Yes, what you write is possible. The Atmel (ex-Temic) chip uses a scheme, where during FLASH sector/block erasure the unchanged bytes are stored in an internal piece of RAM, hidden to the user and written back into FLASH after erasure automatically. So from the user standpoint, any byte of FLASH can be rewritten without changing other bytes. Please check out the datasheet for IAP details.

Please note, that writing a single byte into 5000h means that in fact the whole 5000h-507Fh block is erased/rewritten, hence wears, even if only a single byte is written. Please check the endurance value.

I believe there are some application where the cca.10000 erase/write cycles are appropriate for logging - for these, keeping the log (data) in the internal (code) FLASH is appropriate and very convenient.

Jan Waclawek