Atmel CAVR-4 Manual De Usuario

CAVR-4

Calling convention

Reference Guide

This would result in 

a

 being allocated to 

R16

 (first fit), 

b

 to 

R19:R18

 (alignment), 

c

 to 

R23:R22:R21:R20

 (first fit), and 

d

 to 

R17

 (first fit).

A third example:

void baz(char a, char _ _far * b, int c, int d)

This would give, 

a

 being allocated to 

R16

, 

b

 to 

R22:R21:R20

, 

c

 to 

R19:R18

, and 

d

 to 

the stack.

There is only a limited number of registers that can be used for passing parameters; 
when no more registers are available, the remaining parameters are passed on the stack. 
In addition, the parameters are passed on the stack in the following cases:

●

Structure types: 

struct

, 

union

, and classes if larger than 4 bytes

●

The data type 

double

 (64-bit floating-point numbers)

●

Unnamed parameters to variable length functions; in other words, functions 
declared as 

foo

(

, 

), for example 

printf

.

A function call creates a stack frame as follows:

Note that only the registers that are used will be saved.

Any parameters that did not go into registers.

R4–R15, R24–R27

Area for local variables.

Any temporary values.

High 
address

Low 
address

Y
Stack pointer