[chbot] C question: variables in GCC

[Robin Gilks]

On a 328 why not use the eeprom - it's good for 100,000 read/write cycles.
That's a lot of reboots.


On Fri, Jun 2, 2023 at 12:00 PM Charles Manning <cdhmanning at gmail.com>
wrote:

> 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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