SOS: A Simple Operating System
This is SOS, a Simple Operating System for i386-family
processors. This is as simple as possible to show a way to program a
basic Operating System on real common hardware (PC). The code should
be easily readable and understandable thanks to frequent comments, and
references to external documentation. We chose to implement the basic
features of an OS, thus making design decisions targetting towards
simplicity of understanding, covering most of the OS classical
concepts, but not aiming at proposing yet another full-fledged
competitive OS (Linux is quite good at it). However, for those who
would like to propose some enhancements, we are open to any code
suggestions (patches only, please). And yes, there might be bugs in
the 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 the
journal "Linux Magazine France". Each month, the part of the code
related to the current article's theme is released (see VERSION file),
and the resulting OS can be successfully compiled and run, by booting
it from a floppy on a real machine (tested AMD k7, Cyrix and Intel P4
pentiums), or through an x86 emulator (bochs or qemu). The resulting
OS is available as a multiboot compliant ELF kernel (sos.elf) and as a
floppy image (fd.img). It provides a very very very basic demo whose
aim is to understand how everything works, not to animate sprites on
the 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 VFS
To 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 (see
LICENSE 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 Decotigny
PS: Made with a Mac.
