Code Confidencebuild

Why Configurability?

The eCos component framework places a great deal of emphasis on configurability. The fundamental goal is to allow large parts of embedded applications to be constructed from re-usable software components, which does not a priori require that those components be highly configurable. However embedded application development often involves some serious constraints.

Many embedded applications have to work with very little memory, to keep down manufacturing costs. The final application image that will get blown into EPROM's or used to manufacture ROMs should contain only the code that is absolutely necessary for the application to work, and nothing else. If a few tens of kilobytes are added unnecessarily to a typical desktop application then this is regrettable, but is quite likely to go unnoticed. If an embedded application does not fit on the target hardware then the problem is much more serious. The component framework must allow users to configure the components so that any unnecessary functionality gets removed.

Many embedded applications need deterministic behavior so that they can meet real-time requirements. Such deterministic behavior can often be provided, but at a cost in terms of code size, slower algorithms, and so on. Other applications have no such real-time requirements, or only for a small part of the overall system, and the bulk of the system should not suffer any penalties. Again the component framework must allow the users control over the timing behavior of components.

Embedded systems tend to be difficult to debug. Even when it is possible to get information out of the target hardware by means other than flashing an LED, the more interesting debugging problems are likely to be timing-related and hence very hard to reproduce and track down. The re-usable components can provide debugging assistance in various ways. They can provide functionality that can be exploited by source level debuggers such as gdb, for example per-thread debugging information. They can also contain various assertions so that problems can be detected early on, tracing mechanisms to figure out what happened before the assertion failure, and so on. Of course all of these involve overheads, especially code size, and affect the timing. Allowing users to control which debugging features are enabled for any given application build is very desirable.

However, although it is desirable for re-usable components to provide appropriate configuration options this is not required. It is possible to produce a package which does not provide a single configuration option — although the user still gets to choose whether or not to use the package. In such cases it is still necessary to provide a minimal CDL script, but its main purpose would be to integrate the package with the component framework's build system.