問14
頻出度★☆☆
組込みシステムの特徴の一つであるリアルタイム性の説明として, 適切なものはどれか。
平成23年度 特別試験 ITパスポート試験 問14
This question's explanation and your answers so far.
解説
問 14
Incorrectア
“The property of being usable anytime, anywhere, at the required time and place” describes ubiquitous computing (a ubiquitous society). It refers to the idea of an expanding range of usage situations, in which users can enjoy the benefits of networks and computers anytime and anywhere; it is not about guaranteeing that processing finishes by a deadline. It is true that embedded systems are one of the elements supporting a ubiquitous society, but that is a separate matter from the definition of real-time capability.
- イ
正解 The correct answer is イ. Real-time capability means the property of always completing a process within a time limit (deadline) decided in advance, and choice イ, “the property of completing a given process within a defined time limit,” is exactly that definition. Embedded systems are used where the function itself would not work unless the system responds without delay to an event occurring outside it: braking control and airbag deployment in cars, control of industrial machinery, the response of home appliances to user operations, and so on. What matters is therefore not simply “being fast” but “being guaranteed to finish within the decided time.” Fields where exceeding the deadline leads directly to an accident or a defect are called hard real-time, while fields where some delay is tolerated are called soft real-time. The key point is that the axis real-time capability measures is time — neither the amount of resources nor how unlikely a failure is.
Incorrectウ
“The property of carrying out a given process within tightly constrained resources” describes resource constraints, another important characteristic of embedded systems. An embedded device is strictly limited in the CPU performance, memory capacity, electric power, and cost it can carry, so the design must achieve the intended processing with few resources. This, however, is about the axis of resources, which differs from the axis of real-time capability — finishing within a decided time.
Incorrectエ
“The property of operating without trouble within tightly constrained resources” describes reliability (high reliability, stable operation). Embedded systems often run continuously and unattended for long periods, so they are required not to fail or stop; but that is the viewpoint of reliability — not breaking down, not halting — and it does not include the time viewpoint of a processing deadline. Remember it as something distinct from real-time capability.
ここがポイント
- Real-time capability = the property of guaranteeing that processing is completed within a decided time limit (deadline). Its essence is not “being fast” but “always finishing within the time”; when exceeding the deadline is fatal it is called hard real-time, and when some delay is tolerated it is called soft real-time.
- Embedded systems have several characteristics: besides real-time capability (a constraint on time), there are resource constraints (limited CPU, memory, power, and cost) and high reliability (running for a long period without trouble). Identify which axis the question is asking about, and do not mistake it for the concept of ubiquitous computing (anytime, anywhere).
用語