In the Programming/Erasure Host Command Wait State section of the ROM (Flash Memory for Code Storage) chapter of the user's manual: hardware, for 256-byte programming it states that "FFh should be specified for the bytes that do not need to be programmed." What does this mean?
During programming, a value of FFh prevents the state of the flash memory from being changed. Therefore, FFh should be specified for bytes that do not need to be programmed. (FFh is the value of flash memory in a blank state)