Atmel CAVR-4 Manual De Usuario

CAVR-4

Descriptions of extended keywords

Reference Guide

_ _regvar

The 

_ _regvar

 extended keyword is used for declaring that a global or static variable 

should be placed permanently in the specified register or registers. The registers 

R4–R15

 

can be used for this purpose, provided that they have been locked with the 

--lock_regs

 compiler option; see --lock_regs, page 186 for additional information. 

To declare a global register variable, use the following syntax:

_ _regvar _ _no_init type_name variable_name @ Rlocation

This will create a variable called 

 of type 

, located in 

registers starting from 

Rlocation

, for example:

_ _regvar _ _no_init int counter @ 14;

This will create the 16-bit integer variable counter, which will always be available in 

R15:R14

. At least two registers must be locked. And, as noted in -m, --memory_model, 

page 186, if you lock any registers in your code, the library must be rebuilt with the same 
set of locked registers.

The maximum object size is 4 bytes (32 bits).

Note: It is not possible to point to an object that has been declared 

_ _regvar

. An 

object declared 

_ _regvar

 cannot have an initial value.

_ _root

The __

root

 attribute can be used on a function or a variable to ensure that, when the 

module containing the function or variable is linked, the function or variable is also 
included, whether or not it is referenced by the rest of the program.

By default, only the part of the runtime library calling 

main

 and any interrupt vectors 

are root. All other functions and variables are included in the linked output only if they 
are referenced by the rest of the program.

The 

_ _root

 keyword is placed in front of the type, for example to place 

myarray

 in 

non-volatile memory:

_ _root int myarray[10]; 

The 

#pragma

 

object_attribute

 directive can also be used. The following 

declaration is equivalent to the previous one:

#pragma object_attribute=_ _root

int myarray[10];

Note: The 

_ _root

 keyword cannot be used in combination with the 

typedef

 

keyword.