ZealOS: a modernized TempleOS fork written in HolyC

What ZealOS is, and who the fork is for

ZealOS is a 64-bit operating system forked from TempleOS, the system Terry Davis wrote. The repository's stated guiding principles are transparency, full user control, and adherence to public-domain or open-source implementations. That combination is unusual: most hobby kernels are GPL or MIT, while ZealOS carries the Unlicense, which places the work in the public domain.

The stated audience is narrow and explicit. The README says ZealOS strives to be simple, documented, and to require as little of a knowledge gap as possible, and adds that one person should be able to comprehend the entire system in at least a semi-detailed way within a few days of study. The prerequisites list a working knowledge of the C programming language. So this is not a system you install to browse the web. It is a system you read, modify, and rebuild, with the whole source tree in front of you.

The project also commits to a direction: simplify, don't complicate; make accessible, don't obfuscate. That sentence is the clearest statement of intent in the README, and it explains most of the changes listed against TempleOS.

HolyC to ZealC, and the changes that define the fork

The fork's changelog lists a set of concrete divergences from TempleOS. The language was renamed from HolyC to ZealC. The code was reformatted for readability, and comments and documentation were added. There is also system-wide renaming for clarity. None of these change what the system does at the kernel level, but together they change how approachable the source is, which is the whole point of the fork.

On the graphics side, the changelog claims 60 FPS and VBE graphics with variable resolutions. The README separately lists 32-bit color VBE graphics as a feature in development, pointing at HolyGL, and shows a screenshot captioned 32-bit color. There is a tension here worth naming: the changelog speaks in the past tense about VBE graphics while the feature list still places 32-bit color in development. Anyone planning to build on the graphics layer should check the current state in the source rather than trusting either list.

The rest of the roadmap is honest about being unfinished. The README lists fully functional AHCI support, network card drivers and a networking stack, and UEFI booting as features in development. UEFI booting is described as coming via the BSD2-licensed Limine bootloader and a Public Domain ZealBooter prekernel, and the repository layout confirms both: there is a zealbooter/ directory at the top level alongside src/ and build/.

Building an ISO and booting ZealOS in a VM

There is no package to install and no apt or brew formula. The README's getting started section says to run the build-iso script to create a Distro ISO, and it points at the wiki guide for the details. The repository layout shows a build/ directory at the top level, which is where that script lives.

Before any of that, the prerequisites matter. For running in a VM you need Intel VT-x or AMD-V acceleration enabled in your BIOS settings, because virtualizing a 64-bit operating system properly requires it. On Windows, Hyper-V must be enabled.

git clone https://github.com/Zeal-Operating-System/ZealOS.git
cd ZealOS
./build/build-iso

The exact invocation is not spelled out in the README beyond the name build-iso, so treat the path above as the location rather than a guaranteed command line, and follow the wiki page on building an ISO for the authoritative steps. What you should end up with is a bootable ISO image.

From there, the README links three separate wiki guides: installing in VirtualBox, installing in VMWare, and installing on bare metal. It does not inline those steps. If you are only evaluating the system, a VM is the sane path, and it keeps the ISO off your real disk.

Contributing has two documented routes. The first is the intended one: build the latest ISO, install to a VM, make your changes inside the OS, power off, then run the sync script to merge your changes back into the repository. The second is to edit repository files with an external editor outside the OS. Either way, pull requests go to the master branch.

Where ZealOS stops being the right tool

The feature list is the limitation. Networking is listed as in development, not finished, so anything that assumes a network stack is a bet on unfinished work. AHCI support is likewise listed as in development. UEFI booting is in development too, which means the boot path you get depends on where that work stands at the moment you build.

The knowledge requirement is the second constraint, and it is not a soft one. The prerequisites ask for working knowledge of C, and the system is written in ZealC, a renamed dialect of HolyC. Reading the source means learning that dialect. The README's claim that one person can comprehend the whole system in a few days assumes a reader who already programs in C and is willing to spend those days.

Third, this is a single-person-comprehensible system by design, and that design goal cuts against the things large operating systems accumulate: driver breadth, hardware certification, security review, long-term support guarantees. If you need a machine that runs commercial software, ZealOS is the wrong tool and the README does not pretend otherwise.

How ZealOS relates to TempleOS and ZenithOS

The lineage in the README is specific. Around November 2019, VoidNV forked ZenithOS from TempleOS. That repository was removed in August 2020 and later reuploaded under the ZenithOS organization. In July 2021, ZealOS was forked from ZenithOS.

So the real alternative for most readers is not another modern OS but TempleOS itself, or the archived ZenithOS tree. The difference in approach is the point. TempleOS is the original work by Terry Davis, and the README frames ZealOS's contribution as modernization on top of it: reformatted code, added comments and documentation, system-wide renaming, 60 FPS, VBE graphics with variable resolutions, and the HolyC to ZealC rename. If you want the unmodified original, ZealOS is not it. If you want the same architecture with readability work applied, that is exactly what this fork claims to offer.

The repository also links to archived copies of pre-git ZenithOS releases on mega.nz and to archive.org captures of the removed repository. Those links are the practical route for anyone who wants to compare the three generations directly.

Licence and the cost of keeping up

ZealOS is released under the Unlicense, which is a public-domain dedication rather than a permissive licence with conditions. That has a practical consequence for anyone embedding or redistributing it: there is no attribution requirement and no copyleft. Note that the UEFI path pulls in the Limine bootloader, which the README describes as BSD2-licensed, so the boot chain is not uniformly public domain. If you redistribute a UEFI-capable image, check the terms that apply to Limine separately. This is a description of what the repository states, not legal advice.

Upgrade cost is unusual for a project like this. The last push to the repository was on 2026-09-14, and the most recent release, labelled Pre-Built ISOs, is dated 2025-11-10. There is no versioned release train, no changelog feed you subscribe to, and no dependency graph to update. Upgrading means rebuilding from master and reading the changelog page the README links to. The sync script exists precisely because the intended workflow keeps your changes inside the OS rather than in a branch you merge.

The maintenance burden therefore falls on you, not on a release cadence. If you modify the system in a VM, your changes live in that VM until you sync them out, and merging upstream work afterward is a manual reconciliation.

Editorial conclusion

ZealOS is for programmers who know C and want a small, comprehensible 64-bit system they can read end to end, and for people curious about the TempleOS lineage. It is not for anyone who needs a browser, a package manager or a supported driver set. Before adopting it, verify three things: that your CPU exposes VT-x or AMD-V with Hyper-V enabled on Windows, that the build-iso script produces an ISO on your host, and that you can reach the wiki page for the hypervisor you intend to use, because the README points there instead of documenting installation itself.

Frequently asked questions

Official sources

1. License: Unlicense
2. Project website
3. README
4. Releases
5. Zeal-Operating-System/ZealOS on GitHub