# Interrupt Handling and Synchronization

> **The criterion.** RTOS environments must respond to hardware events instantly: low, bounded interrupt
> latency; a way to defer heavy work out of hard ISRs; and priority-inversion mitigation (priority
> inheritance or ceiling).

## How Revera answers it

On every MCU port the kernel tick and the context switch run off real interrupts (e.g. ARM SysTick → PendSV,
RISC-V machine-trap), so scheduling is **preemptive** — a higher-priority thread woken in an ISR runs as soon
as the ISR returns. **Deferred processing** is the natural idiom: an ISR does the minimum, then wakes a thread
with `tk_sem_post` / `tk_cond_signal` (safe from ISR context), and the heavy work runs at thread priority.
**Priority inversion** is handled by **priority inheritance** on mutexes, on by default.

## Feature · how to turn it on · how to use it · where it works

| Feature | Turn on with | Use these APIs | Where |
|---|---|---|---|
| Priority inheritance (single-level) | default (`TK_MUTEX_PRIO_INHERIT`) | `tk_mutexattr_setprotocol`, then `tk_mutex_lock` / `unlock` | all |
| Deferred (bottom-half) processing | — | `tk_sem_post` / `tk_cond_signal` from an ISR → wakes a thread | all |
| Preemptive switch from an ISR | default | (tick → PendSV / trap; automatic) | preemptive ports |
| Idle / stall observation | — | `tk_kernel_set_idle_hook`, `tk_kernel_on_stall` (weak hook) | all |

*Full signatures and contracts: [API reference — Synchronization / Threads](../reference/api.md).*

## Limits & roadmap

- **Interrupt-latency numbers are not published** — latency is an arch + board + interrupt-controller
  property; measure it on your target.
- Priority-inversion mitigation is **priority inheritance only** (single-level); there is no priority-ceiling
  protocol.
- Revera does not impose a portable ISR framework — ISRs are arch/board code; the kernel gives you the
  wake-a-thread and trace-hook seams, not a driver-model interrupt abstraction.

## Learn more

- **How it works:** [synchronization](../architecture/synchronization.md) · [tick policy](../architecture/tick-policy.md).
- **Run it:** `examples/button_irq` (edge IRQ → thread) · `examples/stall_detect`.
- **Full API:** [API reference](../reference/api.md).