Atmel CAVR-4 Manual De Usuario

CAVR-4

Calling assembler routines from C++

Reference Guide

The C calling convention does not apply to C++ functions. Most importantly, a function 
name is not sufficient to identify a C++ function. The scope and the type of the function 
are also required to guarantee type-safe linkage, and to resolve overloading. 

Another difference is that non-static member functions get an extra, hidden argument, 
the 

this

 pointer.

However, when using C linkage, the calling convention conforms to the C calling 
convention. An assembler routine may therefore be called from C++ when declared in 
the following manner:

extern "C" 

{

  int my_routine(int x);

}

Memory access layout of non-PODs ("plain old data structures") is not defined, and may 
change between compiler versions. Therefore, we do not recommend that you access 
non-PODs from assembler routines.

To achieve the equivalent to a non-static member function, the implicit 

this

 pointer has 

to be made explicit:

class X;

extern "C" 

{

  void doit(X *ptr, int arg);

}

It is possible to “wrap” the call to the assembler routine in a member function. Using an 
inline member function removes the overhead of the extra call—provided that function 
inlining is enabled:

class X 

{

public:

  inline void doit(int arg) { ::doit(this, arg); }

}; 

Note: Support for C++ names from assembler code is extremely limited. This means 
that:

●

Assembler list files resulting from compiling C++ files cannot, in general, be passed 
through the assembler.

●

It is not possible to refer to or define C++ functions that do not have C linkage in 
assembler.