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Real - Time Systems

SREENATH R

Reg. no: 0920021

MBA A

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What is a real-time system?

A real-time system is a software system where the correct functioning of

the system depends on the results produced by the system and the time at

which these results are produced

A real-time system is any information processing system which has to

respond to externally generated input stimuli within a finite and specified

period. the correctness depends not only on the logical result but also the time

it was delivered

failure to respond is as bad as the wrong response!

A single system may have all hard, soft and real real-time subsystems In

reality many systems will have a cost function associated with missingeach deadline

Terminologies:

Hard real-time systems where it is absolutely imperative that responses

occur within the required deadline. E.g. Flight control systems.

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Soft real-time systems where deadlines are important but which willstill function correctly if deadlines are occasionally missed. E.g. Dataacquisition system.

Real real-time systems which are hard real-time and which theresponse times are very short. E.g. Missile guidance system.

Firm real-time systems which are soft real-time but in which there isno benefit from late delivery of service.

Characteristics ofa RTS:

Large and complex vary from a few hundred lines of assembler or Cto 20 million lines of Ada estimated for the Space Station Freedom

Concurrent control of separate system components devices operate inparallel in the real-world; better to model this parallelism by concurrententities in the program

Facilities to interact with special purpose hardware need to be able toprogram devices in a reliable and abstract way

Extreme reliability and safe embedded systems typically control theenvironment in which they operate; failure to control can result in lossof life, damage to environment or economic loss.

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Guaranteed response times we need to be able to predict with

confidence the worst case response times for systems; efficiency is

important but predictability is essential.

Real-time Programming Languages:

Assembly languages

Sequential systems implementation languages e.g. RTL/2, Coral

66, Jovial, C.

Both normally require operating system support.

High-level concurrent languages. Impetus from the software crisis.

e.g. Ada, Chill, Modula-2, Mesa, Java.

No operating system support!

We will consider: Java/Real-Time Java

C and Real-Time POSIX

Ada 95

Also Modula-1 for device driving

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Real-Time Languages and OSs

Typical OS Configuration

Hardware

Operating

System

User Programs

Hardware

Including Operating

System Components

User Program

Typical Embedded Configuration

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A Typical Embedded System

Algorithms for

Digital Control

Data Logging

Data Retrieval

and Display

Operator

Interface

InterfaceEngineering

System

Remote

Monitoring System

Real-Time

Clock

Database

Operators

Console

Display

Devices

Re

al-Time

Compu

ter

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Real - Time System elements

and Design ProcessReal - Time System elements:

Sensors control processes: Collect information from sensors. May buffer

information collected in response to a sensor stimulus

Data processor: Carries out processing of collected information and

computes the system response

Actuator control: Generates control signals for the actuator

Real - Time System Design Process:

Identify the stimuli to be processed and the required responses to these

stimuli.

For each stimulus and response, identify the timing constraints

Aggregate the stimulus and response processing into concurrent

processes. A process may be associated with each class of stimulus and

response

Design algorithms to process each class of stimulus and response. Thesemust meet the iven timin re uirements

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Design a scheduling system which will ensure that processes are

started in time to meet their deadlines

Integrate using a real-time executive or operating system

An Example ofa simple Real Time system: