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name: embedded-rtos description: | Embedded, firmware, and real-time systems architecture: bare-metal vs RTOS (FreeRTOS/Zephyr), real-time scheduling (RMS/EDF), WCET and schedulability, priority inversion, interrupt latency, memory/energy budgets, MMU-less design.

USE WHEN: designing firmware/embedded/IoT/MCU systems, real-time deadlines, "RTOS", "FreeRTOS", "Zephyr", "bare-metal", "WCET", "hard real-time", "priority inversion", "interrupt latency", "deadline", microcontroller architecture.

DO NOT USE FOR: general OS/kernel design (use os-kernel-architecture); cloud/server perf (use hardware-aware-design); app code (use language skills). allowed-tools: Read, Grep, Glob

Embedded / Firmware / RTOS Architecture

Bare-metal vs RTOS vs GPOS — the first decision

Option	When	Cost
Bare-metal (superloop + ISRs)	Tiny MCU, few tasks, hard determinism	No task abstraction; hand-rolled scheduling
RTOS (FreeRTOS, Zephyr, ThreadX)	Many concurrent tasks, deadlines, drivers	Footprint (KBs), scheduling overhead
GPOS w/ RT patch (Linux PREEMPT_RT)	Rich connectivity + soft/firm RT	MBs RAM, MMU, bounded-but-larger latency

Real-time scheduling & schedulability

• Hard vs soft vs firm real-time: missing a hard deadline = system failure.
• RMS (rate-monotonic, fixed priority): schedulable if Σ(Cᵢ/Tᵢ) ≤ n(2^(1/n)−1).
• EDF (earliest-deadline-first, dynamic): optimal, schedulable up to 100% CPU, but harder to reason about overload.
• WCET (worst-case execution time) is the input everything depends on — caches, branch prediction, DMA contention make it hard; measure + analyze.
• Priority inversion → use priority inheritance or priority ceiling on mutexes (the Mars Pathfinder bug). Avoid unbounded blocking.

Architecture constraints that dominate

• Memory: often no MMU → no virtual memory, no process isolation; static allocation preferred (no heap fragmentation); stack sizing per task.
• Interrupt latency: keep ISRs short (top-half), defer to tasks; measure worst-case disable-interrupt windows.
• Power: tickless idle, sleep states, peripheral clock gating, wake sources; energy-per-inference for edge AI (see ai-systems/edge).
• Determinism over throughput: avoid dynamic allocation, unbounded loops, and non-deterministic libraries on hot paths.

When to recommend what

• Hard deadlines + tiny MCU → bare-metal or small RTOS, RMS/EDF, static alloc.
• Connectivity + soft RT → Zephyr or Linux PREEMPT_RT.
• Safety-critical (ISO 26262 / DO-178C / IEC 62304) → certified RTOS, redundancy, formal/coverage analysis (pairs with a safety-critical skill when present).