获取C中的__regvar变量的地址

Question

At the moment I'm working in a poject for an embedded system in which we're using a AVR ATMega8 microproccessor. As it has a quite small memmory area, I'm trying to store some global variables into registers using the __regvar command.

I need to access those variables from multiple files so instead of making all of them external I had thought about making external only a pointer to the first one and through offsets modify all the variables.

For example:

Global 1 --> $Reg4 Global 2 --> $Reg5 Global 3 --> $Reg6 Global 4 --> $Reg7

Pointer = AddressOfGlobal1

And with that I can modify all registers just by adding 1 (1 byte)

The problem is that I can't get the address of that variable:

__no_init __regvar unsigned char ms_flags@ 4; 
__no_init __regvar unsigned char tb_flags@ 5; 
__no_init __regvar unsigned char st_flags@ 6; 
__no_init __regvar unsigned char dp_flags@ 7; 


int myfunction(){


 unsigned char* pointer     = 0;

    pointer = &ms_flags;
}    

Getting the following output:

Error[Pe513]: a value of type "unsigned char __regvar *" cannot be assigned to an entity of type "unsigned char *" C:\\Users\\Alex\\Desktop\\CentroStirling\\Progreso\\MSUIAR\\app\\init.c 39

Answer 1

You can't do that; a value stored in a register has no address , by definition. That's the point, the thing that you're jumping through hoops to achieve.

The draft C99 standard says, in §6.5.3.2:

The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.

If we peel off the compiler-specifics, I'm pretty sure __regvar somehow implies register .