  Linux Assembly HOWTO
  Konstantin Boldyshev  <mailto:konst@linuxassembly.org> and
  Franois-Ren Rideau <mailto:fare@tunes.org>
  v0.5e, February 2000

  This is the Linux Assembly HOWTO.  This document describes how to pro-
  gram in assembly language using FREE programming tools, focusing on
  development for or from the Linux Operating System on IA-32 (i386)
  platform.  Included material may or may not be applicable to other
  hardware and/or software platforms.  Contributions about them will be
  gladly accepted.  Keywords: assembly, assembler, free, macroprocessor,
  preprocessor, asm, inline asm, 32-bit, x86, i386, gas, as86, nasm,
  masm, tasm, OS, kernel, system, small, fast, embedded, hardware, port,
  I/O, syscall, system call, libc, interrupt.
  ______________________________________________________________________

  Table of Contents

















































  1. INTRODUCTION

     1.1 Legal Blurb
     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
     2.3 Linux and assembly

  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 build proper 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 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 your 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 Windows and Co.
     5.4 Your own OS

  6. QUICK START

     6.1 Tools you need
     6.2 Hello, world!
        6.2.1 NASM (hello.asm)
        6.2.2 GAS (hello.S)
     6.3 Producing object code
     6.4 Producing executable

  7. RESOURCES

     7.1 Software projects
     7.2 Tutorials
     7.3 Mailing list
     7.4 Books
     7.5 Somehow related projects
     7.6 General pointers


  ______________________________________________________________________

  1.  INTRODUCTION

  1.1.  Legal Blurb

  Copyright (C) 1999-2000 Konstantin Boldyshev.

  Copyright (C) 1996-1999 Franois-Ren Rideau.

  This document is free; 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 any later
  version.


  1.2.  Important Note

  This is an interactively evolving document: you are especially invited
  to ask questions, to answer questions, to correct given answers, to
  give pointers to new software, to point the current maintainer to bugs
  or deficiencies in the pages.  In one word, contribute!

  To contribute, please contact the Assembly-HOWTO maintainer.  At the
  time of this writing, it is Konstantin Boldyshev
  <mailto:konst@linuxassembly.org> and no more Franois-Ren Rideau
  <mailto:fare@tunes.org>.  I (Far) had been looking for some time for a
  serious hacker to replace me as maintainer of this document, and am
  pleased to announce Konstantin as my worthy successor.






  1.3.  Foreword

  This document aims answering questions of those who program or want to
  program 32-bit x86 assembly using free software
  <http://www.gnu.org/philosophy/>, particularly under the Linux
  operating system.  It may also point to other documents about non-
  free, non-x86, or non-32-bit assemblers, although this is not its
  primary goal.

  Because the main interest of assembly programming is to build the guts
  of operating systems, interpreters, compilers, and games, where C
  compiler fails to provide the needed expressiveness (performance is
  more and more seldom as issue), we are focusing on development of such
  kind of 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 official version of this document is available from
  Linux Assembly <http://linuxassembly.org> and LDP
  <http://linuxdoc.org> sites.


  1.3.2.  Other related documents


  o  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.

  o  Particularly, the interesting feature of free software is that it
     comes with sources that you can consult and correct, or sometimes
     even borrow from.  Read your particular license carefully, and do
     comply to it.

  o  There are FAQs and docs about programming on your favorite
     platform, whatever it is, which you should consult for platform-
     specific issues, not related directly to assembly programming.

  o  Refer to ``RESOURCES'' section of this HOWTO for pointers to
     related documents and projects.


  1.4.  History

  Each version includes a few fixes and minor corrections, that need not
  to be repeatedly mentioned every time.

     Version 0.5e    10 Feb 2000
        url updates, changes in GAS example
     Version 0.5d    01 Feb 2000
        RESOURCE (former POINTERS) section completely redone, various
        url updates.


     Version 0.5c    05 Dec 1999
        New pointers, updates and some rearrangements.  Rewrite of sgml
        source.


     Version 0.5b    19 Sep 1999
        Discussion about libc or not libc continues.  New web pointers
        and and overall updates.


     Version 0.5a    01 Aug 1999
        "QUICK START" section rearranged, added GAS example.  Several
        new web pointers.


     Version 0.5     25 July 1999
        GAS has 16-bit mode.  New maintainer (at last): Konstantin
        Boldyshev.  Discussion about libc or not libc.  Added section
        "QUICK START" with examples of using assembly.


     Version 0.4q    22 June 1999
        process argument passing (argc,argv,environ) in assembly.  This
        is yet another "last release by Far before new maintainer takes
        over".  Nobody knows who might be the new maintainer.


     Version 0.4p    6 June 1999
        clean up and updates.


     Version 0.4o    1 December 1998
        *


     Version 0.4m    23 March 1998
        corrections about gcc invocation


     Version 0.4l    16 November 1997
        release for LSL 6th edition.


     Version 0.4k    19 October 1997
        *


     Version 0.4j    7 September 1997
        *


     Version 0.4i    17 July 1997
        info on 16-bit mode access from Linux.


     Version 0.4h    19 Jun 1997
        still more on "how not to use assembly"; updates on NASM, GAS.


     Version 0.4g    30 Mar 1997
        *
     Version 0.4f    20 Mar 1997
        *


     Version 0.4e    13 Mar 1997
        Release for DrLinux


     Version 0.4d    28 Feb 1997
        Vapor announce of a new Assembly-HOWTO maintainer.


     Version 0.4c    9 Feb 1997
        Added section "DO YOU NEED ASSEMBLY?"


     Version 0.4b    3 Feb 1997
        NASM moved: now is before AS86


     Version 0.4a    20 Jan 1997
        CREDITS section added


     Version 0.4     20 Jan 1997
        first release of the HOWTO as such.


     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 0.3l    11 Jan 1997
        *


     Version 0.3k    19 Dec 1996
        What? I had forgotten to point to terse???


     Version 0.3j    24 Nov 1996
        point to French translated version


     Version 0.3i    16 Nov 1996
        NASM is getting pretty slick


     Version 0.3h    6 Nov 1996
        more about cross-compiling -- See on sunsite: devel/msdos/


     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 0.3f    17 Oct 1996
        *


     Version 0.3c    15 Jun 1996
        *

     Version 0.2     04 May 1996
        *


     Version 0.1     23 Apr 1996
        Francois-Rene "Far" 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"



  1.5.  Credits

  I would like to thank following persons, by order of appearance:

  o  Linus Torvalds <mailto:buried.alive@in.mail> for Linux

  o  Bruce Evans <mailto:bde@zeta.org.au> for bcc from which as86 is
     extracted

  o  Simon Tatham <mailto:anakin@pobox.com> and Julian Hall
     <mailto:jules@earthcorp.com> for NASM

  o  Greg Hankins <mailto:gregh@metalab.unc.edu> and now Tim Bynum
     <mailto:linux-howto@metalab.unc.edu> for maintaining HOWTOs

  o  Raymond Moon <mailto:raymoon@moonware.dgsys.com> for his FAQ

  o  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)

  o  Paul Anderson <mailto:paul@geeky1.ebtech.net> and Rahim Azizarab
     <mailto:rahim@megsinet.net> for helping me, if not for taking over
     the HOWTO.

  o  Marc Lehman <mailto:pcg@goof.com> for his insight on GCC
     invocation.

  o  Abhijit Menon-Sen <mailto:ams@wiw.org> for helping me figure out
     the process argument passing convention

  o  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 is
  some advice from hard-earned experience.



  2.1.  Pros and Cons



  2.1.1.  The advantages of Assembly

  Assembly can express very low-level things:

  o  you can access machine-dependent registers and I/O.


  o  you can control the exact behavior of code in critical sections
     that might otherwise involve deadlock between multiple software
     threads or hardware devices.

  o  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).

  o  you can build interfaces between code fragments using incompatible
     such conventions (e.g. produced by different compilers, or
     separated by a low-level interface).

  o  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)

  o  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!)

  o  you can produce code where (but only on CPUs with known instruction
     timings, which generally excludes all current ....

  o  you can produce hand-optimized code that's perfectly tuned for your
     particular hardware setup, though not to anyone else's.

  o  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

  o  it's long and tedious to write initially,

  o  it's very bug-prone,

  o  your bugs will be very difficult to chase,

  o  it's very difficult to understand and modify, i.e. to maintain.

  o  the result is very non-portable to other architectures, existing or
     future,

  o  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, 3DNOW, SIMD extensions, etc) implies
     potentially completely different optimization techniques.  CPU
     designs already include: Intel 386, 486, Pentium, PPro, Pentium II,
     Pentium III; Cyrix 5x86, 6x86; AMD K5, K6 (K6-2, K6-III), K7
     (Athlon).  New designs keep popping up, so don't expect either this
     listing or your code to be up-to-date.

  o  your code might also be unportable across 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).

  o  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]

  o  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;

  o  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);

  o  And in any case, as says moderator John Levine on comp.compilers
     <news: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:

  o  minimize the use of assembly code,

  o  encapsulate this code in well-defined interfaces

  o  have your assembly code automatically generated from patterns
     expressed in a higher-level language than assembly (e.g. GCC inline
     assembly macros).

  o  have automatic tools translate these programs into assembly code

  o  have this code be optimized if possible

  o  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 Linux kernel sources concerning this: 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 <news:comp.compilers> about
  human vs computer-generated assembly code,
  "The human should always win and here is why.

  o  First the human writes the whole thing in a high level language.

  o  Second he profiles it to find the hot spots where it spends its
     time.

  o  Third he has the compiler produce assembly for those small sections
     of code.

  o  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-mentioned 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.
  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

  o  first try to use a better algorithm;

  o  then try to compile it rather than interpret it;

  o  then try to enable and tweak optimization from your compiler;

  o  then give the compiler hints about how to optimize (typing
     information in LISP; register usage with GCC; lots of options in
     most compilers, etc).

  o  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:

  o  check whether generated code can be obviously enhanced with hand-
     coded assembly (or by tweaking compiler switches)

  o  when that's the case, start from generated code and modify it
     instead of starting from scratch

  o  more generally, use generated code as stubs to modify, which at
     least gets right the way your assembly routines interface to the
     external world

  o  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!



  2.3.  Linux and assembly

  In general case you don't need to use assembly language in Linux
  programming.  Unlike DOS, you do not have to write Linux drivers in
  assembly (this must be done in C).  And with modern optimizing
  compilers, if you care of speed optimization for different CPU's, it's
  much simpler to write in C.  However, if you're reading this, you
  might have some reason to use assembly instead of C/C++.

  You may need to use assembly, or you may want to use assembly.
  Shortly, main practical reasons why you may need to get into Linux
  assembly are small code and libc independence.  Non-practical (and
  most often) reason is being just an old crazy hacker, who has twenty
  years old habit of doing everything in assembly language.

  Also, if you're porting Linux to some embedded hardware you can be
  quite short at size of whole system: you need to fit kernel, libc and
  all that stuff of (file|find|text|sh|etc.) utils into several hundreds
  of kilobytes, and every kilobyte costs much.  So, one of the ways
  you've got is to rewrite some (or all) parts of system in assembly,
  and this will really save you a lot of space.  For instance, a simple
  httpd written in assembly can take less than 800 bytes; you can fit a
  webserver, consisting of kernel and httpd, in 400 KB or less... Think
  about it.


  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 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 into two branches some time ago (GCC 2.8 and
  EGCS), but they merged back, and current GCC webpage is
  <http://gcc.cygnus.com>.

  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/>.


  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
  and %ebp respectively (no letter for %esp).

  The DJGPP Games resource (not only for game hackers) had 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/>, and in the DJGPP Quick
  ASM Programming Guide <http://www.castle.net/~avly/djasm.html>.


  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 build proper 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
  modern GCC has -mpentium and such flags (and PGCC
  <http://goof.com/pcg/> has even more), 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
  performance-critical files will use maximum optimization, whereas
  other files will 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 -W -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.

  The latest version is available from HJLu at
  <ftp://ftp.varesearch.com/pub/support/hjl/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:

  o  Register names are prefixed with %, so that registers are %eax, %dl
     and so on, 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.

  o  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 GAS syntax mov %dx, %ax.

  o  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 AT&T syntax, so the suffix is optional when length
     can be guessed from register operands, and else defaults to 32-bit
     (with a warning).

  o  Immediate operands are marked with a $ prefix, as in addl $5,%eax
     (add immediate long value 5 to register %eax).

  o  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.

  o  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 (no more?), 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 excellent
  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 <http://para.inria.fr/>,
  Gforth <http://www.jwdt.com/~paysan/gforth.html>, 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.  16-bit mode

  The current stable release of binutils (2.9.1.0.25) now fully supports
  16-bit mode (registers and addressing) on i386 PCs.  Still with its
  peculiar AT&T syntax, of course.  Use .code16 and .code32 to switch
  between assembly modes.

  Also, a neat trick used by some (including the oskit authors) is to
  have GCC produce code for 16-bit real mode, using an inline assembly
  statement asm(".code16\n").  GCC will still emit only 32-bit
  addressing modes, but GAS will insert proper 32-bit prefixes for them.



  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 provides cool 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, current version of NASM is 0.98.

  The syntax is Intel-style.  Excellent 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).

  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.
  If you like Intel-style syntax, as opposed to GAS syntax, then it
  should be the assembler of choice...

  Note: There's a ``converter between GAS AT&T and Intel assembler
  syntax'', which does conversion in both directions.


  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.

  Since GAS now supports 16-bit code, and since H. Peter Anvin, well-
  known linux hacker, works on NASM, maybe Linux will get rid of AS86,
  anyway? Who knows!



  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 abandoned 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.

  As an interesting historic remark, on comp.compilers
  <news:comp.compilers>, 1999/07/11 19:36:51, the moderator wrote:
  "There's no reason that assemblers have to have awful syntax.  About
  30 years ago I used Niklaus Wirth's PL360, which was basically a S/360
  assembler with Algol syntax and a a little syntactic sugar like while
  loops that turned into the obvious branches.  It really was an
  assembler, e.g., you had to write out your expressions with explicit
  assignments of values to registers, but it was nice.  Wirth used it to
  write Algol W, a small fast Algol subset, which was a predecessor to
  Pascal.  As is so often the case, Algol W was a significant
  improvement over many of its successors. -John"


  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''.

  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.
  Plain assembler often is 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


  o  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.

  o  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 truly 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 your 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 outputting source code

  o  to generate sine/cosine/whatever lookup tables,

  o  to extract a source-form representation of a binary file,

  o  to compile your bitmaps into fast display routines,

  o  to extract documentation, initialization/finalization code,
     description tables, as well as normal code from the same source
     files,
  o  to have customized assembly code, generated from a
     perl/shell/scheme script that does arbitrary processing,

  o  to propagate data defined at one point only into several cross-
     referencing tables and code chunks.

  o  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 developing 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/metaprogramming.





  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-
  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

  Often you will be told that using libc is the only way, and direct
  system calls are bad.  Believe it, unless of course you're
  specifically writing your own replacement for the libc, adapted to
  your specific language or memory requirements or whatever.

  But you must know that libc is not sacred, and in most cases libc only
  does some checks, then calls kernel, and then sets errno.  You can
  easily do this in your program as well (if you need to), and your
  program will be dozen times smaller, and this will also result in
  improved performance, just because you're not using shared libraries
  (static binaries are faster).  Using or not using libc in assembly
  programming is more a question of taste/belief than something
  practical.  Remember, Linux is aiming to be POSIX compliant, so does
  libc. This means that syntax of almost all libc "system calls" exactly
  matches syntax of real kernel system calls (and vice versa). Besides,
  modern libc becomes slower and slower, and eats more and more memory,
  and so, cases of using direct system calls become quite usual.  But..
  main drawback of throwing libc away is that possibly you will need to
  implement several libc specific functions (that are not just syscall
  wrappers) on your own (printf and Co.).. and you are ready for that,
  aren't you? :)


  Here is summary of direct system calls pros and cons.

  Pros:

  o  smallest possible size; squeezing the last byte out of the system.

  o  highest possible speed; squeezing cycles out of your favorite
     benchmark.

  o  no pollution by libc cruft.

  o  no pollution by C calling conventions (if you're developing your
     own language or environment).

  o  static binaries make you independent from libc upgrades or crashes,
     or from dangling #! path to a interpreter (and are faster).

  o  just for the fun out of it (don't you get a kick out of assembly
     programming?)

  Cons:

  o  If any other program on your computer uses the libc, then
     duplicating the libc code will actually waste memory, not save it.

  o  Size is much better saved by having some kind of bytecode,
     wordcode, or structure interpreter than by writing everything in
     assembly.  (the interpreter itself could be written either in C or
     assembly.)

  o  Services redundantly implemented in many static binaries are a
     waste of memory.  But you can put your libc replacement in a shared
     library.

  o  The best way to keep multiple binaries small is to not have
     multiple binaries, but instead to have an interpreter process files
     with #! prefix.  This is how OCaml works when used in wordcode mode
     (as opposed to optimized native code mode), and it is compatible
     with using the libc.  This is also how Tom Christiansen's Perl
     PowerTools <http://language.perl.com/ppt/> reimplementation of unix
     utilities works.  Finally, one last way to keep things small, that
     doesn't depend on an external file with a hardcoded path, be it
     library or interpreter, is to have only one binary, and have
     multiply-named hard or soft links to it: the same binary will
     provide everything you need in an optimal space, with no redundancy
     of subroutines or useless binary headers; it will dispatch its
     specific behavior according to its argv[0]; in case it isn't called
     with a recognized name, it might default to a shell, and be
     possibly thus also usable as an interpreter!

  o  You cannot benefit from the many functionalities that libc provides
     besides mere linux syscalls: that is, functionality described in
     section 3 of the manual pages, as opposed to section 2, such as
     malloc, threads, locale, password, high-level network management,
     etc.


  o  Consequently, you might have to reimplement large parts of libc,
     from printf to malloc and gethostbyname.  It's redundant with the
     libc effort, and can be quite boring sometimes.  Note that some
     people have already reimplemented "light" replacements for parts of
     the libc -- check them out!  ( Rick Hohensee's libsys
     <ftp://linux01.gwdg.de/pub/cLIeNUX/interim/libsys.tgz>,
     ``asmutils'' project is working on pure assembly libc, can anyone
     send more pointers?)

  o  Static libraries prevent your benefitting from libc upgrades as
     well as from libc add-ons such as the zlibc package, that does on-
     the-fly transparent decompression of gzip-compressed files.

  o  The few instructions added by the libc are a ridiculously small
     speed overhead as compared to the cost of a system call.  If speed
     is a concern, your main problem is in your usage of system calls,
     not in their wrapper's implementation.

  o  Using the standard assembly API for system calls is much slower
     than using the libc API when running in micro-kernel versions of
     Linux such as L4Linux, that have their own faster calling
     convention, and pay high convention-translation overhead when using
     the standard one (L4Linux comes with libc recompiled with their
     syscall API; of course, you could recompile your code with their
     API, too).

  o  See previous discussion for general speed optimization issue.

  o  If syscalls are too slow to you, you might want to hack the kernel
     sources (in C) instead of staying in userland.

  If you've pondered the above pros and cons, and still want to use
  direct syscalls (as documented in section 2 of the manual pages), then
  here is some advice.


  o  You can easily define your system calling functions in a portable
     way in C (as opposed to unportable using assembly), by including
     <asm/unistd.h>, and using provided macros.

  o  Since you're trying to replace it, go get the sources for the libc,
     and grok them.  (And if you think you can do better, then send
     feedback to the authors!)

  o  As an example of pure assembly code that does everything you want,
     examine ``Linux Assembly Projects''.

  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 the environment.  For more details, do examine ``Linux assembly
  resources'', read the sources of C startup code from your libc (crt0.S
  or crt1.S), or those from the Linux kernel (exec.c and 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 free software.  Any
  taker for a replacement?

  Anyway, in all these cases, you'll be better when 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

  o  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,

  o  setup the LDT and/or vm86 mode monitor.

  o  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 are 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 much more) can be found on
  <ftp://x2ftp.oulu.fi/pub/msdos/programming/> (again, the original
  x2ftp site is closing (no more?), 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.  Windows and Co.

  This HOWTO is not about Windows programming, you can find lots of
  documents about it everywhere..  The thing you should know is that
  Cygnus Solutions <http://www.cygnus.com> developed the cygwin32.dll
  library, for GNU programs to run on Win32 platform.  Thus, you can use
  GCC, GAS, all the GNU tools, and many other Unix applications.  Take a
  look on their webpage.



  5.4.  Your own OS

  Control is what attracts many OS developers to assembly, often is what
  leads to or stems from assembly hacking.  Note that any system that
  allows self-development could be qualified an "OS", though it can run
  "on the top" of an underlying system (much like Linux over Mach or
  OpenGenera over Unix).

  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 your own OS) to make it standalone.  When
  your OS is stable, it is time to write your own hardware drivers if
  you really love that.

  This HOWTO will not 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 are on
  the following webpage: <http://www.tunes.org/Review/OSes.html>




  6.  QUICK START

  Finally, if you still want to try this crazy idea and write something
  in assembly (if you've reached this section -- you're real assembly
  fan), I'll herein provide what you will need to get started.

  As you've read before, you can write for Linux in different ways; I'll
  show example of using pure system calls.  This means that we will not
  use libc at all, the only thing required for our program to run is
  kernel.  Our code will not be linked to any library, will not use ELF
  interpreter -- it will communicate directly with kernel.

  I will show the same sample program in two assemblers, nasm and gas,
  thus showing Intel and AT&T syntax.


  6.1.  Tools you need

  First of all you need assembler (compiler): NASM or GAS.  Second, you
  need linker: ld, assembler produces only object code.  Almost all
  distributions include GAS and ld, in binutils package.  As for NASM,
  you may have to download and install binary packages for Linux and
  docs from ``NASM webpage''; however, several distributions (Stampede,
  Debian, SuSe) already include it, check first.

  If you are going to dig in, you should also install kernel source.  I
  assume that you are using at least Linux 2.0 and ELF.


  6.2.  Hello, world!

  Linux/i386 is 32bit and has flat memory model.  A program can be
  divided into sections.  Main sections are .text for your code, .data
  for your data, .bss for undefined data.  Program must have at least
  .text section.

  Now we will write our first program. Here is sample code:


  6.2.1.  NASM (hello.asm)


























  ______________________________________________________________________
  section .data                           ;section declaration

  msg     db      "Hello, world!",0xA ;our dear string
  len     equ     $ - msg                 ;length of our dear string

  section .text                           ;section declaration

                          ;we must export the entry point to the ELF linker or
      global _start       ;loader. They conventionally recognize _start as their
                          ;entry point. Use ld -e foo to override the default.

  _start:

  ;write our string to stdout

          mov     eax,4   ;system call number (sys_write)
          mov     ebx,1   ;first argument: file handle (stdout)
          mov     ecx,msg ;second argument: pointer to message to write
          mov     edx,len ;third argument: message length
          int     0x80    ;call kernel

  ;and exit

          mov     eax,1   ;system call number (sys_exit)
          xor     ebx,ebx ;first syscall argument: exit code
          int     0x80    ;call kernel
  ______________________________________________________________________





  6.2.2.  GAS (hello.S)
































  ______________________________________________________________________
  .data                                 # section declaration

  msg:
          .string       "Hello, world!\n"  # our dear string

  len = . - msg                           # length of our dear string

  .text                                 # section declaration

                          # we must export the entry point to the ELF linker or
      .global _start      # loader. They conventionally recognize _start as their
                          # entry point. Use ld -e foo to override the default.

  _start:

  # write our string to stdout

          movl    $4,%eax         # system call number (sys_write)
          movl    $1,%ebx         # first argument: file handle (stdout)
          movl    $msg,%ecx       # second argument: pointer to message to write
          movl    $len,%edx       # third argument: message length
          int     $0x80           # call kernel

  # and exit

          movl    $1,%eax         # system call number (sys_exit)
          xorl    %ebx,%ebx       # first syscall argument: exit code
          int     $0x80           # call kernel
  ______________________________________________________________________





  6.3.  Producing object code

  First step of building binary is producing object file from source, by
  invoking assembler; we must issue the following:

  For NASM example:

  $ nasm -f elf hello.asm

  For GAS example:

  $ as -o hello.o hello.S

  This will produce hello.o object file.



  6.4.  Producing executable

  Second step is producing executable file itself from object file, by
  invoking linker:

  $ ld -s -o hello hello.o

  This will finally build hello ELF binary.

  Hey, try to run it... Works? That's it. Pretty simple.

  If you get interested and want to proceed further, you may want to
  look through ``Linux assembly projects'', that contain PLENTY of
  source code and examples.
  7.  RESOURCES


  You main resource for Linux assembly programming material is Linux
  Assembly <http://linuxassembly.org> :).

  Here are some resources listed there.


  7.1.  Software projects



  o  asmutils <http://linuxassembly.org/asmutils.html> (unix utils
     written with NASM)

  o  ec64 <http://mars.wiwi.uni-halle.de/ec64/> (C64 emulator)

  o  ELF kickers & tiny Linux executables
     <http://www.muppetlabs.com/~breadbox/software/elfkickers.html>

  o  ASMIX <http://www.lionking.org/~cubbi/serious/asmix.html> (unix
     utils in written with GAS)

  o  eforth 1.0e
     <ftp://ftp.forth.org/pub/Forth/Compilers/native/unix/Linux/linux-
     eforth-1.0e.tar.gz>


  7.2.  Tutorials


  o  Introduction to Linux/i386 Assembly Programming
     <http://linuxassembly.org/intro.html>

  o  Linux assembly tutorial
     <http://www.cs.pdx.edu/~bjorn/CS200/linux_tutorial/> (GAS and GDB
     related)

  o  A Whirlwind Tutorial on Creating Really Teensy ELF Executables for
     Linux
     <http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html>

  o  Jan's Linux & Assembler HomePage <http://bewoner.dma.be/JanW/>
     (mostly about assembly programming with libc)


  7.3.  Mailing list

  If you're are interested in Linux assembly programming (or have
  questions, or are just curious) I especially invite you to join Linux
  assembly programming mailing list.

  List address is  <mailto:linux-assembly@egroups.com>.

  To subscribe send a blank message to  <mailto:linux-
  assembly@egroups.com>.

  List archives are available at  <http://www.egroups.com/list/linux-
  assembly/>.


  7.4.  Books

  Unfortunately there are no ready books I can recommend on the topic.
  However I'm in the progress of writing a book "Linux Assembly
  Programming", which [hopefully] will be published somewhere in 2000.



  7.5.  Somehow related projects


  o  NASM <http://www.cryogen.com/Nasm/> (portable x86 assembler with
     Intel syntax)

  o  BIEW <http://biew.sourceforge.net> (portable console hex
     viewer/editor with built-in disassembler)

  o  UPX <http://wildsau.idv.uni-linz.ac.at/mfx/upx.html> (portable
     executable packer for several formats)

  o  Intel2gas <http://hermes.terminal.at/intel2gas/> (converter between
     AT&T and Intel assembler syntax)

  o  A2I <http://www.multimania.com/~placr/> (converter from AT&T to
     NASM Intel assembler syntax)

  o  Assembly Programming Journal <http://asmjournal.freeservers.com>
     (has articles on Linux assembly)


  7.6.  General pointers


  o  The Art Of Assembly
     <http://webster.cs.ucr.edu/Page_asm/ArtOfAsm.html>

  o  x86 assembly FAQ <http://www2.dgsys.com/~raymoon/faq/>

  o  ftp.luth.se <ftp://ftp.luth.se/pub/msdos/> mirrors the hornet and
     x2ftp former archives of msdos assembly coding stuff

  o  CPU family references: Intel
     <http://www.intel.com/design/pentium/manuals/>, AMD
     <http://www.amd.com/products/cpg/cpg.html>; x86 bugs
     <http://www.xs4all.nl/~feldmann/86bugs.htm>; Alpha
     <http://www.unix.digital.com/faqs/publications/base_doc/DOCUMENTATION/V40D_PDF/>;
     PowerPC assembly
     <http://www.lightsoft.co.uk/Fantasm/Beginners/begin1.html>

  o  Fun stuff: CoreWars <http://www.koth.org>, a fun way to learn
     assembly in general

  o  USENET: comp.lang.asm.x86 <news://comp.lang.asm.x86>;
     alt.os.assembly <news://alt.os.assembly>

  Note: if you know any Linux assembly related project/webpage not
  mentioned here, please send me the url.













