Copyright © 1999-2002 by Konstantin Boldyshev
Copyright © 1996-1999 by Francois-Rene Rideau
$Date: 2002/08/17 08:35:59 $
This is the Linux Assembly HOWTO, version 0.6f. This document describes how to program in assembly language using free programming tools, focusing on development for or from the Linux Operating System, mostly on IA-32 (i386) platform. Included material may or may not be applicable to other hardware and/or software platforms.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1; with no Invariant Sections, with no Front-Cover Texts, and no Back-Cover texts.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License Version 1.1; with no Invariant Sections, with no Front-Cover Texts, and no Back-Cover texts. A copy of the license is included in the GNU Free Documentation License appendix.
This document aims answering questions of those who program or want to program 32-bit x86 assembly using free software, particularly under the Linux operating system. At many places Universal Resource Locators (URL) are given for some software or documentation repository. This document also points to other documents about non-free, non-x86, or non-32-bit assemblers, although this is not its primary goal. Also note that there are FAQs and docs about programming on your favorite platform (whatever it is), which you should consult for platform-specific issues, not related directly to assembly programming.
Because the main interest of assembly programming is to build the guts of operating systems, interpreters, compilers, and games, where C compiler fails to provide the needed expressiveness (performance is more and more seldom as issue), we are focusing on development of such kind of software.
If you don't know what free software is, please do read carefully the GNU General Public License (GPL or copyleft), which is used in a lot of free software, and is the model for most of their licenses. It generally comes in a file named COPYING (or COPYING.LIB). Literature from the Free Software Foundation (FSF) might help you too. Particularly, the interesting feature of free software is that it comes with source code which you can consult and correct, or sometimes even borrow from. Read your particular license carefully and do comply to it.
This is an interactively evolving document: you are especially invited to ask questions, to answer questions, to correct given answers, to give pointers to new software, to point the current maintainer to bugs or deficiencies in the pages. In one word, contribute!
To contribute, please contact the maintainer.
Well, I wouldn't want to interfere with what you're doing, but here is some advice from the hard-earned experience.
Assembly can express very low-level things:
Assembly is a very low-level language (the lowest above hand-coding the binary instruction patterns). This means
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:
Even when assembly is needed (e.g. OS development), you'll find that not so much of it is required, and that the above principles retain.
See the Linux kernel sources concerning this: as little assembly as needed, resulting in a fast, reliable, portable, maintainable OS. Even a successful game like DOOM was almost massively written in C, with a tiny part only being written in assembly for speed up.
As says Charles Fiterman on comp.compilers about human vs computer-generated assembly code:
Languages like ObjectiveCAML, SML, CommonLISP, Scheme, ADA, Pascal, C, C++, among others, all have free optimizing compilers that will optimize the bulk of your programs, and often do better than hand-coded assembly even for tight loops, while allowing you to focus on higher-level details, and without forbidding you to grab a few percent of extra performance in the above-mentioned way, once you've reached a stable design. Of course, there are also commercial optimizing compilers for most of these languages, too!
Some languages have compilers that produce C code, which can be further optimized by a C compiler: LISP, Scheme, Perl, and many other. Speed is fairly good.
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
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. The 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.
There are many reasons to inspect compiler-generated assembly code. Here is what you'll do with such code:
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!
As you probably noticed, in general case you don't need to use assembly language in Linux programming. Unlike DOS, you do not have to write Linux drivers in assembly (well, actually you can do it if you really want). And with modern optimizing compilers, if you care of speed optimization for different CPU's, it's much simpler to write in C. However, if you're reading this, you might have some reason to use assembly instead of C/C++.
You may need to use assembly, or you may want to use assembly. In short, main practical (need) reasons of diving into the assembly realm are small code and libc independence. Impractical (want), and the most often reason is being just an old crazy hacker, who has twenty years old habit of doing everything in assembly language.
However, if you're porting Linux to some embedded hardware you can be quite short at the size of whole system: you need to fit kernel, libc and all that stuff of (file|find|text|sh|etc.) utils into several hundreds of kilobytes, and every kilobyte costs much. So, one of the possible ways is to rewrite some (or all) parts of system in assembly, and this will really save you a lot of space. For instance, a simple httpd written in assembly can take less than 600 bytes; you can fit a server consisting of kernel, httpd and ftpd in 400 KB or less... Think about it.
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.
The original GCC site is the GNU FTP site ftp://prep.ai.mit.edu/pub/gnu/gcc/ together with all released application software from the GNU project. Linux-configured and pre-compiled versions can be found in ftp://metalab.unc.edu/pub/Linux/GCC/ There are a lot of FTP mirrors of both sites everywhere around the world, as well as CD-ROM copies.
GCC development has split into two branches some time ago (GCC 2.8 and EGCS), but they merged back, and current GCC webpage is http://gcc.gnu.org.
Sources adapted to your favorite OS and pre-compiled binaries should be found at your usual FTP sites.
DOS port of GCC is called DJGPP.
There is also an OS/2 port of GCC called EMX; it works under DOS too, and includes lots of unix-emulation library routines. Look around the following site: ftp://ftp-os2.cdrom.com/pub/os2/emx09c.
The documentation of GCC includes documentation files in TeXinfo format. You can compile them with TeX and print then result, or convert them to .info, and browse them with emacs, or convert them to .html, or nearly whatever you like; convert (with the right tools) to whatever you like, or just read as is. The .info files are generally found on any good installation for GCC.
The right section to look for is C Extensions::Extended Asm::
Section Invoking GCC::Submodel Options::i386 Options:: might help too. Particularly, it gives the i386 specific constraint names for registers: abcdSDB correspond to %eax, %ebx, %ecx, %edx, %esi, %edi and %ebp respectively (no letter for %esp).
The DJGPP Games resource (not only for game hackers) had page specifically about assembly, but it's down. Its data have nonetheless been recovered on the DJGPP site, that contains a mine of other useful information: http://www.delorie.com/djgpp/doc/brennan/, and in the DJGPP Quick ASM Programming Guide.
GCC depends on GAS for assembling and follows its syntax (see below); do mind that inline asm needs percent characters to be quoted, they will 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.
Because assembly routines from the kernel headers (and most likely your own headers, if you try making your assembly programming as clean as it is in the linux kernel) are embedded in extern inline functions, GCC must be invoked with the -O flag (or -O2, -O3, etc), for these routines to be available. If not, your code may compile, but not link properly, since it will be looking for non-inlined extern functions in the libraries against which your program is being linked! Another way is to link against libraries that include fallback versions of the routines.
Inline assembly can be disabled with -fno-asm, which will have the compiler die when using extended inline asm syntax, or else generate calls to an external function named asm() that the linker can't resolve. To counter such flag, -fasm restores treatment of the asm keyword.
More generally, good compile flags for GCC on the x86 platform are
gcc -O2 -fomit-frame-pointer -W -Wall
-O2 is the good optimization level in most cases. Optimizing besides it takes more time, and yields code that is much larger, but only a bit faster; such over-optimization might be useful for tight loops only (if any), which you may be doing in assembly anyway. In cases when you need really strong compiler optimization for a few files, do consider using up to -O6.
-fomit-frame-pointer allows generated code to skip the stupid frame pointer maintenance, which makes code smaller and faster, and frees a register for further optimizations. It precludes the easy use of debugging tools (gdb), but when you use these, you just don't care about size and speed anymore anyway.
-W -Wall enables all useful warnings and helps you to catch obvious stupid errors.
You can add some CPU-specific -m486 or such flag so that GCC will produce code that is more adapted to your precise CPU. Note that modern GCC has -mpentium and such flags (and PGCC has even more), whereas GCC 2.7.x and older versions do not. A good choice of CPU-specific flags should be in the Linux kernel. Check the TeXinfo documentation of your current GCC installation for more.
-m386 will help optimize for size, hence also for speed on computers whose memory is tight and/or loaded, since big programs cause swap, which more than counters any "optimization" intended by the larger code. In such settings, it might be useful to stop using C, and use instead a language that favors code factorization, such as a functional language and/or FORTH, and use a bytecode- or wordcode- based implementation.
Note that you can vary code generation flags from file to file, so performance-critical files will use maximum optimization, whereas other files will be optimized for size.
To optimize even more, option -mregparm=2 and/or corresponding function attribute might help, but might pose lots of problems when linking to foreign code, including libc. There are ways to correctly declare foreign functions so the right call sequences be generated, or you might want to recompile the foreign libraries to use the same register-based calling convention...
Note that you can add make these flags the default by editing file /usr/lib/gcc-lib/i486-linux/220.127.116.11/specs or wherever that is on your system (better not add -W -Wall there, though). The exact location of the GCC specs files on system can be found by gcc -v.
GCC allows (and requires) you to specify register constraints in your inline assembly code, so the optimizer always know about it; thus, inline assembly code is really made of patterns, not forcibly exact code.
Thus, you can make put your assembly into CPP macros, and inline C functions, so anyone can use it in as any C function/macro. Inline functions resemble macros very much, but are sometimes cleaner to use. Beware that in all those cases, code will be duplicated, so only local labels (of 1: style) should be defined in that asm code. However, a macro would allow the name for a non local defined label to be passed as a parameter (or else, you should use additional meta-programming methods). Also, note that propagating inline asm code will spread potential bugs in them; so watch out doubly for register constraints in such inline asm code.
Lastly, the C language itself may be considered as a good abstraction to assembly programming, which relieves you from most of the trouble of assembling.
GAS is the GNU Assembler, that GCC relies upon.
Find it at the same place where you've found GCC, in the binutils package. The latest version of binutils is available from http://sources.redhat.com/binutils/.
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 neither worse, nor 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:
Note: There are few programs which may help you to convert source code between AT&T and Intel assembler syntaxes; some of the are capable of performing conversion in both directions.
GAS has comprehensive documentation in TeXinfo format, which comes at least with the source distribution. Browse extracted .info pages with Emacs or whatever. There used to be a file named gas.doc or as.doc around the GAS source package, but it was merged into the TeXinfo docs. Of course, in case of doubt, the ultimate documentation is the sources themselves! A section that will particularly interest you is Machine Dependencies::i386-Dependent::
Again, the sources for Linux (the OS kernel) come in as excellent examples; see under linux/arch/i386/ the following files: kernel/*.S, boot/compressed/*.S, math-emu/*.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 else 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.
Good news are that starting from binutils 2.10 release, GAS supports Intel syntax too. It can be triggered with .intel_syntax directive. Unfortunately this mode is not documented (yet?) in the official binutils manual, so if you want to use it, try to examine http://home.snafu.de/phpr/lhpas86.html.gz, which is an extract from AMD 64bit port of binutils 2.11.
Binutils (18.104.22.168.25+) now fully support 16-bit mode (registers and addressing) on i386 PCs. Use .code16 and .code32 to switch between assembly modes.
Also, a neat trick used by several people (including the oskit authors) is to force GCC to produce code for 16-bit real mode, using an inline assembly statement asm(".code16\n"). GCC will still emit only 32-bit addressing modes, but GAS will insert proper 32-bit prefixes for them.
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 feeding them to GAS. Again and again, see Linux sources for examples.
GAS also has GASP (GAS Preprocessor), which adds all the usual macroassembly tricks to GAS. GASP comes together with GAS in the GNU binutils archive. It works as a filter, like CPP and M4. I have no idea on details, but it comes with its own texinfo documentation, which you would like to browse (info gasp), print, grok. GAS with GASP looks like a regular macro-assembler to me.
The Netwide Assembler project provides cool i386 assembler, written in C, that should be modular enough to eventually support all known syntaxes and object formats.
Binary release on your usual metalab mirror in devel/lang/asm/ directory. Should also be available as .rpm or .deb in your usual RedHat/Debian distributions' contrib.
The syntax is Intel-style. Comprehensive macroprocessing support is integrated.
Supported object file formats are bin, aout, coff, elf, as86, obj (DOS), win32, rdf (their own format).
NASM can be used as a backend for the free LCC compiler (support files included).
Unless you're using BCC as a 16-bit compiler (which is out of scope of this 32-bit HOWTO), you should definitely use NASM instead of say AS86 or MASM, because it runs on all platforms.
Its hand-written parser makes it much faster than GAS, though of course, it doesn't support three bazillion different architectures. If you like Intel-style syntax, as opposed to GAS syntax, then it should be the assembler of choice..
Note: There are few programs which may help you to convert source code between AT&T and Intel assembler syntaxes; some of the are capable of performing conversion in both directions.
AS86 is a 80x86 assembler, both 16-bit and 32-bit, with integrated macro support. It has mostly Intel-syntax, though it differs slightly as for addressing modes.
Current version is 0.16, it can be found at http://www.cix.co.uk/~mayday/, in bin86 package with linker (ld86), or as separate archive.
See the man page and as.doc from the source package. When in doubt, the sources themselves are often a good docs: they aren't very well commented, but the programming style is straightforward. You might try to see how as86 is used in ELKS, LILO, or Tunes 0.0.0.25...
Here's the GNU Makefile entry for using BCC to transform .s asm into both a.out .o object and .l listing:
Remove the %.l, -A-l, and -A$*.l, if you don't want any listing. If you want something else than a.out, you can examine BCC docs about the other supported formats, and/or use the objcopy utility from the GNU binutils package.
There are other assemblers with various interesting and outstanding features which may be of your interest as well.
YASM is a complete rewrite of the NASM assembler under the GNU GPL (some portions are under the "new" BSD License). It is designed from the ground up to allow for multiple syntaxes to be supported (eg, NASM, TASM, GAS, etc.) in addition to multiple output object formats. Another primary module of the overall design is an optimizer module.
It looks promising; it is under heavy development, and you may want to take part. See http://www.tortall.net/projects/yasm/.
FASM (flat assembler) is a fast, efficient 80x86 assembler that runs in 'flat real mode'. Unlike many other 80x86 assemblers, FASM only requires the source code to include the information it really needs. It is written in itself and is very small and fast. It runs on DOS/Windows/Linux and can produce flat binary, DOS EXE, Win32 PE and COFF output. See http://fasm.sourceforge.net.
osimpa is an assembler for Intel 80386 processors and subsequent, written entirely in the GNU Bash command interpreter shell. The predecessor of osimpa was shasm. osimpa is much cleaned up, can create useful Linux ELF executables, and has various HLL-like extensions and programmer convenience commands.
It is (of course) slower than other assemblers. It has its own syntax (and uses its own names for x86 opcodes) Fairly good documentation is included. Check it out: ftp://linux01.gwdg.de/pub/cLIeNUX/interim/. Probably you'll not use it on regular basis, but at least it deserves your interest as an interesting idea.
The Table Driven Assembler (TDASM) is a free portable cross assembler for any kind of assembly language. It should be possible to use it as a compiler to any target microprocessor using a table that defines the compilation process.
It is available from http://www.penguin.cz/~niki/tdasm/.
HLA is a High Level Assembly language. It uses a high level language like syntax (similar to Pascal, C/C++, and other HLLs) for variable declarations, procedure declarations, and procedure calls. It uses a modified assembly language syntax for the standard machine instructions. It also provides several high level language style control structures (if, while, repeat..until, etc.) that help you write much more readable code.
HLA is free and comes with source, Linux and Win32 versions available. On Win32 you need MASM and a 32-bit version of MS-link on Win32, on Linux you nee GAS, because HLA produces specified assembler code and uses that assembler for final assembling and linking.
TALC is another free MASM/Win32 based compiler (however it supports ELF output, does it?).
TAL stands for Typed Assembly Language. It extends traditional untyped assembly languages with typing annotations, memory management primitives, and a sound set of typing rules, to guarantee the memory safety, control flow safety,and type safety of TAL programs. Moreover, the typing constructs are expressive enough to encode most source language programming features including records and structures, arrays, higher-order and polymorphic functions, exceptions, abstract data types, subtyping, and modules. Just as importantly, TAL is flexible enough to admit many low-level compiler optimizations. Consequently, TAL is an ideal target platform for type-directed compilers that want to produce verifiably safe code for use in secure mobile code applications or extensible operating system kernels.
Free Pascal has an internal 32-bit assembler (based on NASM tables) and a switchable output that allows:
The MASM and TASM output are not as good debugged as the other two, but can be handy sometimes.
The assembler's look and feel are based on Turbo Pascal's internal BASM, and the IDE supports similar highlighting, and FPC can fully integrate with gcc (on C level, not C++).
Using a dummy RTL, one can even generate pure assembler programs.
Win32Forth is a free 32-bit ANS FORTH system that successfully runs under Win32s, Win95, Win/NT. It includes a free 32-bit assembler (either prefix or postfix syntax) integrated into the reflective FORTH language. Macro processing is done with the full power of the reflective language FORTH; however, the only supported input and output contexts is Win32For itself (no dumping of .obj file, but you could add that feature yourself, of course). Find it at ftp://ftp.forth.org/pub/Forth/Compilers/native/windows/Win32For/.
Terse is a programming tool that provides THE most compact assembler syntax for the x86 family! However, it is evil proprietary software. It is said that there was a project for a free clone somewhere, that was abandoned after worthless pretenses that the syntax would be owned by the original author. Thus, if you're looking for a nifty programming project related to assembly hacking, I invite you to develop a terse-syntax frontend to NASM, if you like that syntax.
As an interesting historic remark, on comp.compilers,
1999/07/11 19:36:51, the moderator wrote:
You may find more about them, together with the basics of x86 assembly programming, in the Raymond Moon's x86 assembly FAQ.
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.
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 does not fit; C, OCaml, perl, Scheme, might be a better choice in the most cases.
However, there are cases when these tools do not give fine enough control on the machine, and assembly is useful or needed. In these cases you'll appreciate a system of macroprocessing and metaprogramming that allows recurring patterns to be factored each into one indefinitely reusable definition, which allows safer programming, automatic propagation of pattern modification, etc. Plain assembler often is not enough, even when one is doing only small routines to link with C.
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:
CPP is truly not very expressive, but it's enough for easy things, it's standard, and called transparently by GCC.
As an example of its limitations, you can't declare objects so that destructors are automatically called at the end of the declaring block; you don't have diversions or scoping, etc.
CPP comes with any C compiler. However, considering how mediocre it is, stay away from it if by chance you can make it without C.
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.
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 are not writing million-lines assembly programs, are you?).
You can write your own simple macro-expansion filter with the usual tools: perl, awk, sed, etc. It can be made rather quickly, and you control everything. But, of course, power in macroprocessing implies "the hard way".
Instead of using an external filter that expands macros, one way to do things is to write programs that write part or all of other programs.
For instance, you could use a program outputting source code
Think about it!
Compilers like GCC, SML/NJ, Objective CAML, MIT-Scheme, CMUCL, etc, do have their own generic assembler backend, which you might choose to use, if you intend to generate code semi-automatically from the according languages, or from a language you hack: rather than write great assembly code, you may instead modify a compiler so that it dumps great assembly code!
There is a project, using the programming language Icon (with an experimental ML version), to build a basis for producing assembly-manipulating code. See around http://www.eecs.harvard.edu/~nr/toolkit/
The TUNES Project for a Free Reflective Computing System is developing its own assembler as an extension to the Scheme language, as part of its development process. It doesn't run at all yet, though help is welcome.
The assembler manipulates abstract syntax trees, so it could equally serve as the basis for a assembly syntax translator, a disassembler, a common assembler/compiler back-end, etc. Also, the full power of a real language, Scheme, make it unchallenged as for macroprocessing/metaprogramming.
This is the preferred way if you are developing mixed C-asm project. 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 result is in st(0), whole stack caller-saved. The SVR4 i386 ABI specs at http://www.caldera.com/developer/devspecs/ is a good reference point if you want more details.
Note that GCC has options to modify the calling conventions by reserving registers, having arguments in registers, not assuming the FPU, etc. Check the i386 .info pages.
Beware that you must then declare the cdecl or regparm(0) attribute for a function that will follow standard GCC calling conventions. See C Extensions::Extended Asm:: section from the GCC info pages. See also how Linux defines its asmlinkage macro...
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 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...
You can also override the implicit C->asm renaming by inserting statements like
Note that the objcopy 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.
Often you will be told that using C library (libc) is the only way, and direct system calls are bad. This is true. To some extent. In general, you must know that libc is not sacred, and in most 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 libc in assembly programming is more a question of taste/belief than something practical. Remember, Linux is aiming to be POSIX compliant, so does libc. This means that syntax of almost all libc "system calls" exactly matches syntax of real kernel system calls (and vice versa). Besides, GNU libc(glibc) 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. But.. main drawback of throwing libc away is that possibly you will need to implement several libc specific functions (that are not just syscall wrappers) on your own (printf() and Co.).. and you are ready for that, aren't you? :)
Here is summary of direct system calls pros and cons.
If you've pondered the above pros and cons, and still want to use direct syscalls, then here is some advice.
Result is returned in eax, with a negative result being an error, whose opposite is what libc would put into errno. The user-stack is not touched, so you needn't have a valid one when doing a syscall.
Linux Kernel Internals, and especially How System Calls Are Implemented on i386 Architecture? chapter will give you more robust overview.
As for the invocation arguments passed to a process upon startup, the general principle is that the stack originally contains the number of arguments argc, then the list of pointers that constitute *argv, then a null-terminated sequence of null-terminated variable=value strings for the environment. For more details, do examine Linux assembly resources, read the sources of C startup code from your libc (crt0.S or crt1.S), or those from the Linux kernel (exec.c and binfmt_*.c in linux/fs/).
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 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.
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?
Anyway, in all these cases, you'll be better when using GCC inline assembly with the macros from linux/asm/*.h than writing full assembly source files.
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), 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
Again, carefully read the source for the stuff contributed to the DOSEMU project, particularly these mini-emulators for running ELKS and/or simple .COM programs under Linux/i386.
Most DOS extenders come with some interface to DOS services. Read their docs about that, but often, they just simulate int 0x21 and such, so you do "as if" you are in real mode (I doubt they have more than stubs and extend things to work with 32-bit operands; they most likely will just reflect the interrupt into the real-mode or vm86 handler).
DJGPP comes with its own (limited) glibc derivative/subset/replacement, too.
It is possible to cross-compile from Linux to DOS, see the devel/msdos/ directory of your local FTP mirror for metalab.unc.edu; Also see the MOSS DOS-extender from the Flux project from the university of Utah.
Other documents and FAQs are more DOS-centered; we do not recommend DOS development.
Windows and Co. This document is not about Windows programming, you can find lots of documents about it everywhere.. The thing you should know is that Cygnus Solutions developed the cygwin32.dll library, for GNU programs to run on Win32 platform; thus, you can use GCC, GAS, all the GNU tools, and many other Unix applications.
Control is what attracts many OS developers to assembly, often is what leads to or stems from assembly hacking. Note that any system that allows self-development could be qualified an "OS", though it can run "on the top" of an underlying system (much like Linux over Mach or OpenGenera over Unix).
Hence, for easier debugging purpose, you might like to develop your "OS" first as a process running on top of Linux (despite the slowness), then use the Flux OS kit (which grants use of Linux and BSD drivers in your own OS) to make it stand-alone. When your OS is stable, it is time to write your own hardware drivers if you really love that.
This HOWTO will not cover topics such as bootloader code, getting into 32-bit mode, handling Interrupts, the basics about Intel protected mode or V86/R86 braindeadness, defining your object format and calling conventions.
The main place where to find reliable information about that all, is source code of existing OSes and bootloaders. Lots of pointers are on the following webpage: http://www.tunes.org/Review/OSes.html
Finally, if you still want to try this crazy idea and write something in assembly (if you've reached this section -- you're real assembly fan), here's what you need to start.
As you've read before, you can write for Linux in different ways; I'll show how to use direct kernel calls, since this is the fastest way to call kernel service; our code is not linked to any library, does not use ELF interpreter, it communicates with kernel directly.
I will show the same sample program in two assemblers, nasm and gas, thus showing Intel and AT&T syntax.
You may also want to read Introduction to UNIX assembly programming tutorial, it contains sample code for other UNIX-like OSes.
First of all you need assembler (compiler) -- nasm or gas.
Second, you need a linker -- ld, since assembler produces only object code. Almost all distributions have gas and ld, in the binutils package.
As for nasm, you may have to download and install binary packages for Linux and docs from the nasm site; note that several distributions (Stampede, Debian, SuSe, Mandrake) already have nasm, check first.
If you're going to dig in, you should also install include files for your OS, and if possible, kernel source.
Linux is 32-bit, runs in protected mode, has flat memory model, and uses the ELF format for binaries.
A program can be divided into sections: .text for your code (read-only), .data for your data (read-write), .bss for uninitialized data (read-write); there can actually be a few other standard sections, as well as some user-defined sections, but there's rare need to use them and they are out of our interest here. A program must have at least .text section.
Now we will write our first program. Here is sample code:
First step of building an executable is compiling (or assembling) object file from the source:
For nasm example:
For gas example:
This makes hello.o object file.
Second step is producing executable file itself from the object file by invoking linker:
This will finally build hello executable.
Hey, try to run it... Works? That's it. Pretty simple.
Your main resource for Linux/UNIX assembly programming material is:
Do visit it, and get plenty of pointers to assembly projects, tools, tutorials, documentation, guides, etc, concerning different UNIX operating systems and CPUs. Because it evolves quickly, I will no longer duplicate it here.
If you're are interested in Linux/UNIX assembly programming (or have questions, or are just curious) I especially invite you to join Linux assembly programming mailing list.
This is an open discussion of assembly programming under Linux, *BSD, BeOS, or any other UNIX/POSIX like OS; also it is not limited to x86 assembly (Alpha, Sparc, PPC and other hackers are welcome too!).
Mailing list address is <email@example.com>.
To subscribe send a messgage to <firstname.lastname@example.org> with the following line in the body of the message:
Detailed information and list archives are available at http://linuxassembly.org/list.html.
Here are frequently asked questions (with answers) about Linux assembly programming. Some of the questions (and the answers) were taken from the the linux-assembly mailing list.
An answer from Paul Furber:
There's an early version of the Assembly Language Debugger, which is designed to work with assembly code, and is portable enough to run on Linux and *BSD. It is already functional and should be the right choice, check it out!
You can also try gdb ;). Although it is source-level debugger, it can be used to debug pure assembly code, and with some trickery you can make gdb to do what you need (unfortunately, nasm '-g' switch does not generate proper debug info for gdb; this is nasm bug, I think). Here's an answer from Dmitry Bakhvalov:
An additional note from ???:
If you want to set breakpoints across your code, you can just use int 3 instruction as breakpoint (instead of entering address manually in gdb).
If you're using gas, you should consult gas and gdb related tutorials.
Definitely strace can help a lot (ktrace and kdump on FreeBSD), it is used to trace system calls and signals. Read its manual page (man strace) and strace --help output for details.
Short answer is -- noway. This is protected mode, use OS services instead. Again, you can't use int 0x10, int 0x13, etc. Fortunately almost everything can be implemented by means of system calls or library functions. In the worst case you may go through direct port access, or make a kernel patch to implement needed functionality, or use LRMI library to access BIOS functions.
Yes, indeed it is. While in general it is not a good idea (it hardly will speedup anything), there may be a need of such wizardy. The process of writing a module itself is not that hard -- a module must have some predefined global function, it may also need to call some external functions from the kernel. Examine kernel source code (that can be built as module) for details.
Meanwhile, here's an example of a minimum dumb kernel module (module.asm) (source is based on example by mammon_ from APJ #8):
The only thing this example does is reporting its actions. Modify kernel_version to match yours, and build module with:
Now you can play with it using insmod/rmmod/lsmod (root privilidged are required); a lot of fun, huh?
A laconic answer from H-Peter Recktenwald:
An extensive answer from Tiago Gasiba:
An answer from Patrick Mochel:
That's all for now, folks.
Each version includes a few fixes and minor corrections, that need not to be repeatedly mentioned every time.
I would like to thank all the people who have contributed ideas, answers, remarks, and moral support, and additionally the following persons, by order of appearance:
This version of the document is endorsed by Konstantin Boldyshev.
Modifications (including translations) must remove this appendix according to the license agreement.
$Id: Assembly-HOWTO.sgml,v 1.7 2002/08/17 08:35:59 konst Exp $
GNU Free Documentation License