問16
頻出度★☆☆
組込みシステムに求められる特性のうち, 与えられた時間で一定の処理を完了させなければならないことを意味するものはどれか。
平成24年度 秋期 ITパスポート試験 問16
This question's explanation and your answers so far.
解説
問 16
Incorrectア
Safety is the characteristic that a system does not cause harm to human life, bodies, property, or the environment, and that even when a failure or abnormality occurs it does not enter a dangerous state but stops or operates on the safe side (fail-safe). What it covers is whether harm occurs, not whether a process finishes within a time limit. Failing to finish in time may sometimes lead to an accident as a consequence, but that is not the definition of safety itself, so this choice is wrong.
Incorrectイ
Reliability is the degree to which a system keeps operating correctly without failing, and it is evaluated with measures such as MTBF (mean time between failures) and availability. Its focus is on being hard to break and able to keep running, which is a different viewpoint from the time constraint of whether each individual process finishes within its time limit. Even a system that is running without any failure can still miss a deadline, so this does not correspond to the statement in the question.
- ウ
正解 The correct answer is ウ, real-time capability. Real-time capability is the property that a process must always be completed within a fixed time limit (a deadline), and this exactly matches the wording in the question that a certain process must be completed within a given time. Because embedded systems exist to control equipment, there are many situations in which they are meaningless unless they respond to each input within a fixed time. For example, a car airbag cannot protect the occupants unless it deploys within a few tens of milliseconds after a collision is detected, and brake control or motor control breaks down entirely if it is delayed. The important point here is not simply being fast, but always finishing by the fixed time (the timing is guaranteed). Note also that systems where missing the deadline causes a fatal result are called hard real-time, while those where some delay is tolerated and the result is only lower quality (such as video playback) are called soft real-time.
Incorrectエ
Resource constraints refers to the characteristic that the resources an embedded system can carry — CPU performance, memory capacity, storage devices, power consumption, physical size, cost, and so on — are far more tightly limited than those of a general-purpose computer. This is about the precondition that only few resources are available, not about a requirement concerning time such as completing a process within a given time, so this choice is wrong.
ここがポイント
- Real-time capability = the property that a process must always be completed within a fixed time limit (a deadline). When a question uses time expressions such as within a certain time, respond immediately, or by the deadline, choose real-time capability. If missing the deadline is fatal it is hard real-time; if some delay is tolerated it is soft real-time.
- Distinguish the main characteristics required of embedded systems by their viewpoint: real-time capability = time, reliability = not failing and continuing to run, safety = not causing harm (fail-safe), resource constraints = limited resources such as CPU, memory, and power.
用語