About this previous year paper
REAL TIME SYSTEM (NEC013R)
Section-wise Solved Answers & Notes
SECTION – A (10 × 2 = 20 Marks)
Very short, direct answers
(a) Embedded system vs Real-Time system
| Embedded System | Real-Time System |
|---|---|
| Dedicated function | Time-critical response |
| Timing may not be strict | Timing deadlines are mandatory |
| Example: Microwave oven | Example: Airbag system |
(b) Important issues in real-time computing
• Meeting deadlines • Predictability
• Scheduling • Resource sharing
• Fault tolerance
(c) Periodic task
A task that executes at regular, fixed intervals.
Example: Sensor data sampling every 10 ms.
(d) Sporadic task
A task that occurs irregularly but has a minimum inter-arrival time.
Example: Emergency interrupt.
(e) Temporal constraints
Time-related constraints such as deadline, response time, and execution time that must be satisfied.
(f) Why task synchronization is required in RTOS?
To avoid race conditions, data inconsistency, and resource conflicts when multiple tasks share resources.
(g) Predictability vs Validation
| Predictability | Validation |
|---|---|
| System behavior is deterministic | System correctness is verified |
| Timing guarantees | Functional correctness |
(h) Heterogeneous processors
A system using different types of processors (CPU, DSP, GPU) for optimized performance.
(i) Local and remote resources
• Local resources: Located on same processor • Remote resources: Accessed over network or bus
(j) Advantages of multihop protocol
• Extended network coverage • Reduced transmission power
• Improved reliability • Better scalability
SECTION – B (Any 5 × 10 = 50 Marks)
(a) Rate Monotonic (RM) scheduling algorithm
RM is a fixed-priority scheduling algorithm where: Shorter period → higher priority
Utilization bound equation:
U≤n(21/n−1)U \le n(2^{1/n}-1)U≤n(21/n−1)
For large n: U≈0.69U \approx 0.69U≈0.69
Example:
Task with period 10 ms has higher priority than task with period 20 ms.
(b) Bin-packing assignment algorithm for EDF
In EDF, tasks are assigned to processors using bin-packing techniques such as:
First Fit Best Fit
Objective: Minimize processors while meeting deadlines.
(c) Issues in real-time computing & examples
Issues: • Timing constraints
• Scheduling • Resource contention
Examples: Flight control system
Medical monitoring system
(d) Fixed vs Dynamic priority scheduling
| Fixed Priority | Dynamic Priority |
|---|---|
| Priority constant | Priority changes |
| RM algorithm | EDF algorithm |
| More predictable | Higher CPU utilization |
(e) Importance of predictability in RTS Predictability ensures deadlines are always met.
Enforcement methods: • Fixed scheduling policies
• Worst-case execution analysis • Resource access protocols
(f) RMFF vs RMST algorithms
| RMFF | RMST |
|---|---|
| First-Fit strategy | Smallest-Task strategy |
| Faster assignment | Better load balancing |
| Less optimal | More efficient |
(g) Functions of RTOS • Task scheduling
• Memory management • Inter-task communication
• Interrupt handling • Time management
(h) Clock & clock synchronization
Clock: Measures time in RTS.
Clock synchronization: Aligns clocks across processors to ensure consistent timing.
SECTION – C (Any 2 × 15 = 30 Marks)
Q3. Fault tolerance & protocol comparison
Fault tolerance Ability of system to continue operation despite faults.
Fault classification:
Temporal: Permanent, transient, intermittent Output: Omission, timing, response faults
Contention-based vs Token-based protocol
| Contention-based | Token-based |
|---|---|
| Nodes compete for access | Token controls access |
| Higher collision | Collision-free |
| Example: CSMA | Example: Token Ring |
Q4. RTOS vs General-purpose OS & VRTX
RTOS vs GPOS
| RTOS | GPOS |
|---|---|
| Deterministic | Best-effort |
| Deadline-driven | Throughput-driven |
| Embedded use | Desktop/server use |
VRTX RTOS
VRTX is a commercial RTOS with: Preemptive scheduling
Fast context switching Semaphore-based synchronization
Used in industrial and avionics systems.
Q5. Structure of real-time system
Components: • Sensors
• Processor • RTOS
• Actuators • Communication network
Working: Sensors capture data
Processor executes tasks via RTOS Actuators respond within deadlines