Morph-RT

A custom RTOS kernel built from scratch for ARM Cortex-M4.

What is this?

I'm building a real-time operating system kernel to understand how these systems work under the hood. Starting with the basics: task scheduling, memory management, and inter-process communication.

Current status: Task implementation complete, working on task scheduling.

Structure

na-rtos/
├── kernel/
│   ├── inc/           # Public headers
│   └── src/           # Implementation
├── port/arm-cortex-m4/ # Hardware-specific code
├── examples/          # Demo applications
└── tests/             # Unit tests

Building

# Native build (for testing)
make

# ARM build (for hardware)
make TARGET=stm32f4

# Run tests
make test

Design decisions

Memory management: Static allocation only. The kernel doesn't do dynamic allocation - that's the application's job.

Error handling: Functions return status codes, use output parameters for data. No exceptions or global error state.

Circular buffer optimization: Using bit masking instead of modulo for index wrapping. Requires power-of-2 buffer sizes but avoids expensive division operations.

// Instead of: index = (index + 1) % capacity
// We do: index = (index + 1) & mask

API design: Keep it simple. cb_* functions for initialization, self parameter for operations on existing objects.

Current implementation

Circular Buffer

Generic circular buffer that works with any data type. Used for message queues, UART buffers, etc.

Key features:

• Generic void* design
• Power-of-2 size optimization
• Complete API: put, get, peek, clear, size checks
• Proper error handling and memory safety

What's next

1. Task Control Blocks and task lifecycle management
2. Priority-based scheduler with round-robin
3. Context switching (ARM assembly)
4. Basic kernel services
5. Traffic light demo to tie it all together

Hardware target

STM32F4 Discovery board (ARM Cortex-M4). Planning to keep the hardware abstraction clean so porting to other Cortex-M chips should be straightforward.

Why build this?

Mostly curiosity. RTOSes like many things, don't make sense to me, so I thought what better way to understand than to build one. Plus, understanding scheduler internals and memory management at this level is useful even for application development.

License

MIT