Page 7 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes deadline: it is either by the deadline or it is useless! Any system in which the time at which
output is produced is significant. This is usually because the input corresponds to some event in
the physical world, and the output has to relate to that same event.

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Caution The lag from input time to output time must be sufficiently small for acceptable
timeliness.
Real-Time systems span several domains of computer science. They are defence and space systems,
networked multimedia systems, embedded automotive electronics etc. In a real-time system
the correctness of the system behaviour builds upon not only the logical results of the
computations, but also on the physical instant at which these results are produced. A real-time
system changes its state as a function of physical time, e.g., a chemical reaction continues to
change its state even after its controlling computer system has stopped. Based on this a real-time
system can be decomposed into a set of subsystems i.e., the controlled object, the real-time
computer system and the human operator. A real-time computer system must react to stimuli
from the controlled object (or the operator) within time intervals dictated by its environment.
The instant at which a result is produced is called a deadline. If the result has utility even after the
deadline has passed, the deadline is classified as soft, otherwise it is firm. If a catastrophe could
result if a firm deadline is missed, the deadline is hard.

Real-Time systems are becoming pervasive. Typical examples of real-time systems include Air
Traffic Control Systems, Networked Multimedia Systems, and Command Control Systems etc.
In a Real-Time System the correctness of the system behaviour depends not only on the logical
results of the computations, but also on the physical instant at which these results are produced.
Real-Time systems are classified from a number of viewpoints i.e. on factors outside the computer
system and factors inside the computer system. Special emphasis is placed on hard and soft real-
time systems. A missed deadline in hard real-time systems is catastrophic and in soft real-time
systems it can lead to a significant loss. Hence predictability of the system behaviour is the most
important concern in these systems. Predictability is often achieved by either static or dynamic
scheduling of real-time tasks to meet their deadlines. Static scheduling makes scheduling
decisions at compile time and is off-line. Dynamic scheduling is online and uses schedulability
test to determine whether a set of tasks can meet their deadlines. The present paper talks about
static and dynamic scheduling algorithms and operating systems support for these mechanisms.

Example: ABS, aircraft control, ticket reservation system at airport, over-temperature
monitor in nuclear power station, mobile phone, oven temperature controller, Doppler blood-
flow monitor, ECG/arrhythmia monitor.
Although there is no clear dividing line between real-time and non-real-time systems, there are
a set of distinguishing features (listed below) which can assist with an outline classification
schema to identify real-time applications.
Timing: The most common definition of a real-time system involves a statement similar to
‘Real-time systems are required to compute and deliver correct results within a specified period
of time.’ Does this mean that a non-real-time system such as a payroll program, could print
salary cheques two years late, and be forgiven because it was not a real-time system? Hardly so!
Obviously there are time constraints on non-real-time systems too. There are even circumstances
in which the early delivery of a result could generate more problems than lateness of delivery.
A premature newspaper obituary could sometimes create as much havoc as an early green on a
traffic light controller.
Interrupt driven: After the requirement for maximum response delay times, the next characteristic
of real-time systems is their involvement with events. These often manifest themselves in terms

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