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<HTML
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><HEAD
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><TITLE
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>Why Configurability?</TITLE
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CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
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"><LINK
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REL="HOME"
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TITLE="The eCos Component Writer's Guide"
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HREF="cdl-guide.html"><LINK
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REL="UP"
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TITLE="Overview"
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HREF="overview.html"><LINK
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REL="PREVIOUS"
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TITLE="Overview"
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HREF="overview.html"><LINK
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REL="NEXT"
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TITLE="Approaches to Configurability"
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HREF="overview.approaches.html"></HEAD
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><TH
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ALIGN="center"
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>The <SPAN
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>eCos</SPAN
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> Component Writer's Guide</TH
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>Prev</A
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 1. Overview</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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NAME="OVERVIEW.CONFIGURABILITY">Why Configurability?</H1
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><P
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>The <SPAN
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CLASS="APPLICATION"
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>eCos</SPAN
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> component framework places a great deal of emphasis on
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configurability. The fundamental goal is to allow large parts of
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embedded applications to be constructed from re-usable software
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components, which does not a priori require that those components be
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highly configurable. However embedded application development often
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involves some serious constraints.</P
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><P
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>Many embedded applications have to work with very little memory, to
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keep down manufacturing costs. The final application image that will
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get blown into EPROM's or used to manufacture ROMs should contain only
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the code that is absolutely necessary for the application to work, and
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nothing else. If a few tens of kilobytes are added unnecessarily to a
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typical desktop application then this is regrettable, but is quite
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likely to go unnoticed. If an embedded application does not fit on the
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target hardware then the problem is much more serious. The component
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framework must allow users to configure the components so that any
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unnecessary functionality gets removed.</P
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><P
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>Many embedded applications need deterministic behavior so that they
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can meet real-time requirements. Such deterministic behavior can
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often be provided, but at a cost in terms of code size, slower
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algorithms, and so on. Other applications have no such real-time
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requirements, or only for a small part of the overall system, and the
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bulk of the system should not suffer any penalties. Again the
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component framework must allow the users control over the timing
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behavior of components.</P
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><P
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>Embedded systems tend to be difficult to debug. Even when it is
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possible to get information out of the target hardware by means other
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than flashing an LED, the more interesting debugging problems are
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likely to be timing-related and hence very hard to reproduce and track
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down. The re-usable components can provide debugging assistance in
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various ways. They can provide functionality that can be exploited by
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source level debuggers such as gdb, for example per-thread debugging
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information. They can also contain various assertions so that problems
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can be detected early on, tracing mechanisms to figure out what
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happened before the assertion failure, and so on. Of course all of
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these involve overheads, especially code size, and affect the timing.
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Allowing users to control which debugging features are enabled for any
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given application build is very desirable.</P
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><P
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>However, although it is desirable for re-usable components to provide
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appropriate configuration options this is not required. It is possible
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to produce a package which does not provide a single configuration
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option — although the user still gets to choose
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whether or not to use the package. In such cases it is still necessary
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to provide a minimal CDL script, but its main purpose would be to
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integrate the package with the component framework's build system.</P
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>Prev</A
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HREF="cdl-guide.html"
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>Home</A
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>Next</A
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WIDTH="33%"
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ALIGN="left"
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>Overview</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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>Up</A
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>Approaches to Configurability</TD
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