Imagecraft c compiler and development environment for the atmel avr

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ICCV8 for AVR – C Compiler for Atmel AVRThere are two observations about this process. First, the C preprocessor is separatefrom the compiler proper and does textual processing only. There are caveats about#define macros that arise from this. For example, in the macro definition, it isadvisable that you put parentheses around the macro arguments to preventunintended results:#define mul1(a, b) a * b // bad practice#define mul2(a, b) ((a) * (b)) // good practicemul1(i + j, k);mul2(i + j, k);mul1 produces an unexpected result for the arguments, whereas mul2 produces thecorrect result (of course, it is not a good idea to #define simple operations such assingle multiplication, but that is another subject). Second, C files are translated intoassembler files and are then processed by the assembler. In fact, C is sometimescalled a high-level assembler, since the amount of translation between C andassembly is relatively small, compared to the more complex languages such as C++,Java, FORTRAN, etc.Source Code Structures; Header Files etc.Your program must contain a function called main. It is a good practice to partitionyour program into separate source files, each one containing functionally relatedfunctions and data. In addition, when the program is more modular in structure, it isfaster to rebuild a project that has multiple smaller files rather than one big file. Usingthe CodeBlocks IDE, add each file into the Project. Note that if you #include multiplesource files in a main file and only add the main file in the project manager, theneffectively you still have just one main file in your project and will not be getting thebenefits stated above.You should put public function prototypes into public header files that are #includeby other files. Private functions should be declared with the static keyword and thefunction prototypes should be declared either in a private header file or at the top ofthe source file where they appear. Public header files should also contain any globalvariable declarations.Recall that a global variable should be defined in only one place but can be declaredin multiple places. A common practice is to put a conditional declaration such as thefollowing in a header file:(header.h)#ifndef EXTERN#define EXTERN extern72
ICCV8 for AVR – C Compiler for Atmel AVR#endifEXTERN int clock_ticks;Then in one and only one of the source files (say, main.c), you can write#define EXTERN#include “header.h”In all other source files, you would just #include “header.h” without thepreceding #define. Since main.c has EXTERN defined to be nothing, then theinclusion of header.h has the effect of defining the global variable clock_ticks. Inall other source files, the EXTERN is expanded as extern, thus declaring (but notdefining) clock_ticks as a global variable and allowing it to be referenced in thesource files.Use of Global Variables vs. Locals and Function ArgumentsFunctions can communicate using either global variables or function arguments. Onsome processors, it is better to use global variables; on others, it is better to use localvariables and arguments; and on some others, it does not matter at all. The followingsummarizes the current ImageCraft compiler targets but should only be used as aguideline. You should always balance optimization needs with program maintenanceneeds.Generally, using local variables is a better choice for the Atmel AVR, TI MSP 430 andARM targets. ImageCraft compilers for these targets automatically allocate localvariables to machine registers if possible and programs under these RISC processorsrun much faster when machine registers are used. On the Motorola HC11 and HC12/S12, it is a slight win to use local variables. On the HC08/S08, it probably does notmatter at all.On some processors that we do not support, it is much better to use global variables.For example, the 8051 is such an architecture.73
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