startup, firmware, or legacy code on
Intel PCs)
· you can produce reasonably fast code for tight loops to cope with a
bad non-optimizing compiler (but then, there are free optimizing
compilers available!)
· you can produce code where (but only on CPUs with known instruction
timings, which generally excludes all current ....
· you can produce hand-optimized code that's perfectly tuned for your
particular hardware setup, though not to anyone else's.
· you can write some code for your new language's optimizing compiler
(that's something few will ever do, and even they, not often).
2.1.2. The disadvantages of Assembly
Assembly is a very low-level language (the lowest above hand-coding
the binary instruction patterns). This means
· it's long and tedious to write initially,
· it's very bug-prone,
· your bugs will be very difficult to chase,
· it's very difficult to understand and modify, i.e. to maintain.
· the result is very non-portable to other architectures, existing or
future,
· your code will be optimized only for a certain implementation of a
same architecture: for instance, among Intel-compatible platforms,
each CPU design and its variations (relative latency, throughput,
and capacity, of processing units, caches, RAM, bus, disks,
presence of FPU, MMX extensions, etc) implies potentially
completely different optimization techniques. CPU designs already
include Intel 386, 486, Pentium, PPro, Pentium II; Cyrix 5x86,
6x86; AMD K5, K6. New designs keep popping up, so don't expect
either this listing or your code to be up-to-date.
· your code might also be unportable accross different OS platforms
on the same architecture, by lack of proper tools. (well, GAS
seems to work on all platforms; NASM seems to work or be workable
on all intel platforms).
· you spend more time on a few details, and can't focus on small and
large algorithmic design, that are known to bring the largest part
of the speed up. [e.g. you might spend some time building very
fast list/array manipulation primitives in assembly; only a hash
table would have sped up your program much more; or, in another
context, a binary tree; or some high-level structure distributed
over a cluster of CPUs]
· a small change in algorithmic design might completely invalidate
all your existing assembly code. So that either you're ready (and
able) to rewrite it all, or you're tied to a particular algorithmic
design;
· On code that ain't too far from what's in standard benchmarks,
commercial optimizing compilers outperform hand-coded assembly
(well, that's less true on the x86 architecture than on RISC
architectures, and perhaps less true for widely available/free
compilers; anyway, for typical C code, GCC is fairly good);
· And in any case, as says moderator John Levine on comp.compilers,
``compilers make it a lot easier to use complex data structures,
and compilers don't get bored halfway through and generate reliably
pretty good code.'' They will also correctly propagate code
transformations throughout the whole (huge) program when optimizing
code between procedures and module boundaries.
2.1.3. Assessment
All in all, you might find that though using assembly is sometimes
needed, and might even be useful in a few cases where it is not,
you'll want to:
· minimize the use of assembly code,
· encapsulate this code in well-defined interfaces
· have your assembly code automatically generated from patterns
expressed in a higher-level language than assembly (e.g. GCC inline
assembly macros).
· have automatic tools translate these programs into assembly code
· have this code be optimized if possible
· All of the above, i.e. write (an extension to) an optimizing
compiler back-end.
Even in cases when Assembly is needed (e.g. OS development), you'll
find that not so much of it is, and that the above principles hold.
See the sources for the Linux kernel about it: as little assembly as
needed, resulting in a fast, reliable, portable, maintainable OS.
Even a successful game like DOOM was almost massively written in C,
with a tiny part only being written in assembly for speed up.
2.2. How to NOT use Assembly
2.2.1. General procedure to achieve efficient code
As says Charles Fiterman on comp.compilers about human vs computer-
generated assembly code,
``The human should always win and here is why.
· First the human writes the whole thing in a high level language.
· Second he profiles it to find the hot spots where it spends its
time.
· Third he has the compiler produce assembly for those small sections
of code.
· Fourth he hand tunes them looking for tiny improvements over the
machine generated code.
The human wins because he can use the machine.''
2.2.2. Languages with optimizing compilers
Languages like ObjectiveCAML, SML, CommonLISP, Scheme, ADA, Pascal, C,
C++, among others, all have free optimizing compilers that'll optimize
the bulk of your programs, and often do better than hand-coded
assembly even for tight loops, while allowing you to focus on higher-
level details, and without forbidding you to grab a few percent of
extra performance in the above-mentionned way, once you've reached a
stable design. Of course, there are also commercial optimizing
compilers for most of these languages, too!
Some languages have compilers that produce C code, which can be
further optimized by a C compiler. LISP, Scheme, Perl, and many other
are suches. Speed is fairly good.
2.2.3. General procedure to speed your code up
As for speeding code up, you should do it only for parts of a program
that a profiling tool has consistently identified as being a
performance bottleneck.
Hence, if you identify some code portion as being too slow, you should
· first try to use a better algorithm;
· then try to compile it rather than interpret it;
· then try to enable and tweak optimization from your compiler;
· then give the compiler hints about how to optimize (typing
information in LISP; register usage with GCC; lots of options in
most compilers, etc).
· then possibly fallback to assembly programming
Finally, before you end up writing assembly, you should inspect
generated code, to check that the problem really is with bad code
generation, as this might really not be the case: compiler-generated
code might be better than what you'd have written, particularly on
modern multi-pipelined architectures! Slow parts of a program might
be intrinsically so. Biggest problems on modern architectures with
fast processors are due to delays from memory access, cache-misses,
TLB-misses, and page-faults; register optimization becomes useless,
and you'll more profitably re-think data structures and threading to
achieve better locality in memory access. Perhaps a completely
different approach to the problem might help, then.
2.2.4. Inspecting compiler-generated code
There are many reasons to inspect compiler-generated assembly code.
Here are what you'll do with such code:
· check whether generated code can be obviously enhanced with hand-
coded assembly (or by tweaking compiler switches)
· when that's the case, start from generated code and modify it
instead of starting from scratch
· more generally, use generated code as stubs to modify, which at
least gets right the way your assembly routines interface to the
external world
· track down bugs in your compiler (hopefully rarer)
The standard way to have assembly code be generated is to invoke your
compiler with the -S flag. This works with most Unix compilers,
including the GNU C Compiler (GCC), but YMMV. As for GCC, it will
produce more understandable assembly code with the -fverbose-asm
command-line option. Of course, if you want to get good assembly
code, don't forget your usual optimization options and hints!
3. ASSEMBLERS
3.1. GCC Inline Assembly
The well-known GNU C/C++ Compiler (GCC), an optimizing 32-bit compiler
at the heart of the GNU project, supports the x86 architecture quite
well, and includes the ability to insert assembly code in C programs,
in such a way that register allocation can be either specified or left
to GCC. GCC works on most available platforms, notably Linux, *BSD,
VSTa, OS/2, *DOS, Win*, etc.
3.1.1. Where to find GCC
The original GCC site is the GNU FTP site
<ftp://prep.ai.mit.edu/pub/gnu/gcc/> together with all the released
application software from the GNU project. Linux-configured and
precompiled versions can be found in
<ftp://metalab.unc.edu/pub/Linux/GCC/> There exists a lot of FTP
mirrors of both sites. everywhere around the world, as well as CD-ROM
copies.
GCC development has split in two branches recently. See more about
the experimental version, egcs, at <http://www.cygnus.com/egcs/>
Sources adapted to your favorite OS, and binaries precompiled for it,
should be found at your usual FTP sites.
For most popular DOS port of GCC is named DJGPP, and can be found in
directories of such name in FTP sites. See:
<http://www.delorie.com/djgpp/>
There is also a port of GCC to OS/2 named EMX, that also works under
DOS, and includes lots of unix-emulation library routines. See around
the following site: <ftp://ftp-os2.cdrom.com/pub/os2/emx09c/>. Other
URLs listed in previous versions of this HOWTO seem to be as dead as
OS/2.
3.1.2. Where to find docs for GCC Inline Asm
The documentation of GCC includes documentation files in texinfo
format. You can compile them with tex and print then result, or
convert them to .info, and browse them with emacs, or convert them to
.html, or nearly whatever you like. convert (with the right tools) to
whatever you like, or just read as is. The .info files are generally
found on any good installation for GCC.
The right section to look for is: C Extensions::Extended Asm::
Section Invoking GCC::Submodel Options::i386 Options:: might help too.
Particularly, it gives the i386 specific constraint names for
registers: abcdSDB correspond to %eax, %ebx, %ecx, %edx, %esi, %edi,
%ebp respectively (no letter for %esp).
The DJGPP Games resource (not only for game hackers) had this page
specifically about assembly, but it's down. Its data have nonetheless
been recovered on the DJGPP site <http://www.delorie.com/djgpp/>, that
contains a mine of other useful information:
<http://www.delorie.com/djgpp/doc/brennan/>
GCC depends on GAS for assembling, and follow its syntax (see below);
do mind that inline asm needs percent characters to be quoted so they
be passed to GAS. See the section about GAS below.
Find lots of useful examples in the linux/include/asm-i386/
subdirectory of the sources for the Linux kernel.
3.1.3. Invoking GCC to have it properly inline assembly code ?
Because assembly routines from the kernel headers (and most likely
your own headers, if you try making your assembly programming as clean
as it is in the linux kernel) are embedded in extern inline functions,
GCC must be invoked with the -O flag (or -O2, -O3, etc), for these
routines to be available. If not, your code may compile, but not link
properly, since it will be looking for non-inlined extern functions in
the libraries against which your program is being linked !!! Another
way is to link against libraries that include fallback versions of the
routines.
Inline assembly can be disabled with -fno-asm, which will have the
compiler die when using extended inline asm syntax, or else generate
calls to an external function named asm() that the linker can't
resolve. To counter such flag, -fasm restores treatment of the asm
keyword.
More generally, good compile flags for GCC on the x86 platform are
______________________________________________________________________
gcc -O2 -fomit-frame-pointer -W -Wall
______________________________________________________________________
-O2 is the good optimization level in most cases. Optimizing besides
it takes longer, and yields code that is a lot larger, but only a bit
faster; such overoptimization might be useful for tight loops only (if
any), which you may be doing in assembly anyway. In cases when you
need really strong compiler optimization for a few files, do consider
using up to -O6.
-fomit-frame-pointer allows generated code to skip the stupid frame
pointer maintenance, which makes code smaller and faster, and frees a
register for further optimizations. It precludes the easy use of
debugging tools (gdb), but when you use these, you just don't care
about size and speed anymore anyway.
-W -Wall enables all warnings and helps you catch obvious stupid
errors.
You can add some cpu-specific -m486 or such flag so that GCC will
produce code that is more adapted to your precise computer. Note that
EGCS (and perhaps GCC 2.8) have -mpentium and such flags, whereas GCC
2.7.x and older versions do not. A good choice of CPU-specific flags
should be in the Linux kernel. Check the texinfo documentation of
your current GCC installation for more.
-m386 will help optimize for size, hence also for speed on computers
whose memory is tight and/or loaded, since big programs cause swap,
which more than counters any "optimization" intended by the larger
code. In such settings, it might be useful to stop using C, and use
instead a language that favors code factorization, such as a
functional language and/or FORTH, and use a bytecode- or wordcode-
based implementation.
Note that you can vary code generation flags from file to file, so
that performance-critical files use maximal optimization, whereas
other files be optimized for size.
To optimize even more, option -mregparm=2 and/or corresponding
function attribute might help, but might pose lots of problems when
linking to foreign code, including the libc. There are ways to
correctly declare foreign functions so the right call sequences be
generated, or you might want to recompile the foreign libraries to use
the same register-based calling convention...
Note that you can add make these flags the default by editing file
/usr/lib/gcc-lib/i486-linux/2.7.2.3/specs or wherever that is on your
system (better not add -Wall there, though). The exact location of
the GCC specs files on your system can be found by asking gcc -v.
3.2. GAS
GAS is the GNU Assembler, that GCC relies upon.
3.2.1. Where to find it
Find it at the same place where you found GCC, in a package named
binutils.
3.2.2. What is this AT&T syntax
Because GAS was invented to support a 32-bit unix compiler, it uses
standard ``AT&T'' syntax, which resembles a lot the syntax for
standard m68k assemblers, and is standard in the UNIX world. This
syntax is no worse, no better than the ``Intel'' syntax. It's just
different. When you get used to it, you find it much more regular
than the Intel syntax, though a bit boring.
Here are the major caveats about GAS syntax:
· Register names are prefixed with %, so that registers are %eax, %dl
and suches instead of just eax, dl, etc. This makes it possible to
include external C symbols directly in assembly source, without any
risk of confusion, or any need for ugly underscore prefixes.
· The order of operands is source(s) first, and destination last, as
opposed to the intel convention of destination first and sources
last. Hence, what in intel syntax is mov ax,dx (move contents of
register dx into register ax) will be in att syntax mov %dx, %ax.
· The operand length is specified as a suffix to the instruction
name. The suffix is b for (8-bit) byte, w for (16-bit) word, and l
for (32-bit) long. For instance, the correct syntax for the above
instruction would have been movw %dx,%ax. However, gas does not
require strict att syntax, so the suffix is optional when length
can be guessed from register operands, and else defaults to 32-bit
(with a warning).
· Immediate operands are marked with a $ prefix, as in addl $5,%eax
(add immediate long value 5 to register %eax).
· No prefix to an operand indicates it is a memory-address; hence
movl $foo,%eax puts the address of variable foo in register %eax,
but movl foo,%eax puts the contents of variable foo in register
%eax.
· Indexing or indirection is done by enclosing the index register or
indirection memory cell address in parentheses, as in testb
$0x80,17(%ebp) (test the high bit of the byte value at offset 17
from the cell pointed to by %ebp).
A program exists to help you convert programs from TASM syntax to AT&T
syntax. See
<ftp://x2ftp.oulu.fi/pub/msdos/programming/convert/ta2asv08.zip>.
(Since the original x2ftp site is closing, use a mirror site
<ftp://ftp.lip6.fr/pub/pc/x2ftp/README.mirror_sites>). There also
exists a program for the reverse conversion:
<http://www.multimania.com/placr/a2i.html>.
GAS has comprehensive documentation in TeXinfo format, which comes at
least with the source distribution. Browse extracted .info pages with
Emacs or whatever. There used to be a file named gas.doc or as.doc
around the GAS source package, but it was merged into the TeXinfo
docs. Of course, in case of doubt, the ultimate documentation is the
sources themselves! A section that will particularly interest you is
Machine Dependencies::i386-Dependent::
Again, the sources for Linux (the OS kernel), come in as good
examples; see under linux/arch/i386, the following files: kernel/*.S,
boot/compressed/*.S, mathemu/*.S
If you are writing kind of a language, a thread package, etc you might
as well see how other languages (OCaml, gforth, etc), or thread
packages (QuickThreads, MIT pthreads, LinuxThreads, etc), or whatever,
do it.
Finally, just compiling a C program to assembly might show you the
syntax for the kind of instructions you want. See section ``Do you
need Assembly?'' above.
3.2.3. Limited 16-bit mode
GAS is a 32-bit assembler, meant to support a 32-bit compiler. It
currently has only limited support for 16-bit mode, which consists in
prepending the 32-bit prefixes to instructions, so you write 32-bit
code that runs in 16-bit mode on a 32 bit CPU. In both modes, it
supports 16-bit register usage, but what is unsupported is 16-bit
addressing. Use the directive .code16 and .code32 to switch between
modes. Note that an inline assembly statement asm(".code16\n") will
allow GCC to produce 32-bit code that'll run in real mode!
I've been told that most code needed to fully support 16-bit mode
programming was added to GAS by Bryan Ford (please confirm?), but at
least, it doesn't show up in any of the distribution I tried, up to
binutils-2.8.1.x ... more info on this subject would be welcome.
A cheap solution is to define macros (see below) that somehow produce
the binary encoding (with .byte) for just the 16-bit mode instructions
you need (almost nothing if you use code16 as above, and can safely
assume the code will run on a 32-bit capable x86 CPU). To find the
proper encoding, you can get inspiration from the sources of 16-bit
capable assemblers for the encoding.
3.3. GASP
GASP is the GAS Preprocessor. It adds macros and some nice syntax to
GAS.
3.3.1. Where to find GASP
GASP comes together with GAS in the GNU binutils archive.
3.3.2. How it works
It works as a filter, much like cpp and the like. I have no idea on
details, but it comes with its own texinfo documentation, so just
browse them (in .info), print them, grok them. GAS with GASP looks
like a regular macro-assembler to me.
3.4. NASM
The Netwide Assembler project is producing yet another i386 assembler,
written in C, that should be modular enough to eventually support all
known syntaxes and object formats.
3.4.1. Where to find NASM
<http://www.cryogen.com/Nasm>
Binary release on your usual metalab mirror in devel/lang/asm/ Should
also be available as .rpm or .deb in your usual RedHat/Debian
distributions' contrib.
3.4.2. What it does
At the time this HOWTO is written, version 0.98 of NASM is just out.
The syntax is Intel-style. Some macroprocessing support is
integrated.
Supported object file formats are bin, aout, coff, elf, as86, (DOS)
obj, win32, (their own format) rdf.
NASM can be used as a backend for the free LCC compiler (support files
included).
Surely NASM evolves too fast for this HOWTO to be kept up to date.
Unless you're using BCC as a 16-bit compiler (which is out of scope of
this 32-bit HOWTO), you should definitely use NASM instead of say AS86
or MASM, because it is actively supported online, and runs on all
platforms.
Note: NASM also comes with a disassembler, NDISASM.
Its hand-written parser makes it much faster than GAS, though of
course, it doesn't support three bazillion different architectures.
For the x86 target, it should be the assembler of choice...
3.5. AS86
AS86 is a 80x86 assembler, both 16-bit and 32-bit, part of Bruce
Evans' C Compiler (BCC). It has mostly Intel-syntax, though it
differs slightly as for addressing modes.
3.5.1. Where to get AS86
A completely outdated version of AS86 is distributed by HJLu just to
compile the Linux kernel, in a package named bin86 (current version
0.4), available in any Linux GCC repository. But I advise no one to
use it for anything else but compiling Linux. This version supports
only a hacked minix object file format, which is not supported by the
GNU binutils or anything, and it has a few bugs in 32-bit mode, so you
really should better keep it only for compiling Linux.
The most recent versions by Bruce Evans (bde@zeta.org.au) are
published together with the FreeBSD distribution. Well, they were: I
could not find the sources from distribution 2.1 on :( Hence, I put
the sources at my place:
<http://www.tunes.org/~fare/files/asm/bcc-95.3.12.src.tgz>
The Linux/8086 (aka ELKS) project is somehow maintaining bcc (though I
don't think they included the 32-bit patches). See around
<http://www.linux.org.uk/ELKS-Home/> (or
<http://www.elks.ecs.soton.ac.uk>) and
<ftp://linux.mit.edu/pub/linux/ELKS/>. I haven't followed these
developments, and would appreciate a reader contributing on this
topic.
Among other things, these more recent versions, unlike HJLu's,
supports Linux GNU a.out format, so you can link you code to Linux
programs, and/or use the usual tools from the GNU binutils package to
manipulate your data. This version can co-exist without any harm with
the previous one (see according question below).
BCC from 12 march 1995 and earlier version has a misfeature that makes
all segment pushing/popping 16-bit, which is quite annoying when
programming in 32-bit mode. I wrote a patch at a time when the TUNES
Project used as86:
<http://www.tunes.org/~fare/files/asm/as86.bcc.patch.gz>. Bruce Evans
accepted this patch, but since as far as I know he hasn't published a
new release of bcc, the ones to ask about integrating it (if not done
yet) are the ELKS developers.
3.5.2. How to invoke the assembler?
Here's the GNU Makefile entry for using bcc to transform .s asm into
both GNU a.out .o object and .l listing:
______________________________________________________________________
%.o %.l: %.s
bcc -3 -G -c -A-d -A-l -A$*.l -o $*.o $<
______________________________________________________________________
Remove the %.l, -A-l, and -A$*.l, if you don't want any listing. If
you want something else than GNU a.out, you can see the docs of bcc
about the other supported formats, and/or use the objcopy utility from
the GNU binutils package.
3.5.3. Where to find docs
The docs are what is included in the bcc package. I salvaged the man
pages that used to be available from the FreeBSD site at
<http://www.tunes.org/~fare/files/asm/bcc-95.3.12.src.tgz>. Maybe
ELKS developers know better. When in doubt, the sources themselves
are often a good docs: it's not very well commented, but the
programming style is straightforward. You might try to see how as86
is used in ELKS or Tunes 0.0.0.25...
3.5.4. What if I can't compile Linux anymore with this new version ?
Linus is buried alive in mail, and since HJLu (official bin86
maintainer) chose to write hacks around an obsolete version of as86
instead of building clean code around the latest version, I don't
think my patch for compiling Linux with a modern as86 has any chance
to be accepted if resubmitted. Now, this shouldn't matter: just keep
your as86 from the bin86 package in /usr/bin, and let bcc install the
good as86 as /usr/local/libexec/i386/bcc/as where it should be. You
never need explicitly call this ``good'' as86, because bcc does
everything right, including conversion to Linux a.out, when invoked
with the right options; so assemble files exclusively with bcc as a
frontend, not directly with as86.
3.6. OTHER ASSEMBLERS
These are other, non-regular, options, in case the previous didn't
satisfy you (why?), that I don't recommend in the usual (?) case, but
that could prove quite useful if the assembler must be integrated in
the software you're designing (i.e. an OS or development environment).
3.6.1. Win32Forth assembler
Win32Forth is a free 32-bit ANS FORTH system that successfully runs
under Win32s, Win95, Win/NT. It includes a free 32-bit assembler
(either prefix or postfix syntax) integrated into the reflective FORTH
language. Macro processing is done with the full power of the
reflective language FORTH; however, the only supported input and
output contexts is Win32For itself (no dumping of .obj file, but you
could add that feature yourself, of course). Find it at
<ftp://ftp.forth.org/pub/Forth/Compilers/native/windows/Win32For/>.
3.6.2. Terse
Terse <http://www.terse.com> is a programming tool that provides THE
most compact assembler syntax for the x86 family! However, it is evil
proprietary software. It is said that there was a project for a free
clone somewhere, that was abandonned after worthless pretenses that
the syntax would be owned by the original author. Thus, if you're
looking for a nifty programming project related to assembly hacking, I
invite you to develop a terse-syntax frontend to NASM, if you like
that syntax.
3.6.3. Non-free and/or Non-32bit x86 assemblers.
You may find more about them, together with the basics of x86 assembly
programming, in Raymond Moon's FAQ for comp.lang.asm.x86:
<http://www2.dgsys.com/~raymoon/faq/asmfaq.zip>.
Note that all DOS-based assemblers should work inside the Linux DOS
Emulator, as well as other similar emulators, so that if you already
own one, you can still use it inside a real OS. Recent DOS-based
assemblers also support COFF and/or other object file formats that are
supported by the GNU BFD library, so that you can use them together
with your free 32-bit tools, perhaps using GNU objcopy (part of the
binutils) as a conversion filter.
4. METAPROGRAMMING/MACROPROCESSING
Assembly programming is a bore, but for critical parts of programs.
You should use the appropriate tool for the right task, so don't
choose assembly when it's not fit; C, OCAML, perl, Scheme, might be a
better choice for most of your programming.
However, there are cases when these tools do not give a fine enough
control on the machine, and assembly is useful or needed. In those
case, you'll appreciate a system of macroprocessing and
metaprogramming that'll allow recurring patterns to be factored each
into a one indefinitely reusable definition, which allows safer
programming, automatic propagation of pattern modification, etc. A
``plain'' assembler is often not enough, even when one is doing only
small routines to link with C.
4.1. What's integrated into the above
Yes I know this section does not contain much useful up-to-date
information. Feel free to contribute what you discover the hard
way...
4.1.1. GCC
GCC allows (and requires) you to specify register constraints in your
``inline assembly'' code, so the optimizer always know about it; thus,
inline assembly code is really made of patterns, not forcibly exact
code.
Thus, you can make put your assembly into CPP macros, and inline C
functions, so anyone can use it in as any C function/macro. Inline
functions resemble macros very much, but are sometimes cleaner to use.
Beware that in all those cases, code will be duplicated, so only local
labels (of 1: style) should be defined in that asm code. However, a
macro would allow the name for a non local defined label to be passed
as a parameter (or else, you should use additional meta-programming
methods). Also, note that propagating inline asm code will spread
potential bugs in them; so watch out doubly for register constraints
in such inline asm code.
Lastly, the C language itself may be considered as a good abstraction
to assembly programming, which relieves you from most of the trouble
of assembling.
4.1.2. GAS
GAS has some macro capability included, as detailed in the texinfo
docs. Moreover, while GCC recognizes .s files as raw assembly to send
to GAS, it also recognizes .S files as files to pipe through CPP
before to feed them to GAS. Again and again, see Linux sources for
examples.
4.1.3. GASP
It adds all the usual macroassembly tricks to GAS. See its texinfo
docs.
4.1.4. NASM
NASM has some macro support, too. See according docs. If you have
some bright idea, you might wanna contact the authors, as they are
actively developing it. Meanwhile, see about external filters below.
4.1.5. AS86
It has some simple macro support, but I couldn't find docs. Now the
sources are very straightforward, so if you're interested, you should
understand them easily. If you need more than the basics, you should
use an external filter (see below).
4.1.6. OTHER ASSEMBLERS
· Win32FORTH: CODE and END-CODE are normal that do not switch from
interpretation mode to compilation mode, so you have access to the
full power of FORTH while assembling.
· TUNES: it doesn't work yet, but the Scheme language is a real high-
level language that allows arbitrary meta-programming.
4.2. External Filters
Whatever is the macro support from your assembler, or whatever
language you use (even C !), if the language is not expressive enough
to you, you can have files passed through an external filter with a
Makefile rule like that:
______________________________________________________________________
%.s: %.S other_dependencies
$(FILTER) $(FILTER_OPTIONS) < $< > $@
______________________________________________________________________
4.2.1. CPP
CPP is truely not very expressive, but it's enough for easy things,
it's standard, and called transparently by GCC.
As an example of its limitations, you can't declare objects so that
destructors are automatically called at the end of the declaring
block; you don't have diversions or scoping, etc.
CPP comes with any C compiler. However, considering how mediocre it
is, stay away from it if by chance you can make it without C,
4.2.2. M4
M4 gives you the full power of macroprocessing, with a Turing
equivalent language, recursion, regular expressions, etc. You can do
with it everything that CPP cannot.
See macro4th (this4th)
<ftp://ftp.forth.org/pub/Forth/Compilers/native/unix/this4th.tar.gz>
or the Tunes 0.0.0.25 sources
<ftp://ftp.tunes.org/pub/tunes/obsolete/dist/tunes.0.0.0/tunes.0.0.0.25.src.zip>
as examples of advanced macroprogramming using m4.
However, its disfunctional quoting and unquoting semantics force you
to use explicit continuation-passing tail-recursive macro style if you
want to do advanced macro programming (which is remindful of TeX --
BTW, has anyone tried to use TeX as a macroprocessor for anything else
than typesetting ?). This is NOT worse than CPP that does not allow
quoting and recursion anyway.
The right version of m4 to get is GNU m4 1.4 (or later if exists),
which has the most features and the least bugs or limitations of all.
m4 is designed to be slow for anything but the simplest uses, which
might still be ok for most assembly programming (you're not writing
million-lines assembly programs, are you?).
4.2.3. Macroprocessing with yer own filter
You can write your own simple macro-expansion filter with the usual
tools: perl, awk, sed, etc. That's quick to do, and you control
everything. But of course, any power in macroprocessing must be
earned the hard way.
4.2.4. Metaprogramming
Instead of using an external filter that expands macros, one way to do
things is to write programs that write part or all of other programs.
For instance, you could use a program outputing source code
· to generate sine/cosine/whatever lookup tables,
· to extract a source-form representation of a binary file,
· to compile your bitmaps into fast display routines,
· to extract documentation, initialization/finalization code,
description tables, as well as normal code from the same source
files,
· to have customized assembly code, generated from a
perl/shell/scheme script that does arbitrary processing,
· to propagate data defined at one point only into several cross-
referencing tables and code chunks.
· etc.
Think about it!
4.2.4.1. Backends from compilers
Compilers like GCC, SML/NJ, Objective CAML, MIT-Scheme, CMUCL, etc, do
have their own generic assembler backend, which you might choose to
use, if you intend to generate code semi-automatically from the
according languages, or from a language you hack: rather than write
great assembly code, you may instead modify a compiler so that it
dumps great assembly code!
4.2.4.2. The New-Jersey Machine-Code Toolkit
There is a project, using the programming language Icon (with an
experimental ML version), to build a basis for producing assembly-
manipulating code. See around
<http://www.cs.virginia.edu/~nr/toolkit/>
4.2.4.3. TUNES
The TUNES Project <http://www.tunes.org/> for a Free Reflective
Computing System is developping its own assembler as an extension to
the Scheme language, as part of its development process. It doesn't
run at all yet, though help is welcome.
The assembler manipulates abstract syntax trees, so it could equally
serve as the basis for a assembly syntax translator, a disassembler, a
common assembler/compiler back-end, etc. Also, the full power of a
real language, Scheme, make it unchallenged as for
macroprocessing/metaprograming.
5. CALLING CONVENTIONS
5.1. Linux
5.1.1. Linking to GCC
That's the preferred way. Check GCC docs and examples from Linux
kernel .S files that go through gas (not those that go through as86).
32-bit arguments are pushed down stack in reverse syntactic order
(hence accessed/popped in the right order), above the 32-bit near
return address. %ebp, %esi, %edi, %ebx are callee-saved, other
registers are caller-saved; %eax is to hold the result, or %edx:%eax
for 64-bit results.
FP stack: I'm not sure, but I think it's result in st(0), whole stack
caller-saved.
Note that GCC has options to modify the calling conventions by
reserving registers, having arguments in registers, not assuming the
FPU, etc. Check the i386 .info pages.
Beware that you must then declare the cdecl or regparm(0) attribute
for a function that will follow standard GCC calling conventions. See
in the GCC info pages the section: C Extensions::Extended Asm::. See
also how Linux defines its asmlinkage macro...
5.1.2. ELF vs a.out problems
Some C compilers prepend an underscore before every symbol, while
others do not.
Particularly, Linux a.out GCC does such prepending, while Linux ELF
GCC does not.
If you need cope with both behaviors at once, see how existing
packages do. For instance, get an old Linux source tree, the Elk,
qthreads, or OCAML...
You can also override the implicit C->asm renaming by inserting
statements like
______________________________________________________________________
void foo asm("bar") (void);
______________________________________________________________________
to be sure that the C function foo will be called really bar in assem&SHY;
bly.
Note that the utility objcopy, from the binutils package, should allow
you to transform your a.out objects into ELF objects, and perhaps the
contrary too, in some cases. More generally, it will do lots of file
format conversions.
5.1.3. Direct Linux syscalls
This is specifically NOT recommended, because the conventions change
from time to time or from kernel flavor to kernel flavor (cf L4Linux),
plus it's not portable, it's a burden to write, it's redundant with
the libc effort, AND it precludes fixes and extensions that are made
to the libc, like, for instance the zlibc package, that does on-the-
fly transparent decompression of gzip-compressed files. The standard,
recommended way to call Linux system services is, and will stay, to go
through the libc.
Shared objects should keep your stuff small. And if you really want
smaller binaries, do use #! stuff, with the interpreter having all the
overhead you want to keep out of your binaries.
Now, if for some reason, you don't want to link to the libc, go get
the libc and understand how it works! After all, you're pretending to
replace it, ain't you? You might also take a look at how my eforth
1.0e <ftp://ftp.forth.org/pub/Forth/Compilers/native/unix/Linux/linux-
eforth-1.0e.tar.gz> does it.
The sources for Linux come in handy, too, particularly the
asm/unistd.h header file, that describes how to do system calls...
Basically, you issue an int $0x80, with the __NR_syscallname number
(from asm/unistd.h) in %eax, and parameters (up to five) in %ebx,
%ecx, %edx, %esi, %edi respectively. Result is returned in %eax, with
a negative result being an error whose opposite is what libc would put
in errno. The user-stack is not touched, so you needn't have a valid
one when doing a syscall.
As for the invocation arguments passed to a process upon startup, the
general principle is that the stack originally contains the number of
arguments argc, then the list of pointers that constitute *argv, then
a null-terminated sequence of null-terminated variable=value strings.
For more details, read the sources of C startup code from your libc
(crt0.S or crt1.S), the sources of eforth 1.0e, or those of the linux
kernel (exec.c et binfmt_*.c in linux/fs/).
5.1.4. Hardware I/O under Linux
If you want to do direct I/O under Linux, either it's something very
simple that needn't OS arbitration, and you should see the IO-Port-
Programming mini-HOWTO; or it needs a kernel device driver, and you
should try to learn more about kernel hacking, device driver
development, kernel modules, etc, for which there are other excellent
HOWTOs and documents from the LDP.
Particularly, if what you want is Graphics programming, then do join
one of the GGI <http://www.ggi-project.org/> or XFree86
<http://www.XFree86.org/> projects.
Some people have even done better, writing small and robust XFree86
drivers in an interpreted domain-specific language, GAL
<http://www.irisa.fr/compose/gal/>, and achieving the efficiency of
hand C-written drivers through partial evaluation (drivers not only
not in asm, but not even in C!). The problem is that the partial
evaluator they used to achieve efficiency is not itself free software.
Any taker for a replacement?
Anyway, in all these cases, you'll be better off using GCC inline
assembly with the macros from linux/asm/*.h than writing full assembly
source files.
5.1.5. Accessing 16-bit drivers from Linux/i386
Such thing is theoretically possible (proof: see how DOSEMU
<http://www.dosemu.org> can selectively grant hardware port access to
programs), and I've heard rumors that someone somewhere did actually
do it (in the PCI driver? Some VESA access stuff? ISA PnP? dunno). If
you have some more precise information on that, you'll be most
welcome. Anyway, good places to look for more information are the
Linux kernel sources, DOSEMU sources (and other programs in the DOSEMU
repository <ftp://tsx-11.mit.edu/pub/linux/ALPHA/dosemu/>), and
sources for various low-level programs under Linux... (perhaps GGI if
it supports VESA).
Basically, you must either use 16-bit protected mode or vm86 mode.
The first is simpler to setup, but only works with well-behaved code
that won't do any kind of segment arithmetics or absolute segment
addressing (particularly addressing segment 0), unless by chance it
happens that all segments used can be setup in advance in the LDT.
The later allows for more "compatibility" with vanilla 16-bit
environments, but requires more complicated handling.
In both cases, before you can jump to 16-bit code, you must
· mmap any absolute address used in the 16-bit code (such as ROM,
video buffers, DMA targets, and memory-mapped I/O) from /dev/mem to
your process' address space,
· setup the LDT and/or vm86 mode monitor.
· grab proper I/O permissions from the kernel (see the above section)
Again, carefully read the source for the stuff contributed to the
DOSEMU project, particularly these mini-emulators for running ELKS
and/or simple .COM programs under Linux/i386.
5.2. DOS
Most DOS extenders come with some interface to DOS services. Read
their docs about that, but often, they just simulate int $0x21 and
such, so you do ``as if'' you were in real mode (I doubt they have
more than stubs and extend things to work with 32-bit operands; they
most likely will just reflect the interrupt into the real-mode or vm86
handler).
Docs about DPMI and such (and much more) can be found on
<ftp://x2ftp.oulu.fi/pub/msdos/programming/> (again, the original
x2ftp site is closing, so use a mirror site
<ftp://ftp.lip6.fr/pub/pc/x2ftp/README.mirror_sites>).
DJGPP comes with its own (limited) glibc
derivative/subset/replacement, too.
It is possible to cross-compile from Linux to DOS, see the
devel/msdos/ directory of your local FTP mirror for metalab.unc.edu
Also see the MOSS dos-extender from the Flux project
<http://www.cs.utah.edu/projects/flux/> from university of Utah.
Other documents and FAQs are more DOS-centered. We do not recommend
DOS development.
5.3. Winblows and suches
Hey, this document covers only free software. Ring me when Winblows
becomes free, or when there are free dev tools for it!
Well, after all there are: Cygnus Solutions <http://www.cygnus.com>
has developped the cygwin32.dll library, for GNU programs to run on
MacroShit platforms. Thus, you can use GCC, GAS, all the GNU tools,
and many other Unix applications. Have a look around their homepage.
I (Faré) don't intend to expand on Losedoze programming, but I'm sure
you can find lots of documents about it everywhere...
5.4. Yer very own OS
Control being what attract many programmers to assembly, want of OS
development is often what leads to or stems from assembly hacking.
Note that any system that allows self-development could be qualified
an "OS" even though it might run "on top" of an underlying system that
multitasking or I/O (much like Linux over Mach or OpenGenera over
Unix), etc. Hence, for easier debugging purpose, you might like to
develop your ``OS'' first as a process running on top of Linux
(despite the slowness), then use the Flux OS kit
<http://www.cs.utah.edu/projects/flux/oskit/> (which grants use of
Linux and BSD drivers in yer own OS) to make it standalone. When your
OS is stable, it's still time to write your own hardware drivers if
you really love that.
This HOWTO will not itself cover topics such as Boot loader code &
getting into 32-bit mode, Handling Interrupts, The basics about intel
``protected mode'' or ``V86/R86'' braindeadness, defining your object
format and calling conventions. The main place where to find reliable
information about that all is source code of existing OSes and
bootloaders. Lots of pointers lie in the following WWW page:
<http://www.tunes.org/Review/OSes.html>
6. TODO & POINTERS
· find someone who has got some time to takeover the maintenance
· fill incomplete sections
· add more pointers to software and docs
· add simple examples from real life to illustrate the syntax, power,
and limitations of each proposed solution.
· ask people to help with this HOWTO
· perhaps give a few words for assembly on other architectures than
i386?
· A few pointers (in addition to those already in the rest of the
HOWTO)
· 80x86 CPU family references: intel manuals
<http://www.intel.com/design/pentium/manuals/>; bugs
<http://www.xs4all.nl/~feldmann/86bugs.htm>.
· ftp.luth.se <ftp://ftp.luth.se/pub/msdos/> mirrors the hornet and
x2ftp former archives of msdos assembly coding stuff.
· A few starting points on the web about assembly programming: Jannes
Faber's <http://www.fys.ruu.nl/~faber/Amain.html>; QZX's
<http://www.qzx.com/library/>; JanW's <http://bewoner.dma.be/JanW>;
this one (?) <ftp://zfja-gate.fuw.edu.pl/user/net/ka9q/guest/>
· Fun stuff: CoreWars <http://www.koth.org>, a fun way to learn
assembly in general.
· USENET: comp.lang.asm.x86 <news://comp.lang.asm.x86>;
alt.os.assembly <news://alt.os.assembly>.
· And of course, do use your usual Internet Search Tools to look for
more information, and tell me anything interesting you find!
Author's .sig:
## Faré | VN: Уng-Vû Bân | Join the TUNES project! http://www.tunes.org/ ##
## FR: François-René Rideau | TUNES is a Useful, Not Expedient System ##
## Reflection&Cybernethics | Project for a Free Reflective Computing System ##
| HowTo Linux | Linux Zone Home | E-Mail Me | Copyright 1999 Linux Zone
