DESIGN ISSUES
CHIRANTAN ANDHALKAR
ROLL NO : 05
CLOCK SYNCHRONIZATION
• Maintenance of time is important
• Multiple computers have its own local clock ,
keeping the clocks in synchrony is a key issue.
EVENT TRIGGERED SYSTEM
• In outside world when any events occurs it is
detected by the sensor
• This sensor then causes the attached CPU to get
an interrupt
• Interrupt driven
• Works well with soft real time systems.
• For complex systems, it works well if the compiler
can analyze the program and knows about the
system very well once an event occurs.
• Event triggered system can fail under conditions
of heavy load
TIME TRIGGERED SYSTEMS
• A clock interrupt occurs every T milliseconds
• T should be chosen very carefully.
• At each clock tick sensors are sampled and
actuators are driven.
• No interrupts occur other than clock ticks.
• It overcomes the main problem of Event
Triggered System (i.e. failure due to heavy load)
• Higher chances of successfully dealing with crises
.
PREDICTABILITY
• One of the most important properties of real
time systems is that its behavior be
predictable.
• It should be clear at the design time that the
system can meet all of its deadlines, even at
peak load.
• Worst case behavior of the system should be
known.
FAULT TOLERANCE
• Some real time systems have the property
that they can be stopped when a serious
failure occurs. Such systems are known as
fail-safe
• It is especially important that the system be
able to handle the worst case scenarios.
• System must be able to cope with the
maximum number of faults and maximum
load at the same time.
FAULT TOLERANCE cont…..
Different methods used for safety are ::
1. Replication
2. Primary backup scheme
3. Follow the leader
LANGUAGE SUPPORT
• Many real time systems are programmed in in general
purpose language such as C.
• The language should be designed so that maximum
execution time of every task can be computed at
compile time.
• Real time languages need a way to deal with time itself.
• The language should have a way to express minimum
and maximum delays
• Language should also express what should be done if
expected event does not occur in certain time interval.
LANGUAGE SUPPORT cont……
• Recursion should be avoided
• For loop should be used for iterations
• Since periodic events play a big role in a real time
systems, it would be useful to have statements such as
every(25 msec)
{
// statements to be executed
}
• Using such statements, the compiler is able to compute
what percentage of CPU time required by each task.