Atmel CAVR-4 Manual De Usuario

CAVR-4

Part1. Using the compiler

In addition to the parameters visible in a function declaration and definition, there can 
be hidden parameters:

●

A function returning structures or unions larger than four bytes gets an extra hidden 
parameter, which is a default pointer—depending on the used memory 
model—pointing to the location where the result should be stored. This pointer 
must be returned to the called by the assembler function.

●

For non-static C++ member functions, the 

this

 pointer is passed as the first 

parameter (but placed after the return structure pointer, if there is one). Note that 
static member functions do not have a 

this

 pointer.

The called function exits by deallocating auto variables, restoring registers, deallocating 
stack parameters, and finally performing a 

RET

 instruction which pops the return address 

from the return address stack. Note that an interrupt function returns by performing a 

RETI

 function.

A function can return a value to the function or program that called it, or it can be of the 
type 

void

. The return value of a function, if any, can be scalar (such as integers and 

pointers), floating-point, or a structure. A return value can be passed via register or via 
the stack.

For both calling conventions, the following details are valid:

●

The return address stack (RSTACK) and the data stack (CSTACK) are two separate 
stacks. The RSTACK uses the internal I/O port 

SP

, which is declared in the 

ioderivative.h

 include files provided with the product.

●

struct

 and 

union

 values larger than 4 bytes are passed using a pointer; this 

pointer is returned via register

●

The implicit first parameter passed to a function, pointing to the memory to be used 
for storing the return value, is always passed on the stack if the value is larger than 4 
bytes.