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The term real-time is defined by FOLDOC [28] as
following:
- 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.
- 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:
- 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.
- 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:
- 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.
- 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.
Next: 3.2 Virtual-time Simulation
Up: 3. Timing
Previous: 3. Timing
Contents
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Thomas Huining Feng
2004-04-28