[chbot] C question: variables in GCC

[Charles Manning]

Hi Mark

The C start up code works as you say. There are two main sections normally:
.data* variables initialised to non-zero
.bss*   variables initialised to zero

.data* are initialised by copying initialisation values from flash to RAM
at start.
.bss* are normally just initialised to zero by a loop.

If you want untouched memory then you need some area outside of these and
there are at least two ways to do so.

1) Create a new section in the ld script called uninitilised or something.
Create a variable in C and use #pragmas or _attributes() to place the
variable in the uninitialised section.
2) Create a new section in the ld script at a specific location at a fixed
address (eg top of memory). Access it from C by creating a pointer to the
address.
int *magic_place = ((int *)0x1234);
3) Scratch around your memory map and find an untouched address, then
create a pointer to it. ie. a less controlled version of 2.

I would not recommend (3) because if your code changes and the memory map
changes then bad things could happen.



On Fri, Jun 2, 2023 at 6:48 AM Mark Atherton <markaren1 at xtra.co.nz> wrote:

> Hi All,
>
> I am working on a low-power application for an ATmega328P where the
> watchdog is used to repeatedly restart the machine for one particular
> state.
>
> One of the mechanisms (part of a bootloader) requires that the number of
> WD restarts are counted by a U16.
>
> I am pretty sure that C99 normally only supports initialized variables
> (with associated value) and uninitialized variables (which are set to 0).
>
> Is it possible to declare storage in C that isn’t touched by the
> associated startup code ?
>
> Options seem to be to resort to assembler (and possibly have to do
> battle with the linker script), or to find a couple of unused Read/Write
> peripheral registers within the 328 and use them.
>
> Thoughts please.
>
> Thanks, Mark
>
>
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