Complex sentence: The segment index register at 60h is volatile, defaulting to zero and automatically resets to zero after each NACK or STOP, therefore it should be set every time access to data above the first 256-byte segment is performed.
Split into simpler sentences: The segment index register is volatile, defaulting to zero and automatically resetting to zero after each NACK or STOP. Therefore, it must be set every time access to data above the first 256-byte segment is performed.