http://en.wikipedia.org/wiki/Memory-mapped_I/O
Memory-mapped I/O (MMIO) and port I/O (also called isolated I/O or port-mapped I/O abbreviated PMIO) are two complementary methods of performing input/output between the CPU andperipheral devices in a computer.
深入了解内存（一）
http://www.hzlitai.com.cn/article/ARM7-artilcle/cphard/1281.html