THEORY EXAMINATION (SEM–VI) 2016-17 REAL TIME SYSTEM

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

(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

(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

(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

(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

Q4. RTOS vs General-purpose OS & VRTX

RTOS vs GPOS

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