I am trying to use variable attributes specifically provided by AVR flavor of gcc (https://gcc.gnu.org/onlinedocs/gcc/AVR-Variable-Attributes.html#AVR-Variable-Attributes).
The manual says that these special attributes should allow me to force the placement of a variable at the predetermined memory address. They even give an example:
volatile int porta __attribute__((address (0x600)));But when I compile and debug this code example from the above mentioned document, the variable declared with such attribute is placed into a location in SRAM that compiler and linker determine, not at the address 0x600, as requested. Actually, if I remove the attribute entirely from the declaration, the end result does not change - the variable is placed at the same "whatever" address. Same thing happens when I use "io" and "io_low" attributes instead of "address".
I am using gcc toolchain packaged in the latest version Atmel Studio 7.0.19.31 targeted at 8-bit MCUs (ATMega64).
Hence the question: has anyone tried to use these special AVR-specific attributes with any success?
Important notes:
- I am aware that in general to accomplish a placement of a variable at a fixed address in gcc you need to follow a two-step process (using section attribute and then modifying the linker script), but specificially for AVR it seems like these single-step attributes were provided, the question is how to make them work. A two-step process is not an option for me.
- I am aware that in general one can always do this:
*(volatile int*)0x600 = your_data_here;But this is not an option for me either, I need an actual variable declared (because I want to map it onto a bitwise structure to have access to individual bits without explicitly using the masks and logical operations.
So I am really looking for a way to make the provided attributes work, not for a workaround. What am I missing?