You have the option of preserving up to three independent ranges of non-volatile memory (on devices with EEPROM, the EEPROM array may optionally be preserved as well).
Ranges that are designated as "preserved" are read before an erase, and reprogrammed immediately afterwards, thereby preserving the data in these ranges. Any attempt to program data into a preserved range is ignored. When entering an address into the preserved range field (hexadecimal input is expected), the values are masked according to the row size of the device. This ensures that the reprogramming of preserved data does not cause any conditions that disturb programming.