[chbot] C question: variables in GCC

[Charles Manning]

Check the map file when you've done it to make sure the variable turned up
in the right place.

On Fri, Jun 2, 2023 at 10:59 AM Mark Atherton <markaren1 at xtra.co.nz> wrote:

> Thanks Charles,
>
> Your introduction ties up well with my memory, and solution 1 was what I
> had in mind. Thanks for the quick-start, haven't been anywhere the
> linker for ages.
>
> -Mark
>
>
> On 6/2/2023 8:23 AM, Charles Manning wrote:
> > 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
> > <mailto: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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