MSP430 IAR C/C++ Compiler reference guide - Rice University

Calling conventionExample 2This example shows how structures are passed on the stack. Assume that we have thefollowing declarations:struct a_struct { int a; };int a_function(struct a_struct x, int y);The calling function must reserve 4 bytes on the top of the stack and copy the contentsof the struct to that location. The integer parameter y is passed in the register R12. Thereturn value is passed back to its caller in the register R12.Example 3The function below will return a struct.struct a_struct { int a; };struct a_struct a_function(int x);It is the responsibility of the calling function to allocate a memory location—typicallyon the stack—for the return value and pass a pointer to it as a hidden first parameter. Thepointer to the location where the return value should be stored is passed in R12. Thecaller assumes that this register remains untouched. The parameter x is passed in R14.Assume that the function instead would have been declared to return a pointer to thestructure:struct a_struct * a_function(int x);In this case, the return value is a pointer, so there is no hidden parameter. The parameterx is passed in R12 and the return value is also returned in R12.FUNCTION DIRECTIVESNote: This type of directive is primarily intended to support static overlay, a featurewhich is useful in some smaller microcontrollers. The MSP430 IAR C/C++ Compilerdoes not use static overlay, because it has no use for it.The function directives FUNCTION, ARGFRAME, LOCFRAME, and FUNCALL are generatedby the MSP430 IAR C/C++ Compiler to pass information about functions and functioncalls to the IAR XLINK Linker. These directives can be seen if you use the compileroption Assembler file (-lA) to create an assembler list file.For reference information about the function directives, see the MSP430 IAR AssemblerReference Guide.96MSP430 IAR C/C++ CompilerReference Guide