Reserving space in SRAM for SRAM decay experiment (C ; AVR atmega644p ; Atmel Studio 7) - c

Question

I am looking to perform some experiments on an atmega644p looking at evaluating the amount of decay in SRAM between power cycles. My method is to set a number of bytes in SRAM to 0xFF, then when the mcu powers back up, count the number of remaining 1s in these bytes.
For this to work, I need to read and write the array of 1s to/from a known memory address in SRAM. So far I have code which writes the values to a specific address using a pointer set to 0x1000, and then on power up I begin reading the array from this address. However, I need a way of guaranteeing that this section of SRAM memory (say, 0x100 + 64 bytes) is not allocated to other variables/overwritten before it can be read.
I have looked online at the possibility of allocating memory segments - I don't know if this is a good solution in this case, and am not even too sure how to go about doing this. Can anyone suggest a neat way of approaching this?
Please ask any questions for clarification.
Thanks for your help.

Answer 1

If you're using AVR/GNU, then when C app starts it clears whole the memory and initializes global variables as required.
To avoid that, you can configure the linker to exclude all start files using options -nostartfiles -nodefaultlibs -nostdlib
If you're using Atmel Studio you can configure it like this:
After that you can mark your main for it to be called as initialization code:
int main(void) __attribute__((naked, section(".init9")));
Now you'll have the "naked" code, which does not perform ANY initialization.
That means you need at least to initialize the stack pointer and clear register r1 (which is assumed by avr-gcc to contain zero):
int main(void) {
asm volatile (
"clr r1\n" // load zero into r1
"cli" // clear I flag
);
SPL = (uint8_t)RAMEND;
SPH = (uint8_t)(RAMEND >> 8);
... // here goes your code
for(;;); // do not leave main()!
}
After this you'll have ALL global variables uninitialized. You can declare, for example a global array and check its content on startup.

Answer 2

You'll need to come up with a custom region in RAM by reserving space for it in your linker file. It is important that it is marked "no init" or similar, or otherwise .bss initialization and similar might happen on it before main() is called. How to do this is linker-specific.
However, writing software for this seems needlessly cumbersome. Simply use an in-circuit debugger:
Ensure that you are using a debugger which does not power the target.
Download a program which uses no RAM at all into flash. Confirm this by checking the map file.
Set the whole RAM to 0xFF through the debugger.
Remove power while keeping the in-circuit debugger connected.
Wait x time units.
Power up, hit MCU reset in the debugger, memory dump the whole RAM.
Any half-decent tool chain should be able to do this for you.