The document discusses real-time operating systems (RTOS) used in embedded systems, distinguishing between hard and soft real-time systems based on deadline enforcement. It outlines scheduling algorithms, resource allocation, and common features of RTOS, alongside comparisons of various RTOS like FreeRTOS, VxWorks, QNX, ThreadX, and Micrium μC/OS. Each RTOS is evaluated based on its characteristics, applications, advantages, and disadvantages in specific industry contexts.

1. Real-Time Operating Systems
RTOS
BY Abebe B.
For Embedded systems

2. Real-Time Systems
• Result in severe consequences if logical
and timing correctness are not met
• Two types exist
– Soft real-time
• Tasks are performed as fast as possible
• Late completion of jobs is undesirable but not
fatal.
• System performance degrades as more & more
jobs miss deadlines.

3. Real-Time Systems (cont.)
– Hard real-time
• Tasks have to be performed on time
• Failure to meet deadlines is fatal
• Example :
– Flight Control System

4. Hard and Soft Real Time Systems
(Operational Definition)
• Hard Real Time System
– Validation by provably correct procedures
or extensive simulation that the system
always meets the timings constraints
• Soft Real Time System
– Demonstration of jobs meeting some
statistical constraints suffices.
• Example – Multimedia System
– 25 frames per second on an average

5. Most Real-Time Systems are
embedded
• An embedded system is a computer built into
a system but not seen by users as being a
computer
• Examples
– FAX machines
– Copiers
– Printers
– Scanners
– Routers
– Robots

6. Role of an OS in Real Time Systems
• Standalone Applications
– Often no OS involved
– Micro controller based Embedded Systems
• Some Real Time Applications are huge &
complex
– Multiple threads
– Complicated Synchronization Requirements
– File system / Network / Windowing support
– OS primitives reduce the software design time

7. Features of Real Time OS
(RTOS)
• Scheduling.
• Resource Allocation.
• Interrupt Handling.
• Other issues like kernel size.

8. Scheduling in RTOS
• More information about the tasks are
known
– Number of tasks
– Resource Requirements
– Execution time
– Deadlines
• Being a more deterministic system
better scheduling algorithms can be
devised.

9. Scheduling Algorithms in RTOS
• Clock Driven Scheduling
• Weighted Round Robin Scheduling
• Priority Scheduling

10. Scheduling Algorithms in RTOS (cont.)
• Clock Driven
– All parameters about jobs (execution
time/deadline) known in advance.
– Schedule can be computed offline or at
some regular time instances.
– Minimal runtime overhead.
– Not suitable for many applications.

11. Scheduling Algorithms in RTOS (cont.)
• Weighted Round Robin
– Jobs scheduled in FIFO manner
– Time quantum given to jobs is proportional to it’s
weight

12. Scheduling Algorithms in RTOS (cont.)
• Priority Scheduling
– Processor never left idle when there are
ready tasks
– Processor allocated to processes according
to priorities
– Priorities
• Static - at design time
• Dynamic - at runtime

13. Priority Scheduling
• Earliest Deadline First (EDF)
– Process with earliest deadline given highest
priority
• Least Slack Time First (LSF)
– slack = relative deadline – execution left
• Rate Monotonic Scheduling (RMS)
– For periodic tasks
– Tasks priority inversely proportional to it’s period

14. Schedulers
• Also called “dispatchers”
• Schedulers are parts of the kernel
responsible for determining which task
runs next
• Most real-time kernels use priority-
based scheduling
– Each task is assigned a priority based on its
importance
– The priority is application-specific

15. Preemptive Kernels
• The highest-priority task ready to run
is always given control of the CPU
– If an ISR makes a higher-priority task
ready, the higher-priority task is resumed
(instead of the interrupted task)
• Most commercial real-time kernels are
preemptive

16. Preemptive Kernels (cont.)

17. Advantages of Preemptive
Kernels
• Execution of the highest-priority task
is deterministic

18. Resource Allocation in RTOS
• Resource Allocation
– The issues with scheduling applicable here.
– Resources can be allocated in
• Weighted Round Robin
• Priority Based
• Priority inversion problem may occur if
priority scheduling is used

19. Assigning Task Priorities
• Not trivial
• In most systems, not all tasks are
critical
– Non-critical tasks are obviously low-
priorities
• Most real-time systems have a
combination of soft and hard
requirements

20. A Technique for Assigning Task
Priorities
• Rate Monotonic Scheduling (RMS)
– Priorities are based on how often tasks
execute

21. Other RTOS issues
• Interrupt Latency should be very small
– Kernel has to respond to real time events
– Interrupts should be disabled for minimum
possible time
• For embedded applications Kernel Size should
be small
– Should fit in ROM
• Sophisticated features can be removed
– No Virtual Memory
– No Protection

22. Linux for Real Time Applications
• Scheduling
– Priority Driven Approach
• Optimize average case response time
– Interactive Processes Given Highest
Priority
• Aim to reduce response times of processes
– Real Time Processes
• Processes with high priority
• There was no notion of deadlines

23. Interrupt Handling in Linux
• Interrupts were disabled in ISR/critical
sections of the kernel
• There was no worst case bound on
interrupt latency avaliable
– eg: Disk Drivers might disable interrupt for
few hundred milliseconds

24. Interrupt Handling in Linux
(cont.)
• Linux was not suitable for Real Time
Applications
– Interrupts may be missed

25. Other Problems with Linux
• Processes were not preemtible in Kernel Mode
– System calls like fork take a lot of time
– High priority thread might wait for a low priority
thread to complete it’s system call
• Processes are heavy weight
– Context switch takes several hundred
microseconds

26. Part II
BY Abebe B.

27. Characteristics of a real-time
operating system
• Real-time operating systems generally
have the following characteristics:
– Small footprint. Compared to general
OSes, real-time operating systems are
lightweight.
– High performance. RTOSes are typically
fast and responsive.
– Determinism. Repeating inputs end in the
same output.

28. Cont.…
• Safety and security. Safety-critical and
security standards are typically the highest
priority, as RTOSes are frequently used in
critical systems.
• Priority-based scheduling.Tasks that are
assigned a high priority are executed first
followed by lower-priority jobs.
• Timing information. RTOSes are responsible
for timing and providing application
programming interface

29. What are Real-Time Operating
System?
There are several examples of RTOS that
are commonly used in embedded systems:
• FreeRTOS
• VxWorks
• QNX
• ThreadX
• Micrium µC/OS
• etc

30. What are the Best RTOS for
Embedded Systems?
• Choosing the best RTOS for an embedded
system depends on
– the specific requirements of the project,
– Including;
• size of the system,
• performance,
• memory footprint,
• available resources, and real time
requirements.

31. What are the Best RTOS for
Embedded Systems?
• FreeRTOS:
– It is a popular open-source, real-time
operating system that is designed to be small,
portable, and easy to use.
– FreeRTOS provides a rich set of features,
including task management, interrupt
handling, and synchronization mechanisms.
– It is widely used in various industries,
including automotive, aerospace, and
consumer electronics.
https://www.freertos.org/index.html

32. What are the Best RTOS for
Embedded Systems?
• Micrium µC/OS:
– It is a scalable, real-time operating
system that provides a robust set of
features for embedded systems.
Micrium µC/OS is widely used in
various industries, including medical
devices, industrial automation, and
telecommunications.
https://blackberry.qnx.com/en

33. What are the Best RTOS for
Embedded Systems?
• ThreadX:
– It is a small, fast, and efficient real-time
operating system that provides a reliable and
predictable environment for running real-
time applications.
– ThreadX is widely used in various industries,
including automotive, aerospace, and
consumer electronics.
https://www.embeddedtechnology.com/doc/threadx
-rtos-0001

34. What are the Best RTOS for
Embedded Systems?
• VxWorks:
– It is a real-time operating system that
provides a robust set of features for
embedded systems.
– VxWorks is widely used in various industries,
including aerospace, defense, and
telecommunications.
– For nearly 35 years, VxWorks has been used to
ensure the security, safety, and reliability you need to
design and build mission-critical embedded systems
that simply must work.
https://www.windriver.com/products/vxworks

35. What are the Best RTOS for
Embedded Systems?
• QNX:
– It is a real-time operating system that
provides a reliable and predictable
environment for running real-time
applications.
– QNX is widely used in various industries,
including automotive, medical devices,
and industrial automation.

36. Description and Application
RTOS Description Common Applications
FreeRTOS
An open-source RTOS with a lightweight
kernel. Widely used in embedded systems
and microcontrollers.
Embedded devices, IoT,
satellites, drones, robotics,
industrial automation.
VxWorks
A commercial RTOS known for reliability
and scalability. Used in aerospace,
automotive, and more.
Aerospace systems, automotive
control units, industrial
machinery.
QNX
Commercial RTOS with robustness and
fault tolerance. Supports multi-core
processors.
Automotive infotainment,
medical devices, industrial
automation.
ThreadX
Compact and efficient RTOS suitable for
resource-constrained devices.
Consumer electronics, networking
equipment, IoT devices.
Nucleus RTOS
Small-footprint RTOS by Mentor Graphics.
Used in various embedded systems.
Telecommunications, automotive,
consumer electronics.
 These RTOS options cater to different needs, from open-source flexibility
to commercial reliability, making them essential for time-critical applications.

37. Compare
RTOS Description Pros Cons
FreeRTOS
An open-source RTOS with a
lightweight kernel. Widely used in
embedded systems and
microcontrollers.
- Open-source: Easily accessible
and customizable.
- Small footprint: Suitable for
resource-constrained devices.
- Limited features: Basic
functionality; lacks some
advanced features. -
Community support: May
vary in quality.
VxWorks
A commercial RTOS known for
reliability and scalability. Used in
aerospace, automotive, and more.
- Robustness: Proven track record
in critical applications. -
Scalability: Supports complex
systems.
- Cost: Commercial license
required. - Learning
curve: Complex
configuration and setup.
QNX
Commercial RTOS with robustness
and fault tolerance. Supports multi-
core processors.
- Fault tolerance: High reliability
and fault recovery. - Multi-core
support: Efficient utilization.
- Cost: Commercial license
required. - Vendor
dependency: Tied to
BlackBerry.
ThreadX
Compact and efficient RTOS
suitable for resource-constrained
devices.
- Low overhead: Minimal impact
on system resources. - Certified:
Used in safety-critical
applications.
- Proprietary: Closed-
source with licensing fees. -
Limited community:
Smaller user base.
Nucleus
RTOS
Small-footprint RTOS by Mentor
Graphics. Used in various
embedded systems.
- Small footprint: Ideal for
memory-constrained devices. -
Scalability: Supports various
architectures.
- Commercial license:
Requires licensing. -
Vendor-specific: Tied to
Mentor Graphics.

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