Assembly HOWTO

Assembly HOWTO
Franois-Ren Rideau [email protected]
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
______________________________________________________________________

Table of Contents

1. INTRODUCTION

1.1 Legal Blurp
1.2 IMPORTANT NOTE
1.3 Foreword
1.3.1 How to use this document
1.3.2 Other related documents
1.4 History
1.5 Credits

2. DO YOU NEED ASSEMBLY?

2.1 Pros and Cons
2.1.1 The advantages of Assembly
2.1.2 The disadvantages of Assembly
2.1.3 Assessment
2.2 How to NOT use Assembly
2.2.1 General procedure to achieve efficient code
2.2.2 Languages with optimizing compilers
2.2.3 General procedure to speed your code up
2.2.4 Inspecting compiler-generated code

3. ASSEMBLERS

3.1 GCC Inline Assembly
3.1.1 Where to find GCC
3.1.2 Where to find docs for GCC Inline Asm
3.1.3 Invoking GCC to have it properly inline assembly code ?
3.2 GAS
3.2.1 Where to find it
3.2.2 What is this AT&T syntax
3.2.3 Limited 16-bit mode
3.3 GASP
3.3.1 Where to find GASP
3.3.2 How it works
3.4 NASM
3.4.1 Where to find NASM
3.4.2 What it does
3.5 AS86
3.5.1 Where to get AS86
3.5.2 How to invoke the assembler?
3.5.3 Where to find docs
3.5.4 What if I can't compile Linux anymore with this new version ?
3.6 OTHER ASSEMBLERS
3.6.1 Win32Forth assembler
3.6.2 Terse
3.6.3 Non-free and/or Non-32bit x86 assemblers.

4. METAPROGRAMMING/MACROPROCESSING

4.1 What's integrated into the above
4.1.1 GCC
4.1.2 GAS
4.1.3 GASP
4.1.4 NASM
4.1.5 AS86
4.1.6 OTHER ASSEMBLERS
4.2 External Filters
4.2.1 CPP
4.2.2 M4
4.2.3 Macroprocessing with yer own filter
4.2.4 Metaprogramming
4.2.4.1 Backends from existing 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 I/O under Linux
5.1.5 Accessing 16-bit drivers from Linux/i386
5.2 DOS
5.3 Winblows and suches
5.4 Yer very own OS

6. TODO & POINTERS

______________________________________________________________________

1. INTRODUCTION

1.1. Legal Blurp

Copyright (C) 1996,1997 by Franois-Ren 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 Franois-Ren Rideau
<mailto:[email protected]> and now Paul Anderson
<mailto:[email protected]>.

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

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. Litterature from the FSF (free software
foundation) might help you, too.

o 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.

o 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>

o 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 "Far" Rideau <[email protected]> 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-Far-before-new-maintainer-
takes-over (?)

1.5. Credits

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

o Linus Torvalds <mailto:[email protected]> for Linux

o Bruce Evans <mailto:[email protected]> for bcc from which as86 is
extracted

o Simon Tatham <mailto:[email protected]> and Julian Hall
<mailto:[email protected]> for NASM

o Jim Neil <mailto:[email protected]> for Terse

o Tim Bynum <mailto:[email protected]> for maintaining
HOWTOs

o Raymond Moon <mailto:[email protected]> for his FAQ

o Eric Dumas <mailto:[email protected]> 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:[email protected]> and Rahim Azizarab
<mailto:[email protected]> for helping me, if not for taking over
the HOWTO.

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
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:

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

o you can control the exact behavior of code in critical sections
that might involve hardware or I/O lock-ups

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

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

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

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 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 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.

o 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).

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,
``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 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.

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-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

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!

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,
[email protected]), 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:

o 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.

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 att 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 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).

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>

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 ([email protected]) 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

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 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

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 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

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 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 (Far) 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

o fill incomplete sections

o add more pointers to software and docs

o add simple examples from real life to illustrate the syntax, power,
and limitations of each proposed solution.

o ask people to help with this HOWTO

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

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

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

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

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

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

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

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

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

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

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

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

o 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 -- [email protected] -- 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/