RK0 - The Real-Time Kernel '0'

---

Tickless Idle Snapshot

• Added kTicklessEntry and kTicklessExit so SysTick windows expand across idle gaps without losing pending timer expirations.

• Scheduler now parks the CPU when the ready queue is empty, arming a one-shot SysTick calculated from the next timer node and capping it at the hardware reload range.

• Wakes reconstitute the kernel time base by crediting the elapsed ticks back into kTimers, so software timers and sleeps stay bit-for-bit accurate.

• Roadmap: wire the same tickless hooks into STM32 low-power primitives (STOP mode + LPTIM wake) so hardware-offloaded timing replaces SysTick when available.

Test log

  2 ms ::

   Task 3 is waiting at the barrier... 
   5 ms :: 
   Task 1 is waiting at the barrier... 
   5 ms ::
    Task 2 is waiting at the barrier... 
   8 ms ::
   
    Task 3 passed the barrier! 
  10 ms :: Task 1 passed the barrier! 
  11 ms :: Task 2 passed the barrier! 
     TICKLESS: 0% idle (0/19 ticks)
1511 ms :: Task 1 is waiting at the barrier... 
     TICKLESS: 99% idle (1490/1496 ticks)
2011 ms :: Task 2 is waiting at the barrier... 
     TICKLESS: 99% idle (495/497 ticks)
3008 ms :: Task 3 is waiting at the barrier... 
3008 ms :: Task 3 passed the barrier! 
3008 ms :: Task 1 passed the barrier! 
3008 ms :: Task 2 passed the barrier! 
     TICKLESS: 99% idle (995/998 ticks)
4508 ms :: Task 1 is waiting at the barrier... 
     TICKLESS: 100% idle (1498/1498 ticks)
5008 ms :: Task 2 is waiting at the barrier... 
     TICKLESS: 99% idle (499/501 ticks)
6008 ms :: Task 3 is waiting at the barrier... 
6008 ms :: Task 3 passed the barrier! 
6008 ms :: Task 1 passed the barrier! 
6008 ms :: Task 2 passed the barrier! 
     TICKLESS: 99% idle (999/1001 ticks)
7508 ms :: Task 1 is waiting at the barrier... 
     TICKLESS: 99% idle (1497/1498 ticks)
8008 ms :: Task 2 is waiting at the barrier... 
     TICKLESS: 100% idle (500/500 ticks)
9008 ms :: Task 3 is waiting at the barrier... 
9008 ms :: Task 3 passed the barrier! 
9008 ms :: Task 1 passed the barrier! 
9008 ms :: Task 2 passed the barrier! 
     TICKLESS: 99% idle (1000/1001 ticks)

10508 ms :: Task 1 is waiting at the barrier... TICKLESS: 99% idle (1499/1500 ticks) 11008 ms :: Task 2 is waiting at the barrier... TICKLESS: 100% idle (499/499 ticks) 12008 ms :: Task 3 is waiting at the barrier... 12008 ms :: Task 3 passed the barrier! 12008 ms :: Task 1 passed the barrier! 12008 ms :: Task 2 passed the barrier! TICKLESS: 99% idle (999/1001 ticks) 13508 ms :: Task 1 is waiting at the barrier... TICKLESS: 100% idle (1499/1499 ticks)

---

Know it

RK0 is a lean, feature-rich, highly deterministic Real-Time Kernel for deeply embedded solutions.

• See the RK0 Docbook for a thorough design description and usage examples.
• Check the RK0 Blog for some quick reads.

Supported Architectures: ARMv6M (Cortex-M0/0+) and ARMv7M (Cortex-M3/4/7).

---

RK0 Main Features (0.8.0-dev)

• O(1) Scheduler: priority preemptive (RMS)

• Synch Pack:

  • Semaphores (Counting/Binary)
  • Mutexes with fully transitive Priority Inheritance for nested locks
  • Condition Variables
  • Direct Task Notifications

• Priority-aware message-passing

  • Message Queues (Mailboxes) for general asynchronous message-passing
  • Ports for client-server synchronous RPC, message-driven priority inheritance

• Most-Recent Message Protocol

  • Asynchronous, Lock-Free, purpose-built for Real-time Control Loops

• High-Precision Timers:

  • Minimal Tick Handling overhead for Bounded Waiting, Periodic Sleeps and Application Timers.

• Memory Partition:

  • Well-proven, deterministic memory allocator suitable for real-time systems.

• Suits both procedural/shared-memory and message-passing paradigms

• Highly Modular with clean and consistent API.

• (And that wicked cool mascot)

---

Use it

• Emulated hardware: the provided Makefile in this branch targets QEMU
• Nucleo boards

---

System Architecture

If no more details are to be provided, the kernel has a top and a bottom layer. On the top, the Executive manages the resources needed by the application. On the bottom, the Low-level Scheduler works as a software extension of the CPU.

---

Code Quality

RK0 source code compiles cleanly with the following GCC flags:

-Wall -Wextra -Wsign-compare -Wsign-conversion -pedantic

Static Analysis (Cppcheck) is clean with no warnings, errors, or style issues.

---

Dependencies

• ARM-GCC, CMSIS-GCC

---

Copyright (C) 2025 Antonio Giacomelli | All Rights Reserved | www.kernel0.org | 📫