ds28035 flash

 // Example: This will return an error!!  Can't program 1
   // because bit 0 in the the location was previously programmed
   // to zero!
    for(i=0;i<WORDS_IN_FLASH_BUFFER;i++)
    {
        Buffer[i] = 0xFFFF;
    }

    Flash_ptr = Sector[0].EndAddr;
    Length = 13;
    Status = Flash_Program(Flash_ptr,Buffer,Length,&FlashStatus);
    if(Status != STATUS_FAIL_ZERO_BIT_ERROR)//STATUS_FAIL_ADDR_INVALID)
    {
        Example_Error(Status);
    }

例程中，为什么不能写1？please。