Complex sentence: Memory-mapped I/O is the cause of memory barriers in older generations of computers--the 640 KiB barrier is due to the IBM PC placing the Upper Memory Area in the 640--1024 KiB range (of its 20-bit memory addressing).
Split into simpler sentences: Memory-mapped I/O is the cause of memory barriers in older generations of computers. The 640 KiB barrier is due to the IBM PC placing the Upper Memory Area in the 640--1024 KiB range (of its 20-bit memory addressing).