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5.1 Linux

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::

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 assembly.

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.

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 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.

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:

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.

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), 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 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

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 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 has developped the cygwin32.dll library, for GNU programs to run on MacroShit platforms. Thus, you can use GCC, GAS, all the GNU tools, and many other Unix applications. Have a look around their homepage. I (Faré) don't intend to expand on Losedoze programming, but I'm sure you can find lots of documents about it everywhere...

5.4 Yer very own OS

Control being what attract many programmers to assembly, want of OS development is often what leads to or stems from assembly hacking. Note that any system that allows self-development could be qualified an "OS" even though it might run "on top" of an underlying system that multitasking or I/O (much like Linux over Mach or OpenGenera over Unix), etc. Hence, for easier debugging purpose, you might like to develop your ``OS'' first as a process running on top of Linux (despite the slowness), then use the Flux OS kit (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:

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