Specifications

CAVR-4
Part1. Using the compiler
Assembler language interface
105
Hidden parameters
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.
FUNCTION EXIT
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.
Return values
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.