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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  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 . 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) , and ignores some advanced features such as virtual memory and SMP. It is compliant with the original multiboot specification  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  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 . - The C programming language, including common extensions, such as controlling alignment. The source code conforms to C99  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 220.127.116.11 "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  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 . References ----------  ELF: http://refspecs.linuxbase.org/elf/elf.pdf  GNU binutils : http://sourceware.org/binutils/docs-2.29/  Intel combined manuals : https://software.intel.com/en-us/articles/intel-sdm  Multiboot specification : https://www.gnu.org/software/grub/manual/multiboot/multiboot.html  OSDev website : http://wiki.osdev.org/  C99 specification : http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf  https://www.sceen.net/librbraun/  https://www.sceen.net/~rbraun/x15/doc/style.9.html
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