<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Chapter 5. Calling conventions</title><meta name="generator" content="DocBook XSL Stylesheets V1.76.1"><meta name="keywords" content="assembly, assembler, asm, inline, 32-bit, IA-32, i386, x86, nasm, gas, as, as86, yasm, fasm, shasm, osimpa, OS, Linux, Unix, kernel, system, libc, glibc, system call, interrupt, small, fast, embedded, hardware, port, macroprocessor, metaprogramming, preprocessor"><link rel="home" href="Assembly-HOWTO.html" title="Linux Assembly HOWTO"><link rel="up" href="Assembly-HOWTO.html" title="Linux Assembly HOWTO"><link rel="prev" href="meta.html" title="Metaprogramming"><link rel="next" href="dos.html" title="DOS and Windows"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 5. Calling conventions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="meta.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="dos.html">Next</a></td></tr></table><hr></div><div class="chapter" title="Chapter 5. Calling conventions"><div class="titlepage"><div><div><h2 class="title"><a name="s-call"></a>Chapter 5. Calling conventions</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="conventions.html#idp333856">Linux</a></span></dt><dd><dl><dt><span class="section"><a href="conventions.html#idp335104">Linking to GCC</a></span></dt><dt><span class="section"><a href="conventions.html#idp345424">ELF vs a.out problems</a></span></dt><dt><span class="section"><a href="conventions.html#idp349984">Direct Linux syscalls</a></span></dt><dt><span class="section"><a href="conventions.html#idp404400">Hardware I/O under Linux</a></span></dt><dt><span class="section"><a href="conventions.html#idp415184">Accessing 16-bit drivers from Linux/i386</a></span></dt></dl></dd><dt><span class="section"><a href="dos.html">DOS and Windows</a></span></dt><dt><span class="section"><a href="ownos.html">Your own OS</a></span></dt></dl></div>
<div class="section" title="Linux"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idp333856"></a>Linux</h2></div></div></div>
<div class="section" title="Linking to GCC"><div class="titlepage"><div><div><h3 class="title"><a name="idp335104"></a>Linking to GCC</h3></div></div></div>
<p>
This is the preferred way if you are developing mixed C-asm project. Check GCC
docs and examples from Linux kernel <code class="filename">.S</code> files that go through
<span class="application">gas</span> (not those that go through
<span class="application">as86</span>).
</p>
<p>
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.
<code class="literal">%ebp</code>, <code class="literal">%esi</code>, <code class="literal">%edi</code>,
<code class="literal">%ebx</code> are callee-saved, other registers are caller-saved;
<code class="literal">%eax</code> is to hold the result, or <code class="literal">%edx:%eax</code>
for 64-bit results.
</p>
<p>
FP stack: I'm not sure, but I think result is in <code class="literal">st(0)</code>,
whole stack caller-saved.
</p>
<p>
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 <code class="filename">.info</code> pages.
</p>
<p>
Beware that you must then declare the <code class="literal">cdecl</code> or
<code class="literal">regparm(0)</code> attribute for a function that will follow
standard GCC calling conventions. See
<code class="literal">C Extensions::Extended Asm::</code> section from the GCC info
pages. See also how Linux defines its <code class="literal">asmlinkage</code> macro.
</p>
</div>
<div class="section" title="ELF vs a.out problems"><div class="titlepage"><div><div><h3 class="title"><a name="idp345424"></a>ELF vs a.out problems</h3></div></div></div>
<p>
Some C compilers prepend an underscore before every symbol, while others do
not.
</p>
<p>
Particularly, Linux a.out GCC does such prepending, while Linux ELF GCC does
not.
</p>
<p>
If you need to cope with both behaviors at once, see how existing packages do.
For instance, get an old Linux source tree, the Elk, qthreads, or OCaml.
</p>
<p>
You can also override the implicit C->asm renaming by inserting statements like
</p><pre class="programlisting">
void foo asm("bar") (void);
</pre><p>
to be sure that the C function <code class="function">foo()</code>
will be called really <code class="function">bar</code> in assembly.
</p>
<p>
Note that the <span class="command"><strong>objcopy</strong></span> utility 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.
</p>
</div>
<div class="section" title="Direct Linux syscalls"><div class="titlepage"><div><div><h3 class="title"><a name="idp349984"></a>Direct Linux syscalls</h3></div></div></div>
<p>
Often you will be told that using <span class="application">C library</span>
(<acronym class="acronym">libc</acronym>) is the only way, and direct system calls are bad.
This is true. To some extent. In general, you must know that
<span class="application">libc</span> is not sacred, and in <span class="emphasis"><em>most</em></span>
cases it only does some checks, then calls kernel, and then sets errno. You can
easily do this in your program as well (if you need to), and your program will
be dozen times smaller, and this will result in improved performance as well,
just because you're not using shared libraries (static binaries are faster).
Using or not using <span class="application">libc</span> in assembly programming is
more a question of taste/belief than something practical. Remember, Linux is
aiming to be POSIX compliant, so does <span class="application">libc</span>.
This means that syntax of almost all <span class="application">libc</span> "system
calls" exactly matches syntax of real kernel system calls (and vice versa).
Besides, <span class="application">GNU libc</span>(<span class="application">glibc</span>)
becomes slower and slower from version to version, and eats more and more
memory; and so, cases of using direct system calls become quite usual.
However, the main drawback of throwing <span class="application">libc</span> away
is that you will possibly need to implement several
<span class="application">libc</span> specific functions (that are not just syscall
wrappers) on your own (<code class="function">printf()</code> and Co.), and you are
ready for that, aren't you? :-)
</p>
<p>
Here is summary of direct system calls pros and cons.
</p>
<p>
Pros:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<p>
the smallest possible size; squeezing the last byte out of the system
</p>
</li><li class="listitem">
<p>
the highest possible speed; squeezing cycles out of your favorite benchmark
</p>
</li><li class="listitem">
<p>
full control: you can adapt your program/library to your specific language or
memory requirements or whatever
</p>
</li><li class="listitem">
<p>
no pollution by libc cruft
</p>
</li><li class="listitem">
<p>
no pollution by C calling conventions (if you're developing your own language
or environment)
</p>
</li><li class="listitem">
<p>
static binaries make you independent from libc upgrades or crashes, or from
dangling <code class="literal">#!</code> path to an interpreter (and are faster)
</p>
</li><li class="listitem">
<p>
just for the fun out of it (don't you get a kick out of assembly programming?)
</p>
</li></ul></div><p>
</p>
<p>
Cons:
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<p>
If any other program on your computer uses the libc, then duplicating the libc
code will actually wastes memory, not saves it.
</p>
</li><li class="listitem">
<p>
Services redundantly implemented in many static binaries are a waste of memory.
But you can make your libc replacement a shared library.
</p>
</li><li class="listitem">
<p>
Size is much better saved by having some kind of bytecode, wordcode, or
structure interpreter than by writing everything in assembly. (the interpreter
itself could be written either in C or assembly.) The best way to keep multiple
binaries small is to not have multiple binaries, but instead to have an
interpreter process files with <code class="literal">#!</code> prefix. This is how OCaml
works when used in wordcode mode (as opposed to optimized native code mode),
and it is compatible with using the libc. This is also how Tom Christiansen's
Perl PowerTools reimplementation of unix utilities works. Finally, one last way
to keep things small, that doesn't depend on an external file with a hardcoded
path, be it library or interpreter, is to have only one binary, and have
multiply-named hard or soft links to it: the same binary will provide everything
you need in an optimal space, with no redundancy of subroutines or useless
binary headers; it will dispatch its specific behavior according to its
<em class="parameter"><code>argv[0]</code></em>; in case it isn't called with a recognized name,
it might default to a shell, and be possibly thus also usable as an interpreter!
</p>
</li><li class="listitem">
<p>
You cannot benefit from the many functionalities that libc provides besides mere
linux syscalls: that is, functionality described in section 3 of the manual
pages, as opposed to section 2, such as malloc, threads, locale, password,
high-level network management, etc.
</p>
</li><li class="listitem">
<p>
Therefore, you might have to reimplement large parts of libc, from
<code class="function">printf()</code> to <code class="function">malloc()</code> and
<code class="function">gethostbyname</code>. It's redundant with the libc effort, and
can be <span class="emphasis"><em>quite</em></span> boring sometimes. Note that some people have
already reimplemented "light" replacements for parts of the libc - - check
them out! (Redhat's minilibc, Rick Hohensee's
<a class="ulink" href="ftp://linux01.gwdg.de/pub/cLIeNUX/interim/libsys.tgz" target="_top">libsys</a>,
Felix von Leitner's <a class="ulink" href="http://www.fefe.de/dietlibc/" target="_top">dietlibc</a>,
<a class="ulink" href="http://asm.sourceforge.net/asmutils.html" target="_top">asmutils</a>
project is working on pure assembly libc)
</p>
</li><li class="listitem">
<p>
Static libraries prevent you to benefit from libc upgrades as well as from libc
add-ons such as the <span class="application">zlibc</span> package, that does
on-the-fly transparent decompression of gzip-compressed files.
</p>
</li><li class="listitem">
<p>
The few instructions added by the libc can be a
<span class="emphasis"><em>ridiculously</em></span> small speed overhead as compared to the cost
of a system call. If speed is a concern, your main problem is in your usage of
system calls, not in their wrapper's implementation.
</p>
</li><li class="listitem">
<p>
Using the standard assembly API for system calls is much slower than using the
libc API when running in micro-kernel versions of Linux such as L4Linux, that
have their own faster calling convention, and pay high convention-translation
overhead when using the standard one (L4Linux comes with libc recompiled with
their syscall API; of course, you could recompile your code with their API,
too).
</p>
</li><li class="listitem">
<p>
See previous discussion for general speed optimization issue.
</p>
</li><li class="listitem">
<p>
If syscalls are too slow to you, you might want to hack the kernel sources
(in C) instead of staying in userland.
</p>
</li></ul></div><p>
</p>
<p>
If you've pondered the above pros and cons, and still want to use direct
syscalls, then here is some advice.
</p>
<p>
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<p>
You can easily define your system calling functions in a portable way in C
(as opposed to unportable using assembly), by including
<code class="filename">asm/unistd.h</code>, and using provided macros.
</p>
</li><li class="listitem">
<p>
Since you're trying to replace it, go get the sources for the libc, and
grok them. (And if you think you can do better, then send feedback to the
authors!)
</p>
</li><li class="listitem">
<p>
As an example of pure assembly code that does everything you want, examine
<a class="xref" href="resources.html" title="Chapter 7. Resources">Linux assembly resources</a>.
</p>
</li></ul></div><p>
</p>
<p>
Basically, you issue an <code class="function">int 0x80</code>, with the
<code class="literal">__NR_</code>syscallname number (from <code class="filename">asm/unistd.h</code>)
in <code class="literal">eax</code>, and parameters (up to <a class="link" href="conventions.html#six-arg">six</a>)
in <code class="literal">ebx</code>, <code class="literal">ecx</code>, <code class="literal">edx</code>,
<code class="literal">esi</code>, <code class="literal">edi</code>, <a class="link" href="conventions.html#six-arg">
<code class="literal">ebp</code></a> respectively.
</p>
<p>
Result is returned in <code class="literal">eax</code>, with a negative result being an
error, whose opposite is what libc would put into <code class="literal">errno</code>.
The user-stack is not touched, so you needn't have a valid one when doing a
syscall.
</p>
<div class="note" title="Note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3>
<p>
<a name="six-arg"></a>
Passing sixth parameter in <code class="literal">ebp</code> appeared in Linux 2.4,
previous Linux versions understand only 5 parameters in registers.
</p>
</div>
<p>
<a class="ulink" href="http://www.tldp.org/LDP/lki/" target="_top">Linux Kernel Internals</a>,
and especially <a class="ulink" href="http://www.tldp.org/LDP/lki/lki-2.html#ss2.11" target="_top">
How System Calls Are Implemented on i386 Architecture?</a> chapter will give
you more robust overview.
</p>
<p>
As for the invocation arguments passed to a process upon startup, the general
principle is that the stack originally contains the number of arguments
<em class="parameter"><code>argc</code></em>, then the list of pointers that constitute
<em class="parameter"><code>*argv</code></em>, then a null-terminated sequence of
null-terminated <code class="literal">variable=value</code> strings for the
<em class="parameter"><code>environ</code></em>ment. For more details, do examine
<a class="xref" href="resources.html" title="Chapter 7. Resources">Linux assembly resources</a>, read the sources of C startup code from your libc
(<code class="filename">crt0.S</code> or <code class="filename">crt1.S</code>), or those from
the Linux kernel (<code class="filename">exec.c</code> and
<code class="filename">binfmt_*.c</code> in <code class="filename">linux/fs/</code>).
</p>
</div>
<div class="section" title="Hardware I/O under Linux"><div class="titlepage"><div><div><h3 class="title"><a name="idp404400"></a>Hardware I/O under Linux</h3></div></div></div>
<p>
If you want to perform direct port I/O under Linux, either it's something very
simple that does not need OS arbitration, and you should see the
<code class="literal">IO-Port-Programming</code> 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.
</p>
<p>
Particularly, if what you want is Graphics programming, then do join one of the
<a class="ulink" href="http://www.ggi-project.org/" target="_top">GGI</a> or
<a class="ulink" href="http://www.XFree86.org/" target="_top">XFree86</a> projects.
</p>
<p>
Some people have even done better, writing small and robust XFree86 drivers in
an interpreted domain-specific language, GAL, and achieving the efficiency of
hand C-written drivers through partial evaluation (drivers not only not in asm,
but not even in C!). The problem is that the partial evaluator they used to
achieve efficiency is not free software. Any taker for a replacement?
</p>
<p>
Anyway, in all these cases, you'll be better when using GCC inline assembly
with the macros from <code class="filename">linux/asm/*.h</code> than writing full
assembly source files.
</p>
</div>
<div class="section" title="Accessing 16-bit drivers from Linux/i386"><div class="titlepage"><div><div><h3 class="title"><a name="idp415184"></a>Accessing 16-bit drivers from Linux/i386</h3></div></div></div>
<p>
Such thing is theoretically possible (proof: see how
<a class="ulink" href="http://www.dosemu.org" target="_top">DOSEMU</a> 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 sources for various low-level programs under Linux.
(perhaps GGI if it supports VESA).
</p>
<p>
Basically, you must either use 16-bit protected mode or vm86 mode.
</p>
<p>
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.
</p>
<p>
The later allows for more "compatibility" with vanilla 16-bit environments, but
requires more complicated handling.
</p>
<p>
In both cases, before you can jump to 16-bit code, you must
</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<p>
mmap any absolute address used in the 16-bit code (such as ROM, video buffers,
DMA targets, and memory-mapped I/O) from <code class="filename">/dev/mem</code> to your
process' address space,
</p>
</li><li class="listitem">
<p>
setup the LDT and/or vm86 mode monitor.
</p>
</li><li class="listitem">
<p>
grab proper I/O permissions from the kernel (see the above section)
</p>
</li></ul></div><p>
</p>
<p>
Again, carefully read the source for the stuff contributed to the DOSEMU
project, particularly these mini-emulators for running ELKS and/or simple
<code class="filename">.COM</code> programs under Linux/i386.
</p>
</div>
</div>
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