Page 39 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes

Timing constraints can be specified in different terms:

1. deterministic constraints (e.g. response time d” 50 ms)
2. probabilistic constraints (e.g. probability that response time > 50 ms less than 0.1)
3. in terms of some usefulness function (e.g. usefulness function e” 0.8)
In practice hard timing constraints are of the first type: specified as deterministic constraints
(easier to validate than the latter two types).
Steps in validating timing constraints are discussed below:
1. Verify that in each component of the system the timing constraints are specified correctly:
ensure that the timing constraints of each component are:
 Mutually consistent,

 Consistent with the high-level RT requirements.
2. Verify the feasibility of each component with the available HW and SW resources: ensure
that each component in the system can meet its timing constraints if:

 It executes alone
 It has all the required resources.
3. Verify that the whole system behaves as specified: ensure that, when scheduled together
with some scheduling algorithm, the timing constraints of all components (competing for
the available resources) are always met.
Now let us discuss how to validate timing constraints.
1. To verify that in each component of the system the timing constraints are specified correctly:
use formal methods.
2. To verify the feasibility of each component with the available HW and SW resources: use
performance analysis.
3. To verify that a scheduling algorithm can meet the timing constraints of all components,
competing for the available resources is done.

3.5 Hard Real-time Systems and Soft Real-time Systems

Real-time systems are classified as hard or soft real-time systems. Hard real-time systems have
very strict time constraints, in which missing the specified deadline is unacceptable. The system
must be designed to guarantee all time constraints. Every resource management system such as
the scheduler, input-output (I/O) manager, and communications, must work in the correct order
to meet the specified time constraints.
Military applications and space missions are typical instances of hard real-time systems. Some
applications with real-time requirements include telecom switching, car navigation, the medical

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