RTOS

An RTOS, or real-time operating system, is an operating system designed to serve real-time applications by guaranteeing that tasks meet specified timing constraints. It emphasizes deterministic behavior, providing predictable task scheduling and bounded response times, in contrast to general-purpose operating systems where timing can be variable.

Real-time constraints are classified as hard, where missing a deadline constitutes a failure, and soft, where deadlines are important but not strictly fatal. Scheduling policies used in RTOSes include fixed-priority (such as rate-monotonic) and dynamic approaches (such as earliest-deadline-first). Some systems employ tickless scheduling to reduce latency and power consumption.

Core features of an RTOS include task management, inter-task communication and synchronization (semaphores, mutexes, message queues), precise timing services (delays, timers, deadlines), and deterministic memory management. Interrupt handling and bounded interrupt latency are critical, as ISRs may interact with tasks and timing.

Architectures vary: monolithic RTOS kernels provide extensive services in a single address space, while microkernel-based designs keep the kernel minimal and implement services as separate servers. Many RTOSes offer POSIX real-time extensions and, in some domains, ARINC 653 or other domain-specific interfaces. Hardware abstraction layers help portability across platforms.

Common examples include FreeRTOS, Zephyr, VxWorks, QNX, RTEMS, and Integrity. RTOS technology is widely used in embedded systems such as automotive control, aerospace, industrial automation, medical devices, robotics, and consumer electronics, where satisfying timing requirements is essential.

Standards and safety considerations include POSIX real-time extensions, ARINC 653 in avionics, and industry-specific safety norms (for example ISO 26262 in automotive or DO-178C in aviation). Certification focuses on traceability, determinism, and failure handling.