Flash CRC, constant at Flash end

Hello,

I am doing a CRC calculation over the Program Flash of the Cortex M0 STM32F030 CPU.
For this I want to place a constant at the end of the flash (containing the expected CRC) using:
__attribute__((at(address))) variable attribute
It works if I take the last possible word (32 bit) Address - 64 Bytes.
I would like to understand why?
Does anybody know? Probably some generated reagion ist generated which needs minimum size 64 bytes but havend found any info so far.

Thank you.

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