Chapter 3

Question 1

An embedded system has three tasks: sensor acquisition, control algorithm, and
communication. Each task must run independently but share CPU time.
Which RTOS concept best supports this design?

Answer 1

Threads

Question 2

Two threads update the same global variable storing sensor data. Occasionally, corrupted
values appear.
What RTOS mechanism should be used to prevent this issue?

Answer 2

Mutex

Question 3

A thread waits indefinitely for a shared resource and blocks the system when the resource
is never released.
Which RTOS feature prevents this situation?

Answer 3

Mutex timeout

Question 4

A control task must execute every 10 ms with guaranteed maximum latency.
Which system property is most critical?

Answer 4

Deterministic timing

Question 5

An RTOS scheduler decides which thread runs next, not the programmer.
What is the main benefit of this approach?

Answer 5

Deterministic execution control

Question 6

A thread has its own stack in an RTOS.
What is the main purpose of this stack?

Answer 6

Save execution context

Question 7

An RTOS guarantees bounded response times and predictable scheduling.
Which statement best describes such an OS?

Answer 7

Real-time

Question 7

Why are mutexes preferred over disabling interrupts for protecting shared data in RTOS
applications?

Answer 7

Mutexes allow for safe concurrency

Question 8

A thread waits up to 100 ms for a mutex. If unavailable, it continues execution.
What RTOS property does this demonstrate?

Answer 8

Deterministic behaviour

Question 9

Which advantage of RTOS-based concurrency most directly improves software
maintainability?

Answer 9

Cleaner and modular code

Question 10

In a multi-threaded RTOS system, only one thread accesses shared data at a time.
What does this ensure?

Answer 10

Safe concurrency

Question 11

In the FreeRTOS firmware stack, user code interacts mainly with:
A. Vendor drivers directly
B. Hardware registers
C. FreeRTOS APIs
D. MCU peripherals

Answer 11

C) FreeRTOS APIs

Question 12

What is the main role of CMSIS in an ARM-based system?

Answer 12

Hardware abstraction

Question 13

Which application is least appropriate for using an RTOS?
A. Multi-protocol gateway
B. Graphical user interface
C. Periodic ADC sampling
D. Networked motor controller

Answer 13

C) periodic ADC sampling

Question 14

Why can bare-metal systems be more deterministic?

Answer 14

No scheduler overhead

Question 15

An RTOS provides the greatest benefit when:
A. Only one task exists
B. Timing is not critical
C. Multiple tasks compete for CPU
D. Code size must be minimal

Answer 15

Multiple tasks compete for CPU

Question 16

Why should ISRs remain short in FreeRTOS systems?

Answer 16

To minimize latency because they delay task scheduling.

Question 17

Which design best improvides portability across ARM MCUs?
A. Direct register access
B. Vendor-specific HAL only
C. CMSIS with FreeRTOS
D. Inline assembly

Answer 17

C) CMSIS with FreeRTOS

Question 18

When a higher-priority task becomes ready, FreeRTOS will:

Answer 18

Preempt the current task

Question 19

Why is bare-metal often chosen for encoder-based motion control?

Answer 19

Lower overhead

Question 20

Which factor is least relevant when choosing an RTOS?
A. Real-time constraints
B. Available memory
C. Team expertise
D. Screen resolution

Answer 20

D) Screen resolution

Question 21

What is the common property of all embedded
systems?
a) They rely on RTOS for execution.
b) They have a defined exit point.
c) They are designed for desktop environments.
d) They run continuously without an exit point

Answer 21

d) They run continuously without an exit point

Question 22

What is a potential drawback of super loops in real-time systems?

Answer 22

Delays in one function propogate to the next.

Question 23

What is jitter in the context of super loops?

Answer 23

The variability in delay for detecting an event/the variation in response time

Question 24

Which hardware is essential for handling interrupts in ARM Cortex-M cores?

Answer 24

Nested Vector Interrupt Controller (NVIC)

Question 25

What is the role of an Interrupt Service Routine (ISR)?

Answer 25

To handle events with minimal delay

Question 26

What is the advantage of Direct Memory Access (DMA)?

Answer 26

It reduces CPU involvement in data transfer.

Question 27

When is an RTOS not required for a system?

Answer 27

When the system has limited repsonsibilities.

Question 28

What does preemptive scheduling ensure in a system?

Answer 28

The most critical task gets the CPU when needed.

Question 29

What problem does task starvation represent in preemptive scheduling?

Answer 29

Lower-priority tasks not getting CPU time

Question 30

What is a primary benefit of using RTOS tasks over super loops?

Answer 30

Better responsiveness in complex systems

Question 31

What is the role of a scheduler in round-robin scheduling?

Answer 31

It assigns equal CPU time slices to tasks

Question 32

How does FreeRTOS reduce overhead during task switching?

Answer 32

By implementing an efficient kernel.

Question 33

What feature of ARM Cortex-M processors minimizes latency for nested interrupts?

Answer 33

Interrupt-tail chaining.

Question 34

What happens if a task in a super loop takes too long to execute?

Answer 34

It delays the execution of subsequent tasks

Question 35

In a basic super-loop, what mainly dtermines the worst-case response time to an external event?

Answer 35

The duration of the entire loop

Question 36

Why does polling reduce efficiency in embedded systems?

Answer 36

It wastes CPU time when no event occurs.

Question 37

What is the main reason ISRs should be kept short?

Answer 37

To minimize latency and jitter

Question 38

What NVIC feature most directly improves real-time behaviour?

Answer 38

Configurable interrupt priorities.

Question 39

What is the main real-time benefit of using DMA?

Answer 39

Lower CPU involvement

Question 40

In a super-loop combined with interrupts, what should the ISR primarily do?

Answer 40

Set a flag and return

Question 41

Why does each RTOS task have its own stack?

Answer 41

To prevent stack interference

Question 42

In a preemptive RTOS, which task executes when multiple tasks are ready?

Answer 42

The one with highest priority

Question 43

Which condition most strongly justifies choosing an RTOS over a super-loop?

Answer 43

Strict timing guarantees

Question 44

Which of the following resources is NOT shared between tasks in a preemptive RTOS? A. Global variables B. Heap memory C. Peripheral registers D. Task stack

Answer 44

D) Task stack

Question 45

Which sequence best represents the typical steps of a task context switch in a preemptive RTOS? A. Save current task context → select next task → restore next task context B. Disable interrupts → execute ISR → resume task C. Switch stack pointer → restart scheduler → clear registers D. Update priority → flush cache → resume execution

Answer 45

A. Save current task context → select next task → restore next task context

Question 46

When an interrupt occurs, which sequence best describes how the CPU responds? A. Complete instruction → save context → jump to ISR B. Stop execution → clear registers → run ISR C. Disable scheduler → switch task → run ISR D. Save stack → update priority → resume task

Answer 46

A. Complete instruction → save context → jump to ISR