  Assembly HOWTO
  François-René Rideau rideau@ens.fr
  v0.4l, 16 November 1997

  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

  1.  INTRODUCTION

  1.1.  Legal Blurp

  Copyright © 1996,1997 by François-René Rideau.  This document may be
  distributed under the terms set forth in the LDP license at
  <http://sunsite.unc.edu/LDP/COPYRIGHT.html>.

  1.2.  IMPORTANT NOTE

  This is expectedly the last release I'll make of this document.
  There's one candidate new maintainer, but until he really takes the
  HOWTO over, I'll accept feedback.

  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 FAQ.  In one word, contribute!

  To contribute, please contact whoever appears to maintain the
  Assembly-HOWTO.  Current maintainers are François-René Rideau
  <mailto:rideau@clipper.ens.fr> and now Paul Anderson
  <mailto:paul@geeky1.ebtech.net>.

  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
  assemblers, 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 expressivity
  (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.eleves.ens.fr:8080/home/rideau/Assembly-HOWTO> or
  <http://www.eleves.ens.fr:8080/home/rideau/Assembly-HOWTO.sgml>

  but what's in Linux HOWTO repositories should be fairly up to date,
  too (I can't know):

  <ftp://sunsite.unc.edu/pub/Linux/docs/HOWTO/> (?)

  A french translation of this HOWTO can be found around

  <ftp://ftp.ibp.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.  Litterature from the FSF (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 <rideau@ens.fr> 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
        found -fasm option to enable GCC inline assembler w/o -O
        optimizations

     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.

        This is yet another last-release-by-Faré-before-new-maintainer-
        takes-over (?)

  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@poboxes.com> and Julian Hall
     <mailto:jules@earthcorp.com> for NASM

  ·  Jim Neil <mailto:jim-neil@digital.net> for Terse

  ·  Greg Hankins <mailto:gregh@sunsite.unc.edu> for maintaining HOWTOs

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

  ·  Eric Dumas <mailto:dumas@excalibur.ibp.fr> 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.

  ·  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 involve hardware or I/O lock-ups

  ·  you can break the conventions of your usual compiler, which might
     allow some optimizations (like temporarily breaking rules about GC,
     threading, etc).

  ·  get access to unusual programming modes of your processor (e.g. 16
     bit code for startup or BIOS interface on Intel PCs)

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

  ·  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 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 variation (bus width, relative speed and size
     of CPU/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 appearing, 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/> together with all the released
  application software from the GNU project.  Linux-configured and
  precompiled versions can be found in
  <ftp://sunsite.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:

  <http://www.leo.org/pub/comp/os/os2/gnu/emx+gcc/>

  <http://warp.eecs.berkeley.edu/os2/software/shareware/emx.html>

  <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,
  %ebp respectively (no letter for %esp).

  The DJGPP Games resource (not only for game hackers) has this page
  specifically about assembly:

  <http://www.rt66.com/~brennan/djgpp/djgpp_asm.html>

  Finally, there is a web page called, ``DJGPP Quick ASM Programming
  Guide'', that covers URLs to FAQs, AT&T x86 ASM Syntax, Some inline
  ASM information, and converting .obj/.lib files:

  <http://remus.rutgers.edu/~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 have it properly inline assembly code ?

  Be sure to invoke GCC with the -O flag (or -O2, -O3, etc), to enable
  optimizations and inline assembly.  If you don't, your code may
  compile, but not run properly!!!  Actually (kudos to Tim Potter,
  timbo@moshpit.air.net.au), it is enough to use the -fasm flag (and
  perhaps -finline-functions) which is part of all the features enabled
  by -O.  So if you have problems with buggy optimizations in your
  particular implementation/version of GCC, you can still use inline
  asm.  Similarly, use -fno-asm to disable inline assembly (why would
  you?).

  More generally, good compile flags for GCC on the x86 platform are

  ______________________________________________________________________
          gcc -O2 -fomit-frame-pointer -m386 -Wall
  ______________________________________________________________________

  -O2 is the good optimization level. Optimizing besides it yields code
  that is a lot larger, but only a bit faster; such overoptimizationn
  might be useful for tight loops only (if any), which you may be doing
  in assembly anyway; if you need that, do it just for the few routines
  that need it.

  -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.

  -m386 yields more compact code, without any measurable slowdown, (note
  that small code also means less disk I/O and faster execution) but
  perhaps on the above-mentioned tight loops; you might appreciate
  -mpentium for special pentium-optimizing GCC targetting a specifically
  pentium platform.

  -Wall enables all warnings and helps you catch obvious stupid errors.

  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...

  Note that you can add make these flags the default by editing file
  /usr/lib/gcc-lib/i486-linux/2.7.2.2/specs or wherever that is on your
  system (better not add -Wall there, though).

  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 was, 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>

  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 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 sunsite 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, the current NASM version is 0.96.

  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 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.eleves.ens.fr:8080/home/rideau/files/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/Linux8086.html> <ftp://linux.mit.edu/>.

  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 binutil 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.  A patch is published in the Tunes project
  <http://www.eleves.ens.fr:8080/home/rideau/Tunes/> subpage
  files/tgz/tunes.0.0.0.25.src.tgz in unpacked subdirectory LLL/i386/
  The patch should also be in available directly from
  <http://www.eleves.ens.fr:8080/home/rideau/files/as86.bcc.patch.gz>
  Bruce Evans accepted this patch, so if there is a more recent version
  of bcc somewhere someday, the patch should have been included...

  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.  Man pages are also
  available somewhere on the FreeBSD site.  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 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 my patch for compiling Linux with a
  Linux a.out as86 didn't make it to him (!).  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 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 -- you could add that
  yourself, of course).  Find it at
  <ftp://ftp.forth.org/pub/Forth/win32for/>

  3.6.2.  Terse

  Terse is a programming tool that provides THE most compact assembler
  syntax for the x86 family!  See  <http://www.terse.com>.  It is said
  that there was a free clone somewhere, that was abandonned after
  worthless pretenses that the syntax would be owned by the original
  author, and that I invite you to take over, in case the syntax
  interests you.

  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.

  Then, 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.

  Beware that some optimizations that involve passing arguments to
  functions through registers may make those functions unsuitable to be
  called from external (and particularly hand-written assembly) routines
  in the standard way; the "asmlinkage" attribute may prevent a routine
  to be concerned by such optimization flag; see the linux kernel
  sources for examples.

  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. If you could make it without one, don't
  bother fetching CPP (though I wonder how you could).

  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 from <ftp://ftp.forth.org/pub/Forth/> in
  Reviewed/ ANS/ (?), or the Tunes 0.0.0.25 sources 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 existing compilers

  Compilers like SML/NJ, Objective CAML, MIT-Scheme, etc, do have their
  own generic assembler backend, which you might or not want to use, if
  you intend to generate code semi-automatically from the according
  languages.

  4.2.4.2.  The New-Jersey Machine-Code Toolkit

  There is a project, using the programming language Icon, 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 OS project 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 symbolic 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.

  <http://www.eleves.ens.fr:8080/home/rideau/Tunes/>

  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 attribute for a function
  that will follow standard GCC calling conventions (I don't know what
  it does with modified calling conventions).  See in the GCC info pages
  the section: C Extensions::Extended Asm::
  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

  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 1.0c
  <ftp://ftp.forth.org/pub/Forth/Linux/linux-eforth-1.0c.tgz> 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.

  5.1.4.  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
  the GGI project: <http://synergy.caltech.edu/~ggi/>
  <http://sunserver1.rz.uni-duesseldorf.de/~becka/doc/scrdrv.html>

  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 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 repository above, 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/>

  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 sunsite.unc.edu
  Also see the MOSS dos-extender from the Flux project in 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 is: 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://ww.cs.utah.edu/projects/flux/> (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.eleves.ens.fr:8080/home/rideau/Tunes/Review/OSes.html>

  6.  TODO & POINTERS

  ·  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

  ·  find someone who has got some time to takeover the maintenance

  ·  perhaps give a few words for assembly on other platforms?

  ·  A few pointers (in addition to those already in the rest of the
     HOWTO)

  ·  pentium manuals <http://www.intel.com/design/pentium/manuals/>

  ·  cpu bugs in the x86 family <http://www.xs4all.nl/~feldmann>

  ·  hornet.eng.ufl.edu for assembly coders <http://www.eng.ufl.edu/ftp>

  ·  ftp.luth.se <ftp://ftp.luth.se/pub/msdos/demos/code/>

  ·  PM FAQ <ftp://zfja-gate.fuw.edu.pl/cpu/protect.mod>

  ·  80x86 Assembly Page <http://www.fys.ruu.nl/~faber/Amain.html>

  ·  Courseware <http://www.cit.ac.nz/smac/csware.htm>

  ·  game programming <http://www.ee.ucl.ac.uk/~phart/gameprog.html>

  ·  experiments with asm-only linux programming
     <http://bewoner.dma.be/JanW>

  ·  And of course, do use your usual Internet Search Tools to look for
     more information, and tell me anything interesting you find!

  Authors' .sig:

  --    ,                                         ,           _ v    ~  ^  --
  -- Fare -- rideau@clipper.ens.fr -- Francois-Rene Rideau -- +)ang-Vu Ban --
  --                                      '                   / .          --
  Join the TUNES project for a computing system based on computing freedom !
                   TUNES is a Useful, Not Expedient System
  WWW page at URL: http://www.eleves.ens.fr:8080/home/rideau/Tunes/

