Real-time Software Engineering

Software Engineering 10

Computers are used to control a

wide range of systems from simple

domestic machines, through games

controllers, to entire manufacturing

plants.

The software in these systems is

embedded in system hardware, often

in read-only memory, and usually

responds, in real time, to events from

the system’s environment.

Their software must react to events

generated by the hardware and,

often, issue control signals in

response to these events.

Responsiveness in real-time is the

critical difference between

embedded systems and other

software systems, such as web-

based systems or personal software

systems.

In a real-time system, the

correctness depends both on the

response to an input and the time

taken to generate that response.

If the response to a stimulus in a

real-time system is too late, the

system is considered to be incorrect.

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 soft real-time system is a system

whose operation is degraded if

results are not produced according

to the specified timing requirements.

A hard real-time system is a system

whose operation is incorrect if

results are not produced according

to the timing specification.

Embedded systems generally run

continuously. They start when the

system is powered up and do not

terminate until the power is switched

off.

Interactions with the system’s

environment are unpredictable.

Events may not occur when

expected.

There may be physical limitations,

such as battery power, that affect the

design of a system.

Real-time systems often interact

directly with hardware through

specialized hardware interfaces.

Issues of safety and reliability may

may be the most important factors to

be considered when designing the

system.

Real-time systems are often

considered to be reactive systems.

Given a stimulus, the system must

produce a reaction or response

within a specified time.

Periodic stimuli. Stimuli which occur

at predictable time intervals

A sensor may return a value every

250 milliseconds. The system

response depends on that value.

Aperiodic stimuli. Stimuli which

occur at unpredictable times

For example, it is impossible to know

when a power failure will occur.

A general model of an embedded real-

time system

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Real-time systems are usually

organised as a number of parallel

processes.

This allows them to respond in a

timely way to events from the

system’s environment.

It is difficult to design a sequential

process in such a way that you can

guarantee that predict the

responsiveness of the system.

Sensor and actuator processes

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