SOS: A Simple Operating SystemThis   is   SOS,   a   Simple   Operating   System   for   i386-familyprocessors. This is  as simple as possible to show a  way to program abasic Operating System on real  common hardware (PC).  The code shouldbe easily readable and understandable thanks to frequent comments, andreferences to external documentation.  We chose to implement the basicfeatures  of an OS,  thus making  design decisions  targetting towardssimplicity  of  understanding,  covering  most  of  the  OS  classicalconcepts,  but  not  aiming  at  proposing  yet  another  full-fledgedcompetitive OS  (Linux is  quite good at  it). However, for  those whowould  like to  propose some  enhancements, we  are open  to  any codesuggestions (patches only,  please).  And yes, there might  be bugs inthe code, so please send us any bug report, and/or patches !The OS comes as  a set of articles (in french) to  be published in thejournal  "Linux Magazine  France". Each  month, the  part of  the coderelated to the current article's theme is released (see VERSION file),and the resulting OS can  be successfully compiled and run, by bootingit from a floppy on a real  machine (tested AMD k7, Cyrix and Intel P4pentiums), or through an x86  emulator (bochs or qemu).  The resultingOS is available as a multiboot compliant ELF kernel (sos.elf) and as afloppy image (fd.img).  It provides  a very very very basic demo whoseaim is to  understand how everything works, not  to animate sprites onthe screen with 5:1 dolby sound.The initial technical features and lack-of-features of the OS are: - monolithic kernel, fully interruptible, non-preemptible (big kernel   lock), target machines = i386 PC or better - compiles on any host where the gcc/binutils toolchain (target   i586-gnu) is available. Can be tested on real i486/pentium   hardware, or on any host that can run an i486/pentium PC emulator   (bochs or qemu) - kernel loaded by grub, or by a sample bootsector (up to article 2   ONLY) - clear separation of physical memory and virtual memory concepts,   even inside the kernel: no identity-mapping of the physical memory   inside the kernel (allows to move virtual mappings of kernel pages   at run-time, eg to free ISA DMA pages, and to avercome the 4G RAM   barrier) - slab-type kernel memory allocation - no swap, no reverse mapping - VERY simple drivers: keyboard, x86 video memory, IDE disks - logical devices: partitions, FAT filesystem, "hard-coded"   mountpoints only (~ MSDOS) - no network stack - user-level features: ELF loader (no shared libraries), processes,   user threads (kernel-level scheduling only), mmap API, basic VFSTo understand where to look at for what, here is a brief description: - Makefile: the (ONLY) makefile of the OS. Targets are basically   'all' and 'clean' - bootstrap/ directory: code to load the kernel. Both the stuff   needed for a multiboot-compliant loader (eg grub) AND a bootsector   are provided. The bootsector may only be used up to article 2. - sos/ directory: the entry routine for the kernel (main.c), various   systemwide header files, a set of common useful C routines   ("nano-klibc"), and kernel subsystems (kernel memory management,   etc...) - hwcore/ directory: Low-level CPU- and kernel-related routines   (interrupt/exception management, translation tables and segment   registers, ...) - drivers/ directory: basic kernel drivers for various (non CPU)   devices (keyboard, x86 video memory, bochs 0xe9 port, ...). Used   mainly for debugging - support/ directory: scripts and configuration files to build the   floppy images - extra/ directory: a set of configuration files to be customized for   non-x86 host installations (yes, we primarily develop SOS on a ppc, for   the x86 target of course), or for grub-less installations. See   README file in this directory.The code is licensed under the terms of the GNU GPL version 2 (seeLICENSE file).Enjoy !                        David Decotigny, Thomas Petazzoni, the Kos team                                                   http://sos.enix.org/                                        http://david.decotigny.free.fr/                                           http://kos.enix.org/~thomas/                                                   http://kos.enix.org/--David DecotignyPS: Made with a Mac.