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specifies additional characteristics of Ada implementations intended
for real-time systems software. To conform to this Annex, an implementation
shall also conform to the Systems Programming Annex.
The metrics are documentation requirements; an
implementation shall document the values of the language-defined metrics
for at least one configuration of hardware or an underlying system supported
by the implementation, and shall document the details of that configuration.
The metrics do not necessarily yield a simple
number. For some, a range is more suitable, for others a formula dependent
on some parameter is appropriate, and for others, it may be more suitable
to break the metric into several cases. Unless specified otherwise, the
metrics in this annex are expressed in processor clock cycles. For metrics
that require documentation of an upper bound, if there is no upper bound,
the implementation shall report that the metric is unbounded.
1 The specification of
the metrics makes a distinction between upper bounds and simple execution
times. Where something is just specified as ``the execution time of''
a piece of code, this leaves one the freedom to choose a nonpathological
case. This kind of metric is of the form ``there exists a program such
that the value of the metric is V''. Conversely, the meaning of upper
bounds is ``there is no program such that the value of the metric is
greater than V''. This kind of metric can only be partially tested, by
finding the value of V for one or more test programs.
2 The metrics do not cover
the whole language; they are limited to features that are specified
in Annex C, ``Systems Programming''
and in this Annex. The metrics are intended to provide guidance to potential
users as to whether a particular implementation of such a feature is
going to be adequate for a particular real-time application. As such,
the metrics are aimed at known implementation choices that can result
in significant performance differences.
3 The purpose of the metrics
is not necessarily to provide fine-grained quantitative results or to
serve as a comparison between different implementations on the same or
different platforms. Instead, their goal is rather qualitative; to define
a standard set of approximate values that can be measured and used to
estimate the general suitability of an implementation, or to evaluate
the comparative utility of certain features of an implementation for
a particular real-time application.
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