Real-Time and Embedded Systems

Question 1

What is a real-time system?

Answer 1

Any information processing system which must respond to externally generated input stimuli within a finite and specified period. The correctness depends not only on the response but also the time it was delivered. Failure to respond in time is as bad as a wrong response.

Question 2

What are the classes of real-time systems?

Answer 2

Hard real-time
Soft real-time
Firm real-time

Question 3

What is a hard real-time system?

Answer 3

Systems where it is absolutely imperative that responses occur within the required deadline, e.g. a flight control system.

Question 4

What is a soft real-time system?

Answer 4

Systems where deadlines are important but which will still function correctly if deadlines are occasionally missed, e.g. a data acquisition system.

Question 5

What is a firm real-time system?

Answer 5

Systems which are soft real-time but in which there is no benefit from late delivery of service (assuming that there can be something to be gained by late delivery), e.g. on demand video streaming.

Question 6

What are the most popular choices for real-time programming languages?

Answer 6

Java/Real-Time Java
C and Real-Time POSIX
Ada 2012; Ada 202x draft 32

Question 7

What is a bug?

Answer 7

A software fault

Question 8

What are the types of bugs?

Answer 8

Bohrbugs
Heisenbugs

Question 9

What is a bohrbug?

Answer 9

Reproducible and identifiable

Question 10

What is a heisenbug?

Answer 10

Only active under rare conditions (e.g. race conditions)

Question 11

What are the two classes of role of time?

Answer 11

1. Time-aware
2. Reactive

Question 12

What is a time-aware system?

Answer 12

A system which makes explicit reference to time, the system operation references absolute time values.

Question 13

What is a reactive system?

Answer 13

The system must produce output within deadline (as measured from input). The system operation references relative time values and must “keep up-to-date with the environment”. Examples include control systems.

Question 14

What the the different structural and temporal requirements of a RTS?

Answer 14

1. Time-triggered
2. Event-triggered

Question 15

What is meant by a time-triggered RTS?

Answer 15

Computation is triggered by the passage of time (e.g. activity at 9:00am)

Question 16

What is a periodic activity?

Answer 16

Release activity every 25ms

Question 17

What is an event-triggered RTS?

Answer 17

Computation triggered by an external or internal event

Question 18

What is a sporadic activity?

Answer 18

Released activity is sporadic if there is a bound on the arrival interval of the event (e.g. mass storage)

Question 19

What is an aperiodic activity?

Answer 19

Released activity is aperiodic if there is no bound on the arrival (e.g. moving the pointer with a mouse on a PC screen)

Question 20

What are 6 characteristics of a RTS?

Answer 20

1. Guaranteed response time
2. Multiple systems can be controlled at the same time
3. Can interact with specialist hardware
4. Continuous numerical computation
5. Large and complex
6. Reliable and safe - embedded systems often control environment in which failures result in loss of life etc.

Question 21

What is meant by a safe RTS?

Answer 21

Freedom from damage to people, environment and itself, e.g.
System safety analysis must be performed at all stages of its life cycle

Question 22

What is meant by a reliable RTS?

Answer 22

Measure of success vs specification

Question 23

What is a failure?

Answer 23

When the behaviour deviates from that which is specified due to unexpected internal problems, manifesting in the external behaviour

Question 24

What is the difference between an error and a fault?

Answer 24

A mechanical or software fault is the cause which produces an error in the systems behaviour

Question 25

What are the three types of faults?

Answer 25

1. Transient 2. Permanent 3. Intermittent

Question 26

What is a transient fault?

Answer 26

Fault arises randomly and remains in the system for a certain time before disappearing again, e.g. communication systems or adverse reaction in a component due to radioactivity

Question 27

What is a permanent fault?

Answer 27

Faults that remain in the system until they are repaired, e.g. a broken wire.

Question 28

What is an intermittent fault?

Answer 28

A subset of a transient fault. They are transient faults that reoccur, e.g. a heat-sensitive hardware component which works, heats up, stops, cools, and starts working again

Question 29

What is fault prevention and what are the two stages?

Answer 29

Attempts to limit the possibility of a fault creeping into a system before it is operational. The two stages are fault avoidance and fault removal

Question 30

What is fault tolerance?

Answer 30

System is able to continue functioning even if faults are present.

Question 31

What is fault avoidance?

Answer 31

Limits the introduction of faults during system construction

Question 32

How is fault avoidance implemented?

Answer 32

1. Using reliable components 2. Using refined techniques 3. Shielding from interference 4. Rigorous specification of requirements 5. Using proven design techniques 6. Modular design

Question 33

What is fault removal?

Answer 33

The process of finding and removing the causes of (either hardware or software) errors

Question 34

How is fault removal performed? What are some considerations when performing fault removal?

Answer 34

Review designs, verification, code inspections, system testing Tests only show the presence of faults, not their absence It may be impossible to test under realistic conditions

Question 35

What are the three fault tolerance levels?

Answer 35

1. Full fault tolerance 2. Graceful degradation (fail soft) 3. Fail safe

Question 36

What is full fault tolerance?

Answer 36

System continues to operate in the presence of faults (for a limited period) with no significant loss of functionality/performance

Question 37

What is graceful degradation?

Answer 37

System continues to operate in the presence of errors, accepting a partial degradation of functionality/performance during recovery or repair

Question 38

What is fail soft?

Answer 38

System maintains its integrity while accepting a temporary halt in it operation

Question 39

What is redundancy?

Answer 39

It is a type of fault tolerance by having extra components if one fails, often called protective redundancy, these components are not required in a perfect system. The more redundant components are added, the less reliable the system becomes.

Question 40

What trade-off is used in redundancy?

Answer 40

Minimise redundancy while maximising reliability, subject to the cost and size constrains

Question 41

What is exception handling?

Answer 41

A type of error recovery mechanism in which there is no roll back to a previous state. Control is then passed to the handler for recovery procedures to be initiated.

Question 42

What can error handling be used for?

Answer 42

1. Cope with abnormal conditions arising in the environment 2. Enable program design faults to be tolerated 3. Provide a general-purpose error-detection and recovery facility