3.1 The Real-time Concept

The term real-time is defined by FOLDOC [28] as following:

1. Describes an application which requires a program to respond to stimuli within some small upper limit of response time (typically milli- or microseconds). Process control at a chemical plant is the classic example. Such applications often require special operating systems (because everything else must take a back seat to response time) and speed-tuned hardware.

2. In jargon, refers to doing something while people are watching or waiting. ``I asked her how to find the calling procedure's program counter on the stack and she came up with an algorithm in real time.''

(Used to describe a system that must guarantee a response to an external event within a given time. [28])

Definition 1 puts the requirement of time more on the underlying operating system than the specific DCharts implementation. This is because the small upper limit of response time can only be guaranteed if the operating system supports it. For many common-purpose systems that model designers would most probably use, this guarantee is hard to achieve. For example, Linux only provides a very limited support for real-time computation; Windows and many other multi-tasking operating systems perform even more unsatisfactory within the real-time domain. To allow DCharts to be implemented on most systems and platforms, the real-time requirement cannot be formalized as strict as that in definition 1.

Definition 2 makes the real-time concept more general: the DCharts implementations provide real-time support for models within the extent of their capability. The model users watch the simulation/execution of the models and wait for them to respond.

The real-time concept is defined by Webopedia [29] in a similar way:

1. Occurring immediately. The term is used to describe a number of different computer features. For example, real-time operating systems are systems that respond to input immediately. They are used for such tasks as navigation, in which the computer must react to a steady flow of new information without interruption. Most general-purpose operating systems are not real-time because they can take a few seconds, or even minutes, to react.

2. Real time can also refer to events simulated by a computer at the same speed that they would occur in real life. In graphics animation, for example, a real-time program would display objects moving across the screen at the same speed that they would actually move.

Definition 2 is useful for the understanding of the real-time required by DCharts. Similarly, the requirement of ``the same speed'' is not strict. DCharts implementations should provide this support as much as possible, given the restrictions of the operating systems that they are built for.

MSN Encarta [30] also defines the real-time concept, which is stricter than the real-time concept in DCharts:

1. Computing immediacy of data processing: the time in which certain computer systems process and update data as soon as it is received from some external source, e.g. an air-traffic control or antilock brake system. The time available to receive the data, process it, and respond to the external process is dictated by the time constraints imposed by the process.

2. Actual time of occurrence: the actual time during which something happens.

In most cases, model designers may assume that 1 second elapsed in the model simulation or execution is approximately equal to 1 second in reality. Designers with time-critical requirements should turn to the documentation of specific DCharts implementations or operating systems to know whether they suit the need.