1 C Programming for Embedded Systems. Data RAM Pointer Types ...

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P. Klimo Sep 2010 Explanation of the Compilation Process Example: File1.c contains the C source code for the main function. This is the function that executes first. Because of the #include directives the Pre-processor includes the two files, the one in angled brackets resides in the compiler install directory, the one bracketed with “ should be placed in the Project directory. The stdio.h header file declares the library printf function that is used in the main. The header.h is a user created file that defines a new variable type Int16 based on the compiler’s implementation of the integer variable (in this case 16 bit). Unlike a macro, the typedef performs strict type checking. Note the use of the macro construct #ifndef etc that encloses the declarations in the header file. This prevents the Pre-processor to include in any module the same file more than once in case of a circular reference. The two extern statements declare two functions that are defined outside the file1. This enables the Pre-processor to compile the File1.c without any reference to any other files. Following the declaration of the variables, three functions are called by the main. The function getArea that is defined in the File2.c, the function printf that is defined in the Standard Library that must be linked by the linker (either explicitly or by appropriately configuring the Project file) and the function sum that is defined in the assembler File3.asm. The File3.asm includes the assembler code for the function sum. The directive global allows the labels to be exported to the linker. The _sum label (notice the leading underscore) is the starting address of the subroutine sum. The label ?_sum is the address of the run-time function stack created by the main when the function sum is called. In C the function parameters are copied by the caller (in this case the main) onto the function stack in the reverse order. There are two parameters u and v, both one byte wide. The rules between the caller and the callee (the function) are standardised for any C compiler. In this case the second character (v) is passed in the CPU work register (w) and the first parameter (u) is copied to the stack location ?_sum. Having passed the parameters to CPU register an onto the function stack, the control is passed to the subroutine starting at the memory location _sum. The machine code that resides there adds the value of the parameter placed at ?_sum to the value of the working register and returns. Thus when the function sum returns to the caller the w register in the CPU contains the sum of the two input parameters. This value is used by the caller in the function printf. On function return to the caller, it is the responsibility of the caller to “remove” the callee’s function stack so any subsequent attempts to access the callee’s stack (e.g. the previously stored parameter at the memory address ?_sum ) will have an unreliable outcome. Having independently compiled the three source modules File1.c, File2.c and File3.asm, it is the role of the linker to link them together with any pre-compiled library files to produce a single executable file. In the case of embedded system, the format of the executive file normally chosen is the (Intel) hex file that is used by the device programmer to download the code into the microprocessor’s program ROM. 6
P. Klimo Sep 2010 Example: Driving Seven Segment Display Schematic EasyPic 4 Board Task: Drive a single digit Seven Segment display, displaying digits between 0 and 9 at 1 second interval. 7
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