X1 - A minimalist educational operating system
==============================================
X1 is a very small operating system meant to introduce students to low-level
system programming. As a result, it focuses on clarity of explanation. It's
not meant to demonstrate state-of-the-art methods and algorithms, but rather
simple, naive ones that do the job, while providing pointers and references
to the more modern ways.
X1 is a single address space operating system, always running with the
highest privileges. There is no userspace, and no system calls. Despite
that, the operating system is also called the kernel.
Building
--------
X1 expects a Unix-like environment, including make and a GCC compilation
toolchain. It's been tested on a few Linux distributions such as Debian
and Arch.
Building the kernel is done using the make command from the source root
directory. The end result is a statically linked ELF [1] file named x1.
On Debian, the following packages are required :
- build-essential
Meta-package pulling the gcc and make packages, among others.
- gcc-multilib
Multilib support for GCC, which provides the 32-bits static libgcc
library required to link the kernel on 64-bits machines.
Examining the kernel binary
---------------------------
The kernel can be examined with standard GNU binutils tools [2]. Here are
some common command line examples to obtain information directly from the
kernel binary :
- objdump -d x1 | less
Disassemble the kernel machine code.
- readelf -aW x1 | less
Display ELF information, such as headers and sections.
- addr2line -e x1 0x1003f0
Convert an address into a source location.
- nm x1
List symbol names.
Running
-------
X1 targets the x86 32-bits architecture only (i386) [3], and ignores some
advanced features such as virtual memory and SMP. It is compliant with the
original multiboot specification [4] and GRUB is the recommended boot loader.
It only supports legacy BIOS systems (no EFI/UEFI).
A simple way to run the kernel is to use the qemu.sh shell script, which
relies on the QEMU -kernel option to act as a multiboot boot loader.
On Debian, the following packages are required :
- qemu
The well known machine emulator.
When QEMU is running, you may enter (and leave) its monitor prompt using
the Ctrl-a c key sequence. The most useful monitor commands are :
- info registers
Print the content of all core registers.
- info pic
Print the state of the legacy PIC interrupt controller.
If you're interested in the details, the following commands may be of interest :
- info mtree
Print the various emulated address spaces.
- info qtree
Print the emulated device tree.
Getting started
---------------
Even a simple project like X1 requires broad knowledge, spanning from a basic
understanding of processors, memory, assembly and compilation toolchains. The
OSDev website [5] provides a good starting point to find information of decent
quality. Beyond that, readers are encouraged to refer to actual reference
documentation, and of course, search engines.
Here is a non-exhaustive list of topics readers should hopefully develop their
understanding of with this project, and are encouraged to briefly learn about
even before starting playing with X1 :
- Processor architecture, including core registers, machine instructions
and accessing memory through loads and stores.
- Control of the link step through a linker script.
- The concept of an executable format, including partitioning the content into
sections, such as code and data sections. See ELF [1].
- The C programming language, including common extensions, such as controlling
alignment. The source code conforms to C99 [6] with some GNU extensions.
The two files reader should use as their entry points are :
- src/boot_asm.S
The assembly source file containing the very first instructions.
- src/kernel.lds
The linker script used to control the link step.
Sources organization
--------------------
The project sources are split into three directories :
- include
This directory may only contain standard headers, normally provided
by the "implementation" (e.g. the compiler and the C library) in a
hosted environment. Because this is a kernel, the environment is
free standing instead, so any additional standard service must be
added manually. See ISO/IEC 9899:1999 5.1.2.1 "Freestanding environment"
in the C99 specification for all the details.
- lib
This directory contains external code used as a library by the kernel.
These files are copied from the librbraun library [7] and are meant to
provide a tiny and easily embedded "development kit".
- src
This directory contains the actual kernel code.
The coding style used is borrowed from X1's big brother X15 [8].
References
----------
[1] ELF: http://refspecs.linuxbase.org/elf/elf.pdf
[2] GNU binutils : http://sourceware.org/binutils/docs-2.29/
[3] Intel combined manuals : https://software.intel.com/en-us/articles/intel-sdm
[4] Multiboot specification : https://www.gnu.org/software/grub/manual/multiboot/multiboot.html
[5] OSDev website : http://wiki.osdev.org/
[6] C99 specification : http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
[7] https://www.sceen.net/librbraun/
[8] https://www.sceen.net/~rbraun/x15/doc/style.9.html