kosmas12 / SkatzohirOS_x64

A hobby operating system for 64-bit EFI computers.

SkatzohirOS_x64

Welcome to the repository of SkatzohirOS_x64!

What is this?

SkatzohirOS_x64 is a version of SkatzohirOS that is (at least currently) designed to run on PCs that are based on the x86_64 architecture. It currently only supports running on systems that use EFI firmwares.

Licensing

All code under the bootloader and kernel directories is licensed under MIT. Please see LICENSE for details about the MIT license.

The bootloader also uses POSIX-UEFI, which is also licensed under the MIT license.

Code under elfonashelf is licensed under... well, nothing. It was built purely off of common, open knowledge available on the internet, and you can't really put a license on that. So, if you don't feel like making your own ELF library, feel free to use ELF on a shelf with no terms at all.

How to compile and use

SkatzohirOS_x64 currently only supports booting on EFI systems. This means that a computer with a regular Legacy BIOS will not run it. It will also not boot with CSM on an EFI computer. This is because the bootloader is only available as an EFI app, which cannot be booted by a BIOS computer or through CSM. If your computer is set to use only CSM for booting, please set it to only UEFI or Auto mode to run SkatzohirOS_x64.

The requirements for building are:

• clang for creating the object files
• lld to link the object files into an executable
• GNU mtools and dd to create a hard disk image
• GNU make for the build system

NOTE: The Makefile uses Clang as its C compiler. However, you can use any cross-compiler that supports a bare metal x86_64 ELF target. Just make sure to change the value of the CC variable in the Makefile for the kernel and the flags if required.

Building is simple: Just run the build.sh script. If all goes well, this will create the build directory with 3 files: kernel.elf, bootx64.efi and SkatzohirOS_x64.img. SkatzohirOS_x64.img is the hard drive image. You can use it on an emulator like QEMU or burn/flash/write it to physical media to boot on a real computer.

To remove all compiled files, run clean.sh.

The provided CMakeLists.txt file is only used for compatibility with the assistance tools of some IDEs and can not actually be used for building SkatzohirOS_x64 binaries or images.

Layout and technical details

bootloader contains the code for the bootloader. Specifically, it contains the main bootloader code in bootloader.c and the uefi subdirectory contains the UEFI library that I used, POSIX-UEFI. Note that it is mostly only used for easier UEFI builds. The libc APIs that it provides are not used much. Most of the work is done with direct UEFI calls. However, in certain places, printf is used for printing some debugging data and the types and global variables provided by POSIX-UEFI are used all across the code. Porting it to a different UEFI library should not be difficult, however.

elfonashelf contains ELF on a shelf, a simple library that provides some basic data structures and defines to work with ELF executables in a freestanding environment, where such a library can't be expected to exist. Its usage currently is the exact same as the elf.h header that you can find on a lot of POSIX systems. However, a lot of features are not yet implemented. The only things that are defined currently are ELF64 program and file headers. This is prone to change as the OS needs more advanced things.

kernel contains the kernel code. Not much to say here.