Embedded Systems Design - July 2008 - (Page 21)

+++ N O R O Y A LT I E S +++ embeddedsoftware ments that In CleanC, only loops that of using the patinstead of usnot deﬁned follow this ﬁxed pattern tern for example, a ing, or, in general, are recognized as being while loop. If the not well understood. The manifest by the tools. Use iterator is updated CleanC rules a pattern instead of using, in several places,be this can usually specify a limfor example, a while loop. replaced by a ﬁxed ited set of update and not constructs executing the loop that should be body in some iterations. avoided. More rules of this type can A loop with a data-dependent be found elsewhere, such as in the upper bound (the typical while Misra C guidelines available at loop) can be replaced by a manifest www.misra.org.uk. loop over the maximum number of iterations with a condition nested PUTTING CLEANC TO WORK inside it. However, this is only useful Rather than exhaustively delineate these if the iterator is then used in some and other categories of rules for writing index expression. C-code for multiprocessor designs, folHere is an example where the lowing are examples of some of these guideline is followed: rules at work. solutions embOS ® Ev a av l ver ail si ab on le s (RTOS) +++ 8/16/32 bits +++ Preemptive multitasking Zero interrupt latency Easy to use start project included Profiling support included Object/source code available • emWin ® (GUI) Example #1—Guideline: use the manifest loop pattern. Many optimizations rely on analyzing exactly which data element is accessed at which point in time. That requires interpreting how index expressions evolve over the execution of the loops. Therefore, it must be possible to interpret the index expressions, the loop bounds, and how the loop iterator is incremented. To make this interpretation easier, CleanC uses a ﬁxed pattern for manifest loops. A manifest loop is executed in exactly the same way, independent of the input received by the code. More formally, a manifest loop is a loop with an iterator that is initialized with a manifest value. The iterator is incremented or decremented with a constant value in each iteration, and the loop ﬁnishes when the iterator reaches a certain manifest value. A manifest value is a value that is either constant or computed from the iterators of outer loops. Nonmanifest loops, on the other hand, have no restriction on how they should be written. In CleanC, only loops that follow this ﬁxed pattern are recognized as being manifest by the tools. Therefore, it mostly is just a matter for (i=0; i < 100; i = i + 4) for (j=i; j < 150; j++) if (j%5 != 0) +++ 8/16/32 bits +++ and a counterexample (in other words, where the guideline is not followed): i=0; while (i<100) { for (j=i; j<150; j++) { if (j%5 == 0) j++; } i = i + 4; } ANSI "C" source code, no C++ required Supports b/w, grayscale and color 2D graphic library included Variety of fonts included PC simulation included Window Manager/Widgets (opt) emFile (File system) +++ 8/16/32 bits +++ ANSI "C" source code MS-DOS/MS-Windows compatible FAT12, FAT16 and FAT32 support Multiple media support Non FAT file system available Figure 1 shows a screenshot of an example of the guideline “manifest loop pattern” violation as it would appear in the CleanC tool. • Example #2—Restriction: distinguish source ﬁles from header ﬁles. The C language does not formally distinguish between the interface and the implementation of a module. It only provides the #include preprocessor directives, which can be used to separate interface from implementation but which can also For ARM Chips: JTAG debug solution with flash programming phone: 978-874-0299 www.segger.com www.embedded.com | embedded systems design | JULY 2008 21 http://www.misra.org.uk http://www.segger.com http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - July 2008

Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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