--- title: "Real-time system and real-time OS components" subject: "Software Engineering" chapter: 4 key: "semester_5/software_engineering:real-time-os" source: https://www.bctengineer.com/q/semester_5/software_engineering/real-time-os site: BCT Engineering --- # Real-time system and real-time OS components Software Engineering · Chapter 4: Real-time software design - Asked in: 2068 Chaitra, 1+4 marks; year not given, 1+4 marks - Video: [Introduction to RTOS Part 1 - What is a Real-Time Operating System (RTOS)? | Digi-Key Electronics](https://www.youtube.com/watch?v=F321087yYy4) — DigiKey - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/software_engineering/real-time-os - Whole chapter: https://www.bctengineer.com/md/semester_5/software_engineering/chapter-4 ## Question Define a real-time system. Explain the real-time operating system and its components? ## Short answer A real-time system's correctness depends on results and timing; hard real-time fails if a deadline is missed, soft degrades. An RTOS manages processes with priority-based pre-emptive scheduling and bounded response times. Its components are real-time clock, interrupt handler, scheduler, resource manager, and dispatcher. ## Answer A **real-time system** is a software system whose correct functioning depends both on the **results** it produces and on the **time** at which these results are produced. A **hard** real-time system fails if a deadline is missed (e.g. airbag control); a **soft** one only degrades (e.g. video streaming). #### Real-time operating system (RTOS) An RTOS (real-time executive) manages processes and resources in a real-time system so that time-critical processes meet their deadlines. It is usually small, uses **priority-based pre-emptive scheduling**, and has predictable, bounded response times. Examples: VxWorks, FreeRTOS, QNX. Processes are handled at different priority levels: - **Interrupt level** – highest priority, for processes that need a very fast response (e.g. the clock process). - **Clock level** – periodic processes. - **Background level** – low-priority work with no deadline. #### Components of an RTOS (Sommerville) ```text +-----------------+ | Real-time clock | +-----------------+ | +-----------+ v | Interrupt |----> +------------+ | handler | | Scheduler |<--- process +-----------+ +------------+ priority info | v +-------------------+ | Resource manager |<--- available +-------------------+ resource list | v +-------------------+ | Dispatcher |--> ready list +-------------------+ -> processor ``` 1. **Real-time clock** – gives regular timing signals used to schedule periodic processes. 2. **Interrupt handler** – manages aperiodic requests for service (e.g. a sensor event) and passes them on quickly. 3. **Scheduler** – examines the processes that can run and picks the next one according to priority. 4. **Resource manager** – allocates memory and processor resources to the process chosen by the scheduler. 5. **Dispatcher** – starts the execution of a process from the ready list on a free processor. Large real-time systems may also have a **configuration manager** (dynamic reconfiguration, hot-swapping of hardware) and a **fault manager** (detects faults and moves work to backup systems). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A real-time system's correctness depends on: A. Only the results produced B. Only the speed of the processor C. Only user interface design D. Both the results and the time at which they are produced Answer: D. Correct functioning depends on results and on when they are produced. (Key point: Real-time system) 2. Which is a hard real-time example? A. Video streaming B. Web browsing C. Airbag control D. Word processing Answer: C. A hard system fails if a deadline is missed, as in an airbag controller; streaming is soft. (Key point: Hard vs soft) 3. What does the RTOS scheduler do? A. Allocates memory to processes B. Starts the process on a free processor C. Gives regular timing signals D. Examines runnable processes and picks the next by priority Answer: D. The scheduler chooses the next process by priority; the resource manager allocates resources and the dispatcher starts execution. (Key point: Scheduler role) 4. What does the interrupt handler manage? A. Periodic clock ticks B. Aperiodic requests for service such as a sensor event C. Memory allocation D. Fault recovery to backup Answer: B. The interrupt handler deals with aperiodic requests and passes them on quickly. (Key point: Interrupt handler) ## Related in Software Engineering - [Real-time versus ordinary software and OS; data acquisition](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areal-time-vs-non-real-time) — Software Engineering, chapter 4 - [General OS versus real-time OS](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Ageneral-os-vs-rtos) — Software Engineering, chapter 4 - [Designing software for real-time environments](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areal-time-design-considerations) — Software Engineering, chapter 4 ## Related in other subjects - [Basic functions of real-time kernel](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Areal-time-kernel-functions) — Embedded System, chapter 6, asked 2× - [Preemptive vs non-preemptive; real-time scheduling](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apreemptive-nonpreemptive-realtime) — Operating System, chapter 2 - [Rate Monotonic real-time schedule](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Arate-monotonic-schedule) — Operating System, chapter 2 - [Real-time scheduling; two-processor numerical](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Arealtime-two-processor-fcfs-sjf-priority-rr) — Operating System, chapter 2