Atmel CAVR-4 Manual De Usuario

CAVR-4

Type and object attributes

Reference Guide

When declaring objects, note that the IAR-specific attributes work exactly like 

const

. 

One exception to this is that attributes that are declared in front of the type specifier 
apply to all declared objects.

See More examples, page 21.

There are three main reasons for declaring an object 

volatile

:

●

Shared access; the object is shared between several tasks in a multitasking 
environment

●

Trigger access; as for a memory-mapped SFR where the fact that an access occurs 
has an effect

●

Modified access; where the contents of the object can change in ways not known to 
the compiler.

The ISO/ANSI standard defines an abstract machine, which governs the behavior of 
accesses to 

volatile

 declared objects. The AVR IAR C/C++ Compiler considers each 

read and write access to an object that has been declared 

volatile

 as an access. The 

unit for the access is either the entire object or, for accesses to an element in a composite 
object—such as an array, struct, class, or union—the element. For example:

char volatile a;

a = 5;   /* A write access */

a += 6;  /* First a read then a write access */

An access to a bitfield is treated as an access to the underlaying type.

Accesses to 

volatile

 declared objects are subject to the following rules:

1 All accesses are preserved

2 All accesses are complete, that is, the whole object is accessed

3 All accesses are performed in the same order as given in the abstract machine

4 All accesses are atomic, that is, non-interruptable.