# Rtos

> RTOS Internals

- Skill: `j4flmao/rtos` (Agent Skill)
- Install (CLI): `npx skillmds@latest add j4flmao/rtos`
- Raw SKILL.md: https://api.skillmd.com/api/skills/j4flmao/rtos/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: j4flmao (https://skillmd.com/u/j4flmao)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/j4flmao/rtos

---

# Real-Time Operating Systems (RTOS) Mechanics

## Preemptive Scheduling and FreeRTOS Internals
FreeRTOS utilizes a priority-based, preemptive scheduling algorithm. The core data structure is `pxReadyTasksLists`, an array of `List_t` structures, one for each priority level up to `configMAX_PRIORITIES - 1`. The scheduler always selects the highest priority task in the ready state. Context switching is architecture-specific but fundamentally involves saving the CPU context onto the task's stack, updating the TCB (Task Control Block) stack pointer, and restoring the context of the newly selected task via a `PendSV` handler in ARM Cortex-M architectures.

## Priority Inheritance in Mutexes
To prevent priority inversion, FreeRTOS implements priority inheritance for its Mutex objects (which are specialized semaphores). When a high-priority task attempts to acquire a Mutex held by a lower-priority task, the scheduler temporarily elevates the priority of the mutex holder to match the blocked high-priority task. Upon releasing the mutex, the holder's original base priority is restored.

```mermaid
flowchart TD
%%{init: {"theme": "default", "flowchart": {"useMaxWidth": true}}}%%
    subgraph SchedulerFreeRTOSScheduler ["FreeRTOS Scheduler<br><br><br>"]
        Tick[SysTick Interrupt] -->|"xTaskIncrementTick()"| UnblockCheck[Check Delayed Lists]
        UnblockCheck -->|"Yield()"| SwitchContext[Context Switch PendSV]
    end
    subgraph TaskStateTaskStateMachine ["Task State Machine<br><br><br>"]
        Running -->|"vTaskDelay()"| Blocked
        Blocked -->|"xSemaphoreGive()"| Ready
        Ready -->|"Schedule()"| Running
    end
```