嵌入式ARM的mmu操作示例。设置MMU使得外部SDRAM可用，然后对比led测试程序在内部RAM和外部RAM上的运行速度。-A mmu operation demo for embedded arm. This program set the mmu of arm and make external SDRAM usable, then we compare the excutting speed of a led test program between internal SRAM and external SDRAM.