IEEEx: RTSIx Introduction to Real-Time Systems

Quiz

Multiple Choice

• Which of the following is the most important distinguishing feature of a real-time system?

1. very fast hardware and response times 
2. mission critical application
3. temporal correctness

• Which of the following are most likely to be hard real-time systems?

1. weapons delivery system, library of congress database, automatic teller machine
2. weapons delivery system, nuclear power plant controller, seat belt deployment system 
3. automated weed killing system, nuclear power plant controller, pasta sauce bottling factory control

• Real-time systems engineering is most likely to incorporate which of the following?

1. determination of response time requirements and determining hardware and software strategies for meeting those requirements 
2. selection of hardware and configuration of off-the-shelf operating systems products 
3. formal derivation of mathematical models for the schedule

• Which of the following scheduling strategies is the most likely scheduling discipline to be used when “fairness” is the main requirement?

1. round-robin
2. cyclic executive
3. rate-monotonic

Cyclic executive is not fair because one task can be much longer than another, hogging CPU time. Rate-monotonic is not fair because it is preemptive. Round-robin is based on equal sharing of processor time.

• Which of the following scheduling strategies does not require interrupts?

1. round-robin
2. cyclic executive
3. rate-monotonic

• Rate-monotonic scheduling is used for all but the following?

1. determining the theoretical best case scenario for CPU utilization 
2. providing some confidence that deadlines can or cannot be met 
3. for determining how the interrupt priorities should be assigned

It is important not to blindly apply rate-monotonic theorem. In some cases, the lowest frequency task should have the highest priority.

• True or False: It is important to rely only on language constructs for intertask synchronization and communication because they are the most reliable and predictable.

1. True 
2. False

Programming languages cannot be relied upon to provide safe synchronization mechanisms, even when advertised as such. It is better for the real-time systems engineer to carefully study the underlying implementation of these mechanisms for each compiler and environment and consider building these mechanisms from scratch if necessary.

• True or False: The best programming languages for real-time are those that relieve the software engineer from having to understand the core mechanisms of process scheduling and intertask synchronization and communication.

1. True 
2. False

It is generally inadvisable to let the compiler handle these things without “supervision” by the engineer.

• True or False: Testing real-time systems is no different than testing any other type of system.

1. True 
2. False

The requirement for temporal correctness adds a new dimension to testing the system.

• Which of the following is not a challenge that is specific to real-time systems engineer?

1. automatic determination of code execution time is generally not possible at compile time 
2. estimation of costs to complete and schedule is extremely difficult 
3. hardware mechanisms that improve average case performance can adversely impact worst case performance

Schedule and cost estimation is difficult for any kind of system. The other two problems are of special concern to real-time systems engineers.


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