PentOS

A hobby x86_64 operating system written in Rust, featuring a UEFI bootloader, an async kernel, a custom build system, and much more to come.

This is the 5th iteration of HeliumOS, hence the name.

Developer documentation: pentos.yarml.com

Table of Contents

• Dependencies
• Building
• Running in QEMU
• Creating Disk Images
• Building the Documentation
• Build Configuration
• Runtime Configuration
• Acknowledgements

Dependencies

Base Dependencies

The following tools must be installed manually before building:

• cargo / rustup
• nasm
• make

Optional Dependencies

Tool	Purpose
qemu-system-x86_64	Required for quickly running PentOS on a VM (make run-* & make debug).
gdb, tmux, & socat	Required for debugging the kernel (alongside Qemu) (make debug).
mtools, e2fsprogs, & sfdisk	Required to create raw .img disk images (make img-*).
VBoxManage	Required to convert images to .vmdk or .iso format (make vmdk-* / make iso-*).

Automatic dependencies

PentOS depends on other tools, but those not listed above are autmatically managed by the build system itself.

A comprehensive list of all build dependencies can be found in (Cargo.toml).

Additionally OVMF is downloaded and managed automatically by the build system the first time you run make.

Building

First-time setup

Run make once to bootstrap the build system and download OVMF:

make

Building the bootloader, kernel, & user binaries

# Debug build
make build-bootloader-debug
make build-kernel-debug
make build-userbin-debug

# Release build
make build-bootloader-release
make build-userbin-release

Output locations:

• Bootloader: target/uefi/{debug,release}/bootloader.efi
• Kernel: target/kernel/{debug,release}/kernel
• user binaries target/user/{debug,release}/*

Running in QEMU

These targets build everything automatically before launching QEMU.

# Run release build
make run

# Run debug build (no debugger attached)
make run-debug

# Run debug build with GDB + tmux (attaches GDB and a QEMU monitor)
make debug

The configuration Qemu uses for these commands can be found at the root Cargo.toml. The defaults are:

• 12 SMP Cores
• 8GB of RAM
• Resolution set to 1920x1080

Creating Disk Images

Flat

These will create normal directories containing the system

make flat-release   # run/release/flat
make flat-debug     # run/debug/flat

Raw .img

Requires mtools and parted.

make img-release   # run/release/pentos.img
make img-debug     # run/debug/pentos.img

VMDK (VirtualBox / VMware)

Requires VBoxManage. Depends on the .img targets above.

make vmdk-release  # run/release/pentos.vmdk
make vmdk-debug    # run/debug/pentos.vmdk

ISO

Requires VBoxManage. Also depends on the .img targets.

make iso-release   # run/release/pentos.iso
make iso-debug     # run/debug/pentos.iso

Building the Documentation

make doc

This generates rustdoc for the entire workspace under target/doc/. An index.html redirect to the pentos root crate is written automatically.

The hosted version of the developer documentation is available at pentos.yarml.com.

Build Configuration

The following settings in the root Cargo.toml under [workspace.metadata.chef] control build-time behaviour:

Key	Default	Description
ovmf-version	edk2-stable202511-r1	Version of the OVMF firmware to download.
ovmf-source-template	GitHub release URL	URL template used to fetch the OVMF tarball ($$ is replaced by the version string).
ovmf-varsfd-path-template	$$-bin/x64/vars.fd	Path inside the tarball for the VARS firmware volume ($$ is replaced by the version string).
ovmf-codefd-path-template	$$-bin/x64/code.fd	Path inside the tarball for the CODE firmware volume ($$ is replaced by the version string).
install-bootloader	/boot/efi/pentos.efi	Destination used by make install for the bootloader EFI binary.
install-kernel	/boot/efi/pentos.kernel	Destination used by make install for the kernel ELF binary.
qemu-numcores	12	Number of SMP cores used by Qemu.
qemu-mem	8G	Amount of memory allocated to Qemu VM.
qemu-resolution	1920x1080	Maximm resolution supported by the Qemu VM.

Runtime Configuration

Kernel behaviour is controlled by compile-time constants in the config crate (config/src/). The crate is organised into modules:

• config::topology — hardware topology limits (e.g. maximum number of supported CPUs, interrupt controllers, and stack sizes per hart).
• config::task — async executor limits (e.g. maximum number of concurrent urgent tasks).
• config::dev — device driver settings (e.g. PS/2 keyboard retry limits and event queue sizes).

As an example, config/src/topology/hart.rs exposes MAX_HART_COUNT (default 16), which determines the maximum number of CPU cores the OS will boot. The OS will panic at startup if more cores are detected than this constant allows.

Changing any constant in the config crate requires a rebuild of the kernel & bootloader.

Acknowledgements

The following libraries were not taken as direct Cargo dependencies, but code or design from them was copied and adapted into this repository:

• spin (MIT) - The lib/spinlocks crate is heavily inspired by and contains code adapted from spin. It provides SpinMutex, SpinRwLock, and SpinOnce primitives tailored to the kernel environment.
• futures (MIT/Apache-2.0) - The async stream in system/klib/src/task/stream.rs in addition to other system in the future probably

License

PentOS is licensed under the GNU General Public License v3.0 or later.