This is an experimental hobby operating system intended for study of low level languages such as Assembly, C and C++, OS architecture, CPU inner workings and many other things.
This project is also meant to be developed from ground-up. That is, it does not depend on any libraries or software beyond what is absolutely required (such as a compiler). This means writing custom tooling, custom language parsers, bootloader and bootmanager.
As of right now, ZeroK only performs Bootloader chores. Much emphasis has been put on not creating incompatible partition formats or file formats, which is why plenty of time has been spent designing a boot process that respects as many standards as possible. Furthermore, ZeroK aims to be as flexible as possible, and can be installed on any sectored device. Doing this required meticulous designing of the boot process.
The boot process for the OS looks like this:
- Stage 1: Stored in the FAT partition boot record (512 bytes in size). It uses BIOS interrupts to look for and load the next stage sectors. Written in 16-bit assembly code.
- Stage 2: Stored in the reserved FAT sectors, loads and interprets the FAT filesystem looking for the next stage binary file stored regularly on the disk. Written in 16-bit assembly.
- Stage 3: Stored in the disk. Locates an ELF executable file also stored in the FAT filesystem, sets up paging structures, switches the processor to 32 bit mode, then executes the file. Written in both 16-bit and 32-bit assembly.
- Stage 4: Stored in the disk and written completely in C with just a small assembly stub. Responsible for all the bootloader chores, such as identifying hardware, creating memory structures, loading drivers, etc.
The next steps are improving hardware detection in order to load driver files in memory. Also, a memory allocator should definitely be present in these operations to aid flexibility.
Testing of ZeroK can be done on any Virtual Machine or x86 emulator program. A few tools have been tested and proven quite useful:
- VMware, VirtualBox: Serve as a general-purpose testing ground. They don't include simple built-in debugging, but do allow for serial port debugging.
- 86Box, PCem: Can emulate really old hardware combinations. Only allow serial debugging.
- Bochs: Complete x86 emulator and step-by-step machine code debugger.
- YAT + Com0Com: Allow sending and receiving messages through virtual serial ports. Very useful when the video system isn't working properly.
Writing an Operating System from scratch is hard. It requires very specific knowledge of very specific hardware whose documentation isn't always easy to find. It also requires very specific tooling, which is why ZeroK encompasses the creation of the following custom tools and software:
A very simplistic boot manager written purely in Pasme assembly. It can be installed on basically anything that can run x86 code and runs on CPUs even as old as the 8086. Its main purpose is booting ZeroK from whatever block device and whatever partition it gets installed on, be it a primary partition on a hard drive or a logical partition on a floppy disk.
XtBootMgr's source code can be found in the root of the repo under /XtBootMgr.
This is an expanded Assembly Language with a few but much needed quality-of-life improvements. Pasme is used by the whole project. The transpiler source code sub-project can be found under /Tools/Pasme.
A few Pasme features:
- Automatic strings allow you to write messages and other string contents directly in the code without worrying about their names or positioning.
- Automatic variables create storage locations in memory with type-based sizing.
- Stack magement helps you store local variables as well as read function arguments by just declaring them. The transpiler calculates the base pointer offset for you.
- The Preprocessor creates a level above the NASM preprocessor, and allows multi-level macros and includes for complex builds.
This is a CLI tool also written in Java. Most of its functionality relates to reading and burning virtual disks in order to test ZeroK.
DevTK's source code can be found in this repo under /Tools/DevToolkit
DevTK's features:
- Automatic synchronization of local folders and virtual disk folders.
- Mounting and unmounting of virtual disks (.vhd files)
- Reading MBR partition tables from virtual disks and listing them.
- Writing bootloaders to any numbered sector or specific partition.
- Writing code to VBRs and their reserved sectors for installing OSes.
ZeroK requires a custom-built GCC targeting the i386-elf platform for its C code. Build instructions can be found under /Tools/