Acronym · Embedded Software & Tools
RTOS (Real-Time Operating System)
A small kernel that schedules tasks so that timing deadlines are met deterministically, typically with priority-based pre-emption, bounded interrupt latency and predictable synchronisation primitives.
Throughput is traded away for worst-case predictability.
- Also known as
- Real-time kernel
- Type
- Acronym
- Domain
- Embedded Software & Tools, Operating systems
Notes
Hard real-time means a missed deadline is a failure; soft real-time means it only degrades quality, and most marketing blurs the two.
Overview
A real-time operating system (RTOS) is a small operating system kernel that schedules tasks so that time-critical work always meets its deadlines, giving predictable, bounded response times rather than the best average throughput. RTOSes run on microcontrollers and on real-time cores inside larger SoCs, controlling motors, radios, sensors, storage and safety functions.
How it works
An RTOS divides software into tasks, each with a priority. The scheduler always runs the highest-priority task that is ready, and pre-empts a lower-priority task the moment a more urgent one becomes ready, for example when an interrupt signals new data. Kernel operations are designed to take a bounded time, so engineers can calculate the worst-case delay from an event to the response.
Tasks communicate and share resources through queues, semaphores, mutexes and event flags. These must be designed carefully: a classic problem is priority inversion, where a low-priority task holding a shared resource blocks a high-priority one, which RTOSes address with mechanisms such as priority inheritance. Timers, memory management and drivers complete the picture, and many RTOSes add networking, file systems and security libraries.
RTOS kernels are usually small enough to fit in tens of kilobytes. Widely used examples include FreeRTOS, distributed under the MIT open-source licence; Zephyr, an open-source project hosted by the Linux Foundation; and commercial systems such as QNX, used in automotive and industrial products. Systems that need safety certification use RTOSes developed and assessed to standards such as ISO 26262.
Examples
- FreeRTOS running on a Cortex-M microcontroller to handle sensor reading, wireless communication and power management.
- Zephyr running on the radio core of a Bluetooth SoC.
- A safety-certified RTOS on the real-time cores of an automotive SoC controlling braking functions.
- Solid-state drive controller firmware using a small RTOS to schedule flash operations.
Why it matters
For chips with embedded processors, the RTOS is part of the product customers see: silicon vendors ship RTOS ports, drivers and examples so that customers can start quickly. Embedded software and firmware engineers write tasks and drivers, embedded architects choose the RTOS, and functional safety software engineers handle certified versions. Missed deadlines or priority bugs cause intermittent failures that are hard to find.
Related terms
Part of the Embedded Software & Tools glossary, with every term in the topic and the lifecycles, tools and standards that use them.
Where it appears on the Hub
Used on 34 pages of the Hub. In each group, the pages that mention it most come first.
Roles
Skills
Suggest an improvement
Suggest an improvement to this entry
A clearer definition, an example, a correction or a source an editor can check. Editors review every suggestion before the entry changes.
Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary