002 SOS: A Simple Operating System
005 This is SOS, a Simple Operating System for i386-family
006 processors. This is as simple as possible to show a way to program a
007 basic Operating System on real common hardware (PC). The code should
008 be easily readable and understandable thanks to frequent comments, and
009 references to external documentation. We chose to implement the basic
010 features of an OS, thus making design decisions targetting towards
011 simplicity of understanding, covering most of the OS classical
012 concepts, but not aiming at proposing yet another full-fledged
013 competitive OS (Linux is quite good at it). However, for those who
014 would like to propose some enhancements, we are open to any code
015 suggestions (patches only, please). And yes, there might be bugs in
016 the code, so please send us any bug report, and/or patches !
018 The OS comes as a set of articles (in french) to be published in the
019 journal "Linux Magazine France". Each month, the part of the code
020 related to the current article's theme is released (see VERSION file),
021 and the resulting OS can be successfully compiled and run, by booting
022 it from a floppy on a real machine (tested AMD k7, Cyrix and Intel P4
023 pentiums), or through an x86 emulator (bochs or qemu). The resulting
024 OS is available as a multiboot compliant ELF kernel (sos.elf) and as a
025 floppy image (fd.img). It provides a very very very basic demo whose
026 aim is to understand how everything works, not to animate sprites on
027 the screen with 5:1 dolby sound.
029 The initial technical features and lack-of-features of the OS are:
030 - monolithic kernel, fully interruptible, non-preemptible (big kernel
031 lock), target machines = i386 PC or better
032 - compiles on any host where the gcc/binutils toolchain (target
033 i586-gnu) is available. Can be tested on real i486/pentium
034 hardware, or on any host that can run an i486/pentium PC emulator
035 (bochs or qemu)
036 - kernel loaded by grub or by a sample bootsector
037 - clear separation of physical memory and virtual memory concepts,
038 even inside the kernel: no identity-mapping of the physical memory
039 inside the kernel (allows to move virtual mappings of kernel pages
040 at run-time, eg to free ISA DMA pages, and to avercome the 4G RAM
042 - slab-type kernel memory allocation
043 - no swap, no reverse mapping
044 - VERY simple drivers: keyboard, x86 video memory, IDE disks
045 - logical devices: partitions, FAT filesystem, "hard-coded"
046 mountpoints only (~ MSDOS)
047 - no network stack
048 - user-level features: ELF loader (no shared libraries), processes,
049 user threads (kernel-level scheduling only), mmap API, basic VFS
051 To understand where to look at for what, here is a brief description:
052 - Makefile: the (ONLY) makefile of the OS. Targets are basically
053 'all' and 'clean'
054 - bootstrap/ directory: code to load the kernel. Both the stuff
055 needed for a multiboot-compliant loader (eg grub) AND a bootsector
056 are provided.
057 - sos/ directory: the entry routine for the kernel (main.c), various
058 systemwide header files, a set of common useful C routines
059 ("nano-klibc"), and kernel subsystems (kernel memory management,
061 - hwcore/ directory: Low-level CPU- and kernel-related routines
062 (interrupt/exception management, translation tables and segment
063 registers, ...)
064 - drivers/ directory: basic kernel drivers for various (non CPU)
065 devices (keyboard, x86 video memory, bochs 0xe9 port, ...). Used
066 mainly for debugging
067 - support/ directory: scripts and configuration files to build the
068 floppy images
069 - extra/ directory: a set of configuration files to be customized for
070 non-x86 host installations (yes, we primarily develop SOS on a ppc, for
071 the x86 target of course), or for grub-less installations. See
072 README file in this directory.
074 The code is licensed under the terms of the GNU GPL version 2 (see
075 LICENSE file).
077 Enjoy !
079 David Decotigny, Thomas Petazzoni, the Kos team
087 David Decotigny
089 PS: Made with a Mac.