R3 is a proof-of-concept of a static RTOS that utilizes Rust's compile-time function evaluation mechanism for static configuration (creation of kernel objects and memory allocation).

All kernel objects are defined statically for faster boot times, compile-time checking, predictable execution, reduced RAM consumption, no runtime allocation failures, and extra security.
The kernel and its configurator don't require an external build tool or a specialized procedural macro, maintaining transparency.
The kernel is split into a target-independent portion and a target-specific portion. The target-specific portion (called a port) is provided as a separate crate. An application combines them using the trait system.
Leverages Rust's type safety for access control of kernel objects. Safe code can't access an object that it doesn't own.

Features

Traditional uniprocessor tickless real-time kernel with preemptive scheduling
Tasks are kernel objects associated with application threads and encapsulate their execution states. Tasks can be activated by application code or automatically at boot time. Tasks are assigned priorities (up to 2¹⁵ levels on a 32-bit target, though the implementation is heavily optimized for a smaller number of priorities), which can be changed at runtime. A task can enable Priority Boost to temporarily raise its priority to higher than any other tasks. The number of tasks is only limited by memory available.
This kernel provides a unified interface to control interrupt lines and register interrupt handlers. In addition, the Arm M-Profile port supports unmanaged interrupt lines, which aren't masked when the kernel is handling a system call.
This kernel supports common synchronization primitives such as mutexes, semaphores, and event groups. The mutexes can use [the priority ceiling protocol] to avoid unbounded priority inversion and mutual deadlock. Tasks can park themselves.
The kernel timing mechanism drives software timers and a system-global clock with microsecond precision. The system clock can be rewound or fast-forwarded for drift compensation. The timing algorithm has a logarithmic time complexity and is therefore scalable. The implementation is robust against a large interrupt processing delay.
The utility library includes safe container types such as Mutex and RecursiveMutex, which are built upon low-level synchronization primitives.
Supports Arm M-Profile (all versions shipped so far), Armv7-A (no FPU), RISC-V as well as the simulator port that runs on a host system.

Example

```rust

![feature(asm)]

![feature(constfntrait_bound)]

![feature(constmutrefs)]

![feature(constfnfnptrbasics)]

![no_std]

![no_main]

// ----------------------------------------------------------------

// Instantiate the Armv7-M port use r3portarm_m as port;

port::useport!(unsafe struct System); port::usert!(unsafe System); port::usesysticktickful!(unsafe impl PortTimer for System);

impl port::ThreadingOptions for System {}

impl port::SysTickOptions for System { // STMF401 default clock configuration // SysTick = AHB/8, AHB = HSI (internal 16-MHz RC oscillator) const FREQUENCY: u64 = 2000000; }

// ----------------------------------------------------------------

use r3::kernel::{Task, cfg::CfgBuilder};

struct Objects { task: Task, }

// Instantiate the kernel, allocate object IDs const COTTAGE: Objects = r3::build!(System, configure_app => Objects);

const fn configureapp(b: &mut CfgBuilder) -> Objects { System::configuresystick(b);

Objects {
    task: Task::build()
        .start(task_body)
        .priority(2)
        .active(true)
        .finish(b),
}

}

fn taskbody(: usize) { // ... } ```

Explore the examples directory for example projects.

Prerequisites

You need a Nightly Rust compiler. This project is heavily reliant on unstable features, so it might or might not work with a newer compiler version. See the file rust-toolchain to find out which compiler version this project is currently tested with.

You also need to install Rust's cross-compilation support for your target architecture. If it's not installed, you will see a compile error like this:

error[E0463]: can't find crate for `core` | = note: the `thumbv7m-none-eabi` target may not be installed

In this case, you need to run rustup target add thumbv7m-none-eabi.