Real-time systems are those systems in which the correctness of the system depends not only on the logical result of computation, but also on the time at which the results are produced.

1. Real-Time Operating
Systems
DEVANSHI JAIN
15STUJPCS0003
BY

2. Overview
 Introduction
 Classification
 System Characteristics
 Implementing Real-Time
Operating Systems

3. Introduction
 Real-time systems are those systems in which
the correctness of the system depends not
only on the logical result of computation, but
also on the time at which the results are
produced.
 Supports real-time applications by providing
logically correct result within the deadline
required.
 RTOS is key to many embedded systems and
provides a platform to build applications.

4. Classification
Hard real time system
• Tasks have to be
performed on time
• Failure to meet
deadlines is fatal
• Example :
Flight Control System,
medical systems .
Soft real time system
• Tasks are performed
as fast as possible
• Late completion of
jobs is undesirable
but not fatal.
• Example:
Online Databases,
computer games.

5.  Single purpose
 Small size
 Inexpensively mass-produced
 Specific timing requirements
 Do not require interfacing with a
user
System Characteristics

6. Bus-Oriented System

7. System-on-a-Chip
 Many real-time systems are
designed using system-on-a-chip
strategy
 SOC allows the CPU, memory,
memory-management unit, and
attached peripheral ports (i.e., USB)
to be contained in a single
integrated circuit

8. Virtual Memory in Real-
Time Systems
Address Translation

9. Implementing Real-Time
Operating Systems
1. Preemptive, priority-based
scheduling
2. Preemptive kernels
3. Latency must be minimized

10. Event latency

11. Interrupt Latency

12. Dispatch Latency

13. Priority Inversion Problem
 High-level task stalled due to low-level
using shared resources, then a
medium-level task holding up the low-
level one
 Solution: Priority inheritance – give
low-level task high-level priority

14. Scheduling Algorithms in RTOS
 Priority Scheduling
 Processor never left idle when there are
ready tasks
 Processor allocated to processes
according to priorities

15. 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

16. Examples of RTOS
 LynxOS
 OSE
 QNX
 VxWorks
 Windows CE
 RT Linux

17. APPLICATIONS OF RTOS
 Almost all the modern telecommunication
systems make use of RTOS .
 Radar systems, network switching control
systems, satellite monitoring systems, satellite
launch-control and global positioning systems all
have their roots in RTOS.
 Now a days RTOS are increasingly finding use in
strategic and military operations. These are used
in guided missile launching units,track-and-trace
spy satellites, etc.