This document discusses real-time operating systems (RTOS) which are designed to manage applications with strict timing constraints, ensuring predictable and deterministic responses. Key characteristics differentiate them from general-purpose operating systems, such as priority-based preemptive scheduling and fixed time constraints, making them suitable for critical applications like industrial control and medical devices. RTOS are classified into hard, firm, and soft real-time categories based on their tolerance for missed deadlines.

1. There are many operating systems
Real-Time Operating System
Real-time applications usually are executed on top of a Real-time Operating System (RTOS).
Specific scheduling algorithms can be designed. When possible, static cyclic schedules are
calculated off-line.
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.
RTOS is therefore an operating system that supports real-time applications by providing
logically correct result within the deadline required. Basic Structure is similar to regular OS but,
in addition, it provides mechanisms to allow real time scheduling of tasks.
Though real-time operating systems may or may not increase the speed of execution, they can
provide much more precise and predictable timing characteristics than general-purpose OS.
A real-time system is defined as a data processing system in which the time interval required to
process and respond to inputs is so small that it controls the environment. The time taken by the
system to respond to an input and display of required updated information is termed as the
response time. So in this method, the response time is very less as compared to online
processing.
Real-time systems are used when there are rigid time requirements on the operation of a
processor or the flow of data and real-time systems can be used as a control device in a dedicated
application. A real-time operating system must have well-defined, fixed time constraints,
otherwise the system will fail. For example, Scientific experiments, medical imaging systems,
industrial control systems, weapon systems, robots, air traffic control systems, etc.
Design considerations
Designing a proper RTOS architecture needs some delicate decisions. The basic services like
process management, inter-process communication, interrupt handling, or process
synchronization have to be provided in an efficient manner making use of a very restricted
resource budget.
Multi-core architectures need special techniques for process management, memory management,
and synchronization. The upcoming Wireless Sensor Networks (WSN) generate special demands
for RTOS support leading to dedicated solutions. Another special area is given by multimedia
applications. Very high data rates have to be supported under (soft) RT constraints.
The key difference between general-computing operating systems and real-time operating
systems is the need for " deterministic " timing behavior in the real-time operating systems.
Formally, "deterministic" timing means that operating system services consume only known
and expected amounts of time. In theory, these service times could be expressed as mathematical

2. formulas. These formulas must be strictly algebraic and not include any random timing
components. Random elements in service times could cause random delays in application
software and could then make the application randomly miss real-time deadlines – a scenario
clearly unacceptable for a real-time embedded system. Many non-real-time operating systems
also provide similar kernel services.
DIFFERENCE: RTOS v/s General Purpose OS
Determinism - The key difference between general-computing operating systems and real-time
operating systems is the “deterministic " timing behavior in the real-time operating systems.
"Deterministic" timing means that OS consume only known and expected amounts of time.
RTOS have their worst case latency defined. Latency is not of a concern for General Purpose
OS.
Task Scheduling - General purpose operating systems are optimized to run a variety of
applications and processes simultaneously, thereby ensuring that all tasks receive at least some
processing time. As a consequence, low-priority tasks may have their priority boosted above
other higher priority tasks, which the designer may not want. However, RTOS uses priority-
based preemptive scheduling, which allows high-priority threads to meet their deadlines
consistently. All system calls are deterministic, implying time bounded operation for all
operations and ISRs. This is important for embedded systems where delay could cause a safety
hazard. The scheduling in RTOS is time based. In case of General purpose OS, like
Windows/Linux, scheduling is process based.
Preemptive kernel - In RTOS, all kernel operations are preemptible
Priority Inversion - RTOS have mechanisms to prevent priority inversion
Usage - RTOS are typically used for embedded applications, while General Purpose OS are used
for Desktop PCs or other generally purpose PCs.
RTOS CLASSFICATION
RTOS specifies a known maximum time for each of the operations that it performs. Based upon
the degree of tolerance in meeting deadlines, RTOS are classified into following categories
Hard real-time: Degree of tolerance for missed deadlines is negligible. A missed deadline can
result in catastrophic failure of the system
Firm real-time: Missing a deadly might result in an unacceptable quality reduction but may not
lead to failure of the complete system
Soft real-time: Deadlines may be missed occasionally, but system doesn’t fail and also, system
quality is acceptable
For a life saving device, automatic parachute opening device for skydivers, delay can be fatal.
Parachute opening device deploys the parachute at a specific altitude based on various
conditions. If it fails to respond in specified time, parachute may not get deployed at all leading

3. to casualty. Similar situation exists during inflation of air bags, used in cars, at the time of
accident. If airbags don’t get inflated at appropriate time, it may be fatal for a driver. So such
systems must be hard real time systems, whereas for TV live broadcast, delay can be acceptable.
In such cases, soft real time systems can be used.
Important terminologies used in context of real time systems
Determinism: An application is referred to as deterministic if its timing can be guaranteed within
a certain margin of error.
Jitter: Timing error of a task over subsequent iterations of a program or loop is referred to as
jitter. RTOS are optimized to minimize jitter.
Solution
There are many operating systems
Real-Time Operating System
Real-time applications usually are executed on top of a Real-time Operating System (RTOS).
Specific scheduling algorithms can be designed. When possible, static cyclic schedules are
calculated off-line.
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.
RTOS is therefore an operating system that supports real-time applications by providing
logically correct result within the deadline required. Basic Structure is similar to regular OS but,
in addition, it provides mechanisms to allow real time scheduling of tasks.
Though real-time operating systems may or may not increase the speed of execution, they can
provide much more precise and predictable timing characteristics than general-purpose OS.
A real-time system is defined as a data processing system in which the time interval required to
process and respond to inputs is so small that it controls the environment. The time taken by the
system to respond to an input and display of required updated information is termed as the
response time. So in this method, the response time is very less as compared to online
processing.
Real-time systems are used when there are rigid time requirements on the operation of a
processor or the flow of data and real-time systems can be used as a control device in a dedicated
application. A real-time operating system must have well-defined, fixed time constraints,
otherwise the system will fail. For example, Scientific experiments, medical imaging systems,
industrial control systems, weapon systems, robots, air traffic control systems, etc.
Design considerations

4. Designing a proper RTOS architecture needs some delicate decisions. The basic services like
process management, inter-process communication, interrupt handling, or process
synchronization have to be provided in an efficient manner making use of a very restricted
resource budget.
Multi-core architectures need special techniques for process management, memory management,
and synchronization. The upcoming Wireless Sensor Networks (WSN) generate special demands
for RTOS support leading to dedicated solutions. Another special area is given by multimedia
applications. Very high data rates have to be supported under (soft) RT constraints.
The key difference between general-computing operating systems and real-time operating
systems is the need for " deterministic " timing behavior in the real-time operating systems.
Formally, "deterministic" timing means that operating system services consume only known
and expected amounts of time. In theory, these service times could be expressed as mathematical
formulas. These formulas must be strictly algebraic and not include any random timing
components. Random elements in service times could cause random delays in application
software and could then make the application randomly miss real-time deadlines – a scenario
clearly unacceptable for a real-time embedded system. Many non-real-time operating systems
also provide similar kernel services.
DIFFERENCE: RTOS v/s General Purpose OS
Determinism - The key difference between general-computing operating systems and real-time
operating systems is the “deterministic " timing behavior in the real-time operating systems.
"Deterministic" timing means that OS consume only known and expected amounts of time.
RTOS have their worst case latency defined. Latency is not of a concern for General Purpose
OS.
Task Scheduling - General purpose operating systems are optimized to run a variety of
applications and processes simultaneously, thereby ensuring that all tasks receive at least some
processing time. As a consequence, low-priority tasks may have their priority boosted above
other higher priority tasks, which the designer may not want. However, RTOS uses priority-
based preemptive scheduling, which allows high-priority threads to meet their deadlines
consistently. All system calls are deterministic, implying time bounded operation for all
operations and ISRs. This is important for embedded systems where delay could cause a safety
hazard. The scheduling in RTOS is time based. In case of General purpose OS, like
Windows/Linux, scheduling is process based.
Preemptive kernel - In RTOS, all kernel operations are preemptible
Priority Inversion - RTOS have mechanisms to prevent priority inversion
Usage - RTOS are typically used for embedded applications, while General Purpose OS are used
for Desktop PCs or other generally purpose PCs.

5. RTOS CLASSFICATION
RTOS specifies a known maximum time for each of the operations that it performs. Based upon
the degree of tolerance in meeting deadlines, RTOS are classified into following categories
Hard real-time: Degree of tolerance for missed deadlines is negligible. A missed deadline can
result in catastrophic failure of the system
Firm real-time: Missing a deadly might result in an unacceptable quality reduction but may not
lead to failure of the complete system
Soft real-time: Deadlines may be missed occasionally, but system doesn’t fail and also, system
quality is acceptable
For a life saving device, automatic parachute opening device for skydivers, delay can be fatal.
Parachute opening device deploys the parachute at a specific altitude based on various
conditions. If it fails to respond in specified time, parachute may not get deployed at all leading
to casualty. Similar situation exists during inflation of air bags, used in cars, at the time of
accident. If airbags don’t get inflated at appropriate time, it may be fatal for a driver. So such
systems must be hard real time systems, whereas for TV live broadcast, delay can be acceptable.
In such cases, soft real time systems can be used.
Important terminologies used in context of real time systems
Determinism: An application is referred to as deterministic if its timing can be guaranteed within
a certain margin of error.
Jitter: Timing error of a task over subsequent iterations of a program or loop is referred to as
jitter. RTOS are optimized to minimize jitter.