Atmel CAVR-4 Manual De Usuario

CAVR-4

Part1. Using the compiler

Assembler language interface

101

It is often practical to share header files between C and C++. The following is an
example of a declaration that declares a function with C linkage in both C and C++:

#ifdef _ _cplusplus

extern "C"

{

#endif

int f(int);

#ifdef _ _cplusplus

}

#endif

PRESERVED VERSUS SCRATCH REGISTERS

The general AVR CPU registers are divided into three separate sets, which are described
in this section.

Scratch registers

Any function may destroy the contents of a scratch register. If a function needs the
register value after a call to another function, it must store it during the call, for example
on the stack.

For both calling conventions, the following 14 registers can be used as scratch
registers by a function:

R0–R3

R16–R23

, and

R30–R31

Preserved registers

Preserved registers, on the other hand, are preserved across function calls. Any function
may use a preserved register for other purposes, but must save the value prior to use and
restore it at the exit of the function.

For both calling conventions, the following registers are preserved registers:

R4–R15

and

R24–R27

Note that the registers

R4–R15

can be locked from the command line and used for global