Atmel CAVR-4 Manual De Usuario

CAVR-4

Calling convention

Reference Guide

For some registers there are certain prerequisites that you must consider:

●

The stack pointer—register 

Y

—must at all times point to the last element on the 

stack. In the eventuality of an interrupt, everything below the point that the stack 
pointer points to, will be destroyed.

●

If using the 

-v4

 or 

-v6

 processor option, the 

RAMPY

 register is part of the data stack 

pointer.

●

If using a processor option which utilizes any of the registers 

EIND

, 

RAMPX

, or 

RAMPZ

, these registers are treated as scratch registers.

During a function call, the calling function:

●

passes the parameters, either in registers or on the stack

●

pushes any other parameters on the data stack (CSTACK)

Control is then passed to the called function with the return address being automatically 
pushed on the return address stack (RSTACK).

The called function:

●

stores any local registers required by the function on the data stack

●

allocates space for its auto variables and temporary values

●

proceeds to run the function itself.

Parameters can be passed to a function using one of two basic methods: in registers or 
on the stack. It is much more efficient to use registers than to take a detour via memory, 
so the calling convention is designed to utilize registers as much as possible. The 
remaining parameters not passed in registers are passed on the stack.

For both calling conventions, the registers available for passing parameters are:

R16–R23

Parameters are allocated to registers using a first-fit algorithm, using parameter 
alignment requirements according to the following table:

8-bit values

1

R16

, 

R17

, 

R18

, 

R19

, 

R20

, 

R21

, 

R22

, 

R23