RTOS Basic Concepts

REAL TIME OPERATING
SYSTEM (RTOS) CONCEPTS

Pantech Prolabs India Pvt Ltd

What is Real Time?
1) Real time is a level of responsiveness that a user senses
as sufficiently immediate or that enables the computer to
keep up with some external process.
2) Real time describes a human rather than machine sense of
time.
3) It is the class of computers systems that interacts with the
external world in a time frame defined by the external world.
4) It is the system in which the correctness of the
computations not only depends upon the logical correctness
of the computation but also upon the time at which the
result is produced.
5) If the timing constraints of the system are not met system
failure is said to have occurred.

There are two aspects related to timeliness :
1) Objective : The degree to which an entity (application, system, thread) operates in
real time has acceptable timeliness properties according to timeliness specification.

2) Means To that Objective: Achieving an acceptable degree of timeliness may or may
not involve the second aspect of real time computing.(Real time resource management)

What is RTOS?
1) A real time operating system (RTOS) is an operating system that guarantees a certain
capability within a specified time constraint.
2) An OS is a system program that provides an interface between application programs and
the computer system (hardware)
3) The applications where dependability that a certain task will finish before a particular
deadline is just as obtaining the correct results.
4) Besides meeting deadlines RTOS must also be able to respond predictably to unpredictable
events and process multiple events concurrently.

5) A system application/computer/operating system operates in real
time to the degree that those of its actions which have time
constraints are performed with acceptable timeliness.

6) A system is real time the degree that it employs real time resource
management. The resources are explicitly managed for the purpose of
operating in real time.

7) The system operating in real time needs an appropriate balance of
real time resource management & hardware resource capacity.

OS Basics
Four main tasks of OS
 Process Management

 Process creation

 Process loading

 Process execution control

 Interaction of the process with signal
events
 Process monitoring

 CPU allocation

 Process termination

OS Basics
 Inter-process Communication
 Synchronization and coordination

 Deadlock detection

 Process Protection

 Data Exchange Mechanisms

 Memory Management

 Services for file creation, deletion,
reposition and protection
 Input / Output Management

 Handles requests and release subroutines
for a variety of peripherals and read, write
and reposition programs

Types Of RTOS

1) Hard Real Time Systems:
a) If it has a hard deadline for the completion of an action meaning
that the deadline must always be met, otherwise the task has failed.

b) This types of systems deployed in embedded safety-critical
systems in which missed deadline can be catastrophic.

c) Examples: aircraft , train, car, missile and nuclear reactor control
systems.

2) Soft Real Time Systems :

a) Soft real time by default as “Not Hard Real Time.”

b) Missing some deadlines by some amount under some circumstances may be acceptable
rather than failure.

c) In this systems there is usually a rising cost associated with lateness.

d) Soft real time means systems which have reduced constraints on “lateness” but still
must operate very quickly and repeatable.

Architecture of RTOS
The basic architecture of multitasking RTOS includes
a) Program interface b) The Kernel c)Device Drivers
d) Optional service modules

1) The Kernel provides an interrupt handler, task scheduler ,
resource sharing flags and memory management.

2) Primary function : To process interrupts that external and internal
events cause.

3) Scheduler :That sets up the order of execution of your application
code.

Command Interpreter Graphics Interface

Posit and Unix Graphics Programming Interface

Kernal Services Networking

Interrupt Memory
Scheduling Network File
Dispatch Management Graphics
System
ANSI
Remote Support
Compliant
Logic and Real Time Procedure Library
C
character I/O Interposes Clock Calls
Library
Communication
File Support
Loader
Management Streams

Device Drivers
Disc Miscellaneous Network Parallel Serial

Requirement for Good RTOS

1) Multitasking Capabilities: A RT application is divided into multiple
tasks. The separation into tasks helps to keep the CPU busy.

2) Short Interrupt Latency :
Interrupt Latency = Hardware delay to get interrupt signal to the
processor + time to complete the current instruction + time executing
system code in preparation for transferring execution to the devices
interrupt handler.

3) Fast Context Switch : The time between the OS recognizing that the
awaited event has arrived and the beginning of the waiting task is called
context switch time(dispatch latency).This switching time should be
minimum.

4) Control Of Memory Management :an OS should provide way for task
to lock its code and data into real memory so that it can guarantee
predictable response to a interrupt.

5) Proper scheduling : OS must provide facility to schedule properly
time constrained tasks.

6) Fine granularity Timer Services : Millisecond resolution is bare
minimum . Microseconds resolution is required in some cases.

7) Rich set of Inter Task Communication Mechanism :
Message queues, shared memory ,
Synchronization –Semaphores, event flags

Fundamental RTOS capabilities :

a) RTOS vendor must provide worst case execution times for all times
for all OS routines.

b) RTOS must include scheduling and resource sharing algorithms that
enables deterministic behavior.

c) Effective RTOS must also be able to handle preemptive scheduling.
It enables high priority tasks to execute without being blocked by
lower priority tasks.

RTOS Concepts
1) Scheduling and Dispatching :
a) Scheduling is the creation of a schedule Ie. A ordered list specifying
how contending accesses to one or more sequentially reusable
resources will be granted.

b) A schedule is some criteria such as timeliness.

c) Dispatching is granting access to the currently most eligible
contending entity.

d) Eligibility is the entities position in a schedule or in the absence of
schedule , other eligibility parameters such as priority and deadline.

e) More commonly RTOS/OS they can be used for dispatching from a
heap of unscheduled contending entities (highest priority or earliest
deadline) this is called dispatching rule.

2) Schedulable and Non-Schedulable Entities : A computing system has
a mixture has schedulable & Non-schedulable entities.
a) Schedulable Entities : Threads, tasks, processes in application and
system s/w are scheduled by scheduler.

b) Non-Schedulable Entities: Interrupt handlers,OS commands, packet
level N/W communication services ,OS scheduler. They execute
continuously , periodically or in response to events, their timeliness is
the responsibility of the system design and implementation not of the
scheduler.

3) Timeliness Specification : There are two levels
a) For Individual Entities : Schedulable ones such as threads ,non
schedulable ones such as interrupt routines.
b) For sets of Entities : For sets of entities have collective timeliness
specification.

4) Schedulable Entity Timeliness Specifications
a) First level of timeliness specification for a schedulable entity
(thread) is that it may have one or more completion time
constraints(deadlines).
b) Second level of timeliness specification is that currently unable
set of such threads is scheduled such that to optimize collective
timeliness.ie.to meet all deadlines.

5) Completion Time Constraint : It is predicate which applies to some portion(frequently all) of
a threads locus of execution, that portion is called the time constraints scope. It is the part of
the logic of the application.

6) Deadline : It is the completion time constraint which specifies the timeliness of the threads
transit through the deadline scope whether the threads execution point reaches the end of the
scope before the deadline time has occurred ie.whether the deadline is met.

7) Hard Deadline: It is completion time constraint ie.the threads
execution point reaches the end of the deadline scope before the
deadline time occurs, then time constrained portion of threads
execution is timely otherwise that portion is not timely.

8) Soft Deadline: It is completion time constraint ie.the threads
execution point reaches the end of the deadline scope before the
deadline time occurs, then time constrained portion of threads
execution is more timely otherwise that portion is less timely.

9) Soft Time Constraint :
a) It is any relationship between the time when the threads execution
point reaches the end of the that time constraints scope and the utility
to the system of when it does so.
b) A thread for which there is no relationship between the timing of its
execution and its utility to the system is a non real-time system.

10) Non-schedulable Entity Timeliness Specifications :
a) The completion time constraints of non-schedulable entities
are Hard & soft upper bounds on their execution
latencies(duration) which correspond to hard and soft deadlines
of schedulable entities.

11) Upper Bound: An upper bound is completion time constraint.
It specifies the timeliness of Non-schedulable entity’s execution
point reaches the end of the scope before the upper bound time
has occurred in which case the upper bound is satisfied.

a) Hard Upper Bound : Non-schedulable entity’s execution point
reaches the end of the upper bound scope before the upper bound time
occurs. Time constrained portion of the entity’s execution is timely,
otherwise the portion is not timely.

b) Soft Upper Bound : Non-schedulable entity’s execution point reaches
the end of the upper bound scope before the upper bound time occurs.
Time constrained portion of the entity’s execution is more timely,
otherwise the portion is less timely.

12) Predictability :
a) It is to the degree that is known in advance.
b) Determinism : the property is known exactly in advance.
c) Maximum Entropy : Nothing at all is known in advance about the property.

13) Predictability of Timeliness Optimality :
a) It’s the performance of the system of whole. ie.meet all deadlines , minimize the number of
missed deadlines.
b) The predictability of collective timeliness optimality, rather than predictability of each
individual threads completion with respect to its own time constraint.

14) Hard Real Time :
a) Case where for schedulable entities some time constraints are hard deadlines.
b)Timeliness component of scheduling criterion is always meet all hard deadlines.
c)For non-schedulable entities some upper bounds are hard, system has designed &
implemented so that all hard upper bounds are always satisfied.

15) Soft Real Time :
a) Time constraints are soft such as classical lateness function.
b) Any scheduling criteria may be used such as minimizing the no. of missed deadlines.

Features Of RTOS
1) Multitasking : There are two ways to achieve Multitasking
to share CPU time between two or more tasks.

A) Pre-emptive Multitasking :
a) An external tick interrupt ,interrupts task at an
indeterminate point and passes control to kernel program.
b) The kernel will save the state of the interrupted task ad
then determine which task it should run next.
c) The Kernel restore the state of task and pass control of the
CPU to that task.
d) Task will continue to run until it is interrupted by next
external tick interrupt or voluntarily gives up allotted time
slice.

B) Non-preemptive Multitasking :

a) In Non-preemptive multitasking a task is designed to relinquish
control of the CPU to the kernel at regular intervals.

b) To the implementation of non-preemptive multitasking is the use
of a message queue.

c) The kernal manages the message queue which is essentially a
queue of message numbers & small amount of data.

.

2) Process thread can be prioritized :
a) A OS schedules threads/or processes based on their relative priorities.
b) Processes are forced to relinquish the processor if a higher priority process becomes ready to
run.
c) Highest priority unable process will be executing on the system at all times

3) Sufficient number of interrupt levels :
a) Advantage of interrupt threads is that they permit the OS to be more responsive to the
devices it services.
b) ISR typically disable other interrupts while executing using interrupt threads permits
interrupts to be responded to at more regular intervals

Task Management

The main function of tasks within a Real-Time Multitasking
Executive is the time-critical processing of external or
internal events

 Task States
 Task Switching

Process Scheduling Types
 Types of Scheduling Policies
 Deadline driven – ideal but not available

 Cooperative – relies on current process to give up

the CPU
 Pre-emptive priority scheduling – higher priority

tasks may interrupt lower priority tasks
 Static – priorities are set before the system
begins execution
 Dynamic – priorities can be redefined at run
time

Process Scheduling Types
 Many priorities levels in a RTOS is better to
have for complex systems with many threads
At least 128 levels
A different priority level can be set for each
task or thread
 Round Robin – give each task an equal share of the
processor
 Implemented when all tasks or threads have the
same priority level
 May be implemented within a priority range

Real time Applications Used Today:

1) Military : For gathering data from a wide variety of real
time sensors (satellite, air,surface,subsurface) and delivering
it to analysis capabilities and weapon systems

2) Telecommunications: Real Time access to information
from vast array of devices including computers,set top
boxes,PDAs,Pagers and wireless phones.

3) Aviation: Improved real time capabilities will increase
safety and efficiency by identifying dangerous situations and
enabling flexible routing to relieve congestion and prevent
collisions.

4)Automotive : Commuters will receive real time data , GPS location information and
operational data via ground stations and satellites , thus routing the driver around
accidents , roadwork or traffic congestion in real time.

5) Scientific : For monitoring and instrument control.eg.seismic information is
increasingly distributed on a vast scale ,enabling scientist to verify even small scale
nuclear detonations as well as helping smaller remote countries prepare for weather and
seismic disasters.

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RTOS Basic Concepts

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