Real Time Operating Systems Questions and Answers

Answer: A

Correct answer: the timing of its results as well as their values

A right answer delivered late is a wrong answer in a real time system, which is what distinguishes the field from ordinary computing. Nothing follows about the speed of the hardware or the presence of a kernel - a system with a deadline of a second is as real time as one with a deadline of a microsecond.

Answer: B

Correct answer: constitutes a system failure

A hard deadline is one whose breach means the system has failed, whatever else it is doing correctly - an airbag firing late, a motor commutation missed. A soft deadline degrades the result instead, and the distinction determines how the system must be analysed and how much margin it needs.

Answer: C

The dropped frame degrades the experience without making the system fail or the result worthless, which is the definition of soft. A firm deadline would make the late frame valueless but harmless, and a hard one would make the miss a failure.

Answer: D

A bound that can be stated and relied upon is what allows a deadline to be proved, and a kernel advertises its operations as taking a fixed number of cycles or a time independent of the number of tasks. Being fast on average is a different and weaker property.

Answer: A

A task released every ten milliseconds but actually running between 9.7 and 10.4 has 0.7 milliseconds of jitter, which matters greatly to a control loop and not at all to a display update. Latency is the separate quantity measured from a triggering event.

Answer: B

Speed is neither necessary nor sufficient: a slow system with generous deadlines is real time and a fast one with unbounded worst-case behaviour is not. Predictability rather than throughput is the property being claimed.

Answer: C

Everything the kernel needs to suspend and later resume the task lives there, which is why switching between tasks reduces to saving one block and restoring another. The code stays in flash and is shared by every invocation.

Answer: D

Correct answer: blocked

Ready means able to run and waiting only for the processor; running means in possession of it; blocked means waiting for something else and therefore not a candidate for scheduling at all. Most kernels add a suspended state, but these three are the minimum any preemptive kernel must provide.

Answer: A

Correct answer: waits for a resource or a timeout

Blocking means the task cannot proceed until something happens, so the scheduler stops considering it. Being displaced by a higher priority task is different: that task returns to ready, since it could run if the processor were free, and it is scheduled again as soon as it is.

Answer: B

It is still perfectly able to run and needs nothing but the processor, which is exactly what the ready state means. Confusing ready with blocked is the commonest error in reading a state diagram, and it matters because a blocked task needs an event to release it.