Real-time

Real-time

1. Definition

A real-time operating system (RTOS) is one in which the correctness of a result depends not only on the computed value but also on the time at which it is produced. Every task must complete within its deadline; a late answer is a wrong answer. The OS must therefore be deterministic and predictable — not merely fast, but reliably bounded in the worst case.

Example: an airbag controller has roughly 20-30 ms to sense a crash and fire the bag. A response of 50 ms is technically "faster than a human notices" but completely wrong for the passenger.

2. Types of Real-Time Systems

Type	Deadline miss means	Examples
Hard real-time	Catastrophic failure — loss of life, mission or money	Airbag, ABS brakes, pacemaker, flight control, nuclear reactor control, missile guidance
Firm real-time	Result has no value after the deadline, but no disaster	Robotics picking arm, automated trading quotes, industrial batching
Soft real-time	Quality degrades gracefully; occasional miss is tolerable	Video/audio streaming, VoIP, online gaming, virtual meetings

3. Requirements and Characteristics of an RTOS

• Deterministic scheduling — the worst-case response time of every task is known and provable in advance.
• Priority-based pre-emptive scheduling — a high-priority task pre-empts a lower one immediately.
• Bounded interrupt latency and dispatch latency — small, predictable worst-case delays.
• Small, efficient kernel — a footprint of kilobytes and fast context switches.
• Priority inheritance / ceiling protocols — prevent priority inversion (a low-priority task holding a lock needed by a high-priority task).
• No or controlled virtual memory — page faults are unpredictable, so pages are locked in RAM or paging is disabled altogether.
• Predictable device handling — interrupt-driven I/O with bounded service times, often direct control of hardware.
• Reliability and graceful degradation — watchdog timers, fail-safe states, redundancy, built-in self-tests.
• Task synchronisation primitives — semaphores, mutexes, message queues, event flags, all with bounded latencies.
• Compact, ROM-able code — runs on microcontrollers with limited RAM/flash.

4. Scheduling in Real-Time Systems

Algorithm	Policy	Property
Rate Monotonic (RM)	Fixed priority; shorter period gets higher priority	Optimal among fixed-priority algorithms; schedulable if $U ≤ n × (2^{1/n} - 1)$ (≈ 0.693 for large n)
Earliest Deadline First (EDF)	Dynamic priority; task with nearest deadline runs first	Optimal for uniprocessor pre-emptive systems; schedulable if $U ≤ 1$
Least Laxity First	Run the task with least slack (deadline − remaining work)	Good but heavy overhead

Utilisation of a task set: $U = Σ (execution time / period)$.

Worked example: three periodic tasks — T1 (C = 1 ms, P = 4 ms), T2 (C = 2 ms, P = 6 ms), T3 (C = 2 ms, P = 12 ms).

EDF can schedule it ($U ≤ 1$). For RM the bound for n = 3 is $3 × (2^{1/3} - 1) ≈ 0.78$, so 0.75 also fits RM — but note that RM is conservative: values above the bound may still be schedulable.

5. RTOS vs General-Purpose OS

Basis	RTOS	General-purpose OS (Windows/Linux desktop)
Primary goal	Meet deadlines, predictability	Throughput, fairness, convenience
Scheduling	Priority-based, deterministic, often RM/EDF	Time-sliced round robin / fair-share (CFS)
Worst-case latency	Bounded and documented	Not guaranteed — can be tens of ms
Kernel size	Tiny (KB)	Large (MB), hundreds of drivers
Memory	No paging for time-critical code; static allocation	Virtual memory with demand paging
Priority inversion	Handled by protocol (inheritance/ceiling)	Usually ignored
User interface	Minimal or none	Rich GUI, multitasking for humans
Examples	VxWorks, QNX Neutrino, FreeRTOS, RTEMS, µC/OS-II, PREEMPT-RT Linux	Windows 11, Ubuntu desktop, macOS

6. Applications

Automotive (engine control, ABS, airbag, ADAS), medical devices (pacemakers, ventilators, insulin pumps), aerospace and defence (flight control, UAVs, missile systems), industrial automation (PLCs, SCADA, CNC, robotic arms), telecommunications (base stations, switches, routers), consumer electronics (set-top boxes, cameras, printers), railways and traffic signalling — plus soft real-time uses such as streaming media and online gaming.

7. Exam Points

• Define a real-time operating system and differentiate hard, firm and soft real-time with examples — 5 marks, almost certain.
• List the characteristics/requirements of an RTOS — 5 marks, frequent.
• Explain Rate Monotonic and EDF scheduling with the utilisation conditions — 5-7 marks.
• Numerical: compute utilisation of a periodic task set and test RM/EDF schedulability — 5 marks.
• Differentiate an RTOS from a general-purpose OS — 5 marks.
• What is priority inversion and how is it solved? — 3 marks.
• Why is paging avoided in hard real-time systems? — 3 marks.