ctOS is a bit 32 bit Unix-like operating system that I developed as a toy project
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ctOS - a toy 32-bit operating system

What is ctOS

This repository contains the source code for a toy operating sytem called ctOS that I created many years ago (most of the work was actually done in 2011 and 2012, judging from timestamps of old backups that I found). As I have not been able to find the time for a couple of years now to actually continue working on it, and also have other interests now, I decided to publish the code "as-is" on GitHub, hoping that it might be useful for other OS hobbyist developers (and if it be only to learn from my mistakes).

So I invested some time to dig out the old source code and find my way through the build system. Back in 2012, the build was done on a 32 bit system and I mostly used the QEMU emulator in version somewhere around v1.0. So I had to migrate the build scripts to create 32 bit code on a 64 bit host (easy) and to adapt the code so that the kernel would run on a recent version like 2.5 of QEMU (not so easy, as QEMU has changed a lot in the meantime, unveiling some errors in my code that I had to fix). Once all the tests turned to green again, I spent the last couple of weeks adding some additional functionality, like the ability to boot on UEFI only systems and quite a few additional C library functions.


Currently, the following features have been implemented:

  • Boot using GRUBs multiboot configuration (has support for multiboot and multiboot2)
  • Support for BIOS and (U)EFI based systems
  • Basic ACPI support
  • Support for SMP with up to eight CPUs
  • Virtual memory and multi-tasking
  • Built-in kernel debugger to inspect CPU state, memory contents, threads, kernel configuration etc. (launch with F1)
  • 32 bit protected mode (64 bit not supported and unlikely that I will ever find the time to port it)
  • Virtual memory and multi-tasking
  • Kernel threads and threaded interrupt handling (queues)
  • POSIX Signal handling
  • Job control a la POSIX
  • Pipes
  • RAM disk
  • Device drivers:
    • VGA (using VBE)
    • Keyboard
    • RTC
    • Ethernet adapter (RTL8139)
    • IDE hard drive
    • AHCI hard drive
    • PCI bus
  • Support for ext2 file system
  • Interrupt routing via PIC, APIC and MSI
  • A full networking stack, supporting ARP, ICMP, IP, UDP, TCP and DNS resolution
  • A simple user-space command line utility
  • A basic POSIX compatible C library
  • A small floating point math library

I did also port a few userspace tools (dash, wget, elvis, most of the coreutils, a BASIC interpreter, ...) to work with ctOS. For licensing reasons, the required patches and build scripts are not contained in this repository but in a separate repository - please have a look at the respective documentation on how to build the ports. The screenshot below, by the way, shows elvis running in ctOS on an QEMU emulator and editing main.c of the kernel code.

ctOS Elvis


I have tested ctOS with three emulators (QEMU, VirtualBox and Bochs) and also did a few tests on three different real machines. The easiest way to run ctOS is to use the emulator QEMU. Please make sure that you get a comparatively recent version of QEMU installed (Ubuntu 16.04 for instance comes with QEMU 2.5 which works fine).

The binary distribution (see the release page) comes as a GZIPPED TAR files. To install and run, simply enter

gzip -d ctOS.bin.tar.gz
tar xvf ctOS.bin.tar

This should bring up a QEMU window with a GRUB menu inside. Select the first option (ctOS from ramdisk) and hit enter. You should then see the first kernel messages from ctOS, telling you that a few subsystems have been initialized, a root partition has been mounted and the init process has been started. If everything works fine, you should see the prompt


from the user space command line interface. Entering 'help' gives you a list of available commands and you can start to play around.

Additional run targets and tests

The run script that comes with ctOS supports different run targets which are combinations of emulators and settings. Currently, run targets for QEMU, VirtualBox and Bochs are included, covering different configurations (attached drives, CPUs, networking). You can get a full list of supported targets by running

./bin/run.sh help

The file TESTING.md contains a few advanced scenarios that you can try out.


Instructions to build ctOS from the source can be found in the file BUILD.md