Imagecraft c compiler and development environment for the atmel avr

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ICCV8 for AVR – C Compiler for Atmel AVRStringsAs explained in Program Data and Constant Memory, the separation of program anddata memory in the AVR’s Harvard architecture introduces some complexity. Thissection explains this complexity as it relates to literal strings.The compiler places switch tables and items declared as __flash into programmemory. The last thorny issue is the allocation of literal strings. The problem is that, inC, strings are converted to char pointers. If strings are allocated in the programmemory, either all the string library functions must be duplicated to handle differentpointer flavors, or the strings must also be allocated in the data memory. TheImageCraft compiler offers two options for dealing with this.Default String AllocationThe default is to allocate the strings in both the data memory and the programmemory. All references to the strings are to the copies in data memory. To ensure theirvalues are correct, at program startup the strings are copied from the programmemory to the data memory. Thus, only a single copy of the string functions isneeded. This is also exactly how the compiler implements initialized global variables.If you wish to conserve space, you can allocate strings only in the program memory byusing __flash character arrays. For example:__flash char hello[] = “Hello World”;In this example, hello can be used in all contexts where a literal string can be used,except as an argument to the standard C library string functions as previouslyexplained.The printf function has been extended with the %S format character for printing outFLASH-only strings. In addition, new string functions have been added to supportFLASH-only strings. (See String Functions.)Allocating All Literal Strings to FLASH OnlyYou can direct the compiler to place all literal strings in FLASH only by selecting theBuild Options - Target “Strings In FLASH Only” checkbox. Again, be aware that youmust be careful when calling library functions. When this option is checked, effectivelythe type for a literal string is __flash char *, and you must ensure the functiontakes the appropriate argument type. Besides new __flash char * related StringFunctions, the cprintf and csprint functions accept __flash char * as theformat string type. See Standard IO Functions.118
ICCV8 for AVR – C Compiler for Atmel AVRStacksThe generated code uses two stacks: a hardware stack that is used by the subroutinecalls and interrupt handlers, and a software stack for allocating stack frames forparameters, temporaries and local variables. Although it may seem cumbersome,using two stacks instead of one allows the most transparent and code efficient use ofthe data RAM.Since the hardware stack is used primarily to store function return addresses, it istypically much smaller than the software stack. In general, if your program is not callintensiveand if it does not use call-intensive library functions such as printf with %fformat, then the default of 16 bytes should work well. In most cases, a maximum valueof 40 bytes for the hardware stack is sufficient unless your program has deeplyrecursive routines.The hardware stack is allocated at the top of the data memory, and the software stackis allocated a number of bytes below that. The size of the hardware stack and the sizeof the data memory are controlled by settings in the target tab of Build Options -Target. The data area is allocated starting at 0x60, 0x100 or 0x200, depending onthe device, after the IO space. This allows the data area and the software stack togrow toward each other.If you select a device target with 32K or 64K of external SRAM, then the stacks areplaced at the top of the internal SRAM and grow downward toward the low memoryaddresses. See Program and Data Memory Usage.Stack CheckingA common source of random program failure is stack overflowing other data memoryregions. Either of the two stacks can overflow, and Bad Things Can Happen (tm) whena stack overflows. You can use the Stack Checking Functions to detect overflowsituations.119
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Imagecraft c compiler and development environment for the atmel avr

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