SimpleOS

SimpleOS is a from-scratch, educational bare-metal operating system for the Raspberry Pi family, built using Assembly, C, and (eventually) Rust. The goal is to explore low-level systems programming while creating a minimal, efficient kernel that runs directly on the hardware — no Linux, no firmware blobs, just the metal and myself.

The repository is structured to support both 32-bit and 64-bit architectures in a single codebase where possible:

SimpleOS/
├── 32bit/          # Raspberry Pi 1, Zero, 2, 3 (ARMv7/AArch32)
├── 64bit/          # Raspberry Pi 4, 5 (AArch64)
├── common/         # Shared code (planned)
└── README.md

The user-facing API and high-level design will be virtually identical across both branches, with architecture-specific differences isolated to boot code, UART drivers, and low-level initialisation.

Development Status

We are actively developing the 64-bit version for Raspberry Pi 4 & 5. The kernel currently:

• Boots in AArch64 EL1
• Initialises the PL011 UART for early console output
• Provides a simple echoing terminal
• Parks secondary cores safely
• Builds with a clean, freestanding toolchain

We are actively developing the 32-bit version for Raspberry Pi 1, 2b & 3. The kernel currently:

• Boots in AArch32 EL1
• Initialises the PL011 UART for early console output
• Provides a simple echoing terminal
• Parks secondary cores safely
• Builds with a clean, freestanding toolchain

Roadmap

Phase 1 – Core Boot & Console (Current Focus)

• Multi-core safe boot (core 0 runs, others parked)
• Reliable early UART console (PL011 for QEMU reliability, conditional GPIO mux for real hardware)
• Basic puts / putc / getc with echo loop
• stdio and stdlib building for more diverse terminal output.
• Robust exception vector table (synchronous, IRQ, FIQ, SError)
• Hex terminal colors
• System timer interrupts

Phase 2 – Memory & Scheduling

• Physical memory manager (frame allocator)
• Virtual memory with page tables
• Simple round-robin scheduler
• Heap allocation (custom allocator)

Phase 3 – Multi-Core & Concurrency

• Wake secondary cores
• Per-core stacks and data
• Spinlocks and basic synchronisation

Phase 4 – Rust Integration

• no_std Rust crate for higher-level kernel code
• Mixed C/Rust build system
• Safe abstractions over hardware (drivers in Rust)

Phase 5 – Drivers & Features

• Framebuffer/graphics output
• GPIO & basic input
• SD card / filesystem
• USB & keyboard input

Long-Term

• Simple shell
• User-mode processes
• Networking

Prerequisites

To build and test SimpleOS, you will need:

• Toolchain:
  • For 64-bit: aarch64-none-elf-gcc (ARM GNU Toolchain, Apple Silicon build recommended on macOS)
  • For 32-bit: arm-none-eabi-gcc
• QEMU: qemu-system-aarch64 for 64bit or qemu-system-arm for 32bit (QEMU version 8.0 or newer recommended)
• Make: I provide an included makefile which simplifies the kenel compilation process if you wish to compile from source.
• macOS/Linux recommended (Windows via WSL works)

On macOS with Homebrew:

brew install qemu

Then download and install the ARM GNU Toolchain from developer.arm.com

Building & Running

Inside the 64bit directory

make clean
make
make run          # Uses -nographic -serial stdio for direct terminal output

You should see:

Hello, World!

followed by an interactive echo loop. To quit QEMU: Ctrl+A then X (or Ctrl+C in some configurations).

Real Hardware

Build with GPIO pin muxing enabled:

make real

Copy kernel8.img to an SD card (replace the stock one on a fresh Raspberry Pi OS image, or use a minimal boot setup). On Pi 4/5, disable Bluetooth if using PL011 on GPIO 14/15: Add to config.txt:

dtoverlay=disable-bt

Boot the Pi — output appears on the serial console (3.3V TTL USB adapter on GPIO 14/15).

Contributing

Feel free to open issues, suggest features, or submit pull requests. This is an educational project, so clean, well-commented code and explanations are highly valued.

License

MIT License — see LICENSE for details.