页面置换--问题描述及设计思想：在进程运行过程中，若其所要访问的页面不在内存需把它们调入内存，但内存已无空闲空间时，为了保证该进程能正常运行，系统必须从内存中调出一页程序或数据，送磁盘的对换区中。但应将哪个页面调出，所以需要根据一定的算法来确定。-Page replacement- problem descr iption and design: the process is running, if they want to access the memory page is not required to put them into memory, but memory is no longer the free space, in order to ensure that the process can function properly, the system must memory transfer from one program or data on disk for delivery area. But what the page should be transferred out, so it is necessary to determine according to certain algorithms.